Integrated Adsorption-Photocatalytic Decontamination of Oxytetracycline from Wastewater Using S-Doped TiO₂/WS₂/Calcium Alginate Beads

Abstract

Integrated wastewater treatment processes are needed due to the inefficient removal of emerging pharmaceuticals by single methods. Herein, an adsorbent-photocatalyst integrated material was fabricated by coupling calcium alginate with sulfur-doped TiO₂/tungsten disulfide (S-TiO₂/WS₂/alginate beads) for the removal of oxytetracycline (OTC) from aqueous solution by an integrated adsorption-photocatalysis process. The semiconductor S-TiO₂/WS₂ hybrid photocatalyst was synthesized with a hydrothermal method, while the integrated adsorbent-photocatalyst S-TiO₂/WS₂/alginate beads were synthesized by blending S-TiO₂/WS₂ with sodium alginate using calcium chloride as a precipitating agent. The physicochemical characteristics of S-TiO₂/WS₂/alginate beads were analyzed using X-ray diffraction , scanning electron microscopy, elemental mapping, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy. The integrated adsorption-photocatalysis process showed enhanced removal from 92.5 to 72%, with a rise in the OTC concentration from 10 to 75 mg/L respectively. The results demonstrated that the adsorption of OTC onto S-TiO₂/WS₂/alginate beads followed the Elovich kinetic model and Redlich–Peterson isotherm models. The formations of H-bonds, cation bridge bonding, and n-π electron donor-acceptor forces were involved in the adsorption of OCT onto S-TiO₂/WS₂/alginate beads. In the integrated adsorption-photocatalysis, surface-adsorbed OTC molecules were readily decomposed by the photogenerated active radical species (h⁺, O₂^•−, and HO^•). The persulfate addition to the OTC solution further increased the photocatalysis efficacy due to the formation of additional oxidizing species (SO₄^•⁻, SO₄⁻). Moreover, S-TiO₂/WS₂/alginate beads showed favorable efficiency and sustainability in OTC removal, approaching 78.6% after five cycles. This integrated adsorption-photocatalysis process offered significant insight into improving efficiency and reusability in water treatment.

1. Introduction

With the discovery of penicillin by Alexander Fleming in the 1940s, there came a big revolution in therapeutic medicines [1]. Now, pharmaceutical companies are developing newer antibiotics for treating infectious diseases, leading to a dynamic entrance of antibiotics into all aspects of human and animal living [2,3]. However, the excessive use of antibiotics such as sulfamethoxazole, trimethoprim, ciprofloxacin, and oxytetracycline and their unethical disposal in the environment has led to the observance of antibiotic residues in soil, aquatic bodies, microorganism, animals, and human beings [4]. Among different antibiotics, oxytetracycline (OTC) is one of the largely used antibiotics owing to its affordability, availability, and efficiency [5,6]. In the report of Shen et al. [7], the concentration of OTC was reported to be 2.98 g kg⁻¹ in manures when its concentration was 8.76 mg kg⁻¹ in the feeds. Once released into the environment, these antibiotics are distributed into the air, water, soil, or sediment. Scientists are worried that OTC residues could produce resistant bacterial species and also trigger the production of antibiotic-resistant genes. These, when present in drinking water or irrigation water, gradually enter the food chain and can upset the human intestinal system’s microflora, leading to certain infections [8,9]. In addition to this, acute toxicity of OTC has been observed in small aquatic organisms such as water fleas, zebrafish, and goldfish [10]. OTC can also interfere with gene expression and affect the production of steroid hormones from cholesterol [11]. Its effect on the reproduction of fish has also been confirmed [12]. Thus, due to these potential high-risk dangers of OTC, its removal from the environment is a significant issue of concern.

Many techniques, such as membrane processes (reverse osmosis, nanofiltration, and ultrafiltration), adsorption, electrochemical, and photodegradation, are employed to remove antibiotics from water. However, most of these techniques have their respective demerits and advantages. A single process is either inefficient or time-consuming and expensive; therefore, an integrated approach may be effective, fast, and economical for removing pharmaceuticals. Integrated adsorption-photocatalysis could be a promising treatment for pharmaceuticals due to its high removal efficacy, low energy consumption, and environmental friendliness. With this integrated process, adsorption can help in the binding of pollutants to the surface of the material, while photocatalysis can decompose the adsorbed pollutants and regenerate material for further adsorption-photocatalysis [13,14]

