Rates of microbial elemental sulfur oxidation and ~(18)O and ~(34)S isotopic fractionation under varied nutrient and temperature regimes

摘要

Globally escalating excess supplies of elemental S (S ~0) are often stored in large outdoor S ~0 blocks that are exposed over time to the environment. Here S ~0 oxidation experiments were conducted under ambient conditions to investigate (i) the rate of S ~0 oxidation at a range of ambient temperatures (6-32°C) in the presence and absence of Acidithiobacillus thiooxidans and (ii) the O- and S-isotope fractionation during oxidation to identify sources of O incorporated into the resulting SO42- Experiments were conducted using four different δ ~(18)O _(H2O) values in a nutrient and nutrient free solution. Results showed that >95% of total SO42- generated could be attributed to autotrophic microbial activity. Experiments conducted in the nutrient solution showed oxidation rates increased with temperature (Q _(10)≈1.7-1.9); while experiments conducted in a nutrient-free solution showed no increase in oxidation rate with temperatures between 12 and 32°C. The contribution of water-derived O to SO42- by S ~0 oxidation ranged from 84% to 97% for all treatments. The final δ _(18)O _((SO4)) value indicated nearly all SO42- oxygen originates from water, and the ε _(18)O _(SO4-H2O) was estimated to be between -0.9‰ and -6.2‰ with a mean of -3.6±2.7‰ for the nutrient tests and +1.1‰ to -3.4‰ with a mean of -1.5±2.4‰ for nutrient free tests. A relationship between the proportions of water O incorporated from O _2 or water into the SO42- and temperature was not observed. The δ ~(34)S data showed that the oxidation of S ~0 produces SO42- with a δ ~(34)S value that is nearly indistinguishable from the parent S ~0. The findings suggest that SO42- accumulation in S ~0 block effluent would be minimized by limiting H _2O influx and preventing microbial inhabitation of the block.