Redox Dependence of Thioarsenate Occurrence in Paddy Soils and the Rice Rhizosphere.

In flooded paddy soils, inorganic and methylated thioarsenates contribute substantially to arsenic speciation besides the much-better-investigated oxyarsenic species, and thioarsenate uptake into rice plants has recently been shown. To better understand their fate when soil redox conditions change, that is, from flooding to drainage to reflooding, batch incubations and unplanted microcosm experiments were conducted with two paddy soils covering redox potentials from EH -260 to +200 mV. Further, occurrence of thioarsenates in the oxygenated rice rhizosphere was investigated using planted rhizobox experiments. Soil flooding resulted in rapid formation of inorganic thioarsenates with a dominance of trithioarsenate. Maximum thiolation of inorganic oxyarsenic species was 57% at EH -130 mV and oxidation caused nearly complete dethiolation. Only monothioarsenate formed again upon reflooding and was the major inorganic thioarsenate detected in the rhizosphere. Maximum thiolation of mono- and dimethylated oxyarsenates was about 70% and 100%, respectively, below EH 0 mV. Dithiolated species dominated over monothiolated species below EH -100 mV. Among all thioarsenates, dimethylated monothioarsenate showed the least transformation upon prolonged oxidation. It also was the major thiolated arsenic species in the rhizosphere with concentrations comparable to its precursor dimethylated oxyarsenate, which is especially critical since dimethylated monothioarsenate is highly carcinogenic.

Methylated Thioarsenates and Monothioarsenate Differ in Uptake, Transformation, and Contribution to Total Arsenic Translocation in Rice Plants.

Methylated and inorganic thioarsenates have recently been reported from paddy fields besides the better-known oxyarsenates. Methylated thioarsenates are highly toxic for humans, yet their uptake, transformation, and translocation in rice plants is unknown. Here, hydroponic experiments with 20 day old rice plants showed that monomethylmonothioarsenate (MMMTA), dimethylmonothioarsenate (DMMTA), and monothioarsenate (MTA) were taken up by rice roots and could be detected in the xylem. Total arsenic (As) translocation from roots to shoots was higher for plants exposed to DMMTA, MTA, and dimethylarsenate (DMAV) compared to MMMTA and monomethylarsenate (MMAV). All thioarsenates were partially transformed in the presence of rice roots, but processes and extents differed. MMMTA was subject to abiotic oxidation and largely dethiolated to MMAV already outside the plant, probably due to root oxygen loss. DMMTA and MTA were not oxidized abiotically. Crude protein extracts showed rapid enzymatic reduction for MTA but not for DMMTA. Our study implies that DMMTA has the highest potential to contribute to total As accumulation in grains either as DMAV or partially as DMMTA. DMMTA has once been detected in rice grains using enzymatic extraction. By routine acid extraction, DMMTA is determined as DMAV and thus escapes regulation despite its toxicity.