Environ. Sci. Technol. 2003, 37, 5027-5033
Microbial and Nutrient Investigations into the Use of in Situ Layers for Treatment of Tailings Effluent †
ANDREA H. M. HULSHOF, D A V I D W . B L O W E S , †,* CAROL J. PTACEK,† AND W. DOUGLAS GOULD‡ Department of Earth Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada, Biotechnology Laboratory, CANMET, 555 Booth Street, Ottawa K1A 0G1, Canada
The release of acidic drainage, containing high concentrations of dissolved metals, is associated with mining districts throughout the world. Remediation of acidic drainage at active and abandoned mines remains a significant challenge. A potential alternative technique to prevent the release of acidic drainage is the addition of labile organic carbon to mine wastes during deposition, creating large in situ treatment systems. Organic carbon can enhance bacterially mediated sulfate reduction and subsequent metal sulfide precipitation, treating metal-contaminated water prior to discharge from the impoundment. Two laboratory column experiments were conducted using simulated mine drainage water. The columns contained tailings derived from the Kidd Creek Metallurgical site in Timmins, Ontario, and reactive materials mixed to a 4:1 volumetric ratio. The average sulfate reduction rate observed in the woodchip column was 0.009 mmol L-1 day-1 g-1 organic matter and in the pulp waste column 0.018 mmol L-1 day-1 g-1 organic matter. Residence times were 14 days in the woodchip column, resulting in the average removal of 500 mg L-1 (5.2 mmol L-1) SO4 and 60 mg L-1 (1.1 mmol L-1) Fe, and 13 days in the pulp waste column, resulting in the average removal of 600 mg L-1 (6.2 mmol L-1) SO4 and the complete removal of 100 mg L-1 (1.8 mmol L-1) Fe. In both columns, sulfate reduction was coupled with an increase in alkalinity and pH and the complete removal of 80 mg L-1 (1.2 mmol L-1) Zn and other metals. Populations of sulfate-reducing bacteria within both columns increased by 3-4 orders of magnitude, and bacterial activity was up to 5 times greater than in the unamended tailings. The woodchip material contained lower concentrations of labile C, N, and P than the pulp waste, possibly accounting for the lower sulfate reduction rates and metal removal capacity observed.
Introduction Acid mine drainage (AMD) from mine wastes is an insidious problem in mining districts throughout the world. Acidic * Corresponding author phone: (519) 888-4567 ext 4878; fax: (519) 746-3882; e-mail:
[email protected]. † University of Waterloo. ‡ CANMET. 10.1021/es020822r CCC: $25.00 Published on Web 09/30/2003
2003 American Chemical Society
drainage results from the oxidation of pyrite (FeS2) or other sulfide minerals by oxygen or ferric iron through reactions of the forms
FeS2(s) + 7/2O2(aq) + H2O(l) f Fe2+(aq) + 2SO42-(aq) + 2H+(aq) (1) FeS2(s) + 14Fe3+(aq) + 8H2O(l) f 15Fe2+(aq) + 2SO42-(aq) + 16H+(aq) (2) followed by the oxidation of Fe(II) to Fe(III) and the precipitation of a hydrous ferric oxide, Fe(OH)3:
Fe2+ + 1/4O2 + 5/2H2O f Fe(OH)3(s) + 2H+
(3)
Rapid sulfide oxidation occurs above the water table in mine tailings impoundments and waste rock piles, where gas-phase transport of O2 is rapid. Below the water table sulfide oxidation is limited by the low diffusion coefficient of oxygen in water. An alternative approach to the prevention of mine drainage is the direct addition of organic carbon to the saturated tailings as layers or coblended zones below the water table (Figure 1). Organic carbon, when blended directly with the tailings within the impoundment, has the potential to promote bacterially mediated sulfate reduction and subsequent metal sulfide precipitation, remediating acid mine drainage at the source. Sulfate-reducing bacteria catalyze the oxidation of organic carbon coupled with the reduction of SO4 to H2S through the reaction
SO42- + 2CH2O f H2S + 2HCO3-
(4)
where CH2O represents a generic organic carbon compound (1). The release of H2S into pore waters containing high concentrations of metals can result in the precipitation of metals through the reaction
Me2+ + HS- f MeS + H+
(5)
Results expected in sulfate-reducing environments include decreased concentrations of iron, sulfate, and heavy metals coupled with an increase in pH and alkalinity (2-4). In this paper we focus on column experiments designed to evaluate the effectiveness of the direct addition of reactive materials to mine tailings, to evaluate potential inhibition on bacterial growth by tailings constituents, and to evaluate whether sufficient levels of treatment could be obtained within residence times typical of mill tailings impoundments. A simulated mine drainage solution containing elevated concentrations of dissolved SO4, Fe(II), and other metals was pumped through columns containing a mixture of mill tailings and organic matter. Organic matter was added to the tailings to provide an organic carbon source and to promote the reproduction and growth of sulfate-reducing bacteria (SRB). This approach has the potential to create a large treatment system within waste disposal facilities and to decrease the exposure of aquatic ecosystems to metals and acidity, ultimately improving aquatic habitats in streams receiving mine drainage (Figure 1). These treatment zones could be used in conjunction with existing mine waste remediation programs including the use of fine-grained covers, constructed wetlands, reactive barriers, and/or downstream water treatment facilities.
Materials Potential subsurface treatment layers were evaluated using mine tailings derived from the concentrator at the Kidd Creek VOL. 37, NO. 21, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Addition of organic carbon to the tailings within the impoundment to enhance sulfate reduction and the precipitation of sulfide minerals. Metallurgical Site located in Timmins, Ontario, and two different reactive materials, woodchips from a nearby forest product mill and waste solids derived from a nearby pulp and paper plant (pulp waste). The concentrations of phosphorus and nitrogen of the organic amendments were measured as percent dry weight. The pulp waste contained 0.68 wt % P and 2.96 wt % N, and the woodchip material contained 0.016 wt % P and 0.10 wt % N. The respiration rate of the organic carbon materials was determined using US Compost Council Procedure ASA 41-2.2 (5). This procedure assesses the lability of the organic carbon on the basis of capturing carbon dioxide in a laboratory incubation (a 12 h equilibration period at 34 °C). The concentrator tailings were composed of 10-20 wt % pyrite, 1-2 wt % pyrrhotite, and 1-2 wt % combined sphalerite and chalcopyrite. The remaining 75-85 wt % of the tailings was nonsulfide minerals, including 8 wt % carbonates in which ankerite-dolomite and siderite predominated with lesser amounts of calcite (
