PhD Thesis

Effects of sulfur on arsenic speciation in paddy soils and rice grains

Jiajia Wang (05/2016-09/2020)

Support: Britta Planer-Friedrich

Arsenic (As) is a carcinogenic metalloid ubiquitously present in soil environments and its accumulation in rice grains poses a health threat to millions of people. The geochemical behavior and bioavailability of As are largely determined by its speciation. Current research on As speciation in rice paddies mainly focuses on inorganic and methylated oxyarsenic species, but does not consider thioarsenates, in which sulfur takes the place of oxygen in oxyarsenic species. The general assumption is that thioarsenates only form in environments with excess dissolved sulfide. This paradigm was recently challenged by the hypothesis that As thiolation could be mediated by reduced sulfur bound to surfaces of minerals or organic matter, suggesting thioarsenates may be more widespread than currently assumed.

Therefore, the aim of this thesis was to reveal whether and to what extent thioarsenates contribute to As speciation in paddy soil pore waters, as well as to decipher soil parameters that are important for their formation. Additionally, the effects of sulfate fertilization on As speciation in pore waters and As accumulation in rice grains were examined.

In the first study, a novel diethylenetriamine-pentaacetic acid-based sampling and analytical method was developed, enabling the simultaneous determination of thioarsenates and their respective oxyarsenic species. On the basis of field, mesocosm and soil incubation studies across various paddy soils from major rice cultivation areas in Italy, France and China, thioarsenates were revealed as important but previously overlooked contributors to As speciation in rice paddy pore waters. Thioarsenates were observed throughout the cropping season and in quantities comparable to methylated oxyarsenates. On regional scale, soil pH values represented an easy-to-measure parameter indicative for As thiolation potentials under anaerobic conditions. Inorganic thioarsenates occurred predominantly in alkaline soils, controlled by the presence of zero-valent sulfur. Methylated thioarsenates occurred predominantly in acidic soils, related to the presence of their precursors, methylated oxyarsenates.

The second study showed that for paddy soils where conditions change from anoxic during flooding to oxic during drainage, the importance and occurrence of thioarsenates in pore waters were highly dependent on soil redox potentials (EH). Inorganic thioarsenates formed rapidly upon soil flooding with a dominance of trithioarsenate, while they dethiolated almost completely after soil oxidation. Thiolation of methylated oxyarsenic played an important part over a wide range of soil redox potentials, with total thiolation of mono- and dimethylated As up to 70% and 100% below EH 0 mV, respectively. Dithiolated species dominated over monothiolated species below EH −100 mV. Among all thioarsenates, dimethylated monothioarsenate showed the least transformation upon prolonged oxidation and also represented the major thiolated As species in the oxygenated rice rhizosphere, which is especially critical since dimethylated monothioarsenate is highly carcinogenic.

While natural sulfate contents in paddy soils could support substantial As thiolation already, combined results from the first and third study showed that sulfate fertilization could further increase thioarsenate formation, especially for paddy soils with low initial zero-valent sulfur formation. Sulfate fertilization also increased As methylation, but largely decreased the bioavailability of inorganic oxyarsenic during rice cultivation. From an agronomic point of view, the third study evidenced sulfate fertilization as an effective measure to mitigate the accumulation of cancerogenic inorganic As in rice grains, irrespective of seeding practices and water managements.

Altogether, these three studies clearly illustrated the importance of sulfur in influencing As speciation in paddy soils and its accumulation in rice grains, revealing sulfurmediated As thiolation as an important and heretofore unaccounted contributor to arsenic biogeochemistry in rice paddies. Future studies are needed to clarify whether thiolation is good or bad for producing rice low in toxic As. Furthermore, thioarsenates need to be considered in low-sulfate terrestrial environments thus to paint a more accurate picture of global As cycling.