Potential Impacts of Tailings and Tailings Cover. Fertilization on Arsenic Mobility in Surface and. Ground Waters

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1 Potential Impacts of Tailings and Tailings Cover Fertilization on Arsenic Mobility in Surface and Ground Waters Sierra Rayne * and Kaya Forest Water Treatment Technology Program, Thompson Rivers University, Box 3010, 900 McGill Road, Kamloops, British Columbia, Canada, V2C 5N3; Department of Chemistry, Okanagan College, Penticton Campus, 583 Duncan Avenue West, Penticton, British Columbia, Canada, V2A 8E1 Corresponding author: ( ) rayne.sierra@gmail.com or srayne@tru.ca; (phone) +1 (250) Water Treatment Technology Program, Thompson Rivers University Department of Chemistry, Okanagan College, Penticton Campus 1

2 A number of mining sites worldwide, particularly gold mines, have tailings management facilities (TMFs) that contain high levels of arsenic. Current closed mine site regulatory agencies tend to prefer revegetation of TMFs as part of the mandated reclamation activities. At many sites, often in polar regions, vegetation is difficult to establish either directly on the tailings or on the coarse-rock covers due to nutrient poor soils, phytotoxicity problems, and/or a less than optimum climate. Addition of phosphorus-based fertilizers to the tailings and/or cover material is commonly considered in order to promote the revegetation process and ideally allow the site owners to discharge their closure duties as rapidly as possible. However, due to the similar geochemistry of arsenic and phosphorus oxyanion species, this type of mine closure strategy may have unintended consequences regarding arsenic mobility on and off the site. This document reviews the current state-of-the-art regarding mobilization of arsenic by phosphate ions, and identifies relevant risks and opportunities of using this information to better manage closed mine sites. Several studies have shown that arsenate (AsO 3-4 ) and phosphate (PO 3-4 ) compete for sorption sites because of their similar chemical behavior. Thus, arsenate sorption may be significantly depressed where phosphate concentrations are elevated. 1 Arsenic may also coprecipitate with mineral phases that are forming, thereby providing an effective mechanism for removing arsenic from solution. For example, coprecipitation with ferrihydrite can significantly reduce arsenic concentrations. 2,3 The similarity of AsO 3-4 to other tetrahedral oxyanions, such as PO and SO 4, allows As(V) substitution in a number of minerals (e.g., Ca 3 (AsO 4 ) 2 :4H 2 O, scorodite (FeAsO 4 :2H 2 O), Mn 3 (AsO 4 ) 2 :8H 2 O, AlAsO 4 :2H 2 O, and Cu 3 (AsO 4 ) 2 :6H 2 O), which helps aid in the net removal of arsenic from solution in mine site drainage. 4 Where phosphate is present, it 2

3 may compete with arsenate for positions in these crystal lattices, thereby reducing the potential for arsenic immobilization. In early studies on stabilizing arsenical wastes, the use of phosphate in combination with lime was suggested, 5 which was proposed to take advantage of the stability of a solid solution of calcium phosphate and calcium arsenate. Although phosphate in a lime system would stabilize arsenic under thermodynamic controls, if redissolution occurred, the presence of phosphate would contribute to an increased arsenic mobility in soils and mine wastes via competition for inner-sphere complex adsorption sites with metal oxides. 6,7,8 More recent batch and column laboratory experiments investigating the adsorption and transport of As(V) in heterogeneous iron oxide containing soils have shown the effects on As(V) mobility and recovery increase in the order ph<pore water velocity<phosphate concentration. 9 Higher As(V) adsorption to heterogeneous iron oxide containing soils is generally observed at lower ph values, with distribution coefficients decreasing by almost an order of magnitude in moving from ph 7 to 9. Additions of up to 0.1 mm (ca. 3 mg/l) orthophosphate greatly reduce the As(V) adsorption capacity of the soil. As noted above, this is because phosphate is able to effectively compete with As(V) for adsorption sites, and thereby significantly reduce adsorption of As. In the absence of phosphate, As(V) would be considered a relatively immobile anion. In the presence of 0.25 mm phosphate (ca. 8 mg/l), it would be considered a relatively mobile anion. Specific studies on the revegetation of arsenic contaminated mine soils have also found that while phosphorus containing soil amendments were added to promote root and shoot growth, the 3

