New CdS–Bentonite Composites with Photocatalytic Properties

Abstract

Cadmium sulfide is an important II-VI semiconductor known for its valuable photocatalytic properties ascribable to its band gap energy, which allows light absorption in the visible domain. Nonetheless, the application of cadmium sulfide in wastewater organic pollutant degradation is restricted due to its high toxicity to humans, soil, and marine life. To address this issue, we developed new composite materials by depositing CdS on a bentonite support in a 1:9 mass ratio to develop a photocatalyst with lower toxicity. In the first step, bentonite was activated using an aqueous HCl solution; for the deposition of CdS powder, we proposed the trituration method and compared it with chemical precipitation and hydrothermal synthesis, using thioacetamide as a sulfide ion source. The modified bentonite underwent characterization using X-ray diffraction, scanning electron microscopy, X-ray fluorescence, UV-Vis, and FTIR spectroscopy. The photocatalytic activity was tested in the degradation of Congo red (CR), a persistent diazo dye. The efficiency of removing CR with CdS–bentonite composites depended on the deposition method of CdS, and it was higher than that of pristine CdS and of only adsorption onto acid-activated bentonite. The photocatalytic degradation mechanism was estimated by the scavenger test using ethylenediaminetetraacetic acid disodium salt, ascorbic acid, ethanol, and silver nitrate as radical scavengers.

1. Introduction

The clay minerals belong to phyllosilicates (layered silicates or sheet silicates), a large group of minerals that are important native materials utilized in different domains of industry (such as in coatings and cosmetics, in the synthesis of nanomaterials, in water purification, etc.) due to characteristics like high abundance, low cost, and environmental friendliness [1,2]. The layers of phyllosilicates have different compositions and coordination, i.e., silica tetrahedral (SiO₄, T) sheets and alumina octahedral (AlO₆, O) sheets. Bentonite is a typical 2:1 (T-O-T) clay mineral, having montmorillonite as the main component [3]. Due to the isomorphic replacement of octahedral and/or tetrahedral cations by cations with a lower charge, the layers have a slight negative charge, which is compensated by the exchangeable cations located in the interlayer spaces (“galleries”) between the clay layers [2,4].

The II-VI semiconductor cadmium sulfide (CdS) is a material of interest because of its tunable light emission and size-dependent optoelectronic characteristics. It is used in fields and products such as photoconductors, transistors, light-emitting diodes, sensors, solar cell windows, biological indicators, photocatalysts, and laser communication because of its characteristics, which include a strong electron affinity, a wide and direct band gap of about 2.4 eV, and remarkable transparency [5].

Many studies have dealt with the photocatalytic activity of CdS, and high efficiency has been reported [6,7,8]. It was observed that the aggregation of CdS nanoparticles and the recombination of photoexcited charge carriers can result in a decrease in photocatalytic activity. Several techniques have been performed to avoid nanoparticles’ aggregation, like the use of capping or functionalizing agents [8,9,10], deposition onto particles of other inorganic materials [11,12], or incorporation in layered materials [13,14]. The addition of CdS to layered inorganic materials can lead to distinctive chemical and physical characteristics [13,14]. The typical method for producing CdS/clay nanocomposites involves a cation exchange process using cadmium acetate, octanoate, nitrate, chloride, or iodide, followed by a sulfurization process using hydrogen sulfide or sodium sulfide [1]. CdS/clay nanocomposites (e.g., CdS/laponite, saponite, hectorite, and montmorillonite [2,14]) were prepared mainly by the hydrothermal method [13]. Besides the two-step synthesis of CdS/clay nanocomposites, a combination of an ion exchange and a hydrothermal process was also used [13,14]. The nanocomposites resulting from the last method display structural features comparable to the clay host, albeit altered by the incorporation of CdS into the clay layers [14]. Two other synthesis routes were proposed for Cd/bentonite nanocomposite, which involved a complex between Cd(II) and thiourea formation followed by its thermal decomposition [13,14]. The obtained CdS/clay composites were used as photocatalysts in the production of hydrogen by water splitting under visible light irradiation [13].

The reason for CdS synthesis in a confined space is related to the variation in semiconductor properties based on particle size. During the growth of CdS particles in the interlayer of phyllosilicates, their dimensions are constrained in the z-axis, potentially resulting in an anisotropic semiconductor with distinctive photophysical and photocatalytic properties [1]. Furthermore, clay is not only a support that allows CdS to be obtained as an anisotropic semiconductor, but its layers have a significant ability to adsorb oxygen through their hydroxyl groups, thus improving the separation of charge carriers. It can also promote the production of superoxide radicals. Both of these properties result in a significant improvement in the photocatalytic efficiency of CdS nanoparticles [15].

In this study, we reported obtaining a new CdS–bentonite composite by trituration in the presence of Triton X-100. The properties of the resulting composite were compared with those of the composites obtained by chemical precipitation and hydrothermal synthesis, respectively. Thioacetamide was used as a precursor for the sulfide ion in all syntheses. The resulting powders were characterized by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), UV-vis, and infrared spectroscopy (FTIR), as well as by specific surface area measurements. The photocatalytic properties of CdS–bentonite composites were evaluated in the photodegradation of a diazo dye, Congo red.

