Photophysical properties, antimicrobial activity, energy consumption and financial cost of recycling of TiO2 quantum dots via photomineralization processes of Vat Green dye and industrial dyeing effluents

This study synthesized three distinct variants of titanium dioxide quantum dots (TS) using an advanced precipitation method, fine-tuning the calcination temperatures to TS1 = 290 °C, TS2 = 300 °C, and TS3 = 350 °C. These modifications aimed to explore the relationship between calcination temperature and the material properties of the quantum dots. Comprehensive characterization techniques such as XPS, TEM, and XRD were employed. These analyses revealed high crystallinity and purity of the TS catalysts alongside their unique elongated morphological structure. Further, notable differences were found as the surface area, band gap energy, and crystallite size at these varying calcination temperatures were meticulously measured. Specifically, at 290 °C, the surface area was 357.14 m2/g, the band gap energy was 3.20 eV, and the crystallite size was 4.0 nm. At 300 °C, these values altered to 325.20 m2/g, 3.15 eV, and 5.0 nm, respectively, and at 350 °C, they further changed to 262.23 m2/g, 3.07 eV, and 6.5 nm. The photocatalytic efficacy of these quantum dots in degrading Vat Green dye, a common pollutant in industrial effluents, was investigated. Utilizing a 100-W xenon photoreactor, the photodegradation process was assessed under simulated and natural sunlight conditions. Intriguingly, a direct correlation was observed between the increase in crystallite size of the TS samples and a decrease in their photocatalytic activity. The effectiveness of this degradation process was evaluated by the COD limits prescribed by the environmental regulations of the Kingdom of Saudi Arabia. Additionally, the study was extended to include the sustainability aspect by recycling the TS samples up to 10 times and assessing their enduring efficacy using FTIR, TOC, and COD analyses. The study evaluated the photocatalytic process's energy consumption and financial costs, focusing on the mineralization efficiency of the Vat Green dye and its antibacterial activity against various bacteria, yeast, and fungi.