Photocatalytic Degradation of Tetracycline Hydrochloride Using TiO₂/CdS on Nickel Foam Under Visible Light and RSM–BBD Optimization

Abstract

This study investigates the photocatalytic degradation of tetracycline hydrochloride (TCH) using a TiO₂/CdS composite nanocatalyst synthesized on flexible nickel foam via a dipping–pull method. By comparing the photocatalytic degradation of TCH by TiO₂/CdS with different precursor ratios, it was found that TiO₂/CdS-1.43% exhibited better photocatalytic degradation performance. The X-ray diffraction (XRD) pattern of the TiO₂/CdS composite retains the characteristic peaks of both TiO₂ and CdS, indicating the successful formation of the composite. According to the analysis of ultraviolet–visible spectroscopy (UV–Vis), the absorption edge of TiO₂/CdS is approximately 530 nm. The transmission electron microscopy (TEM) images show Cd and S evenly, densely distributed in TiO₂/CdS, further validating its successful synthesis. X-ray photoelectron spectroscopy (XPS) analysis reveals that Cd and Ti elements exist in the forms of Cd²⁺ and Ti⁴⁺, respectively. TiO₂/CdS loading uniformity on the nickel foam was assessed using super-depth microscopy. The removal efficiency of 10 L of 20 mg/L TCH solution achieved 53.89%, respectively, under response surface methodology—Box–Behnken design (RSM–BBD) optimal conditions (28 g catalyst, 325 rpm, pH = 9.04 within 150 min). Furthermore, five successive cycling experiments demonstrated strong stability, with a catalyst loss of only 4.44%. Finally, free radical scavenging experiments revealed that ·O₂⁻ radicals are the primary active species. This study highlights the potential of TiO₂/CdS composites supported on nickel foam for efficient photocatalytic degradation of antibiotic pollutants in water.

1. Introduction

Antibiotic pollution threatens global water environments, particularly with TCH, widely used in medicine and agriculture, significantly impacting human health and the ecological environment [1,2]. The persistence of TCH in water bodies can foster antibiotic-resistant bacteria, leading to long-term risks [3]. Various technological approaches are currently employed, including membrane filtration, material adsorption, and photocatalytic oxidation. Membrane filtration technology utilizes porous membranes to efficiently remove antibiotics from wastewater [4,5]. Material adsorption techniques use modified activated carbon, biochar, and other materials to adsorb antibiotics from water [6,7]. Photocatalytic oxidation is considered one of the most promising methods for removing antibiotics from wastewater due to its simplicity, lack of secondary pollution, and strong oxidation-reduction capabilities [8,9].

TiO₂ has garnered extensive research attention due to its high efficiency, low cost, non-toxicity, and strong physical stability [10,11,12]. For instance, TiO₂ demonstrates excellent photocatalytic efficiency, with reaction rates often exceeding 10⁻² to 10⁻³ mol·L⁻¹·min⁻¹ in the degradation of organic pollutants like phenol and methyl orange. Additionally, TiO₂ exhibits a high quantum efficiency of 50–80% under UV light, indicating its ability to convert a significant proportion of incoming photons into useful electron–hole pairs, thereby boosting catalytic activity. TiO₂ maintains its integrity in extreme conditions, with a melting point of 1832 °C and a pH stability range of pH 3–12, enabling its use in a variety of environments, from acidic to alkaline. These properties, combined with TiO₂’s non-toxic nature, make it an ideal material for applications in photocatalysis. However, TiO₂ primarily acts as a photocatalyst activated by ultraviolet light, and ultraviolet energy constitutes only a portion of solar energy. Moreover, the recombination of light-induced electron–hole pairs is a major cause of energy loss in photocatalytic reactions, leading to reduced photocatalytic efficiency and energy utilization [13,14,15]. Combining TiO₂ with other semiconductors to form heterojunctions enhances the light absorption range and the generation of photoinduced electron–hole pairs, making it one of the effective methods to address the low photocatalytic activity of TiO₂ under visible light [16,17].

CdS is a photocatalyst dopant that exhibits strong visible light responsiveness, high photocatalytic activity, and excellent compatibility [18,19]. This is primarily due to CdS favorable bandgap, which is typically around 2.4–2.5 eV, allowing them to absorb visible light (around 400–500 nm) efficiently. Integrating CdS into TiO₂ results in a co-doping system with a compatible band structure, which introduces new energy levels within the TiO₂ bandgap, narrows the bandgap width, and creates oxygen vacancies that boost catalytic activity [20,21,22]. Compared with traditional photocatalysts, TiO₂/CdS features smaller microcrystal sizes, larger surface areas, a more favorable distribution of defect density, higher surface-active species density, and improved charge carrier separation [23,24,25].

