Tailings and Mine Waste Conference

Methods for removing selenium from aqueous systems Moore, Lucas; Mahmoudkhani, Amir Nov 30, 2011

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Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 Methods for Removing Selenium from Aqueous Systems Lucas Moore, Ph.D., Amir Mahmoudkhani, Ph.D. Kemira Oil and Mining, Atlanta, USA Abstract Selenium is among the list of oxyanions that lead to contamination in mining aqueous waste streams.  Though the elemental forms are toxic, the aqueous oxyanions are more so.  The most common forms of selenium released during mining processes are the aqueous forms, selenite and selenates. The common treatment technologies to date can be summed up in these major categories: media filtration, chemical treatment, and biomediated removal.  These methods can often involve many expensive processing steps that may also be limited by variables such as total dissolved solids, presence of other ions, and ability to maintain microbial growth.  We have developed an innovative chemical technology that can successfully reduce selenates to a level below the EPA recommendations.  This new technology offers a unique and viable solution which is transparent to the above mentioned limitations. Introduction Selenium is a naturally occurring metalloid that belongs to the chalcogen group.  Selenium is widely applied in global industries such as electronics, fertilizers, fungicides, antidandruff shampoo, and many more.  Commercial quantities of selenium are generated during copper electrolytic refining (USGS 2007; Lenz 2009).  Selenium, in small quantities (0.1-0.5 ppm dry weight) is a micronutrient that is part of everyday life.  Having said that, selenium becomes toxic at concentrations > 3 ppm dry weight. The National Primary Drinking Water Standard is 50 ppb for total selenium and the National Fresh Water Quality Standard is 5 ppb for total selenium (EPA 2001; EPA 2011). British Columbia has placed a standard of 2 ppb for total selenium. In nature, selenium is most commonly observed as selenate, selenite, or selenide. (Figure I) Though complexed selenium is of low toxicity, selenate (SeVI) and selenites (SeIV) are very toxic. These two forms of selenium are generally found in water, and display bioaccumulation and bioavailability.  Under acidic conditions, the extremely toxic and corrosive hydrogen selenide gas can be generated from selenium containing species.  The presence of selenates and selenites in waste water is an immediate problem. If left untreated, selenium will bioaccumulate and pose a threat to all aquatic life downstream. The EPA listed selenium in at least 508 of the 1,636 sites listed on the National Priorities List (ATSDR 2011). The National Priorities List is a list of sites containing high levels of hazardous waste in the USA and these sites are financially eligible for the Federal Superfund program.  Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 Figure I:  Most common oxidation states of selenium Chemically, selenium behaves very similarly to sulfur. Thus, selenium is most often associated with sulfur containing ores such as pyrite, sphalerite, and chalcopyrite (Adams 2005). These sulfate/sulfite containing ores are prevalent in the mining industry of metals such as copper, nickel, silver, lead, and uranium. Consequently, selenium contamination has become an emerging issue in such mining processes. (Figure II)  Selenium contamination typically occurs in the aqueous stream, and this stream must be treated prior to discharge. However, exposure to environmental conditions that the contaminants were not exposed to prior to unearthing could result in the contaminants becoming mobile while still in a tailing pond/pit or even the initial mine site.   Figure II:  Example of a mine site and possible routes for contamination. Selenium is also a major impurity in the production of sulfuric acid and mining/utilization of fossil fuels. Coal, for example, has been reported to contain 0.4-24 ppm selenium prior to processing or usage (Lemly 2004). (Table I) During coal processing/usage, selenium may become further concentrated as seen in the fly ash (0.2-500 ppm). As with coal, oil is also a source of selenium contamination.  Crude oil can contain levels of selenium between 500-2000 ppb, while oil shale has been reported to contain levels between 1.3-5.2 ppm. Refinery waste water has been reported with levels of selenium contamination as high as 75 ppb. Such mining and industrial activities have led to an increase in selenium contamination that can be found from urban to rural areas, mountains to plains, deserts to rainforests, and arctic to tropics. Hence, treating the waste streams from these processes have become of high interest. Selenium is not only a localized threat, but also a global threat that is increasing as various industrial activities increase. (Figure III) Table I:  An example of the selenium concentrations found during the mining, processing and usage of coal. Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 Material/Waste Selenium Concentration Earth's Crust 0.2 ppm Surface Water 0.2 ppb Coal 0.4-24 ppm Coal Storage Pile (Leachate) 1-30 ppb Coal Cleaning (Process Water) 15-63 ppb Coal Cleaning (Solid Waste) 2.3-31 ppm Coal Cleaning (Solid Waste Leachate) 2-570 ppb Coal Burner Ash (Bottom Ash) 7.7 ppm Precipitator Ash (Fly Ash) 0.2-500 ppm Scrubber Ash (Fly Ash) 70-440 ppm Fly Ash (Leachate) 40-610 ppb Flue Gas Desulfurization (Process Water) 1-2700 ppb Flue Gas Desulfurization (Sludge) 0.2-19 ppm Boiler Cleaning Water 5-151 ppb Coal Ash Slurry 50-1500 ppb Ash Settling Ponds 87-2700 ppb Ash Pond Effluents 2-260 ppb Ash Disposal Pit (Leachate) 40-950 ppb Coal Gasification (Solid Waste) 0.7-17.5 ppm Coal Gasification (Solid Waste Leachate) 0.8-100 ppb Gasification (Solid Waste Leachate) 0.8-100 ppb Coal Liquifaction (Process Water) 100-900 ppb Coal Liquification (Solids Waste) 2.1-22 ppm   Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011  Figure 3:  A map providing various sites that have reported high levels of selenium contamination as of 2004. (Coal mining/combustion, oil refining, phosphate mining, gold mining, silver mining, nickel mining, metal smelting, and landfill leachate) Existing Treatment Technologies Elemental selenium is relatively insoluble in aqueous systems and not biologically active, which makes removal much simpler and prevents bioaccumulation. Having said that, the most common form of selenium released during the previously mentioned mining processes are the aqueous forms, selenite and selenates, which are water soluble oxyanions. The most common technologies to date can be summed up in these major categories: media filtration, chemical treatment, and biomediated removal. However, it is important to mention that to date, there is not an ultimate solution for the challenging environmental contamination with selenium (Golder 2009). Physical Treatment Media filtration is a physical treatment method.  These can be as simple as filtering through sand (Kuan 1998), clay (Goh 2004), titanium dioxide (Zhang 2009), or can be as exotic as filtering through ion exchange resins or a membrane (reverse osmosis and nanofiltration) (Stripeikis 2001). Many of these media are commonly used in the water treatment industry. Two common problems associated with filtration media are the increased amount of waste, and the potential for fouling or scaling of the membrane. Many of these types of methods also have sensitivities to other ions such as nitrates, sulfates, and chlorides, which lead to the inability to remove selenates. Membrane filtration is a method that is commonly used, but can be quite costly. In a closed gold mine in California, a reverse osmosis system was applied (Golder 2009).  In this case, the water was already contaminated and contained in a waste pond where aqueous waste was no longer being produced.  In an effort to prevent such contamination from reaching the drinking water reservoir, Golder was contracted to implement the reverse osmosis system.  In this case, 100 US gallons/min of water was treated.  Due to a high level of total dissolved solids, only 40% of the selenium was removed. Another method is ion exchange resins, which work by reversibly exchanging a more desirable ion with a contaminated one.  These ion exchange resins can be altered to fit either a specific ion, or left broad enough to remove a series of ions.  Having said that, there are cases where the resin cannot Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 distinguish between ions as well.  Due to the similar chemical nature and reactivity of sulfates and selenates, it is quite difficult to separate the two using an ion exchange resin; thus, a significant performance decrease is observed in sulfate rich environments.  This performance decrease can be overcome by forcing the formation of a barium scale via the addition of BaCl2.  A combination of precipitation/ion exchange can reduce selenium contamination levels from 1000 ppm to 0.1 ppm. Resins can also be cleaned and reused, leaving a concentrated selenium waste to be disposed of.  As mentioned previously, the large quantities of waste generated is a significant concern when considering such a treatment method. Chemical Treatment Chemical treatment can be categorized into three classes: precipitation (Zhang 2008, Rovira 2008, Hayashi 2009, Geoffroy 2010), cementation, and coagulation (Golder 2009). The treatment works by adjusting the physical or chemical properties of the dissolved contaminant or suspended matter in a way that will enhance the ability to agglomerate.  The particles can then be removed by flotation, filtration, or gravity settling.  Coagulants (Ferrous, Ferric, Aluminate) work by altering the surface charge of the contaminants, thus allowing for the agglomeration of the particles into a flocculated precipitate.  The floc size can be increased by the addition of a polymeric flocculant, such as polyacrylamides. Selenites are quite easily removed using any of these methods; however, selenates are not as reactive.  