Schwertmannite maintains effective stabilization of arsenic in flooded paddy soil: Insights from a two-season rice experiment and underlying mechanisms.

Rice (Oryza sativa L.) tends to accumulate elevated levels of arsenic (As) in grain, threatening food safety and human health. Schwertmannite (Sch), with its high As adsorption capacity, has been suggested as a potential soil amendment to mitigate As accumulation in paddy soils. However, due to the metastable state of schwertmannite and the high iron reduction potential of paddy soil, it remains unclear whether schwertmannite can maintain a persistent stabilizing effect on As, as well as the underlying mechanisms of such stabilization. In this study, two-season pot experiments with rice were conducted to evaluate the potential of one-time application of schwertmannite for sustainable remediation of As-contaminated paddy soil, and the mechanism underlying its persistent retention of soil As was further investigated under laboratory conditions. It was found that, compared with the corresponding control, the application of 0.5-1 % (w/w) schwertmannite reduced As accumulation in grains by 60.1-62.5 % in the first growing season and by 27.0-28.9 % in the second season. On the one hand, secondary minerals maintained a certain adsorption capacity for As (8.85-16.08 mg/g), thus continuously immobilizing As. On the other hand, the formation of a considerable amount of residual As (90.0-98.8 %) on schwertmannite or secondary minerals enhanced the ability of solid minerals to stably retain As, thereby reducing the risk of As re-release. In addition, secondary minerals did not undergo phase transformation in a reducing environment, which was beneficial for maintaining the mineral structure and the stable presence of residual As in the solid mineral structure. These findings provide valuable insights into the mechanisms by which schwertmannite effectively immobilizes As in two-season rice cultivation, and hold important implications for its practical application in the remediation of arsenic-contaminated paddy soils. DATA AVAILABILITY: Data will be made available on request.