Overlooked\nRoles and Transformation of Carbon-Centered\nRadicals Produced from Natural Organic Matter in a Thermally Activated\nPersulfate System

The\npresence and induced secondary reactions of natural organic\nmatter (NOM) significantly affect the remediation efficacy of in situ\nchemical oxidation (ISCO) systems. However, it remains unclear how\nthis process relates to organic radicals generated from reactions\nbetween the NOM and oxidants. The study, for the first time, reported\nthe vital roles and transformation pathways of carbon-centered radicals\n(CCR<sup>•</sup>) derived from NOM in activated persulfate\n(PS) systems. Results showed that both typical terrestrial/aquatic\nNOM isolates and collected NOM samples produced CCR<sup>•</sup> by scavenging activated PS and greatly enhanced the dehalogenation\nperformance under anoxic conditions. Under oxic conditions, newly\nformed CCR<sup>•</sup> could be oxidized by O<sub>2</sub> and\ngenerate organic peroxide intermediates (ROO<sup>•</sup>) to\ncatalytically yield additional <sup>•</sup>OH without the involvement\nof PS. Nuclear magnetic resonance (NMR) and Fourier transform ion\ncyclotron resonance mass spectrometry (FT-ICR-MS) results indicated\nthat CCR<sup>•</sup> predominantly formed from carboxyl and\naliphatic structures instead of aromatics within NOM through hydrogen\nabstraction and decarboxylation reactions by SO<sub>4</sub><sup>•–</sup> or <sup>•</sup>OH. Specific anoxic reactions (i.e., dehalogenation\nand intramolecular cross-coupling reactions) further promoted the\ntransformation of CCR<sup>•</sup> to more unsaturated and polymerized/condensed\ncompounds. In contrast, oxic propagation of ROO<sup>•</sup> enhanced bond breakage/ring cleavage and degradation of CCR<sup>•</sup> due to the presence of additional <sup>•</sup>OH and self-decomposition. This study provides novel insights into\nthe role of NOM and O<sub>2</sub> in ISCO and the development of engineered\nstrategies for creating organic radicals capable of enhancing the\nremediation of specific contaminants and recovering organic carbon.