Cadmium (Cd) and arsenic (As) in the rice paddy soil are a serious public concern for people whose diet is chiefly based on rice. For rice cultivation in some temperate and (sub)tropical areas, paddy soil undergoes a long flooding (reducing) period followed by a short draining (oxidizing) period, which can alter solubility of Cd in the soil. In contrast to Cd, soil As chemistry is contrastingly different and the solubility increases more in the reducing conditions where As occurs mainly as As(III). Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy was used to understand chemical species of Cd and As in relation to their solubility and redox gradients in paddy soils.
Our study found that the formation and dissolution of CdS is primarily controlled by redox potential and sulfur (S) in the soil. In the reducing period, the CdS proportion in the high-S soil was rapidly increased to 30% at day 4 and reached 90% at day 29, whereas CdS in the low-S soil did not exceed 35%. In the following oxidizing period, CdS in the soils underwent oxidative dissolution, but was not completely dissolved remaining at < 20% to the total Cd. Our study also found that S impurity in zero-valent iron (ZVI) particles enhances the formation of CdS in reduced soils. Cadmium K-edge µ-XAFS spectra for selected soil particles revealed that the proportion of CdS ranged from 20 to 87% (ave. 53 ± 22%) in the soil with low-S ZVI and from 64 to 98% (ave. 84 ± 14%) in the soil with high-S ZVI.
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% As2S3-like species, regardless of initial S treatments. In the following soil reoxidation, As2S3-like species were sensitive to oxidation and disappeared completely in the first 2 days when the Eh value increased rapidly above 160 mV. About 50% of As(III) to the total As persisted over 32 days of soil reoxidation period (Eh > 400mV), suggesting some mechanisms against oxidation of As(III) such as physical sequestration in soil microsites. Arsenic species and distribution in Fe plaque of the rice rhizosphere soil were investigated using micro-XANES spectroscopy. The result demonstrated that oxidation of As(III) to As(V) occurred faster in the Fe-plaque than the soil matrix, and As was sequestered in iron mottles originating from Fe-plaque around the roots. The ratio of As(V) to As(III) decreased toward the outer-rim of the subsurface Fe mottles where lepidocrocite occurred preferentially. These results provide direct evidence that speciation of As in proximity to roots depends on spatial and temporal redox variations in the soil matrix.