Enhanced Photocatalytic Cr(VI) Reduction in Aqueous Solution with Black TiO₂ Under Visible Light Irradiation

The photocatalytic reduction of hexavalent chromium (Cr(VI)) is an advanced method for remediating toxic chromium pollution in water. Cr(VI) is highly poisonous, carcinogenic, and water-soluble, making it a significant environmental and health concern. Photocatalysis offers a sustainable approach to reducing Cr(VI) to its less toxic and insoluble trivalent state (Cr(III)) using light energy and a photocatalyst. Titanium dioxide (TiO₂) is one of the most widely used photocatalysts due to its high catalytic efficiency, non-toxicity, chemical stability, resistance to photo corrosion, abundance, and cost-effectiveness properties. However, due to its wide bandgap (~3.2 eV), it can adequately work under UV light only and exhibit relatively less quantum efficiency. To overcome these limitations, black TiO₂ (BT) with oxygen vacancies, Ti³⁺ sites, enlarged surface area, and active sites may be considered as potential alternatives. Herein, BT was synthesized via chemical reduction using NaBH₄ as a reducing agent. In total, 40 mg photocatalytic efficiently reduced 60 ppm of Cr(VI) within 60 min of visible light (450 nm ) illumination at room temperature using EDTA-2Na (500 ppm) as a hole scavenger. The 1,5-Diphenylcarbazide (DPC) colorimetric method was utilized to detect the reduction of Cr(VI) to Cr(III). From UV-vis DRS spectra, a clear red shift was observed for BT compared to TiO₂, suggesting the improved light absorption capability and Tauc plot disclosed reduced energy bandgap (~1.5–2.5 eV), which further accelerated the transition of photo-generated electrons from the valance band to the conduction band.