Natural organic matter adsorption conditions influence photocatalytic reaction pathways of phosphate-treated titanium dioxide nanoparticles

Abstract

Titanium dioxide (TiO₂) nanoparticles have been widely studied for water treatment applications; however, natural organic matter (NOM) is often reported to hamper the efficiency of the nanoparticles toward the degradation of target pollutants. Phosphate treatment has been proposed as a potentially facile solution to this problem, as phosphate competes for TiO₂ surface sites to diminish the NOM adsorption. However, the potential importance of the conditions of the NOM exposure and the residual NOM remaining after phosphate treatment have not been fully explored. Here, we investigate the reactivity of phosphate-treated TiO₂ nanoparticles with NOM coatings adsorbed from two background water chemistries, deionized water (TiO₂–NOM_DIW) and moderately hard water (TiO₂–NOM_MHW). Thorough characterization by size exclusion chromatography revealed that the adsorbed NOM was only partially displaced after phosphate treatment, with a higher adsorbed mass and wider variety of NOM species persisting on TiO₂–NOM_MHW compared to TiO₂–NOM_DIW. Although the remaining adsorbed NOM did not significantly influence the degradation rate of phenol as a model pollutant, remarkably distinct effects were observed in the degradation of catechol as an oxidative byproduct of phenol, with TiO₂–NOM_MHW hindering catechol degradation and TiO₂–NOM_DIW accelerating catechol degradation. The suppressed reactivity for TiO₂–NOM_MHW was attributed to hindrance of the physical adsorption of catechol to the TiO₂ surface by the NOM_MHW layer as well as changes in the reactive oxygen species profile as measured by electron paramagnetic resonance (EPR) spectroscopy, whereas the enhanced reactivity for TiO₂–NOM_DIW was attributed to higher hole formation, suggesting participation of the NOM_DIW layer in electron transfer processes. This research highlights the critical importance of the NOM surface coating in directing the mechanisms for pollutant degradation in photocatalytic nano-enabled water treatment applications.