Nickel foam, a three-dimensional metal foam structure, is characterized by high electrical conductivity that enables rapid electron transfer, along with excellent stability and flexibility, making it well suited for use in various structured photocatalytic reactors [26,27,28]. To overcome challenges associated with powdered catalysts, such as loss and clogging in water systems, TiO₂/CdS can be effectively loaded onto nickel foam, thereby improving both catalyst recoverability and catalytic performance [29]. Additionally, the porous structure of nickel foam significantly increases the specific surface area of the catalyst, providing more active sites; the porosity of nickel foam facilitates the uniform distribution of reactants and light, promoting contact between the reactants and the catalyst [30,31,32].

The TiO₂/CdS nanocomposite, formed by integrating TiO₂ and CdS, capitalizes on the remarkable stability of TiO₂ and the efficient visible—light absorption capacity of CdS. When irradiated by visible or UV light, electron–hole pairs are generated in CdS, and the photoexcited electrons can transfer to TiO₂, improving the charge separation efficiency and minimizing recombination. Remarkable achievements have been made in recent research. For instance, an efficient IO-TiO₂-CdS composite with a honeycomb-like 3D inverse opal TiO₂ structure was synthesized, which could degrade over 99% of tetracycline hydrochloride (30 mg/L) within 20 min under visible light irradiation [33]. Another study focused on the beta zeolite-supported Ti³⁺-TiO₂/CdS heterojunction. Zeolite served as an effective support, leading to a relatively high surface area of approximately 63.2 m² g⁻¹. This heterojunction achieved a photocatalytic degradation efficiency of 78% in merely 3 min for tetracycline, with a corresponding rate constant as high as 0.788 min⁻¹, far exceeding that of the individual components [34]. Using carbon fibers (CFs) as the fixing substrate, TiO₂/CdS heterojunction was constructed on the CF surface. The resulting CFs/TiO₂/CdS bundles could be woven into macroscopic cloth, which demonstrated remarkable photocatalytic activities, degrading a variety of pollutants such as 95.44% methylene blue, 64.95% acid orange 7, 91.37% tetracycline hydrochloride, and removing 90.70% hexavalent chromium after 120 min under visible light irradiation [35].

In this study, we applied a TiO₂/CdS film onto nickel foam using the dipping–pull method. TiO₂/CdS composite was characterized through XRD, UV–Vis, TEM, and XPS analyses. The loading state of TiO₂/CdS on the nickel foam was observed using super-depth microscopy. The photocatalytic performance of the TiO₂/CdS catalyst for degrading organic pollutants, with TCH as the target contaminant, was evaluated in a medium-scale reactor. The effect of the TiO₂ to CdS precursor ratio on the degradation of the TCH solution was investigated. To maximize the degradation rate, RSM–BBD was employed to optimize key operational parameters in the water treatment process, including catalyst dosage, stirring speed, and pH, while establishing relevant models. Furthermore, the reusability, recoverability, and major active species of the TiO₂/CdS composite were assessed. Based on this, the photocatalytic reaction mechanism of TiO₂/CdS was analyzed. This research provides insights into the practical application of TiO₂/CdS composites supported on nickel foam for the photocatalytic degradation of antibiotic pollutants in water.

2. Experiment

2.1. Chemicals and Materials

CdS (purity 99%), Ti(OBu)₄ (purity 99%), and TCH were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Ethylene glycol (purity 99%) was purchased from Tianjin Fuyu Fine Chemicals Co., Ltd. (Tianjin, China). Glacial acetic acid (purity 99%) was purchased from Shanghai Runjie Technology Development Co., Ltd. (Shanghai, China). NaOH (purity 96%) was purchased from Tianjin Fuchen Chemical Reagents Co., Ltd. (Tianjin, China). HCl (purity 10%) was purchased from Shenzhen Bida Environmental Protection Technology Co., Ltd. (Shenzhen, China). Nickel foam mesh was sourced from Quanzhou Yunzongcheng New Materials Co., Ltd. (Quanzhou, China). Benzoquinone (BQ), isopropanol alcohol (IPA), and sodium oxalate (AO) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). All chemicals are analytical grade and used without further purification. The light source used in the experiment was purchased from Shenzhen Qinming Technology Co. Ltd. (Shenzhen, China). Deionized water used in the experiments was made in our lab.

