THE POTENTIAL FOR METAL RELEASE BY REDUCTIVE DISSOLUTION OF WEATHEREDMINE 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 sul phide minerals. The aerobic and anaerobic degradation of such organic- carbon-rich waste can release soluble organic compounds to infiltratin g precipitation water. In laboratory experiments, and in natural setti ngs, 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, oxidat ion of sulphide minerals typically results in the formation of abundan t ferric-bearing secondary precipitates near the tailings surface. The se secondary precipitates may contain high concentrations of potential ly toxic metals, either coprecipitated with or adsorbed onto ferric (o xy)hydroxides. Reductive dissolution reactions, resulting from the add ition of the organic-carbon covers, may remobilize metals previously a ttenuated 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 th e 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study indicates that sulphide minerals o riginally present in the vadose zone at the time of tailings depositio n have been replaced by a series of secondary precipitates. The most a bundant 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 tr ace metals including Ni, Cr and Cu are associated with these secondary minerals. To assess the masses of trace metals associated with each o f the dominant secondary mineral phases, a series of extraction proced ures was used. The masses of metals determined in three fractions (wat er soluble, reducible and residual) suggest that the greatest accumula tion of metals is in the reducible fraction. These measurements indica te that high concentrations of metals are potentially available for re lease by reductive dissolution of the ferric-bearing secondary mineral s. The actual mass of metals that can be released by. this mechanism w ill depend on a number of site-specific characteristics, particularly the intensity of the reducing conditions established near the tailings surface.