Enhanced metal recovery by efficient agglomeration of precipitates in an up-flow fixed-bed bioreactor

In this study, a single-stage up-flow fixed-bed sulfidogenic bioreactor was operated for 288 days (six stages), treating mine-impacted water at different concentrations of nickel (Ni), cobalt (Co) and other associated metals. The effect of metals on the sulfate reducing activity, metal recovery, extracellular proteins and microbial diversity was evaluated. The bioreactor configuration showed a positive synergistic effect on both sulfate reducing activity and metals recovery, which could treat up to 200 mg/L Ni. Over 99% of Ni and Co, as well as over 91% of the other metals in the influent, precipitated and settled in the bioreactor regardless of the initial metal concentration. The scanning electron microscopy (SEM) analysis of the precipitates showed the presence of extracellular polymeric substances (EPS), which may have helped to agglomerate the metal sulfide precipitates increasing their ‘particle size’ from 0.1 to 0.5 μm to 10–100 μm. Different extracellular proteins associated to these EPS increased in abundance upon variations of the bioreactor operation. These included proteins involved in enzymatic reactions and metal binding, such as periplasmic [NiFeSe] hydrogenase, standing out when Ni and Co was added. The biofilm characterization showed the dominance of metal-tolerant SRB genera (e.g., Desulfomicrobium spp. and Desulfovibrio spp.), but also of other non-SRB, demonstrating that a higher microbial diversity may help the biofilm endure higher metal concentrations.