Precipitation of mixed metal residues from wastewater utilising biogenic sulphide

Conventional metal extraction processes leave residual metals in solution which are lost as pollutants in process wastewaters. Biogenic sulphide precipitation is applied successfully to processes for the extraction of base metals and sulphates from solution, and may also be applicable to the low concentrations found in wastewater streams. Experiments were performed to precipitate metals out of synthetic wastewater by sparging with biogenic sulphide gas and by mixing with sulphide rich supernatant from a sulphate-reducing bioreactor. The composition of the metal precipitates generated was investigated. Metal removal from wastewater sparged with biogenic H2S(g) varied depending on the metal and the ratio of metal:sulphide. Maximum removal efficiencies were 30% for iron and 100% for gold and copper. Mixing the bioreactor supernatant with the metal solutions achieved 100% iron, 96% gold and 99% copper removal. The precipitates produced comprised mainly metal sulphides, but also contained unidentified complexes which may include nitrates, chlorides and carbonates.