In order to remove the bulk of the selenate contamination with a chemical treatment method, a reduction step must be incorporated.  Another major disadvantage of most chemical treatment possibilities is in the high quantity of chemicals being consumed, consequently leading to the need to treat the resulting solid waste.  The literature suggests there is often an inconsistency with reducing selenium to the regulated limit. One way to remove selenates was referenced in the previous section. Barium can be used to form a scale/precipitate of barium selenates/selenites that can be filtered.  Zero-valent iron can also be used to remove selenates, as well as selenites (Zhang 2008, Rovira 2008, Hayashi 2009).  The iron can first reduce selenate to selenite, forming ferric and ferrous hydroxides. In turn, the ferric and ferrous hydroxides can complex with selenites, resulting in a precipitate.  At Barrick’s Richmond Hill Mine, ferric sulfate was used to precipitate “selenium” at a pH of 4.5 and this reduced selenium concentrations to 12-22 ppb. Electrocoagulation (Mavrov 2006), photoreduction, and adsorption to acid-treated peanut shells (El- Shafey 2007) are other methods that have been discussed in the literature. However, these methods are very much still in the lab stages and have not progressed into actual trials. Biotreatment It has been claimed that an effective method for removing selenite and selenate from aqueous systems is via the microbial reduction of selenates to elemental selenium (Cohen 2006, Harrison 2010). As with the previously mentioned methods, there are problems associated with the biomediated reduction. Often, the presence of other ions such as nitrates can decrease the effectiveness of the biological systems in reducing the selenates and selenites. If such a problem occurs, a pretreatment step will be necessary prior to introducing mine-produced water into the biological treatment area. Consequently, the capital expense will be further increased. In active microbial reduction, process water is added to the bottom of a reactor where the water flows upward into a microbial “sludge”. It is in this microbial “sludge” where the selenates and selenites are reduced to selenium, which is then removed from the top of the reactor. The literature lists molasses, wood chips, and distiller’s grains as possible media for such microbial activity (Golder 2009). Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 However, one of disadvantages of the microbial reduction process is the elevated concentrations of total suspended solids; a successful microbial reduction would require pretreatment. Such a process has been applied at the USEPA Kennecott site for 6 months, and data suggests a decrease in selenium from 1950 ppb to less than 2 ppb.  This process was also applied at Duke Energy in North Carolina in the presence of high total dissolved solids, resulting in a 99.3% reduction in selenium after 9 months. In addition to the high capital costs associated with this technique, another disadvantage is the microbial maintenance of parameters such as nutrients, energy, and temperature that is required to sustain adequate reduction. Another biomediated route to removing selenium is via biofilms. The literature suggests that this is a route that can also be applied to selenium removal.  The use of Desulfomicrobium sp. was proven to decrease the selenate concentration to sub-micromolar concentrations when lactate and sulfate were used as the growth media (Hockin 2006).  In limited levels of sulfate concentrations, the dominant species of selenium measured is selenide; however, at an excess of sulfates, the selenate is enzymatically reduced to selenium.  It is important to mention that a disadvantage to this method is the decrease of activity observed in the presence of elevated nitrate levels. Unlike the majority of the other biomediated pathways, passive microbial reduction has a relatively low capital expense. It is successful at reducing both selenite and selenate with minimal supervision required, but will leave a large amount of waste.  This method is also temperature sensitive and requires a significant increase in process time. There are other biotreatment methods, but many of them require anaerobic conditions that may be problematic on the industrial scale with the amount of mining water being released (Oremland 1989, Lee 2007). Results and Discussion In-Situ Solidification – Chemisorption Treatment Method The focus of this work is to introduce an innovative approach using in-situ solidification – chemisorption method for treatment of contaminated process waters. Such a method involves the collaboration of the chemical and physical treatment methods mentioned above. An insoluble amorphous sorbent possessing active sites for the chemisorption of oxyanionic species, such as selenate, was prepared by the chemical modification of an inorganic silica based polymeric material. (Figure IV) Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011  Figure IV: Schematic representation of in-situ solidification – chemisorption method. The addition of a promoter (Pr-1) will induce the cross-linking of the inorganic polymer in solution, thus allowing for maximization of the amount of active sites available, hence increasing the potential contact and interaction with the selenates.  