DEET Degradation by Titanium Dioxide–Zeolite Nanocomposites: Effects of Aggregation State, Water Chemistry, and Natural Organic Matter

Immobilization of titanium dioxide nanoparticles (TiO₂ NPs) facilitates their removal and reuse in water treatment applications. Composite materials of electrostatically-bound TiO₂ NPs and zeolite particles have been proposed, but limited mechanistic studies are available on their performance in complex media. This study delineates the relative importance of homo- and heteroaggregation, water chemistry, and surface fouling by natural organic matter (NOM) on the photocatalytic degradation of diethyltoluamide (DEET) by TiO₂-zeolite composites. Zeolite adsorbs a portion of the DEET, rendering it unavailable for degradation; corrections for this adsorption depletion allowed appropriate comparison of the reactivity of the composites to the NPs alone. The TiO₂-zeolite composites showed enhanced DEET degradation in moderately hard water (MHW) compared to deionized water (DIW), likely attributable to the influence of HCO₃⁻, whereas a net decline in reactivity was observed for the TiO₂ NPs alone upon homoaggregation in MHW. The composites also better maintained reactivity in the presence of NOM in MHW, as removal of Ca²⁺ onto the zeolite mitigated fouling of the TiO₂ surface by NOM. However, NOM induced partial dissociation of the composites. DEET byproduct formation, identified by quadrupole–time of flight (QTOF) mass spectrometry, was generally unaffected by the zeolite, while NOM fouling favored de-ethylation over hydroxylation products. Overall, the most significant factor influencing TiO₂ reactivity toward DEET was NOM adsorption, followed by homoaggregation, electrolytes (here, MHW versus DIW), and heteroaggregation. These findings can inform a better understanding of NP reactivity in engineered water treatment applications.