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Geochemical assessment of the bioavailability of platinum group metals for phytomining Shi, Peipei 2016

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GEOCHEMICAL ASSESSMENT OF THE BIOAVAILABILITY OF                   PLATINUM GROUP METALS FOR PHYTOMINING  by Peipei Shi  B.S., Lanzhou University, 2013  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF APPLIED SCIENCE in The Faculty of Graduate and Postdoctoral Studies (Mining Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April 2016  © Peipei Shi, 2016 ii  Abstract Phytomining is suggested as a novel technology to obtain platinum group metals (PGMs) nanoparticles from plants grown on the mineralized soil, rock, or on mine wastes. The primary determinant of metal uptake by a plant is the bioavailability of the metal in the soil-plant system. In this thesis project, the bioavailability of PGMs of several PGM-rich materials is assessed with respect to phytomining. Notably, feed, concentrate and tailings from North American Palladium (NAP) in Canada, and two gossan samples from Broken Hill (BH) mineral complex in Australia are assessed in the context of phytomining.  Geochemical techniques are used to obtain the mineralogy, the total concentration of PGMs, and concentrations of PGMs extracted by chemicals. These methods assess the bioavailability of PGMs in the samples and enable the estimation regarding the available concentrations of Pd, Pt, Au and Cu to plants. Soil-associated factors such as pH, salinity (EC), and cation exchange capacity (CEC) have been shown to influence indirectly the bioavailability of PGMs. Thus, soil-associated factors were analyzed. Additionally, a selection model for the substrate of phytomining is proposed. The criteria for choosing substrate for the phytomining of Pd include available Pd concentration, pH, EC, CEC and available Cu concentration.   This thesis concludes that the PGM species that can be extracted by ammonium acetate are the best indicators of their natural availability to plants. Those PGMs that can be extracted by fulvic acid and citrate-dithionite are good indicators as they can be soluble in soils. According to the selection model, the available Pd concentration of BH gossan 1 is higher than 2 mg/kg. Its low EC, high CEC, and proper pH make it a suitable substrate for plant growth. It is the best “one” of the five samples for phytomining of Pd. One thing to note is that high Cu-tolerant plant species should be chosen to grow on BH gossan 1 due to its high Cu concentration available to plants. iii  Preface Under the supervision of Dr. John Meech and Dr. Marcello Veiga, I was responsible for developing the test program, data collection and conducting analysis as well as interpreting the results.  Chapter 3. ICP-AES analysis was conducted by Acme lab in Vancouver, Canada. XRD analysis was performed by Dr. Mati Raudsepp at the Department of Earth, Ocean & Atmospheric Sciences at UBC. Bo Xu assisted with the selective extraction experiments. The solutions and residues of selective extraction experiments were analyzed by SGS Lab in Vancouver. Soil nutrient analysis was done by Pacific Soil Analysis Inc. in Vancouver.  Chapter 4. A version of this material has been published [Shi, P, Meech, J.A., Anderson, C.W.N., & Veiga, M. (2015). Resource assessment for phytomining of platinum group metals. In Proceedings of the 2015 Sustainable Industrial Processing Summit & Exhibition]. iv  Table of Contents Abstract .................................................................................................................................... ii Preface ..................................................................................................................................... iii Table of Contents ................................................................................................................... iv List of Tables .......................................................................................................................... vii List of Figures ......................................................................................................................... ix List of Abbreviations .............................................................................................................. xi Acknowledgements ............................................................................................................... xii Chapter 1: Introduction ..................................................................................................... 1 1.1 Background ............................................................................................................. 1 1.2 Statement of the problem ........................................................................................ 2 1.3 Objectives ............................................................................................................... 3 1.4 Value of this research .............................................................................................. 3 1.5 Organization of this research .................................................................................. 4 Chapter 2: Literature Review ............................................................................................ 6 2.1 Introduction ............................................................................................................ 6 2.2 Platinum group metals ............................................................................................ 6 2.2.1 Introduction ..................................................................................................... 6 2.2.2 Resources, reserves and production ................................................................ 9 2.2.3 Mineralogy and ore deposits .......................................................................... 11 2.2.4 Processing ...................................................................................................... 13 2.2.5 Potential targets for the phytomining of PGMs............................................. 15 2.2.6 Chemical dissolution of PGMs ..................................................................... 15 2.3 Phytomining .......................................................................................................... 18 2.3.1 Introduction ................................................................................................... 18 v  2.3.2 Phytomining of gold and palladium .............................................................. 19 2.3.3 Soil-associated factors ................................................................................... 20 2.3.3.1 Nutrients ................................................................................................... 20 2.3.3.2 Cation exchange capacity ......................................................................... 22 2.3.3.3 Soil salinity ............................................................................................... 23 2.3.3.4 pH ............................................................................................................. 23 2.3.3.5 Heavy metals in soils ................................................................................ 26 Chapter 3: Sampled Sites and Methods .......................................................................... 28 3.1 Locations of two sampled sites ............................................................................. 28 3.1.1 North American Palladium (NAP) ................................................................ 28 3.1.2 Broken Hill (BH) gossan ............................................................................... 30 3.2 Methods ................................................................................................................ 33 Chapter 4: Results and Discussion .................................................................................. 38 4.1 Particle size analysis ............................................................................................. 38 4.2 Mineralogy ............................................................................................................ 39 4.2.1 X-ray diffraction analysis .............................................................................. 39 4.2.2 SEM analysis ................................................................................................. 42 4.3 PGMs and Au concentration ................................................................................. 51 4.3.1 Total concentration of PGMs and Au ............................................................ 51 4.3.2 Selective extraction experiments ................................................................... 52 4.3.2.1 Exchangeable cations ............................................................................... 52 4.3.2.2 Metallic PGMs ......................................................................................... 52 4.3.2.3 Fe oxides .................................................................................................. 53 4.3.2.4 Reverse aqua regia ................................................................................... 53 4.3.2.5 Non-extractable cations ............................................................................ 53 vi  4.4 Soil associated factors .......................................................................................... 56 4.4.1 Nutrients, pH, EC and CEC .......................................................................... 56 4.4.2 Heavy metals ................................................................................................. 59 4.5 The selection model .............................................................................................. 60 Chapter 5: Conclusion ...................................................................................................... 64 References .............................................................................................................................. 66 vii  List of Tables Table 2. 1 Some important properties of the PGMs as compared with gold (Au) (Gunn, 2014) ................................................................................................................................................... 8 Table 2. 2 Platinum metal reserves worldwide in 2015 (U.S. Geological Survey, 2016a) ..... 10 Table 2. 3 Palladium metal reserves worldwide in 2006 (Diego, 2007) ................................. 10 Table 2. 4 Platinum production worldwide from 2013 to 2015 (U.S.Geological Survey, 2016b) .................................................................................................................................................. 11 Table 2. 5 Palladium production worldwide from 2013 to 2015 (U.S. Geological Survey, 2016c) ...................................................................................................................................... 11 Table 2. 6 Some potential sites worldwide for sampling for the phytomining of PGMs........ 15 Table 2. 7 Some specific plants that can accumulate higher concentration of metals ............ 18 Table 2. 8 Typical concentrations of macronutrients and micronutrients sufficient for plant growth ..................................................................................................................................... 21 Table 2. 9 Different characteristics of soils with different CEC ranges .................................. 22 Table 2. 10 The classes of salinity in soils .............................................................................. 23 Table 2. 11 Soil pH and influence on plant growth................................................................. 25 Table 2. 12 Lime application rates to raise soil pH ................................................................. 26 Table 2. 13 Dutch standards for soil contamination assessment of total concentration.......... 27  Table 4. 1 Particle size distribution of NAP feed, con, tailings, and BH gossan samples ...... 38 Table 4. 2 XRD analysis (wt. %) for samples from NAP ....................................................... 41 Table 4. 3 XRD analysis (wt. %) for gossan 1 and gossan 2 from BH ................................... 41 Table 4. 4 Minerals of BH gossan 1 and gossan 2 observed by SEM .................................... 42 Table 4. 5 Total concentrations of PGMs and Au in all samples ............................................ 52 Table 4. 6 Pd%, Au%, Cu%, Fe% and Ni% extracted by chemicals in NAP feed ................. 54 Table 4. 7 Pd%, Au%, Cu%, Fe% and Ni% extracted by chemicals in NAP con .................. 55 Table 4. 8 Pd%, Au%, Cu%, Fe% and Ni% extracted by chemicals in NAP tailings ............ 55 Table 4. 9 Pd%, Au%, Cu%, Fe% and Ni% extracted by chemicals in BH gossan 1............. 55 Table 4. 10 Pd%, Au%, Cu%, Fe% and Ni% extracted by chemicals in BH gossan 2........... 55 Table 4. 11 Availability of Pd, Pt, Au and Cu in all samples to plants .................................... 56 viii  Table 4. 12 % Metals extracted from BH gossan 1 with citrate-dithionite at 50℃ ................ 56 Table 4. 13 % Metals extracted from BH gossan 2 with citrate-dithionite at 50℃ ................ 56 Table 4. 14 Available concentrations of the primary nutrients in all samples and .................. 58 Table 4. 15 The pH, EC and CEC of all samples .................................................................... 58 Table 4. 16 Soil pH and its influence on plant growth (Whiting et al., 2014) ........................ 58 Table 4. 17 Comparisons with Dutch standards for heavy metals in soils ............................. 59 Table 4. 18 Summary of characteristics of all the samples ..................................................... 63 Table 4. 19 The suitability of all samples assessed by the selection model ............................ 63   ix  List of Figures Figure 2. 