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Article

Mesoporous SBA-15-Supported Ceria–Cadmium Composites for Fast Degradation of Methylene Blue in Aqueous Systems

by
Dănuţa Matei
1,
Abubakar Usman Katsina
2,*,
Diana-Luciana Cursaru
1 and
Sonia Mihai
1
1
Faculty of Petroleum Technology and Petrochemistry, Petroleum-Gas University of Ploiesti, 100680 Ploiesti, Romania
2
Department of Pure and Industrial Chemistry, Bayero University, PMB 3011, Kano 700006, Nigeria
*
Author to whom correspondence should be addressed.
Water 2025, 17(12), 1834; https://doi.org/10.3390/w17121834
Submission received: 19 April 2025 / Revised: 14 June 2025 / Accepted: 17 June 2025 / Published: 19 June 2025
(This article belongs to the Special Issue Advanced Design and Synthesis of Novel Photocatalyst Materials for Wastewater Remediation)
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Abstract

A composite photocatalyst of ceria–cadmium supported on mesoporous SBA-15 silica was synthesized and employed for the aqueous methylene blue (MB) degradation. The composites were prepared using an incipient wetness impregnation technique and a conventional sol–gel approach with triblock copolymer P123 as a structure-directing agent for SBA-15 preparation, enabling the uniform dispersion of CeO2 and Cd species within the SBA-15 framework. The physicochemical properties of both CeO2/SBA-15 and Cd-CeO2/SBA-15 composites were analyzed using small-angle and wide-angle XRD, FT-IR spectroscopy, SEM, TEM, EDX spectroscopy, N2 physisorption at 77 K, and UV-Vis spectroscopy. The findings revealed that the SBA-15 support retained its well-ordered hexagonal mesostructure in both the ceria–SBA-15 and SBA-15-supported cadmium–ceria (Cd-CeO2) composites. The highest degradation efficiency of 96.40% was achieved under optimal conditions, and kinetic analysis using the Langmuir–Hinshelwood model indicated that the MB degradation process followed pseudo-first-order kinetics, with a strong correlation coefficient (R2 = 0.9925) and a rate constant (k) of 0.02532 min−1. Under irradiation, the Cd-CeO2/SBA-15 composites exhibited superior photocatalytic activity compared to the pristine components, owing to the synergistic interaction between ceria and cadmium, enhanced light absorption, and improved charge carrier separation. The recyclability test demonstrated that the degradation efficiency decreased slightly from 96.40% to 94.86% after three cycles, confirming the stability and reusability of Cd-CeO2/SBA-15 composites. The photocatalytic process demonstrated a favorable electrical energy per order (EE/O) value of 281.8 kWh m−3, indicating promising energy efficiency for practical wastewater treatment. These results highlight the excellent photocatalytic performance and durability of the synthesized Cd-CeO2/SBA-15 composites, making them promising candidates for facilitating the photocatalytic decomposition of MB and other dye molecules in water treatment applications.