2. Materials and Methods

2.1. Materials and Reagents

The clay studied was a commercially purchased food-grade bentonite for winemaking. The high-purity reagents were purchased from Sigma-Aldrich (Sigma–Aldrich Co. Merck Group, Darmstadt, Germany): hydrochloric acid, HCl 37%; cadmium acetate, Cd(CH₃COO)₂∙2H₂O; thioacetamide, CH₃CSNH₂; polyethylene glycol tert-octylphenyl ether (Triton X-100), t-oct-C₆H₄-(OCH₂CH₂)_xOH, x = 9–10; disodium salt of 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene-1-sulfonic acid) (Congo Red), C₃₂H₂₂N₆Na₂O₆S₂; disodium ethylenediaminetetraacetate dihydrate (EDTA-2Na), C₁₀H₁₄N₂Na₂O₈·2H₂O; ethanol, C₂H₅OH; ascorbic acid, C₆H₈O₆; silver nitrate, AgNO₃) and were used as received, without further purification.

2.2. Synthesis of CdS–Bentonite Nanocomposites

Acid-activated bentonite (AAB). A mass of 5 g of bentonite was mixed with 100 mL of 0.5 M HCl solution and sonicated for 30 min in an ultrasonic bath (MRC D80H Ultrasonic Cleaner). Afterwards, the mixture was heated under reflux and magnetically stirred for 6 h. The powder was separated by centrifugation and washed with water until a neutral pH was reached.

Bentonite activated with acid in the presence of Triton X-100 (AAB-TX). A mass of 5 g of bentonite was mixed with 100 mL of 0.5 M HCl solution and 2 mL of Triton X-100 (TX). Further activation was performed according to the procedure described for AAB.

CdS-AAB 1. A mass of 0.45 g AAB was triturated with 1 mL of TX. To the obtained paste, 0.092 g of Cd(CH₃COO)₂∙2H₂O (0.346 mmol) and 0.026 g of thioacetamide (0.346 mmol) were added. A light beige paste was obtained, which was left for a few days, triturated intermittently, and kept at the same consistency by adding water. In time, the color of the paste turned light yellow. The final paste was mixed with 50 mL of H₂O and magnetically stirred for 15 min. The yellow powder was filtered, washed with H₂O, and dried at room temperature.

CdS-AAB 2. A mass of 0.45 g of AAB was dispersed in 50 mL of H₂O and magnetically stirred for 60 min at room temperature. Then, 0.092 g of Cd(CH₃COO)₂∙2H₂O (0.346 mmol) were dissolved in the mixture, which was stirred further for another 60 min, and 0.026 g of thioacetamide (0.346 mmol) were added. The mixture was heated for 2 h, at 80 °C, under magnetic stirring. After cooling, the obtained orange powder was filtered, washed with H₂O, and dried at room temperature.

CdS-AAB 3. A mass of 0.45 g AAB was dispersed in 100 mL H₂O and magnetically stirred for 60 min at room temperature. Then, 0.092 g of Cd(CH₃COO)₂∙2H₂O (0.346 mmol) were dissolved in the mixture, which was stirred further for another 60 min, and 0.026 g of thioacetamide (0.346 mmol) were added. The mixture was transferred to a hydrothermal reactor (CIT-HTC230-V200) and heated for 4 h at 180 °C. After cooling, the obtained orange powder was filtered, washed with H₂O, and dried at room temperature.

CdS. A mass of 2.66 g Cd(CH₃COO)₂∙2H₂O (10 mmol) was dissolved in 100 mL of H₂O. Then, 0.75 g of thioacetamide (10 mmol) were added to the obtained solution, and the mixture was heated for 2 h, at 80 °C, under magnetic stirring, an orange precipitate being obtained over time. After cooling, the orange powder was filtered off, and the precipitate was washed with H₂O and dried at room temperature.

2.3. Characterization of Powders

The powders were investigated by small-angle and wide-angle X-ray diffraction performed on a Rigaku Miniflex 2 diffractometer (Rigaku Corporation, Tokyo, Japan) with Ni-filtered CuKα radiation in the range of 2θ, 3–10° (small-angle) and 10–70° (wide-angle), with a scan rate of 0.5°/min and 2°/min, respectively, and a step of 0.02°. The morphology of composite materials was studied using a Tescan Vega 3LMH scanning electron microscope (Brno, Czech Republic). Energy dispersive X-ray spectroscopy (EDX) was performed on the SEM equipped with a Bruker X-ray energy dispersive detector (Bruker Nano, GmbH, Berlin, Germany) to identify the material composition. The X-ray fluorescence analysis was achieved using an Olympus InnovX Delta scanner (Woburn, MA, USA) with an Au/Ta anode and a solid-state Silicon Drift Detector (SDD). The UV–visible diffuse reflectance spectra (DRS) of powders were recorded in the range of 220–850 nm on a Jasco V 550 spectrophotometer (Jasco Corporation, Tokyo, Japan) with an integrating sphere using MgO as the reference. The FTIR spectra were recorded on an Agilent Cary 630 FTIR spectrometer (Agilent Scientific Instruments, Santa Clara, CA, USA) with a ZnSe ATR in the wavenumber range of 4000–650 cm⁻¹. The specific surface area was determined on a Micromeritics Tristar II Plus porosimeter (Micromeritics Instrument Corporation, Norcross, GA, USA). The photocatalytic/adsorption properties of the obtained powders were estimated by monitoring the color of Congo red azo dye solution using the Jasco V 550 spectrophotometer (200–900 nm).