Among photocatalytic materials, TiO₂ has been extensively studied owing to its chemical inertness, non-toxic nature, strong oxidizing potential, wide commercial availability, and high stability against photochemical corrosion [15,16]. TiO₂, owing to its wide band gap (≈3.2 eV) and high charge recombination rate, is only active in UV light; thus, considerable efforts are being made to modify it and make it active in visible light. Several strategies, such as the functionalization of TiO₂ with photosensitizers, doping with metal or non-metals, and fabrication of composites with polymers, metal oxide, or compounds that lower the band gap, are being employed. Guo et al. [17] reported that altering the surface bulk structure with heteroatoms is highly effective in band gap reduction of TiO₂, while Zhou et al. [18] showed that C-, N-, and S-doped TiO₂ is up to six times more photochemically efficient than pure TiO₂. Among these heteroatoms, sulfur can cause a significant change in the bulk structure owing to its larger ionic radii. It can be presumed that further reduction in the band gap of S-doped TiO₂ can be achieved using composite fabrication with metal, metal oxide, sulfides, polymers, etc. Among these, tungsten disulfide (WS₂), a type of transition metal dichalcogenide, has recently attracted attention owing to its unique structure of unsaturated atoms on the surfaces and edges, resulting in high photocatalytic activity with a wide spectral range, i.e., from near-infrared to UV, biocompatibility, biodegradability, and tunable morphology [19,20,21]. Fatima et al. [21] showed that pure WS₂ nanosheets efficiently degrade 90% of the antibiotic nitrofurantoin in 2 hrs. Similarly, Ali et al. [22] reported that WS₂ immobilized on chitosan and polycaprolactone is highly efficient at degrading neomycin antibiotics under UV light. Recently, it has been shown that non-metal-doped TiO₂ may behave as a p-type semiconductor. Thus, its combination with n-type WS₂ (band gap energy 1.5–2.8 eV), leading to a p-n heterojunction photocatalyst, effectively lowers the recombination rates of charge carriers [23]. From the above discussion, it can be concluded that the combination of S-TiO₂ and WS₂ will show high visible light absorption with a lower charge recombination rate and, thus, will be a highly efficient photocatalyst due to its synergistic and additional effects [24,25,26,27].

Although WS₂ is a promising catalyst, its hydrophobic and agglomerative nature may hinder WS₂ photocatalytic properties in wastewater [28]. Moreover, the complete recovery of the powdered S-TiO2/WS₂ catalyst from the solution might be a challenge, affecting reproducibility and cost-effectiveness. Several techniques such as polymer-catalyst film or supported catalyst on glass plates have been tried to overcome this problem [29]. However, these techniques often limit catalyst activity due to embedded catalysts, showing a much lower exposed catalyst surface for reaction. In this study, an S-TiO₂/WS₂ catalyst was incorporated into modified calcium alginate to fabricate porous macro-sized beads of S-TiO₂/WS₂/alginate, which can be easily recovered after reaction completion. Although calcium alginate is not a catalyst, it is a well-known biosorbent that has been widely investigated for the adsorption of pollutants [30]. In addition to this, it is expected that the hydrophilic nature of alginate will facilitate the movement of pollutants along with water molecules on all corners of the catalyst. The S-TiO₂/WS₂/alginate adsorbent-photocatalyst evaluated for OTC catalytic decomposition in natural sunlight showed much-enhanced degradation. In short, the novelty of this work is the design of S-TiO₂/WS₂/alginate adsorbent-photocatalyst beads.

2. Results and Discussion

2.1. Synthesis and Characterization

Figure 1 shows the schematic diagram for synthesizing the S-TiO₂/WS₂/alginate beads. Calcium-substituted sodium alginate was used as adsorbent support for the S-TiO₂/WS₂ photocatalyst for the degradation of the OTC pollutant. The hydrophilic nature of alginate adsorbs the dissolved contaminant, and thus the pollutant degradation will not only take place on the surface but also in the deeply embedded S-TiO₂/WS₂, thereby enhancing the photocatalytic efficiency. The catalyst anchored on or deeply embedded inside the alginate beads solves the problem of reproducibility, as it can be quickly recovered after the reaction for repeated use.

The SEM images of the calcium alginate (CA) beads, S-TiO₂/alginate beads, and S-TiO₂/WS₂/alginate beads are presented in Figure 2 (and in Figure S1). All the beads showed a roughly spherical morphology of sizable dimensions in the range of 1–1.5 mm. Pure CA has a distinct contracted/wrapped morphology with distinct folds of ~50 µm thickness (Figure 2a). It showed irregular spongy structures with compact pores and corrugations at higher magnifications. The topographical features showed only a single constituent of CA, thereby confirming the absence of any other impurities. In the case of the S-TiO₂/alginate and S-TiO₂/WS₂/alginate beads, the folds are less distinct at lower magnifications, and the morphology shows a bumpy appearance with embedded globules-like structures. For the S-TiO₂/alginate beads, small, scattered agglomerates of particles can be seen at higher magnifications, which are presumably TiO₂. In contrast, the S-TiO₂/WS₂/alginate beads show a much rougher appearance with the appearance of a much greater number of agglomerates, probably consisting of TiO₂ and WS₂. It can be presumed that most of the TiO₂ and WS₂ are embedded, and some agglomerates can also be seen on the surface.