1. Introduction

The rapid industrialization and urbanization in recent decades have led to a significant increase in the discharge of toxic organic substances like dyes into water bodies. Methylene blue (MB; C16H18ClN3S) is a synthetic dye commonly used in various industries like the textile [1], paper [2], and plastic [3] industries. Its presence in wastewater poses a significant environmental threat due to its toxicity, persistence, and potential carcinogenic effects [4,5]. Despite representing a small fraction of wastewater organic load, dyes are visually detectable even at low concentrations, making their removal essential for maintaining the aesthetic and ecological value of water bodies. Traditional chemical oxidation treatments, including the use of chlorine [6], ozone [7], or hydrogen peroxide [8], often produce toxic intermediates, further complicating water purification efforts. As a result, developing effective, sustainable, and advanced solutions for dye removal has become a priority. Semiconductor-based photocatalysis has emerged as a promising eco-friendly approach for degrading persistent organic pollutants [9,10]. By utilizing light energy, semiconductor photocatalysts generate reactive species capable of converting harmful dyes into harmless end products like CO2 and H2O [11].
The efficiency of photocatalytic systems depends on factors such as surface area, charge separation efficiency, and light absorption capacity [12,13]. Extensive research has demonstrated that anchoring metal oxides on mesoporous materials significantly enhances their photocatalytic activity for removing pollutants from water, due to increased surface area, improved dispersion, and better light-harvesting efficiency [14]. Among various semiconductor materials, ceria (CeO2) has garnered significant attention due to its unique redox properties, oxygen storage capacity, and ability to generate reactive oxygen species [15,16]. However, pristine ceria often demonstrates limited photocatalytic performance, primarily due to fast charge recombination and limited surface area [17,18]. To overcome these limitations, the incorporation of ceria into mesoporous supports such as SBA-15 silica [19,20] has been shown to enhance its photocatalytic properties in various photocatalytic applications.
Mesoporous silica materials like SBA-15 are widely studied photocatalyst supports, valued for their large surface area [21], tunable pores [22], and excellent thermal stability [23]. Incorporating active catalytic components can significantly enhance its photocatalytic efficiency. In a related study [24], TiO2 immobilized on SBA-15 exhibited high photocatalytic efficiency for solar-driven pesticide degradation in water, attributed to the enhanced adsorption and electron delocalization properties of SBA-15. Another report [25] compared TiO2, ZnO, and SnO2 supported on ordered mesoporous silica for the removal of cationic MB from neutral aqueous media via UV-assisted adsorption and photodegradation.
Additionally, recent efforts to overcome the intrinsic limitations of pristine CeO2 involve doping CeO2 with transition metals, including cadmium (Cd), nickel (Ni), and iron (Fe), which can extend visible light absorption, enhance charge separation, and promote reactive species generation [26,27]. For instance, transition metals of Fe, Cu, Ni, Cr, and Co were introduced into TiO₂ uniformly dispersed within SBA-15 using a two-step synthesis approach [28]. Separately, ZnO nanoparticles were synthesized and anchored onto MCM-41 and SBA-15 supports via a simple solvothermal impregnation method [29].Although cadmium’s toxicity is a concern, its incorporation into a stable mesoporous matrix like SBA-15 mitigates these risks [30,31]. According to our literature search, the combination of Cd and CeO2 on an SBA-15 support has rarely been explored, making this work the first report on SBA-15-supported cadmium-doped ceria (Cd-CeO2/SBA-15) composites specifically designed for the photocatalytic degradation of MB in aqueous systems.
In this study, we report the synthesis and characterization of SBA-15-supported Cd-CeO2 composites via a facile impregnation technique for their application in the photocatalytic degradation of MB in aqueous systems. The aim is to harness the synergistic effects of ceria and Cd species within a robust SBA-15 support to achieve enhanced charge separation and photocatalytic efficiency [32,33,34]. The study evaluates key parameters, including catalyst dosage and reaction conditions, demonstrating the potential of this composite material as a sustainable solution for water pollution mitigation.

2. Materials and Methods

The catalysts were prepared using ethanol (96%, Chemical Ch-C, Iasi, Romania), polyethylene glycol (Sigma Aldrich, Darmstadt, Germany), and Pluronic P123 triblock copolymer (EO20PO70EO20, 5800, Sigma Aldrich, Chemie GmbH, Eschenstrasse, Germany), with tetraethyl orthosilicate (TEOS, 98%, Sigma Aldrich, Chemie GmbH, Eschenstrasse, Germany) serving as the silica precursor and organic template. Cerium nitrate hexahydrate (99%, Sigma Aldrich, Chemie GmbH, Eschenstrasse, Germany) and cadmium acetate dihydrate (98%, Sigma Aldrich, Chemie GmbH, Eschenstrasse, Germany) were used as cerium oxide and cadmium precursors, respectively. All substances were utilized in their as-purchased state, with no further treatment.

2.1. Synthesis of SBA-15 Support

The SBA-15 mesoporous framework was synthesized following a modified method presented by Zhao et al. [35]. Briefly, a transparent solution was obtained by dissolving 6 g of Pluronic P123 in deionized water and 35% HCl under continuous stirring at 35 °C for 6 h. Thereafter, 12.49 g of TEOS was added gradually, yielding a gel-like mixture. The homogeneous solution was sealed in a Teflon bottle and subjected to hydrothermal aging at 100 °C for 24 h. The product was filtered, washed thoroughly, and dried at 60 °C. Calcination at 550 °C for 5 h in air removed the organic template, yielding SBA-15 mesoporous powder.

2.2. Synthesis of Cd-CeO2/SBA-15 Composites

The fabrication of CeO2/SBA-15 composites followed a procedure adapted from Zhan et al. [36]. Initially, a solution containing 3 g of SBA-15 and 1 g of Ce(NO3)3·6H2O dissolved in 20 mL of ethanol was stirred for 2 h and then oven-dried at 127 °C. The dried material was calcined at 550 °C for 5 h in air. To synthesize Cd-CeO2/SBA-15 composites, the incipient wetness impregnation method was employed. Specifically, a cadmium precursor solution of Cd(CH3COO)2·2H2O in an ethanol medium was used to impregnate 1 g of the prepared CeO2/SBA-15 with continuous agitation (2 h, ambient conditions). The resulting mixture was then dried in an oven at 100 °C for 48 h.

