Synthesis, Characterization, and Photocatalytic Activity of Titanium Dioxide/Reduced Graphene Oxide Nanocomposites for Diclofenac Degradation: Process Optimization via Response Surface Methodology

Abstract

Water pollution has emerged as a significant contemporary issue, particularly concerning pharmaceutical compounds, which have posed severe challenges to public health and the broader ecosystem. In this study, the synthesis, characterization, and photocatalytic activity of titanium dioxide/reduced graphene oxide (TiO2/rGO) nanocomposites were investigated for the degradation of diclofenac (DCF), a prominent pharmaceutical pollutant, in aqueous solutions under solar light. The nanocomposites were synthesized using a hydrothermal method with varying rGO content (1,2,3,4,6 wt.%), and their crystalline structures, morphological features, optical properties, surface compositions, and thermal stabilities were determined. The incorporation of rGO into the TiO2 matrix enhanced the visible-light absorption and reduced the bandgap energy from 3.15 eV (pristine TiO2) to 2.8 eV. The photocatalytic degradation of DCF was optimized using response surface methodology (RSM) with a central composite design (CCD), considering the catalyst dose, initial DCF concentration, and solution pH as the key operational parameters. The TiO2/rGO nanocomposite absorbed sunlight and generated electron-hole pairs, where rGO enhanced charge separation and extended light absorption, while TiO2 produced reactive oxygen species (center dot OH, center dot O-2(-), h(+)) that attacked DCF molecules. Sequential oxidation broke DCF into intermediate compounds, leading to complete mineralization into CO2, H2O, NH3, and Cl- ions through continuous radical-mediated oxidation processes, as confirmed by LC-MS analysis. The optimal TiO2/rGO(4%) nanocomposite (TG4) exhibited superior photocatalytic activity compared to pristine TiO2, achieving > 99% DCF degradation under optimal conditions (pH 5.5, catalyst dose of 150 mg/L, and initial DCF concentration of 6 mg/L). ANOVA identified pH as the most influential parameter, with a distinctive bell-shaped response curve centered at a pH of 5.5. The TG4 dosage had significant non-linear effects, whereas the initial DCF concentration exhibited an inverse relationship with the degradation efficiency. The linear decrease in total organic carbon (TOC) over time suggests the effective mineralization of DCF and its intermediate compounds. The nanocomposite remained efficient until the fourth cycle, indicating the reusability of the photocatalyst. A potential degradation pathway was also elucidated based on LC-MS analysis. This study highlights the potential of TiO2/rGO nanocomposites for the effective photocatalytic treatment of pharmaceutical-contaminated water.