Hydrophobicity-driven kinetic modeling of sulfamethazine transformation by TiO₂/high-silica zeolite composites for...

The environmental risks of sulfamethazine (SMT) and its transformation products during photocatalytic treatment remain poorly understood, particularly regarding their phase-specific behavior and ecotoxicity. To address this, we developed a hydrophobicity-driven kinetic model to simulate the fate of SMT and its intermediates in a TiO₂/high-silica zeolite composite system. A novel quantification method was introduced, classifying transformation products into hydrophobic (I₁) and hydrophilic (I₂) groups based on their adsorption affinity. This allowed phase-specific tracking and kinetic modeling. Hydrophobic intermediates (I₁), sharing common structural traits, followed a unified adsorption isotherm and were efficiently retained by the zeolite. Hydrophilic intermediates (I₂), more prevalent in later stages, were more persistent due to weaker interactions with the composite and lower reactivity. Notably, decomposition rate constants for both I₁ and I₂ dropped sharply in the early stages, indicating preferential degradation of more reactive compounds, and later stabilized as less reactive species accumulated. The model incorporated Langmuir-Hinshelwood kinetics to account for competitive interactions in the aqueous phase and aligned well with experimental observations. In combination with the kinetic model, ECOSAR-based ecotoxicity analysis showed that the composite significantly reduced predicted algal toxicity compared to conventional TiO₂. This was mainly due to the retention of over 60 % of the more toxic hydrophobic intermediates in the solid phase. These results demonstrate the potential of TiO₂/high-silica zeolite composites and underscore the value of hydrophobicity-driven modeling in understanding transformation dynamics and reducing ecotoxicological risks in advanced water treatment.