Insights into ROL-driven and ROS-mediated metalloid oxidation and sequestration in the soil-rice iron barrier system

This study investigated how reactive oxygen species (ROS) mediated by radial oxygen loss (ROL) function in arsenic (As) remediation through soil-rice iron barriers. Using high- and low-ROL rice varieties combined with zero-valent iron (ZVI) and steel slag amendments, As transformation pathways at soil-root interfaces were examined. ZVI increased soil amorphous Fe content by 22.8 %-49.3 % and iron plaque Fe by 59.9 %, while steel slag increased iron plaque weakly crystalline Fe by 62.4 %. Despite higher ROL, minimal As(III) oxidation occurred in rhizosphere compared to bulk soil due to counteracting effects of Fe(III)-reducing bacterial activity. However, substantial oxidation occurred on root surfaces, with As(V)/total As proportion reaching 59.3 %-74.3 %, attributed to hydroxyl radical accumulation driven by iron plaque thickening and amorphous Fe enrichment. In situ visualization using ROS-capturing membranes confirmed strong ROS accumulation specifically at root surfaces with negligible signals in surrounding soil. Path analysis established a positive relationship between hydroxyl radical production and As(V) proportion at iron plaque interfaces (path coefficient = 0.74, p < 0.01), which indirectly reduced grain inorganic As. These findings elucidate the critical role of localized ROS processes in As immobilization and provide a mechanistic foundation for developing effective rhizosphere-targeted remediation strategies in contaminated paddy systems.