Overlooked Roles and Transformation of Carbon-Centered Radicals Produced from Natural Organic Matter in a Thermally Activated Persulfate System

The presence and induced secondary reactions of natural organic matter (NOM) significantly affect the remediation efficacy of in situ chemical oxidation (ISCO) systems. However, it remains unclear how this process relates to organic radicals generated from reactions between the NOM and oxidants. The study, for the first time, reported the vital roles and transformation pathways of carbon-centered radicals (CCR^•) derived from NOM in activated persulfate (PS) systems. Results showed that both typical terrestrial/aquatic NOM isolates and collected NOM samples produced CCR^• by scavenging activated PS and greatly enhanced the dehalogenation performance under anoxic conditions. Under oxic conditions, newly formed CCR^• could be oxidized by O₂ and generate organic peroxide intermediates (ROO^•) to catalytically yield additional ^•OH without the involvement of PS. Nuclear magnetic resonance (NMR) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) results indicated that CCR^• predominantly formed from carboxyl and aliphatic structures instead of aromatics within NOM through hydrogen abstraction and decarboxylation reactions by SO₄^•- or ^•OH. Specific anoxic reactions (i.e., dehalogenation and intramolecular cross-coupling reactions) further promoted the transformation of CCR^• to more unsaturated and polymerized/condensed compounds. In contrast, oxic propagation of ROO^• enhanced bond breakage/ring cleavage and degradation of CCR^• due to the presence of additional ^•OH and self-decomposition. This study provides novel insights into the role of NOM and O₂ in ISCO and the development of engineered strategies for creating organic radicals capable of enhancing the remediation of specific contaminants and recovering organic carbon.