Abstract
The irrefutable and fast progressing environmental deterioration predicaments due to human activities have become a severe global concern. The environmental deteriorations come in many different forms including air pollution, water pollution, soil pollution, and thermal pollution to name a few. In the recent years of emergence of large number of pharmaceutical manufacturing industries, water pollution as a result of pharmaceutical products has become a global calamity. The production of large amounts of pharmaceutical products is accelerated by the frequently emerging infectious diseases (EIDs), pandemics, and periodic infections. In the past few decades, highly contagious and deadly viral pandemics have been witnessed, and this includes the recent and ongoing COVID 19 and the Spanish flue (H1N1) experienced in the 1918 with records of 7 million and 50 million, deaths respectively on a global scale.
In an effort to develop various effective medicaments for infectious diseases, large amounts of medical wastes and pharmaceutical compounds are inevitably produced and discarded into the environment. This phenomenon leads to a rapid surge in contaminants of emerging concerns (CECs) with the potential to enter the aquatic ecosystem and pose ecotoxicological threats to human and animals. Antiviral drugs (ATVs) such as hydroxychloroquine (HCQ) and oseltamivir (OS) were amongst the repurposed drugs used to mitigate the severity of the infectious COVID 19. Moreover, the occurrence of these CECs such as the ATVs in the environment, specifically in aquatic milieu, pose concerning health issues related to the development of multi drug resistance infections.
Generally, different CECs including ATVs find their ways into wastewater treatment plants (WWTPs) through drainage and sewage system, where most of these pollutants disseminate into the environment. In most developing countries, the wastewater treatment facilities rely on the use of activated sludge systems for mass water purifications. These kinds of water treatment systems present poor removal efficiency when targeting pharmaceutical pollutants such as ATVs, which results in
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detectable traces of a plethora of potentially toxic bioactive pharmaceuticals in effluents leaving the treatment facilities.
Due to the health implications arising from the occurrences of ATVs such as HCQ and OS in drinking and environmental waters, it is essential to develop new sustainable and dependable paradigms for environmental remediation. In this work, an efficient advanced oxidation process (AOP) in the form of photocatalysis is developed and hyphenated onto a lab--scale conventional activated sludge wastewater treat system. This was designed for the removal of hydroxychloroquine sulfate and oseltamivir phosphate used as ATVs pollutants models in wastewater. The activated sludge and wastewater were collected from real conventional WWTPs, and then inoculated and acclimatized to simulated WWTP fed with synthetic wastewater. Acclimatization processes were conducted for 30 days at which the parameters such as mixed liquor suspended solids (MLSSs), mixed liquor volatile suspended solids (MLVSSs), sludge volume index (SVI), and chemical oxygen demand (COD) reached optimal targeted vales of 2500 mg.L--1 (both MLSSs and MLVSSs), 80 to 150 mL.g--1, and >80%, correspondingly.
In this work, efficient photocatalytic materials based on graphitic carbon nitride (g--C3N4) as a primary substrate have been fabricated, where a Schottky--junction photocatalysts was formed using different weight percentages (wt%) of Nb2CTx (1, 3, and 5 wt%) as a co--catalyst. The formation of these composites was in effort to improve light absorption ability and enhance charge carrier separations of the g--C3N4. Layered two--dimensional (2D) structure was formed between the Nb2CTx@g--C3N4, resulting in interfacial surface interactions. The as--prepared Nb2CTx@g--C3N4 was further functionalized with an acyl functionalized calix[4]arene (Cx--COCl) supramolecular complex to facilitate host--guest and sensitization effects on the prepared photocatalytic materials. The photocatalysts g--C3N4, Cx--COCl/g--C3N4, Nb2CTx@g--C3N4, and Cx--COCl/Nb2CTx@g--C3N4 were used in the photoreactor system for the photodegradation of biologically treated effluent.
