Abstract
Environmental pollution by emerging organic contaminants poses a significant threat to the ecosystems, especially the aquatic environments. These organic contaminants are introduced into the aquatic environment via human excretion and as pharmaceutical waste, leading to their presence in wastewater treatment plants (WWTPs). The increased presence of these pharmaceutical drugs in the environment is due to their high demand in treating pandemic-causing diseases such as Human Immunodeficiency Virus and Acquired Immunodeficiency Syndrome (HIV/AIDS), coronavirus, and influenza viruses. The removal of organic contaminants by the WWTP has been reported worldwide. However, in South Africa, the traditional WWTPs are reported to be inefficient as studies reveal their presence in surface and underground water. Therefore, it is important to develop efficient, cost-effective and sustainable treatment technologies for the removal of these contaminants in conjunction with the WWTP. This study focused on synthesising and characterising advanced photocatalytic nanocomposites, specifically the CB[7]-TiNbCTx@α-Bi2O3/ZnSe (CTBZ) S-scheme heterojunction system. The materials were analysed using a combination of spectroscopic and microscopic techniques to evaluate their structural, optical, and photoelectrochemical properties.
The crystallinity of the synthesised pristine cubic ZnSe, monoclinic Bi2O3, α-Bi2O3/ZnSe, TiNbCTx@α-Bi2O3/ZnSe, CTBZ composites was confirmed through the co-existence of their crystal planes and bands as observed in X-ray diffraction (XRD), Selected Area Electron Diffraction (SAED), and Raman spectroscopy. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analyses validated the expected characteristic morphologies and formation of the interfaces in the heterostructure materials. Ultraviolet-Visible Diffuse Reflectance Spectroscopy (UV-Vis DRS) measurements indicated that α-Bi2O3 and ZnSe possess band gaps of 2.7 eV and 2.41 eV, respectively. The formation of an α-Bi2O3/ZnSe heterostructure and TiNbCTx@α-Bi2O3/ZnSe ternary system resulted in the bandgap red-shifting, suggesting enhanced optical properties. Additionally, the optoelectronic properties of the CTBZ samples exhibited excellent optical characteristics as well. Photoluminescence (PL) and photoelectrochemical studies
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demonstrated that CTBZ-7% effectively reduces the recombination of photoinduced charges more efficiently than other as-synthesised photocatalysts in this study. Photoelectrochemical evaluations demonstrated notable enhancements in charge separations of the composites, particularly in the quaternary S-scheme heterojunction CTBZ-7%. Linear sweep voltammetry (LSV) and Nyquist's plots demonstrated a significant reduction in charge transfer resistance (84 Ω) following the addition of the double transition metal MXene - TiNbCTx, which led to improved charge carrier lifetimes (299 ms). The quaternary CTBZ-7% demonstrated enhanced optical and photoelectrochemical performance relative to other photocatalysts, indicating its potential for increased generation of reactive oxygen species essential for photocatalytic reactions. The incorporation of cucurbit[7]uril (CB[7]) significantly increased the electron-hole recombination rates and enhanced the redox capacity of the CB[7]-TiNbCTx@α-Bi2O3/ZnSe (CTBZ-7%) composite material.
The photocatalytic performance of α-Bi2O3, ZnSe, BZ, TBZ-7%, and CTBZ-7% was evaluated for the photodegradation of hydroxychloroquine (HCQ) in aqueous media under visible light irradiation. Optimum conditions occurred at pH 7, with catalyst loading of 20 mg and an HCQ concentration of 5 ppm. Under these conditions, the CTBZ-7% heterostructure achieved 92% overall HCQ degradation within 50 minutes, outperforming other photocatalysts due to the inclusion of the CB[7] macrocycle. CB[7] facilitated pollutant capture and proximity to the active sites. The kinetic rate constant for CTBZ-7% was 2.2x10-1 s-1, surpassing the rates of pristine (α-Bi2O3 and ZnSe), binary (BZ), and ternary composites (TBZ-7%). Superoxide radicals were identified as the primary reactive species, and CTBZ-7% demonstrated excellent stability over four degradation cycles. Liquid Chromatography-Mass Spectrometry (LC-MS) analysis revealed nine HCQ degradation fragments with lower m/z values, which have been reported to be removable from wastewater. These findings highlight the potential of CTBZ-7% for WWTPs and emphasise the advantages of visible-light-active photocatalysts for sustainable solar energy utilization.