Bachelor Thesis

Effects of organic Carbon and Sulfate on the Formation of Thioarsenates in Paddy Soils

Jens Hamberger (10/2019-10/2020)

Support: Britta Planer-Friedrich, Alan Nicol

Thioarsenates have recently been discovered as a new group of arsenic species in paddy soils. Thioarsenates can be separated in two groups: methylated and inorganic thioarsenates. For the formation of inorganic thioarsenates reduced sulfur is mandatory. Methylated thioarsenates rely on the organic arsenic species as reactants, which get methylated by sulfur-reducing bacteria and demethylated by methanogens. However, formation pathways and involved reactants for both thioarsenate groups are still not fully understood. The effects of sulfate and organic carbon on thiolation of arsenic were investigated in this thesis. Sulfate addition is assumed to increase the concentration of reduced sulfur and favoring sulfate-reducing bacteria. Organic carbon favors no specific group of bacteria but enhances overall microbial activity. In the present experiment, two Italian paddy soils (Veronica and Fornazzo) were incubated anoxically with different amounts of glucose or rice straw as main OC source and different amounts of sulfate. The redox potential was measured daily, and samples were taken once at nominal -59 ± 23 mV for rice straw incubations and at nominal -101 ± 47 mV for glucose incubations. In total, eleven arsenic species were detected, seven of them were organic and inorganic thioarsenates. The amount of added organic carbon primarily increased the release of arsenic in the porewater and therefore the overall thioarsenate concentration. Additionally, the rice straw treatment leads to high percentages of methylation (up to 50 % of total arsenic concentration). Against our expectations, sulfate addition did not increase the share of thioarsenates further, most likely because of the immediate re-oxidation of formed sulfide back to sulfate. To fully understand the influence of different organic carbon sources, their concentration, sulfur and nitrate concentrations, and the effects of microbial communities on the presence of thioarsenates, all factors must be further studied.