Efficient Photocatalytic Degradation of Methylene Blue and Methyl Orange Using Calcium-Polyoxometalate Under Ultraviolet Irradiation

Abstract

With the increasing demand for eco-friendly water treatment solutions, the development of novel photocatalysts such as calcium polyanion (Ca-POM) plays a vital role in mitigating industrial wastewater pollution. In this research, calcium polyanion, H₆₀N₆Na₂Ca₂W₁₂O₆₀ (Ca–POM), was successfully synthesized via a self-assembly reaction from metal-oxide subunits. The synthesized Ca–POM was verified to have a polycrystalline structure with a broad size distribution, with an average particle diameter of approximately 623.62 nm. Powder X-ray diffraction (XRD) analysis confirmed the polycrystalline structure of the Ca–POM, with a calculated band gap energy of 3.29 eV. The photocatalytic behavior of the Ca-POM sample was tested with two model dyes, methylene blue (MB) and methyl orange (MO). The reaction mixture was then exposed to ultraviolet (UV) irradiation for durations ranging from 20 to 140 min. The synthesized cluster demonstrated photocatalytic efficiency (PCE%) values of 81.21% for MB and 25.80% for MO. This work offers a valuable basis for applying Ca–POM as a heterogeneous photocatalyst for treating industrial wastewater organic pollutants and highlights the potential of Ca–POM in sustainable water treatment applications.

1. Introduction

The rapid and ongoing expansion of modern industries, such as paper, plastics, printing, beauty products, and many other sectors, has led to significant environmental challenges. The wastewater generated by these industries, which contains large quantities of chemical pollutants, is often discharged into aquatic systems [1,2]. Among all these pollutants, dyes are the most notorious chemical pollutant because of their widespread use, ease of use, competitive price, and good solubility in water. About 15–20% of used dyes are discharged into the water system. Most dyes are toxic to the ecosystem, highly stable, and do not biodegrade [3,4]. Additionally, their presence leads to a decrease in the concentration of dissolved oxygen in water, further negatively affecting aquatic life.

Several chemical, microbial, and physical methods have been used to purify wastewater from different dyes [5,6]. Adsorption technology using different adsorbents is commonly applied [7,8,9,10], with the most effective and common one being activated carbon. However, while adsorption used in water treatment is successful, it faces some difficulties, such as preparation, economic cost, and the need for filtration to remove the adsorbent, which can later cause secondary environmental pollution. Therefore, there is an urgent need to research and develop new technologies to address wastewater pollutants, which are characterized by high efficiency and low costs and do not generate secondary pollutants [11,12].

Photocatalysis, a process which enhances oxidation, has been successfully utilized to decompose contaminants [13]. The distinctive and superior advantages of this method include complete catalytic decomposition, rapid reaction rates, ambient reaction conditions, and, most notably, the absence of secondary pollutants under ideal conditions during the treatment process, which has led to its extensive adoption in wastewater treatment on a large scale [14]. TiO₂ and ZnO particles have been widely employed as photocatalysts for wastewater treatment with unique advantages of high efficiency and low costs, along with remarkable stability and non-toxicity [15].

Polyoxometalates (POMs) are metal-oxide clusters in their anionic form, often referred to as polyacids, formed through condensation self-assembly processes using high oxidation states of early-transition metals, such as tungsten, vanadium, or molybdenum. These transition metals in POM anions typically exhibit d⁰ or d¹ electronic configurations [16,17,18,19].

The structural variations and diverse chemical and physical properties of these compounds make them promising materials for a wide range of applications, including catalysts, sensors, and magnetic and photocatalytic materials [20,21]. POMs are also known for their excellent oxidative and thermal stability, which compares favorably to other catalyst materials, in addition to their affordability and minimal environmental impact. As a result, POM clusters have garnered significant attention, particularly in catalysis and photocatalytic applications [22].

