SYNTHESIS AND CHARACTERISATION OF TiO2/ZnO NANOCOMPOSITES IMMOBILIZED ON KAOLIN FOR THE TREATMENT OF TANNERY WASTEWATER

Abstract

In this study, the adsorption effect on selected physicochemical parameters from tannery wastewater onto the beneficiated kaolin, kaolin/TiO2, kaolin/ZnO and kaolin/TiO2/ZnO nanocomposites was investigated by employing the batch adsorption technique. The effects of pH value on crystal size of TiO2 and ZnO nanoparticles prepared by sol-gel method were also examined. The kaolin/TiO2 (KT), kaolin/ZnO (KZ) and kaolin/TiO2/ZnO (KTZ11, KT21 and KTZ12) nanocomposites were prepared bywet impregnation methods. The prepared adsorbents were characterized using X-ray diffraction (XRD), nitrogen gas adsorption-desorption Brunauer-Emmett-Teller (BET), Fourier transform infrared (FTIR), High Resolution Scanning Electron Microscopy (HRSEM), Energy Dispersive X-ray spectroscopy (EDX), High Resolution Transmission Electron Microscopy (HRTEM) and X-ray Photoelectron Spectroscopy (XPS). The results of the characterization showed the morphology, phase identification, surface area, functional groups as well as the adsorption process involved of the nanoadsorbents. From XRD patterns, it was found that the crystallite size of the synthesized TiO2 at pH 4-12 and ZnO at pH 6-12 calcined at 450 oC were from 5.67 to 15.02 nm and 11.84 to 24.82 nm respectively. The kaolinite (plate-like), anatase (tetragonal) and zincite (hexagonal) shapes of kaolin, TiO2 and ZnO were further confirmed by the HRSEM and HRTEM. The BET results showed that the surface areas of kaolin/TiO2 and kaolin/ZnO nanocomposites increased compared to the surface area of the beneficiated kaolin. The XRD and HRTEM results demonstrated that the TiO2 and ZnO nanoparticles were distributed within the kaolin framework. The EDX analysis also established that the loading correlated to the anchored different ratios of the nanoparticles on the beneficiated kaolin by wet impregnation method. The adsorption method was established to investigate the influence of some experimental parameters such as contact time, adsorbent dosage and temperature. The adsorption isotherm studies were investigated at different temperatures (30-70 oC). The adsorption data were fitted to Jovanovic, Halsey, Flory-Huggins and Redlich-Peterson isotherm models. The results indicated that Jovanovic isotherm model best fitted the data in the temperature range studied with correlation coefficient, R2 > 0.999. However, the Flory-Huggins adsorption isotherm explained the spontaneous adsorption system. The experimental results at different contact times were subjected to kinetic adsorption models (fractional power, Bangham and Avrami) and adsorption mechanism determinations (Boyd and intra-particle diffusion). The results showed that the kinetic adsorption process followed Banghham kinetic model. The mechanism for the adsorption of tannery wastewater onto the adsorbents indicates that external mass transfer is the rate-determining step since the plots of Boyd and intra-particle diffusion did not pass through the origin. Thermodynamic studies showed that the adsorption system is temperature dependent since increase in temperature showed increase in the adsorption of the pollutants, indicating that the adsorptivity was endothermic and spontaneous. The values of the thermodynamic parameters; change in enthalpy (∆H) and change in entropy (∆S), calculated from Vant’s Hoff plots confirmed the spontaneity of the adsorption process. A mechanistic pathway for the synthesized nanocomposites was presented and the nanocomposite filter produced was resistant to temperature above 900 oC. The flow rate optimized by changing the proportion of kaolin and saw dust for the filter pot formulation was observed and this served as the controller of the flow rate and percentage porosity of the filter pot. The bactericidal effects of TiO2 and ZnO nanoparticles in the composites explained the nanoscale and physicochemical properties of the nanoparticles along with surface area of the kaolin. The filtrates using the nanocomposites filter pots gave concentrations of some investigated physicochemical parameters less than the WHO and NESREA standards. This illustrates that the filter pots nanocomposites could be a potentially safe and viable adsorbent in water filters for water purification systems.