Photochemical Degradation of Organic Carbon in Acidic Mining Lakes

The investigated coal mining lakes of the Lusatian lignite mining district and the mid-German lignite district are geogenically acidified caused by pyrite and marcasite oxidation. All lakes are characterized by high ionic strength and extremely high concentrations of dissolved ferric iron. In oxic water layers, up to 10% of total iron is present as Fe(II). Mean concentrations of dissolved organic carbon (DOC) are extremely low in the investigated Lusa-tian lakes (<0.1 mmol/L). During summer months, DOC concentrations below the detection limit were observed in epilimnetic layers accompanied by increased concentrations of ferrous iron. It is suggested that photochemical reactions are responsible for depletion of DOC and formation of ferrous iron as it has been described for acidic soft-water lakes. The coupling of DOC degradation and ferrous iron production was verified with sunlight exposure experiments. Samples of acidic mining lakes were irradiated in quartz glass vessels. Up to 50% of DOC was transformed to CO2 after 300 min of irradiation with natural sunlight, and the light-induced production of Fe(II) was up to 450% (compared to 100% Fe(II) concentration before irradiation). As a consequence of acidification and photochemical reactions, the carbon budget of investigated mining lakes is kept low because organic carbon is finally oxidized to carbon dioxide which leaves epilimnetic water layers as a gas.