The EDAX analysis of the calcium alginate beads shows the existence of C, O, and Ca; the S-TiO₂/alginate beads show C, O, Ca, and Ti; while the S-TiO₂/WS₂/alginate beads show the presence of C, O, Ca, Ti, W, and S (Figure 3). This confirms the successful replacement of Na with Ca in the sodium alginate during the reaction with CaCl₂. The existence of C, O, Ca, Ti, W, and S in the elemental mapping analysis also suggests efficacy of the synthesis methodology (Figure 4).

The XRD patterns of CA, S-TiO₂/alginate, and S-TiO₂/WS₂/alginate beads are presented in Figure 5. All the samples showed a broad peak in the region of 2θ of ~20° owing to the presence of amorphous calcium alginate. The S-TiO₂/alginate beads show the peaks of rutile and anatase phases of TiO₂. The anatase peaks appeared at 24.21° (101), 48.17° (004), and 62.9° (200), while the rutile phase peaks were observed at 28.11° (110), 36.09° (101), 45.34° (101), and 54.23° (211) [31]. The XRD spectra of the S-TiO₂/alginate and S-TiO₂/WS₂/alginate beads showed a similar pattern, displaying new peaks for the WS₂ at 2θ° of 13.95 (002), 33 (100), and 44.6 (006) [32]. However, the peak intensity of the TiO₂/WS₂ is low due to it being embedded in the polymeric matrix. Similar reports of highly diffused peaks of fillers in the polymer matrix have been previously reported [33].

The surface composition of the S-TiO₂/WS₂/alginate beads analyzed using XPS showed the presence of C1s, O1s, Ti2p, W4f, Ca2p, and S2p peaks corresponding to the presence of carbon, oxygen, titanium, tungsten, calcium, and sulfur (Figure 6). The atomic percentages of C, O, Ti, W, Ca, and S elements are 45.4, 42.6, 9.3, 2.3, 0.4, and 0.1%. The C1s spectra can be deconvoluted into three peaks at 284.72, 286.02, and 289.48 eV, corresponding to the (C–C), (C–O), and (COOH), which are obvious functionalities of alginate [34]. The high percentages of the C–C and C–O peaks compared to COOH are due to the major carbon skeleton in rings, C–O in rings and interconnections between rings, and the carbon bonded to the hydroxyl group. The O1s spectra consist of three peaks at 530.93, 532.84, and 535.36 eV corresponding to the oxygen of Ti–O bonds in TiO₂ or C=O, C–O, and associated water molecules, respectively [35,36,37]. The Ti 2p3/2 and Ti 2p1/2 peaks at 458.01 and 463.72 eV corresponds with the TiO₂ [38]. The presence of WS₂ is confirmed by the W 5p3/2, W 4f3/2, and W 4f7/2 peaks at 38.44, 35.15, and 33.03 eV, and the S 2p1/2 and S 2p3/2 peaks at 162 and 161.85 eV. The presence of Ca2p peaks at 352.42 and 356.03 eV and, subsequently, no observance of Na confirms the successful replacement of Ca with Na during the reaction of composite beads with the CaCl₂ aqueous solution.

The PL emission spectra of the CA beads, S-TiO₂/alginate beads, and S-TiO₂/WS₂/alginate beads are included in Figure 7. The PL spectra indicate the charge separation rate and migration efficiency of the charge carriers. The decrease in the PL intensity of the S-TiO₂/alginate beads compared to the CA beads and a further reduction in loading with WS₂ suggests the highest photocatalytic activity of the S-TiO₂/WS₂/alginate beads. The excitation wavelengths of the S-TiO₂/alginate and ternary S-TiO₂/WS₂/alginate beads fall in the visible light region, indicating their visible light activity. The peaks in area 450–500 nm is related to the charge recombination from the conduction band to the recombination center at the ground state. Thus, the lowest peak intensity of the S-TiO₂/WS₂/alginate beads suggests their lowest charge recombination and hence the longer lifetime of the photogenerated carrier [39].

