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

Abstract

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.