British Columbia Mine Reclamation Symposium

Leonardite and biochar for mine impacted water and soils Stewart, K. J.; Janin, A. 2014

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 LEONARDITE AND BIOCHAR FOR MINE IMPACTED WATER AND SOILS  K.J Stewart, PhD.1 A. Janin, PhD.2  1Research Associate, 2Industrial Research Chair, Yukon Research Centre, Yukon College,Whitehorse, YT     ABSTRACT  Immobilization of metals using soil amendment processes is increasingly being considered as an effective and low cost remediation alternative in the mining industry.  Both leonardite, a carbon-rich material rich in humic acids and biochar, an organic material that has undergone pyrolysis, have shown to adsorb heavy metals, such as Cd and Zn and promote plant growth.  We examined the potential to use leonardite and biochar for metal sequestration in mining impacted water and soils, by determining their capacity to adsorb metals in water, sequester metals in tailings and promote plant growth.  Biochar removed up to 95% Cd and 90% Zn from synthetic water and resulted in a 74% reduction of Cd and 18% of Zn leached from columns containing tailings. Whereas, leonardite only adsorbed 38% Cd and 29% Zn from synthetic water and resulted in column leachate with higher concentrations of heavy metals.  Leonardite amendments caused decreases in pH and mobilization of metals from tailings may be due to acidification.  Above and belowground growth of 2 different northern native herb species (Lupinus arcticus and Hedysarum alpinum) in amended tailings were examined.  Amendments had little influence on growth with only the leonardite and lime treatment showing increased belowground biomass.  This initial trial demonstrates that both amendments show potential for on-going management of contaminated waters and tailings, however, additional liming agents are likely necessary with leonardite.  KEYWORDS Leonardite, biochar, remediation, heavy metals, tailings, sequestration  INTRODUTION In situ immobilization of metals using soil amendment processes is increasingly being considered as an effective and low cost remediation alternative (Mench et al. 2007, Kumpiene et al. 2008, Fellet et al. 2011).  Leonardite is a carbon-rich material derived from the oxidation of Lignite and is rich in humic acid whereas biochar is a product that results from the oxygen limited, pyrolysis of various biological ingredients, such as wood, fish or animal bone. Several studies have found biochar amendments result in significant decreases in the bioavailability of heavy metals associated with mining impacted soils (Namgay et al. 2006, Fellet et al. 2011, Beesley et al. 2010) and simultaneously improve physical, chemical and biological soil properties (Laird et al. 2010).  Leonardite is known to improve soil conditions (Lao et al. 2005, Zeledόn- Torunõ et al. 2005, Madejόn et al. 2010) and has potential to significantly reduce metal bioavailability due to high metal adsorption capacity (Lao et al. 2005, Zeledόn- Torunõ et al. 2005).   The study of solubility and bioavailability of metals in contaminated soils or water is important in remediation activities because they represent the most labile fractions subject to leaching and to being uptaken by plants and microorganisms (Adriano 2001).  Both leonardite and biochar have shown to adsorb heavy metals, such as Zn, Pb and Cd from contaminated waters (Lao et al. 2005, Chen et al. 2011, Zeledόn- Torunõ et al. 2005, Kolodynska et al. 2012, Regmi et al. 2012).  The removal of metals from both water and soils is highly pH dependent (Zeledόn- Torunõ et al. 2005).  While metal precipitation requires alkaline pH, metal adsorption is less pH dependent. Alteration of the pH by liming is a frequent remediation practice for trace element polluted systems (Adriano 2001, Madejόn et al. 2009) however the effects of liming gradually reduce over time due to the dissolution and leaching of the liming agent (Ruttens et al. 2010).  Biochars and leonardite are both highly recalcitrant and their effects may persist over long time periods (Steiner et al. 2007).   Phytostabilization of mine tailings is highly difficult, not only due to phytotoxic effects of elevated heavy metal concentrations, but also due to extreme pH values, low fertility, low water-holding capacity and unfavorable substrate structure (Fellet et al. 2011).  In highly degraded soils amendments can not only assist in trace element stabilization, but may also directly impact plant cover establishment and the long-term improvement of soil quality (Pérez-de-Mora et al. 2005, Pérez-de-Mora et al. 2006). Both leonardite and biochar are known to promote seed germination and viability, increase plant biomass and the rate of root development (Chen and Aviad 1990, Nardia et al. 2002, Jones et al. 2012). In addition, these materials can help reduce metals toxicity and revegetation contaminated areas (Pérez-de-Mora et al. 2007).   