Ferrihydrite decreased the soluble As but increased the proportion of colloidal As in solution of paddy soils under redox change

Ferrihydrite, a metastable iron (Fe) mineral with high surface reactivity, is widely utilized for in situ arsenic (As) immobilization in soil remediation. However, its impact on the generation and composition of particulate As in paddy soils, especially under redox changes, remains poorly understood. This study examined the effects of exogenous ferrihydrite on the speciation, abundance and composition of colloidal As in four contaminated paddy soils during Fe transformation through reduction and subsequent re-oxidation. The findings indicated that exogenous ferrihydrite significantly facilitated the stabilization of solid-phase Fe and As, reducing As release into the solution by up to 91.9 % during these processes, particularly under anoxic conditions. Notably, ferrihydrite decreased the concentration of total As and concurrently colloidal As (10 kDa–450 nm), but significantly increased the proportion of colloidal As in the solution (< 450 nm) during both reduction and reoxidation periods. Further quantitative analysis revealed pH-dependent positive correlations between colloidal As and both Fe and organic matter (OM) across all soils during redox transitions. Relative weight analysis revealed that the formation of colloidal As was primarily controlled by the availability of total As in the solution, followed by the interaction between Fe and OM. This study offers quantitative insights into the abundance and composition of As-carrying particles in paddy soils under exogenous ferrihydrite and redox changes, highlighting the crucial role of nano-scale As–Fe–OM interactions in regulating As biogeochemical cycling. • Ferrihydrite promoted the transformation of solid Fe and As into more stable forms. • Ferrihydrite restrained As release from solid to solution under redox change. • Ferrihydrite decreased colloidal As concentration but increased its proportion. • Colloidal Fe, OM and As in soil solution were significantly positively correlated.