1 Abundance (atom fraction) of the chemical elements in the Earth’s upper continental crust as a function of atomic number (Haxel et al., 2002) ..................................... 7 Figure 2. 2 Price ($/ounce) of platinum and gold and the gold/platinum ratio (Cammarosano, 2014) ......................................................................................................................................... 9 Figure 2. 3 The distribution of the main PGM mining districts, mines and other deposits (Gunn, 2014) ........................................................................................................................... 14 Figure 2. 4 (a) Eh–pH diagram for Pt in aqueous solution containing a mixed ligand system consisting of [ΣPt] = 10−9 M, [ΣS]= 10−3 M, [ΣN] = 5 × 10−4 M and [ΣCl] = 0.5 M; (b) Eh–pH diagram for Pd in aqueous solution containing a mixed ligand system consisting of [ΣPd] = 10−9 M, [ΣS]= 10−3 M, [ΣN] = 5 × 10−4 M and [ΣCl] = 0.5 M. (Colombo et al., 2008) ..... 17 Figure 2. 5 Effects of pH (1:5 CaCl2) on nutrient availability ................................................ 25  Figure 3. 1 Lac des Iles (LDI) mine (Peck et al., 2015) ......................................................... 29 Figure 3. 2 Mine Block Intrusion (MBI) (Biggar, 2010) ........................................................ 29 Figure 3. 3 Location of the Broken Hill Pd-Zn-Ag deposit and Mulga Springs deposit (Impact Minerals Limited, 2013) ............................................................................................ 31 Figure 3. 4 Location of the Ni-Cu-PGE prospects of Round Hill, Mulga Springs, Little Darling Creek, Red Hill and other deposits (Golden Cross Resources, 2013) ....................... 32 Figure 3. 5 First extraction scheme ......................................................................................... 36 Figure 3. 6 Second extraction scheme .................................................................................... 37  Figure 4. 1 Particle size distribution curves of NAP feed, con, tailings, and BH gossan samples .................................................................................................................................... 39 Figure 4. 2 Micrography of Back-scattered Electrons and EDS spectra showing: barite, spot 1 in (a) and (b); galena, spot in (c) and (d); chalcopyrite, spot in (e) and (f); Fe-Ni sulfide, spot 2 in (g) and (h). ....................................................................................................................... 45 Figure 4. 3 Micrography of Back-scattered Electrons and EDS spectra showing: Fe-Cr-Ni alloy, spot 1 in (a) and (b); iodargyrite, spot 2 in (a) and (c); bismuth, spot 1 in (d) and (e); Pd-Pt alloy, spot 2 in (d) and (f). ............................................................................................. 46 x  Figure 4. 4 Micrography of Back-scattered Electrons and EDS spectra showing: native Pt, spot 1 in (a) and (b); Pd telluride, spot 1 in (c) and (d). .......................................................... 47 Figure 4. 5 Micrography of Back-scattered Electrons and EDS spectra showing: Pt-Au alloy, spot 1 in (a) and (b); sperrylite, spot 2 in (a) and (c); Pd-Cu oxides, spot 1 in (d) and (e); Pt-Au alloy, spot 2 in (d) and (f). ............................................................................................ 48 Figure 4. 6 Micrography of Back-scattered Electrons and EDS spectra showing: Pd-Cu oxide, spot 1 in (a) and (b). The following six EDS X-ray maps show the spatial distributions of elements in sample BH2-4. ..................................................................................................... 49 Figure 4. 7 Micrography of Back-scattered Electrons and EDS spectra showing: Pd-Cu oxide, spot 1 in (a) and (b); Pt-Au alloy, spot 2 and spot 3 in (a), (c) and (d). The following eight EDS X-ray maps show the spatial distributions of elements in sample BH2-2-4. ................. 51 Figure 4. 8 A possible selection model for the substrate of phytomining ............................... 62  xi  List of Abbreviations BH   Broken Hill CEC   Cation Exchange Capacity EC   Electrical Conductivity ICP-AES   Inductively Coupled Plasma Atomic Emission Spectroscopy ICP-MS   Inductively Coupled Plasma Mass Spectrometry INAA   Instrumental Neutron Activation Analysis LDI   Lac des Iles  MBI   Mine Block Intrusion  NAP   North American Palladium PGE   Platinum Group Element PGMs   Platinum Group Metals SEM   Scanning Electron Microscope  XRD   X-ray Diffraction   xii  Acknowledgements First, I offer my enduring gratitude to my first supervisor, Dr. John Meech, who has inspired me to continue my work in this field. He often encouraged me to have confidence in my ideas and abilities. Many thanks for his willingness to help me to solve problems both in academic study and in life.  I would like to offer special thanks to Dr. Chris Anderson, my co-supervisor, for his continous supervision and constructive guidance on this thesis. I also thank Dr. Marcello Veiga, my second supervisor, for his generous support, advice regarding my thesis, and wonderful music.   Addtionally, I would like to thank Dr. Mati Raudsepp for his support on the XRD and SEM analysis, as well as Acme lab, SGS lab, and Pacific Soil Analysis Inc. for conducting ICP-MS and ICP-AES analysis and soil nutrients analysis. I would like to acknowledge Bo Xu for his generous help with the selective extraction experiments. I also appreciate the faculty, staff and my fellow students at NBK Institute of Mining Engineering for their support and encouragement during my time at UBC.  Finally, special thanks are extended to my parents for their consistent encouragement and incredible support during this period of study. 1  Chapter 1: Introduction  1.1 Background Platinum group metals (PGMs) are used in a broad range of important applications in several industrial sectors including the automotive, chemical, petroleum and medical industries, with by far the greatest use being in the manufacture of automotive catalytic converters; more than 50% of total PGMs production is used to make autocatalysts (Johnson Matthey, 2013). With the number of vehicles increasing and tighter emission standards being brought into effect, the demand for such metals is growing rapidly. Traditionally, PGMs are obtained from ores containing relatively high concentration of the desired element. Usually such ores are from large PGM deposits that are concentrated in a limited number of countries such as South Africa and Russia (U.S. Geological Survey, 2016a; U.S. Geological Survey, 2016c). Mining activities and the production of metals have increased many folds over the past 20 years due to a continuous rise in the world’s population, and this increase has been associated with the gradual depletion of high-grade, easily accessible resources (Sheoran et al., 2013). Worldwide, there are large amounts of PGMs in small-scale mineralized rock or in mining tailings, while it is uneconomic if exploited by the traditional mining methods. Therefore, as supplies for PGMs become constrained by the decreasing ore grade, and demand continues to increase, the development of novel technologies through which to recover PGMs has become globally important.  Some plants can hyperaccumulate metals to a concentration that is much higher than the substrate concentration. Phytomining involves the use of hyperaccumulating plants to extract valuable metals from the substrate (Sheoran et al., 2013). Hyperaccumulating plants naturally absorb metals such as nickel, cadmium and manganese etc., in situations where most of the metals are bioavailable in soil solution for plant adsorption (Baker and Brooks, 1989). This phenomenon may also be induced in some high biomass plant species (e.g. Brassica junca) by the addition of chemicals to solubilized metals, such as gold, lead, zinc and uranium, to make them available for plant uptake (Ebbs et al., 1998; Anderson et al., 1999). Most of the work has focused on the capture of metals such as gold, nickel, zinc, cadmium and mercury, 2  while there have only been a limited number of studies conducted with regard to PGMs accumulation by plants. (McGrath et al., 2000; Anderson et al., 2005; Tangahu et al., 2011).   Phytomining for PGMs involves growing plants on PGM-rich materials that are capable of selectively incorporating PGMs into their cellular structures. Theoretically, the plants can then be harvested and subjected to controlled pyrolysis in order to yield a material with stabilized PGM nanoparticles that can later be used in catalytic reactions. Parker et al. (2014) reported the first use of living plants to recover palladium and the production of catalytically active palladium nanaoparticles. These nanoparticles showed excellent catalytic activity across a range of coupling reactions, also producing higher yields than commercial Pd catalyst. This process reduces the number of production steps compared to traditional catalyst production methods. Phytomining, therefore, offers an innovative opportunity to increase the recovery of these valuable metals from materials not currently exploited, and could help develop a new range of naturally-derived catalysts. Catalysts developed within this project can be used in a variety of chemical reactions relevant to a broad cross-section of the automotive, chemical and pharmaceutical industries. For example, in automotive industries, platinum (Pt), along with palladium (Pd), and rhodium (Rh) are coated onto a substrate housed in the exhaust system and act as catalysts to convert toxic vehicle emissions, such as carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx), to less harmful substances (Dietz, 2014).  1.2 Statement of the problem  In the soil, a large portion of metals are bound up to different mineralogical components that are unavailable to plants. A smaller portion is in simpler, more soluble form, which is exchangeable cation and available to plants. The mineralogical component can gradually change into available compounds by chemical weathering and biological process (Dairy Soils and Fertiliser Manual, 2013). Thus, when referring to the availability of a metal, one considers how easily plants can access the metal in a form they require. The primary determinant of metal uptake by a plant is the availability of the metal in the soil-plant system. This is defined as the bioavailability of a target metal. The type and nature of constituent minerals, pH, organic matter, cation exchange capacity and competing ions have all been 3  shown to influence PGM solubility, and thus bioavailability (Ko et al., 2008; Wilson-Corral et al., 2013). As such, the primary interest of this study is the assessment of the bioavailability of PGMs of mine samples for the phytomining. X-ray diffraction analysis, scanning electron microscopy, inductively coupled plasma analysis, selective extraction experiments, and soil fertility analysis were conducted to predict the plant bioavailability based on geochemical characteristics of the samples.    1.3 Objectives The main purpose of the study is to establish geochemical procedures to assess the bioavailability of PGMs for phytomining, and to develop an assessment model for the selection of the best sample. Comparative assessment for each of the samples provides a supplement for the work regarding the extractability of Pd and Au, and plant growth trials done by Dr. Chris Anderson in Massy University, New Zealand.  Specifically, the following objectives are addressed: 1. A mineralogical study of two types of samples from different deposits to determine the PGM-bearing minerals;  2. Assessment of the geochemical conditions of the associations of the PGMs with minerals, and the availability of PGMs to plants using selective extraction methods; 3. Provide indirect information about the possibility of bioavailability of the PGMs for plants. 4. Development of a model for the selection of the best sample for phytomining.  1.4 Value of this research This research will contribute to the geochemical assessment of the bioavailability of PGMs for phytomining. The selection model of the substrate of phytomining will provide an overall, systematic and efficient method for the selection of the best sample. Additionally, the mineralogy and PGMs association of BH gossan samples have been studied, providing knowledge about platinum group minerals in this yet-unexploited ore.   4  1.5 Organization of this research  This thesis is presented in five chapters:  Chapter 1 - Introduction  Chapter 1 provides an overview of the study, the problem, the objectives and value of this research. Phytomining offers an innovative opportunity to increase the recovery of PGMs from materials not currently exploited, and could help develop a new range of naturally-derived catalysts. The bioavailability of the metal in the soil-plant system is the primary determinant of metal uptake by a plant.  Chapter 2 - Literature review  Chapter 2 presents an overview of PGMs, world reserves and production of PGMs, and the processing of PGM ores. This overview reveals the potential sites for phytomining of PGMs worldwide. The chemical dissolution of PGMs presents the interactions of Pt and Pd with ligands, e.g., hydroxide, chloride, sulfide, ammonia, cyanide, humic and fulvic acids. The phytomining of precious metals is usually an induced process, which means chemicals should be added to make the target metal soluble. Soil factors that influence plant growth and thus indirectly influence PGMs adsorption are reviewed. Soil properties such as pH, salinity (EC) and cation exchangeable capacity (CEC) provide the criterion for the assessment model.  Chapter 3 - Materials and methods   Two sampled sites were investigated in the study. The samples from North American Palladium (NAP) are sulfide ore, while those from Broken Hill (BH) are surface-exposed gossan. The analytical methods employed in this work are presented: X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis, particle size analysis, selective extraction experiments, and soil fertility 5  analysis. The mineralogy, total metal concentration, metal concentration extracted by chemicals, and soil-associated factors of all the samples were investigated by these methods.  