2.3. Characterization

The framework characteristics of SBA-15 and its CeO2- and Cd-CeO2-modified counterparts were analyzed by Fourier-transform infrared (FT-IR) spectroscopy (Schimadzu, Tokyo, Japan) and X-ray diffraction (XRD; Bruker, Karlsruhe, Germany). FT-IR spectra were recorded in the range of 4000–400 cm−1 using a Shimadzu IRTracer-100 spectrophotometer (Tokyo, Japan). X-ray diffraction analyses, including small-angle (SA-XRD) and wide-angle X-ray diffraction (WA-XRD), were conducted on a Bruker D8 Advance diffractometer (Karlsruhe, Germany). The instrument utilized CuKα radiation (λ = 1.5418 nm) with a graphite monochromator, operating at 40 kV and 40 mA. SA-XRD patterns were recorded over a 2θ range of 0–3°, and WA-XRD patterns over 10–70°, at a scan speed of 0.1° per 5 s. Phase identification was conducted using Diffracplus Basic software (version 3.2) and the PDF-ICDD 2-2008 database, while quantitative analysis was performed with Diffracplus TOPAS 4.1 software.
Surface area and pore characteristics of SBA-15, CeO2/SBA-15, and Cd-CeO2/SBA-15 were determined through N2 adsorption–desorption isotherms at 77 K, measured with a Quantachrome Nova 2200e analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). Prior to the analysis, the samples were degassed at 473 K for 4 h under vacuum. Data analysis was performed using NovaWin software (version 1.0) (Boynton Beach, FL, USA).
Morphological differences between SBA-15 and its modified composites were revealed through dual-beam FIB-SEM analysis (Scios 2 HIVAC system, Thermo Fisher, Brno, Czech Republic) at various magnifications. Elemental analysis was carried out using an EDAX energy-dispersive X-ray (EDX) detector Thermo Fisher, Brno, Czech Republic) attached to the same setup. High-resolution transmission electron microscopy (HRTEM) was conducted with an FEI Tecnai G2 F-20 TWIN Cryo-TEM (FEI American Company, Brno, Czech Republic) operated at 200 kV with magnifications of 20,000× and 80,000×.
The optical properties of the synthesized materials were analyzed using UV-Vis spectroscopy (Schimadzu, Tokyo, Japan). This technique was employed to investigate the absorption spectrum, which provides insights into the electronic structures and optical bandgaps of the as-synthesized materials, essential for evaluating its photocatalytic potential. Measurements were performed over the 200–500 nm range to enable detailed analysis of light absorption behavior.

2.4. Photocatalytic Study

The photocatalytic performance of the synthesized composites was assessed by monitoring the degradation of aqueous methylene blue (MB) solution using a Toption photochemical reactor (TOPTION INSTRUMENT Co., Ltd., Xi’an, China).The lamp emits a broad spectrum of 400–700 nm, with 150 W lamp power and a measured light intensity of 65 mW/cm2 at the sample surface. To attenuate IR radiation, a liquid water filter was implemented in the optical path. The reactor temperature was maintained near ambient (~25 ± 2°C) throughout the experiment.
Catalyst dosages of 0.4 g L−1, 0.6 g L−1, and 0.8 g L−1 were investigated at an initial dye concentration of 28.8 mg/L and pH 6.7. In a typical experiment, the catalyst was dispersed in 50 mL of the MB solution under constant stirring, after which the mixture was exposed to irradiation using the photoreactor. To establish baseline performance, CeO2/SBA-15 was subjected to identical testing protocols. The degradation was tracked over a period of 120 min, with aliquots taken every 15 and 30 min to evaluate the methylene blue removal efficiency using a Shimadzu 3600iPlus UV-Vis spectrophotometer. A free radical assessment was performed using ascorbic acid, 2Na-EDTA, and isopropyl alcohol as scavengers to detect the catalytically active species responsible for the breakdown process. These scavengers helped elucidate the roles of superoxide radicals, hydroxyl radicals, and holes in the photocatalytic activity. Furthermore, a recyclability study was performed to evaluate the stability and reusability of the photocatalyst through four consecutive cycles of MB degradation conducted under consistent experimental conditions. Following each cycle, the catalyst was collected by filtration, meticulously washed with ethanol and deionized water, and dried prior to reuse.

