Bachelor Thesis

Effect of redox changes and sulfate application mode on arsenic thiolation in paddy soil incubations

Laura Wegner (12/2017-10/2018)

Support: Britta Planer-Friedrich, Jiajia Wang

Thioarsenates have been recently discovered in rice grain and paddy fields. Since arsenic speciation has a great impact on its toxicity, the formation and stability of thioarsenates in paddy fields was investigated during an incubation experiment with paddy soil spiked with 4 mg/kg arsenate. The soil was incubated under reducing conditions in glass bottles for 30 days, followed by an oxidation phase of 25 days. Two different sulfate application modes were used to examine, whether a low, continuous supply of sulfate leads to a difference in the thiolation of arsenic compared to the addition of the same amount of sulfate at one time. Eleven arsenic species were detected, seven of them were organic and inorganic thioarsenates. During the reducing phase, the share of thioarsenates increased to 60 % of the arsenic species. After aeration, it decreased to only 23% of the species. No statistically significant difference in thiolation was observed between the two sulfate application modes. The reason for this might have been the low content of iron of the soil combined with a too high sulfate application. Iron was almost completely titrated out of the soil solution by sulfide. The redox potental had a great impact. Thioarsenates formed under reducing conditions via the thiolation of arsenite, monomethylarsenate or dimethylarsenate and were decomposed after the development of oxic conditions. The organic dithiolated and inorganic species were highly sensitive to oxygen, while the organic arsenic species containing one thio-group, especially DMMTA, showed greater stability after exposure to air.