Abstract: Acid mine drainage (AMD) causes environmental pollution that affects many countries having historic or current mining industries. Preventing the formation or the migration of AMD from its source is generally considered to be the preferable option, although this is not feasible in many locations, and in such cases, it is necessary to collect, treat, and discharge mine water. There are various options available for remediating AMD, which may be divided into those that use either chemical or biological mechanisms to neutralise AMD and remove metals from solution. Both abiotic and biological systems include those that are classed as “active” (i.e., require continuous inputs of resources to sustain the process) or “passive” (i.e., require relatively little resource input once in operation). This review describes the current abiotic and bioremediative strategies that are currently used to mitigate AMD and compares the strengths and weaknesses of each. New and emerging technologies are also described. In addition, the factors that currently influence the selection of a remediation system, and how these criteria may change in the future, are discussed.

Abstract: Constructed wetland treatment system effectiveness has been limited by the alkalinity-producing, or acidity-neutralizing, capabilities of systems. Anoxic limestone drains (ALD's) have allowed for the treatment of approximately 300 mg/L net acidic mine drainage, but current design guidance precludes using successive ALD's to generate alkalinity in excess of 300 mg/L because of concerns with dissolved oxygen. “Compost” wetlands designed to promote bacterially mediated sulfate reduction are suggested as a means of generating alkalinity required in excess of that produced by ALD's. Compost wetlands create two basic needs of sulfate reducing bacteria; anoxic conditions resulting from the inherent oxygen demand of the organic substrate, and quasi-circumneutral pH values resulting from the dissolution of the carbonate fraction of the compost. However, sulfate reduction treatment area needs are generally in excess of area availability and/or cost effectiveness. Second generation alkalinity-producing systems demonstrate that a combination of existing treatment mechanisms has the potential to overcome current design concerns and effectively treat acidic waters ad infinitum. Successive alkalinity-producing systems (SAPS) combine ALD technology with sulfate reduction mechanisms. SAPS promote vertical flow through rich organic wetland substrates into limestone beds beneath the organic compost, discharging the pore waters. SAPS allow for conservative wetland treatment sizing calculations to be made as a rate function based on pH and alkalinity values and associated contaminant loadings. SAPS potentially decrease treatment area requirements and have the further potential to generate alkalinity in excess of acidity regardless od acidity concentrations.

Abstract: A permeable reactive barrier was installed in August 1995 at the Nickel Rim Mine near Sudbury, Ontario, Canada, for the passive remediation of groundwater contaminated with acid mine drainage. The reactive component of the barrier consists of a mixture of municipal and leaf compost and wood chips: the organic material promotes bacterially-mediated sulphate reduction. Hydrogen sulphide, a product of sulphate reduction, may then complex with aqueous ferrous iron and precipitate as iron sulphide. This study presents the solid phase sulphur chemistry of the reactive wall after two years of operation, and discusses the formation and accumulation of iron sulphide minerals in the reactive material. The results from the solid-phase chemical analysis of core samples indicate that there is an accumulation of reduced inorganic sulphur in the reactive wall, with levels reaching 190 mu mol g (super -1) (dry weight) by July 1997.