Solar-light-active mesoporous Cr-TiO₂ for photodegradation of spent wash: an in-depth study using QTOF LC-MS

A dark-coloured effluent called "spent wash" is generated as an unwanted product in sugarcane-based alcohol distilleries. Most distilleries discharge this effluent into soil or water without any treatment, causing water and soil pollution. Herein, we report chromium-doped TiO₂ (Cr-TiO₂) as a photocatalyst for the degradation of spent wash colour under natural sunlight. Cr-doped TiO₂ nanoparticles were prepared using an aqueous titanium peroxide-based sol-gel method with titanium isopropoxide as the Ti precursor and chromium nitrate as the Cr precursor. To observe the effect of dopant on sol-gel behaviour and physicochemical properties, the Cr concentration was varied in the range 0.5-5 wt%. The crystallization temperature and time were optimized to obtain the required phase of Cr-TiO₂. The physicochemical characteristics of the Cr-doped TiO₂ catalyst were determined using X-ray diffraction, FE-SEM, FETEM, TG, XPS, the Brunauer-Emmett-Teller (BET) method, FT-IR, Raman, PL, ICP-MS, and UV visible spectroscopy. A shift in the absorption edge of TiO₂ by doping with chromium suggested an increase in visible light absorption due to a decrease in the effective band gap. The application potential of the Cr-TiO₂ catalyst was studied in the degradation of sugar-based alcohol distillery waste under natural sunlight, and the results were compared with those of undoped TiO₂ and Degussa P25 TiO₂. Degradation of the spent wash solution was monitored using UV-visible, gel permeation chromatography (GPC), and QTOF LC-MS. GPC and LC-MS showed significant changes in the molecular weight of spent wash colour-forming compounds due to the degradation reaction. QTOF LC-MS analysis suggested that acids, alcohols, glucosides, ketones, lipids, peptides, and metabolites were oxidized to low-molecular-weight counterparts. From the results, 5% Cr-TiO₂ showed the highest degradation rate among all Cr-TiO₂ samples, undoped TiO₂, and Degussa P25 TiO₂ under identical reaction conditions, with nearly 68-70% degradation achieved in 5 h.