In recent years, many POM clusters exhibited the properties of photoactive materials [22,23]. In most cases, the oxidized form of POM materials shows a highly intense absorption peak, with a high molecular absorption coefficient (Ꜫ ≥ 1 × 10⁴ L mol⁻¹ cm⁻¹), which is attributed to the ligand-to-metal charge transfer (LMCT) which occurs from the HOMO to LUMO (O²⁻–M⁶⁺) within the POM cluster. Upon exposure to the near-UV light region (250–400 nm) [24], various organic compounds undergo photocatalytic oxidation. The role of POM materials in photocatalysis was first studied by Pope and Papaconstantinou [18,22]. Numerous homogeneous and heterogeneous organic reduction and oxidation reactions are photocatalyzed by different types of POMs [25,26]. Various POMs have been utilized as photocatalysts for the removal of dyes and heavy metals from wastewater [27,28]. Examples include SiW₁₂O₄₀²⁻, PW₁₂O₄₀³⁻, and P₂Mo₁₈O₆₂⁻, which have proven to be highly effective photocatalysts in the presence of O₂ [29]. POM-based materials have also been successfully used as environmental catalysts for water purification [30]. For example, Na₂HPW₁₂O₄₀ and H₄SiW₁₂O₄₀ have been used as homogeneous catalysts for the removal of orange II dye [31], and K₃PW₁₂O₄₀ has been effective in degrading several dye contaminants when exposed to visible light in the presence of H₂O₂ [27]. Nanotubes developed from Zn_1.5 [PW₁₂O₄₀] were efficiently used for safranine T dye degradation [32], and novel POMs have been synthesized and employed in the photodegradation of rhodamine-B dye [33]. These successes have established POMs as effective materials for dye removal in water purification.

Prompted by the success of POMs in various applications, numerous composite materials based on POMs have been synthesized and effectively used as photocatalysts for the removal of organic dyes [34,35,36]. Nanoparticles of activated carbon coated with POM materials were also successfully applied for photocatalytic degradation and the adsorption of three different dyes; MB was one of them [37]. The high photocatalytic efficiency of POM compounds in removing various dyes has led to their widespread use in the removal of methylene blue dye.

It is well known that catalyst composition, optical band gap, radiation source, exposure time, and other parameters can all influence photodegradation performance. The structure and composition of POMs have significantly impacted the determination of photoactivity in organic dyes. For example, Ag₅BW₁₂ showed a photoactivity efficiency of 39.3% in terms of RhB degradation [38], while, for PMo₁₁V, MB degradation reached 50.8% [39]. Cu-POM-TiO₂ showed the maximum MB degradation of 41.6%, while, for Zn-POM-TiO₂, the degradation was 35.3%. The novelty of this work lies in the synthesis of new POMs, which are superior to the reported POMs in terms of their photoactive efficiency and ease of preparation [40].

Therefore, in the present study, calcium polyanion, H₆₀N₆Na₂Ca₂W₁₂O₆₀ (Ca–POM), was successfully synthesized under various reaction conditions, as described in the literature [41], and characterized by measuring its surface area, porosity, and thermal stability. The photocatalytic degradation activity of synthesized Ca-POM was tested using MB and MO as model pollutants and compared with other reagents to assess the photocatalytic performance.

2. Experimental Methods

2.1. Materials

Calcium chloride, hydrochloric acid, ammonium chloride, methylene blue, and methyl orange with purity ≥ 99% were obtained from Sigma-Aldrich, with excellent purity.

2.2. Synthesis of H₆₀N₆Na₂Ca₂W₁₂O₆₀ Cluster

The POM precursor Na₁₁H[H_(2−x)Bi₂W₂₀O₇₀(HWO₃)_x]₄₆H₂O, where (x = 1.4) was used to synthesize Ca-POM according to literature methods [41] under specific reaction conditions, specifically 1.49 g of the POM precursor (0.225 mmol) was mixed with 10 mL of H₂O at 80 °C, then combined with 0.08 g of CaCl₂ in 15 mL of H₂O solution. With constant stirring, a 0.1 M HCl solution was added dropwise into the resultant solution to adjust the pH. The detailed synthesis parameters, including temperature, precursor quantities, and pH, are summarized in

Table 1. Summary of reaction conditions for the preparation of Ca-POM.