Chapter 4 - Results and discussion   XRD analysis results indicated that NAP feed and tailings contained mainly silicates, while concentrate mostly consisted of sulfides. BH gossan 1 and gossan 2 were dominated by goethite and hematite. The SEM analysis revealed that barite is common in the two gossan samples. Native Pt, Pt-Au alloy and Pd telluride were detected in both gossan samples. Sperrylite and Pd-Cu oxide were also found in BH gossan 2. Pt and Au concentration in NAP concentrate is the highest of all samples. All six PGMs concentration in BH gossan 1 and gossan 2 were found to be very high. The selective extraction analysis indicates that most of the Pd and Pt were not in Fe-oxide phases, and shows the available concentration of Pd, Pt, Au and Cu to plants. The results of soil-associated factors indicate that the pH and EC of NAP concentrate are not suitable for plant growth. Potential toxicity problems with respect to heavy metals should also be taken into account. A selection model is also proposed to provide guidelines for suitability assessment of media for phytomining. NAP feed, concentrate and tailings, BH gossan 1 and gossan 2 were assessed by the selection model to check their suitability for the phytomining of PGMs.  Chapter 5 - Conclusion  Conclusion from this research is addressed in bullet format.  6  Chapter 2: Literature Review   2.1 Introduction Well-defined planning is necessary in order to ensure the success and economic viability of a phytomining operation. The first important step is to identify the resource that contains the target metals. Thus, the first part of this literature review examines the abundance, properties, and prices of PGMs, and analysis of the world resources, reserves, production, and the processing of PGMs, with a particular focus on the geographical regions from where the metals derive. The potential sites for phytomining of PGMs worldwide were given in this part. The total concentration of a metal does not represent the available concentration to the plant. The target metal should be dissolved in the soil for plant uptake. Thus, the chemical dissolution of PGMs is also reviewed. The second part presents the background of phytomining with a focus on the phytomining of gold and palladium. No public data with respect to phytomining of Pt was found. The conditions of the resource such as its nutrients, pH, salinity, CEC and heavy metals concentration that have been shown to influence indirectly the bioavailability of PGMs are also reviewed.   This thesis focuses particularly on palladium, platinum and gold due to the limited applications in industry and lack of reported data of the other four PGMs. Despite this, commentary on the other PGMs is provided where relevant.  2.2 Platinum group metals  2.2.1 Introduction Platinum group metals is a term used to refer collectively to the six metallic elements of platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir) and osmium (Os) that are clustered together in the Periodic Table. They are commonly abbreviated as PGMs by metallurgists and engineers (Gunn, 2014). PGMs are among the rarest of metals (Figure 2.1). The average concentration in the Earth’s crust of Pt and Pd is 37 ppb and 6.3 ppb respectively (Cox, 1989). Together with gold and silver, the PGMs have been referred to as noble, or 7  precious metals.    Figure 2. 1 Abundance (atom fraction) of the chemical elements in the Earth’s upper continental crust        as a function of atomic number (Haxel et al., 2002)  Some of the physical properties of PGMs are listed in Table 2.1. All six of the platinum group metals are silvery-white lustrous metals, although osmium has a bluish tinge (Hartley, 2013). Platinum and palladium are highly resistant to heat and corrosion, and are soft and ductile. Rhodium and iridium are harder to work while ruthenium and osmium are hard, brittle and almost unworkable. All PGMs, commonly alloyed with one another or with other metals, can act as catalysts which are exploited in a broad range of industrial applications (Gunn, 2014). Platinum and palladium are of major commercial significance, while, of the other PGMs, rhodium is the next most important. Other properties include resistance to chemical attack, excellent high-temperature characteristics, and stable electrical properties. All these properties are exploited for specific industrial applications (Hunt and Lever, 1969).     8  Table 2. 1 Some important properties of the PGMs as compared with gold (Au) (Gunn, 2014) Element Units Platinum Palladium Rhodium Iridium Ruthenium Osmium Gold Symbol Pt Pd Rh Ir Ru Os Au Atomic Number  78 46 45 77 44 76 79 Atomic Mass  195.08 106.42 102.91 192.22 101.07 190.23 196.97 Density at 25℃ kg m-3 21450 11995 12420 22550 12360 22580 19281 Melting Point ℃ 1769 1554 1960 2443 2310 3050 1064 Electrical resistivity at 25℃ µΩ•m 106 105 45 51 71 92 22 Hardness (Mohs Scale)  4-4.5 4.75 5.5 6.5 6.5 7 2.5-3  Platinum is unaffected by acids except for hot aqua regia, in which it readily dissolves, whereas iridium and rhodium are unaffected even by aqua regia (Hartley, 2013). Palladium, even in the bulk state, is attacked by hot concentrated nitric and sulphuric acids, particularly in the presence of oxygen and nitrogen oxides. In its powder form, palladium dissolves slowly in hydrochloric acid in the presence of oxygen. Osmium is barely affected by aqua regia, oxidizing acids, hydrochloric or sulphuric acid, but does dissolve in molten alkalis and oxidizing fluxes. Ruthenium is insoluble in all acids including aqua regia, although if potassium chlorate is added to the aqua regia, ruthenium is oxidized explosively (Yu, 2009).  The PGMs are pronouncedly chalcophile (sulphur-related), and therefore, they are often found associated with sulphide minerals (Rao and Reddi, 2000). PGMs are also known to be siderophile (iron-related). Thus, Ru and Rh are found in chromites. The association of PGMs with tellurides, selenides, arsenides and antimonides is also well known (Buchanan, 2012). The silicate minerals are, however, generally free of PGMs minerals (Crocket, 1979).  The price of platinum has varied widely throughout the 20th century and into the 21st century (Figure 2.2). Over a century ago, it was inexpensive enough to be used to "adulterate" gold. However, in 2008, it was over twice as valuable as gold. Currently Pt price is lower than gold price. The traditional use of gold as a monetary metal has supported a price near platinum, despite being far more abundant than platinum. PGMs are used in a diverse range of industrial applications as well as in jewelry.  9   Figure 2. 2 Price ($/ounce) of platinum and gold and the gold/platinum ratio (Cammarosano, 2014)  2.2.2 Resources, reserves and production  Most of the platinum in the world is mined in the Bushveld igneous complex (South Africa), the Ni-Cu-PGM sulphide deposits of Norilsk in the Russian Arctic and placer deposits in the Ural mountains (Russia), Sudbury (Ontario, Canada), the Stillwater complex (Montana, USA), the Hartley mine (Zimbabwe), Northern Territory (Australia), the Zechstein copper deposit in Poland, and Jinchuan (China) (Cabri, 1981; Rao and Reddi, 2000; Buchanan, 2012). With the growing interest in PGMs, many new deposits are being explored. Uncertainty regarding the long-term sustainability of production from these major mining areas has led to investment in the search for new deposits (Rao and Reddi, 2000). The global extent of recorded platinum metal reserves worldwide in 2014 is summarized in Table 2.2. South Africa has over 95% of the world’s platinum metal reserves.    10  Table 2. 2 Platinum metal reserves worldwide in 2015 (U.S. Geological Survey, 2016a) Country Tons Percentage (%) South Africa 63,000 95.3 Russia 1,100 1.7 United States 900 1.3 Canada 310 0.5 Others 800 1.2 Worldwide 66,110 100  Much like with platinum, the largest palladium reserves in the world, accounting for 75.8%, are in the Bushveld Complex of South Africa. Other extensive deposits can be found in the Stillwater Complex in Montana, United States, and the Thunder Bay District, Ontario, Canada. Recycling also provides an important source of palladium, mainly from scrap catalytic converters (Johnson Matthey, 2015). There is little up-to-date information on palladium reserves by country. The latest available information can be found is the world Palladium reserves in 2006 (Table 2.3). South Africa and Russia together have over 93% of the world's palladium reserves.  Table 2. 3 Palladium metal reserves worldwide in 2006 (Diego, 2007) Country Tons Percentage (%) South Africa 22,861 75.8 Russia 5,235 17.4 North America 1,748 5.8 Others 317 1 Total 30,161 100  Since PGMs reserves exist in only a few regions, production also occurs in a limited numbers of countries. The majority of raw materials are in South Africa and Russia, where the biggest reserves are found. South Africa is the largest producer of PGMs, accounting for about 70.2% of the world’s platinum and 35.2% of the world’s palladium in 2015 (Tables 2.4 & 2.5). Russia produced 12.9% and 38.6% respectively. In Norilsk-Talnakh (Russia) and Sudbury 11  (Canada), PGMs are produced as by-products of nickel mining (Steinweg, 2008).  Table 2. 4 Platinum production worldwide from 2013 to 2015 (U.S.Geological Survey, 2016b) Country 2013 2014 2015 tons % tons % tons % South Africa 131.0 71.4 94.0 63.7 125.0 70.2 Russia 25.5 13.9 23.0 15.6 23.0 12.9 Zimbabwe 12.4 6.8 12.5 8.5 12.5 7.0 Canada 7.0 3.8 8.5 5.8 9.0 5.1 United States 3.7 2.0 3.7 2.5 3.7 2.1 Others 3.8 2.1 5.8 3.9 4.8 2.7 Worldwide 183.4 100.0 147.5 100.0 178.0 100.0  Table 2. 5 Palladium production worldwide from 2013 to 2015 (U.S. Geological Survey, 2016c) Country 2013 2014 2015 tons % tons % tons % South Africa 75.0 37.0 58.4 30.3 73.0 35.2 Russia 80.0 39.5 83.0 43.0 80.0 38.6 Canada 16.5 8.1 20.0 10.4 24.0 11.6 United States 12.6 6.2 12.4 6.4 12.5 6.0 Zimbabwe 9.6 4.7 10.1 5.2 10.0 4.8 Others 8.9 4.4 9.0 4.7 8.0 3.9 Worldwide 202.6 100.0 192.9 100.0 207.5 100.0  2.2.3 Mineralogy and ore deposits  There are over 100 platinum group minerals that include sulfides, tellurides, antimonides, arsenides, as well as all sorts of native metals and alloys (Cabri, 2002; Xiao and Laplante, 2004). Cabri’s two books: “Platinum-Group Elements: Mineralogy, Geology, Recovery’’ (1981) and ‘‘The Geology, Geochemistry, Mineralogy and Mineral Beneficiation of Platinum-Group Elements’’ (2002) provide more details about the PGM minerals. The three primary minerals associated with platinum group minerals are pyrrhotite, chalcopyrite (CuFeS2), and pentlandite ((Ni,Fe)9S8), such as in the South African Merensky Reef deposit. In the Stillwater J-M Reef orebody (USA), the principal sulphide minerals are chalcopyrite and pentlandite. In this deposit, most platinum minerals are associated with copper sulfides, 12  and palladium is associated with the nickel sulfides. Platinum has long been known to exist in the arsenide form (sperrylite, PtAs2) in nickel–copper sulfides in the Sudbury area (Canada) (Cabri, 2002).  Typical issues that come up when carrying out mineralogical studies on PGMs include their extremely low concentrations, their fine size particles, difficulties in their detection and identification, and sample representativity (Xiao and Laplante, 2004). In other words, the range of minerals present, their relative densities, shape, particle size, and associations present a challenge to metallurgists in the design and optimization of the extraction process (Petruk and Hughson, 1977; Cabri, 1981; Henley, 1983).   The distribution of primary deposits, mines, and other selected occurrences of PGMs worldwide are shown in Figure 2.3. Magmatic PGM deposits, found in mafic and ultramafic igneous rocks, are of two principal types: PGM-dominant deposits that are associated with sparse, dispersed sulfide mineralization (Maier, 2005); and nickel-copper sulfide deposits, in which the PGMs occur in association with sulfide-rich ores (Naldrett, 2010).  PGM-dominant deposits In PGM-dominant deposits, PGMs are the principal commercial products with minor amounts of nickel and copper derived from sparsely disseminated (up to five volume percent) sulfides, chiefly pyrrhotite, pentlandite and chalcopyrite. Typical PGM-dominant deposits have been developed in the Bushveld Complex in South Africa, which is the largest layered igneous complex in the world (Gunn, 2014). The mineralogy of these deposits is quite variable, including a diverse range of alloys, sulfides, tellurides and arsenides. Other PGM-dominant deposits are found in the Great Dyke in Zimbabwe, the Stillwater Complex in the USA, and the Lac des Iles Intrusive Complex in Canada.     Nickel-copper-dominant deposits Magmatic nickel-copper deposits are the most important sources of nickel worldwide (Naldrett, 2010). Copper, cobalt and the PGMs, mainly palladium, are important by-products. Gold, silver, chromium, sulfur, selenium, tellurium and lead are also recovered from some 13  deposits. The dominant ore minerals are pyrrhotite, pentlandite and chalcopyrite, which constitute more than 10% by volume of the host rock in magmatic nickel-copper deposits. Two nickel-copper districts are predominant globally, namely Sudbury in Ontario, Canada and Norilsk-Talnakh in the polar region of Russia, with each containing more than10 million tonnes of nickel metal (Gunn, 2014).  Other deposit types High PGMs concentration is known in several other geological environments. Although very high grades may occur, most deposits are small, and the mining of these PGMs is not currently considered economically viable. Alluvial placer deposits of PGMs, mainly related to Alaskan-type intrusions, were worked for many decades in Colombia, Alaska, British Columbia, the Urals and central New South Wales (Tolstykh et al., 2005). They were the world’s principal sources of platinum until the discoveries at Sudbury in late 19th century and in the Bushveld Complex in early 20th century (Cawthorn, 2006; Turner et al., 2015). Currently however, significant PGMs production from alluvial deposits is restricted to the Russian Far East (Johnson Matthey, 2013).  