Photocatalytic degradation of orange II sodium dye in textile wastewater using TiO₂-supported with activated carbon

Abstract

Azo dyes are the most used dyes in the textile industry. Azo dyes are also the most harmful in the environment due to their soaring toxicity, and bio-recalcitrance for common microbial wastewater treatment. Photocatalytic degradation using titanium dioxide (TiO₂) has been found effective in their treatment in wastewater. However, the large energy band gap of TiO₂ (2.80 - 3.20 eV) makes TiO₂ absorb less than 5 % in the visible spectrum. Supporting TiO₂ with a non-metal can enhance the photocatalytic activity of TiO₂ and allow the use of a larger amount of solar energy. A non-metal such as carbon is mostly used as photocatalyst support due to its stability, mechanical resistance, and elevated superficial area. In this study, the photocatalytic degradation process of orange II sodium dye was investigated by using TiO₂-supported biochar nanoparticles as photocatalyst. TiO₂-supported biochar composites were synthesised by an ultrasound process by mixing TiO₂ and biochar using the ratio 3:2 (60% biochar and 40% TiO₂). TiO₂ was synthesized from TiCl₃ via a hydrothermal process and biochar was obtained by carbonization of Acacia saligna (Port Jackson Willow) leaves. Biochar, TiO₂ and TiO₂-supported biochar nanoparticles were characterised by UV-Vis spectrophotometry, Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS), Fourier transformer infrared (FTIR), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET). Compared with the synthesised TiO₂ (band gap = 2.81 eV), the energy band gap of TiO₂-supported biochar composite was measured to be 2.11 eV, showing comparatively more promise as a solar active photocatalyst. Results from FTIR and SEM-EDS confirmed that TiO₂ was successfully immobilized on the biochar external surface. The BET results showed curves of TiO₂ and TiO₂ supported biochar composites exhibiting small hysteresis phenomena which represent a typical type-IV isotherm attributed to mesoporous material with low porosity. Furthermore, the XRD results revealed the presence of rutile and anatase crystalline phases in the TiO₂-supported biochar composites, due to the doping of biochar. The photocatalytic activity of TiO₂ and TiO₂-supported biochar composites were studied for the removal of orange II sodium dye. Batch experiments were conducted to obtain the optimum conditions for the dye degradation. The effect of parameters such as pH, photocatalyst loading and initial dye concentration was determined. From the results obtained in this study, the optimum conditions obtained were 120 min contact time for both nanoparticles using 200 mg/L for TiO₂-supported biochar at pH 6.8 and TiO₂ at pH 4, respectively. The highest degradation efficiency of orange II sodium was found to be 20.75 % using TiO₂ at pH 4 and 83.48 % using TiO₂-supported biochar at pH 6.8. The photocatalytic degradation of orange II sodium followed the pseudo-first-order kinetic (r² = 0.9914) with the lowest EEo of 136.49 kWh/m³ using TiO₂-supported biochar at pH 6.8; thus, reducing the process’ cost. TiO₂-supported biochar composites were found to be effective in the photocatalytic degradation of orange II sodium dye in a neutral solution. In addition, TiO₂-supported biochar composites showed good stability without any significant loss over three reusability cycles.