Utilization of tannery wastewater as a source of Cr doped into TiO₂ for improving its activity under visible light in the Congo red degradation

The use of the Cr contained in tannery wastewater as a dopant for improving TiO2 activity under visible light has been studied systematically in the present paper. The doping was conducted by reacting titanium tetra-isopropoxide (TTIP) as TiO2 precursor and Cr-containing tannery wastewater through the sol-gel method. The concentration of Cr in the wastewater was varied giving mole ratio of TiO2 to Cr as 1:0.25, 1:0.5, and 1:1. The Cr-doped TiO2 photocatalysts obtained were characterized by UV/Vis diffuse reflectance, Fourier Transform infrared, X-ray diffraction, X-ray fluoresence, and Scanning electron microscopy machines. The effect of the Cr doping on the TiO2 activity under visible light was evaluated through Congo red dye photodegradation by batch experiment. The results reveal that Cr doping on TiO2 has successfully proceeded which can reduce the gap energy shifting into the visible region, and further enhance TiO2 activity under visible light. The interesting result is also found that doping TiO2 with Cr by the mole ratio of TiO2 to Cr = 1:1, has converted TiO2 into CrTi2O5 crystal that also shows a photocatalyst character. The most effective degradation of 10 mg/L Congo red in 50 mL solution which is about 98%, can be obtained by applying 20 mg of TiO2-Cr(1:0.5) photocatalyst weight, solution pH at 5 in 60 min. It is clearly inferred that the toxic Cr in the leather tannery wastewater can be utilized to form a more valuable material, preventing environmental pollution by Congo red dye.