Structure-driven photocatalytic degradation pathways of organic contaminants of emerging concern over TiO₂/CeO₂/g-C₃N₄: A molecular reactivity and life cycle perspective.

Organic contaminants of emerging concern (OCECs) exhibit diverse molecular structures that critically determine their environmental persistence and degradation pathways. Most photocatalysis research remains focused on catalyst design, with limited mechanistic interpretation at the molecular level. Herein, we investigate structure-reactivity-sustainability relationships in visible-light-driven photocatalytic degradation of indigo carmine (IC), tetracycline (TTC), and ibuprofen (IBU) using a recyclable TiO₂/CeO₂/g-C₃N₄ (TiCe-gCN) heterojunction photocatalyst. The ternary composite exhibited enhanced activity and stability attributed to superior charge separation. Degradation kinetics followed pseudo-first-order behavior with rate constants in the order IC (0.0397 min⁻¹) > TTC (0.0215 min⁻¹) > IBU (0.0157 min⁻¹), reflecting intrinsic molecular reactivity differences. Mechanistic interpretation was achieved by coupling LC-MS transformation product analysis with DFT-derived descriptors including condensed Fukui indices, electrostatic potential maps, and HOMO-LUMO energy gaps. The analysis revealed distinct structure-dependent degradation behaviors: IC exhibited delocalized radical susceptibility facilitating rapid oxidative bond cleavage; TTC displayed localized reactivity leading to sequential hydroxylation and ring-opening; and IBU showed localized radical accessibility at benzylic positions promoting oxidative dealkylation. This is further strengthened using EPR spin-trapping experiments, providing evidence on the charge carrier and reactive species that participate in the photocatalytic process. Beyond degradation performance, life cycle assessment (LCA) was conducted to evaluate the environmental sustainability of the TiCe-gCN system, identifying electricity consumption during catalyst synthesis as the dominant burden across all impact categories. Projected grid decarbonization could reduce global warming potential (GWP) by 54.9% by 2040, highlighting the critical role of energy infrastructure in determining the sustainability of photocatalytic water treatment.