2.2.4 Processing The concentration process of PGM ores consists of crushing, grinding, and froth flotation of the PGM bearing minerals (Locmelis et al., 2010; Gunn, 2014). PGMs grains usually float together with sulfides, either as discrete grains, or attached to sulfide grains, or as inclusions within them. The sulfide concentrate is treated by smelting and refining to recover the base metals (e.g., Ni, Cu, and Co). Afterwards, the PGMs are extracted as a residual precious metal concentrate from the base–metal refinery (Evans, 2002).    14   Figure 2. 3 The distribution of the main PGM mining districts, mines and other deposits (Gunn, 2014) 15  2.2.5 Potential targets for the phytomining of PGMs Some small deposits that have high PGM grades are worthy of phytomining trial due to uneconomic mining with traditional methods. According to the map that indicates the distribution of major PGM deposits, mines, and other selected occurrences of PGMs worldwide, a table (Table 2.6) that speaks to the potential targets for phytomining of PGMs is established. The PGM-dominant deposits are the first consideration due to their high grade of PGMs and relatively lower concentration of Cu and Ni compared with Ni-Cu dominant deposits and other deposits.  Table 2. 6 Some potential sites worldwide for sampling for the phytomining of PGMs Country Potential sites Canada Sudbury area  Lac des Iles Intrusive Complex near Thunder Bay (North American Palladium Ltd.) Coldwell Complex in northwest Ontario (Stillwater, the Marathon PGM-Copper project)  U.S. Stillwater and East Boulder mines (Stillwater Mining Company) Duluth Complex in Minnesota (Twin Metals Minnesota LLC.) Australia Munni Munni (Platina Resources Ltd.) Panton (Panoramic Resources Ltd.) PGE-Ni-Cu gossan in Broken Hill mineral complex South Africa Bushveld Complex Great Dyke Stella Intrusion Finland Kevitsa mine (First Quantum Ltd.) Penikat Complex Portimo Complex  2.2.6 Chemical dissolution of PGMs  Pt and Pd can occur in either the +2 or the +4 valence state in aqueous solution. Based on Pearson’s hard-soft acid-base theory, Pt and Pd are classified as a soft acid in their cationic forms, which is the same with gold (Pearson, 1963). Thus, when in cationic form, Pt and Pd prefer to form stable complexes with soft ligands, e.g., chloride, cyanide, amines, thiosulfate, bisulfide and with some “hard” ligands, including hydroxyl groups, ammonium and 16  carboxylic acids (Mountain and Wood, 1988; Wood, 2002).   Redox potential (Eh) is used as a measure of the oxidizing potential of solutions. The Eh value gives an indication of how likely a given redox reaction will occur. Higher Eh positive values suggest that reaction is more likely to occur spontaneously without the need of extra energy. Another parameter measured in solutions is pH. This term is the degree of the acidity of a solution in terms of the amount of hydrogen (H+). The pH and Eh are important with respect to the reactions in solutions.  Colombo et al. (2008) investigated the interactions of Pt and Pd with different inorganic ligands, the formation of stable PGE species, and the thermodynamic stability of these species in the environment. The Eh-pH diagrams (Figure 2.4) built in this study indicate that Pt and Pd are able to form stable complexes with hydroxide, chloride, sulfide and ammonia. They concluded that hydroxide species can contribute to the transport of PGEs in oxidizing environments such as runoff waters, freshwater, seawater and soil solutions, whereas bisulphide complexes could transport Pt and Pd in reducing environments. Ammonia species appear to be significant under near-neutral to basic oxidizing conditions. Chloride species are likely to be important under oxidizing, acidic and saline environments such as seawater and road-runoff waters in snowmelt conditions. Mixed ammonia–chloride species may also contribute to the transport of Pt and Pd in highly saline solutions.  Cyanide forms very stable complexes with Pt (II) and Pd (II) under reducing and oxidizing conditions. Hartley and Robinson (1973) reports the two major complexes as M(CN)2 and M(CN)42- , where M is Pt or Pd. Pt and Pd can also form stable complexes with thioligands, e.g., thiosulphate, thiourea and thiocyanate (Walton, 2002).  Fulvic acid is naturally occurring organic compounds usually found in soils and plants. It has a small molecule and can readily dissolve and bond mineral and nutritional elements into its own molecular structure. Nutrients and minerals that have been chelated by fulvic acid are then in the simplest ionic forms and ideal be absorbed by plant roots. The relevance of these organic compounds for Pt and Pd mobility lies in the functional groups they contain, which 17  can form complexes with Pt and Pd (Wood, 2002). Bowles and Gize (2005) found that Pt and Pd can be taken into humic and fulvic acid solutions exposed to the metal foils for about a year at room temperature. These organic acids can also contribute to gold solubility.                               (a)                    (b)  Figure 2. 4 (a) Eh–pH diagram for Pt in aqueous solution containing a mixed ligand system consisting of [ΣPt] = 10−9 M, [ΣS]= 10−3 M, [ΣN] = 5 × 10−4 M and [ΣCl] = 0.5 M; (b) Eh–pH diagram for Pd in aqueous solution containing a mixed ligand system consisting of [ΣPd] = 10−9 M, [ΣS]= 10−3 M, [ΣN] = 5 × 10−4 M and [ΣCl] = 0.5 M. (Colombo et al., 2008)  18  2.3 Phytomining  2.3.1 Introduction  Individual plants have been recognized as having the ability to accumulate higher concentrations of metals than other plants growing in the same environment for hundreds of years. This characteristic is known as phytoextraction that uses plants to extract metals from the metalliferous soil (Chaney et al., 1998; Garbisu and Alkorta, 2001). The two major applications of phytoextraction are phytoremediation and phytomining. Phytoremediation refers to work where metal contaminants are stabilized or recovered for secure disposal (McGrath and Zhao, 2003), while phytomining deals with the recovery of valuable metals such as gold (Au), platinum (Pt), nickel (Ni) and thallium (Tl) via cropping for monetary return (Brooks et al., 1998; Anderson et al., 1999; Anderson et al., 1999). Some specific plants that can accumulate higher concentration are given in Table 2.7 (Walton, 2002; Sheoran et al., 2009; Aquan, 2015) .  Table 2. 7 Some specific plants that can accumulate higher concentration of metals Elements Plant species Concentration mg/kg dry matter Copper Haumaniastrum                                Katangense                                           Ipomea alpine 8356 (1)Gold (induced-hyperaccumulation) Brassica juncea, Berkheya coddii Chicory                                                      C. linearis 10 (.001)Nickel  alyssum bertolonii                                Berkheya coddii                                 Streptanthus polygaloides 13,400 (2)               17,000 (2) Palladium (induced-hyperaccumulation) Brassica juncea, Cannabis sativa (Hemp) 13 (0.001) Silver Brassica juncea                                    Medicago sativa  -  The process of phytomining involves: (1) growing high-biomass plants that can efficiently extract metals from the metalliferous soils; (2) after sufficient growth, the plants are harvested and dried; (3) plant material is dried, and burn to ash, and the ash is smelted to 19  extract the metals (Brooks et al., 1998; Robinson et al., 1999; Sheoran et al., 2009). The effective extraction of metals from metalliferous soils requires the target metals be available to plants. The assessment of the bioavailability of target metals is based on the extractable concentration of the target metals, which is effected indirectly by soil-associated factors such as nutrient availability, cation exchange capacity, soil salinity, pH and heavy metals concentration (Wilson-Corral et al., 2013).   2.3.2 Phytomining of gold and palladium  Due to the extremely low solubility of gold in soil solution, no known plant is believed to naturally accumulate high levels of this metal, and hence plant uptake has to be induced. Anderson et al. (1998) were the first to report unusually large amount of gold uptake by plants, with the induction of up to 57 ppm of gold in Brassica juncea from mine tailings through treatment of the substrate with thiocyanate. They found that the key point that limited plant uptake of gold was the limited solubility of this metal.   Only a limited number of studies have been conducted into PGMs accumulation by plants. As with gold, the uptake of PGMs by plants also has to be induced. Walton (2002) investigated the induced uptake of Pd and Au from mine tailings containing 61.4 ppb Au and 315 ppb palladium. Nearly 1600 ppb and 7700 ppb palladium were accumulated in the plant Berkheya coddii using cyanide as the chemical amendment. This may have been the first lab experiment for Pd phytoextraction. It is of note that no public data with respect to Pt phytoextraction from mine samples was found.  The use of total concentration as criterion to assess the potential bioavailability of these metals to plants is untenable. The key variable for the successful plant uptake of gold and palladium involves the dissolution of these metals within the soil solution. The solubility of metal is related to its geochemical properties and mineral associations. Metals within different minerals require different reagents to dissolve. Selective extraction involves extracting metals selectively through the use of appropriate reagents (Tessier et al., 1979). This method provides information about the mineral associations and geochemical availability of metals.  20  Gray et al. (1996) investigated the mineral associations of platinum and palladium in the lateritic regolith in west Australia with selective extraction methods. The reagents used included ammonium acetate (for exchangeable and extractable cations), hydroxylamine hydrochloride with nitric acid (for Mn oxides) or hydrochloric acid (for amorphous Fe oxides), and ammonium citrate with sodium dithionite (for crystalline Fe oxides). The results indicated that most of the Pt, and a small proportion of the Pd, are associated with goethite and hematite.     2.3.3 Soil-associated factors “However, extremes of metal concentration, high or low pH, and salinity that are typical of mining or mineralized materials make them challenging media for plant growth” (Meech and Anderson, 2014). If plants are to grow to their potential, the soil must provide a suitable environment for plant growth. The chemical properties of the soil play a significant role in metals uptake and plant growth (Brady and Weil, 1996). In this part, the chemical properties of the soil (nutrients, cation exchange capacity, soil salinity, pH and heavy metals) and their effects on plant growth will be reviewed.  2.3.3.1 Nutrients Apart from atmospheric oxygen and soil-derived water, plants typically require 14 essential nutrients (Maathuis, 2013). Six of these are needed in relatively large amounts, and are commonly referred to as “macronutrients” (Maathuis, 2009): nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), phosphorous (P), and sulfur (S). The nutrients belonging to the second group called “micronutrients” or “trace elements” are needed at much smaller concentrations in plant tissues, and comprise of chloride (Cl), boron (B), copper (Cu), manganese (Mn), iron (Fe), zinc (Zn), molybdenum (Mo), and nickel (Ni) (Hodges, 2010; Marschner, 2012). Plant concentrations of these essential elements vary from species to species. Nonetheless, Table 2.8 (Epstein, 1965) provides the general requirements needed for plants.   21  Table 2. 8 Typical concentrations of macronutrients and micronutrients sufficient for plant growth Element Typical concentration (mg/kg)  sufficient for plant growth N 15,000 K 10,000 Ca 5,000 Mg 2,000 P 2,000 S 1,000 Cl 100 Fe 100 B 20 Mn 50 Zn 20 Cu 6 Mo 0.1 Ni 0.1  According to Hodges (2010), soils contain large amounts of all the elements, but only a very small percentage of these total amounts are actually available to plants. As Alloway (2013) described, total concentrations include all forms of the element in a soil. The available concentration of an element in the soil is the fraction of that element presented as free ions, soluble complexes, or in readily desorption forms. The availability of nutrients to plants is determined by the total content of the element, the form and chemical properties of the element, the soil pH, and the adsorptive capacity of the soil (Brady and Weil, 1996; Hodges, 2010; Alloway, 2013). Although total concentrations are often a poor indication of the potentially plant-available, or bioavailable fractions of a metal in a soil, they do provide a useful indication of whether a soil has anomalously high or low concentrations. Alloway (2013) suggests that the total concentration will indicate whether a soil is contaminated and geochemically enriched, and thus poses the possible risk of toxicity to some species of plants. On the other hand, low total concentrations indicate that there is a greater chance of that element being deficient in soils where soil conditions do not favor plant availability.   