TREATMENT OF PORE WATER IN MINE TAILINGS USING ORGANIC CARBON AMENDMENTS

Field and laboratory experiments were conducted to evaluate pore-water treatment in pyrite- and dolomite-rich mine tailings using organic carbon amendments to support sulfate reduction. A field experiment was conducted over four years to evaluate the effectiveness of three organic carbon sources for promoting sulfate reduction and metal(loid) attenuation. Six field-scale experimental cells were constructed by amending fresh tailings with varied mixtures of peat, spent-brewing grain (SBG) and municipal biosolids (MB) at rates of 5 and 10 vol. %. Three laboratory columns were packed with tailings amended with a 1:1 (vol.) mixture of peat and SBG at proportions of 0, 2 and 5 vol. %. A simulated tailings pore-water solution was passed through the columns for 540 days. The geochemistry, microbiology, and mineralogy of the field cells and laboratory columns were monitored with time. Organic carbon amendments containing peat + SBG and peat + SBG + MB supported DOC concentrations > 200 mg L^­-1 and pore-water SO₄ concentrations decreased from > 3000 to < 500 mg L^­-1 in the field experiments. Removal of sulfate in these cells was accompanied by H₂S production, increases in δ³⁴S-SO₄ values by > +30 ‰, undersaturation of pore-water with respect to gypsum [CaSO₄∙2H₂O], and most-probable number (MPN) populations of sulfate reducing bacteria (SRB) > 10⁶ cells g^­-1. Decreases in the mass transport of Zn, Ni, Sb and Tl were attributed to the precipitation secondary Fe-S and Zn-S phases in field cells that supported sulfate reduction. Organic carbon amendments evaluated during the laboratory column experiments generally supported S₂O₃ reduction and/or disproportionation and decreased mass transport of S₂O₃, Cu, Mn, Ni, Pb, Tl and Zn. Reformation of S₂O₃ was observed within columns under low Fe and Zn input concentrations. Organic carbon amendments resulted in slight increases in Mo mobility, whereas the discharge of Sb exhibited limited variation among columns. Increased organic carbon availability supported initial increases in Fe and As mobility in the field and laboratory experiments. However, subsequent attenuation of Fe and As was observed for some organic carbon amendments. These experiments demonstrate the potential for pore-water treatment by amending mine tailings with organic carbon.