Condition	Range	Optimized Condition
Amount of POM precursor	0.5–2 g	1.49 g
pH	4–7	6
Temperature	60–90 °C	80 °C
Reaction time	1–2 h	2 h

. The adjusted mixture was then heated under reflux for 2 h at 100 °C with vigorous stirring. The turbid mixture was filtered out after cooling it to ambient temperature. Then, 5 mL of 1 M NH₄Cl was added. The mixture was allowed to remain at room temperature. After 5 days, trigonal colorless crystals of H₆₀N₆Na₂Ca₂W₁₂O₆₀ were obtained as tungstate polyhedral polyanion [W₁₂O₄₂].

2.3. Material Characterization

The mineralogy of Ca-POM was identified using X-ray powder diffraction (XRD) (Shimadzu XRD-6000 Japan) with a Ni filter, Cu-Kα radiation source (λ = 1.5418 Å) at 30 kV, and a step size of 2θ° = 0.02. The surface morphology of the POM cluster was investigated by scanning electron microscopy (SEM) at a 10 kV accelerating voltage. The IR spectra of the materials were recorded by an FT-IR instrument to describe the functional group of Ca-POM. The textural properties of the cluster (including specific pore size distribution, pore volume, and surface area) were investigated using conventional N₂ adsorption procedures. The multipoint BET technique was used to determine a particular surface area. Thermal stability was determined using a thermal gravimetric analyzer (TGA). Samples weighing 15–20 mg were placed in 100 L alumina pans with a flow rate of N₂ of 100 mL/min and a heating rate of 5 ± 1 °C/min.

2.4. Photocatalytic Degradation

The photocatalytic behavior of the Ca-POM cluster was examined for dye degradation under UV irradiation. To establish the photocatalyst’s adsorption–desorption equilibrium, 20 mg of the photocatalyst POM was dispersed in 25 mL of dye solution of methylene blue (MB) or 20 mg/L of methyl orange (MO). For 10 min, the mixture (organic dyes + Ca-POM) was agitated for 10 min using an ultrasonic scattering device. This mixture was then placed in quartz glass tubes under an ultraviolet lamp (UVGL-58 Handheld UV Lamp) with a wavelength of 354 nm and power of 6 W at a separation distance of 15 cm. A double-beam UV-visible spectrophotometer (Jenway, 6800) was used to measure the absorbance of the mixture over a time range of 20 to 140 min.

The photocatalytic degradation efficiency (PCE%) of the debasement of organic dyes was determined using the following equation:

$$\mathrm{PCE}\%={\displaystyle \frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_{\mathrm{o}}}}\times 100={\displaystyle \frac{{\mathrm{A}}_{\mathrm{o}}-{\mathrm{A}}_{\mathrm{t}}}{{\mathrm{A}}_{\mathrm{o}}}}\times 100$$

(1)

where C_o denotes the dye’s initial concentration, and C_t denotes the dye’s post-UV concentration. Additionally, the values of A_o and A_t represent the dye’s initial and post-irradiation residual absorbance, respectively.

3. Results and Discussions

3.1. Characterization of Ca-POM Cluster

The FTIR spectrum’s vibrational bands of the synthesized polyoxometalate cluster are shown in Figure 1. The three asymmetric vibrations characteristic to the W-O bonds were 696.49, 865.19, and 938.72 cm⁻¹, which is consistent with results from previous studies. The very strong bands at 1669, 1403, and 1327 cm⁻¹ corresponded to the H–O–H bending mode, while the vibration band at 3175 cm⁻¹ confirmed the presence of hydrogen-bonded O–H stretching modes of water, which is in line with the findings reported by Atta et al. [41].

The XRD pattern of the powder Ca–POM cluster was obtained at 2θ° angles from 10 to 60° and is displayed in Figure 2. The diffraction peaks in the figure indicate that the Ca–POM sample is polycrystalline and has a monoclinic crystal structure in which space group P21/m has lattice parameters a = 12.18 Å, b = 17.40 Å, c = 14.45 Å, and β° = 114.03° [41]. The Miller indices (h k l) for each diffraction peak in the XRD pattern were calculated using the Check Cell program [42] and are depicted in Figure 2.