2.4. Photocatalytic Properties of CdS-AAB Nanocomposites

The photocatalytic properties of the nanocomposites, in comparison with the CdS powder, were tested in the degradation of CR azo dye, following the procedure previously published [8,9]. Summarily, photocatalyst powder (50 mg or 100 mg) was dispersed in 100 mL of CR solution (30 mg/L), and the mixture was stirred for 60 min in dark conditions to achieve the adsorption–desorption equilibrium. Subsequently, the suspension was exposed to visible light using a 45 W mercury vapor lamp simulating the solar spectrum, which was measured with a high-resolution HR4000CG-UV-NIR spectrometer (Ocean Optics, Orlando, FL, USA). Solution samples were withdrawn at regular intervals, and their absorbance was measured at 498 nm wavelength using a Jasco V 550 spectrophotometer. For comparison, the adsorption properties of AAB were determined similarly, but the samples were not exposed to a light source.

To highlight the influence of the active species generated in the photocatalytic reaction that contributed to the CR degradation, we carried out experiments in the presence of various scavengers—namely ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), ascorbic acid (AA), ethanol (EtOH), and AgNO₃—in order to capture the holes (h⁺), superoxide radicals (·O₂⁻), hydroxyl radicals (HO·), and respective electrons (e⁻) [16]. We introduced 2 mL of each scavenger as 1 mM solutions in the photocatalytic reaction.

3. Results and Discussion

3.1. Characterization of Bentonites and Bentonite-Based Nanocomposites

3.1.1. X-Ray Diffraction (XRD) and X-Ray Fluorescence (XRF)

Bentonites typically consist of clay minerals, mainly montmorillonite, and secondarily kaolinite and illite, along with cristobalite, carbonates, zeolites, iron oxides and hydroxides, and remnants of quartz and feldspar [17,18]. Row clays are usually activated prior to use in order to improve surface area and porosity because they frequently have poor sorption performances [19]. Acid activation of clay minerals is an efficient technique considering the short processing time and minimal reagent requirement. The prospective uses of acid-activated clays as adsorbents and catalysts are currently attracting a lot of interest [20]. Acid treatment may cause significant changes in the structure and chemical composition of aluminosilicates because of the dissolution of structural ions and subsequent restructuring of the framework. Thus, the clay mineral surface is made acidic through acid treatment, which then causes the leaching of metal ions from the clay mineral lattice, resulting in an expansion of the external surface area and the establishment of permanent porosity [21]. We also performed the acid activation of bentonite in the presence of Triton X-100 (sample denoted AAB-TX), because, for the subsequent deposition of CdS on bentonite by trituration, we used this nonionic surfactant. The acid-activated bentonite samples were characterized by XRD, XRF, FTIR, and diffuse reflectance spectroscopy (DRS).

The wide-angle XRD patterns of acid-activated bentonite samples (AAB and AAB-TX) (Figure 1A) present the characteristic peaks of montmorillonite (M) and cristobalite (C) [17]. In the case of bentonite activation in the presence of Triton X-100 (TX), a new diffraction peak can be observed at 2θ = 9.02°, and the peak shifts from 14.13° to 18.20°.

Figure 1. Wide-angle XRD patterns of AAB and AAB-TX (A) and for CdS–bentonite nanocomposites in comparison with that of CdS powder (B). Inset-small-angle XRD patterns of CdS–bentonite nanocomposites.

Previous research has shown that Triton X-100, a nonionic surfactant, primarily modifies the bentonite surface. This is most likely because TX molecules with the extended length of a single molecule in the range of 4–4.5 nm and forming micelles in solution with diameters in the range of 8–12 nm are too large to be intercalated into the interlayer space [22,23]. The distance between the SiO₄ (T) and AlO₆ (O) sheets of 13.04 Å for AAB decreased to 9.85 Å for AAB-TX, confirming that TX molecules were not intercalated in the bentonite structure.

The crystallographic composition of acid-activated bentonites determined by XRD was correlated with the chemical composition determined by XRF analysis (Table 1).

Table 1. Mineral composition of bentonites obtained using XRF.

Sample	Components Concentration (% wt)
	SiO2	Al2O3	Fe2O3	Other Oxides
Raw bentonite	70.57	26.11	3.04	0.28
AAB	71.19	25.42	3.07	0.32
AAB-TX	72.66	24.73	2.46	0.15

XRF analysis demonstrated small quantities of iron oxide and traces of other metal oxides (TiO₂, V₂O₅, ZrO₂, ZnO). The chemical composition of acid-activated bentonite samples changed slightly as a result of the acid activation compared to the initial composition, namely the solubilization of Al(III) with the increase in the SiO₂/Al₂O₃ mass ratio from 2.7 in raw bentonite to 2.8 in AAB and 2.94 in AAB-TX. The content of Fe₂O₃ remained approximately constant in AAB compared with the raw bentonite but decreased in the case of AAB-TX. It is interesting that, in the presence of TX, larger amounts of iron and other metallic oxides were solubilized.

