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The leaked arsenic-containing chemical warfare agent has caused severe contamination to the surrounding soil and water. In this study, iron oxyhydroxide (FeOOH) with different crystalline phases was used to stabilize arsenic. The results revealed that α/β- mixed crystalline iron oxyhydroxide (MIX-FeOOH) had better adsorption performance for As(V) and phenylarsonic acid (PAA) in water, with the adsorption capacity 71.4 and 54.7 mg g -1 at 50 mg L -1 equilibrium concentration, respectively. The adsorption mechanism was proved to be inner-sphere complexation, electrostatic interaction, and hydrogen bonding. Meanwhile, the oxygen vacancies on FeOOH could increase the isoelectric point and further promote the adsorption capacity through inner-sphere complexation. In arsenic contaminated soil, when the addition amount of MIX-FeOOH was 5%, the bioavailability of arsenic in As(V) and PAA contaminated soil was significantly reduced after 28 days, and the stabilization rate reached 77.2% and 76.5%, respectively. After 7 days of remediation, 17.1% and 11.9% of the most mobile portions of As(V) and PAA could be converted into poorly mobile portions, respectively. The stabilization mechanism includes inner-sphere complexation, mineral adsorption, and coprecipitation. In summary, this study can provide technical support for the remediation practice of arsenic-containing warfare agent contaminated sites., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)