IN SITU NEUTRALIZATION OF ACIDIC PIT LAKES: PROCESSES IN THE SEDIMENT AND LIMITING FACTORS

A strategy to neutralize acidic pit lakes was tested in a field mesocosm of 4500 m3 volume in the Acidic Pit Mine Lake 111 in Germany. After addition of the substrates Carbokalk and straw a neutral sediment layer formed, in which microbial sulphate and iron reduction as well as sulphide precipitation occurred. The neutralisation rate was limited by the precipitation of iron sulphides rather than by microbial reactions. Oxidation of H2S by ferric iron in the anoxic sediment lowered the net sulphate reduction rate. During the experiment precipitation of ferric minerals formed an acidic sediment layer at the sediment surface. In that layer sulphides did not precipitate, but H2S diffused into the overlying water where it was oxidised