The environmental impact of mine wastes — Roles of microorganisms and their significance in treatment of mine wastes

Mine wastes have been generated for several centuries, and mining activity has accelerated significantly during the 20th century. The mine wastes constitute a potential source of contamination to the environment, as heavy metals and acid are released in large amounts. A great variety of microorganisms has been found in mine wastes and microbiological processes are usually responsible for the environmental hazard created by mine wastes. However, microorganisms can also be used to retard the adverse impact of mine wastes on the environment. tionally, the mine drainage as well as the waste itself can be treated with alkali to increase pH and precipitate metals. The main drawback of this method is that it has to be continuously repeated to be fully effective. There may also be negative effects on beneficial microorganisms. Several other treatment methods have been developed to stop weathering processes thereby reducing the environmental impact of mine wastes. One approach has been to influence the waste deposit itself by reducing the transfer of oxygen and water to the waste. This can be achieved by covering the waste or by placing it under water. Vegetating the cover will probably also decrease the transfer of oxygen and water, and will give the deposit area a more aesthetical appearance. her main approach to reduce the environmental impact of mine wastes is to treat the drainage water. Various methods aim at using microorganisms for this in natural or engineered systems. Sulfate-reducing bacteria, metal-transforming bacteria and metal accumulating microorganisms are some examples. Often, some kind of reactor design is needed to effectively control these processes. Recently, much interest has been focused on the use of natural or artificial wetlands for treatment since this generally is a low-cost and low-maintenance method. Bacterial sulfate-reduction and microbial metal accumulation are processes wanted in such systems. Few studies have dealt with long-term effects of wetland systems, but there are some indications that the wetland material has to be replaced for effective treatment. Furthermore, bacterial iron reduction may take place instead of sulfate-reduction in some wetlands. eral, the activity of microorganisms is neglected in the design of mine waste treatment systems, and the treatments are created merely from a technical point of view. This can result in situations where unexpected microbial processes take over, and, in the worst scenario, the overall effect is opposite to the desired.