3. Results

3.1. SEM/EDX Analysis

The SEM micrographs in Figure 1a reveal that the SBA-15 material consists of particles with a well-organized hexagonal structure, characteristic of the mesostructural arrangement of amorphous SBA-15 as documented in the literature [37]. Similarly, Figure 1b,c show a relatively uniform distribution of particles within the interstitial spaces of SBA-15 after impregnation, which can be attributed to the incorporation of the CeO2 phase and Cd species into the mesopores. The mesoscopic structures and ordering of the as-synthesized SBA-15 mesopores are effectively retained upon the introduction of CeO2 and Cd species. The EDX spectra in Figure 1d confirm the presence of silica, carbon, cerium, cadmium, and oxygen atoms as the constituent elements in the as-synthesized Cd-CeO2/SBA-15 particles. The detected carbon signal originates from the sample coating applied for SEM analysis. For comparison, the EDX spectra of the pristine SBA-15 support and CeO2/SBA-15 composites are included in the Supplementary File (Figure S1), confirming the absence of Cd and Ce element species in the control sample (SBA-15).

3.2. FTIR Analysis

Figure 2 below shows the FT-IR profiles of SBA-15, CeO2/SBA-15, and CeO2/SBA-15 samples. The FTIR spectra of the three samples show characteristic absorption bands of functional groups with similar vibrational frequencies, confirming the structural and chemical integrity of the mesoporous SBA-15 material. Vibrational modes associated with Si-O-Si stretching, symmetric Si-O stretching, and Si-O bending vibrations of the SBA-15 framework were observable around 1080 cm−1, 800 cm−1, and 467 cm−1, respectively [38,39]. Additional bands observed in the range of 500–600 cm−1 in the FT-IR spectrum of CeO2/SBA-15 can be attributed to Cd-O or Ce-O-Si groups, indicating successful dispersion of metal oxides on the mesoporous silica surface [40]. The uniformity and intensity of these bands highlight the effective integration of Cd-CeO2 within the SBA-15 matrix without significant disruption to the silica ordering network of SBA-15. In addition to the characteristic Si-O-Si and Ce-O bands, new absorption peaks appeared at 1425 cm−1 and 1550 cm−1 in the spectra of Cd-CeO2/SBA-15. These bands are typically associated with the asymmetric and symmetric stretching modes of the carboxylate group (-COO−), which likely originate from residual cadmium acetate or the coordination of Cd2+ ions with oxygen-containing groups on the silica surface. Their presence supports the successful incorporation of Cd species and suggests potential interactions with CeO2 or SBA-15, which potentially influence the electronic structure and contribute to improved photocatalytic performance.

3.3. XRD Analysis

The diffraction patterns obtained from the as-prepared Cd-CeO2/SBA-15 photocatalysts were analyzed in both small-angle and wide-angle regions to investigate the structural and crystalline properties of the composite. Figure 3a shows small-angle XRD (SA-XRD) patterns, revealing diffraction peaks indexed to the (100) and (200) planes of the hexagonal p6mm space group [41,42]. These features confirm the successful formation of the typical hexagonal symmetry and ordered mesoporous architecture of amorphous SBA-15 across all samples during synthesis and calcination. Additionally, the retention of these reflections after CeO2 and Cd incorporation indicates that the mesostructural integrity of SBA-15 was preserved. Figure 3b shows a wide-angle XRD (WA-XRD) region, exhibiting distinct diffraction peaks at 2θ values of approximately 28.55°, 33.08°, 47.48°, 56.33°, 59.09°, and 69.40°, which are consistent with the cubic fluorite structure of pure cerium oxide (CeO2) (JCPDS card No. 34-0394) [42]. These angles matched the (111), (200), (220), (311), (222), and (400) crystal planes, respectively. The peak broadening in the XRD pattern indicates CeO2 crystallites with nanoscale dimensions. Additionally, distinct diffraction peaks at 2θ values corresponding to the (111), (200), (220), and (311) planes were observed, confirming the formation of a cubic CdO crystal structure and a monoclinic Cd(OH)2 phase (JCPDS card No. 05-0640). The XRD peaks for the (311) and (222) planes are very close and overlap, making them indistinguishable. This is due to their similar interplanar spacings, which are common for this material’s structure. The results indicate that the Cd-CeO2/SBA-15 composite successfully combines ordered mesoporosity with distinct crystalline phases.