2.2. Adsorption Studies

The adsorption of the OTC onto the S-TiO₂/WS₂/alginate beads was conducted as a function of adsorption time, OTC concentration, and solution pH. The equilibrium time analysis results are depicted in Figure 8a, and the plot demonstrates that initially, OTC was adsorbed quickly due to abundant vacant sites on the S-TiO₂/WS₂/alginate beads, but later the adsorption became slower due to the saturation of active sites, hence resulting in the attainment of equilibrium within 120 min. The results show that at equilibrium, the adsorption efficiency of the S-TiO₂/WS₂/alginate beads was 47 and 32.4% at 10 and 25 mg/L OTC concentrations, respectively. Moreover, to find the rate of OTC removal by the S-TiO₂/WS₂/alginate beads, the kinetic data were applied to the pseudo-first order, pseudo-second order, and Elovich models. The non-linear equations and the obtained data from the respective plots (Figure 8b) are depicted in

Table 1. Adsorption kinetics parameters for OTC onto S-TiO₂/WS₂/alginate beads.

Kinetic Model	Parameters	10 mg/L	25 mg/L
Pseudo-first order: $q_t=qe\left(1-e^{\left(-k_{1}t\right)}\right)$	qe (exp) (mg g−1): qe (cal) (mg g−1): k1(min−1): R2: RMSE: χ2	4.70 4.697 0.019 0.980 0.196 0.560	8.10 7.550 0.026 0.960 0.449 0.411
Pseudo-second order: $q_t=\frac{q_e^2{k}_{2}t}{\left[k_2\left(q_e\right)t+1\right]}$	qe (cal) (mg g−1): k2 (g mg−1 min−1): R2: RMSE: χ2	4.738 0.0036 0.987 0.156 0.269	8.889 0.0034 0.986 0.260 0.097
Elovich model: $q_t=\frac{1}{\beta }\ln \left(\alpha {\beta }_t\right)$	a (mg g−1 min−1): β (mg g−1): R2: RMSE: χ2	0.185 0.706 0.988 0.155 0.106	0.453 0.492 0.992 0.196 0.083

where q_e and q_t: OTC adsorption capacities at equilibrium and at time t (min), k₁ and k₂: Pseudo-first order and Pseudo-second order rate constants, α and β: adsorption and desorption rate constants for the Elovich model, χ²: chi-square, and RMSD: root-mean-square deviation.

. The estimated parameters from kinetics modeling showed that the calculated adsorption capacity (q_e^cal) for the pseudo-second order is much more consistent with the experimental adsorption capacity (q_e^exp) compared to the q_e^cal of the pseudo-first order. The best kinetic model describes the OTC adsorption rate onto the S-TiO₂/WS₂/alginate beads based on higher R² values, and the lower chi-square (χ²) and root-mean-square deviation (RMSD) are the Elovich model and pseudo-second order at both concentrations. The fitting of the Elovich model and pseudo-second order indicating the adsorption of OTC onto the S-TiO₂/WS₂/alginate beads was due to chemisorption and ion exchange [40]. These results reveal that the OTC adsorption rate onto S-TiO₂/WS₂/alginate beads decreases exponentially with an increase in the OTC amount on the adsorbent (heterogeneous system) [41].

The correlation between the OTC concentration and removal efficiency can be explored by fitting the experimental equilibrium data to the adsorption isotherm models. A detailed study of varying OTC concentrations from 10 to 150 mg/L is shown in Figure 9a. The adsorption efficiency of the S-TiO₂/WS₂/alginate beads gradually reduced with the rise in the OTC mass in the solution. The adsorption of OTC onto the S-TiO₂/WS₂/alginate beads decreased with the increase in OTC concentration due to the rise in the drug molecules in the solution. In contrast, the active site on the S-TiO₂/WS₂/alginate beads remained constant [42].

The OTC adsorption equilibrium data on the S-TiO₂/WS₂/alginate beads were further fitted to the non-linear Langmuir, Freundlich, Temkin, and Redlich–Peterson isotherm models, as reported in

Table 2. Adsorption isotherm parameters for OTC onto S-TiO₂/WS₂/alginate beads.

