Abstract
Recent studies point out the fact that most of the conventional municipal wastewater treatment plants are not designed to deal with new emerging pollutants (such as antibiotics) as these are still prevalent in the effluent and hence accumulate in the environment. Therefore, there is an urgent need for innovative and effective methods that will result in the complete or efficient removal of organic pollutants with only a few or no drawbacks. Membrane-based technologies have shown promising results in water treatment. The use of photocatalysts and biocatalysts has also proved to be effective in the removal of organic pollutants. This research therefore presents the fabrication of a photo-enzyme integrated (P-g-C3N4/laccase) hollow fibre membrane for the removal of ciprofloxacin and sulfamethoxazole. Graphitic carbon nitride (g-C3N4) was prepared by calcination of melamine in a muffle furnace at a temperature of 600ºC. The doping of g-C3N4 with phosphorus was carried out using different compositions of phosphoric acid (2%, 4%, and 6% wt.%).
The composite samples were characterised using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), and ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS). Photocatalytic degradation studies of ciprofloxacin and sulfamethoxazole were also performed and monitored using UV-Vis and liquid chromatography coupled with mass spectrometry (LC-MS). The optimum composite, i.e. 2%P-g-C3N4 was incorporated into HPEI/PES together with laccase enzyme from Rhus vernicifera. The wet-spinning technique for the phase inversion method was utilised to fabricate a multifunctional photo-enzyme integrated membrane (P-g-C3N4/Lac/HPEI/PES). The properties and structure of the prepared hollow fibre membranes were evaluated using contact angle analysis, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), correlative light electron microscopy (CLEM), atomic force microscopy (AFM), tensile strength, water-intake capacity, and pure water flux.
The EDX analysis displayed the presence of C, O, and N for pristine g-C3N4. After doping, the presence of C, O, N, and P was detected which confirmed the doping of pristine g-C3N4 with phosphorus. The UV-Vis DRS analysis showed a shift from 2.70
viii
eV to 2.48 eV in the band gap after doping g-C3N4 with phosphorus. The degradation of sulfamethoxazole by P-g-C3N4 was found to be significantly higher (70%) in contrast to that of the pristine g-C3N4 which was just over 50%. For ciprofloxacin, the degradation by P-g-C3N4 improved slightly to 60% versus 50% for the pristine g-C3N4. The degradation process was accompanied by the production of degradation products such as anthralin acid (m/z 307) for ciprofloxacin and monohydroxylated I10 (m/z 269) for sulfamethoxazole. The modified multifunctional hollow fibre membranes showed increased surface roughness which contributed to the significantly higher water flux of 90 kg/m2·h in contrast to that of the pristine PES at 54 kg/m2·h. The hydrophilicity also improved after modification as the contact angle decreased from 72 ± 1.01 to 42 ± 2.26. The modified hollow fibre membranes showed an enhanced removal of ciprofloxacin (77%) and sulfamethoxazole (80%). Moreover, antifouling properties towards BSA fouling were FRR = 89%, Rr = 5%, Rir = 10% and Rt = 15%. Hence, this work proved the hypothesis that the combination of the individual system, enzymes, photocatalyst, and hollow fibre membrane could be a promising route in the treatment of wastewater.