2.2. Preparation Process

The preparation process is shown in Figure 1. Titanium tetrabutoxide was mixed with anhydrous ethanol in a volume ratio of 1:4 and subjected to ultrasonic oscillation for 5 min. A specific mass of cadmium sulfide powder was weighed using an electronic balance, ensuring that the mass ratio of CdS in the mixed solution was 1.43%. The TiO₂ used in this experiment was maintained at a mass fraction of 7.02% in the mixed solution. Ice acetic acid solution was added to adjust the pH to approximately 4, followed by continuous ultrasonic treatment for 10 min. A magnetic stirrer was used to stir the solution at a constant temperature of 60 °C for 30 min. Then, it was sealed and aged for over 24 h to obtain a stable, homogeneous, and clear orange–yellow solution.

The nickel foam was cut into a rectangle measuring 215 mm × 140 mm. Then, the nickel foam was soaked in anhydrous ethanol for 1 h under ultrasonic oscillation to clean it and remove the oxidation layer from the surface. It was then kept in anhydrous ethanol for an additional 24 h. After soaking, the nickel foam was placed in a beaker containing deionized water and subjected to ultrasonic treatment for 30 min for further cleaning. The nickel foam was then heat-treated in a muffle furnace at 400 °C for 10 min. Comparing before and after treatment, the pores on the nickel foam became more permeable, allowing for a larger surface area to load the catalyst. The film coating was applied to the nickel foam using the impregnation method. After coating, the foam was placed in a drying oven at 80 °C to dry, and the coating process was repeated to achieve the desired catalyst loading. Finally, the nickel foam loaded with the catalyst was placed in the muffle furnace and held at 500 °C for 60 min to enhance the bonding strength, successfully loading TiO₂/CdS onto the nickel foam.

2.3. Characterization

The powder X-ray diffraction (XRD) patterns were recorded using an XD-3 (Shimadzu, Kyoto, Japan) with Cu-Kα radiation. The UV–Vis diffuse reflectance spectra were obtained directly using a UV–Vis spectrophotometer (UV-2550, Shimadzu, Kyoto, Japan) with BaSO₄ as a reference. X-ray surface morphology and dispersibility of the samples were recorded by high-resolution transmission electron microscopy (Tecnai F20, Portland, OR, America). Photoelectron spectroscopy (XPS) was conducted using an AXIS-ULTRA DLD (Shimadzu, Kyoto, Japan) with an Al Kα X-ray source (hν = 1486.8 eV). The nickel foam loaded withTiO₂/CdS was observed and analyzed using a super depth microscope. (VHX-6000, Keyence Corporation, Osaka, Japan).

2.4. Photocatalytic Experiments

The photocatalytic degradation experiment was conducted in a 10 L cylindrical acrylic glass photoreactor, which was made of polymethyl methacrylate, as shown in Figure 2. A 24 W visible-light LED array, with its emission wavelength peaking at around 450 nm, was selected as the light source. The central layout of the LED array ensures that the visible light uniformly illuminates the surrounding nickel foam, improving the uniformity of light exposure during the reaction. A magnetic stirrer drives the stirring bar to mix the TCH solution, facilitating contact between the TCH and the active sites of TiO₂/CdS on the nickel foam. The nickel foam loaded with the TiO₂/CdS composite nanocatalyst was fixed around the light source using a mounting fixture, with a distance of 100 mm from the foam to the center of the LED array.

Ten liters of a 20 mg/L TCH solution was added to the reactor and stirred for 30 min. In the laboratory, maintain both the room temperature and the solution temperature at 25 °C each time. This helps minimize the impact of temperature fluctuations on the experiment. The pH of the TCH solution was adjusted to the desired value using 10% HCl or NaOH pellets. According to the preliminary experiment, the adsorption–desorption equilibrium between the catalyst and the target pollutant was reached in 30 min. Before the dark reaction, a 10 mL sample was extracted from the reactor for concentration analysis, and the solution was stirred in a dark environment for 30 min to achieve adsorption/desorption equilibrium between the photocatalyst and the TCH molecules. During the light reaction, 10 mL samples were extracted every 20 min from the reactor for concentration analysis.