We examined the potential to use leonardite and biochar for metal sequestration in mining impacted water and soils, by determining the capacity of leonardite and biochar to adsorb metals in water, sequester metals in tailings and promote plant growth.    METHODS Metal adsorption capacity Leonardite and biochar ranging from 10g/L to 50g/L were exposed for 24 hours to synthetic contaminated water containing either Cd or Zn and pH was adjusted at 7. Water metal concentrations before and after exposure to the treatments were measured and compared. The data was used to determine the adsorption capacity of each treatment.   Metal sequestration property Tailings contaminated with Cd and Zn from Keno Hill Mining District (KHMD) were mixed with 4 treatments: 1) Lenoardite (6%v/v), 2) Biochar (6% v/v), 3) Leonardite and Biochar (each at 6% v/v) and 4)  Leonardite (6%v/v ) and dolomite lime (54.6% CaCO₃, 41.5% MgCO₃ at 484 g/m2).  The mixture was contained in PVC pipes (4 columns + 1 unamended control). Water was passed through the mixture continuously at a low flow rate for two months to simulate water infiltration through mine tailings. Data was compared to assess the capacity of the biochar and leonardite to sequester metals leached from tailings.  Plant growth Tailings contaminated with Cd and Zn from KHMD were mixed with the same 4 treatments as above. All treatments were also fertilized with a 19:19:19 fertilizer at a rate of 110 kg/ha.  There were 10 replicates (i.e. individual containers with two seedlings) for each treatment.  The and above and belowground growth of two different northern native herb species (i.e. Arctic Lupin (Lupinus arcticus) and Alpine Sweetvetch (Hedysarum alpinum)) were examined across each of the 4 treatments over a two month greenhouse experiment.  The greenhouse conditions and watering (6 ml DI per replicate every second day) were controlled to reflect typical summer growing conditions in the Keno area.  RESULTS Both biochar and leonardite showed metal adsorption capacity.  Up to 95% of Cd initially present in solution is removed by biochar after 24 hours of contact versus 38% by leonardite.  Adsorption of Zn showed a similar trend with up to 90% of Zn removed by biochar after 24 hours of contact versus 29% by leonardite.  With 10 g/L of leonardite or biochar, Cd loading was 49.2 mg Cd/g leonardite and 110.6 mg Cd/g biochar (Figure 1). Therefore, biochar was able to adsorb more than 10% of its mass in Cd.  Figure 1. Cadmium loading on leonardite and biochar after 24 hrs with leonardite and biochar concentrations ranging from 10-50 g/L. With 10 g/L of leonardite or biochar, Zn loading was up to 19.6 g Zn/g leonardite and 51.9 g Zn/g biochar (Figure 2).  We did not observe any increase in metal removal by increasing the amount of leonardite or biochar above 10g/L (Figures 1 and 2).   Figure 2.  Zinc loading on leonardite and biochar after 24 hrs with leonardite and biochar concentrations ranging from 10-50 g/L. Solutions with an initial pH of 7 containing leonardite and biochar ranging in a solid to liquid ratio of 10-50% were held with 24 hours of contact between the water and each adsorbent.  Both leonardite and biochar lowered the pH, however, leonardite lowered the pH to 2.9-3.4, while biochar lowered the pH less to 5.6-6.4 (Figure 3).  Figure 3.  Change in pH after 24 hours of contact with leonardite or biochar with solid to liquid ratios ranging from 10 to 50%.  Initial pH of 7 is shown as a reference line. We observed a similar trend in pH change with leachate from the metal sequestration column experiment.  Columns amended with leonardite resulted in a leachate with a much lower pH compared to the unamended control in the first month of column operation (Figure 4). Biochar had very little effect on the pH with leachate having a similar pH to the unamended control. Addition of lime did not raised the pH in the first month of operation. However, after 45 days, leachate from the column amended with lime showed an increase in pH towards that of the control.   Figure 4.  pH of leachate from tailings columns amended with biochar, lenoardite, biochar and leonardite and leonardite and lime over 60 days.   The column amended with leonardite had leachate with higher Cd and Zn compared with both the column amended with biochar and the unamended column (Figure 5).  