Chiang Mai Journal of Science

Preparation of Dye-sensitized POM and Application in Treatment of Cotton Fabric Dyeing Wastewater

HIGHLIGHTS

• Developed a Dye-POM-TEOA system using eosin Y-sensitized H₃PW₁₂O₄₀ for photocatalysis.
• Enhanced phenol degradation: Achieved 82% phenol degradation in 2 hrs under simulated sunlight.
• Degradation process follows first-order kinetic model.
• Eosin Y broadens POM's absorption spectrum, improving visible light use.
• Dominant ·OH oxidation: Hydroxyl radicals (·OH) play a primary role in phenol oxidation.

1. INTRODUCTION

The global textile enterprises will discharge a large amount of printing and dyeing wastewater in the production and processing process. These wastewater contains harmful substances such as benzene rings, dyes, surfactants, etc. [1-3], resulting in high organic matter content, chroma and chemical oxygen demand (COD), and poor biodegradability and high toxicity [4-6]. It poses a serious threat to human health and also causes extremely bad pollution to the surrounding environment [7]. At present, there are many physical and chemical methods to treat printing and dyeing wastewater, such as flocculation technology, membrane separation technology, activated carbon adsorption, chemical oxidation, electrochemical oxidation and so on [8-11]. As a new sewage treatment method in recent years, photocatalytic oxidation technology can effectively degrade organic pollutants under normal temperature and pressure conditions and has broad application prospects [12-14].

Polyoxometalate (POM) as a catalyst, exhibits good redox ability and photoelectrochemical properties. It has a variety of characteristics, including structural diversity (such as Keggin, Dawson, Waugh, etc.), modifiability and adjustable denaturation, excellent redox properties, high thermal stability, relatively large molecular weight (near 10³ to 10⁴) and soluble in water [15,16]. Among them, the POM with Keggin structure is the most common, the research on its structure and properties is also the most in-depth. In the 1960s, studies have shown that in the presence of some precious metals, heteropoly blue (POM^-) produced by photocatalytic reduction of heteropoly acid can be used to catalyze water to produce hydrogen [17-19].

However, due to the limited absorption spectral characteristics of POM, it can usually only be excited by (near) ultraviolet light, resulting in a ligand-metal charge transfer transition (LMCT), which leads to reoxidation reaction and spectral sensitization is a method to expand the absorption spectrum and improve the utilization of visible light [20-22]. Typical Jameson dyes such as fluorescein, Eosin Y, Rhodamine B, etc., can not only broaden the absorption spectrum of POM, but also promote electron transition and separation, thus effectively improving the catalytic reaction rate [23]. At present, Eosin Y has been used to sensitize TiO₂, SnO₂, titanate, graphene and other materials [24-27], but it has not been reported that dye sensitized POM is used to treat cotton textile printing and dyeing wastewater. The dye-sensitized POM system has more obvious advantages compared to traditional photocatalysts (such as TiO₂). Specifically, it does not rely on wide-bandgap semiconductors and can achieve efficient visible light response through molecular sensitization. Moreover, the POM structure is diverse and its redox properties can be adjusted, which makes it more adaptable and flexible when dealing with organic pollutants in complex wastewater substrates.

In this paper, the structure and catalytic properties of H₃PW₁₂O₄₀ sensitized by Eosin Y (EY) were studied. The effects of photocatalyst dosage and initial concentration of EY on the catalytic effect and the corresponding photocatalytic reaction kinetics were investigated, and the reaction mechanism was analyzed and discussed.

2. MATERIALS AND METHODS

2.1 Chemicals and Instrumental Techniques

2.1.1 Materials

Na₂WO₄·2H₂O (Sinopharm Chemical Reagents Co., Ltd.), Na₂HPO₄·12H₂O (Sinopharm Chemical Reagents Co., Ltd.), Eosin Y (Titan Group), Triethanolamine (TEOA, Tianjin Guangfu Reagents Co., Ltd.), Phenol solution (Tianjin Guangfu Reagents Co., Ltd.).

2.1.2 Characterization

X-ray diffraction (XRD) patterns of the catalysts were obtained on Bruker axs D8 Discover (Cu K_ɑ= 1.5406 Å, 40 kV, 200 mA). Power diffraction was scanned at 8°/min in the range of 10°-80°.

Ultraviolet-visible and near-infrared spectroscopy spectra of the materials were carried out using Agilent Cary6000i spectrometer operated at 1 cm^-1 resolution in 400-4000 cm^-1 region.

