###### Abstract  
This work compares two technologies for the remediation of metal-polluted mine tailings based on lab-scale bioleaching experiments performed in (a) conventional agitated slurry-phase reactors and (b) in situ electrokinetic percolation. While ex situ bioleaching in agitated reactors has been widely studied, only a few previous works have studied the in situ option that couples bioleaching and electrokinetics. Real mine tailings from an abandoned sphalerite mine in southern Spain were used. The leaching medium was externally generated in a bioreactor using an autochthonous acidophilic culture and then added to tailings in batch experiments. This medium enabled metal leaching from mine tailings without the stringent operating conditions required by a classic bioleaching process. Metal removal efficiencies and kinetic rate constants after 15 d of treatments were calculated. Additionally, advantages or disadvantages between the two methods were discussed. The results for the innovative EK-percolation method showed rates and efficiencies that were comparable to, and in some cases better than, those achieved with conventional stirred slurry systems. Boleaching; slurry agitated reactor; electrokinetic percolation; metal pollution; mine tailing; soil remediation +
Footnote †: journal: Applied sciences  
-heap leaching. However, this method requires a high permeability in the mine tailings, making it challenging for low-permeability materials [6]. One of the ways to improve in situ metal bioleaching in these types of materials could be by combining it with electrokinetic remediation (EK), which takes advantage of the electrokinetic fluxes, electroosmosis, and electromigration to effectively transport leaching solutions and/or metals throughout soils with a low permeability. The application of EK for the removal of heavy metals in mine tailings has been extensively studied [9; 10; 11], but this technique is only able to transport the already solubilized heavy metals, leaving the insoluble metals in the soil. The combination of in situ bioleaching and electrokinetics has only been reported in a few previous works [12; 13; 14]. Our research team has recently reported on an electrokinetically assisted percolation treatment using a leaching medium generated by acidophilic microbial activity in an external bioreactor and characterized by a high redox potential and high concentrations of Fe(III) and sulfuric acid [15]. This study demonstrated that horizontal EK-percolation of the leaching medium, under increasing voltage gradients, enhances in situ metal recovery from tailings. Moreover, that leaching medium could alternatively be used in stirred-tank suspended systems or vertical-column percolation systems to chemically leach the metals from the solid material [16; 17; 18], thus avoiding some operational problems regarding the adequate conditions for microbial activity, e.g., temperature and pH changes, oxygen supply for aerobic metabolism, or physical damage from mechanical stirring. Despite the effectiveness of EK-percolation, its novelty and the scarcity of additional reported works means that further evaluation is required to assess its efficiency, kinetics, advantages, and disadvantages compared to conventional ex situ bioleaching using an agitated slurry-phase reactor. In this context, the present work compares the two mentioned bioleaching methodologies for the removal of metals from highly polluted real mine tailings in terms of metal extraction efficiency and process kinetics, also using mathematical modeling of the bioleaching processes. Note that it is not the objective of this work to deeply look into the influence of factors and mechanisms of conventional bioleaching or electrokinetics, as there are many previously published studies on the biological mechanisms of bioleaching in mine tailings and the effect of the distinct factors [6; 7], and on the mechanisms and factors that influence the electrokinetic mobility of metals that are already available in the liquid medium [11].