The as--prepared materials were characterized using microscopic, spectroscopic, and chromatographic analytical techniques. The formation of the accordion--like
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structure of Nb2CTx MXene upon etching of Nb2AlC MAX phase was confirmed by the used of scanning electron microscopy (SEM). Furthermore, the presence of all the expected materials in the composites were confirmed using Fourier transform infrared (FT--IR) spectroscopy, energy dispersive spectroscopy (EDS), and x--ray photoelectron spectroscopy (XPS) analyses. High Brunauer--Emmett--Teller (BET) N2 adsorption--desorption surface area of 9.15 m2.g--1 was observed for the Nb2CTx@g--C3N4 compared to the 4.09 and 5.24 m2.g--1 surface are for the Nb2CTx MXene and g--C3N4, respectively. This was credited to the formed 2D--2D layered structure for the composite materials.
The optical and photoelectrochemical properties were assessed using ultraviolet--visible diffuse reflectance (UV--Vis DRS) and electrochemical impedance spectroscopy (EIS). UV--Vis DRS analysis for the pristine g--C3N4 presented an optical absorption edge at 480 nm which red--shifted upon the formation of 1, 3, and 5 wt% Nb2CTx@g--C3N4 MXene--based Schottky--junctions. Furthermore, the functionalization using the calix[4]arene complex (Cx--COCl) further enhanced the light absorption with an absorption edge at 574 nm for 5--Cx--COCl/Nb2CTx@g--C3N4 (containing 5wt% Nb2CTx and 10w% of Cx--COCl). Subsequently, a narrow energy bandgap of 1.79 eV was observed for 5--Cx--COCl/Nb2CTx@g--C3N4 compared to the 2.62 eV for g--C3N4.
Excellent photoelectrochemical properties were observed for the Cx--COCl/Nb2CTx@g--C3N4 composite materials in comparison with g--C3N4, Cx--COCl/g--C3N4, and Nb2CTx@g--C3N4 (composed of 1, 3, and 5 wt% Nb2CTx). Photocurrent density of 5--Cx--COCl/Nb2CTx@g--C3N4 was observed to be 2.3 folds higher than the pristine g--C3N4, which signifies improved charge separations for the composite materials. Low charge transfer resistance (RCT) value of 110.7 Ω.cm2 was observed for 5--Cx--COCl/Nb2CTx@g--C3N4 composite relative to the 825.5 Ω.cm2 observed for the pristine g--C3N4, validating the enhanced electrical conductivity for the composite materials. The improved optoelectronic and photoelectrochemical properties of the composite materials was ascribed to the introduction of Nb2CTx MXene as co--catalyst and the sensitization with Cx--COCl calix[4]arene complex.
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The removal of HCQ and OS from the simulated WWTP was conducted using the acclimatized activated sludge and the optimized photocatalytic reactor system. In a 250 mL working solution, a pH of 4, catalyst loading of 20 mg, and initial pollutant concentration of 5 mg.L--1 were obtained as optimum operating parameters for the photoreactor. The biological treatment in the hyphenated system resulted in biodegradation efficiency of 62.5 and 58.1% for HCQ and OS, respectively. Moreover, the photocatalytic reactor system increased the removal efficiencies to 97.3 and 92.6% for HCQ and OS respectively under visible light irradiation using 5--Cx--COCl/Nb2CTx@g--C3N4. The principal radicals towards the degradation of the pollutants were found to be e− and •O2− which facilitate reduction redox reactions. Excellent recyclabilities were observed for Cx--COCl/Nb2CTx@g--C3N4 photocatalyst towards the photodegradation of HCQ and OS over 5 cycles.
Liquid chromatography mass spectrometry (LC--MS/MS) was employed to study the by--products formed during the photodegradation of HCQ and OS. The pollutants HCQ and OS fragmented into relatively smaller organic molecules 2--aminobenzaldehyde and 3--hydroxycyclohexa--2,5--dien--1--ylium, respectively. The detected fragments exhibited smaller m/z vales signifying the effective decomposition of the parent molecules. This study demonstrates the benefits of combining biological and photocatalytic wastewater treatment technologies to maximize the efficacy of the treatment system, thus paving a way for incorporation of tertiary water treatment technologies in conventional WWTPs.