22  2.3.3.2 Cation exchange capacity The cations and anions in the soil can be: (1) absorbed (taken up) by plant roots; (2) leached from the soil via the soil water; (3) adsorbed (attached) to the surfaces of negatively and positively charged soil particles (Dairy Soils and Fertiliser Manual, 2013). The soil’s capacity to adsorb nutrients in the form of cations is called cation exchange capacity (CEC) (Brady and Weil, 1996). It is expressed in units of milliequivalents per 100 grams (meq/100g) of soil. For a plant to absorb nutrients, the nutrients must be dissolved. Nutrients in the ionic form can be attracted to any opposite charges present in the soil. Clay minerals and organic matter have a large number of negative charges on their surfaces, and thus, attract cations and contribute to a higher CEC (Spectrum Analytic Inc., 2015; McKenzie et al., 2004). Cations held on the clay and organic matter particles in soils can be replaced by other cations; thus, they are exchangeable (Brady and Weil, 1996). The exchangeable cations calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+) are the main ions associated with CEC in soils (Rayment and Higginson, 1992). CEC can significantly affect the inherent fertility, and long-term productivity of a soil (Hodges, 2010). Table 2.9 (Spectrum Analytic Inc., 2015) shows that soils with different CEC ranges display different characteristics.   Table 2. 9 Different characteristics of soils with different CEC ranges Soils with CEC 1-10 meq/100g Soils with CEC 11-50 meq/100g • Nutrients leaching more likely         • Less lime required to correct a given pH               • Low water-holding capacity    • Greater capacity to hold nutrients              • More lime required to correct a given pH       • High water-holding capacity          CEC affects the management of soil for crop production (Chapman, 1965). According to Hodges (2010), a soil with a low CEC has a low water holding capacity, is more likely to leach potassium and magnesium (and other cation), and thus requires more frequent lime and fertilizer additions. Such soils will have lower yield potentials than soils with higher CEC under the same level of management, but productivity can be improved through intensive management. Soils with high CEC may have high water holding capacity, low leaching potential for cationic nutrients, and less frequent need for lime and fertilizers (except N). Nevertheless, such soils are difficult to irrigate and maintain good aeration.  23  2.3.3.3 Soil salinity The accumulation of soluble salts of sodium, magnesium and calcium in the soil to the extent that soil fertility is severely reduced is called salinization (Tóth et al., 2008). The amount of salts in the soil is referred as salinity, and it can be estimated by measuring the electrical conductivity (EC) of an extracted soil solution (McKenzie, 1988). It is expressed in units of deciSiemens per metre (dS/m). The classes of salinity are presented in Table 2.10 (Soil Management Guide, 2015). High salt concentrations result in a high osmotic potential of the soil solution, so the plant has to use more energy to absorb water. Under conditions of extreme salinity, plants may be unable to absorb water and will wilt, even when the surrounding soil is saturated. Moreover, it is harmful to the plant when it absorbs water containing excess salts. Additionally, an excess of one ion limits the uptake of another ion. There are no quick or easy solutions to soil salinity. Saline soils can only be reclaimed by rinsing the salts down and out of the root zone (Soil Management Guide, 2015).   Table 2. 10 The classes of salinity in soils EC range (dS/m) Soil salinity 0-2 Non saline 2-4 Slightly saline 4-8 Weakly saline 8-15 Moderately saline >15 Strongly saline  2.3.3.4 pH The degree of soil acidity or alkalinity is expressed as the soil pH (Brady and Weil, 1996). It measures the concentrations of positively charged hydrogen ions (H+) in the soil solution on a logarithmic scale ranging from 0 to 14. Values of less than 7 are acidic, and greater than 7 are alkaline. Soil pH can affect the availability of plant nutrients (Brady and Weil, 1996). Natural soil pH reflects the combined effects of soil-forming factors (parent material, time, climate, and organisms). Soil pH affected by land use and management (Hodges, 2010). For example, areas of forestland tend to be more acidic than areas of grassland. Addition of nitrogen and 24  sulfur fertilizers can lower soil pH over time. In very acid or alkaline soils, some plant nutrients convert to forms that are unavailable for plants to absorb. This can result in nutrient deficiencies. Figure 2.5 (Dairy Soils and Fertiliser Manual, 2013) shows the nutrient availability at different levels of soil pH. Note that in strongly acidic soils (pH (CaCl2) less than 4.0), the availability of macronutrients (nitrogen, phosphorus, potassium, sulphur, calcium and magnesium) as well as molybdenum is curtailed. When the pH (CaCl2) is greater than 6.5, levels of available iron, manganese, boron, copper, zinc and aluminum are so low that it may lead to deficiencies in the plants.  Most common plants can tolerate a range of pH. Table 2.11 (Whiting et al., 2014) shows that soils with a pH of 6.0 to 7.5 are acceptable for most plants. Soils with a pH of 8.3 or higher normally have a very high sodium content (such soils are referred to as sodic) (Whiting et al., 2014). Soils with a pH of 3.5 or lower have very high sulphide concent (Alloway, 2013). A soil’s pH is primarily the result of a combination of the parent material of the soil and climate (Whiting et al., 2014). Nevertheless, it can also be influenced by other factors including organic matter and fertilizer. Additionally, the application of limes and elemental sulfur are ways to increase and decrease the pH, respectively (Dairy Soils and Fertiliser Manual, 2013; Whiting et al., 2014). Lime application rates to raise soil pH are presented in Table 2.12 (Whiting et al., 2014).  25   Figure 2. 5 Effects of pH (1:5 CaCl2) on nutrient availability  Table 2. 11 Soil pH and influence on plant growth Soil Reaction pH Plant Growth Alkaline soil >8.3 Too alkaline for most plants 7.5 Iron availability becomes a problem in alkaline soils 7.2 6.8 to 7.2 - "near neutral"                           6.0 to 7.5 - acceptable for most plants Neutral soil 7.0 Acid soil 6.8 6.0 -  5.5 Reduced soil microbial activity <4.6 Too acid for most plants    26  Table 2. 12 Lime application rates to raise soil pH  Existing soil pH Lime application rate              (pound per 1,000 square feet) Sandy Loamy Clayey 5.5 to 6.0 20 25 35 5.0 to 5.5 30 40 50 4.5 to 5.0 40 55 80 3.5 to 4.5 50 70 80  2.3.3.5 Heavy metals in soils Heavy metals are defined as the elements that have densities greater than 5 g cm−3 (Adriano, 2001). As described in Bothe (2011), several heavy metals such as iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), cobalt (Co), or molybdenum (Mo) are essential for the growth of plants; others have a single function, and are found only in some organisms such as vanadium (V) and nickel (Ni). The remainders of the heavy metals are always toxic to plants: cadmium (Cd), lead (Pb), uranium (U), thallium (Tl), chromium (Cr), silver (Ag), and mercury (Hg). Arsenic (As) and selenium (Se) are non-heavy metals. Heavy metals affect the physiological and biochemical processes of plants and thus result in the continued decline of plant growth (Chatterjee and Chatterjee, 2000; Öncel et al., 2000; Oancea et al., 2005).  In non-heavy metal soils, the concentrations of Zn, Cu, Pb, Ni, Cd, and Cr range between 0.0001 and 0.065%, whereas Mn and Fe can reach 0.002% and 10.0%, respectively (Ernst, 1974). Except iron, all heavy metals above a concentration of 0.1% in the soil become toxic to plants (Bothe, 2011). However, each plant species has a particular threshold value for each heavy metal, after which point, it exerts toxicity (Ernst, 1982).  Various criteria for the assessment and remediation of contaminated soils have been developed. Some countries have also made modifications to develop their own regulations based on the soil qualities they feel are most important. However, the Dutch authorities have continually upgraded their soil quality criteria in light of new scientific work, especially the ecotoxicology of listed substances and their impacts on species in the ecosystem. Thus, Dutch standards are commonly used by many national governments and local authorities who 27  lack their own formal guidelines in assessing contaminated sites, or monitoring sites (Chen, 1998). The Dutch standards for assessing soil contamination on the basis of the total concentration of heavy metals in the soil are listed in Table 2.13 (Ministry of Housing - Netherlands, 1994).  Table 2. 13 Dutch standards for soil contamination assessment of total concentration               of heavy metals in soils Elements Intervention value   mg/kg soils As 55 Ba 625 Cd 12 Cr 380 Co 240 Cu 190 Hg 10 Pb 530 Mo 200 Ni 210 Zn 720 Notes: Intervention value is to identify serious contamination of soils and to indicate when remedial action is necessary. 28  Chapter 3: Sampled Sites and Methods  3.1 Locations of two sampled sites 3.1.1 North American Palladium (NAP) North American Palladium is a platinum group metals producer that has operated Lac des Iles (LDI) mine (Figure 3.1) in northern Ontario, Canada since 1993 (Peck et al., 2015). It mainly produces palladium. In addition to palladium, other productions include platinum, gold, copper and nickel. The mine lies in the southern end of the Lac des Iles intrusive complex, in a roughly elliptical intrusive package measuring 3 km long by 1.5 km wide, termed the Mine Block Intrusion (MBI) (Figure 3.2). It began with the open pit mining of the Roby Zone, and it added underground production in 2006 via ramp access to the Roby High Grade Zone (Peck et al., 2015). In 2010, mining of the Offset Zone was begun. LDI processed 2.6 million tons (Mt) of ore at a head grade of 2.7 g/t of palladium in 2014. The mineral reserves calculated in their 2014 Technical Report total 15.0 Mt grading 2.77 g/t Pd and indicate a mine life to 2019 (Peck et al., 2015).  The MBI is comprised of rocks with a very wide range of mafic and ultramafic compositions, ranging from anorthosite to clinopyroxenite, leuco-gabbronorite to melanonorite, and includes magnetite-rich gabbro (McCombe et al., 2009). Sulfide mineralogy is dominated by pyrite with lesser amounts of pyrrhotite, chalcopyrite, pentlandite and millerite (Peck et al., 2015). The majority of platinum-group minerals in the Roby Zone and Offset Zone deposits occur either interstitially to sulfides in association with gangue minerals such as plagioclase, amphibole, chlorite, orthopyroxene, and talc, or at sulfide-silicate boundaries (Yu et al., 2010; Huminicki, 2013). To the present, the average relative proportions of the identified PGE minerals in the known mineralized zones in the MBI have not been quantified. However, the relative abundance of PGE-bearing minerals in the mill feed and concentrates from the recently mined zones (Roby, Offset) on the property are palladium tellurides > palladium antimonides > palladium sulfides > sperrylite (platinum arsenide) > gold-silver alloys (Yu et al., 2010; Huminicki, 2013). PGE grades in the MBI show varying degrees of correlation with nickel and copper concentrations. Many zones display a general moderate to strong 29  positive correlation between palladium, copper, and nickel but others (e.g., North VT Rim Zone) show no significant, positive correlation between these metals (Peck et al., 2015).   Figure 3. 1 Lac des Iles (LDI) mine (Peck et al., 2015)   Figure 3. 2 Mine Block Intrusion (MBI) (Biggar, 2010) MBI 30  3.1.2 Broken Hill (BH) gossan  The Broken Hill deposit in western New South Wales, Australia has one of the richest accumulations of lead, zinc and silver in the world (Figure 3.3). Mining by Broken Hill Proprietary Company Limited began in 1883 and continues until today in the South and North operations by Perilya Ltd (Morland and Webster, 1998; Leary and Sims, 2009). The Broken Hill ore body consists of a series of closely-spaced sulfides-rich deposits separated by quartz- and garnet-rich host Willyma Supergroup rocks. In addition to the world-class deposits of zinc, lead, and silver, it also contains minor occurrences of iron, manganese, copper, cobalt, nickel, tungsten, uranium and tin (Leary and Sims, 2009). The main minerals mined are galena and sphalerite, from which lead, zinc and silver are extracted. Two types of sulfides deposits, which may be massive in character, exist at Broken Hill; lead-rich zones or lodes and zinc-rich zones or lodes. Sulfide mineralogy of the lead lodes consists of galena, pyrite and lesser amounts of pyrrhotite, sphalerite, and chalcopyrite. Zinc lodes contain sphalerite and pyrite with lesser amounts of pyrrhotite, galena, arsenopyrite and chalcopyrite (Leary and Sims, 2009). In the early days and upper parts of the mine, many of the sulfide minerals were converted by supergene oxidation to a large suite of oxide minerals some of which were first recognized as new minerals from their formation in the Broken Hill District (Beeson, 1990). Some deposits have deep weathered zones: gossanous quartz-garnet-gahnite rocks, with abundant Mn and Fe oxides at the surface, and secondary Ag enrichment at depths associated with oxides (goethite and coronadite) and carbonates (dolomite, cerussite, and smithsonite); leached sulfides mark the transition into underlying sulphide ore (Mackenzie and Davies, 1990).   A number of very rich, but rather small, platinum group element (PGE) deposits in the vicinity of Broken Hill, have been known for some time (Andrew, 1922). These groups of deposits occur approximately 20km to the east of Broken Hill and from north to south are Round Hill, Mulga Springs, Little Darling Creek and Red Hill (Figure 3.4). Mingaye (1889) was the first to detect platinum in small parcels of ore from Little Darling Creek. It was considered likely that sperrylite (PtAs2) was present in the ore samples, but in too finely divided form (Mingaye, 1892). In the 1990s, analysis of the gossan, which is intensely oxidized, weathered or decomposed rock on the surface of deposit, from Mulga Springs 31  returned 19.6 g/t Pt, 50 g/t Pd, 3 g/t Rh, 3 g/t Os, 4.4 g/t Ir, 2 g/t Ru, 0.57 g/t Au, 0.34% Ni and 0.71% Cu (Meech and Anderson, 2014). According to Elvy (1998) the gossan samples collected from Mulga Springs are dominated by goethite and hematite; the observed PGM minerals are froodite, sperrylite, paolovite, palladium-copper alloys, palladium-tellurium-mercury alloy, platinum alloys, and unnamed palladium tellurides; other minerals detected include barite, galena, iodargyrite, arsenopyrite, gold, bismuth, azurite, cerussite, chrysocolla, and malachite.   