Based on the larger distance between the SiO₄ (T) and AlO₆ (O) sheets and the better preservation of the chemical composition of the raw bentonite in the case of AAB than in the AAB-TX sample, we chose to deposit CdS on AAB. The resulting CdS–bentonite composites were structurally investigated by wide-angle XRD (Figure 1B), which confirmed the presence of both bentonite [17] and CdS [24]. The new peak at 2θ = 18.20°, which was found in the XRD pattern of AAB-TX, could also be found in the wide-angle XRD pattern of CdS-AAB 1 obtained by trituration, which involved the addition of TX during synthesis.

Two differences can be observed in the small-angle XRD patterns (Figure 1B-inset) of the samples. First, the diffraction peak characteristic of bentonite from 6.77° of AAB corresponding to the (0 0 1) crystal plane was shifted toward lower 2θ values (5.35° and 5.95° for CdS-AAB 1 and CdS-AAB 2, respectively). The interplanar spacing, d, was different—1.65 nm for CdS-AAB 1 and 1.48 nm for CdS-AAB 2, compared to 1.30 nm in the case of AAB. The increase in the interplanar space of bentonite is a consequence of the insertion of cations (Cd²⁺) between the anionic sheets of the clay [25], and a greater increase can be correlated with a higher amount of CdS. The shift of the peak toward lower 2θ values with the increase in the quantity of CdS in the interlayer space of AAB has also been observed by other authors [13].

The intercalation process may be altered by the nature of the interlayer space; if the interlayer space is insufficient to accommodate reactants like sulfide precursor, CdS particles may form beyond the interlayer space [1]. Other authors reported the synthesis of CdS both inside and outside the interlayer space and a correlation between the CdS particle dimension and the interlayer space size, but confirming the growth of CdS nanoparticles inside the clay interlayer space is difficult [1].

We also synthesized CdS by chemical precipitation, which, according to XRD analysis, had the structure of a zinc blende (cubic close-packed, JCPDS 10-0454) (Figure 1B). Thus, in the XRD pattern of CdS (Figure 1B-olive curve), three peaks can be observed, i.e., at 2θ = 26.31° (1 1 1), 43.83° (2 2 0), and 52.03° (3 1 1) [24,26]. The crystallite size for CdS, determined from the (1 1 1) Bragg reflection, is 7.16 nm, while for the CdS deposited onto bentonite, it is 1.94 (CdS-AAB 1), 6.33 (CdS-AAB 2), and 8.94 nm (CdS-AAB 3). The intensity of the diffraction peaks assigned to CdS is the lowest in the XRD pattern of CdS-AAB 1, probably due to poor crystallinity. The largest crystallite size was observed for CdS deposited on bentonite by the hydrothermal method (CdS-AAB 3), probably due to the higher synthesis temperature (180 °C compared to 80 °C for chemical precipitation). In this case, CdS is probably formed outside the bentonite interlayer space.

The synthesis of CdS by trituration, using a surfactant as a solvent, was previously reported [8]. This technique is considered to be an application of the reverse micelles method; namely, surfactant molecules form water-filled micelles, which function as “nanoreactors,” in which CdS nanoparticles are formed. A similar method was used for the synthesis of CdS-AAB 1. Thioacetamide generated sulfide ions over time, in a slow process, which combine with the Cd(II) ions adsorbed by bentonite. The synthesis of CdS can be observed by the specific color change in bentonite. A quite similar method for the synthesis of CdS in the presence of a surfactant was reported by Chen et al. as a solid-state synthesis; the authors used thiourea as a precursor for the sulfide ion [27]. Both methods resulted in a CdS nanopowder that was highly efficient in removing pollutants from aqueous solutions.

3.1.2. Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) Spectroscopy

The morphology of CdS–bentonite nanocomposites was investigated by SEM, and the distribution of CdS particles on clay was studied by EDX analysis (Figure 2). SEM images of acid-activated bentonites (Figure 2A,B) showed particles with irregular shapes and dimensions on the order of microns, even for the AAB-TX sample for which activation was performed in the presence of TX surfactant (Figure 2B). For CdS–bentonite composites, SEM characterization demonstrated a morphology similar to that of AAB (Figure 2A), but the clay aggregates had smaller dimensions (Figure 2C,G,E). Through elemental mapping carried out on the CdS–bentonite samples, we highlighted the presence of CdS particles, which were distributed quite uniformly on the bentonite (Figure 2D,F,H).

Figure 2. SEM images of AAB (A), AAB-TX (B), CdS-AAB 1 (C), elemental mapping for CdS-AAB 1 (D), CdS-AAB 2 (E), elemental mapping for CdS-AAB 2 (F), CdS-AAB 3 (G), and elemental mapping for CdS-AAB 3 (H).

Based on EDX analysis in two different areas of each sample, the CdS loading of bentonite for the composite samples was calculated based on both cadmium and sulfur content present in the samples (Table 2). The results revealed a higher amount of CdS in CdS-AAB 1 (19.7 ± 1.3% wt) than in other samples that have a similar metallic sulfide content (14.2 ± 1.1% wt for CdS-AAB 2 and 14.7 ± 0.5% wt for CdS-AAB 3). The highest amount of CdS in CdS-AAB 1 corresponds to the larger clay interplanar distance and could be correlated with the higher photocatalytic effectiveness of this composite.