Fluorescence spectra of the materials were carried out using Shimadzu F-4600 Fluorescence spectrophotometer.

Photocatalysis using 350 W xenon lamp to simulate the sunlamp light source, the light source is MC-X10 Xenon lamp (Beijing Maisheng Technology Co., LTD.)

2.2 Catalysts Preparation

2.2.1 H₃PW₁₂O₄₀ preparation

Add 12 g Na₂WO₄·2H₂O and 1 g Na₂HPO₄·12H₂O to the beaker, add concentrated HCl while heating and stirring, and keep the solution pH ≤ 3. When it is observed that phosphotungstic acid begins to dissociate from the solution, stop heating and naturally cool to room temperature. An appropriate amount of acetone was added to the cooled solution, and the mixture was allowed to stand for 0.5 h after full stirring to fully stratify the different components. H₃PW₁₂O₄₀ was prepared by separating the lower oil from the separation funnel and drying at 85 °C for 2 h to remove the residual water and organic solvent.

2.2.2 Dye sensitization

The photocatalytic reactor was 500 ml, and the standard reaction solution was prepared according to the following method : 3.0 g of phosphotungstic acid, 5 mL of Eosin Y (EY) dye solution (5.0 × 10^-2 mol/L), and an appropriate amount of TEOA (8.0 mL, 0.2 mol/L) were added to the reactor in turn in the dark environment, and then the phenol solution was added to stir evenly, so that the total volume of the reaction solution was maintained at 500 ml, and the phenol concentration was 20 mg/L.

TEOA acts as a sacrificial electron donor, regenerating the oxidized dye (EYox) and sustaining the photocatalytic cycle. The amount of TEOA used (8.0 mL, 0.2 mol/L) was chosen based on preliminary optimization experiments to ensure efficient electron transfer without excessive consumption.

2.2.3 Activity test

The dye-sensitized phosphotungstic acid was fully mixed with phenol solution under magnetic stirring. The 350 W xenon lamp was used to simulate the solar light source, and the air pump was used to continuously introduce air into the reaction system for aeration.

Phenol was used to simulate cotton fabric printing and dyeing wastewater. The degradation rate of phenol was analyzed by spectrophotometry. The absorbance of the sample was measured by an ultraviolet-visible spectrophotometer at a wavelength of 270 nm, and the degradation rate was calculated according to the following formula:

$\eta \mathrm{=}\frac{A_0\mathrm{-}{A}_t}{A_0}\mathrm{\text{×100}}\text{\%}$ (1)

In the formula: A₀ is the absorbance of the sample before illumination, A_t is the absorbance of the sample when the illumination time is t.

All experiments were performed in triplicate, and the results were presented as mean ± standard deviation.

3. RESULTS AND DISCUSSION

3.1 Characterization of Catalyst

According to the IR spectra of H₃PW₁₂O₄₀ in Figure 1, there are four absorption peaks (1042 cm^-1, 937 cm^-1, 861 cm^-1 and 752 cm^-1) between 500-1300 cm^-1, which are caused by the symmetrical stretching vibration of the central PO₄ tetrahedron (P-Oa), the stretching vibration of W and terminal oxygen in the octahedron (W-Od), the stretching vibration of W-Ob-W (W-Ob), and the stretching vibration of W-Oc-W (W-Oc), respectively. This shows that the prepared H₃PW₁₂O₄₀ still maintains the basic structure of the Keggin structure [28].

From the XRD pattern of H₃PW₁₂O₄₀ in Figure 2, it can be seen that the characteristic diffraction peaks are located at 9.26°, 10.57°, 20.45°, 25.19°, 28.92° and 34.3°, which further indicates that the prepared heteropoly acid is H₃PW₁₂O₄₀ with Keggin structure [28].

3.2 Charge Transfer Between Dye and POM

The experimental observation showed that the standard reaction solution was reddish before illumination, which was caused by the color of the dye. After exposure to visible light, the color of the system gradually changed to light blue. This change indicates that charge transfer occurs between the dye and POM under light conditions, forming heteropoly blue (POM^-, blue) [29]. Figure 3 The visible absorption spectra of the reaction solution under different illumination time show that the absorbance in the wavelength range of 550~900 nm gradually increases with the increase of illumination time. This observation is consistent with the phenomenon of color change, which proves that EY transfers electrons to POM under light excitation, so that the concentration of POM^- increases continuously.

