Sustainable photocatalytic removal of cosmetic preservative parabens over a dual S-scheme TiO2/ZnO/g-C3N4 catalyst under natural sunlight irradiation

Abstract

Parabens, such as MeP or EtP, are common ingredients of widely used synthetic preservatives in cosmetics or personal care products. Extensive use of those products eventually leads to their persistence in aquatic environments, posing ecological and health risks. In the present work, a ternary dual S-scheme photocatalyst, TiO₂/ZnO/g-C₃N₄ (TZC), was rationally synthesized and comprehensively characterized using XRD, FTIR, FESEM, HRTEM, XPS, TGA, UV-vis DRS, PL, and N₂ sorption analyses. The results confirmed the successful construction of a well-coupled heterojunction with optimized band alignment, facilitating efficient charge transfer and visible-light utilization. Under sunlight irradiation, the TZC composite exhibited markedly enhanced photocatalytic activity, achieving 97.6% and 98.3% degradation of MeP and EtP, respectively, within 90 min of sunlight exposure. Kinetic analysis revealed first-order degradation behaviour, while radical quenching experiments identified ˙OH as the predominant reactive oxygen species responsible for the photocatalytic oxidation of parabens. The superior performance was attributed to the synergistic effects of the dual S-scheme charge transfer, which promoted the spatial separation of photogenerated charge carriers and preserved a strong redox potential. The heterogeneity test showed the catalyst was purely active in the solid phase and recyclable up to six cycles with a marginal decrease in efficiency of ∼8%. Furthermore, LC-MS analysis enabled the identification of intermediate degraded species, and a plausible degradation pathway was proposed. This study not only demonstrates the efficacy of a dual S-scheme TZC heterojunction in the solar-driven removal of cosmetic parabens but also provides mechanistic insights into its potential for sustainable water purification applications.