Thiolated arsenic species observed in rice paddy pore waters

The accumulation of carcinogenic arsenic in rice, the world’s main staple crop, represents a health threat to millions of people. The speciation of arsenic controls its mobility and bioavailability and therefore its entry into the food chain. Inorganic and methylated oxyarsenic species have been a focus of research, but arsenic characterization in the field has largely ignored thioarsenates, in which sulfur takes the place of oxygen. Here, on the basis of field, mesocosm and soil incubation studies across multiple paddy soils from rice cultivation areas in Italy, France and China, we find that thioarsenates are important arsenic species in paddy-soil pore waters. We observed thioarsenates throughout the cropping season, with concentrations comparable to the much-better-investigated methylated oxyarsenates. Anaerobic soil incubations confirmed a large potential for thiolation across a wide diversity of paddy soil types in different climate zones and with different parent materials. In these incubations, inorganic thioarsenates occurred predominantly where soil pH exceeded 6.5 and in the presence of zero-valent sulfur. Methylated thioarsenates occurred predominantly at soil pH below 7 and in the presence of their precursors, methylated oxyarsenates. High concentrations of dissolved iron limited arsenic thiolation. Sulfate fertilization increased thioarsenate formation. It is currently unclear whether thiolation is good or bad for rice consumption safety. Nevertheless, we highlight thiolation as an important factor to arsenic biogeochemistry in rice paddies.