Table 2. CdS content of CdS–bentonite composites calculated from EDX analysis.

Sample	CdS (% wt)
CdS-AAB 1	19.7 ± 1.3
CdS-AAB 2	14.2 ± 1.1
CdS-AAB 3	14.7 ± 0.5

3.1.3. Diffuse Reflectance Spectroscopy

The optical properties of CdS–bentonite nanocomposites were studied by diffuse reflectance spectroscopy (DRS). The UV-vis absorption spectra of nanocomposites in comparison with those of acid-activated bentonites and CdS were recorded (Figure 3).

Figure 3. UV-vis spectra of activated bentonites (A), CdS–bentonite composites (B), and CdS deposited onto AAB after subtracting the bentonite spectrum from each composite spectrum (C); Kubelka–Tauc plot for the estimation of CdS band gap energy (n = 2) (D).

A comparison between AAB and AAB-TX (Figure 3A) revealed the presence of a new band in the AAB-TX spectrum, which was probably derived from a shoulder located at about 254 nm in the AAB spectrum, whose intensity increased. By comparing the spectra of CdS-AAB 1-CdS-AAB 3 with that of CdS (Figure 3B), it is evident that the characteristic CdS band located at 474 nm is shifted to a lower wavelength. The absorption band is the largest in the spectrum of CdS-AAB 2 and the narrowest for CdS-AAB 1, which is in agreement with the light yellow and less intense color of the CdS-AAB 1 sample.

The shift in the characteristic band of CdS to a lower wavelength can be correlated with the smaller CdS particles in the resulting composites, as also demonstrated by XRD.

By subtracting the bentonite spectrum from each composite spectrum, the spectra of the deposited CdS were obtained (Figure 3C). It is interesting to note that the absorption band for the CdS-AAB composites is split into two bands, except for CdS-AAB 1, which has a single band in its spectrum, located at the same wavelength as it is in the composite spectrum. It should also be observed that the decrease in absorption intensity around 440 nm is probably due to the existence of a band at 437 nm in the bentonite spectrum, the intensity of which was mathematically subtracted.

Estimation of the band gap energy. The band gap energy (E_g) can be determined from electronic spectra using different approaches. Thus, the easiest method to estimate the band gap energy is based on the position of the absorption maximum in the UV-vis spectrum, expressed as wavelength, using the following equation:

$$E_g= {\displaystyle \frac{h \times c}{\lambda }}= {\displaystyle \frac{1240 \mathrm{eV}·\mathrm{nm}}{\lambda \left(\mathrm{nm}\right)}}$$

(1)

where h = 6.626 × 10⁻³⁴ J·s is Plank’s constant; c = 2.99 × 10⁸ m/s is the light velocity; λ = absorption edge wavelength, nm; and 1 eV = 1.6 × 10⁻¹⁹ J (conversion factor) [26]. Quite similar values of 2.99 eV (CdS-AAB 2 and CdS-AAB 3, λ = 415 nm) and 2.92 eV (CdS-AAB 1, λ = 425 nm) were determined for the prepared CdS–bentonite composites. All values are higher than the value of 2.62 eV (λ = 474 nm) determined for CdS, thus demonstrating the broadening of the band gap with decreasing particle size. However, the calculation of the band gap energy using the Kubelka–Munk and Tauc methods [26] is considered more accurate.

According to certain research, the band gap energy may still be overestimated when using the reflectance versus wavelength plot, known as the traditional Tauc plot. In order to achieve more precise values of optical band gap energy, the Kubelka–Munk function was combined with the standard Tauc’s equation [28,29]. The value of Eg obtained for CdS deposited on AAB (Figure 3D) for CdS-AAB 2 (2.34 V) is almost equal to the value determined for the CdS nanopowder (2.33 V). For CdS-AAB 3 (2.43 V), and especially for CdS-AAB 1 (2.59 V), the increase in band gap energy can be correlated with the smaller size of the CdS nanoparticles.

3.1.4. Fourier Transform Infrared (FTIR) Spectroscopy

The CdS–bentonite nanocomposites were analyzed by ATR-FTIR spectroscopy in the 4000–650 cm⁻¹ domain (Figure 4).

Figure 4. ATR-FTIR spectra of activated bentonites (A) and CdS–bentonite composites in comparison with that of CdS powder (B).

In the FTIR spectrum of activated bentonites (Figure 4A), the following bands assigned to -OH groups can be identified: OH stretching of inner hydroxyl groups of montmorillonite (3623 cm⁻¹ and 3630 cm⁻¹ for AAB and AAB-TX, respectively), OH stretching of water (3384 cm⁻¹), and deformation band of adsorbed water molecules (1629 cm⁻¹) [30]. The intensity of the stretching band assigned to water molecules is very low due to the removal of water from raw bentonite during the activation process. Conversely, the intensity of the stretching vibration of the inner hydroxyl groups remains intense in the activated bentonite sample and CdS–bentonite nanocomposites, demonstrating the existence of inner OH groups.

In the FTIR spectrum of AAB-TX (Figure 4A), one can notice the presence of some additional bands from TX used during bentonite activation, such as 2945 cm⁻¹ and 2870 cm⁻¹ (C-H stretching), 1509 cm⁻¹ (C=C aromatic), 1457 cm⁻¹ (CH₂ bending), and 1243 cm⁻¹ (C-O-C stretching).

