Environ Sci Technol. 2019 Dec 3;53(23):13666-13674. doi: 10.1021/acs.est.9b04158. Epub 2019 Nov 13.
Iron (hydr)oxide coating at rice roots, so-called iron plaque (IP), is often an important barrier for uptake of inorganic oxyarsenic species and their accumulation in rice grains. Sorption of methylated thioarsenates, which can co-exist with inorganic and methylated oxyarsenates in paddy soils, was not studied yet, even though these toxic species were detected in xylem and grains of rice plants before. Hydroponic experiments at pH 6.5 with 20 day-old rice plants showed lower net arsenic enrichment in IP for plants exposed to monomethylthioarsenate (MMMTA) compared to monomethylarsenate (MMA) and no enrichment for dimethylmonothioarsenate (DMMTA). Goethite was the dominant mineral phase in our IP. Sorption experiments with synthesized goethite and ferrihydrite revealed a 30-times-higher sorption capacity for MMMTA to amorphous ferrihydrite than to crystalline goethite, comparable to methylated oxyarsenates. No evidence for direct MMMTA binding was found. Instead, we postulate that MMMTA transformation to MMA is a prerequisite for removal. DMMTA showed very little sorption, even to amorphous ferrihydrite, which is in line with a lack of direct binding and reported slow transformation to dimethylarsenate. Our study implies that IP is no effective barrier for methylated thioarsenates and that especially DMMTA is very mobile with a high risk of uptake in rice plants.
水稻根部的铁（氢）氧化物涂层，即所谓的铁斑（IP），通常是无机含氧砷化物吸收和在稻米中积累的重要屏障。尽管这些有毒物种以前在水稻植物的木质部和谷粒中被检测到，但对于在稻田土壤中与无机和甲基化的含氧砷酸盐共存的甲基硫代砷酸盐的吸附，尚未进行研究。在 pH 值为 6.5 的水培实验中，用 20 天大的水稻进行实验，与甲基砷酸盐（MMA）相比，暴露于单甲基硫代砷酸盐（MMMTA）的植物在 IP 中的净砷富集较低，而二甲基单硫代砷酸盐（DMMTA）则没有富集。针铁矿是我们 IP 中的主要矿物相。用合成的针铁矿和水铁矿进行的吸附实验表明，与结晶针铁矿相比，MMMTA 对无定形水铁矿的吸附能力高 30 倍，与甲基化的含氧砷酸盐相当。没有发现直接结合 MMMTA 的证据。相反，我们假设 MMMTA 向 MMA 的转化是去除的先决条件。DMMTA 的吸附性很小，即使是无定形水铁矿也是如此，这与缺乏直接结合以及报道的向二甲基砷酸盐的缓慢转化一致。我们的研究表明，IP 不是甲基硫代砷酸盐的有效屏障，特别是 DMMTA 具有很高的迁移性，在水稻植物中吸收的风险很高。
