Carbon dioxide treatment of low density sludge: a new remediation strategy for acidic mining lakes?

Acid mine drainage (AMD) is treated at several points in the Lausitz lignite mine district (Saxony, Germany) in treatment plants. The remaining alkaline low density sludge (LDS) was deposited in acidic mining lakes without having an impact on the lake water quality. Batch experiments show that alkalinity can be raised using LDS from acid mine drainage treatment plants together with CO₂. Batch experiments were conducted using lake water and deposited LDS from the mining lake Spreetal-Nordost with varying concentrations of CO₂. Also the duration of gas contact as well as the LDS–water ratio was changed in the batch experiments. The gas contact time and the partial pressure of CO₂ are the relevant parameters controlling the alkalinity in the lake water at the end of the experiments. The Ca and Mg concentrations of the pore water are relevant for higher pH values. Therefore, dissolved CO₂ can form bicarbonate or carbonate complexes, thus alkalinity rises. A second factor for alkalinity gain is the calcite content of the sludge, because CO₂ triggers the dissolution of carbonates. Therefore, unused calcite in the sludge can raise the alkalinity more effectively by the application of carbon dioxide. Furthermore, it was shown that remobilization of trace elements will not affect the water quality.