The most intense band assigned to bentonite is situated around 1000 cm⁻¹ and is attributed to Si–O–Si bonds (asymmetric stretching in longitudinal mode and Si–O stretching of cristobalite). Other bands are identified at 917 (Al-OH deformation), 790 (Si–O symmetric stretching of cristobalite), and 708 cm⁻¹ (Si–O) [30]. The bands assigned to bentonite, which are consistent with the crystallographic composition revealed by XRD, were also found in the prepared CdS–bentonite composites. In the FTIR spectrum of CdS (Figure 4B), one can see very weak bands assigned to water molecules (a broad band at 3300 cm⁻¹) and acetate ion (1543 cm⁻¹ and 1420 cm⁻¹). The bands observed in the FTIR spectra of CdS–bentonite composites can be assigned to bentonite with or without a slight shift compared to those in the AAB spectrum.

3.1.5. Specific Surface Area

The specific surface area, S_BET, was determined by the BET method in the 0.05–0.25 relative pressure range. A large difference between S_BET values was observed: 9 m²/g for CdS-AAB 1 and 40 m²/g for CdS-AAB 2. The lower specific surface area of CdS-AAB 1 in comparison with that of CdS-AAB 2 could be explained by the adsorption of TX molecules on the activated bentonite surface.

3.2. Photocatalytic Properties

The photocatalytic properties of CdS–bentonite nanocomposites were assessed in the removal of Congo red (CR) from the aqueous solution. CR is suspected to be toxic and interferes with aquatic photosynthesis [31]. The results obtained for the CdS-AAB 1 nanocomposite were particularly good, prompting us to continue the experiments using a higher photocatalyst concentration, specifically for CdS-AAB 2. To obtain information about the mechanism of CR removal in the presence of CdS-AAB powders, we used scavenger molecules.

Dark experiments were the basis for determining the adsorption capacity of the studied materials. Therefore, considering that the systems reach equilibrium after 60 min, the adsorption capacity was calculated as follows:

$$Q_e= {\displaystyle \frac{{(C}_i- C_e)\times V}{m}}$$

(2)

where C_i and C_e are the initial concentration and the concentration after 60 min, respectively, for CR solutions (mg/L; measured at 498 nm); V is the volume of CR solution (L); and m is the mass of adsorbent (g).

The adsorption capacity, determined for 50 mg of material in 100 mL of 30 mg/L CR solution, is represented in Figure 5A. The highest adsorption capacity was determined for CdS-AAB 1, followed by CdS-AAB 3, with CdS-AAB 2 showing almost the same adsorption capacity as CdS powder. We observed that a higher amount of CdS in the composite leads to a better adsorption capacity of the composite powder. Compared to composites, the adsorption capacity of AAB is lower. The lower capacity of AAB to adsorb CR can be explained based on the electrical charge and structure of CR. Thus, it is known that clays have a high capacity to adsorb positively charged molecules due to the negative charge of the atomic layers in their structure. However, acid-activated clays have the capacity to adsorb anionic dyes due to the positive charge of the surface, as reported by other studies [32]. It is likely that the linear structure of the CR molecule also contributes to a moderate adsorption capacity. The increase in adsorption capacity for the composites may be due to a further increase in the positive surface charge and the presence of Cd(II) ions to which the CR molecules can coordinate.

Figure 5. Adsorption capacity of CdS–bentonite composites compared to CdS and AAB (A); CR removal efficiency of CdS-AAB composites compared to CdS and AAB (50 mg powder/100 mL CR solution) (B); CR removal efficiency of CdS-AAB 2, CdS and AAB (100 mg powder/100 mL CR solution) (C).

The bleaching of CR solution was estimated using the C_t/C_i ratio (where C_t and C_i are the concentration of CR at a certain time, t, and the initial concentration, respectively). The efficacy in the removal of CR from the solution was estimated as the removal efficiency (RE%):

$$RE\%= {\displaystyle \frac{C_i-C_t}{C_i}} \times 100 = {\displaystyle \frac{A_i-A_t}{A_i}}\times 100$$

(3)

where A_i, A_t are the absorbance values for CR solutions measured at 498 nm for a certain time, t, and the initial concentration (based on Lambert–Beer law) [8].

The CR removal efficiency can be observed in Figure 5B, which highlights the best effectiveness of CdS-AAB 1 in the bleaching of the CR solution.

After the experiment in dark conditions, which consists of CR adsorption on powder (50 mg/100 mL of CR solution), CdS-AAB 1 had the highest removal efficiency (88.8%), i.e., the highest adsorption capacity (Figure 5B), although its specific surface area was lower than that of CdS-AAB 2. The removal efficiency values after 60 min in the dark for CdS (62.4%) and CdS-AAB 2 (63.1%) were comparable and lower than that of CdS-AAB 3 (73.3%), all being superior to AAB (53.56%). After 120 min of photocatalytic process, the efficiency of Cd-AAB composites (92% for CdS-AAB 1, 84.7% for CdS-AAB 2, and 87.1% for CdS-AAB 3) was superior to both CdS (76.2%) and AAB (53.5%).