The concentration of TCH in the solution was measured using an ultraviolet–visible spectrophotometer. Following the methodology of previous studies, the concentration changes in TCH were assessed using this spectrophotometer. The solution reached adsorption–desorption equilibrium within 30 min after adding TiO₂/CdS, after which the degradation stabilized. Therefore, the adsorption equilibrium time was designated as 30 min. The photocatalytic activity of the TiO₂/CdS composite material was evaluated under the illumination of the 24 W visible light LED array. The degradation rate percentage was calculated using Equation (1).

$$D=(1-{\displaystyle \frac{C_{\mathrm{t}}}{C_0}})\times 100\%$$

(1)

where D represents the degradation rate of antibiotics at time t in the photocatalytic degradation system. C₀ is the initial concentration of TCH, and C_t is the concentration of TCH at time t.

2.5. Experimental Design and Optimization

Three key parameters were analyzed to evaluate the impact of catalyst dosage, solution stirring speed, and pH on the TCH degradation rate using TiO₂/CdS photocatalysis. Design–Expert (version 13.0.1.0 64-bit) optimized these parameters using the response surface methodology—Box–Behnken design. The BBD can assess the potential interactions between parameters and their effects on the degradation rate. Equation (2) represents the relationship between the variables:

$$Y={\beta }_0+{\displaystyle \sum _{i=1}^k{\beta }_i{x}_i+}{\displaystyle \sum _{i=1}^k{\beta }_{\mathit{ii}}{x}_i^2+{\displaystyle \sum _{i=1}^k{\displaystyle \sum _{j=1}^k{\beta }_{ij}}}}{x}_i{x}_j+\epsilon$$

(2)

where β₀ represents the intercept term, β_i represents the linear factor coefficients, β_ii represents the quadratic terms, β_ij represents the interaction terms, and Y represents the degradation percentage.

Table 1. Independent variables and coded values of the BBD model.

Independent Variables	Symbols	Coded Levels
		−1	0	1
Catalyst Dose	A	20	30	40
Agitating rate	B	100	300	500
pH	C	6	9	12

shows the BBD model independent variables and coded values.

2.6. Reusability of TiO₂/CdS

After conducting the photocatalytic degradation reaction on the same nickel foam loaded with TiO₂/CdS, it was slowly rinsed with deionized water and placed in a drying oven at 100 °C for 60 min to ensure complete drying. The photocatalytic degradation reaction was then repeated. This process was performed for five consecutive degradation experiments using TCH at an initial concentration of 20 mg/L to assess the stability of the catalyst. The mass of the nickel foam loaded with TiO₂/CdS was recorded before and after each cycle to calculate the catalyst mass loss during the five cycles.

3. Results and Discussion

3.1. Characterization of the Photocatalyst

XRD analysis compared the crystal structures of TiO₂, CdS, and TiO₂/CdS composite samples. The XRD patterns are shown in Figure 3. The diffraction peaks at 25.30°, 37.80°, 48.02°, 53.89°, 55.02°, 62.66°, and 68.81° correspond to the anatase TiO₂ (JCPDS No. 21-1272) (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), and (1 1 6) crystal planes [36]. The diffraction peaks at 26.39°, 43.53°, and 51.69° correspond to the cubic phase of CdS (JCPDS No. 80-0019) (1 1 1), (2 2 0), and (3 1 1) crystal planes [37]. The XRD pattern of the TiO₂/CdS composite retains the characteristic peaks of both TiO₂ and CdS, indicating the successful formation of the composite [38]. Additionally, the peak of TiO₂ at 25.30° overlaps with the (1 1 1) crystal plane of CdS. No significant changes in the characteristic peaks of TiO₂ and CdS were observed, indicating that CdS exists in a separate phase rather than within the lattice of TiO₂.

Figure 4a shows the absorption spectra of all samples in the UV–Vis range. All samples except for TiO₂ exhibit strong light absorption in both the ultraviolet and visible ranges. The absorption edge of TiO₂ is approximately 410 nm, while that of TiO₂/CdS is at 530 nm. The bandgap widths of TiO₂ and CdS were obtained using the extrapolation method, as shown in Figure 4b [39]. The bandgap width of TiO₂ is 3.07 eV, and that of TiO₂/CdS is 2.77 eV, which is lower than the 3.18 eV of TiO₂/Cd prepared by previous researchers [40]. Compared with pure TiO₂, the TiO₂/CdS composite has a significantly narrower bandgap, which enables better visible light absorption.

