Abstract: To achieve effective remediation of As contamination in paddy soils, chitosan-based nanoscale zero-valent iron (C-NZVI) was prepared by liquid phase reducing method using biodegradable chitosan as the stabilizer. The adsorption kinetics and adsorption isotherms of As(Ⅲ) with C-NZVI were studied. The effects of typical competitive anions and molecules on As(Ⅲ) adsorption efficiency by C-NZVI were also tested. On this basis, the fortification effects of flooding and antagonistic fertilizers on As dissolution in paddy soil were addressed. C-NZVI was then employed for ectopic adsorption of soluble As in effluent from flooded paddy soils followed by complementary in situ immobilization of soil residual As. The results showed that the process of As(Ⅲ) adsorption with C-NZVI could be well described by pseudo-first-order kinetics and Langmuir isothermal adsorption. The maximum adsorption capacity of C-NZVI towards As(Ⅲ) was 145.09 mg/g. With the presence of K₂HPO₄, H₃BO₃, Na₂SiO₃ and CH₃COOH at 0.05-0.50 mmol/L, the efficiency of As(Ⅲ) adsorption by C-NZVI was still as high as 99%. When the tested paddy soil was subject to flooding and subsequent application of NH₄H₂PO₄, (NH₄)₂C₂O₄ and Na₂SiO₃, the cumulative level of soil soluble As (18.1 mg/kg) accounted for 30.0% of the total soil As concentration. Up to 91.3%-99.8% of the effluent As from submerged paddy soil could then be removed with C-NZVI, which reduced the total soil As level by 43.4%-52.6%. To further stabilize soil As in the labile pool, 1% and 5% C-NZVI was applied, and the non-specifically bound As was decreased remarkably by 94.7%-100%. The results highlightedthat the combination of ex situ removal and in situ passivation of soil As with C-NZVI under flooding condition can provide a useful strategy for efficient remediation of As contamination in paddy soil.