The existing literature studies have shown that [30], EY aqueous solution will produce a fluorescence emission peak centered at about 538 nm under the excitation of 500 nm, which is consistent with the experimental results of fluorescence spectroscopy in Figure 4. At the same time, the fluorescence emission intensity of EY increases with the increase of H₃PW₁₂O₄₀ addition, which indicates that POM has no fluorescence quenching effect on the dye, which is consistent with the results reported in the literature [30]. Because the triplet excited state (³EY*) of EY has the characteristics of long lifetime and high formation rate [31], it can be inferred that the photo-sensitive electron transfer of EY is mainly realized by the excited triplet state (³EY*), that is, the single excited state forms a triplet state through inter-gap hopping (ISC), and then injects electrons into the LUMO orbit of POM. The photoinduced electron transfer process between dye EY and POM can be expressed by Eqs. (2) - (4):

EY + hʋ → ¹EY* → ³EY* (2)

³_{EY* + POM → POM}^- _{+EYox (3)}

EY_ox + TEOA → EY + TEOA_ox (4)

3.3 Control Experiment

In order to verify the effect of photocatalytic degradation of phenol by dye EY sensitized phosphotungstic acid, the following control experiments were carried out: The experimental conditions of 3.0 g phosphotungstic acid, 5 mL Eosin Y (5.0 × 10^-2 mol/L), 8.0 mL TEOA, xenon lamp simulated solar light source were used as the standard group, and the control group was not added with phosphotungstic acid, Eosin Y and no light. It can be seen from Table 1 that the degradation rate of phenol in the standard group reached 82 % after 2 h of simulated illumination, and the degradation rate of phenol by non-sensitized phosphotungstic acid was about 12 %, which may be due to the oxidation of phosphotungstic acid itself. Eosin Y itself has no degradation rate for phenol, and the standard group has a degradation rate of about 13 % for phenol under dark conditions. It can be seen that the simultaneous presence of light, Eosin Y and phosphotungstic acid is a necessary condition for phenol degradation.

3.4 Effect of Initial Concentration of Phenol on Its Degradation

The reaction solutions with different phenol concentrations (10 ~ 30 mg/L) were prepared, and other conditions such as phosphotungstic acid and EY were unchanged. The degradation rate of phenol was calculated after 2 h of illumination. The results are shown in Figure 5.

As shown in Figure 5, the degradation rate of phenol increased first and then decreased with the increase of its initial concentration. When the initial concentration of phenol is relatively low (10 mg/L and 15 mg/L), the degradation reaction rate is slow, so that the degradation rate of phenol at the initial stage of the reaction is lower than that of the initial concentration of 20 mg/L. When the initial concentration of phenol exceeds 20 mg/L, the degradation rate of phenol begins to decrease with the further increase of phenol concentration. This is because the photolysis time and the amount of catalyst in the reaction solution are fixed, so the amount of active substances produced after photoexcitation is also certain. In this case, the increase of phenol concentration will lead to the decrease of the active substances that each phenol molecule can contact, thus reducing the degradation rate of phenol.

This trend indicates that the generation rate of the active species is limited under a fixed catalyst dosage. When the phenol concentration is below 20 mg/L, the reaction is restricted by the collision probability between the substrate and the active species; while above this concentration, the excess substrate leads to a relative insufficiency of the active species, thereby inhibiting the degradation efficiency. Therefore, 20 mg/L is the optimal initial concentration for the reaction system.

3.5 Effect of Dye EY Concentration

From Figure 6, it can be seen that the effect of dye Eosin Y as a photosensitizer on the catalytic activity of phenol degradation is first increasing with the concentration of dye EY and then decreasing. When the EY degree was 5.0 × 10^-2 mol/L, the degradation rate of phenol was the highest after photolysis for 2 h. Without EY, the degradation rate of phenol by H₃PW₁₂O₄₀ was about 12 %.

POM itself cannot be directly excited by visible light. When the concentration of EY is small, the absorption of light increases with the increase of EY molecules, the concentration of excited state ³EY* increases, and more and more POM^- is produced, thus the activity of phenol degradation is also improved. However, when the concentration exceeds a certain concentration (> 5.0 × 10^-2 mol/L), the degradation rate of phenol decreases with the increase of the initial concentration of EY, which is due to the concentration quenching effect. When the concentration of EY is relatively high, the excited state

³EY* is more likely to deactivate (deactivate) through gap crossing and non-radiation transfer, which reduces the utilization efficiency of the dye. At the same time, studies have shown that [32] EY will undergo different degrees of aggregation dominated by dimerization in solution. When the concentration is high, this aggregation is more serious, and multiple aggregation states such as trimerization occur. EY aggregation may not be fully involved in the reaction resulting in a decrease in the degradation rate of phenol.

