Neutralizing extremely acidic mining lakes by chemical and microbial treatment – Mesocosm Studies

This study is investigating the buffering of extremely acidic mining lakes in the Lusatian open cast lignite mining area and treatment options for these lakes through mesocosm experiments. First results concerning the different buffering systems in the lakes will be shown and discussed. In acidic lakes with similar pH and from the same mining area, the strength of the buffering system can be very different. The most important buffers are aluminum and iron(III) ions. Two types of alternative acidity removal treatments are possible, namely chemical neutralization or microbiological alkalinity production. With chemical treatment, alkaline material in the form of soda is applied. The microbiological treatment acts as the reverse of the acidification processes i.e. the reduction of Fe(III) and sulfate. These processes are carried out by bacteria in anoxic conditions such as found in sediments. To enhance these microbial processes, the bacteria need an increased supply of organic carbon as substrate. This requires addition of organic materials such as agricultural wastes (saprobization) or organics produced within the water column of the lake through enhanced primary production achieved through fertilization (eutrophication). Both of these remediation strategies have disadvantages. Chemical treatment is not self-sustaining, biological sulfate reduction processes are hindered by the low pH values and primary production is limited by an extremely low nutrient supply. Therefore a combined chemical/biological treatment options are being investigated. First results show an enhancement in primary production as well as in iron reduction when combined with chemical neutralization. Introduction In Lusatia (north-eastern Germany), there are several hundred flooded open cast lignite pits forming a new lake district (Nixdorf et al. 1997) During mine operation, the groundwater level is lowered (by pumping) and a vast quantity of overburden as well as waste material exposed. The exposure of these materials to oxygen results in the chemical and microbiological oxidation of iron sulfide (pyrite and marcasite) and consequent acidification (1). Following mine closure, the water level rises again and fills the mined holes. Hence, the oxidized iron is hydrolyzed and is a secondary source of acidification (2). Overall, for each mole of iron sulfide, 4 moles of H-ions (= acid) are produced