AMD
When solid-phase pyrite in rock is exposed to oxygenated water acid drainage is typically formed by the oxidation of pyrite (FeS2) containing reduced iron and sulfur species. As a result of this oxidation, the concentration of both reduced iron and acid in the drainage water increases significantly. Additionally, the simultaneous presence of reduced iron and oxygen promotes the growth of acidophilic, autotrophic iron- and sulfur-oxidizing bacteria such as Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, and Sulfolobus acidocaldarius.
The oxidation of Fe2+ serves as the rate-limiting step in biologically-mediated acid production. In most waste rock piles with acid drainage, T. ferrooxidans and other iron oxidizers often out-compete other indigenous heterotrophic bacteria species present due to low organic levels. However, under the right environmental conditions, indigenous and inoculated heterotrophic microbes can increase in concentration at faster rates than autotrophic bacteria. Additionally, in the absence of oxygen, certain heterotrophic microbes are capable of coupling organic carbon oxidation to ferric iron reduction.
Significant reductions in acidophilic, iron- and sulfur-oxidizing bacteria were obtained in column and pilot tests by the addition of organic materials. Iron oxidation was inhibited as the treatment reduced dissolved oxygen levels with concomitant increase in indigenous and inoculated non-iron-oxidizing heterotrophic bacteria. Furthermore, as the system became anoxic, the much of the oxidized iron species was consumed resulting in a pH increase. As the applied organics and microbial materials decrease the populations of acidophilic bacteria it leads to the dominance of heterotrophic bacteria in biofilms that coat rock surfaces. This has secondary beneficial effects of metal sulfide precipitation and lowering the redox environment, further inhibiting growth of iron- and sulfur-oxidizing species. Results suggest that biologically and chemically mediated acid drainage can be reversed by dispersion of organic treatment materials and microbes into the subsurface.
The oxidation of Fe2+ serves as the rate-limiting step in biologically-mediated acid production. In most waste rock piles with acid drainage, T. ferrooxidans and other iron oxidizers often out-compete other indigenous heterotrophic bacteria species present due to low organic levels. However, under the right environmental conditions, indigenous and inoculated heterotrophic microbes can increase in concentration at faster rates than autotrophic bacteria. Additionally, in the absence of oxygen, certain heterotrophic microbes are capable of coupling organic carbon oxidation to ferric iron reduction.
Significant reductions in acidophilic, iron- and sulfur-oxidizing bacteria were obtained in column and pilot tests by the addition of organic materials. Iron oxidation was inhibited as the treatment reduced dissolved oxygen levels with concomitant increase in indigenous and inoculated non-iron-oxidizing heterotrophic bacteria. Furthermore, as the system became anoxic, the much of the oxidized iron species was consumed resulting in a pH increase. As the applied organics and microbial materials decrease the populations of acidophilic bacteria it leads to the dominance of heterotrophic bacteria in biofilms that coat rock surfaces. This has secondary beneficial effects of metal sulfide precipitation and lowering the redox environment, further inhibiting growth of iron- and sulfur-oxidizing species. Results suggest that biologically and chemically mediated acid drainage can be reversed by dispersion of organic treatment materials and microbes into the subsurface.