3.4. N2 Physisorption Analysis

The porosity and surface characteristics of SBA-15, CeO2/SBA-15, and Cd-CeO2/SBA-15 were analyzed through nitrogen adsorption–desorption measurements at 77 K. As shown in Figure 4, all samples exhibited type IV isotherms, consistent with mesoporous materials based on the 2015 IUPAC classification [43]. An H1-type hysteresis loop was observed in the nitrogen adsorption–desorption isotherm of the SBA-15 support, indicating a narrow range of uniform cylindrical mesopores with a highly ordered pore structure [44]. Upon the incorporation of CeO2 and cadmium species, atypical H5-type hysteresis loops were observed in the isotherms of the CeO2/SBA-15 and Cd-CeO2/SBA-15 composites, characteristic of partially blocked mesoporous structures. The observed decreases in BET surface area, pore size, and pore volume (Table 1) indicate partial blockage and surface coverage of mesoporous channels, likely resulting from the deposition of CeO2 and cadmium species. Despite these changes, the mesoporous framework of SBA-15 was retained, as evidenced by the persistence of type IV isotherms. Modification with CeO2 and subsequent Cd doping transformed the originally unimodal mesostructured (4.74 nm) into a bimodal pore system with a dominant 3.71 nm feature, indicating effective incorporation of metal oxides within the SBA-15 channels. This shift highlights the effective dispersion of metal oxide nanoparticles within the mesopores, confirming the successful functionalization of SBA-15 while preserving its structural integrity. These textural properties are crucial for enhancing catalytic activity by providing accessible active sites for photocatalytic reactions.

3.5. UV-Vis Analysis

Figure 5 shows the UV-Vis spectra and Tauc plots of (αhν)1/2 against the optical bandgap energy (hν) of the CeO2/SBA-15 and Cd-CeO2/SBA-15 composites. Figure 5a shows the UV absorption band observed around 328 nm, which is attributed to the charge transfer transition in CeO2 particles [45]. Meanwhile, the absorption band of Cd-CeO2/SBA-15 shifts a little beyond 450 nm, extending beyond that of the undoped CeO2/SBA-15, due to the presence of Cd species. Figure 5b exhibits the optical bandgap energy (Eg) values as determined using a Kubelka–Monk plot. The calculated Eg values for the CeO2/SBA-15 and Cd-CeO2/SBA-15 catalysts were determined. A little decrease in Eg was observed, dropping from 3.16 eV for CeO2/SBA-15 to 3.02 eV for Cd-CeO2/SBA-15 composites, attributed to the relatively weak interaction between Cd species and the CeO2/SBA-15 system. However, the redshift in the absorption band and the corresponding decrease in Eg with Cd doping suggests interactions between Cd species and highly dispersed CeO2 on SBA-15 support, facilitating electron transfer.

3.6. Photodegradation of MB

The light-driven effectiveness of the as-prepared Cd-CeO2/SBA-15 composites was evaluated through the degradation of aqueous MB solutions under visible light irradiation. The degradation process was monitored using UV-Vis spectrophotometry, with the absorbance of MB recorded at regular intervals. Prior to light irradiation, a 30 min dark adsorption period was applied, during which a small percentage of MB dye was removed by the catalyst, indicating a measurable contribution from adsorption. This step was necessary to ensure that the observed degradation was primarily photocatalytic. Figure 6 shows the effect of catalyst dosage, expressed in g L−1, on the photocatalytic degradation efficiency of MB. An increase in dosage from 0.4 to 0.8 g L−1 improved the degradation rate, likely due to the increased number of active sites and light-absorbing centers.
Figure 6a–c,e illustrate the degradation performance for different catalyst doses (0.4 g L−1, 0.6 g L−1, and 0.8 g L−1). Notably, the 0.8 g L−1 Cd-CeO2/SBA-15 composite achieved the highest degradation efficiency, reaching approximately 96.4% within 120 min. This superior performance is attributed to the optimal catalyst amount, which maximized light absorption and photocatalytic reactions [46]. In contrast, the 0.4 g L−1 and 0.6 g L−1 doses achieved degradation efficiencies of 70% and 81%, respectively. However, further increasing the catalyst dose beyond 0.8 g L−1 led to diminished efficiency due to catalyst aggregation, which hindered effective light penetration [47].
The effect of MB concentration on the photodegradation efficiency of Cd-CeO2/SBA-15 composites was also assessed, as shown in Figure 6d. The composite exhibited a high degradation efficiency of 96.4% for a 20 ppm MB solution. However, an increase in the concentration to 35 mg/L led to a notable decline in degradation efficiency, despite unchanged experimental conditions, indicating that higher MB concentrations may hinder light absorption and active site availability [48,49]. Although solution pH and catalyst surface charge are known to significantly affect photocatalytic processes—especially for charged pollutants like methylene blue—this study did not assess the pH effect systematically. In general, at pH values above the point of zero charge (PZC), the surface of oxide-based catalysts tends to be negatively charged, which would favor the adsorption of cationic dyes like MB via electrostatic attraction. This, in turn, can enhance the generation of reactive oxygen species near the surface. Future work will involve determining the PZC of Cd-CeO2/SBA-15 and evaluating its performance under different pH conditions to better understand the role of surface charge and solution chemistry in photocatalytic degradation.
Previous studies have shown that bare SBA-15 exhibits negligible photocatalytic activity, highlighting the enhancement from Cd-CeO2 incorporation [50]. For instance, Ali et al. (2022) assert that SBA-15 catalysts exhibit no selectivity toward isomerization products due to their inactivity at the relatively low temperatures required for their reaction [51]. In another work, Au nanoparticles were incorporated to enhance the application of SBA-15 in catalysis because it did not show any catalytic activity due to its lack of photoactive properties and low acidic strength [52]. The Cd-doped CeO2/SBA-15 composites demonstrated significantly superior photocatalytic activity compared to the undoped CeO2/SBA-15 and pristine CeO2. This improvement can be attributed to the synergistic effect of the Cd, CeO2, and SBA-15 components. Cd incorporation extended the light absorption, which enhanced photocatalytic reactions. The high surface area of SBA-15 provided abundant adsorption sites for MB, while CeO2 facilitated charge separation and reactive oxygen species generation. Moreover, Figure 6f shows that the degradation kinetics followed a pseudo-first-order model, with apparent rate constants of 0.01054 min−1(0.4 g L−1 catalyst), 0.01343 min−1 (0.6 g L−1 catalyst), and 0.02532 min−1 (0.8 g L−1 catalyst). The increasing rate constant values with higher catalyst doses further confirmed the enhanced photocatalytic activity. It is worth noting that while the pseudo-first-order kinetic model was applied to the standard 28.8 mg/L MB for comparative analysis, variations in initial dye concentration can influence the apparent rate constant due to factors such as surface saturation and light attenuation. A detailed kinetic study on the concentration dependence of the rate constant will be considered in future work.

