Bachelor Thesis

Mobilization of arsenic from paddy soils with different sources of organic carbon and sulfate addition

Emma Stieff (03/2021-09/2021)

Support: Britta Planer-Friedrich, Alan Nicol

Arsenic (As) is a metalloid for which some species are associated with an increased cancer risk for humans, and it can be taken up into rice plants through the porewater of contaminated soils. This is why As mobilization from rice paddies into the porewater needs to be investigated. In this study, two Italian paddy soils (Veronica and Fornazzo) were incubated for 1, 3, 5, 7, 10, 15 and 30 days without addition of external organic carbon (control) and with addition of different sources of organic carbon (acetate, glucose, rice straw), each without and with sulfate addition (1mM).

The results revealed that the As total concentration in the porewater of Fornazzo soil did not respond to sulfate addition. However, in the porewater of Veronica soil, sulfate addition caused an increase of As total concentration in the end of incubation. It is either explainable by formation of thioarsenates and the low sorption affinity of this group of species or competition for binding sites between, for example, arsenate and phosphate.

A lower-than-expected influence of external carbon addition on As total concentrations was observed. The carbon addition increased reductive dissolution of iron but As concentrations in the porewater were not increased compared to the control. Contrary to expectations, only the glucose treatment without sulfate addition in Veronica soil reached a slightly higher value of 0.39 ± 0.01 μM. The maximum of 1.14 ± 0.02 μM in Fornazzo soil was reached in the control without sulfate addition. Moreover, in the porewater of Fornazzo soil, As total concentrations 8 times as high as in Veronica were reached. The differences in total As concentrations between the two soils are probably explained by the internal carbon content that is twice as high in Fornazzo soil (47 g/kg) compared to Veronica soil (20 g/kg). This leads to the hypothesis that the determining factor of As mobilization in paddy soils is the internal carbon pool of the soil.