Towards recycling remediated cyanidation tailings water to the mineral biooxidation process – Impact on microbial community and its performance

Abstract

Biooxidation is a well-established pretreatment technology used in refractory gold mineral processing to enhance metal value recovery. The technology is microbially-mediated and is dependent on appropriate quality water for optimum function. The influence of remediated cyanidation tailings effluent and increasing thiocyanate (SCN−) concentration on a biooxidation microbial community was evaluated in small-scale batch tests. Changes in ferrous iron (Fe2+) oxidation activity, microbial growth and microbial community structure were evaluated. Two detoxified tailings wastewater samples, one sourced from an industrial operation and another from a laboratory system, were evaluated and used as the liquid matrix for medium preparation. These were compared with a standard laboratory medium described for the cultivation of acidophilic biooxidation organisms. Additionally, the effect of increasing SCN− concentration over the range of 0–5 mg/L across the different nutrient matrices. Similar microbial growth and Fe2+ oxidation activity with a nominal shift in community structure were observed in nutrient media containing remediated tailings wastewater relative to experiments conducted using defined medium prepared with deionised water (dH2O). However, a lag in the onset of Fe2+ oxidation activity was observed in systems cultured in media containing remediated effluent. This was attributed to the coupled presence of anions, NO3– and SCN−, in conjunction with additional constituents within the remediated effluent matrix. Exposure to SCN− concentrations up to 1 mg/L across all nutrient media demonstrated marginal differences in Fe2+ oxidation performance, although the observed lag in Fe2+ oxidation activity was sustained in remediated effluent tests. At a SCN– concentration of 1.4 mg/L Fe2+ oxidation activity within the defined medium system was inhibited, however systems cultivated in remediated tailings water exhibited sustained Fe2+ oxidation performance at 1.4 mg SCN− mg/L. Exposure to SCN− concentrations above 1.4 mg/L resulted in negligible Fe2+ oxidation activity, lower cell concentrations and a significant increase in growth lag time in detoxified tailings water media. Microbial community structure data illustrated that Leptospirillum ferriphilum was the dominant Fe2+ oxidising species within all systems; and its decline in abundance at elevated SCN− concentrations corresponded to poor Fe2+ utilisation. The abundance (and inferred growth rate) of sulfur-oxidising Acidithiobacillus caldus remained constant across the SCN− range explored. Abundance of Acidiplasma cupricumulans was found to increase at SCN− concentrations above 1 mg/L; however, it did not contribute toward Fe2+ oxidation. Results from this study indicated that inclusion of remediated tailings wastewater within the biooxidation circuit would be feasible causing negligible disruption to process performance; however, SCN− concentrations should not exceed 1 mg/L.