Acid mine drainage remediation options: a review

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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.

Section snippets

Acid mine drainage: nature of the problem

Acidic sulfur-rich wastewaters are the by-products of a variety of industrial operations such as galvanic processing and the scrubbing of flue gases at power plants (Johnson, 2000). The major producer of such effluents is, however, the mining industry. Waters draining active and, in particular, abandoned mines and mine wastes are often net acidic (sometimes extremely so). Such waters typically pose an additional risk to the environment by the fact that they often contain elevated concentrations

“Source control” vs. “migration control” options

Given the axiom that “prevention is better than cure”, it is generally preferable, although not always pragmatic, to preclude the formation of AMD in the first instance. Such techniques are known collectively as “source control” measures (Fig. 1) and will be described only briefly.

In as much as both oxygen and water are required to perpetuate the formation of AMD, it follows that by excluding either (or both) of these, it should be possible to prevent or minimise AMD production. One way in

Active technologies

The most widespread method used to mitigate acidic effluents is an active treatment process involving addition of a chemical-neutralising agent (Coulton et al., 2003b). Addition of an alkaline material to AMD will raise its pH, accelerate the rate of chemical oxidation of ferrous iron (for which active aeration, or addition of a chemical oxidising agent such as hydrogen peroxide, is also necessary), and cause many of the metals present in solution to precipitate as hydroxides and carbonates.

Significant biological processes

The basis of bioremediation of AMD derives from the abilities of some microorganisms to generate alkalinity and immobilise metals, thereby essentially reversing the reactions responsible for the genesis of AMD. While in aerobic wetlands constructed to treat AMD, macrophytes such as Typha and Phragmites spp. are the most obvious forms of life present, their direct roles in improving water quality have been questioned (Johnson and Hallberg, 2002).

Microbiological processes that generate net

Active biological systems: sulfidogenic bioreactors

Off-line sulfidogenic bioreactors represent a radically different approach for remediating AMD (Johnson, 2000, Boonstra et al., 1999). These engineered systems have three potential advantages over passive biological remediation: (i) their performance is more predictable and readily controlled; (ii) they allow heavy metals, such as copper and zinc, present in AMD to be selectively recovered and reused; and (iii) concentrations of sulfate in processed waters may be significantly lowered. On the

Remediation options: factors in decision making

The choice of which option to use to remediate AMD is dictated by a number of economical and environmental factors. Sometimes the true environmental cost of a remediation system is not immediately apparent. One such cost is the amount of fossil fuel energy needed to transport liming materials, often long distances from source to mine sites (such as at the Wheal Jane mine in Cornwall, where the lime is transported several hundred kilometres from a site in the midlands of England). Traditionally,

Acknowledgements

The authors would like to acknowledge the financial support received from the LINK directorate (Grants # BTL/70/21 and 5/BRM18412) in supporting their research programmes into bioremediation of mine waters.

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