Differentiation between hydroxyl radical generation and organic pollutants degradation in paddy soils oxygenation:...

Hydroxyl radicals (^•OH) is one of the prime reactive species for abiotic transformations of pollutants during redox oscillation in paddy wetland‌. However, the effect of O₂ diffusion rates in wetland on ^•OH generation, as well as its subsequent effects on pollutant removal, remains largely uninvestigated. This study reveals a significant promotion of ^•OH generation and nonphotochemical transformations of organic pollutants by controlling O₂ permeability rates combining in-situ experiments. Batch experiments confirmed that air supply rates determined the generation rates of ^•OH (7.7 ± 0.48-58.04 ± 5.01 μM h^-1), but did not alter the cumulative concentration of ^•OH upon complete oxidation of soil. Interestingly, despite no significant difference in cumulative ^•OH concentration among different air supply rates, higher air supply rates resulted in more efficient pollutant degradation. The presence of soil organic matter, especially dissolved organic matter (DOM), is a key cause of the hindered imidacloprid removal under low air supply rates. Coupled with Fourier transform ion cyclotron resonance mass spectrometry, we found that the lability of DOM increased during the oxidation period, which proved the potential reaction between DOM and ^•OH. In summary, our study links two previously isolated factors (O₂ supply and soil fractions) and enriches the understanding of multi-factor regulatory mechanisms of the contaminant fates in paddy fields. Importantly, increasing O₂ supply and transfer is low-cost and sustainable strategies for soil remediation in subsurface environments with frequent redox fluctuations.