1 Introduction
In the later decades, pollution of water sources with different species of organic pollutants was reported extensively as an associated result of the increase in chemical and pharmaceutical industries (Wang et al., 2020; Asadi-Ghalhari et al., 2022). Pharmaceutical residuals, petrochemicals, pesticides, dyes, surfactants, and drugs were reported extensively as essential organic pollutants that cause toxic and harmful effects on human, aquatic life, and wildlife (Mostafa et al., 2021; Abukhadra et al., 2022a). The discharged pharmaceutical residuals and their metabolite products attracted strong interest as agents causing hazardous environmental and health issues (Zhong et al., 2020; Fan et al., 2022; Tai et al., 2022). Commonly used antibiotics represent an essential category of the detected pharmaceutical residuals and organic pollutants in aqueous environments (Abukhadra et al., 2022a; Dhiman, 2022). Ciprofloxacin (CFX), levofloxacin, azithromycin, clarithromycin, and cefixime are the mostly detected antibiotic residuals in the aqueous environment and are associated with significant side effects according to the WHO (2017) (Rocha et al., 2017; Asadi-Ghalhari et al., 2022).
Ciprofloxacin (C17H18FN3O3) (CFX) is one of the fluoroquinolone analog antibiotics applied widely during the treatment of infectious diseases (bone, respiratory system, and gastrointestinal tract) and as an antibacterial agent against Gram-negative and Gram-positive bacilli (Abukhadra et al., 2022a; Falyouna et al., 2022; Guo et al., 2022). CFX exhibits poor metabolic stability, and about 75% of the delivered dosage in the animal or human body get rid of parent compounds (Abukhadra et al., 2020; Asadi-Ghalhari et al., 2022). Therefore, it is widely detected in the sewage systems of urban regions and hospitals at concentrations up to 150 μg/L and 30 mg/L in the discharged effluents of pharmaceutical factories (Peng Sun et al., 2009; Asadi-Ghalhari et al., 2022; Parmar and Srivastava, 2022). The existence of CFX in soil, water, and food chains is associated with remarkable health side effects such as kidney failure, fatty liver, vomiting, nausea, shivering, headache, and diarrhea (Zhang et al., 2021; Asadi-Ghalhari et al., 2022; Falyouna et al., 2022). Additionally, CFX residuals exhibit considerable catastrophic effects in supporting the resistance of common pathogens against the application of antibiotics (Xiang et al., 2020; Al-Musaw et al., 2021). Regarding its impact on the aquatic ecosystem, the CFX molecules cause perturbation in the nitrogen cycle as well as the translation and replication of chloroplasts in addition to its strong inhabitation impacts on the photosynthesis system, which negatively affects the growth rate of algae (Yin et al., 2017; Xiang et al., 2020; Falyouna et al., 2022). Therefore, the elimination of antibiotics as CFX effectively from the aqueous environments and the water supplies is a significant environmental challenge and a hot research topic.
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Ozonation, advanced oxidation, adsorption, nano-filtration, membrane separation, ultrafiltration, ion exchange, flocculation, biological degradation, and coagulation are well-known remediation methods of organic compounds including pharmaceutical residuals (Grisales-Cifuentes et al., 2021; Abukhadra et al., 2022a). However, the adsorption elimination of the drug and pharmaceutical residuals was recommended to avoid the toxic properties of the resulting intermediate compound during oxidation and degradation (Boral et al., 2021; Grisales-Cifuentes et al., 2021; Salam et al., 2022). Moreover, the adsorption removal of pharmaceutical residuals is a simple, affordable, effective, safe, available, and reusable method at the industrial scale (Wang et al., 2020; Boral et al., 2021; Mostafa et al., 2021). However, there are several factors that control the selection of the appropriate absorbent, such as the production cost, fabrication procedures, availability of its precursors, adsorption capacity, reusability, kinetic rate, biodegradability, mechanical stability, surface reactivity, adsorption affinity, and environmental safety (Jiang et al., 2020; Abukhadra et al., 2022a). Therefore, innovative adsorbents based on natural raw materials that exhibit high availability and low cost were evaluated widely in the later periods, especially the carbonaceous or carbon-based structures (Giraldo et al., 2020; Ramirez et al., 2020; Boral et al., 2021; Grisales-Cifuentes et al., 2021).
Recently, most of the common ranks of natural coal were assessed as potentially low-cost and effective adsorbents of different species of synthetic dyes (Simate et al., 2016; Shaban et al., 2017; Surip et al., 2020). This demonstrates the activity of the coal chemical groups to act as adsorption centers for different organic molecules. As a mineralogical term, coal refers to an organic-rich sedimentary rock containing different macerals (cellulose, lignite, and resin) and inorganic impurities (Yu et al., 2018; Tang et al., 2019). As a chemical term, natural coal can be identified as having a series of aromatic polycyclic hydrocarbons in which the structural aromatic rings connect with different forms of oxygenated chemical groups (carbonyl, hydroxyl, phenolic, and carboxyl groups) that show significant adsorption activity (Kurková et al., 2004; Flores et al., 2019; Fonseca et al., 2020). Recently, some studies were carried out to enhance the surface activity and physicochemical properties of coal by different chemical and physical methods (Shaban et al., 2017; Ibrahim et al., 2021). This involved the chemical activation, thermal activation, demineralization, and metal oxide surface decoration of the coal structure (Simate et al., 2016; Shaban et al., 2017; Abukhadra et al., 2022a).
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The previous studies reported remarkable enhancement effects of chemical or surficial modification processes on the properties of carbon-based materials. This includes activation of essential chemical groups, incorporation of new active chemical groups (oxygenated groups), and enhancement in the surface area (Jawad et al., 2018; Surip et al., 2020; Abukhadra et al., 2022b). Oxidation of coal by sulfuric acid was signified as a promising chemical activation method that enhances the electronegativity of its surface and induces the incorporation of additional active oxygenated groups into its structure (Jawad et al., 2018; Abukhadra et al., 2022b). Moreover, the modification of coal by sulfuric acid is a cost-effective process and can be performed by a single modification or activation step (Abukhadra et al., 2022b). Also, the previous studies demonstrated a significant effect of the modification of coal by nitric acid as low-cost oxidation reactions on the organic structure of coal by enhancing its hydrophilicity and providing the structure with numerous oxygen-containing functional groups (Shi et al., 2012; Elkady et al., 2020).
However, the efficiency of acid oxidation or modification of coal is controlled by the oxidation conditions, chemical activators used, and the concentrations of the used acid (Shi et al., 2012; Fonseca et al., 2020; Ibrahim et al., 2021). Unfortunately, the adsorption properties of acid-oxidized coal have not been assessed in satisfactory studies considering the types of the used acid and the species of organic pollutants. Therefore, this study involved, for the first time, the production of two acid-oxidized coal samples by sulfuric acid (S.C) and nitric acid (N.C) as enhanced low-cost adsorbents for CFX residuals from the aqueous environments. The adsorption properties were followed considering the main experimental factors in addition to detailed theoretical kinetic and equilibrium studies. Equilibrium behavior was illustrated based on the steric parameters (active site’s density, number of adsorbed CFX per site, and theoretical saturation adsorption capacities) and energetic parameters (adsorption energies, enthalpy, internal energy, and entropy) to assess the impact of the modification processes on the surface of coal as adsorbents in terms of the adsorbent/pollutant interface.
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