Evaluation of the effects of application of iron materials on the accumulation and speciation of arsenic in rice grain grown on uncontaminated soil with relatively high levels of arsenic

• We examined the effect of Fe materials on the arsenic uptake by rice.
• Fe materials significantly decreased arsenic species in the soil solution.
• Increase in free iron oxide in the soil reduced the arsenic uptake by rice.
• Application of Fe materials effectively reduced arsenic in rice grain.
• Application of Fe materials is a promising agronomic practice for reducing arsenic.

The Codex Committee adopted a maximum level of 0.2 mg kg−1 for inorganic arsenic in polished rice at the 37th Session of the Commission (Codex Alimentarius Commission, 2014). It is thus necessary to establish agronomic management practices for reducing the accumulation of As in rice grain even in As-uncontaminated soil. We examined the effect of the application of different kinds of iron (Fe) materials on As uptake in rice plants grown in bottomless concrete frames filled with soil collected from an area surrounding a formerly As-polluted region. We assessed the concentration and speciation of As in soil solution throughout the rice cultivation period. Application of Fe materials had a significant effect on the concentration of arsenite, arsenate, inorganic As (sum of arsenite and arsenate) and total As, in the soil solution. The concentration of these forms of As in soil solution treated with Fe materials was significantly lower than the control. The lowest concentrations of As were observed in the plot to which a metal Fe powder composed mainly of a zero-valence Fe was applied throughout the rice cultivation period. The application of metal Fe powder and Fe oxide material composed mainly of ferrihydrite significantly reduced the amount of available As in the soil. The amount of acid ammonium oxalate extractable Fe was significantly increased by the application of metal Fe powder and Fe oxide material compared to that of the control and converter furnace slag. Concentrations of all forms of As in soil solution showed a significant positive correlation with the amount of available As and a significant negative correlation with the amount of acid ammonium oxalate extractable Fe in soil; this indicated that the increase of acid ammonium oxalate extractable Fe by application of Fe materials retarded the release of As from the soil solid phase to soil solution by fixing As and Fe. The concentrations of arsenite, dimethylarsinic acid (DMA), the sum of As species, and total As in grain with applied metal Fe powder and Fe oxide material were significantly lower than the control, reflecting the concentration of soluble As species in soil solution. These results strongly suggest that the application of metal Fe powder and Fe oxide material effectively reduced As accumulation in rice grain, making it a promising agronomic management practice for reducing the risk of As accumulation.