3.6.1. Free Radical Scavenging

To identify the dominant reactive species responsible for MB degradation, scavenger experiments were conducted using isopropanol (IPA), disodium ethylenediaminetetraacetate (Na2-EDTA), and ascorbic acid as scavengers for hydroxyl radicals (•OH), superoxide radicals (•O2−), and holes (h+), respectively [53]. Figure 7 shows that the degradation efficiency of MB significantly decreased in the presence of IPA, indicating that •OH radicals played an influential role in the MB degradation process. A small reduction in degradation efficiency was observed when ascorbic acid was introduced, suggesting that holes also contributed to the degradation process. In contrast, the addition of Na2-EDTA had a minimal effect, implying that •O2− had a lesser contribution compared to •OH and h+. These results suggest that the photocatalytic decomposition of MB using Cd-CeO2/SBA-15 was primarily driven by •OH, with h+ playing a secondary role, while •O2− had a negligible impact under the experimental conditions.

3.6.2. Possible Degradation Mechanism

The enhanced photocatalytic activity of Cd-CeO2/SBA-15 can be attributed to improved charge separation and extended visible light absorption induced by Cd incorporation. Under visible light irradiation, electrons in the valence band (VB) of CeO2 are excited to the conduction band (CB), generating electron–hole (e−–h+) pairs. However, pristine CeO2 suffers from rapid recombination of these charge carriers, limiting its efficiency. The introduction of Cd2+ species creates shallow trap states and modifies the local electronic environment, thereby improving the charge carrier separation. Cd2+ may also introduce intermediate energy levels between the VB and CB of CeO2, allowing for a narrowed bandgap and facilitating electron excitation under visible light. Once separated, the photogenerated electrons react with adsorbed O2 to form superoxide radicals (•O2−), as shown in equation 1, while holes (h+) oxidize water to form hydroxyl radicals (•OH). These reactive oxygen species are primarily responsible for the degradation of MB dye, as supported by the scavenger test results.
e−+O2→•O2−
h++H2O→•OH+H+
Furthermore, the high surface area and mesoporosity of SBA-15 provide accessible active sites and enhance reactant diffusion, supporting the overall degradation process. Mechanisms were also observed in related systems such as SrTiO3 nanofibers modified with Fe2O3, Cr2O3, and CuO, which showed reduced bandgaps and increased hydrogen evolution rates [54]. Free radical scavenging tests confirm that the role of •OH radicals in the MB degradation mechanism is predominant, because the presence of IPA (•OH scavenger) significantly inhibited MB degradation. The relatively smaller impact of AA and EDTA suggests that superoxide radicals and photogenerated holes play secondary but synergistic roles.
The band edge positions of CdO and CeO2 were estimated using the Mulliken electronegativity method [55,56]. Based on reported electronegativities and measured bandgap energies, the conduction band (CB) and valence band (VB) positions for CdO were estimated to be −0.30 eV and +1.90 eV, respectively, while those of CeO2 were −0.54 eV and +2.66 eV (vs. NHE). This alignment supports the formation of a Type II heterojunction between CdO and CeO2, facilitating photogenerated electron migration from CdO to CeO2 and enhancing charge separation efficiency. The improved charge dynamics and extended visible-light absorption arising from this configuration contribute significantly to the superior photocatalytic performance observed in MB degradation. To assess the energy efficiency of the photocatalytic process, the electrical energy per order (EE/O) was calculated using the pseudo-first-order kinetic rate constant, following standard methods [57]. The EE/O represents the energy required to degrade 90% of MB dye in 1 m3 of water. Under the optimal conditions (MB concentration = 6.68 × 10−5 M, catalyst dosage = 0.8 g L−1, k = 0.02532 min−1), the EE/O was estimated to be 281.8 kWh m−3. This value compares favorably with other reported photocatalytic systems, as shown in Table 2.

