Sulfur-induced remobilisation of arsenic from natural organic matter
Anne Eberle (11/2016-08/2018)
Support: Britta Planer-Friedrich, Johannes Besold, Katharina Palmer, Carolin Kerl
Arsenic, a common contaminant in mining wastewater, has been shown to be efficiently retained in treatment peatlands. However, high sulfate input concentrations from the mining wastewaters together with reducing conditions in the peatlands can promote formation of arsenic-sulfur species, so-called thioarsenates, which are known to be more mobile than arsenic oxyanions. Whether thioarsenate in situ formation could contribute to arsenic remobilisation from peat has not been studied so far. A peatland used for treating mining wastewater from a gold mine in Finnish Lapland was therefore chemically characterised and two desorption experiments were conducted to investigate the influence of zero-valent sulfur (ZVS) and sulfide on the remobilisation of arsenic bound to natural organic matter via sulfhydryl-groups and the formation of thioarsenates. One experiment was done with sulfide-reacted and arsenite-loaded purified model peat under different pH conditions and another one with peat from the Finnish treatment peatland under sterile and non-sterile conditions. In pore water of the treatment peatland, up to 9 % of arsenic were found to be thioarsenates, where high concentrations of total arsenic, sulfide and ZVS occurred. These high arsenic concentrations were only found near the inflow of the peatland, suggesting that the peatland still shows good arsenic retention and a sudden release is rather unlikely. In the desorption experiments, thioarsenates were formed under all conditions apart from low pH. Sulfide or ZVS addition, high sulfide saturation of the peat and increasing pH led to formation of up to 98 % thioarsenates and comparatively low percentages of oxyanions. In all experiments, di- and trithioarsenate were formed due to the high reduced sulfur concentrations. Similar results of ZVS and sulfide treatments indicate that it is not decisive which sulfur species is added. Further, additional sulfide formation by microbes did not alter thioarsenate formation substantially. It was shown that at neutral to alkaline conditions, thioarsenates can be formed even without ZVS in solution. This suggests that the solid phase plays a crucial role in thioarsenate formation. Mobilisation of thioarsenates could occur either by reaction of sulfhydrylbound arsenic with aqueous ZVS and sulfide at the peat surface and subsequent release into solution or by desorption of arsenite and rapid reaction with ZVS and sulfide in solution. In conclusion, thioarsenate formation is a possible mobilisation pathway, but the exact mechanism still remains elusive. As the percentage of thioarsenates was low in the field (< 9 %), thioarsenate formation might not be the only process for overall arsenic mobilisation in the studied treatment peatland. However, the desorption experiments with peat from the treatment peatland showed a potential thioarsenate formation of up to 90 % without and up to 98 % with sulfur addition.