An investigation into the influence of microbes on the cycling of sulfur in a net neutral oxidation reservoir in Sudbury

The generation of acid and H2S associated with microbial cycling of intermediate sulfur species, (sulfur oxidation intermediates; SOIs), is a global mining industry management issue. Both the role of bacteria in SOI transformations as well as comprehensive understanding of the SOIs that can occur within mining wastewaters, are poorly constrained. Key impediments to the industry’s ability to improve wastewater sulfur management have been securing a better understanding of the specific sulfur oxidation intermediate (SOI) species that occur in wastewaters, as well as microbial transformations of these sulfur species. One of the significant prohibitions on the advancement of knowledge in the area of SOI transformations in mine waste waters has been the lack of analytical methods for these species as well as lack of understanding of the controls on these transformations. A significant step forward was established through the development of robust analytical methods using derivatization and HPLC analysis to characterize sulfite (SO32-), thiosulfate (S2O32-), sulfide (H2S) as well as elemental sulfur (S0). These methods enabled assessment of these sulfur compounds in >60 seasonally and spatially varying wastewater samples collected from Sept 2014 to May 2016. Results identified SOIs were present in all wastewater samples and there were seasonal variations in both concentrations and occurrence of specific SOIs. The mass balance analysis of bulk water samples show that the total sulfur concentration varies seasonally in the system. Higher total sulfur occurred during spring and summer (8.4-13.1 mM) with lower (5.3-10.8 mM) total sulfur observed during the fall and winter sampling campaigns. Further, the proportion of the total sulfur pool associated with sulfate, indicative of complete oxidation of sulfur, were highest during spring and summer (75-100%) with a decreasing trend through fall (60-75%) and lowest in the winter under ice (10-20%); suggesting temperature may be an important ecological control on sulfur redox biogeochemistry. Corresponding to the observed decreasing seasonal sulfate trend, an increasing trend in the proportion of unanalyzed sulfur species (e.g. S4O62-, S2−n+1, SnO62-) was also observed, increasing from 0-25% (spring, summer) to 80-90% under ice. Further, elemental sulfur (S0), which emerged as an important part of the sulfur cycle in these waters, ranged in proportional abundance from 25-99% of the analyzed sulfur species. Elemental sulfur increased during the fall and winter (75-99%), compared to 25-65% during the spring and summer. Enrichment of sulfur oxidizing microbes (SOM) was conducted to determine whether SOM’s were present in endemic waters, and if so, what were the controls on these microbes in terms of cycling SOI’s and producing protons. Enrichment experiments were successful from all >60 water samples collected indicating the presence of these bacteria throughout the system over seasonal scales. These SOM catalyzed sulfur transformations consistent with the seasonal SOI characterization results which indicates that SOM are likely important players in sulfur cycling within mine wastewaters. Consumption of thiosulfate was limited to SOM enrichments from waters which were 10 °C or warmer (i.e. spring/summer) and generated sulfate and unanalyzed SOIs in lower and higher proportions respectively than those observed in summer field samples. Consistent with winter field results evidencing lower concentrations of sulfur and sulfate occurrence, winter SOM enrichments only partially consumed thiosulfate and cycled sulfur through different reactions compared to those catalyzed by warmer SoM enrichments. Analysis of SOI and endemic microbial communities provide a key assessment link in mine environmental management. The new methods that were developed enable more accurate determination of SOI in mining wastewaters. Assessment of SOI within mining waste waters demonstrate that simple H2S/ SO42- measurements will not comprehensively represent sulfur reactions and therefore accurately predict water quality outcomes that occur. Similarly, microbial sulfur metabolism was shown to be possible throughout space and time, but with differing seasonal implications for S cycling in these waters. The inclusion of SOI and SOM understanding into mine wastewater biogeochemical sulfur models will provide prophylactic rather than reactive management strategies.

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