The nickel foam loaded with different layers of TiO₂/CdS was observed and analyzed using a super depth microscope. The super depth of field microscope, with its large depth of field, can clearly image uneven samples. It has the unique ability to keep different heights in focus simultaneously, making it ideal for observing 3D microstructures such as catalysts. Moreover, it offers high-resolution views, enabling detailed morphological analysis and precise defect detection at the micro level. From Figure 5, it can be seen that the nickel foam has a dense pore structure. As the number of coating layers increases, the coverage of TiO₂/CdS on the nickel foam also increases, and the thickness of the TiO₂/CdS film on the nickel foam grows in Figure 5b–f. The surface area per unit volume was measured, as shown in Figure 5g. The number of TiO₂/CdS plating layers on the nickel foam mesh exhibits a linear correlation with the TiO₂/CdS catalyst dosage applied to the nickel foam mesh. This relationship can be expressed through the equation presented in Figure 5h.

The TEM of the samples was observed, as shown in Figure 6a,b, revealing that the TiO₂/CdS loaded on the nickel foam has a high-density particle structure, which facilitates the entry and loading of CdS. TEM images show the lattice fringes of TiO₂/CdS, as depicted in Figure 6e,f. Among them, a lattice fringe of 0.23 nm corresponds to the CdS (2 2 0) plane, while a 0.35 nm lattice fringe corresponds to the TiO₂ (1 0 1) plane [41]. The precise contact interface between CdS and TiO₂ indicates low interface losses during the electron transport process, and the close interface contact is beneficial for the interfacial charge transfer between CdS and TiO₂. In Figure 6d, the X-ray energy spectrum shows that TiO₂/CdS contains elements S, Cd, Ti, and O, indicating that CdS forms a deposit on the surface of TiO₂. Moreover, the Cd and S elements are uniformly and densely distributed within the TiO₂/CdS composite, suggesting good loading and high dispersion of CdS within the TiO₂ particles. Combined with the results from XRD, this confirms the successful synthesis of TiO₂/CdS.

XPS was used to study the chemical states of the surface elements in the composite material, as shown in Figure 7. The full spectrum reveals the presence of Ti, O, Cd, C, and S elements in the TiO₂/CdS composite material, as illustrated in Figure 7a–c, which display the high-resolution spectra of Cd 3d and S 2p, respectively. The characteristic peaks at binding energies of 404.49 eV and 411.30 eV correspond to Cd 3d₅₂ and 3d₃₂, indicating that the Cd element in CdS exists in the form of Cd²⁺. The binding energy difference of 6.8 eV between the Cd 3d₅₂ and 3d₃₂ peaks further confirms that the state of Cd is Cd²⁺ [21]. The characteristic peaks at binding energies of 164.18 eV and 168.31 eV correspond to S 2p₁₂ and S 2p₃₂, indicating that the S element in CdS exists in the form of S²⁻ [42]. The Ti 2p characteristic peaks near binding energies of 457.87 eV and 463.56 eV correspond to Ti 2p₃₂ and Ti 2p₁₂, which are typically attributed to Ti⁴⁺, as shown in Figure 7d [43]. Furthermore, the binding energy difference of 5.7 eV between Ti 2p₃₂ and Ti 2p₁₂ further indicates that the Ti element exists in the form of Ti⁴⁺ [44]. Figure 7e shows the characteristic peaks at binding energies of 531.02 eV, which correspond to Ti–O bonds in the TiO₂ lattice, respectively [45].

3.2. Quality Ratio

By adding 1 g, 3 g, 5 g, and 10 g of CdS powder to four identical 50 mL mixtures of tetrabutyl titanate and 200 mL of anhydrous ethanol, mixed solutions with CdS mass ratios of 0.48%, 1.43%, 2.37%, and 4.63% were obtained. A 70 × 36 mm nickel foam was then subjected to a three-time dipping–pull method, followed by heat treatment and other steps, resulting in four pieces of nickel foam loaded with different mass ratios of TiO₂/CdS. These were used for photocatalytic degradation of an 80 mL TCH solution with an initial concentration of 20 mg/L, as shown in Figure 8. The subsequent preparation involved using a mixed solution with a CdS mass ratio of 1.43% to fabricate TiO₂/CdS. In the paper, for the XRD, UV–Vis, TEM, and XPS tests and photocatalytic experiments using the TiO₂/CdS composite, the TiO₂/CdS was prepared from a precursor solution with a CdS mass ratio of 1.43%.

