Abstract
Among the reported organic pollutants that have been detected in water systems,
tetracycline has gained a lot of awareness regarding its unintentional presence in
water. Tetracycline is an effective antibiotic that is extensively used for treating
bacterial infections found in animals and humans due to its low and cost broad
spectrum of activity. Tetracycline as an emerging pollutant it is not regulated meaning
there are no set limits for its presence in water. Moreover, complete removal of
tetracycline is not achieved by the existing traditional wastewater treatments. Hence,
there is a great need to explore new technologies that can effectively remove
tetracycline in the water system.
The advanced oxidation processes (AOPs) has recently been used to remove organic
pollutants in aquatic environments. This method is preferred because of the in-situ
formation of reactive radicals such as the hydroxyl radicals (.OH) and the superoxide
(.O2-) radicals that oxidise the pollutants into less toxic compounds such as carbon
dioxide CO2 and H2O. Therefore, this study will be focused on the development of a
photocatalytic membrane for the degradation of tetracycline in water system.
Graphitic carbon nitride/niobium oxide (g-C3N4/Nb2O5) heterostructure was prepared
via an in-situ hydrothermal method within a hyperbranched polyethyleneimine (HPEI)
template for the first time. This heterostructure was embedded on polyethersulfone
(PES) membrane via phase inversion method. The heterostructure was first
synthesised and characterized using Fourier Transform Infrared Spectroscopy
(FTIR), Raman spectroscopy, X-ray Diffraction Spectroscopy (XRD), Transmission
Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Ultraviolet-Visible
Diffuse Reflectance Spectroscopy (UV-Vis DRS) and photolumiscence studies (PL).
Water contact angle analysis, pure water flux and SEM were employed to determine
the hydrophilicity, water flux and morphological properties of the fabricated
nanocomposite membrane respectively. Liquid chromatography-mass spectrometer
(LC-MS) was employed to monitor the photodegraded by products. Furthermore, the
Total Organic Carbon (TOC) analysis was used determine the organic content of the
tetracycline solution before and after photodegradation. The structural and morphological properties showed the formation of orthorhombic
phase of Nb2O5 that consisted of well dispersed crystalline nanospheres having an
average diameter of 53 nm which coexisted with g-C3N4 in forming a heterostructure.
Photoluminescence data of the heterostructure revealed less recombination rate in
comparison with the pristine g-C3N4 and Nb2O5. Thus, revealing improved charge
separation of the g-C3N4/Nb2O5 heterostructure. The nanocomposite membranes
showed improved hydrophilicity from 60.3° up to 48.07° as compared to pure PES
(65.80°). Also, the addition of the heterostructure to the PES matrix was found to
change the morphological features of the nanocomposite membrane resulting in large
macrovoids and reduced pore size. Parameters that influence the degradation rate of
tetracycline were optimized, these included working pH and the effect of initial
concentration. From the analysis it was observed that tetracycline degrades better in
a solution concentration of 5 mg/L at the working pH of 10. An improved
photodegradation efficiency was observed for the modified membrane as compared
to the pristine PES membrane. Furthermore, the modified nanocomposite membrane
reached degradation efficiency of 88% as compared to 60% of PES, this degradation
was accompanied by the presence of 7 by products (tetracycline, carbonyl
oxycamptotheicin, 3-methoxycatchol, benzoate, 2,3-butadienoate and
methoxyacetylene) such as phenols and organic acids. The photodegradation of
tetracycline followed the first order kinetics.