Dual-Mode Synergy-Driven Free Radical Chain Reactions in Hard Self-Activated Paddy Soil

Abstract

The self-activation of oxygen to produce free radicals is inefficient in soils with low active iron and low microbial activity. While biochar and agricultural amendments are commonly employed, they fail to rapidly initiate and sustain free radical generation due to limited electron transfer ability. To address this challenge, a dual-mode synergistic mechanism was presented for boosting hydroxyl radical (•OH) production in hard self-activated paddy soil. This process depended on rapid oxygen activation and highly stable electron transfer via an Fe(0/II)/graphitized-biochar (Fe/G-biochar), enabling a significant enhancement in the •OH yield. During the first stage (12–24 h), oxygen activation was enhanced by the heterostructured Fe(0/II) species and the thin carbon layer in Fe/G-biochar, leading to rapid •OH generation. During the second period (36–96 h), highly stable •OH production was driven by microbial iron reduction, facilitated by the Fe feed from prior dissolution and the residual Fe/G-biochar geoconductor. Finally, metabolic marker genes of genera such as Methylocystis, Methanosarcina, and Kineosporia are associated with the G-biochar matrix-centered geoconductor function. In contrast, genera including Cellulomonas, Novosphingobium, and Burkholderia are linked to the operation of the insoluble Fe-based geoconductor function. This work provides a promising strategy to overcome the intrinsic limitations of self-activation of paddy soil.