This trend may suggest that Cd and Zn are mobilized from the tailings by the leonardite.  Biochar appears to reduce the amount of Cd and Zn leached from the tailings.  Compared to the unamended control, leachate from the column with biochar had a 73.8% reduction in Cd and a 18.27% reduction in Zn.       Figure 5.  Cadmium and Zinc concentrations of leachate from tailings columns amended with biochar, lenoardite, biochar and leonardite and leonardite and lime over 70 days.   Both northern native herb species showed poor growth after two months in the greenhouse with higher belowground biomass than aboveground biomass.  We did not detect any influence of the treatments on the below and aboveground biomass accumulation for L. arcticus, but found significantly higher belowgound biomass for H. aplinum grown in tailings amended with leonardite and lime (Figure 6).   Figure 6.  Belowground and aboveground biomass of Lupinus Arcticus (Arctic Lupin) and Hedysarum alpinum (Alpine Sweetvetch) following two months of growth in a greenhouse in tailings treated with leonardite and lime, leonardite, leonardite and biochar, biochar and fertilizer only.  Bars are means with standard error.  There were no significant differences in above or belowground biomass with treatment for L. arcticus or aboveground biomass for H. aplinum (ANOVA, p>0.05 for all comparisons).  Belowground biomass of H. aplinum was significantly higher in tailings treated with leonardite and lime compared with all other treatments (ANOVA, TukeyHSD, p<0.01 for all comparisons). Both species showed signs of potential heavy metal toxicity with chlorosis and a purpling and reddening of leaves after approximately 30 days of growth in the tailings (Figure 7).  After 2 months of growth stem death was evident for some replicates of both herbs.   Figure 7.  Hedysarum alpinum (a) and Lupinus arcticus (b) showing signs of chlorosis (i.e. yellowing and purpling of leaves) after approximately 30 days of growth in tailings likely due to heavy metal toxicity.  DISCUSSION Both biochar and leonardite demonstrated metal adsorption of Cd and Zn.  However, adsorption rates for biochar were considerably higher in synthetic water and Cd and Zn concentrations in leachate from tailings with biochar were reduced compared to the unamended control.  Leonardite amended tailings resulted in the mobilization of Cd and Zn from the tailings, which was likely due to acidification of the water flowing within the tailings.   We also observed that both biochar and leonardite lowered pH of mining contaminated water and tailings.  However, leonardite lowered the pH to a much greater extent and likely accounts for the potential acidification and subsequent mobilization of Cd and Zn from leonardite amended tailings columns. The poor growth of both northern native herb species on the tailings is most likely due to heavy metal toxicity.  Other studies that have examined vegetation establishment of the UKHM tailings have also found chlorosis in species such as, Arctostaphylos uva-ursi, Carex aquatilis, and Equisetum arvense (Clark & Hutchinson, 2005).  Zinc toxicity symptoms are known to include chlorosis and a reddening of younger leaves (Reichmann, 2002).  However, the increased belowground biomass for H. aplinum in the leonardite and lime treatment suggests that leonardite may improve root development.  Leonardite is known to stimulate root growth with both increased root length and development of secondary roots (Chen and Aviad 1990) and can reduce soil pH around roots helping to convert unavailable nutrients to plant accessible forms (Vaughan and Donald, 1976).  However, in the context of remediation and phytostabilization of mine tailings decreases in pH associated with leonardite are undesirable and a liming agent may need to be considered.  In the tailings amended with both leonardite and lime, pH appeared to equilibrate after 45 days and further studies are needed to examine the longer-term influence of a combined leonardite and lime treatment.  Biochar shows strong potential for metal removal in both mining impacted water and tailings. This initial trial demonstrates that both leonardite and biochar have potential as amendment technologies for remediation and restoration in northern Canada.     a) b)ACKNOWLEGEMENTS We would like to thank Raymond Potié of Wapaw Bay Resources for his collaboration on this project.  We would also like to thank Hiromi, Isobel Ness, Annie-Claude Letendre, Justine Viros and Carrie Boles for laboratory and greenhouse assistance.  The project was funded by an NSERC Applied Research and Development Grant. 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