[Translocation Influence and Immobilization Mechanisms of Iron-containing Alkaline Materials on Cd and As in Paddy...

Arable land polluted by cadmium （Cd） and arsenic （As） is a widespread problem. We applied zero-valent iron （ZVI） and ZVI mixed with limestone （ZVI + LS） as soil amendments， combined with two types of water management strategies， including flooding （F） and alternate wetting and drying （AWD）， to study their immobile ability to prevent Cd and As simultaneously in paddy soil and rice systems. This study lays a theoretical foundation for the reuse of contaminated soils. The dosages of ZVI were set at 1%（ZVI1） and 4%（ZVI4） of the soil mass， while the dosage of LS was 0.3% when mixed with ZVI （ZVI1 + LS and ZVI4 + LS）. The results showed that F management， compared to AWD management， raised soil pH by 0.24-0.85. The immobile effect of F management combined with ZVI4 + LS was the most effective treatment. Specifically， the available fraction （T-EC-Cd） and residual fraction Cd （T-RES-Cd） in the soil decreased by 12.33% and 2.94%， respectively. In contrast， the readily reducible iron and manganese fraction Cd （T-FE-Cd）， organic fraction Cd （T-OM-Cd）， and crystalline iron oxide fraction Cd （T-FEC-Cd） increased by 12.06%， 1.94%， and 1.27%， respectively. The application of ZVI could promote the formation of the amorphous iron oxide fraction of Cd， while LS facilitated the precipitation of CdCO₃ and Cd（OH）₂ through the speciation changes of Cd in soil colloids and the adsorption mechanisms by amendment. However， the immobile effect of ZVI4 under AWD management was the most effective treatment for As， mainly due to the formation of more Fe-As co-precipitates， such as Fe₂As₄O₉ and FeHAsO₄. The changes in rice quality and physiological and biochemical indexes influenced by the amendments were consistent with the trends in soil Cd and As bioavailability. AWD management combined with the ZVI4 + LS treatment significantly reduced the accumulation of Cd and As in rice roots and their translocation to the grains， which were both less than the national safety standards （GB 2762-2022）. The primary pathway for Cd accumulation in the grains changed from root uptake to a combination of root uptake （27.47%） and Cd remobilization from the straw （72.53%）， while As accumulation remained predominantly through straw remobilization.