3.6.3. Recyclability Test

The stability and reusability of the synthesized Cd-CeO2/SBA-15 photocatalyst were evaluated through four successive degradation cycles of MB under identical reaction conditions. After each cycle, the photocatalyst was recovered by centrifugation, washed thoroughly with distilled water and ethanol, and dried before reuse. The degradation efficiency showed a slight decrease over the four cycles, indicating good photocatalytic stability. While the Cd-CeO2/SBA-15 composite exhibited excellent stability over four photocatalytic cycles, the potential for Cd2+ leaching under prolonged visible light exposure remains an important environmental consideration. Specifically, Figure 8 shows that the efficiency remained above 90% after the first and second cycles, and gradually decreased to 55% by the fourth cycle, suggesting minimal catalyst deactivation. The reduction in activity could be attributed to surface fouling, partial loss of the catalyst during post-application recovery, or structural modifications over repeated use. However, further leaching analysis to quantify any release of cadmium ions using techniques such as ICP-OES is recommended to confirm this and ensure the environmental safety of Cd-containing photocatalysts in real-world applications. Overall, the results demonstrate that the photocatalyst retains significant activity after multiple cycles, highlighting its potential for practical wastewater treatment applications.

4. Conclusions

An SBA-15-supported ceria–cadmium composite was fabricated through the standard incipient wetness impregnation approach. Analytical characterization revealed that the introduction of CeO2 and Cd species into the SBA-15 framework retained the material’s mesoporous architecture, with no significant disruption to its long-range structural order. The resulting Cd-CeO2/SBA-15 composites demonstrated superior photocatalytic activity compared to both pristine ceria and CeO2/SBA-15 composites. The enhanced photocatalytic performance is attributed to the high surface area of the SBA-15 support, improved charge separation, and the generation of reactive oxygen species.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17121834/s1, Figure S1. EDX mapping for (a) SBA-15 Support and (b) CeO2/BA-15 composites.

