Tokyo University of Agriculture and Technology, Japan.
Tokyo University of Agriculture and Technology, Japan.
Environ Pollut. 2018 Jul;238:617-623. doi: 10.1016/j.envpol.2018.03.039. Epub 2018 Mar 31.
A substantial amount of sulfate is often supplied in paddy fields with concomitant applications of chemical fertilizers and manure for rice growth. It is unclear how solubility and speciation of arsenic (As) are affected by the levels of soil sulfate and their relationship to soil redox status and sulfur (S) and iron (Fe) speciation in a short cycle of soil reducing (flooding) and oxidizing (drying) periods. The objective of this study was to investigate the solubility of As in relation to chemical speciation of As and S in different levels of soil sulfate through a time series of measurements during a 40-day reduction period (Eh < -130 mV) followed by a 32-day reoxidation period (Eh > 400 mV) using X-ray absorption fine structure (XAFS) spectroscopy. An excess of sulfate decreased extractable and dissolved As in the soil reducing period due to retardation of soil reduction process that decreased soluble As(III) in the soil solid phase. The As species at the end of soil reducing period were 38-41% As(V), 46-51% As(III), and 11-13% AsS-like species, regardless of initial S treatments. In the following soil reoxidation, AsS-like species were sensitive to oxidation and disappeared completely in the first 2 days when the Eh value increased rapidly above 160 mV. The addition of extra sulfate to the soil did not result in the formation of neither reduced S species nor AsS-like species. About 50% of As(III) to the total As persisted over 32 days of soil reoxidation period (Eh > 400 mV), suggesting some mechanisms against oxidation of As(III) such as physical sequestration in soil microsites. This study demonstrates that the extra SO in paddy soils can help mitigate the dissolution of As in reduction and reoxidation periods.
大量硫酸盐通常随化肥和粪肥一起施用于稻田，以促进水稻生长。目前尚不清楚土壤硫酸盐水平如何影响砷（As）的溶解度和形态，以及它们与土壤氧化还原状态以及硫（S）和铁（Fe）形态之间的关系。本研究的目的是通过在 40 天的还原期（Eh < -130 mV）期间进行一系列测量，然后在 32 天的再氧化期（Eh > 400 mV）期间使用 X 射线吸收精细结构（XAFS）光谱法研究不同硫酸盐水平下 As 的溶解度与 As 和 S 的化学形态之间的关系。过量的硫酸盐会在土壤还原期降低土壤中可提取和溶解的 As，这是由于减缓了土壤还原过程，从而降低了土壤固相中的可溶性 As(III)。在土壤还原期结束时，As 的形态分别为 38-41% As(V)、46-51% As(III)和 11-13% AsS 样物质，而与初始 S 处理无关。在随后的土壤再氧化过程中，AsS 样物质对氧化很敏感，当 Eh 值迅速增加到 160 mV 以上时，在最初的 2 天内完全消失。向土壤中添加额外的硫酸盐不会导致形成还原 S 物质或 AsS 样物质。在 32 天的土壤再氧化期（Eh > 400 mV）中，约有 50%的总 As 以 As(III)形式存在，这表明存在一些防止 As(III)氧化的机制，例如在土壤微生境中的物理固定。本研究表明，稻田土壤中的额外 SO 可以帮助减轻还原和再氧化期 As 的溶解。