Isotherm Model	Parameters	Values
Langmuir $q_e=\frac{q_m{k}_L{C}_e}{1+k_L{C}_e}$	qm (mg g−1): KL (L mg−1): R2: RMSE: χ2:	27.692 0.023 0.938 1.413 1.602
Freundlich $q_e=k_F{C}_e^{\frac{1}{n}}$	n: Kf (mg g−1) (mg L−1)−1/nF: R2: RMSE: χ2:	2.102 2.175 0.989 0.575 0.161
Temkin $q_e=B_t \ln \left(K_t{C}_e\right)$	Bt (J mg−1) Kt (L mg−1): R2: RMSE: χ2:	497.485 0.375 0.936 1.441 1.018
Redlich–Peterson $q_e=\frac{K_{RP} C_e}{1+{\alpha }_{RP} C_e{}^{\beta }}$	$K_{RP}$ × 10−5 (L/g): ${\alpha }_{RP}$ × 10−5 (L/mg): β: R2: RMSE: χ2:	33.760 15.518 0.524 0.989 0.570 0.161

where C_e: equilibrium concentration, q_e: equilibrium capacity, q_m: monolayer adsorption capacity, k_F and 1/n: Freundlich constant related to adsorption intensity and adsorption capacity, K_t: equilibrium binding constant, B_t: heat of adsorption, $K_{RP}$ (L/g) and ${\alpha }_{RP}$: Redlich–Peterson isotherm constants, and β: exponent reflecting the heterogeneity of the beads.

. The isotherm parameters and error function values are obtained from the plots shown in Figure 9b. Based on the lower error functions RMSD, χ² values, and higher R² value, the Freundlich and Redlich–Peterson isotherm models are most suited to the OTC adsorption data on the S-TiO₂/WS₂/alginate beads. These isotherm modeling results demonstrate that the adsorption of the OTC occurred on the heterogeneous surface of the S-TiO₂/WS₂/alginate beads through combined Freundlich and Redlich–Peterson isotherm assumptions [43,44].

2.3. Photocatalysis

After identifying the adsorption of OTC, the degradation properties of the S-TiO₂/WS₂/alginate beads were investigated to find the effectiveness of the synthesized material as a catalyst under solar light irradiation. The OTC solution was prepared for adsorption onto the S-TiO₂/WS₂/alginate beads in the dark (90 min). Then, photocatalytic degradation of the OTC onto the S-TiO₂/WS₂/alginate beads was started in solar light irradiation. Figure 10a shows the OTC degradation at different concentrations. The photocatalysis reduced from 92.5 (C/C₀: 0.075) to 72% ((C/C₀: 0.28) as the OTC concentration increased from 10 to 75 mg/L. The results reveal that OTC decomposition reduces with an increase in the number of OTC molecules in the solution due to the rise in the solution density, which in turn reduces the interaction between the solar light and S-TiO₂/WS₂/alginate beads. Thus, the fewer active radicals produced are not enough to decompose all the OTC molecules. Moreover, active sites on the S-TiO₂/WS₂/alginate beads remained constant while the OTC molecules increased simultaneously with the increase in the initial concentrations [45].

To obtain further insights on the OTC degradation onto the S-TiO₂/WS₂/alginate beads at various concentrations, the first-order kinetic model [ln(C/C₀) = −k₁ t] was applied. The plot ln(C/C₀) vs. t for the first-order kinetics are depicted in Figure 10b. The values of the first-order rate constant k₁ are 0.0122, 0.0102, 0.007, and 0.0075 min⁻¹ at 10, 25, 50, and 75 mg/L concentration, respectively. The highest rate constant value of 0.0122 min⁻¹ demonstrated that the fastest degradation of OTC was at 10 mg/L concentration.

OTC shows different ionic forms at different solution pH; thus, a change in pH values can control the degradation process. Moreover, the difference in the S-TiO₂/WS₂/alginate beads surface charge with the change in the OTC solution pH may affect the degradation rate. The degradation of OTC at varying pH is shown in Figure 11a. As shown in Figure 11a, the OTC decomposition rose with the increase in the OTC solution pH, and optimum removal was observed at pH 6–7. This behavior of the OTC degradation onto the S-TiO₂/WS₂/alginate beads might be controlled by the different ionic forms of OTC, as its pKa values are 3.57, 7.49, and 9.88. In addition, OTC exists in negatively charged molecular form (OTC⁻) and zwitterionic (OTC^{+ −}) in a basic medium and neutral medium, while it appears positively charged in an acidic medium (OTC⁺) [46]. In acidic conditions, excessive H⁺ is present in the solution, which generates a positive charge on the OTC and S-TiO₂/WS₂/alginate beads; thus, there is an electrostatic repulsion between the OTC⁺ and protonated S-TiO₂/WS₂/alginate beads. Moreover, H₂O₂ may be produced under acidic conditions due to photogenerated ^•O₂ reacting with the h⁺. As a solution pH reaches neutral, a better interaction occurs between OTC and S-TiO₂/WS₂/alginate beads, leading to a higher OTC removal [47]. Although at a higher pH, both OTC and S-TiO₂/WS₂/alginate beads may have a negatively charged surface, and a reasonable photocatalysis decomposition of the OTC is observed. This might be due to the presence of ⁻OH, which helps in ^•OH radicals production in a basic medium [48,49].