This result indicates that when the EY concentration is too low, the generation of the excited state ³EY* is insufficient, and the electron injection efficiency is low; when the concentration is too high, the excited state is prone to inactivation through concentration quenching, and the aggregation of dye molecules reduces the proportion of effective participation in the reaction.

3.6 Effect of H₃PW₁₂O₄₀ Addition on Photocatalytic Degradation of Phenol

Different amounts of H₃PW₁₂O₄₀ (0.1 g ~ 0.5 g) were added to the reaction solution, and the phenol degradation rate was calculated after 2 h of illumination. The results are shown in Figure 7.

From Figure 7, it can be seen that as the amount of catalyst H₃PW₁₂O₄₀ added increases from 0.1 g to 0.3 g, the degradation rate of phenol solution gradually increases until it reaches the maximum value. This is due to the increase of the amount of H₃PW₁₂O₄₀ added. The amount of active species such as holes (h*) and hydroxyl radicals (·OH) generated in the reaction system also increases, thereby increasing the degradation rate of phenol, when the catalyst concentration exceeds 0.3g, the degradation rate of phenol solution gradually decreases slowly. This may be due to the increase of the content of active substances such as h* and ·OH in the solution when the addition amount of H₃PW₁₂O₄₀ exceeds 0.3g, and the collision reaction occurs between them. Deactivation, which led to a decrease in the degradation rate of phenol.

3.7 Kinetics of Catalytic Degradation of Phenol by Dye-sensitized H₃PW₁₂O₄₀

(1) Langmmuir-Hinshelwood kinetic model

The reaction rate of photocatalytic oxidation can be described by Langmmuir-Hinshelwood kinetic equation:

$r\mathrm{\text{=-}}\frac{d{C}_t}{\mathrm{dt}}\mathrm{=}\frac{\mathrm{kK}{C}_t}{\mathrm{\text{1+}}K{C}_t}$ (5)

$\frac{1}{r}=\frac{1}{\mathrm{kK}}\text{∙}\frac{1}{C_t}+\frac{1}{k}$ (6)

r (mg·L^-1·h^-1) — Reaction rate

C_t (mg·L^-1) — The concentration of the reactants when the reaction time is t

K (L·mg^-1) — Adsorption equilibrium constant

k (mg·L^-1·h^-1) — Apparent reaction rate equilibrium constant

It can be seen from the above equation that when the concentration of phenol is very low, that is, when KCt<<1,

$r\mathrm{\approx }\mathrm{Kk}{C}_t=K^{\text{'}}{C}_t$ ₍₇₎

$\ln (C_0\text{/}{C}_t\text{)=}{K}^{\text{'}}t$ (8)

Where K'=Kk, this is a first-order reaction, that is, the reaction rate is proportional to the solute concentration, called the quasi first-order reaction rate equilibrium constant.

(2) Kinetics model of phenol degradation catalyzed by H₃PW₁₂O₄₀

The initial concentration of phenol was changed to 16 mg/L, 18 mg/L, 20 mg/L and 22 mg/L respectively, and photocatalytic degradation reaction was carried out under light. ln(C₀/C_t) and light time t were plotted, and the results were shown in Figure 8.

It can be observed from Figure 8 and Figure 9 that there is a significant linear correlation between ln(C₀/C_t) and light time t when the initial concentration of phenol solution is 16-22 mg/L. This observation is in agreement with the Langmuir-Hinshelwood quasi-first-order kinetic equation, which further verifies the applicability of this equation under these conditions. At the same time, the reciprocal of the initial photocatalytic degradation rate of phenol solution also presents a significant linear correlation with the reciprocal of its initial concentration, which means that the initial photocatalytic reaction rate will show a corresponding increasing trend with the increase of the initial concentration of phenol solution. The first-order reaction kinetics equation and parameters of the phenol degradation reaction catalyzed by H₃PW₁₂O₄₀ are shown in Table 2 below.