The BET surface area analysis of the cluster sample yielded the following values: the sample’s surface area was 0.2031 m²/g, with a pore size of 908.857 Å and a pore volume of 0.001746 cm³/g. The relatively low surface area (0.2031 m²/g) suggests that, while Ca–POM may not have the high surface area typically associated with enhanced photocatalytic performance, it still shows photocatalytic activity due to its electronic and structural properties, including the band gap energy and the material’s ability to generate reactive oxygen species under UV light. The large pore volume of 908.857 Å supports a sufficient surface area for the adsorption of larger organic molecules like dyes, which is essential for photocatalytic degradation. The pore volume, although small, suggests the ability of the sample to adsorb and interact with contaminant molecules during UV exposure.

The thermal stability of the synthesized polyoxometalate cluster was evaluated using TGA measurements in the temperature range of 20–700 °C, as demonstrated in Figure 3. The thermal analysis showed no significant weight change from 300 to 700 °C. A single weight loss step in the temperature range of 20–300 °C was attributed to the release of bonded water molecules; the residual mass at 700 °C was determined to be 88.5%, aligning with the results of previous studies [41].

The morphology of the Ca–POM cluster was examined using SEM, as shown in Figure 4. Figure 4a shows that the prepared POM molecules appear white, polycrystalline, and generally spherical, with a wide distribution of sizes. The average particle diameter was approximately 623.62 nm, as depicted in Figure 4b.

The UV–Vis absorbance of the Ca–POM cluster within the 200–500 nm range is shown in Figure 5. The figure indicates that the prepared POM clusters exhibit strong absorbance in the UV-Vis range, with two peaks at 260 nm and 330 nm. The energy band gap of the prepared POM was determined from the absorbance spectrum using the Tauc equation [43,44,45].

(hvα)^1/2 = A(hv − E_g)

(2)

where v is the vibration frequency, α is the coefficient of absorption, E_g is the energy band gap in (eV), and A and h are the proportional and Planck constants, respectively.

The experimental data of hν are plotted against (αhν)^1/2 in Figure 6, and the resulting relationship shows a good fit. This indicates that the allowed indirect transitions are responsible for the absorption observed in the synthesized POM. The intercept of the graph was used to determine the optical energy band gap (Eg), which was found to be 3.29 eV. Similar results have been reported in previous studies; for example, the energy band gap of zinc oxide nanoparticles was found to be 3.15 eV, while, for Ag₃PW₁₂O₄₀ polyoxometalate, the value was 3.12 eV [46].

3.2. Photocatalysis Investigations of Ca–POM

By evaluating the photocatalytic degradation of MB and MO dyes in the produced POM, the photocatalytic activity of POM was evaluated. A comparison of the performance of the absorbance spectra of each dye with and without the addition of the catalyst at different UV irradiation times is shown in Figure 7 and Figure 8. Figure 7a illustrates the two distinct absorbance peaks for MB at 570 and 660 nm after zero and 140 min. This means that, without a catalyst, 32% and 33.31% of MB degradation were achieved after 140 min for absorbance bands at 570 and 660 nm, respectively, which points to the effect of UV light over time. Figure 7b displays the changes in the absorption spectrum of the MB solution after treatment with Ca-POM. The complete disappearance of the 660 nm wavelength after 20 min of adding the catalyst to the MB dye solution and reductions of 80% in absorption peaks at 570 nm over time are strong indicators of the ability of the catalyst to decompose the dye molecules into smaller molecules. Conversely, no new absorption peaks appeared during the reaction, which confirms that no new compounds were formed during the reaction, supporting the hypothesis of successful dye degradation.

As shown in Figure 7b, the appearance of two peaks in the UV-Vis spectrum of MB at time = 0, right after adding the catalyst, can indeed be due to the aggregation behavior of the dye. As MB tends to form dimers and other aggregated species, these aggregates can exhibit different absorption characteristics compared to the monomeric form of the dye [47], resulting in split absorption peaks. One peak typically corresponds to monomeric MB (around 664 nm), while the second may be due to the aggregated or dimeric form, causing a red shift in the absorption.

