Development of photocatalytic ceramic membrane for the removal of pharmaceuticals from water

Abstract

The use of advanced hybrid wastewater treatment techniques in the elimination of emerging contaminants such as pharmaceuticals has recently attracted significant interest as a viable technique for industrial scale wastewater treatment. Advanced hybrid wastewater treatment techniques are based on the integration of different advanced wastewater treatment techniques such as membrane technology, advanced oxidation processes and bioremediation. Each approach addresses different aspects of contaminants removal, resulting in a robust system that enhances efficiency by minimizing the drawbacks of individual technique and enhancing their advantages. The aim of this study was to prepare a hybrid wastewater treatment technique involving photocatalysis and ceramic membrane technology and evaluate their efficiency in the removal of tetracycline as a model wastewater pollutant by utilising both photocatalysis and filtration processes. The main technical barriers hindering the application of photocatalysis at industrial scale are: (1) limited utilization of solar spectrum by most photocatalysts; (2) high electron and hole recombination rate, which lowers quantum efficiency and photocatalytic activity; (3) poor adsorption towards non-polar organic pollutants; (4) challenging recovery of the used nanoparticles from the uniform suspension; and (5) low thermal stability. Therefore, this work focused on the fabrication of surface defective heterojunction nanocomposites from TiO2 and ZnO in order to enhance the photocatalytic activity through improved light absorption capabilities and charge separation process. High energy requirements for sintering, high cost of starting materials, and limited effectiveness in pollutant mineralization are identified as significant drawbacks in the industrial application of ceramic membrane technology. Therefore, this study focussed on fabrication of affordable and sustainable ceramic membranes from coal fly ash, an industrial solid waste product, and natural sand, a commonly available and inexpensive natural material. Firstly, surface defective TiO2-ZnO heterojunction nanocomposite were synthesized using the conventional sol-gel approach, coupled with aluminium foil-assisted NaBH4 reduction. Titanium tetra-isopropoxide and zinc acetate were used as precursors at different molar ratios while isopropyl alcohol and water were used as solvents. Subsequently, ceramic membranes were fabricated by the uniaxial compaction of the starting material (coal fly ash, natural sand, dextrin (pore former) and carboxymethyl cellulose (binder)) using a manual hydraulic pressing machine. The coal fly ash and natural sand were mixed in different proportions and the fabricated ceramic membranes were sintered at different temperatures of 800, 100, and 1200 ℃. Finally, the novel TiO2-ZnO functionalized coal fly ash-based ceramic membrane was fabricated utilizing co-pressing and co-sintering processes. The photoactive layer was formed by placing the grounded mixture of pre weighed photocatalyst (TiO2-ZnO) and pre-weighed ceramic membrane precursor material on the mold's base, and the supporting layer was formed by adding pre-weighed ceramic membrane precursor material to the mold. The structure, morphology, surface properties, and optical characteristics of the synthesized materials were studied using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), UV-vis absorption spectra, Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), Thermogravimetric analysis (TGA) and Transmission electron microscopy (TEM). The viii properties of the optimized membranes were further studied to ascertain their mechanical strength, chemical stability, porosity, water absorption, water permeability, and shrinkage behaviour. The XRD analysis confirmed the successful formation of black BTiO2-ZnO heterostructure, while SEM revealed the structural morphology as pseudo-spherical with slight agglomeration. BTiO2-ZnO was found to be more efficient than BTiO2 and BZnO for the removal of tetracycline with degradation efficiencies of 63, 58, and 56 % respectively. The effects of process parameters such as the amount of photocatalyst, pollutant's concentration, and the initial solution pH on photocatalytic degradation study were systematically explored. The results confirm that the formation of the heterostructure from BTiO2 and BZnO could offer a facile route to improving the catalytic degradation of tetracycline. X-ray fluorescence (XRF) analysis of the starting materials (coal fly ash and natural sand) confirmed the presence of SiO2 and Al2O3, two key inorganic materials required for the fabrication of ceramic membranes. ICP OES analysis revealed that coal fly ash contains heavy metals, including arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and mercury (Hg). The levels of arsenic and chromium exceeded the WHO limits. Therefore, the heavy metals present in the raw coal fly ash were successfully leached out using acetic acid prior to the fabrication process. The membrane fabricated with 20 wt% sand content and at 1000 °C sintering temperature had pore size of 0.64 µm and 34.7% porosity, exhibited good mechanical strength (7.71 MPa), and exceptional chemical stability. The membrane also showed a remarkable water permeability of 32.23 L/m2 /h. The ceramic membrane functionalized with 0.5 g of TiO2 -ZnO and sintered at 850 °C exhibited good thermal, chemical stability, and porosity required for ultrafiltration applications. The optimum photocatalytic efficiency of the functionalized photocatalytic substrate achieved within 100 min under the visible light was estimated at 77% and removal efficiency by filtration was estimated at 57%. Moreover, the functionalized membrane was found to be stable with 73% photocatalytic degradation efficiency after 5 consecutive cycles of reusability test, with negligible loss of efficiency. In the filtration studies, tetracycline was simply adsorbed and filtered without degradation complicating desorption from surface-active areas. The scale-up of photocatalytic ceramic membranes and their utilization in real industrial applications will confirm their robustness when used in a hybrid photocatalysis and filtration reactor system that optimize its synergistic photocatalytic, adsorption, and filtration properties. In general, the use of advanced hybrid wastewater treatment techniques, specifically through the use of photocatalytic ceramic membranes fabricated from cost-effective starting materials, presents a promising and sustainable alternative for the removal of pharmaceuticals from water.