Peroxydisulfate-Driven Reductive Dechlorination as Affected by Soil Constituents: Free Radical Formation and Conversion.

We report a previously unrecognized but efficient reductive degradation pathway in peroxydisulfate (PDS)-driven soil remediation. With supplements of naturally occurring low-molecular-weight organic acids (LMWOAs) in anaerobic biochar-activated PDS systems, degradation rates of 12 γ-hexachlorocyclohexanes (HCH)-spiked soils boosted from 40% without LMWOAs to a maximum of 99% with 1 mM malic acid. Structural analysis revealed that an increase in α-hydroxyl groups and a diminution in pKa1 values of LMWOAs facilitated the formation of reductive carboxyl anion radicals (COO•-) via electrophilic attack by SO4•-/•OH. Furthermore, degradation kinetics were strongly correlated with soil organic matter (SOM) contents than iron minerals. Combining a newly developed in situ fluorescence detector of reductive radicals with quenching experiments, we showed that for soils with high, medium, and low SOM contents, dominant reactive species switched from singlet oxygen/semiquinone radicals to SO4•-/•OH and then to COO•- (contribution increased from 30.8 to 66.7%), yielding superior HCH degradation. Validation experiments using SOM model compounds highlighted critical roles of redox-active moieties, such as phenolic - OH and quinones, in radical formation and conversion. Our study provides insights into environmental behaviors related to radical activation of persulfate in a broader soil horizon and inspiration for more advanced reduction technologies.

Methane-associated micro-ecological processes crucially improve the self-purification of lindane-polluted paddy soil.

Reductive dechlorination, an efficient pathway for complete removal of organic chlorinated pollutants (OCPs), is commonly reported to be coupled to oxidation of methane (CH4) or methanogenesis in anaerobic environments. However, the relationship between dechlorination and CH4-associated bioprocesses is unclear. Based on the hypothesis that CH4 supplementation could facilitate OCP dechlorination, we investigated the role of CH4-associated bioprocesses in the self-purification of flooded lindane-spiked paddy soils. Four treatments were conducted for up to 28 days: sterilized soil (S), sterilized soil + CH4 (SC), non-sterilized soil (NS), and non-sterilized soil + CH4 (NSC). Results indicated that both sterilization and addition of CH4 promoted lindane degradation and CH4 emissions in the flooded paddy soils. In the NS treatment, lindane had the lowest degradation rate when CH4 emissions were barely detected; while in the SC treatment, lindane had the highest degradation rate when CH4 achieved its highest emissions from anaerobic soil. Also, sterilization led to microbial diversity loss and functional recession, but increased ferrous ion [Fe(II)] concentrations compared to non-sterilized soils. Methanogenic communities and mcrA gene recovered faster than the majority of microorganisms (e.g., Fe bacteria, Bdellovibrionaceae, Rhizobiaceae, Dehalogenimonas) or functional genes (e.g., Dhc, Geo, narG, nirS). Collectively, we assume the enhanced removal of lindane may partly be due to both abiotic dechlorination promoted by chemical Fe redox processes and methanogenesis-derived biotic dechlorination. Revealing the coupling between dechlorination and CH4-associated bioprocesses is helpful to resolve both pollution remediation and mitigation of CH4 emissions in anaerobic contaminated sites.