Enhanced Adsorptive and Photocatalytic Degradation of Synthetic Melanoidin Using Silver-Doped TiO₂ Nanoparticles

Abstract

Herein, silver-doped titanium dioxide nanoparticles (Ag-TiO2 NPs) were synthesized via a solution based sol-gel method followed by calcination at 450 oC for 2 h and were utilized in the treatment of synthetic melanoidin solution (SMS). The structural and morphological characterization, conducted through X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR), and Ultraviolet-visible (UV–Vis) spectroscopy, confirmed the successful integration of silver within the TiO₂ matrix. Ag-TiO₂ NPs showed improved physicochemical properties, such as a reduced band gap of 2.8 eV, as compared to undoped TiO₂. When exposed to visible light (λ > 400 nm), Ag-TiO2 NPs decolorized SMS by more than 85% in 180 min at pH 7 and an initial melanoidin concentration of 200 mg/L. Also, chemical oxygen demand (COD) and total organic carbon (TOC) were significantly reduced to 60% and 40%. The conversion of recalcitrant melanoidin into more biodegradable byproducts was confirmed by the significant improvement in the biodegradability index (BOD5/COD) from 0.1 to 0.3. The stability of the catalyst was confirmed through reusability testing, color removal efficiency ranged between 85% to 75% after five cycles. According to kinetic studies, the principal mechanism of degradation is surface-mediated melanoidin adsorption followed by plasmon-induced charge separation, which promotes the production of reactive oxygen species and preserves catalytic performance by lowering surface fouling. Collectively, these findings underscore Ag-TiO2 NPs as a robust, reusable, and ecologically sustainable solute ion for large-scale melanoidin-rich industrial effluents.