3.3. RSM–BBD

The results of the variance analysis shown in

Table 2. Analysis of variance (ANOVA) for the TCH photodegradation rate regression model.

Source	Sum of Squares	df	Mean Square	F-Value	p-Value
Model	1338.61	9	148.73	115.99	<0.0001	significant
A	20.67	1	20.67	16.12	0.0051
B	108.71	1	108.71	84.78	<0.0001
C	7.74	1	7.74	6.04	0.0436
AB	3.13	1	3.13	2.44	0.1620
AC	9.86	1	9.86	7.69	0.0276
BC	68.81	1	68.81	53.66	0.0002
A2	56.92	1	56.92	44.39	0.0003
B2	830.49	1	830.49	647.66	<0.0001
C2	151.79	1	151.79	118.38	<0.0001
Residual	8.98	7	1.28	1.28
Lack of Fit	6.68	3	2.23	2.23	3.88	not significant
Pure Error	2.30	4	4	0.5740
Cor Total	1347.58	16	16

indicate that the model is significant in elucidating the photocatalytic degradation of TCH. The high F-value (115.99) and extremely low p-value (<0.0001) indicate that the model is highly effective in explaining the variations in TCH degradation rates. The three factors, including catalyst dosage (A), agitating rate (B), and solution pH (C), are all highly significant, reaffirming their critical roles in influencing TCH degradation rates. The positive coefficients of these factors suggest that increasing the catalyst dosage has a beneficial effect on enhancing removal efficiency, while higher agitating rates and alkaline pH values also improve photocatalytic degradation rates. The analysis of variable interactions (AB, AC, BC) demonstrates their high significance, underscoring the importance of considering these factors’ synergistic effects on TCH degradation rates. The significant quadratic terms (A², B², C²) indicate a nonlinear relationship between the independent variables and the response. This nonlinear relationship highlights the need to understand further how variations in these parameters collectively affect the degradation rate of TCH. The non-significant goodness-of-fit value (p = 0.1117) suggests that the model fits the experimental data well, indicating that the model adequately represents the relationship between the variables and the response. This confirms the reliability of the model regarding TCH degradation rates. The binary polynomial formula (Equation (2)) predicts the response variable while accounting for the independent factors and their interactions. To maximize the degradation rate of TCH, the solver in Design–Expert was used to solve Equation (3) and provide the necessary values. This mathematical formula can predict the values required to achieve the maximum TCH degradation rate.

$$\begin{array}{c}Y=-69.441+1.441\times A+0.278\times B+12.840\times C+0.0004\times AB+0.052\times AC\\ -0.007\times BC-0.037A^2-0.0004B^2-0.667C^2\end{array}$$

(3)

Figure 9a–f illustrate the relationships among the three factors and the degradation rate of TCH. To simplify the analysis, each figure selects two independent factors while keeping the remaining factor fixed at its midpoint value. The contour plots effectively demonstrate the interactions between the selected factors, where elliptical or saddle-shaped contours indicate significant interactions, while circular contours suggest negligible interactions. Additionally, the three-dimensional plots in Figure 9a–c provide a deeper understanding of these relationships. Figure 10a also presents the optimization results. Figure 10b shows the curve fitting of experimental and predicted TCH degradation rates along with the normal distribution of the residuals. The differences between the experimental and expected results are minimal, and the residual analysis confirms the model’s reliability. The residuals in Figure 10c appear as a straight line, indicating that the errors follow a normal distribution.

The analysis of this dataset reveals the intricate relationships between various factors and the degradation rate. The catalyst dosage (Factor 1, A) is a crucial parameter affecting photocatalytic degradation, controlled by applying different layers of TiO₂/CdS catalysts onto the foam nickel using the dip-coating method. The range of catalyst dosage (20–40 g) was determined through preliminary experiments and extensive literature review. This dosage range was selected to assess its suitability for degrading TCH. Generally, an increase in catalyst dosage is associated with enhanced degradation capability; for instance, at 29 g (13 layers), the degradation of a 20 mg/L TCH solution reached 53.84%. This phenomenon is expected as a more significant catalyst dosage provides more active sites for the photocatalytic reaction [46,47,48]. However, this relationship may not be linear. As the number of catalyst layers increases, the pores of the foam nickel can become clogged, hindering sufficient contact between active radicals and the TCH solution. Consequently, an excessive catalyst dosage may not yield better results, indicating an optimal range for catalyst dosage [49]. A possible reason for this is that increasing the catalyst amount may lead to easier detachment of the catalyst from the foam nickel, resulting in an excess of catalyst particles in the solution, which could scatter light and adversely affect the overall enhancement of the photocatalytic degradation rate [50,51].

