Evaluation on the Feasibility of Microbially Enhanced Electrokinetic Removal of Multiple Heavy Metals from Tailing Soil

Abstract

Lab-scale experiments were conducted to evaluate the efficacy of the enhanced electrokinetics combined with anaerobic and aerobic bioleaching processes for remediation of tailing soil. The tailing soil contained multiple heavy metals, such as As, Cd, Co, Cu, Cr, Fe, Mn, Ni, Pb, and Zn. To begin with, the applicability of both anaerobic and aerobic bioleaching was investigated as a pretreatment step to transform them into more mobile forms. In the anaerobic bioleaching, the supply of carbon source into the soil stimulated anaerobic metal reducing bacteria with dissolution of heavy metals, As, Fe, Mn, and Zn. In the aerobic bioleaching, on the other hand, indigenous sulfur oxidizing bacteria oxidized elemental sulfur with acidifying soil and mobilizing heavy metals, such as As, Fe, Mn, Pb, and Zn. To compare the removal efficiencies between individual and combined processes, individual electrokinetics was conducted as a control with only electrolyte conditioning and the tailing soils pretreated by bioleaching were sequentially applied to the electrokinetics to simulate the combined process. Generally speaking, the microbially enhanced electrokinetics showed higher removal efficiencies of almost all the heavy metals tested than the individual electrokinetics, because bioleaching process converted the heavy metals to be labile. One more synergistic effect of the combined processes was that bioleaching enhanced the rate of acidification of tailing soil, which offers a more suitable condition for electrokinetics. The highest removal efficiency of Co, Mn, Ni, and Zn was accomplished in the electrokinetic process combined with anaerobic bioleaching (72.5%, 66.4%, 57.9%, and 76.7%, respectively), whereas that of As, Cd, and Pb was rendered in the process combined with aerobic bioleaching (27.2%, 46.6%, and 25.0%, respectively). The results indicate that the performance of the combination between bioleaching and electrokinetics is significantly dependent on the species of heavy metals to be removed. In addition, this study verifies that the electroosmotical injection of carbon sources is feasible to activate the indigenous microorganisms in the anaerobic bioleaching coupled with the electrokinetics.