The optimum pH for the OTC onto S-TiO₂/WS₂/alginate beads degradation rate can be determined by fitting the data to the first-order kinetic model. The plot ln(C/C₀) vs. t for the first-order kinetics is depicted in Figure 11b. The first-order rate constants k₁ are 0.0027, 0.0047, 0.0086, 0.0080 min⁻¹, and 0.0068 at pH 2, 4, 6, 7, and 9, respectively. The highest k₁ value was 0.0086 min⁻¹ at pH 6, indicating that pH 6 is the best condition to achieve the optimum photocatalysis of OTC.

2.4. Adsorption and Photocatalysis Mechanism

The interaction between the OTC molecules and S-TiO₂/WS₂/alginate beads is essential for its removal, either by adsorption or photocatalysis. Various functional groups on the surface of OTC and S-TiO₂/WS₂/alginate beads help in the interactions through several mechanisms such as H-bonding, electrostatic, n–π electron donor-acceptor, ion-exchange van der Waals forces, etc. However, the H-bonding and n–π electron donor-acceptor mechanisms are possible to a more considerable extent due to the presence of the double bond benzene ring, oxygen and nitrogen, sulfur-containing functional groups on the OTC molecules, and S-TiO₂/WS₂/alginate beads [50].

The adsorption of the OTC onto the S-TiO₂/WS₂/alginate beads was ~32.4% (at 25 mg/L conc.) at the equilibrium. The removal reached to ~82% after the illumination of the solution under solar light. These results demonstrate that active radical species were produced under solar light irradiation and oxidation-reduction of the OTC occurred. Calcium alginate is not a catalyst, but it helps in the binding of OTC molecules, which are simultaneously decomposed by the active radicals produced by S-TiO₂/WS₂. A detailed mechanism of the active radicals generation is shown in Figure 12. Initially, S-TiO₂/WS₂/alginate beads are exited after the absorption of photons and generate electron-hole pairs (e⁻/h⁺). The valance band (VB) and conduction band (CB) energies of the WS₂ are higher than the S-TiO₂ [51]. Therefore, the photogenerated e⁻ migrate from WS_2(CB) to S-TiO_2(CB) and convert O₂ into ^−•O₂ anionic radicals. Simultaneously, the photogenerated h⁺ migrate from S-TiO_2(VB) to WS_2(VB) and produce HO^• radicals after reacting with H₂O and OH. These O₂^−• and HO^• radicals decompose the S-TiO₂/WS₂/alginate beads’-surface-adsorbed OTC molecules [52]. The OTC degradation potential of the photogenerated active radical species may be different. Quenching experiments were performed to identify the highest OTC degradation contributing to the oxidizing radical species. The photogenerated electrons were captured by K₂S₂O₈ and EDTA-2Na was applied for h⁺ quenching, and HO^• radicals were eliminated with ethanol. The effect of the active radicals captures agents on OTC degradation is shown in Figure 13a. The OTC photocatalysis degradation onto the S-TiO₂/WS₂/alginate beads was ~82% in the absence of trapping agents. A significant reduction in OTC degradation of 68 and 64% in the presence of ethanol and EDTA-2Na was observed. The reduction in OTC degradation in the presence of ethanol and EDTA-2Na revealed that h⁺ and HO^• radicals were actively involved in the photocatalysis process. However, OTC degradation in the presence of K₂S₂O₈ increased up to 90%, indicating the involvement of another mechanism. K₂S₂O₈ is a well-known electron acceptor. The photogenerated electrons were trapped by K₂S₂O_8, which inhibits recombination with h⁺, and additional oxidizing species may produce (S₂O₈²⁻ + e⁻ → SO₄^•− + SO₄⁻) [53].

2.5. Stability and Reusability

The photocatalysis property of the S-TiO₂/WS₂/alginate beads was tested for up to five cycles to determine the stability and reusability under similar experimental conditions. The reusability efficiency of the S-TiO₂/WS₂/alginate beads is shown in Figure 13b. The results illustrate that the photocatalytic degradation efficiency of the freshly used S-TiO₂/WS₂/alginate beads material was 82.68 and changed to 78.6% after the five cycles. A slight reduction in efficacy reveals that S-TiO₂/WS₂/alginate beads are highly stable and maintain good photocatalytic activity after multiple uses.

