Effects of iron amendments on the speciation of arsenic in the rice rhizosphere after drainage

Applications of iron-(Fe-) bearing materials represent an effective countermeasure for decreasing the dissolution of arsenic (As) in soil under anaerobic conditions. In this study, we investigated the effects of Fe amendments (ferrihydrite-based and zero-valent iron-(ZVI-) based materials) on the speciation of As in rice cultivated soils and root-attached materials including Fe plaque when the soil shifts from anaerobic to aerobic conditions. Rice (Oryza sativa L.) was cultivated in pots filled with soil under continuous flooding conditions, and root distribution in the soil was restricted inside a cylinder made by nylon mesh. Soil and root samples were collected after drainage at different growth stages of the rice plants, which are represented by intermittent drainage and drainage at harvest. The speciation of As was determined by As K-edge X-ray absorption near edge structure (XANES) spectroscopy. The proportion of arsenite did not differ between the bulk soil and root-attached materials including Fe plaque, whereas a larger proportion of dimethylarsinic acid was found in the root-attached materials regardless of the application of Fe amendments. Observation of soil thin-sections showed that the application of Fe amendments caused an increase in Fe (hydr)oxide deposition around the roots as well as on the soil particles. In addition to Fe (hydr)oxide, sulfide was found to be associated with As under anaerobic conditions, notably for the ZVI-amended soil at the time of intermittent drainage. The concentration of As in the soil solution and As uptake by rice grains decreased, while As speciation near the roots was not influenced by the application of Fe amendments. In conclusion, Fe amendments mitigated As dissolution in the soil solution by providing a sorption site for As in bulk soil without altering As speciation near the roots.