Figure 3. 3 Location of the Broken Hill Pd-Zn-Ag deposit and Mulga Springs deposit (Impact Minerals Limited, 2013) 32   Figure 3. 4 Location of the Ni-Cu-PGE prospects of Round Hill, Mulga Springs, Little Darling Creek, Red Hill and other deposits (Golden Cross Resources, 2013)  33  3.2 Methods Feed, concentrate, and tailings samples (3 kg each) were collected from the mineral processing plant of LDI mine, North America Palladium (NAP) , Canada, and two types of gossan samples (1 kg each) were collected at Mulga Springs, Broken Hill (BH), Australia.   Inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis for whole-rock chemistry, and Instrumental Neutron Activation Analysis (INAA) for PGMs (Ir, Os, Pd, Pt, Rh, Ru) and Au were conducted by Acme Analytical Labs Ltd. in Vancouver.   The mineralogy was examined through quantitative X-ray powder diffractometry (XRD) at the department of Earth, Ocean & Atmospheric Sciences at the University of British Columbia. The samples were reduced to the optimum grain-size range for quantitative X-ray analysis (<10 μm) by grinding under ethanol in a vibratory McCrone Micronizing Mill for 7 minutes. Step-scan X-ray powder-diffraction data were collected over a range 3-80° 2θ with CoKa radiation on a Bruker D8 Advance Bragg-Brentano diffractometer equipped with a Fe monochromator foil, 0.6 mm (0.3°) divergence slit, incident- and diffracted-beam Soller slits and a LynxEye-XE detector. The long fine-focus Co X-ray tube was operated at 35 kV and 40 mA, using a take-off angle of 6°.  The platinum group mineral association of BH gossan 1 and gossan 2 was studied by scanning electron microscope (SEM) at the department of Earth, Ocean & Atmospheric Sciences at the University of British Columbia. Four polished thick sections for gossan 1 and 2 respectively embedded in epoxy resin were prepared by Vancouver Petrographics Ltd. They were carbon-coated and investigated by a Philips XL-30 scanning electron microscope equipped with a Bruker Quantax 200 energy-dispersion X-ray (EDX) microanalysis system, XFlash 6010 SDD detector, and Robinson cathodoluminescence detector. Minerals were acquired at 15 kV, a working distance of 10-11 mm, and measurement time of 30 s. Micrographs of back-scattered electrons, energy dispersive X-ray spectrometers (EDS), and element mapping were obtained to present the results.  34  The mineral association and bioavailability of Pt, Pd, and Au in all samples was investigated by selective extraction methods at the Coal and Mineral Processing Laboratory of the Mining Engineering Department at the University of British Columbia. Cu and Ni were also analyzed due to their phytotoxicity that may limit plant growth. Ammonium acetate (NH4C2H3O2), fulvic acid, citrate-dithionite and reverse aqua regia were used to extract exchangeable cations, metallic PGMs, partial crystalline Fe oxides (i.e., hematite and goethite), and sulfides, respectively. Ammonium acetate (NH4C2H3O2) can extract readily exchangeable species (Ferreira and Veiga, 1995). It is believed that exchangeable metals in soil are most available to plants (Castilho and Rix, 1993). Citrate-dithionite was used by Gray et al. (1996) to selectively dissolve goethite and hematite in order to explore the mineral associations of platinum and palladium. No literature information was found on fulvic acid to study the selective extraction of precious metals (e.g., Au and PGMs). The work conducted for this thesis may be the first trial to use fulvic acid in selective extraction for Pt, Pd, and Au. Reverse aqua regia is a mixture of nitric acid and hydrochloric acid in a molar ratio of 3:1. Nitric acid can dissolve sulfides (Tessier et al., 1979). When sulfides are dissolved, nitric acid and hydrochloric acid provide complexing ligands for Pt and Pd that are associated with sulfides (Colombo et al., 2008).   All the samples were crushed and ground at first. The particle size was analyszed by sieves and cyclosizer. The sieves were used to determine the distribution of coarser, larger-sized particles (> 38um), and the cyclosizer was used to determine the distribution of finer particles (< 38 um). Secondly, screen the sample with a 200 mesh (75 µm) sieve. Weigh 3 g sieved sample into a 150 mL bottle. Thirdly, the selective extraction was conducted.  The first and second extraction scheme adapted from Gray et al. (1996) are described as follows:  First extraction scheme (Figure 3.5)  1. Exchangeable cations: add 50 mL 1 M NH4C2H3O2 into the bottle. Place the bottle in a shaker and shake for 1 hr. 35  2. Filter the mixture. Wash the solid residues with deionized water (30-50 ml). Measure and record the volume of the final filtered solution. 3. Dry and weigh the solid residues. 4. Extractable cations: add 50 mL fulvic acid to the solid residues in a 150 mL bottle. Place the bottle in a shaker and shake for 5 hrs., heated at 50℃. 5. Repeat procedures 3 and 4.  6. Dissolution of goethite and hematite: add 50 mL citrate-dithionite (50 mL 0.3 M ammonium citrate and 1 g sodium dithionite) to the solid resides in a 150 mL bottle. Place the bottle is placed in a shaker and shake for 16 hrs., heated at 50℃. 7. Repeat procedures 3 and 4. 8. Dissolution of sulfides: add 30 ml 65% HNO3, 10 mL 30% HCl and 10 mL H2O2 to the solid residues in a 150 mL bottle. Place the bottle in a shaker and shake for 16 hrs., heated at 50℃. 9. Repeat procedures 3 and 4. 10. Send the final solid residues and each solution to be analyzed at SGS Lab, Vancouver.  Second extraction scheme (Figure 3.6) (Only the gossan samples were analyzed as they have considerable amount of iron oxides)  Citrate-dithionite (CD) sequential extraction   1. Add 50 ml citrate-dithionite. Place the bottle in a shaker and shake for 4 hrs., heated at 50℃. 2. Filter the mixture. Wash the solid residues with deionized water (30-50 ml). Measure and record the volume of final filtered solution. 3. Dry and weigh the solid residues.  4. Repeat procedures 2, 3 and 4 three times at 4 hrs., 8 hrs. and 16 hrs. in sequence. 5. Send final solid residues and each solution to be analyzed at SGS Lab, Vancouver.  Nutrient content, pH, EC and CEC were examined to assess the sample fertility for plant-growth trials. The analyses of nutrients, pH, EC and CEC were conducted by Pacific Soil Analysis Inc. in Vancouver. The analyzed nutrients include total N and S, available P, K, 36  solution analyzed solution analyzed solution analyzed solution analyzed Ca, Mg, Na, SO4-S, NH4-N and NO3-N, and exchangeable Ca, Ma, Na, and K.                        filter                                        filter                                    filter                                  filter     Figure 3. 5 First extraction scheme           Citrate- dithionite (Holmgren, 1967) Fulvic acid 1 M NH4C2H3O2  shake 16 hrs.  50℃  shake 16 hrs. 50℃  shake 5 hrs. 50℃  HNO3-HCl-H2O2 shake 1 hr. room temperature Exchangeable Metallic PGMs Sulfides Goethite and hematite  Residue 3 g sample < 200 mesh 37  shake 8 hrs.  50℃  shake 16 hrs. 50℃  solution analyzed solution analyzed solution analyzed solution analyzed                      filter                      filter                                   filter                               filter     Figure 3. 6 Second extraction scheme  shake 4 hrs. 50℃  Citrate- dithionite  Citrate- dithionite  Citrate- dithionite  Citrate- dithionite  shake 4 hrs. 50℃  Pt, Pd, Au in goethite and hematite Residue 3 g sample <200 mesh Pt, Pd, Au in goethite and hematite Pt, Pd, Au in goethite and hematite Pt, Pd, Au in goethite and hematite 38  Chapter 4: Results and Discussion  4.1 Particle size analysis Particle size distribution of NAP feed, con and tailings indicate a dominance of fine particles (Figure 4.1). The particle size distribution of BH gossan samples indicates a homogeneous distribution of coarse size to fine size (Figure 4.1). The gossan samples were field-collected as large pieces of rock and then crushed at Massey University to generate media that is suitable for studies of plant growth (Meech and Anderson, 2014). The particle size of NAP samples were distributed up to 425 µm, while for BH gossan samples, the particle sizes were crushed to a much coarser level of up to 4750 µm (Table 4.1).   Table 4. 1 Particle size distribution of NAP feed, con, tailings, and BH gossan samples  Micron(um) Cumulative weight percent passing (%) NAP Feed NAP Con NAP Tailings BH  Gossan 1 BH  Gossan 2 4750 - - - 85.3 90.6 3350 - - - 65.8 72 2360 - - - 54.5 59.5 1700 - - - 47.3 50.8 1180 - - - 42.5 44.9 850 - - - 36.5 37.7 425 93.5 99.9 99.6 27.9 27.2 210 87.2 90.6 97.2 20.3 18.4 150 80.2 84 93.1 17.2 15.2 106 67.6 73.8 79.5 14.2 12.2 75 53.8 58.4 52.5 11.5 9.7 53 32 45.9 33.4 9.0 7.4 38 19.3 32.6 22.2  - - 32 13.6 27.6 14.6  - - 24.4 11 22 10.7  - - 17.3 7.7 15.4 7.4  - - 12.6 4.5 7.9 4.8  - - 9 2 3.4 2.8  - - 0 0 0 0  0 0  39   Figure 4. 1 Particle size distribution curves of NAP feed, con, tailings, and BH gossan samples  4.2 Mineralogy 4.2.1 X-ray diffraction analysis  X-ray diffraction analysis (XRD) showed that NAP feed and tailings contained mainly silicate minerals: actinolite (Ca2(Mg,Fe)5Si8O22(OH)2), andesine ((Na,Ca)(Si,Al)4O8) and clinochlore ((Mg,Fe2+)5Al(Si3Al)O10(OH)8) (Table 4.2). The three minerals accounted for 75.8% and 76.4% of the feed and tailings by mass, respectively. However, NAP concentrate (con) was dominated by the sulfide minerals: pyrite (FeS2), chalcopyrite (CuFeS2) and pentlandite ((Fe,Ni)9S8). These three minerals accounted for 71.3% of the mass of the sample. The XRD analysis did not detected sulfide minerals in the NAP tailings. According to information from the NAP mine, the majority of platinum group minerals occurs either interstitially to sulfides as grains or are associated with sulfides at sulfide-silicate boundaries (Yu et al., 2010; Huminicki, 2013).   The BH gossan samples consisted mainly of Fe-oxide minerals: goethite (α−Fe3+Ο(ΟΗ)) and hematite (α−Fe2Ο3), entirely accounting for 90.6% and 90.5% of gossan 1 and gossan 2 by mass, respectively (Table 4.3). Additionally, more goethite (58.4%) was found in BH gossan 1, while more hematite (49.4%) was found in BH gossan 2. Other minerals in the BH gossan 01020304050607080901005 50 500 5000Cumulative Weight Percent Passing (%)Passing Size (um)FeedConTailingsGossan 1Gossan 240  samples were quartz, jarosite, talc and plagioclase. There were no PGM minerals detected by XRD in the gossan samples.   The different density between the two types of gossan samples may be due to the different main contents of goethite and hematite. The important factors influencing the quantitative relationship between goethite and hematite are temperature, activity of water, organic matter, pH, and the release rate of Fe during weathering (Schwertmann, 1985). Both goethite and hematite can form from ferrihydrite and their formations compete with each other (Schwertmann, 2008). Hematite production by transformation of ferrihydrite takes place under weakly acid to weakly alkaline conditions and requires the presence of some water within the ferrihydrite aggregates (Schwertmann and Fischer, 1966; Schwertmann et al., 1999). Under alkaline conditions, ferrihydrite dissolves to release soluble FeIII species (Fe(OH)-4 from which the goethite forms (Schwertmann and Cornell, 2008). Hematite is favored over goethite in warmer and drier pedoclimates because these promote dehydration over dissolution of the ferrihydrite. Hydrous ferric oxides, like goethite, are strong adsorbers of metals such as Pt, Pd, Au and Ag under many conditions (Jenne, 1968).   41  Table 4. 2 XRD analysis (wt. %) for samples from NAP Mineral Ideal Formula NAP Feed  NAP Con  NAP Tailings  Actinolite Ca2(Mg,Fe)5Si8O22(OH)2 40.1 8.4 36.4 Andesine (Plagioclase) (Na,Ca)(Si,Al)4O8 25.0 4.9 28.3 Clinochlore (Mg,Fe2+)5Al(Si3Al)O10(OH)8 10.7 2.5 11.7 Plagioclase NaAlSi3O8 – CaAl2Si2O8 8.7 0.0 10.3 Quartz SiO2 6.8 2.0 7.4 Talc Mg3Si4O10(OH)2 3.9 9.0 2.5 Biotite K(Mg,Fe)3(AlSi3O10)(OH)2 1.5 0.0 1.6 Ankerite-Dolomite Ca(Fe2+,Mg,Mn)(CO3)2-CaMg(CO3)2 1.4 0.0 1.0 Pyrite FeS2 0.7 33.2 0.0 Ulvöspinel ? Fe2+2TiO4 0.7 0.0 0.8 Chalcopyrite CuFeS2 0.5 26.3 0.0 Pentlandite (Fe,Ni)9S8 0.0 11.8 0.0 Millerite NiS 0.0 1.8 0.0 Merenskyite ? (Pd,Pt)(Te,Bi)2 0.0 0.1 0.0 Total   100.0 100.0 100.0  Table 4. 3 XRD analysis (wt. %) for gossan 1 and gossan 2 from BH Mineral Ideal Formula BH Gossan 1 BH Gossan 2 Goethite  α−Fe3+Ο(ΟΗ) 58.4 41.1 Hematite α−Fe2Ο3 32.2 49.4 Quartz SiO2 7.2 7.7 Plagioclase  NaAlSi3O8 – CaAlSi2O8 1.4 0.0 Jarosite  K2Fe63+ (SO4)4(OH)12 0.8 0.6 Talc Mg3Si4O10(OH)2 0.0 1.2 Total 100.0 100.0 42  4.2.2 SEM analysis  Minerals of BH gossan 1 and gossan 2 observed by SEM are listed in Table 4.4.  Table 4. 4 Minerals of BH gossan 1 and gossan 2 observed by SEM BH Gossan 1 BH Gossan 2 Barite (BaSO4) Barite (BaSO4) Fe-Cr-Ni alloy Fe-Cr-Ni alloy Galena (PbS) Galena (PbS) Iodargyrite (AgI) Iodargyrite (AgI) Chalcopyrite (CuFeS2) Chalcopyrite (CuFeS2) Native Pt Native Pt Pt-Au alloy Pt-Au alloy Pd telluride Pd telluride Fe-Ni sulfide Native Bi - Pd-Pt alloy - Sperrylite (PtAs2) - Pd-Cu oxide  Barite (Figure 4.2) crystals and patches are reasonably common in both BH gossan samples. They were enclosed in goethite. Crystals of galena (Figure 4.2) up to 20 um in size were observed from both BH gossan samples. Sn was detected in the galena grain of BH gossan 2. Chalcopyrite (Figure 4.2) grains were found in both BH gossan samples. One grain of Fe-Ni sulfide (Figure 4.2) associated with barite was located in BH gossan 1. It may be violarite (FeNi2S4), which is a weathering product of nickel sulfide minerals. A few grains of Fe-Cr-Ni alloy (Figure 4.3), associated with iodargyrite or hematite, were found in both BH gossan samples. They may be the weathering product of chromite. Irregular grains of iodargyrite (Figure 4.3), associated with Fe-Cr-Ni alloy or hematite, were common in both BH gossan samples. Clusters of native bismuth (Figure 4.3), associated with Pd-Pt alloys, were located in BH gossan 2.  