Near-Surface Hydroxyl Radical Hotspots Mobilize Cadmium and Immobilize Arsenic during Paddy Soil Drainage

Alternating flooded (anoxic) and drained (oxic) conditions restructure redox chemistry in paddy soils, but how drainage shapes vertical distributions of reactive oxygen species (ROS) and metal mobility remains unclear. Using soil-slope incubations of three contaminated paddy soils, coupled with in situ ROS spatial imaging (IS-ROS-SI), diffusive gradients in thin films (DGT), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we resolved a transient oxidative front forming ∼1 to 5 cm below the water-soil interface during early drainage. Within this zone, steep O₂ gradients and rapid Fe(II) oxidation generated hydroxyl radical (•OH) hotspots that accelerated CdS oxidation, increasing Cd solubility by up to 2.6-fold, while concurrently oxidizing As(III) to As(V) and enhancing adsorption to Fe/Mn (oxyhydro)oxides, thereby reducing As solubility by as much as 48%. The Random Forest model identified pH, the O₂ penetration depth (aggregation-controlled), and mineral-bound Fe(II) speciation as the primary controls on •OH production. This mechanistic insight into ROS-driven redox transformations along vertical profiles reveals a dual role of drainage-induced •OH in enhancing Cd mobilization while suppressing As release, with implications for managing redox-sensitive contaminants and improving rice grain safety in contaminated paddy fields.