The effect of heavy metals on thiocyanate biodegradation by an autotrophic microbial consortium enriched from mine tailings

Bioremediation systems represent an environmentally sustainable approach to degrading industrially generated thiocyanate (SCN^-), with low energy demand and operational costs and high efficiency and substrate specificity. However, heavy metals present in mine tailings effluent may hamper process efficiency by poisoning thiocyanate-degrading microbial consortia. Here, we experimentally tested the tolerance of an autotrophic SCN^--degrading bacterial consortium enriched from gold mine tailings for Zn, Cu, Ni, Cr, and As. All of the selected metals inhibited SCN^- biodegradation to different extents, depending on concentration. At pH of 7.8 and 30 °C, complete inhibition of SCN^- biodegradation by Zn, Cu, Ni, and Cr occurred at 20, 5, 10, and 6 mg L^-1, respectively. Lower concentrations of these metals decreased the rate of SCN^- biodegradation, with relatively long lag times. Interestingly, the microbial consortium tolerated As even at 500 mg L^-1, although both the rate and extent of SCN^- biodegradation were affected. Potentially, the observed As tolerance could be explained by the origin of our microbial consortium in tailings derived from As-enriched gold ore (arsenopyrite). This study highlights the importance of considering metal co-contamination in bioreactor design and operation for SCN^- bioremediation at mine sites. KEY POINTS: • Both the efficiency and rate of SCN^- biodegradation were inhibited by heavy metals, to different degrees depending on type and concentration of metal. • The autotrophic microbial consortium was capable of tolerating high concentrations of As, potential having adapted to higher As levels derived from the tailings source.