Investigations on pyrite oxidation in mine spoils of the Lusatian lignite mining district

The impact of organic waste material and fly ash on microbial and chemical pyrite oxidation was investigated in a field experiment, as well as in colu mn tests under laboratory conditions. For the field experiment, pyritic min e spoil was ameliorated with fly ash and treated either with mineral fertil iser, with sewage sludge or with compost. Independent of treatment, during the 18 months following application, the pyrite-S contents decreased steadi ly in the top spoil (0-30 cm depth). However, high variations of the pyrite -S content were observed. Compared to other pyrite oxidation studies, the p yrite content of the mine spoil at the experimental site was low. Therefore , a model spoil with a higher pyrite content, derived from Tertiary strata of the overburden sequence in the same open-cast mine, was used for the col umn experiments. For the first column experiment, the model spoil was mixed with fly ash and mineral fertiliser, reflecting the common reclamation pra ctice in the Lusatian open-cast lignite mining district. Columns with this spoil were either inoculated with different cell numbers of autochthonous a cidophilic bacteria, isolated from the model spoil, or with a commercial st rain of Thiobacillus ferrooxidans. The ratio of sulphate-S to total S was u sed as a measure for the degree of pyrite oxidation. The ratio of sulphate- S to total S increased within 28 days of incubation. The increase was relat ed to the inoculated cell numbers of bacteria, but independent of the origi n of the bacteria. It can be stated, that autochthonous bacteria from the m odel spoil oxidised pyrite at a similar rate as did the commercial T. ferro oxidans strain. For the second column test, mineral fertiliser, sewage slud ge or compost were applied to the model spoil. The columns were inoculated with autochthonous bacteria, isolated from the model spoil. Application of sewage sludge and compost seemed to promote the weathering of pyrite, as th e ratio of sulphate-S to total S increased more rapidly in these treatments compared to control or mineral fertiliser application. Both experiments sh owed an increase of cell numbers of inoculated bacteria, independent of the ratio of sulphate-S to total S.