Abstract
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.
Highlights
The chronic exposure to arsenic (As) is associated with an increased risk of carcinoma in humans.Increases in the As concentration in paddy rice are of great concern, especially in Asian countries where rice is consumed as a staple food [1]
The application of FB and ZVI to paddy soil resulted in a decrease in the dissolution of As into the soil solution under anaerobic conditions
At the time of intermittent drainage, formation of As sulfide was an important mechanism to restrict As dissolution in soil solution when soil was amended with ZVI
Summary
The chronic exposure to arsenic (As) is associated with an increased risk of carcinoma in humans.Increases in the As concentration in paddy rice are of great concern, especially in Asian countries where rice is consumed as a staple food [1]. Paddy rice is flooded during growing, which leads to the development of anaerobic conditions, leading to microbial reduction of arsenate (As-5) to arsenite (As-3). Since the adsorption affinity of As-3 to soil is weaker than that of As-5 [2], the As concentration in the soil solution is increased with the development of anaerobic conditions [3,4,5]. Iron (Fe) (hydr)oxide, which adsorbs As, undergoes reductive dissolution, and the As-3 associated with Fe (hydr)oxides is released into the solution as the adsorption phase diminishes [6]. When rice is grown as an upland crop, soil conditions remain aerobic and the As concentration is significantly lower than in paddy rice [5,7,8] because the dissolution of As in the soil solution is
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