The agitating rate of the magnetic stirrer (Factor 2, B) significantly impacts the degradation rate of TCH. As the agitating rate increased from 100 rpm to 300 rpm, the degradation rate of TCH rose from 25.5% to 53.61%. This increase in degradation rate can be attributed to the kinetic energy gained by the TCH solution, which improved the intimate interface contact between reactants and facilitated sufficient interaction between active radicals and the TCH solution. The probability of successful collisions between active radicals and TCH during the photocatalytic process increases with the agitating rate [52]. However, it is noteworthy that in this study, the performance slightly declined when the agitating rate was raised to 500 rpm. This may be due to the agitating rate exceeding the optimal value, resulting in ineffective collisions between reactants [53].

The pH value (Factor 3, C) affects the structure and surface properties of TCH in aqueous solution, as well as the surface charge of the photocatalyst, thus influencing the removal efficiency of pollutants [54,55,56]. The data indicate that the pH value significantly impacts degradation efficiency, with different values yielding varying results. Considering the response variable (degradation rate), the dataset shows a wide range of degradation efficiency, from 25.5% to 54.2%. With 30 g of catalyst, an agitating rate of 300 rpm, and pH = 9, the highest degradation efficiency reached 53.84%. Conversely, with 30 g of catalyst, an agitating rate of 100 rpm, and pH = 6, the lowest degradation efficiency was 25.5%. The dataset suggests that alkaline conditions can enhance photocatalytic removal efficiency. However, with 30 g of catalyst, an agitating rate of 500 rpm, and pH = 12, the degradation efficiency dropped to 33.1%. This decrease of 20.74% when the pH increased from 9 to 12 is attributed to the TCH negative surface charge becoming a double negative charge, significantly increasing repulsion from the photocatalyst particles. Thus, an optimal pH value exists while increasing the pH, which enhances the formation of hydroxyl radicals and the degradation efficiency. The point of zero charge (pzc) of the TiO₂/CdS composite material is 6.25. Although TCH and the catalyst surface carry negative charges at pH = 9, the degradation rate is still higher [57]. A possible reason for this phenomenon is the higher concentration of adsorbed OH⁻, which promotes the formation of hydroxyl radicals, thereby enhancing the oxidative effect on TCH [58].

3.4. Reusability

The reusability of photocatalytic materials is a critical factor in assessing the performance of photocatalysts. This study investigated the reusability of TiO₂/CdS composite materials by cleaning and drying nickel foam loaded with TiO₂/CdS. This was followed by conducting five consecutive degradation experiments using an initial concentration of 20 mg/L TCH to evaluate the stability of the catalyst.

As shown in Figure 11a, the cyclic experimental results of the TiO₂/CdS composite material indicate that although there is a slight decrease in photocatalytic degradation efficiency after the third cycle, the degradation rate of TCH remains relatively stable during the subsequent two cycles. After five consecutive cycles of degrading the TCH solution, the degradation efficiency decreased to 25.03%. Figure 11b shows the change in mass of the nickel foam loaded with TiO₂/CdS during the continuous cycling experiments. It is evident that the mass significantly decreased after the first cycle, while the mass gradually reduced during the subsequent four cycles. This is attributed to the partial detachment of TiO₂/CdS from the nickel foam during the first cycle. Notably, most TCH degradation occurred within the first 60 min of the first cycle, as loosely adsorbed TiO₂/CdS particles were lost with the flow of the solution, which increased the contact area with the TCH solution, leading to a marked improvement in degradation efficiency. However, as more particles detached, the solution’s transparency decreased, resulting in a decline in degradation efficiency after 90 min of the first cycle. In the following four cycles, the concentration of TCH steadily reduced during the photocatalytic reaction, indicating minimal detachment of TiO₂/CdS particles, which correlates with the slow reduction in the mass of the nickel foam loaded with TiO₂/CdS. The observed decrease in photocatalytic performance may be due to surface area loss from nanoparticle agglomeration and particle detachment from the nickel foam. There may also be slight changes in the structure of TiO₂/CdS or accumulation of reaction byproducts on its surface during the photocatalytic process, which could affect the degradation efficiency in multiple cycles.

