title: Current approaches for mitigating acid mine drainage.
abstract: AMD is one of the critical environmental problems that causes acidification and metal contamination of surface and ground water bodies when mine materials and/or over burden-containing metal sulfides are exposed to oxidizing conditions. The best option to limit AMD is early avoidance of sulfide oxidation. Several techniques are available to achieve this. In this paper, we review all of the major methods now used to limit sulfide oxidation. These fall into five categories: (1) physical barriers,(2) bacterial inhibition, (3) chemical passivation, ( 4) electrochemical, and (5) desulfurization.We describe the processes underlying each method by category and then address aspects relating to effectiveness, cost, and environmental impact. This paper may help researchers and environmental engineers to select suitable methods for addressing site-specific AMD problems.Irrespective of the mechanism by which each method works, all share one common feature, i.e., they delay or prevent oxidation. In addition, all have limitations.Physical barriers such as wet or dry cover have retarded sulfide oxidation in several studies; however, both wet and dry barriers exhibit only short-term effectiveness.Wet cover is suitable at specific sites where complete inundation is established, but this approach requires high maintenance costs. When employing dry cover, plastic liners are expensive and rarely used for large volumes of waste. Bactericides can suppress oxidation, but are only effective on fresh tailings and short-lived, and do not serve as a permanent solution to AMD. In addition, application of bactericides may be toxic to aquatic organisms.Encapsulation or passivation of sulfide surfaces (applying organic and/or inorganic coatings) is simple and effective in preventing AMD. Among inorganic coatings,silica is the most promising, stable, acid-resistant and long lasting, as compared to phosphate and other inorganic coatings. Permanganate passivation is also promising because it creates an inert coating on the sulfide surface, but the mechanism by which this method works is still unclear, especially the role of pH. Coatings of Fe-oxyhydroxide, which can be obtained from locally available fly ash are receiving attention because of its low cost, self-healing character, and high cementation capacity. Among organic coatings, lipids and natural compounds such as humic acid appear to be encouraging because they are effective, and have a low environmental impact and cost. Common advantages of organic vs. inorganic coatings are that they work best at low pH and can prevent both chemical and biological oxidation.However, organic coatings are more expensive than inorganic coatings. Furthermore,while organic coatings are effective under laboratory conditions, they often fail under field conditions or require large amounts of reagents to insure effectiveness.Electrochemical cover technology may become a suitable technique to prevent AMD, but the mechanism by which this technique operates is still under investigation.Limitations of this method include the initial capital cost and ongoing costs of anodes and cathodes.Desulfurization is an alternative process for managing large-scale sulfide wastes/tailings. This process can separate sulfide minerals into a low-volume stream, leaving mainly waste with low sulfur content that will be non-acid-generating. The attractiveness of desulfurization is that it is simple and economic.Our review has clearly disclosed that more information is needed for most of the AMD-mitigation techniques available. Silica passivation has shown promise, butmore extensive field-testing is needed to reduce it to commercial viability. Silica is the dominant element in fly ash, and therefore, its use as a low-cost, easily accessible coating should be evaluated. Permanganate passivation also requires further study to understand the role of pH. The secondary formation of Fe-oxyhydroxide minerals from Fe-oxyhydroxides, from the standpoint of their phase transformation,stability and effectiveness, should be assessed over longer experimental periods. All inorganic coatings are designed to inhibit abiotic oxidation of pyrite; however, their effect on biotic pyrite oxidation is not well known and should be further studied.Currently, there is no information available on longer-term field application of organic reagents. Such information is needed to evaluate their lifetime environmental and performance effects. Future studies require spectroscopic analyses of all coating types to achieve a better understanding of their surface chemistry. In addition,a thorough mineralogical and geochemical characterization of waste materialsis essential to understand the acid generating potential, which can indeed help to select better prevention measures.From having performed this review, we have concluded that no single method is technologically mature, although the majority of methods employed are promising for some applications, or at specific sites. Combining techniques can help ac~Ie:eAMD containment in some cases. For example, applying dry cover (e.g., sml) mcombination with liming material or a bactericide, or applying inorganic coatings(e.g., silica) along with organic reagents (e.g., lipids or humic acid) may be moreeffective than utilizing any single technique alone.
authors.affiliation: Department of Environmental Engineering, Kunsan National University, Jeonbuk, Republic of Korea. prafulla.iitkgp@gmail.com
authors.forename: Prafulla Kumar
authors.initials: PK
authors.lastname: Sahoo
authors.forename: Kangjoo
authors.initials: K
authors.lastname: Kim
authors.forename: Sk Md
authors.initials: SM
authors.lastname: Equeenuddin
authors.forename: Michael A
authors.initials: MA
authors.lastname: Powell
mesh_terms: D004322:Drainage; D017895:Manganese Compounds / Q000737:chemistry; D008903:Minerals / Q000737:chemistry*; D008906:Mining; D010087:Oxides / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry; D013440:Sulfides / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000633:toxicity*; D014877:Water Pollution, Chemical / Q000517:prevention & control*
chemical_list: D017895:Manganese Compounds; D008903:Minerals; D010087:Oxides; D013440:Sulfides; D014874:Water Pollutants, Chemical; C048856:permanganic acid; D012822:Silicon Dioxide