Electrowinning and bioremediation processes for toxic metals removal from Acid Mine Drainage

One of the most important problems affecting mining companies around the world is the treatment of acid mine drainage (AMD) which contain sulphuric acid, dissolved heavy metals, sulphates, iron precipitates and their pH can be very low. The presence of heavy metals in the AMD excludes the possibility for re-use and involves environmental problems. Main aim of the present experimental work was to compare and to interpret two different innovative technologies such as the electrowinning and the bioremediation by sulphate-reducing bacteria (SRB), applied for toxic metals removal (Zn, Ni, Cd, Cu, Mn and Fe) from AMD samples. The AMD under study coming from the zinc mine located in Montevecchio Mine (Italy). The present study consists in a preliminary work: the experimental tests have been conducted by synthetic solutions. The application of the electrowinning process demonstrates the technical feasibility to remove toxic metals from AMD samples; in fact, at the end of the process, all the metal concentrations decrease under the recommended limit suggested from EC. In particular, as far as Zn electrowinning, it was possible to achieve high Zn removal: 98% Zn after 90 minutes and 99% Zn after 2 hours with an energetic consumption of 20 kWh/kg and 33 kWh/kg, respectively. The treatment of AMD by the SRB is based on the ability of these bacteria to reduce sulphates to hydrogen sulphide, which binds readily with most metals to form an insoluble precipitate. Investigated was the kinetics of the Cd precipitation by SRB of genera Desulfovibrio and Desulfotomaculum. Achieved results demonstrate the 98-99% elimination of Cd by bacterially produced H2S after 30 minutes.