Author Contributions

Conceptualization, D.M. and A.U.K.; methodology, D.M.; software, D.M.; validation, S.M., D.M., and D.-L.C.; formal analysis, D.M.; investigation, A.U.K. and S.M.; resources, S.M. and D.-L.C.; data curation, A.U.K.; writing—original draft preparation, D.M.; writing—review and editing, A.U.K.; visualization, S.M.; supervision, D.-L.C. and A.U.K.; project administration, D.-L.C. and S.M.; funding acquisition, D.-L.C. and S.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 17 01834 g001aWater 17 01834 g001b
Figure 1. SEM images of (a) the as-prepared SBA-15, (b) CeO2/SBA-15, and (c) Cd-CeO2/SBA-15 and (d) EDX mapping for Cd-CeO2/SBA-15.
Figure 1. SEM images of (a) the as-prepared SBA-15, (b) CeO2/SBA-15, and (c) Cd-CeO2/SBA-15 and (d) EDX mapping for Cd-CeO2/SBA-15.
Water 17 01834 g001aWater 17 01834 g001b
Water 17 01834 g002
Figure 2. FTIR spectra for SBA-15, CeO2/SBA-15, and Cd-CeO2/SBA-15.
Figure 2. FTIR spectra for SBA-15, CeO2/SBA-15, and Cd-CeO2/SBA-15.
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Water 17 01834 g003
Figure 3. (a) SA-XRD and (b) WA-XRD for SBA-15, CeO2/SBA-15, and Cd-CeO2/SBA-15.
Figure 3. (a) SA-XRD and (b) WA-XRD for SBA-15, CeO2/SBA-15, and Cd-CeO2/SBA-15.
Water 17 01834 g003
Water 17 01834 g004
Figure 4. N2 adsorption–desorption isotherms for (a) SBA-15, (b) CeO2/SBA-15, and (c) Cd-CeO2/SBA-15; and BJH pore size distributions for (d) SBA-15, (e) CeO2/SBA-15, and (f) Cd-CeO2/SBA-15 composites.
Figure 4. N2 adsorption–desorption isotherms for (a) SBA-15, (b) CeO2/SBA-15, and (c) Cd-CeO2/SBA-15; and BJH pore size distributions for (d) SBA-15, (e) CeO2/SBA-15, and (f) Cd-CeO2/SBA-15 composites.
Water 17 01834 g004
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Figure 5. (a) UV-Vis absorption spectra and (b) Kubelka–Monk plot for optical bandgap determination for CeO2/SBA-15 and Cd-CeO2/SBA-15.
Figure 5. (a) UV-Vis absorption spectra and (b) Kubelka–Monk plot for optical bandgap determination for CeO2/SBA-15 and Cd-CeO2/SBA-15.
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Figure 6. Absorbance data of MB removal for Cd-CeO2/SBA-15 using (a) 0.4 g L−1, (b) 0.6 g L−1, and (c) 0.8 g L−1; time-dependent UV-Vis absorbance plots for (d) effect of MB concentration and (e) effect of catalyst dose; and (f) pseudo-first-order kinetics of MB degradation over 0.4 g L−1, 0.6 g L−1, and 0.8 g L−1.
Figure 6. Absorbance data of MB removal for Cd-CeO2/SBA-15 using (a) 0.4 g L−1, (b) 0.6 g L−1, and (c) 0.8 g L−1; time-dependent UV-Vis absorbance plots for (d) effect of MB concentration and (e) effect of catalyst dose; and (f) pseudo-first-order kinetics of MB degradation over 0.4 g L−1, 0.6 g L−1, and 0.8 g L−1.
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Figure 7. Free radical trapping/scavenging test.
Figure 7. Free radical trapping/scavenging test.
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Figure 8. Recyclability test for Cd-CeO2/SBA-15 composites.
Figure 8. Recyclability test for Cd-CeO2/SBA-15 composites.
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Table 1. Surface sorption properties for SBA-15, CeO2/SBA-15, and Cd-CeO2/SBA-15 composites.
Table 1. Surface sorption properties for SBA-15, CeO2/SBA-15, and Cd-CeO2/SBA-15 composites.
CatalystSBET (m2/g)Pore Size Distribution
DP (nm)VP (cm3/g)
SBA-15581.274.740.638
CeO2/SBA-15403.133.710.447
Cd-CeO2/SBA-15330.173.040.367
Table 2. Comparison of photocatalytic performance and energy efficiency (EE/O) of various catalysts for MB degradation.
Table 2. Comparison of photocatalytic performance and energy efficiency (EE/O) of various catalysts for MB degradation.
PhotocatalystMB (mg/L)Dosage (g L−1)Lamp Power (W)Volume (L)k (min−1)EE/O (kWh m−3)Reference
Cd–CeO2/SBA-1528.80.81500.050.0253281.8This work
N-TiO21014500.1-1.6 × 104[58]
Co3O4-hBN 1013000.10.0155670[59]
ZnO 100.250.150.0347[60]
Fe3O4/Graphene 200.4360.10.419.29[61]
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Matei, D.; Katsina, A.U.; Cursaru, D.-L.; Mihai, S. Mesoporous SBA-15-Supported Ceria–Cadmium Composites for Fast Degradation of Methylene Blue in Aqueous Systems. Water 2025, 17, 1834. https://doi.org/10.3390/w17121834

AMA Style

Matei D, Katsina AU, Cursaru D-L, Mihai S. Mesoporous SBA-15-Supported Ceria–Cadmium Composites for Fast Degradation of Methylene Blue in Aqueous Systems. Water. 2025; 17(12):1834. https://doi.org/10.3390/w17121834

Chicago/Turabian Style

Matei, Dănuţa, Abubakar Usman Katsina, Diana-Luciana Cursaru, and Sonia Mihai. 2025. "Mesoporous SBA-15-Supported Ceria–Cadmium Composites for Fast Degradation of Methylene Blue in Aqueous Systems" Water 17, no. 12: 1834. https://doi.org/10.3390/w17121834

APA Style

Matei, D., Katsina, A. U., Cursaru, D.-L., & Mihai, S. (2025). Mesoporous SBA-15-Supported Ceria–Cadmium Composites for Fast Degradation of Methylene Blue in Aqueous Systems. Water, 17(12), 1834. https://doi.org/10.3390/w17121834

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