Nonradical oxidation in persulfate activation by graphene-like nanosheets (GNS): Differentiating the contributions of singlet oxygen (¹O₂) and sorption-dependent electron transfer

Nonradical reactions induced by nanocarbon-driven peroxydisulfate (PDS) activation recently emerge a promising strategy of groundwater remediation or wastewater treatment, whereas the involved reaction pathways remain controversial. We here investigate and differentiate the two representative nonradical mechanisms of PDS activation on graphene-like nanosheets (GNS). The template-induced GNS exhibited a high porosity of over 1200 m² g⁻¹ and a few-layered turbostratic structure with graphitic microcrystals. The nonradical oxidation system (aqueous ¹O₂ and nonaqueous electron transfer) of GNS/PDS was highly-reactive, and outperformed graphene- or metal-based activators. Although singlet oxygen (¹O₂) was detected in the bulk solution, the contribution of ¹O₂ to the entire oxidation were limited (0.004–0.43% and ~6% at neutral and alkaline pH, respectively), determined by chemical probes and steady-state kinetics. Different to the reference product (benzoquinone) of phenol oxidation by photosensitive singlet oxygenation, intermediate product in GNS/PDS was benzoic acid which barely resulted from ¹O₂ oxidation. Furthermore, surface-confined electron transfer was the major oxidation pathway verified by electronic measurement. Surface adsorption rather than hydrophobic effect of organic substrates on GNS accelerated a two-electron transfer. The nonradical oxidation exhibited a dissociation constant (pKa)-dependent mechanism because of competitive adsorption with S₂O₈²⁻ anions. The findings facilitate a thorough understanding of carbon-catalyzed persulfate activation during in situ chemical oxidation (ISCO) and provide novel insights for the selective removal of aqueous organic contaminants in a nonradical manner.