Grains of native Pt (Figure 4.4) were found enclosed in goethite in both BH gossan samples. A few grains of Pt-Au alloy (Figure 4.5) were located in both BH gossan samples. Ir was detected in the alloys associated with Pd-Cu oxides in BH gossan 2 (Figure 4.6). One grain of sperrylite (Figure 4.5) was observed in BH gossan 2. One grain of Pd-Pt alloy (Figure 4.3), 43  associated with native bismuth, was located in BH gossan 2. Crystals of Pd telluride (Figure 4.4) with Bi were detected in both BH gossan samples. Grains of Pd-Cu oxide (Figures 4.5, 4.6 & 4.7) were found in BH gossan 2. Ag was also detected in the Pd-Cu oxides associated with Pt-Au alloys (Figures 4.6 & 4.7).  In this SEM study, lots of barite grains were found in both gossan samples, and they were surrounded by goethite and hematite. Some grains of iodargyrite were either separated as independent grains or associated with Fe-Cr minerals. Galena was also common in both samples. Some grains of platinum group minerals were observed in both samples. Certain amounts of Cu and Ni were found associated with platinum group minerals. According to Elvy (1998), the observed platinum group minerals in the Mulga Springs gossan included froodite (PdBi2), palladium-copper alloys, palladium-tellurium-mercury alloy, paolovite (Pd2Sn), platinum alloys, sperrylite (PtAs2) and unnamed palladium tellurides. Among them, palladium-copper alloys platinum alloys, sperrylite (PtAs2) and unnamed palladium tellurides were found in BH samples. Except for the above-observed minerals, others such as azurite (Cu3(CO3)2(OH)2), cerussite (PbCO3), chrysocolla, and malachite were also detected by Elvy (1998).   Locmelis et al. (2010) and Suárez et al. (2010) studied the platinum group element (PGE) distribution in the oxidized sulfide zone of Great Dyke, Zimbabwe and the Aguablanca Ni–Cu deposit, SW Spain. The main iron oxides and oxyhydroxides in the samples were analyzed quantitatively by electron microprobe. They both classified the PGE-bearing minerals in groups including: (1) Platinum group minerals hosted or associated with base metal sulfides consisting of arsenides including sperrylite and Pt-Pd-tellurides; (2) Platinum group minerals formed by an initial stage of oxidation or secondary platinum group minerals; (3) PGE-oxides/hydroxides; (4) Fe (±Ni–Cu)-oxides with variable concentrations of Pt and Pd, and PGE-hydroxides, and (5) PGE bearing goethite, hematite and silicates containing traces of all six PGEs. In the BH gossan samples, minerals in the groups (1), (2), (3) were detected by SEM.   Lottermoser and Ashley (1996) investigated the barite-rich rock in the Proterozoic Willyama 44  Supergroup, Olary Block, Australia, which is contiguous with the Broken Hill deposit. Sulphur and strontium isotopic data, and REE distribution indicate that barite is precipitated from the injection of high-temperature, acidic, oxidized hydrothermal fluids into alkaline waters of sabkha-type environments. Sabkhas are supratidal, forming along arid coastlines and are characterized by evaporite-carbonate deposits with some siliciclastics (Smithson et al., 2002). The fluids evolved via mixing processes to lower temperatures and higher pH values, and precipitated silica and Fe-oxide rich products. This may provide an explanation regarding the existence of numerous barite grains in the BH gossan samples.   The SEM analysis reveals that barite is common in the two gossan samples. As for platinum group minerals, native Pt, Pt-Au alloy and Pd telluride were detected in both gossan samples. Sperrylite and Pd-Cu oxide were also found in gossan 2. Certain amounts of Cu and Ni were found associated with platinum group minerals. Other common minerals observed include iodargyrite, galena, chalcopyrite and Fe-Cr-Ni alloy. Minor Fe-Ni sulfide in gossan 1, and native bismuth in gossan 2 were also found.                  45    Figure 4. 2 Micrography of Back-scattered Electrons and EDS spectra showing: barite, spot 1 in (a) and (b); galena, spot in (c) and (d); chalcopyrite, spot in (e) and (f); Fe-Ni sulfide, spot 2 in (g) and (h).  46    Figure 4. 3 Micrography of Back-scattered Electrons and EDS spectra showing: Fe-Cr-Ni alloy, spot 1 in (a) and (b); iodargyrite, spot 2 in (a) and (c); bismuth, spot 1 in (d) and (e); Pd-Pt alloy, spot 2 in (d) and (f). 47   Figure 4. 4 Micrography of Back-scattered Electrons and EDS spectra showing: native Pt, spot 1 in (a) and (b); Pd telluride, spot 1 in (c) and (d).               48    Figure 4. 5 Micrography of Back-scattered Electrons and EDS spectra showing: Pt-Au alloy, spot 1 in (a) and (b); sperrylite, spot 2 in (a) and (c); Pd-Cu oxides, spot 1 in (d) and (e); Pt-Au alloy, spot 2 in (d) and (f). 49    Figure 4. 6 Micrography of Back-scattered Electrons and EDS spectra showing: Pd-Cu oxide, spot 1 in (a) and (b). The following six EDS X-ray maps show the spatial distributions of elements in sample BH2-4. 50      51   Figure 4. 7 Micrography of Back-scattered Electrons and EDS spectra showing: Pd-Cu oxide, spot 1 in (a) and (b); Pt-Au alloy, spot 2 and spot 3 in (a), (c) and (d). The following eight EDS X-ray maps show the spatial distributions of elements in sample BH2-2-4.   4.3 PGMs and Au concentration 4.3.1 Total concentration of PGMs and Au  The total concentration of PGMs and Au determined by INAA are shown in Table 4.5. The Au concentration is also considered as it can be made soluble and accumulated by plants. Pt and Au concentrations in NAP con are the highest of all samples. All six PGM grades in BH gossan 1 and gossan 2 are very high.   The PGM and Au concentrations in BH gossan 1 are greater than those in BH gossan 2. Particularly, the Pd concentration is almost twice of that found in BH gossan 2. Notably, the difference between Pt concentration in BH gossan 1 and gossan 2 is not as large as that of Pd in the two samples. With the exception of several grains of Pd tellurides, no other Pd minerals were found in BH gossan 1. Gray et. al (1996) investigated the mineral phases 52  hosting platinum group elements in a lateritic regolith, Ora Banda Sill, and suggested that Pd tends to be incorporated into Al-rich goethite, and that Pt tends to be incorporated into hematite. This may provide an indication regarding the difference of Pd in the two gossan samples. As for Pt, hematite in gossan 2 (49.4%) was found in greater amounts than that in gossan 1 (32.2%). However, this did not result in more Pt in gossan 2.   Table 4. 5 Total concentrations of PGMs and Au in all samples Element Unit Sample NAP Feed NAP Con NAP Tailings BH Gossan 1 BH Gossan 2 Pd ppb 20700 267000 700 48000 28000 Pt ppb 1700 22000 100 15000 13000 Rh ppb <5 180 <5 4000 2100 Ir ppb <1 11 <1 7100 2300 Os ppb <56 <110 110 3800 1500 Ru ppb <150 <420 <50 3000 1200 Au ppb 12500 22000 120 2500 940   4.3.2 Selective extraction experiments 4.3.2.1 Exchangeable cations Ammonium acetate was used to extract exchangeable cations. These are the PGMs weakly adsorbed onto the minerals, which can be exchanged by ammonium acetate.  They are naturally soluble species. The results (Table 4.6, 4.7, 4.8, 4.9 & 4.10) show that from 0.1% to 2% of Pd in NAP samples and BH samples were dissolved by ammonium acetate. Almost no Pt in all samples was dissolved by ammonium acetate.   4.3.2.2 Metallic PGMs  Fulvic acid was used to extract metallic PGMs. Wood (1996) found that fulvic acid may form soluble complexes with Pd and Pt. Small proportions of Pd in NAP samples and BH samples were dissolved by fulvic acid. Almost no Pt in NAP samples and BH samples was dissolved by fulvic acid.  53  4.3.2.3 Fe oxides Citrate-dithionite can dissolve amorphous and partially crystalline Fe oxides (i.e., hematite and goethite). From 8.7% to 12.3% of Pd and from 22.9% to 24% of Pt in BH samples were dissolved by citrate-dithionite indicating association of these metals with the Fe oxides. The proportions of Pt dissolved by citrate-dithionite in both BH samples were similar, due to Pt being associated with Fe oxides in both BH samples. The different dissolving percentages of Pt and Pd in two samples may suggest that the proportions of Pt and Pd associated with Fe oxides in two samples are different.  4.3.2.4 Reverse aqua regia Reverse aqua regia is supposed to extract sulfides in NAP samples. As for BH samples, the major minerals are goethite and hematite, which can also be dissolved by reverse aqua regia. Considering that citrate-dithionite is also able to dissolve goethite and hematite, ambiguities remained with respect to the proportions dissolved by reverse aqua regia in BH samples.   Large amounts of Pd and Au in NAP samples were dissolved by reverse aqua regia. This suggests Pd and Au associations with sulfides. In contrast, the Pt solubility in reverse aqua regia was found to be considerably lower in the NAP samples. This can either indicate that most of the Pt is not in sulfides or the chloride concentration was not high enough to keep Pt in the solution. The proportions of Au and Cu dissolved in NAP feed, con, and tailings are similar, probably due to Au being associated with Cu sulfides.  4.3.2.5 Non-extractable cations Lots of Fe in NAP feed and tailings remained in the residues. This agrees with the XRD analysis results that up to 70% of NAP feed and tailings were the silicate minerals actinolite (Ca2(Mg,Fe)5Si8O22(OH)2), andesine ((Na,Ca)(Si,Al)4O8) and clinochlore ((Mg,Fe2+)5Al(Si3Al)O10(OH)8). These silicate minerals that cannot be dissolved by reverse aqua regia contain high quantities of Fe.   The exchangeable species extracted by ammonium acetate are naturally available to plants. 54  Cations dissolved by fulvic acid and citrate-dithionite are expected to be extractable from the soil environment. Those extracted by reverse aqua regia are not necessarily available to plants as they are in the lattice of sulfides. The availability of Pd, Pt, Au and Cu of all samples are showed in Table 4.11.   Due to the high surface area, goethite has a high adsorption capacity of precious metals ions (Dikikh et al., 2007; Jean-Soro et al., 2013). Large amounts of goethite and hematite were not dissolved by citrate-dithionite. Veiga et al. (1991) conducted the selective extraction experiments with respect to hydrous ferric oxides and also found that the dissolution of hematite and other crystalline hydrous ferric oxides, such as goethite, by citrate and Na-dithionite was unsuccessful. Therefore, a second extraction scheme, citrate-dithionite sequential extractions, was performed trying to dissolve all goethite and hematite.  In the second extraction scheme, 97.7% and 96.5% of Fe were dissolved by citrate-dithionite in BH gossan 1 and gossan 2 (Table 4.12 & 4.13). However, more than 87% of Pd and 68% Pt in both BH gossan samples were not extracted. This suggests that most of the Pd and Pt are not associated with Fe-oxide phases.   Table 4. 6 Pd%, Au%, Cu%, Fe% and Ni% extracted by chemicals in NAP feed NAP Feed Pd % Pt %1 Au % Cu % Fe % Ni % Ammonium acetate 2.0 - 0.4 2.0 0.0 1.5 Fulvic acid 0.1 - 0.2 0.2 0.0 1.6 Citrate-dithionite 0.2 - 0.4 0.5 5.2 3.7 Reverse aqua regia 86.7 - 98.6 97.0 18.0 67.6 Not extracted 11.1 - 0.4 0.4 76.8 25.6 Total 100.0 - 100.0 100.0 100.0 100.0  1 Pt concentrations are lower than the detect limitation of ICP-AES, 10 ug L-1.       55  Table 4. 7 Pd%, Au%, Cu%, Fe% and Ni% extracted by chemicals in NAP con NAP Con Pd % Pt % Au % Cu % Fe % Ni % Ammonium acetate 0.1 0.0 0.2 0.3 0.0 8.5 Fulvic acid 0.0 0.0 0.3 0.2 0.0 9.9 Citrate-dithionite 0.0 0.0 1.6 0.4 5.0 1.3 Reverse aqua regia 77.1 17.9 96.3 99.1 90.4 79.9 Not extracted 22.7 82.2 1.7 0.1 4.6 0.4 Total 100.0 100.0 100.0 100.0 100.0 100.0            Table 4. 8 Pd%, Au%, Cu%, Fe% and Ni% extracted by chemicals in NAP tailings NAP Tailings Pd % Pt %1 Au % Cu % Fe % Ni % Ammonium acetate 1.8 - 7.9 27.9 0.0 0.0 Fulvic acid 1.1 - 8.2 0.8 0.2 0.0 Citrate-dithionite 1.9 - 17.5 0.9 3.5 0.0 Reverse aqua regia 90.7 - 60.1 68.7 15.7 24.9 Not extracted 4.5 - 6.4 1.7 80.7 75.1 Total 100.0 - 100.0 100.0 100.0 100.0   1 Pt concentrations are lower than the detect limitation of ICP-AES, 10 ug L-1.                       Table 4. 9 Pd%, Au%, Cu%, Fe% and Ni% extracted by chemicals in BH gossan 1  BH Gossan 1 Pd % Pt % Au % Cu % Fe % Ni % Ammonium acetate 1.3 0.0 4.4 4.3 0.0 0.6 Fulvic acid 0.2 0.0 3.4 0.0 0.1 0.6  Citrate-dithionite 12.3 22.9 13.7 28.3 34.8 25.6 Reverse aqua regia 79.7 53.5 74.8 54.2 57.6 61.7 Not extracted 6.5 23.5 3.6 13.2 7.5 11.5 Total 100.0 100.0 100.0 100.0 100.0 100.0  Table 4. 10 Pd%, Au%, Cu%, Fe% and Ni% extracted by chemicals in BH gossan 2 BH Gossan 2 Pd % Pt % Au % Cu % Fe % Ni % Ammonium acetate 1.2 0.0 12.0 14.8 0.0 0.6 Fulvic acid 0.1 0.0 5.9 0.0 0.0 0.7 Citrate-dithionite 8.7 24.0 9.2 26.9 36.2 34.1 Reverse aqua regia 81.5 56.6 68.5 51.7 44.4 48.9 Not extracted 8.5 19.4 4.5 6.6 19.4 15.7 Total 100.0 100.0 100.0 100.0 100.0 100.0  56  Table 4. 11 Availability of Pd, Pt, Au and Cu in all samples to plants Sample Availability Pd (mg/kg ) Pt (mg/kg) Au (mg/kg) Cu (mg/kg) Feed Exchangeable 0.56  - 0.05 30.80 Extractable 0.08  - 0.08 10.30 Con Exchangeable 0.23  - 0.03 283.33 Extractable 0.12  - 0.28 476.67 Tailings Exchangeable 0.05  - 0.04 83.33 Extractable 0.09  - 0.14 6.67 Gossan 1 Exchangeable 0.49 0.00 0.04 645.43 Extractable 4.89 2.92 0.14 4248.53 Gossan 2 Exchangeable 0.24 0.00 0.06 1400.00 Extractable 1.87 3.36 0.08 2550.00  Table 4. 12 % Metals extracted from BH gossan 1 with citrate-dithionite at 50℃ Gossan 1 Pd % Pt % Cu % Fe% Ni% 4 h 3.9 21.4 14.4 39.3 27.4 8 h 6.6 27.5 24.0 73.7 63.5 16 h 9.4 29.6 39.5 97.4 92.9 32 h 10.1 31.9 53.4 97.7 93.7 Not extracted 88.9 68.1 46.6 2.3 6.3 Total 100.0 100.0 100.0 100.0 100.0          Note: Au concentrations are lower than the detection limit of ICP-AES, 1ug g-1.  