4 fertilizers also affected soluble arsenic concentrations in the pore waters and arsenic uptake into the cover vegetation. 10 Plant tissue arsenic concentrations and soluble arsenic follow similar trends, suggesting that water soluble arsenic is a good predictor for plant uptake of arsenic. 11 In addition, reduced arsenic species such as arsenite have lower retention capacities on clay minerals (e.g., montmorillonite and kaolinite) than arsenate. 12 Addition of nutrients to mine tailings pore water via surface fertilizer additions may induce sufficiently reducing conditions (depending on what the relative nutrient limitations are) to generate reduced As(III) species that display lower sorption behavior in the tailings mass. Furthermore, induced reducing conditions in tailings pore water from surface fertilizer additions could lead to the dissolution of Fe(III) oxyhydroxides into the more soluble Fe(II) speciations. The dissolution of Fe(III) oxyhydroxides would also provide a mechanism by which sorbed arsenic would be re-released in the tailings seepage and runoff. Arsenic sorbed on relatively insoluble Fe(III) would likely be mobilized if the iron substrate was dissolved. However, there may also be an opportunity to consider intentionally applying dilute phosphate solutions to tailings as wash waters to increase the rates of arsenic release. If arsenic leaching from tailings is identified as a potential long-term site water treatment issue, the intentional application of phosphate containing fertilizers (i.e., either via sprinklers, a.k.a. fertigation or fertilizer irrigation, or through dry application to the tailings surface) during the summer months may serve the dual purposes of promoting more rapid revegetation as well as flushing the tailings of arsenic more rapidly than would occur naturally. This flushing action may, in the 4

5 long term, generate a lower volume of water with higher arsenic concentrations requiring treatment. This may be a means of reducing the long-term site water treatment requirements (without needing to increase water quality capacity), and the more concentrated waste stream may result in more cost-effect treatment practices. 5

6 References 1 Jain, A., Loeppert, R.H. (2000). Effect of competing anions on the adsorption of arsenate and arsenite by ferrihydrite. Journal of Environmental Quality, 29, Fuller, C.C., Davis, J.A., Waychunas, G.A. (1993). Surface chemistry of ferrihydrite: Part 2. Kinetics of arsenate adsorption and coprecipitation. Geochimica et Cosmochimica Acta, 57, Richmond, W.R., Loan, M., Morton, J., Parkinson, G.M. (2004). Arsenic removal from aqueous solution via ferrihydrite crystallization control. Environmental Science and Technology, 38, Myneni, S.C.B., Traina, S.J., Logan, T.J., Waychunas, G.A. (1997). Oxyanion behavior in alkaline environments: Sorption and desorption of arsenate in ettringite. Environmental Science and Technology, 31, Laguitton, D. (1976). Arsenic removal from gold-mine waste waters: Basic chemistry of the lime addition method. CIM Bulletin, Roy, W.R., Hasset, J.J., Griffin, R.A. (1986). Competitive coefficients for the adsorption of arsenate, molybdate and phosphate mixtures by soils. Soil Science Society of America Journal, 50, Goldberg, S. (1986). Chemical modeling of arsenate adsorption on aluminum and iron oxide minerals. Soil Science Society of America Journal, 50, Barrow, N.J. (1974). On the displacement of adsorbed anions from soil: 2. Displacement of phosphate by arsenate. Soil Science, 117, Williams, L.E., Barnett, M.O., Kramer, T.A., Melville, J.G. (2003) "Adsorption and 6

7 transport of arsenic(v) in experimental subsurface systems." Journal of Environmental Quality, 32, Heeraman, D.A., Claasen, V.P., Zasoski, R.J. (2001) Interaction of lime, organic matter and fertilizer on growth and uptake of arsenic and mercury by Zorro fescue. Plant and Soil, 234, Sadiq, M. (1986) Solubility relationships of arsenic in calcareous soils and its uptake by 12 corn. Plant and Soil, 91, Frost, R.R., Griffin, R.A. (1977). Effect of ph on adsorption of arsenic and selenium from landfill leachate by clay minerals. Soil Science Society of America Journal, 41,

How Arsenic Chemistry Determines Remediation Efficacy as well as Fate and Transport Russ Gerads Business Development Director

Northwest Remediation Conference October 5, 2017 How Arsenic Chemistry Determines Remediation Efficacy as well as Fate and Transport Russ Gerads Business Development Director www.brooksapplied.com Arsenic