The blue shift observed in the characteristic peak of MP after UV irradiation is due to the degradation of MB’s chromophoric structure under photocatalytic conditions [48] in which specific bonds in MB, such as C=N and C=S, are broken. This disturbs the conjugated system of MB, leading to a decrease in the absorption intensity and a shift in wavelength, causing the observed blue shift in the absorption maximum. Additionally, the process involves the removal of methyl groups from nitrogen atoms in MB, producing less-conjugated degradation products. These degradation products absorb at shorter wavelengths, which further contributes to the blue shift.

Regarding the behavior of the MO dye with the catalyst, the absorption of the dye was monitored over time before the addition of the catalyst, as shown in Figure 8a. It was demonstrated that the dye was only absorbed once at 450 nm and that the photocatalytic process achieved 18% MO degradation after 140 min, even in the absence of the catalyst. Adding the catalyst to the dye solution, as shown in Figure 8b, led to a clear decline in the absorption spectrum at 450 nm, with no new absorption band appearing, meaning that no new molecules had formed through the reaction.

Comparing the photocatalytic efficiency of the catalyst for the two dyes, MB and MO, Figure 7 and Figure 8 show that the efficiency of the catalyst was greater with MB. The absorbance at 660 nm for MB disappeared within 20 min of adding the catalyst, while the absorbance at 570 nm was reduced by over 80%, compared to the case of MO, in which the ratio almost reached 26%.

The photocatalytic efficiency (PCE%) of MB and MO degradation over the Ca–POM cluster was evaluated using Equation (1) and is illustrated in Figure 9. As shown in Figure 9a, the photocatalytic efficiency of MB degradation was zero before the addition of the POM catalyst. The photocatalytic ability of the POM synthesized in this work increased with time, where the increase was gradual until the 80th minute. It then stabilized between 80 and 100 min before resuming and reaching the highest value at 140 min, at which point the PCE% value for MB degradation was 81.21%. In the case of the catalyst’s performance with MO dye, as shown in Figure 9b, it is clear that there was no photodegradation of the dye before the addition of the Ca–POM catalyst. In contrast, the dye photodegradation process took place as soon as the catalyst was added, as the percentage of PCE% increased significantly between 0 and 20 min. However, the increase in PEC% after 20–40 min was insignificant, and, after 140 min, the PEC% was 25.80%.

Furthermore, the experimental findings of this photodegradation were studied according to different kinetic models, and it was found that pseudo-first-order Equation (3) provided the best fit [49]. It was therefore used to determine the kinetic performance of the photodegradation process for each dye and assay the degradation rate constant.

$$ln\left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_{\mathrm{o}}}}\right)=-\mathrm{kK}t=-{\mathrm{k}}_{app}\mathrm{t}$$

(3)

where k, K, and k_app are constants representing the degradation rate, the equilibrium adsorption, and the apparent kinetic rate, respectively. The linear relationship between irradiation times, t versus ln (C_t/C₀), was plotted for each of the dyes, as shown in Figure 10. Based on this, the values of k_app were estimated, from the slopes of each plot, to be 1.17 × 10⁻² and 3.58 × 10⁻⁴ min⁻¹ for MB and MO, respectively. This is documented in Table 2. The photocatalytic activity of MB (Figure 10a) was greater than that of MO (Figure 10b), as confirmed by the reaction rate constant (k_app) value, which was significantly higher for MB compared to MO.

3.3. Photocatalytic Mechanism

For the Ca-POM catalyst used, the primary photocatalytic mechanism under UV-vis light irradiation can be explained by the following equations, displayed schematically in Scheme 1.

The absorption of POM molecules in the UV-Vis light irradiation causes electrons to move from the valence band (VB) region on the surface of the Ca–POM molecules to the conduction band (CB) region in Scheme 1, leaving positively charged holes in the VB region, due to the band gap energy alignment, after which, the excited electrons in the CB region can be scavenged by dissolved oxygen molecules in water, generating (.O²⁻) molecules. Meanwhile, the positive holes in the VB region react with water molecules to generate (.OH) [50]. As a result, the active species (.O²⁻ and .OH) generated during photodegradation oxidize the dye molecules (MB or MO), breaking them down into smaller degradation products [51].