Table 4. 13 % Metals extracted from BH gossan 2 with citrate-dithionite at 50℃ Gossan 2 Pd % Pt % Cu % Fe % Ni % 4 h 4.4 20.0 14.4 35.1 24.9 8 h 8.2 26.0 23.6 70.2 59.7 16 h 10.6 28.0 37.8 96.1 91.8 32 h 12.6 30.7 63.1 96.5 92.5 Not extracted 87.4 69.3 36.9 3.5 7.5 Total 100.0 100.0 100.0 100.0 100.0           Note: Au concentrations are lower than the detection limit of ICP-AES, 1ug g-1.  4.4 Soil associated factors 4.4.1 Nutrients, pH, EC and CEC  According to the nutrients analysis the levels of N, K, P and Mg are very low (Table 4.14). 57  As the samples have low nutrient reserves, adequate fertilization program must be implemented before cultivation of the substrate.   The pH of wet NAP con was 3.6 (Table 4.15), and as such, according to Dairy Soils and Fertiliser Manual (2013) the availability of macronutrients (nitrogen, phosphorus, potassium, 57ulphur, calcium and magnesium) as well as molybdenum in the soil is curtailed at pH of 3.6. Thus, the soil was too acid for the survival of most plants (Table 4.16). This acidity may be due to the presence of acid-generating sulphides in this substrate. Both the NAP feed and NAP tailings were slightly alkaline. In normal soil with this pH, iron availability would be a problem for plant growth. However, as the selected samples had high concentrations of metals, iron availability might not be a problem. BH gossan 1 is acceptable for most plants, whereas BH gossan 2 is too alkaline for most plants. When the pH is greater than 8.5, levels of available nitrogen, calcium, magnesium, iron, manganese, copper, zinc and aluminium are very low, and this could possibly lead to deficiencies in plants.   The electrical conductivity (EC) of all samples except NAP con were lower than 2 dS/m (Table 4.15), suggesting they don’t have problems of salinity. The EC of NAP con reached as high as 9 dS/m. According to Manitoba Agriculture, Food and Rural Initiatives soil management guide (2008), this is moderately saline, meaning that plant growth can be limited by holding water more tightly than the plants can extract it.  The cation exchange capacity (CEC) of the BH gossan samples was the highest, 38.8 meq/100g, while those of NAP feed, con and tailings were below 10 meq/100g (Table 4.15). According to Spectrum Analytic Inc. (2015), soils with a CEC of 11-50 meq/100g have a greater capacity to hold nutrients and water than do those with CEC of 1-10 meq/100g. Low CEC soils are more likely to develop potassium and magnesium (and other cations) deficiencies while high CEC soils are less susceptible to the leaching losses of these cations (Rayment and Higginson, 1992). This indicates that an adequate fertilization program should be implemented when cultivating on NAP samples. Although normally, CEC is a function of clay and organic matter in the soil, in this particular situation it may be a function of the weathering of Fe.  58  Table 4. 14 Available concentrations of the primary nutrients in all samples and  typical concentrations sufficient for plant growth Available concentration (mg/kg) Samples Typical concentration sufficient for plant growth NAP Feed NAP Con NAP Tailings BH Gossan 1 BH Gossan 2 N 24 9 10 32 36 15,000 K 44 4 18 155 660 10,000 Ca 500 750 700 3,950 1,050 5,000 Mg 20 140 17 80 90 2,000 P 3 3 3 3 1 2,000 S 125 2,083 55 57 66 1,000 Na 210 17 60 44 70 -  Table 4. 15 The pH, EC and CEC of all samples Parameter Unit Samples NAP Feed NAP Con NAP Tailings BH Gossan 1 BH Gossan 2 pH  - 7.8 3.6 8.0 6.5 8.5 EC dS/m 1.7 9.0 0.4 0.7 0.7 CEC meq/100g 5.0 8.4 4.2 38.8 38.8  Table 4. 16 Soil pH and its influence on plant growth (Whiting et al., 2014) Soil Reaction pH Plant Growth Alkaline soil >8.3 Too alkaline for most plants 7.5 Iron availability becomes a problem in alkaline soils 7.2 6.8 to 7.2 – “near neutral”                           6.0 to 7.5 – acceptable for most plants Neutral soil 7.0 Acid soil 6.8 6.0 -  5.5 Reduced soil microbial activity <4.6 Too acid for most plants   59  4.4.2 Heavy metals The total concentrations of heavy metals in all samples were compared to the Dutch standards (Ministry of Housing – Netherlands, 1994) for assessing soil contamination on the basis of the total concentration of heavy metals in the soil in Table 4.17. None of the heavy metals concentrations of NAP tailings exceeded the intervention values of the Dutch standards. Cu and Ni contents in the other four samples went beyond the intervention values of the Dutch standards. This indicates that plants may have Cu and Ni toxicity problems when they grow on these substrates. Ba and Co concentrations of BH gossan 1 and gossan 2 were also very high. The phytotoxicity of Cu has been proposed as an important variable that will affect the yield of PGMs during phytomining, since toxicity due to Cu uptake will influence the rate of transpiration and thus the degree of PGM uptake (Wilson-Corral et al., 2013; Aquan, 2015). The exchangeable (naturally soluble) and extractable (chemically soluble) Cu (Table 4.11), those of NAP con, BH gossan 1 and gossan 2 exceed 190 mg/kg, are problems to plants. Thus, potential problems with heavy metals toxicity should be taken into account when growing plants on NAP feed, NAP con, BH gossan 1 and BH gossan 2.  Table 4. 17 Comparisons with Dutch standards for heavy metals in soils Elements Dutch standards intervention value Total concentrations in samples mg/kg soils NAP Feed NAP Con NAP Tailings BH Gossan 1 BH Gossan 2 As 55 3.7 <0.5 <0.5 36.6 23.7 Ba 625 82 23 88 1140 1142 Cd 12 0.1 3.7 <0.1 0.5 1 Cr 380 294 109 253 239 315 Co 240 78.3 2138.6 38.6 243.8 351.4 Cu 190 1157.5 >10000 170.6 >10000 >10000 Hg 10 <0.01 0.04 <0.01 0.07 0.1 Pb 530 3.3 130.8 1.3 12.9 10 Mo 200 5 36.2 0.8 27.7 35 Ni 210 1051.4 >10000 147.6 3088.5 6800.4 Zn 720 20 384 16 36 153  60  4.5 The selection model   A possible selection model of the substrate for phytomining is proposed in Figure 4.18. This model is applied to select the best “one” from among the five samples in this work with respect to phytomining of Pd. Anderson et al. (2005) derived a model to quantify the relationship between the substrate and plant gold concentrations. The model was described by the following equation: y = 51.314 In (x) + 62.882 (y represents plant metal concentration, and x represents soil metal concentration). Anderson et al. (2005) estimated that to yield a crop with a gold concentration of 100 mg/kg, a gold concentration of 2 mg/kg is required in the substrate. The total concentration does not represent the available concentration to the plant. 2 mg/kg should be considered as the available Au concentration to plants, due to the similar chemical characteristics of PGMs and Au, 2 mg/kg will be used as the available threshold concentration (exchangeable and extractable concentration) for the phytomining of Pd.  Soil factors are important when considering the suitability of the substrate for plant growth, which will affect indirectly the availability of PGMs to plants. The pH can affect the solubility of target metals and plant nutrients. Target metals may not be dissolved in very high pH. CEC determines the ability of the soil to hold cations. EC measures soil salinity, which influences the water absorption of plants. The pH, EC and CEC are the most important chemical properties of soils. If any of these three parameters are not suitable for plant growth, it will take much effort to make amendments. For example, for EC, there are no quick or easy solutions to soil salinity. The only method is rinsing the salts down. Therefore, appropriate values of pH, EC and CEC would prove favourable for plant growth. Nutrient problems can usually be solved by adding fertilizers.      It was found that Cu is usually associated with PGM minerals in the samples studied and it can be accumulated by plants. Aquan (2015) investigated phytomining of Pd and Au with the BH gossan samples and identified that the high Cu accumulation in plants affect plant growth due to the onset of toxic effects. Hence, Cu concentration is included in the assessment of substrates suitable for the phytomining of PGMs. According to the Dutch standards for assessing soil contamination, the value that identifies serious contamination of soils is 190 61  mg/kg for Cu. Since the total concentration does not represent the concentration available to plants, it is assumed that the available concentration of Cu to plants is the portion exchangeable and extractable and was used for assessment in the selection model. The model can also be used for the selection of other metals as long the target metal threshold concentration is applied.   The main characteristics of the five samples are summarized in Table 4.18. According to the selection model, the suitability of the five samples is presented in Table 4.19. The available Pd concentration of NAP feed, con, and tailings are lower than 2 mg/kg. The available Pd concentration of BH gossan 1 is higher than 2 mg/kg. Its low EC, high CEC, and proper pH make it a suitable substrate for plant growth. It is the best “one” of the five samples. One thing to note is that high Cu-tolerant plant species should be chosen to grow on BH gossan 1 due to its high Cu concentration available to plants. The available Pd concentration of BH gossan 2 is also higher than 2 mg/kg. However, the pH (8.5) is slight alkaline. This means amendments should be applied to enhance its pH before planting plants. Besides, high Cu-tolerant plant species should also be chosen to grow on BH gossan 2 due to its high Cu concentration.62   Figure 4. 8 A possible selection model for the substrate of phytomining  63  Table 4. 18 Summary of characteristics of all the samples Characteristics NAP Feed NAP Con NAP Tailings BH Gossan 1 BH Gossan 2 Available Pd and Pt  Pd 0.64 mg/kg, Pt 0 mg/kg                 Pd 0.35 mg/kg, Pt 0 mg/kg                        Pd 0.14 mg/kg, Pt 0 mg/kg Pd 5.38 mg/kg, Pt 2.92 mg/kg Pd 2.11 mg/kg, Pt 3.36 mg/kg pH 7.8 3.6 8.0 6.5 8.5 EC 1.7 dS/m 9.0 dS/m 0.4 dS/m 0.7 dS/m 0.7 dS/m CEC 5.0 meq/100g 8.4 meq/100g 4.2 meq/100g 38.8 meq/100g 38.8 meq/100g Exchangeable Cu  31 mg/kg 283 mg/kg 83 mg/kg 645 mg/kg 1400 mg/kg Nutrients N, K, P, Ca, Mg and S concentration were lower than required concentration.  N, K, P, Ca and Mg concentration were lower than required concentration.  N, K, P, Ca, Mg and S concentration were lower than required concentration.  N, K, P, Ca, Mg and S concentration were lower than required concentration.  N, K, P, Ca, Mg and S concentration were lower than required concentration.  Minerals Mainly contained silicates (actinolite, andesine and clinochlore) Mainly contained sulfides (pyrite, chalcopyrite and pentlandite) Mainly contained silicates (actinolite, andesine and clinochlore) Mainly contained Fe oxides (63ssociat and hematite) Mainly contained Fe oxides (63ssociat and hematite) PGMs minerals - Merenskyite, associated with sulfides - Native Pt, Pt-Au alloy,                 Pd telluride Native Pt, Pt-Au alloy, Pd telluride, Pd-Pt alloy, sperrylite, Pd-Cu oxide  Table 4. 19 The suitability of all samples assessed by the selection model Samples NAP Feed  NAP Con NAP Tailings BH Gossan 1  BH Gossan 2  Pd concentration >  2 mg/kg × × × √ √ pH: 6.0-7.5 × × × √ × EC < 2 dS/m √ × √ √ √ CEC: 11-50       meq/100g × × × √ √ Cu concentration < 190 mg/kg × × √ × × 64  Chapter 5: Conclusion  • Mineralogical studies found - NAP feed and tailings contained mainly silicate minerals, while NAP con was dominated by sulfide minerals. NAP con was detected to contain platinum group minerals (Merenskyite). - BH gossan samples consisted mainly of Fe-oxide minerals. - The different ratios of goethite and hematite in BH gossan 1 and gossan 2 result in their different density. - Platinum group minerals such as native Pt, Pt-Au alloy and Pd telluride were detected in SEM in both BH gossan samples. Sperrylite and Pd-Cu oxide were also found in BH gossan 2. Certain amounts of Cu and Ni were found associated with platinum group minerals. - Significant amount of barite was found in both BH gossan samples by SEM. The formation of barite may be due to the precipitation from the the injection of high-temperature, acidic, oxidized hydrothermal fluids into alkaline waters of sabkha-type environments.   • Selective extraction experiments found - Little portion of Pd and Pt were found associated with exchangeable fractions in NAP samples and BH samples. - Most of Pd was found associated with sulfides in NAP samples, while not enough data suggest that Pt is associated with sulfides in NAP samples. - Most of the Pd and Pt in the BH gossan samples are not associated with goethite and hematite. - Large amounts of Cu were found associated with exchangeable and extractable fractions in NAP samples and BH samples, which are problems to plants. - Selective extraction analysis is prediction of the bioavailability of PGMs to plants. 65  - The PGM species that can be extracted by ammonium acetate are the best indicators of their natural availability to plants. Those PGMs that can be extracted by fulvic acid and citrate-dithionite are good indicators as they can be soluble in soils.  • Fertility analysis found  - The NAP con has low pH and high salinity, which makes it unsuitable as a       direct growth media for plants. - BH gossan 1 has low EC, high CEC, and proper pH, which makes it a suitable substrate for plant growth.  • Selection model derived  - The available Pd concentration of NAP feed, con, and tailings are lower than 2 mg/kg. - The available Pd concentration of BH gossan 1 is higher than 2 mg/mkg. Its low EC, high CEC, and proper pH make it a suitable substrate for plant growth. It is the best “one” of the five samples for phytomining of Pd. One thing to note is that high Cu-tolerant plant species should be chosen to grow on BH gossan 1 due to its high Cu concentration available to plants. - The available Pd concentration of BH gossan 2 is also higher than 2 mg/kg. However, the pH (8.5) is slight alkine. 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