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Enhanced Photocatalytic Cr(VI) Reduction in Aqueous Solution with Black TiO2 Under Visible Light Irradiation
* 1 , 1 , * 1 , 2 , 3 , 1
1  Department of Applied Chemistry, Mie University, Tsu, Mie 514-8507, Japan.
2  Environmental Preservation Center, Mie University, Tsu, Mie 514-8507, Japan
3  Department of Applied Chemistry, Mie University, Tsu, Mie 514-8507, Japan
Academic Editor: Ioannis Konstantinou

Abstract:

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.

Keywords: Black TiO2; Cr(VI) reduction; EDTA-2Na; 1,5-DPC colorimetric detection; Visible light irradiation
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