Nevertheless, the dense structure of TiO₂/CdS loaded on the nickel foam still maintained its effectiveness, as evidenced by the relatively small loss in removal efficiency. There was no significant change in the XRD pattern (Figure 11c), indicating that the TiO₂/CdS composite material is stable even after multiple use cycles.

The main active species responsible for the degradation of TCH were analyzed through radical scavenging experiments. A total of 1 mM benzoquinone (BQ), isopropanol (IPA), and sodium oxalate (AO) were used as scavengers for ·O₂⁻, ·OH, and h⁺, respectively. As shown in Figure 12a,b, the effects of different scavengers on the photocatalytic degradation capacity of the TiO₂/CdS composite material were investigated. The addition of benzoquinone and ascorbic acid significantly decreased the photocatalytic degradation rate of TCH, indicating that ·O₂⁻ and h⁺ are essential for the degradation process. However, the addition of isopropanol did not significantly reduce degradation efficiency, suggesting that the ·OH radical has a minimal effect on the degradation of TCH. The degradation rate of TCH without any scavengers was 59.65%, while the degradation rates with the addition of benzoquinone, isopropanol, and ascorbic acid were 34.42%, 56.31%, and 44.06%, respectively. The contributions of the radicals to the photocatalytic degradation of TCH using TiO₂/CdS loaded on nickel foam were calculated. The results indicated that the contribution rates of ·O₂⁻, ·OH, and h⁺ were 50.06%, 16.23%, and 29.99%, respectively. The scavenging experiment results suggest that ·O₂⁻ is the primary active species in the TiO₂/CdS photocatalytic system, while ·OH and h⁺ play a supporting role.

Figure 13 illustrates the photocatalytic reaction mechanism of TiO₂/CdS. Under visible light irradiation, both TiO₂ and CdS are excited to produce photogenerated electrons and holes. The conduction band edge potential of CdS is more negative than that of TiO₂, and the electrostatic field facilitates the transfer of electrons from CdS to the conduction band of TiO₂. Meanwhile, the valence band edge potential of CdS is also more negative than that of TiO₂, allowing holes from the valence band of TiO₂ to transfer to the valence band of CdS. This spatial separation of electrons and holes is one of the essential methods to reduce their recombination. The electrons in the conduction band of TiO₂ react with dissolved oxygen in water to generate superoxide radicals, while the holes in the valence band of CdS react with water to produce ·OH hydroxyl radicals. The results of the radical scavenging experiments indicate that O₂⁻ plays a major role in the photocatalytic degradation of TCH using TiO₂/CdS. Furthermore, during the photocatalytic reaction, active species decompose TCH into intermediate products and ultimately convert it into CO₂ and H₂O.

4. Conclusions

This study successfully developed an efficient TiO₂/CdS composite material supported on nickel foam for photocatalytic degradation of tetracycline hydrochloride (TCH). Characterization techniques such as XRD, UV–Vis, TEM, and XPS were employed to comprehensively analyze the TiO₂/CdS composites, revealing that these materials exhibit excellent photocatalytic activity. Under optimized conditions, with a TiO₂ to CdS mass ratio of 3.8:1, the catalyst achieved a maximum degradation rate of 90.03% for 80 mL of 20 mg/L TCH solution within 150 min. The response surface methodology–Box–Behnken design (RSM–BBD) was successfully applied to optimize the operational parameters, with the best degradation rate of 53.89% obtained at a catalyst dosage of 28 g, an agitation rate of 325 rpm, and a pH of 9.04.

Additionally, the TiO₂/CdS composites supported on nickel foam demonstrated good stability and reusability, maintaining a degradation rate of 25.03% after five consecutive cycles with a minimal mass loss of 4.44%. Radical scavenger experiments revealed that superoxide radicals (·O₂⁻) played the dominant role in the photocatalytic degradation of TCH, with the order of contribution being ·O₂⁻ > h⁺ > OH.

Overall, the TiO₂/CdS composite material supported on nickel foam not only exhibited excellent photocatalytic degradation efficiency but also showed promising stability and reusability, indicating its potential for water treatment applications. However, future research could focus on optimizing the synthesis methods, exploring different loading strategies, and investigating the performance of these materials in more complex water matrices. Furthermore, combining this system with other advanced oxidation technologies could enhance the degradation efficiency, providing more effective solutions for practical applications in environmental remediation.