Key Roles of Iron (Oxyhydr)oxides and Organic Matter in Controlling the Mobility of Ni, Cr, and Pb in Redox-Fluctuating Basalt-Derived Paddy Soils

The mobility of nickel (Ni), chromium (Cr), and lead (Pb) in basalt-derived paddy soils under redox-fluctuating conditions governs their accumulation in rice. However, the influence of redox-induced changes in soil components, particularly iron (Fe) (oxyhydr)oxides and organic matter (OM), on the mobility of these metals remains poorly understood. This study revealed that Fe redox cycling serves as the fundamental driving force controlling the release and immobilization of Ni, Cr, and Pb. Fe(III) reduction and OM complexation under anoxic conditions increased metal mobility, while Fe(II) oxidation and OM stabilization under oxic conditions reduced metal mobility. A kinetic model revealed that Ni and Pb mobility was predominantly controlled by Fe cycling, with Pb showing the strongest coupling (81.5-93.5% influence). For Cr, although Fe cycling remained the underlying driver, a greater proportion (82.6-83.6%) was associated with OM interactions. These findings advance risk assessment and inform redox-based remediation strategies for contaminated paddy soils.