3.8 Mechanism Analysis of Photocatalytic Degradation of Phenol Solution

Literature studies have shown that the main oxidants in the photochemical reaction of POM are ·OH free radicals, photoinduced hole h⁺, or the synergistic effect of the two [19,20]. In order to further explore the catalytic mechanism of ·OH and h⁺ in the degradation of phenol by H₃PW₁₂O₄₀, KBr was selected as the ·OH catcher [33] and KI was selected as the h⁺ catcher [34], and different amounts of KBr (0.2 mol/L) and KI (0.2 mol/L) were added to the standard reaction solution. The 2 h photolysis reaction was performed. The effects of the addition amounts of self ·OH and h⁺ catcher on the degradation rate of phenol are shown in Figure 10.

As shown in Figure 10, it can be observed that whether KBr or KI is added, the degradation rate of phenol shows a decreasing trend. This phenomenon indicates that ·OH (hydroxyl radical) and h⁺ (hole) both play a key role in the degradation of phenol in the solution system using H₃PW₁₂O₄₀ as photocatalyst [35]. In order to further explore the primary and secondary effects of ·OH and h⁺ on oxidative degradation of phenol in the catalytic degradation process and their relationships, we can adopt the L-H quasi-first-order kinetic reaction model for analysis.

The results of Figure 11 show that when different amounts of KBr and KI are added to the reaction system for catalytic degradation of phenol by H₃PW₁₂O₄₀, the degradation process of phenol always follows the L-H pseudo-first-order kinetic equation, and it can be seen from Figure 12 that the inhibitory effects of KBr and KI on the photocatalytic degradation process are significantly different.

Further combined with Figures 10, 11 and 12, it can be seen that the inhibitory effect of hydroxyl catcher KBr on the photocatalytic degradation of phenol is significantly stronger than that of hole catcher KI. This observation indicates that the oxidation of ·OH (hydroxyl radical) is dominant in the photocatalytic degradation process compared to the oxidation of h⁺ (hole). TEOA acts as a sacrificial electron donor, reducing the oxidized dye (EYox) back to its ground state (EY), thereby regenerating the dye and sustaining the photocatalytic cycle (Eq. 11).

In summary, it can be preliminarily speculated that the photocatalytic degradation pathways of phenol are as follows:

EY + hʋ → ¹EY* → ³EY* (9)

³_{EY* + POM → POM}^- _{+ EYox (10)}

EY_ox + TEOA → EY + TEOA_ox (11)

^{POM - + O}₂ ^{→ POM-O}₂^{- → POM + O}₂^{- (12)}

O₂^- + H⁺ → HO₂* (13)

2HO₂* → H₂O₂ + O₂ (14)

H₂O₂ + hʋ → 2·OH (15)

·OH+ phenol → resorcinols and quinones → carboxylic acids →CO₂+H₂O

4. CONCLUSIONS

In this study, Dye-POM-TEOA photocatalytic system was prepared by sensitizing H₃PW₁₂O₄₀ with Eosin Y. Dye sensitization could significantly improve the degradation of phenol by H₃PW₁₂O₄₀. The photocatalytic degradation of H₃PW₁₂O₄₀ phenol in cotton dyeing wastewater was simulated with phenol. The photocatalytic degradation of H₃PW₁₂O₄₀ phenol after dye sensitization was 82%. In the degradation reaction of phenol, the oxidation of ·OH is more dominant than that of h⁺, and the degradation process conforms to the first-order kinetic model. Although this system demonstrates excellent photocatalytic performance under laboratory conditions, in practical applications, issues such as the recovery and reutilization efficiency of the catalyst, the adaptability to different types of pollutants, and the long-term operational stability need to be considered. These factors will directly affect the economic feasibility and practical promotion value of this technology.

Future research can further explore the application effect of this system in actual dyeing wastewater, optimize the stability and recycling performance of the catalyst, and deeply investigate its degradation mechanism, in order to promote its practical application in industrial wastewater treatment.

ACKNOWLEDGEMENTS

This research was supported by Shanxi University.

AUTHOR CONTRIBUTIONS

Yan Zhang: Conceptualization, Methodology, Resources, Project administration. Jun Lu: Data curation, Writing-Original draft preparation. Liu Qian: Formal analysis, Writing-Reviewing. Zhang Huai-yi: Formal analysis, Writing-Reviewing. Liu Chang: Formal analysis, Writing-Reviewing. Yue-tao Liu: Formal analysis, Writing- Reviewing and Editing.

CONFLICT OF INTEREST STATEMENT

There are no conflicts to declare.

DATA AVAILABILITY STATEMENT

All data are available from the corresponding author upon request.

FUNDING

This research was financially supported by the Shanxi University Research Fund (Grant Number 2024012121421).

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