Table 2. Comparison of values for the apparent rate kinetic constant (k_app) and photocatalytic efficiency (PCE %) with values for dye removal reported in the literature.

Catalytic	Dye	ƛ irra (nm)	Optical Band Gap/eV	PCE%	kapp (min−1)	Time/min	Reference
Ca–POM	MB	254	3.29	81.21	1.17 × 10−2	140	Current work
Ca–POM	MO	254	3.29	25.80	3.58 × 10−4	140	Current work
NiO-ZnONCs	MB	365	3.69	72	1.50 × 10−2	80	[51]
Cr2O3-CNT NPs	MB	>420	-	68	9.80 × 10−4	120	[52]
ZnO NPs	MB	254		71.1	4.30 × 10−3	150	[53]
TCPP/CuPOM/TiO2	MB	UV	3.2	49	-	80	[54]
TCPP/ZnPOM/TiO2	MB	UV	3.2	44	-	80	[54]
CdS nanocrystals	MB	UV	2.91	35	3.42 × 10−2	30	[55]
PMo11V	MB	UV	-	50.8	-	120	[39]
Cu-POM-TiO2	MB	UV	-	41.58	2.9 × 10−2	180	[40]
Zn-POM-TiO2	MB	UV	-	35.34	2.6 × 10−2	180	[40]

Table 2. Comparison of values for the apparent rate kinetic constant (k_app) and photocatalytic efficiency (PCE %) with values for dye removal reported in the literature.

Catalytic	Dye	ƛ irra (nm)	Optical Band Gap/eV	PCE%	kapp (min−1)	Time/min	Reference
Ca–POM	MB	254	3.29	81.21	1.17 × 10−2	140	Current work
Ca–POM	MO	254	3.29	25.80	3.58 × 10−4	140	Current work
NiO-ZnONCs	MB	365	3.69	72	1.50 × 10−2	80	[51]
Cr2O3-CNT NPs	MB	>420	-	68	9.80 × 10−4	120	[52]
ZnO NPs	MB	254		71.1	4.30 × 10−3	150	[53]
TCPP/CuPOM/TiO2	MB	UV	3.2	49	-	80	[54]
TCPP/ZnPOM/TiO2	MB	UV	3.2	44	-	80	[54]
CdS nanocrystals	MB	UV	2.91	35	3.42 × 10−2	30	[55]
PMo11V	MB	UV	-	50.8	-	120	[39]
Cu-POM-TiO2	MB	UV	-	41.58	2.9 × 10−2	180	[40]
Zn-POM-TiO2	MB	UV	-	35.34	2.6 × 10−2	180	[40]

4. Comparative Study

Table 2 summarizes the photocatalytic efficiency (PCE %) value for the photocatalyst synthesized and applied in this study along with values for dye removal reported in the literature. The Ca–POM catalyst, previously used to remove MB from aqueous samples, has a higher PCE% than the other catalysts [51,52,53,54,55], as shown in Table 2. As a result, its low cost, ease of preparation from small units, and high performance in degrading MB from polluted water give this catalyst a significant advantage over other photocatalysts.

5. Conclusions

The cluster of Ca–POM was successfully constructed through self-assembly using a chemical process. The synthesized POMs exhibited a polycrystalline structure, with a wide size distribution and a particle diameter of 623.62 nm. The catalyst’s band gap energy was 3.29 eV, which revealed that the spectral response of POM extended into the visible region. The surface area of the sample was approximately 0.2031 m² g⁻¹, with a pore size of 908.857 Å and a pore volume of 0.001746 cm³ g⁻¹. Under UV irradiation, the photocatalytic efficiency of the synthesized Ca–POM cluster was 81.21% for MB and 25.80% for MO. This work provides valuable theoretical guidance and an experimental basis for the application of POM materials in water treatment, particularly for organic pollutant removal. This study also confirms the possibility of fabricating more efficient and selective photocatalytic materials for the removal of a wide range of contaminants.