Tailings Pore Water Research Articles

The potential for metal release by reductive dissolution of weathered mine tailings

Remediation programs proposed for decommissioned sulphide tailings may include the addition of a cover layer rich in organic-carbon material such as sewage sludge or composted municipal waste. These covers are designed to consume oxygen and prevent the oxidation of underlying sulphide minerals. The aerobic and anaerobic degradation of such organic-carbon-rich waste can release soluble organic compounds to infiltrating precipitation water. In laboratory experiments, and in natural settings, biotic and abiotic interactions between similar dissolved organic compounds and ferric-bearing secondary minerals have been observed to result in the reductive dissolution of ferric (oxy)hydroxides and the release of ferrous iron to pore waters. In weathered tailings, oxidation of sulphide minerals typically results in the formation of abundant ferric-bearing secondary precipitates near the tailings surface. These secondary precipitates may contain high concentrations of potentially toxic metals, either coprecipitated with or adsorbed onto ferric (oxy)hydroxides. Reductive dissolution reactions, resulting from the addition of the organic-carbon covers, may remobilize metals previously attenuated near the tailings surface. To assess the potential for metal release to tailings pore water by reductive dissolution reactions, a laboratory study was conducted on weathered tailings collected from the Nickel Rim mine tailings impoundment near Sudbury, Ontario, Canada. This site was selected for study because it is representative of many tailings sites. Mineralogical study indicates that sulphide minerals originally present in the vadose zone at the time of tailings deposition have been replaced by a series of secondary precipitates. The most abundant secondary minerals are goethite, gypsum and jarosite. Scanning electron microscopy, coupled with elemental analyses by X-ray energy dispersion analysis, and electron microprobe analysis indicate that trace metals including Ni, Cr and Cu are associated with these secondary minerals. To assess the masses of trace metals associated with each of the dominant secondary mineral phases, a series of extraction procedures was used. The masses of metals determined in three fractions (water soluble, reducible and residual) suggest that the greatest accumulation of metals is in the reducible fraction. These measurements indicate that high concentrations of metals are potentially available for release by reductive dissolution of the ferric-bearing secondary minerals. The actual mass of metals that can be released by this mechanism will depend on a number of site-specific characteristics, particularly the intensity of the reducing conditions established near the tailings surface.

The pore-water geochemistry and the mineralogy of the vadose zone of sulfide tailings, Waite Amulet, Quebec, Canada

Low-pH waters, requiring treatment because of high concentrations of dissolved metals, are being discharged from a decommissioned tailings impoundment at the former Waite Amulet Zn Cu mine in northeastern Quebec, Canada. A detailed study of the vadose zone of the tailings comparing mineralogical and geochemical analyses of the tailings solids with geochemical analysis of the tailings pore water and pore gas indicates the presence of three zones within the tailings. These zones include an upper sulfide-depleted zone in which the sulfides have been extensively removed and oxidation is largely complete, an intermediate zone, where sulfide oxidation and acid neutralization reactions are occurring, and an unoxidized zone below the depth of oxygen penetration, where sulfides are unoxidized. High aqueous concentrations of dissolved solids are being displaced downward from the sulfide-depleted zone, through the unoxidized zone toward the water table. Sulfide-oxidation and acid-neutralization reactions have generated concentrations of dissolved solids high enough to lead to the precipitation of a series of secondary solid phases. Precipitation-dissolution reactions involving these solid phases control the concentrations of the major ions in the tailings pore water. Concentrations of dissolved metals are controlled by precipitation-dissolution, crystal-structural replacement, and adsorption/coprecipitation reactions. Predictions based on the observed data and numerical simulations suggest that, in the absence of an effective remedial program, sulfide oxidation and H + production will continue for several centuries.

Application of Sphagnum Peat, Calcium Carbonate, and Hydrated Lime for Immobilizing Uranium Tailings Leachate

ABSTRACTBatch experiments, mineralogical studies, and geochemical modeling were conducted to evaluate the effectiveness of sphagnum peat, calcium carbonate, and hydrated lime in removing dissolved concentrations of As, Mo, NO3, and U present in uranium-tailings pore water at Gunnison, Colorado. Amounts of As, Mo, and U removal by sphagnum peat, calcium carbonate, and hydrated lime at 5.0, 2.5, and 2.5 wt.%, respectively, were typically above 97 %. Nitrate removal ranged between 55 and 80 %. Significant contaminant removal was achieved by sphagnum peat alone at pH 3.18. Addition of 2.5 wt.% calcium carbonate to the sphagnum peat-tailings system resulted in formation of soluble uranyl carbonato complexes, which decreased the amount of U(VI) fixed onto sphagnum peat by a factor of 2. Addition of 2.5 wt.% hydrated lime to the acidic tailings increased Mo concentrations by a factor of 2 under moderately alkaline conditions (pH 12). During neutralization of tailings-pore water, precipitation of ferric oxyhydroxides may provide additional removal of As, Mo, and U(VI) from solution through adsorption and coprecipitation processes.

Early diagenesis of copper and molybdenum in mine tailings and natural sediments in Rupert and Holberg inlets, British Columbia

The distribution and behaviour of Cu and Mo in interstitial waters collected from Cu- and Mo-rich mine tailings and natural sediments in Rupert and Holberg inlets (Vancouver Island, British Columbia) are discussed. Dissolved Fe, Mn, [Formula: see text], and Σ H2S distributions and solid-phase Cu, Mo, Mn, and organic carbon data are used to constrain the interpretation of Cu and Mo diagenesis in both tailings and natural sediment facies. Cu is released to solution at or near the sediment–water interface in both facies either by decomposing labile organic material or as a probable result of oxidation of sulphide minerals within the tailings. Mo is significantly enriched (up to 0.6 μmol L−1) in tailings pore water, probably as a result of dissolution of soluble Mo oxides produced initially by oxidation of MoS2 during milling of the ore.In natural sediments, both Cu and Mo are removed from solution at depth, probably by precipitation as their sulphides or in solid solution with an iron sulphide phase. Similar reactions appear to be occurring at depth in the tailings. The benthic flux of copper from the tailings into overlying seawater is similar in magnitude to that observed in natural sediments; there is no indication from the present data that remobilization of copper from the tailings is perturbing the pre-mine Cu distribution in Rupert Inlet. Unlike that of Cu, the benthic flux of Mo is unique to the tailings but is insufficient to produce a measureable anomaly in inlet waters because of the short water residence time and the relatively high Mo concentration characteristic of seawater.