It is noteworthy that, starting from AAB, which has the lowest efficiency, and continuing with its combination with CdS, a semiconductor with good photocatalytic activity, we obtained composites with an efficiency higher than that of bentonite or CdS. It is particularly remarkable that in the CdS-AAB composites, the amount of CdS is small (in the range of 14.2–19.7% wt), having a higher efficiency than pure CdS, which makes the material much more environmentally friendly. A fairly similar efficiency, over 90%, has been previously reported for CdS nanopowder obtained by trituration [8].

By using a higher quantity of powders (i.e., 100 mg/100 mL of CR solution), some differences can be observed (Figure 5C). Thus, after the experiment in the dark, the quantity of CR adsorbed by AAB and CdS-AAB 2 is quite similar, with RE values of 55.1% (CdS-AAB 2) and 50.6% (AAB), while an increase can be observed for CdS (72.2%). After 120 min, the highest amount of CR was removed by CdS powder (84.8%), a slightly better result than in the previous experiment, but the AAB removal efficiency remained approximately the same (55.9%). The efficiency of CdS-AAB 2 increased to 70.7%, which is almost equal to that of CdS, but it is more environmentally friendly given the low CdS content.

The UV-vis spectra of powders after the photocatalytic/adsorption experiments (Figure 6) confirmed their color change during the CR removal process. Thus, the band due to the adsorbed dye is weak for CdS compared to AAB, and an intermediate behavior can be observed for CdS-AAB composites, demonstrating the presence of moderate quantities of adsorbed dye and confirming the removal of CR both by adsorption and photodegradation.

Figure 6. UV-vis spectra of powders after the removal of CR from aqueous solutions.

The main process in the case of calcium bentonite clays (formed from the smectite clay mineral montmorillonite) mainly involves ion exchange and electrostatic interactions on the surface of the clay particles. Hence, changes in the composition of the clay can be made to enhance its ability to adsorb anionic dyes [33]. Many studies have addressed the removal of cationic dyes by adsorption onto clays [21,34], but fewer have considered the adsorption of anionic dyes [32,33]. In the adsorption of a dye onto a clay, two potential adsorption mechanisms may occur: electrostatic interactions between the surface groups of the clay and the dye functional groups and/or a chemical reaction between the clay and the dye [32].

Considering the acid activation of bentonite, it is expected that its surface is protonated, which favors interaction with CR, an anionic dye, and supports the mechanism through electrostatic interactions. On the other hand, the absorption band located at approximately 660 nm in the AAB spectrum after removal of CR indicates a color change in CR, which suggests a chemical interaction. The deep red color (absorption at 660 nm) may also be due to the color change in CR caused by increasing the pH.

To compare the photocatalytic efficiency, the pseudo-first-order model was employed for quantitative evaluation, with the rate constant, k_app values presented in Table 3. The rate constants were calculated as follows [10]:

$$\mathit{ln}{\displaystyle \frac{C}{C_0}} = k_{app}\times t$$

(4)

where C₀ is the initial concentration of CR (after the experiment in dark) (mg/L); C is the concentration of CR at time t; and k_app is the pseudo-first-order reaction apparent rate constant.

Table 3. Kinetic parameters for the photocatalytic degradation of CR (30 mg/L).

Sample	CdS-AAB 1	CdS-AAB 2	CdS-AAB 3	CdS	CdS-AAB 2	CdS
Catalyst mass	50 mg	50 mg	50 mg	50 mg	100 mg	100 mg
kapp (10−3 s−1)	1.24	4.31	2.82	2.82	2.26	2.80
Standard error of kapp (10−4)	7.69 × 10−5	1.01 × 10−3	9.79 × 10−4	2.59 × 10−4	3.22 × 10−4	3.54 × 10−4
R2	0.985	0.819	0.674	0.967	0.925	0.940

A good fit with the pseudo-first-order kinetic model can be observed, except for CdS-AAB 3 (Table 3). The values of the rate constants are similar, except for CdS-AAB 1 and CdS-AAB 2 (50 mg/100 mL solution). It can be observed that, for CdS-AAB composites, the rate constant value depends on the amount of CdS, the adsorption capacity, and the ratio of photocatalyst to dye. For CdS, however, the reaction rate seems to be independent of the CdS/CR ratio.

Effect of scavengers. We used four scavengers to elucidate the CR degradation mechanism in the presence of CdS–bentonite composites. The addition of a certain scavenger can cause a significant reduction in photocatalytic activity, thus suggesting that the captured radicals are the main active species in the photocatalytic system [35].

Ethanol, ethylene diaminotetraacetic acid disodium salt (EDTA), ascorbic acid (AA), and AgNO₃ were used as scavengers of hydroxyl radicals (·OH), holes (h·), superoxide radicals (·O₂⁻), and electrons (e⁻), respectively. The effect of scavengers on CR removal is questionable, since CdS–bentonite composites can act as both adsorbents and photocatalysts. Therefore, the added scavengers may also modify the adsorption capacity of the AAB component of CdS-AAB composites. However, as noted by other researchers [16], the results should be considered carefully, since scavengers may interact with the substrate on the surface of the photocatalyst and not only react with the target reactive species.

