Molecular insights into phenolic polymerization during the transition from high-valent iron oxidation to electron transfer.

Polymerization has emerged as a promising pathway for total organic carbon (TOC) removal, yet the molecular identities of polymerization products and mechanisms governing their formation remain largely unexplored. Here, multi-walled carbon nanotubes (MWCNTs) were employed to induce a transition in ferrate (Fe(VI)) oxidation from a high-valent iron pathway to an electron transfer-mediated pathway during phenol degradation. Compared with the Fe(VI)-alone system, the Fe(VI)/MWCNTs system significantly improved both phenol degradation and TOC removal. Mechanistic investigations demonstrated that interfacial electron transfer facilitated polymerization reactions, resulting in the accumulation of abundant polymeric deposits on the MWCNT surface and consequently improving TOC removal. Furthermore, nontarget high-resolution tandem mass spectrometry was employed to characterize polymerization products. Up to 35 polymers were identified in the Fe(VI)/MWCNTs system, featuring higher oligomerization degrees (predominantly 4-8 units), dominant C-O linkages, and diverse cyclic structures. In contrast, only six oligomers were detected in the Fe(VI)-alone system, predominantly low-degree oligomers (mainly dimers and trimers) generated through C-C coupling. Meanwhile, electron transfer enabled more transformation routes, including hydroxylation, dehydrogenation, and dehydration condensation, thereby fundamentally promoting the formation of structurally diverse high-molecular-weight polymers. These findings highlight the necessity of clarifying polymeric products and pathways at the molecular level in polymerization-based oxidation processes.