2.6. Comparison of Adsorption and Photocatalytic Properties

We compared the adsorption and photocatalytic properties of the S-TiO₂/WS₂/alginate beads for OTC removal with previously reported research articles.

Table 3. Comparison of adsorption and photocatalytic properties of various materials for the removal of OTC.

Materials/(Light Source)	Adsorption (A) /Photocatalytic (P) Efficiency	Conditions						Ref.
		pH	Mass (g)	Time (min.)	Temp. (°C)	Conc. (mg/L)	Volume (mL)
POPD/TiO2/fly ash (VL)	73% (P)	-	0.10	60	30	10	50	[54]
Br(15%)/g-C3N4 (VL)	75% (P)	-	0.25	120	-	10	250	[55]
Co-B co-doped TiO2/SiO2 (VL)	37% (P)	-	0.05	100	-	5	5	[56]
N-ZnO/CdS/GO (VL)	50 % (P)	-	0.50	100	25	15	100	[57]
Co3O4/TiO2/GO (SL)	65% (P)		0.50	90		10	200	[58]
Nitrilotriacetic acid modified magnetic chitosan	337.5 (A)	8.0	0.25	350	25	100	50	[59]
Modified magnetic chitosan	92 mg/g (A)	8.0	0.25	350	25	100	50
Lanthanum modified magnetic humic acid	21.90 mg/g (A)	6.0	0.05	30	25	10	50	[60]
Hydroxyapatite/Aluminosilicates composite	57.39 mg/g (A)	5.0	0.25	400 h	35	10	250	[61]
GO functionalized magnetic particles	45 mg/g (A)	6.0	0.02	20	-	50	10	[62]
Co2+/F− co-doped TiO2-SiO2 (UV)	42% (P)	-	-	40	-	100	-	[63]
Fe3O4@C@TiO2	87.3 (A)	-	0.050	120	25	50	1000	[64]
S-TiO2/WS2/alginate bead (SL)	82 (P)	6	0.10	240	24	25	100	This study

where VL: visible light, SL: solar light, and UV: ultraviolet light.

shows that the adsorption of the OTC is not very high. However, integrated adsorption photocatalysis offers a significantly high removal of OTC from an aqueous solution. Moreover, experimental conditions play a vital role in the adsorption of OTC. The results in Table 3 show that the materials’ OTC removal efficiency changes as the experimental conditions change.

3. Materials and Methods

3.1. Chemicals

Na₂WO₄.2H₂O (98%) was obtained from Fisons scientific equipment (Loughborough, UK). TiCl₄ (99.9%), CaCl₂ (≥76.5%), and Na₂S,xH₂O (30% Na₂S) were obtained from Sigma-Aldrich (Saint Louis, MO, USA) and BDH Chemicals Ltd (Mumbai, India). Sodium alginate was received from Techno Pharma, Gujarat, India.

3.2. Synthesis of S-TiO₂/WS₂/Alginate Beads

The synthesis of S-TiO₂/WS₂/alginate beads was carried out in a three-step process: firstly, the synthesis of S-TiO₂, then S-TiO₂/WS₂, and finally, the fabrication of S-TiO₂/WS₂/alginate beads. With a typical procedure, 2.1 g of Na₂S was dissolved in 50 mL water in an ice bath and stirred for 30 min. Then, 5 mL of TiCl₄ was mixed in the Na₂S solution. Thereafter, 50 mL of 10 M NaOH was added dropwise to the above dispersion, and the precipitate was stirred for 3 h. The precipitate was heated for 24 h in a Teflon hydrothermal reactor at 125 °C. The white S-TiO₂ precipitate was centrifuged and washed several times with deionized water to remove the excess sodium ions and unreacted sulfur. Thereafter, the S-TiO₂ precipitate was washed a further three times with ethanol and acetone. The washed S-TiO₂ precipitate was dried at 80 °C for 16 h.

A binary S-TiO₂/WS₂ nanocomposite was synthesized using the following procedure. Initially, 1.33 g of Na₂WO₄.2H₂O was dissolved in 80 mL of water, and 2.27 g of S-TiO₂ was mixed and stirred for 30 min. After that, 2.251 g of Na₂S (dissolved in 20 mL of deionized water) was added, and the solution pH was fixed to 5.5. After 30 min of stirring, the solution was heated at 160 °C in a Teflon-lined hydrothermal vessel for 18 h. After the cooldown of the vessel, the S-TiO₂/WS₂ precipitate was centrifuged and washed with plenty of deionized water, followed by ethanol and acetone. The S-TiO₂/WS₂ was dried at 105 °C for 18 h. The dried S-TiO₂/WS₂ was further calcined at 600 °C for 3.5 h. A similar methodology was used to prepare WS₂ but in the absence of S-TiO₂.