Considering the removal of CR as a photocatalytic degradation, the normalized degradation (ND %) after 60 min in relation to the degradation in the absence of a scavenger (NS) can be calculated as follows [35]:

$$ND \%= {\displaystyle \frac{C_0-C_{t = 60 \min + scavenger}}{C_0-C_{t = 60 \min }}} \times 100= {\displaystyle \frac{the amount of decomposed dye with scavenger addition}{the amount of decomposed dye}} [\%]$$

(5)

where C_t and C_i are the concentrations of CR at a certain time, t, and initial concentration, respectively.

The addition of ethanol had the strongest effect (ND = 23.6%) in the photodegradation of CR, suggesting that the main active species involved in photocatalysis are hydroxyl radicals (Figure 7A). Acetic acid also influenced the photocatalytic process (ND = 87.6%), but to a lesser extent. This behavior confirmed the involvement of superoxide anions in the photocatalytic process. Since clays are known to promote the production of superoxide radicals, it is possible that the influence of ascorbic acid was underestimated due to a higher concentration of superoxide. The influence of Ag(I), which is an electron scavenger, was the lowest (ND = 94.9%).

Figure 7. Effect of scavengers on the photocatalytic degradation of CR in the presence of CdS-AAB 2: normalized degradation after 60 min (A); variation in CR concentration (B) (NS—no scavenger).

The drastic reduction in photocatalytic activity in the presence of EDTA, known as a hole scavenger, means that the reduction in hole concentration led to a decrease in the removal efficiency of CR (Figure 7A,B). This behavior could imply the involvement of holes in the degradation of CR, but this would not explain the increase in CR concentration upon addition of EDTA. It is expected that EDTA, due to its high ability to coordinate to metal ions, may be coordinated to bentonite cations (i.e., Ca²⁺, Mg²⁺) and thus interfere with dye adsorption, reducing the adsorption capacity of AAB. It is obvious that, after an initial decrease during the experiment in the dark, the amount of CR in the solution increased by adding EDTA, with a desorption occurring that was not observed in the case of the other scavengers.

Studies have shown that reactive oxygen species (ROS), i.e., hydroxyl radicals, superoxide ion radicals, hydrogen peroxide, and singlet oxygen mainly contribute to the degradation of pollutants in the presence of a semiconductor. Hydroxyl and superoxide ion radicals are formed in the redox reaction due to photogenerated electrons and holes [36]. The role of EDTA in the photocatalytic process is to trap holes, the highly reactive positive charges on the photocatalyst surface [37]. However, a decrease in reaction time has also been reported in the presence of EDTA as a scavenger, which is explained by the enhanced generation of photogenerated electrons. Thus, while moderate EDTA concentrations (10–20 mM) can improve photocatalyst performance, higher concentrations (as 30 mM) have the effect of passivation of semiconductor active catalytic sites due to strong interactions, ultimately hindering photocatalytic activity [38].

The photocatalytic activity of CdS and CdS-based composites has been extensively described in many studies, both for the photodegradation of organic dyes [39] and also in other reactions, such as the generation of hydrogen [40].

A semiconductor like CdS is exposed to light during photocatalysis, which causes photon absorption and excitation of an electron from the valence band to the conduction band, creating a positive hole in the valence band. The charge carriers have the ability to move to the catalyst surface and start secondary reactions with the materials that are adsorbed there. For instance, the positive holes in the valence band can oxidize adsorbed water or hydroxide ions and produce hydroxyl radicals, which in turn oxidize the organic pollutants. Also, the photoexcited electron in the conduction band can react with oxygen to form superoxide radicals or hydroperoxide radicals, which can participate in the degradation of organic pollutants [41].

The composites obtained by CdS deposition onto clays through different methods were also tested in the removal of pollutants from aqueous solutions. Wang et al. showed that clay mineral/CdS photocatalysts based on one-dimensional clay minerals have more photocatalytic activity for CR degradation than two-dimensional ones [10]. Boukhatem et al. demonstrated that CdS-montmorillonite composites obtained by the hydrothermal method have a superior efficiency in the removal of methylene blue and Rhodamine 6G compared to those of Na-montmorillonite and CdS [42]. Zhang et al. studied the CdS–bentonite composite as a photocatalyst in hydrogen production by water splitting [13]. They reported that the intercalation of CdS nanoparticles into the bentonite interlayer space facilitated the transfer of photogenerated electrons from CdS to the bentonite layer, and this was conducive to the enhancement of hydrogen generation.

4. Conclusions

In order to obtain materials that are more environmentally friendly than CdS and that have a high capacity for removing pollutants from wastewater, we combined the adsorbent properties of bentonite with the photocatalytic properties of CdS. We proposed trituration in the presence of a surfactant for the deposition of CdS on bentonite, and the resulting composites were characterized by XRD, XRF, SEM, EDX, DRS, and FTIR. The composite prepared by trituration contained the highest number of CdS particles with lower crystallinity. CdS-AAB 1 also exhibited the best photocatalytic properties for Congo red removal from wastewater when compared with the other composites. Using the same amount of photocatalyst, 50 mg/100 mL of CR solution, CdS-AAB 1 achieved a dye removal efficiency of 92.04%, while CdS had an efficiency of only 76.22%, although the proposed composite contained five times less CdS with known toxicity. Since bentonite, the major component in the developed composite, does not exhibit toxicity and is used in the wine industry, the toxicity of the CdS–bentonite composite is lower, and it can be successfully applied in wastewater treatment.