A sequential bioreactor adaption strategy enhanced the precipitation of metals from tailings’ leachates

This study demonstrated the adaption of a sulfidic microbial community to mixed metal mine wastes, i.e., tailings leachate solutions, while maintaining bioreactor performance. This overall process occurred using a sulfate reducing bioreactor operated in continuous mode for 172 days with ethanol as electron donor and, at first, a synthetic mine solution and then, tailings leachate solutions. Remarkably, the addition of a succession of leachates increased the sulfate removal efficiency, which reached values around 84.7 ± 6.9% after the sulfate adaptation period. This increase was also in line with an increase in the soluble chemical oxygen demand removal efficiency and with a slight increase in the pH of the reactor. This high activity supported metal removal efficiencies close to 100% for most metals, except Mn, Ca and Mg whose precipitation was lower (80, 50 and 38%, respectively). The bacterial community in the bioreactor was initially diverse but evolved to be dominated by SRB, in particular Desulfovibrionaceae. Interestingly, the addition of the tailings leachates enriched for the presence of methanogens in the bioreactor, consistent with the consumption of the accumulated acetate.