Encapsulation of the chemisorbed contaminant is believed to occur due to the irregular nature of the cross-linking process, thus producing an immobilized amorphous solid mass.  The resulting solid mass can be removed from solution with relative ease by gravitational settling, filtration, or other conventional solid removal methods. Adsorption is a process where the substance, contaminant in this case, is transferred from the liquid phase (solution) to the solid surface. Adsorption involves the inter-phase accumulation/concentration of substances at the surface or interface, which can be between any two phases such as liquid-solid. Chemical adsorption or chemisorption takes place as a result of chemical bond being formed between the solute (dissolved species) and the adsorbent, comparable with those leading to the formation of chemical compounds. (Figure V) There are many factors affecting adsorption such as nature of the adsorbent, nature of adsorbate, nature of the solvent, and others. Adsorption processes are capable of removing contaminants if the adsorbent (solid surface) is selected carefully and the solution chemistry is controlled.    Figure V: Chemisorption of selenium oxyanionic species on inorganic polymeric sorbent Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 The new water treatment technology developed here is based on in-situ solidification of a sorbent and chemisorption of contaminant species onto the resulting sorbent.  An amorphous solid is formed by in- situ cross-linking of a modified inorganic polymer based on silicates. The process, as schematically shown in Figure VI, involves the formation of the sorbent and the chemisorption of contaminated species onto sorbent active sites. This single stage treatment takes place in a continuously stirred (400 – 500 rpm) mixing tank. Inorganic polymeric system is dosed at 20 – 1000 ppm to the contaminated water based on the level of contaminant. After 1 – 3 hours mixing, the aliquot is transferred into a gravity settling tank to allow for precipitation of the suspended solids. The clear supernatant is decontaminated process water and may be discharged, or undergo further treatment if necessary. Faster solid separation may be achieved by filtration and/or the use of common organic flocculants such as anionic polyacrylamides. The separated solids from this process may be safely landfilled.  Figure VI: Schematic representation of water treatment in this work. Treatment of Selenium Containing Water Samples Optimization of this new technology for the treatment of selenate containing water led to the development of a series of inorganic polymeric materials that ranged in their chemical compositions and affinities toward cross-linking in aqueous media. (Figure VII)  When using the inorganic polymer type 1 (IP-1*) to treat water samples containing ppm levels of selenate, the efficiency of selenate removal was more than 99% under our process conditions. As with most treatment technologies, a reduction in efficiency was observed as the initial selenium level is decreased to the lower ppb range; however, some treatments successfully reduced selenate levels to below 1 ppb. Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011  Figure VII:  Efficiency of the inorganic polymer systems for selenate removal from water samples. To better understand the technology and to customize it for individual aqueous systems, the treatment method was further developed by optimizing the ratios of the best performing sorbent (IP-1) and the sorption promoting chemical (promoter, Pr-1). IP-1* consists of IP-1 and Pr-1 at an optimized ratio that maximized selenate removal; it resulted in > 99% selenate removal. The efficiency of the treatment process was monitored via ICP analyses of decontaminated water samples, as well as XRF analyses to evaluate the quantity of absorbed contaminant. The IP-1* technology was applied towards more realistic mine conditions.  A dosage demand curve was generated by applying IP-1* towards water samples that contained 1100 ppm sulfates, 1087 ppm total dissolved solids (TDS), and a conductivity of 1625 uS/cm.  Concentrations of IP-1* ranging from 300 ppm to 6000 ppm were applied, and these studies each yielded selenium concentrations reduced from 1000 ppb to below 5 ppb.  (Figure VII) 0 1 2 3 4 5 6 0 1000 2000 3000 4000 5000 6000 7000 S e le n iu m  R e m a in in g  ( p p b ) Treatment Concentration (ppm) IP-1* Selenium Removal  Figure VIII:  IP-1* dosage demand curve in the presence of other ions. Conclusions A new chemisorption technology that incorporates both physical and chemical treatment methods was developed. This technology was evaluated for removing selenium from aqueous systems. The sorbent was formed in-situ by use of silicate-based inorganic polymeric materials. Selenium oxyanions were then chemically adsorbed onto the active sites within the cavities of the sorbent material, thus removing Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 > 99% of the total selenium from water. The chemical nature of this treatment system is diverse, robust, and it was customized via the addition of a promoter to further increase efficiency of selenium removal. Silicate-based inorganic polymers provide an economical and versatile solution for treatment of contaminated mining process waters within an operational and environmental-friendly process. The sensitivities that are commonly associated with the existing treatment technologies, such as elevated concentrations of common cations and anions (e.g. Ca2+, Fe2+, Cl- and SO42-) were not observed in the initial screening of this inorganic polymer system. Ongoing work evaluating this technology on actual contaminated mine process waters shows promising initial data. Experimental Materials Sodium selenate (Na2SeO4) were purchased from Aldrich and used as received with no further purification. Lab-made aqueous solutions containing selenium were prepared by dissolution of the above chemicals in the city of Atlanta tap water. Caution! Sodium selenate is extremely toxic and should be handled and disposed according to regulations for toxic substances. Instruments In this study, a Thermo Scientific ICP-AES system model iCAP 6500 equipped with a charge injection device (CID) detector and a CETAC ASX-520 autosampler was used for determination of the selenium species in water samples. Low detection limits (1 ppb for selenium) were achieved by pre- concentration of 100 mL aqueous samples. Quantitative elemental analyses of trace elements were conducted on a Bruker S4 Explorer wavelength-dispersive X-ray fluorescence spectrometer. Element distributions of selenium before and after treatments were used for qualitative and quantitative analyses of chemisorption of the contaminant species on inorganic polymeric solid sorbent. References Adams, D. J.; Pennington, P., 2005. Selenium and Arsenic Removal from Mining Wastewaters.  Proceedings of the SME Annual Meeting, Denver, Colorado, Preprint 05-53. ATSDR, 2011. 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Goh, K. H.; Lim, T. T., 2004,  Geochemistry of inorganic arsenic and selenium in a tropical soil: effect of reaction time, pH, and competitive anions on arsenic and selenium adsorption.   Chemosphere, 55, 849-859. Golder Associates Inc., 2009, Literature Review of Treatment Technologies to Remove Selenium from Mining Influenced Water; Technical Bulletin No. 08-1421-0034 Rev. 2; Tech Coal Limited, Office: Lakewood, CO. Proceedings Tailings and Mine Waste 2011 Vancouver, BC, November 6 to 9, 2011 Harrison, T.; Sandy, T.; Leber, K.; Srinivasan, R.; McHale, J.; Constant, J.  2010, Characterization and treatment of selenium in water discharged from surface coal mining operations in West Virginia.  Proceedings of the SME Annual Meeting, Phoenix, Preprint 10-142. Hayashi, H.; Kanie, K.; Shinoda, K.; Muramatsu, A.; Suzuki, S.; Sasaki, H., 2009,  pH-dependence of selenate removal from liquid phase by reductive Fe (II)-Fe(III) hydroxysulfate compound, green rust.  Chemosphere, 76, 638-643. Hockin, S.; Gadd, G. M., 2006,  Removal of selenate from sulfate-containing media by sulfate reducing bacterial biofilms. Enviromental Microbiology, 8, 816-826. Kuan, W. H.; Lo, S. L.; Wang, M. K.; Lin, C. F., 1998,  Removal of Se (IV) and Se (VI) from water by aluminum-oxide coated sand.  Water Research, 32, 915-923. Lee, J. H.; Han, J.; Choi, H.; Hur, H. G., 2007,  Effects of temperature and dissolved oxygen on Se(IV) removal and Se (0) precipitation by Shewanella sp. HN-41. Chemosphere,  68, 1898-1905. Lemly, A. D., 2004, Aquatic selenium pollution is a global environmental safety issue.  Ecotoxicology and Environmental Safety, 59, 44-56. Lenz, M.; Lens, P. N. L., 2009, The essential toxin: The changing perception of selenium in environmental sciences. Science of the Total Environment, 407, 3620-3633. Mavrov, V.; Stamenov, S.; Todorova, E.; Chmiel, H.; Erwe, T., 2006,  New hybrid electrocoagulation membrane process for removing selenium from industrial wastewater.  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Mineral Commodity Summaries-Selenium 2007. http://minerals.usgs.gov/minerals/pubs/commodity/selenium/. Zhang, L; Liu, N.; Yang, L.; Lin, Q., 2009,  Sorption behavior of nano-TiO2 for the removal of selenium ions from aqueous solution.   Journal of Hazardous Materials, 170, 1197-1203. Zhang, Y.; Amrhein, C.; Chang, A.; Frankenberger, W. T., 2008, Effect of zero-valent iron and a redox mediator on removal of selenium in agricultural drainage water.  Science of the Total Environment, 407, 89-96. 


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