Finally, the S-TiO₂/WS₂/alginate beads were prepared by stirring 0.30 g of S-TiO₂/WS₂ in 5 mL of water for 15 min, and 10 mL of sodium alginate (2% w/w in water) was added to it dropwise. The whole dispersion was left to stir for 30 min at 90 °C for the beads preparation. Finally, the beads were dropped in 10 mL of 2% w/w CaCl₂ aqueous solution to replace Na⁺ with Ca²⁺. A similar methodology was used to prepare the WS₂/alginate and S-TiO₂/alginate beads but in the absence of S-TiO₂ and WS₂, respectively. Based on the obtained beads yield, about 33% calcium alginate, 13% WS₂, and 54% S-TiO₂ were present in the S-TiO₂/WS₂/alginate beads.

3.3. Materials Characterization

Scanning electron microscopy (SEM) images of the CA, S-TiO₂/alginate, and S-TiO₂/WS₂/alginate beads were recorded with a JSM7600F, JEOL (Tokyo, Japan) in SEI mode at 10 kV. The X-ray photoelectron spectroscopy (XPS) analysis of the S-TiO₂/WS₂/alginate beads was performed with an ESCALAB 250, Thermo Fisher Scientific, Warrington, UK, at a monochromatized Al Kα X-ray source λ¼ 1486.6 eV. The X-ray diffraction (XRD) patterns of the CA, S-TiO₂/alginate, and S-TiO₂/WS₂/alginate beads were recorded with ALTIMA-IV, RIGAKU (Tokyo, Japan) spectroscopy. The photoluminescence (PL) spectra of CA, S-TiO₂/alginate, and S-TiO₂/WS₂/alginate beads were recorded with an RF-5301PC Shimadzu (Kyoto, Japan).

3.4. Adsorption and Photocatalytic Degradation of OTC

The adsorptive removal of the OTC onto prepared beads was investigated in the dark by mixing the 0.1 g beads in a 100 mL aqueous solution under continuous shaking conditions. The adsorption equilibrium time analysis was performed at 10 mg/L and 25 mg/L concentrations at pH 6 for a time range from 5 to 240 min. The effect of the OTC concentration was investigated for a concentration range from 10 mg/L to 50 mg/L at pH 6 for 180 min. After identifying the optimum adsorption conditions, integrated adsorption-photocatalysis experiments were conducted under solar light (intensity 710 ± 35 × 10³ LUX). Before starting the photocatalysis, the OTC solution and catalyst beads were left in the dark for 90 min, and then the solution was shifted to solar light to observe the photocatalysis. The OTC’s ongoing adsorption and photocatalytic degradation were observed using the HACH (Loveland, CO, USA) DR-6000 UV-visible spectrophotometer at 353 nm.

4. Conclusions

This work reports an integrated adsorption-photocatalysis process for removing OTC using S-TiO₂/WS₂/alginate beads. A facile method was used for the synthesis of S-TiO₂/WS₂/alginate beads. The results revealed that the OTC removal was controlled by various factors such as OTC concentration, reaction time, and solution pH. The adsorption equilibrium was established within 120 min, while the highest removal of the OTC was observed at pH 6 at 240 min of the interaction time with the S-TiO₂/WS₂/alginate beads in an integrated process. The adsorption kinetics data were best fitted to the Elovich model, suggesting that the OTC adsorption rate onto S-TiO₂/WS₂/alginate beads decreases exponentially with an increase in the OTC amount on the adsorbent (heterogeneous system). The non-linear adsorption isotherm modeling of the OTC equilibrium data indicated that the Redlich–Peterson model is the most suitable for explaining the interaction between the OTC and S-TiO₂/WS₂/alginate beads. Moreover, the results demonstrated that adsorption promoted OTC photocatalysis. As the OTC molecules are adsorbed onto the surface of S-TiO₂/WS₂/alginate beads, the photogenerated active radical species decompose the pollutant molecules simultaneously. The active radical scavenger study revealed that photocatalysis of the OTC reduced in the presence of the hole (EDTA-2Na) and hydroxyl radicals (ethanol) trapping agent. An enhancement in the OTC degradation rate was observed in the presence of an electron-capturing agent (K₂S₂O₈) due to the formation of extra oxidizing species (SO₄^•− and SO₄⁻). The reusability and stability study demonstrated that S-TiO₂/WS₂/alginate is an efficient and durable material. These results may provide new insight into the integrated adsorption-photocatalysis method’s potential for large-scale pharmaceutical wastewater treatment.