Preparation of PdCu Catalyst and the Catalytic Degradation of Methylene Blue and Rhodamine B with PMS

Abstract

Spherical Cu₂O nanoparticles were obtained by reducing copper acetate in N,N-dimethylformamide (DMF) system using glucose as the reducing agent and polyvinylpyrrolidone (PVP) as the surfactant, with which spherical PdCu nanocatalysts were thus synthesized by disproportionation. The catalyst was used for the activation of peroxymonosulfate (PMS) and showed an excellent degradation effect on rhodamine B and methylene blue-contained printing and dyeing wastewater with good stability. Additionally, the surface morphology analysis of the catalyst was carried out by SEM and TEM. The structure was characterized by XRD and FT-IR. The valence state and composition of the catalyst were characterized by XPS. The catalytic performance of the prepared catalysts was investigated with methylene blue and rhodamine B used as target pollutants. The results showed that the catalytic reduction efficiency of PdCu nanocatalyst for the two pollutants could reach 99% at 20 °C, when catalyst concentration was 60 mg/L and PMS concentration was 1.0 g/L and 0.6 g/L, respectively. The degradation efficiency of the catalyst was significantly reduced when Cl⁻, ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$ and HA were present in the water. The degradation efficiency was above 90% when the pH was in the range of 5–11. The excellent performance of the PdCu/PMS system in the treatment of RhB-contained wastewater was further confirmed by taking into account of the data of free radical quenching experiment and the results of electron paramagnetic resonance (EPR) experiment. After three cycles, the removal rate of MB and RhB could still be maintained at more than 90%, which proved its excellent recyclability due to its remarkable stability and efficiency.

1. Introduction

In recent years, with the rapid development of the economy, the printing and dyeing industry has discharged 20% of the wastewater around the world [1,2], and it has been reported that the discharge of printing and dyeing wastewater in China accounts for more than one-third of industrial wastewater. Printing and dyeing factories often use a large number of chemical compounds, such as dyes, colorants, surfactants, and mercerizing agents [3], to produce brightly colored garments, and almost 10,000 dyes are used each year [4]. Currently, about 40% of the colorant chemicals used worldwide are organically bound chlorine, which is known to have potentially carcinogenic properties [5]. Anthraquinone and azo dyes are the most consumed dyes in the global textile industry, accounting for 70–90% of total dye consumption [6]. Due to their toxicity, carcinogenicity and mutagenicity, these organic pollutants are likely to cause some unimaginably disastrous consequences on the environment and human health even with low concentrations in water [7]. Dyes are the main organic pollutants emitted in the production process of printing, dyeing, and finishing. Several studies have reported that there are more than 100,000 commercially available 86 dyes with an estimated annual production of over 7 × 10⁵ tons of dyestuff [8]. However, the inefficiencies of the dyeing process result in the release of 10 to 15% of the dye with wastewater, equivalent to 2% of the total dye yield [9]. Therefore, the treatment of textile wastewater is essential before it is discharged into the environment.

At present, there are three main treatment technologies for PDW, including physical treatment (adsorption [10], membrane separation [11,12], and magnetic separation [13]), chemical treatment (coagulation and flocculation [14], chemical oxidation [15], photocatalytic oxidation [16,17], and electrochemical), and biological treatment (anaerobic treatment, aerobic treatment, and anaerobic-aerobic combined treatment) [18,19]. In recent decades, advanced oxidation processes (AOPs) have received considerable attention due to their strong oxidizing ability, rapid reaction rate, and broad applicability on the degradation of organic compounds [20]. Peroxymonosulfate (PMS), as a relatively safe, strong, and stable oxidant, was commonly used to produce sulfate radicals [21,22]. Advanced oxidation technology produces reactive oxygen species (ROSs), including hydroxyl radicals ($\bullet \mathrm{O}\mathrm{H}$), sulfate radicals (${\mathrm{S}\mathrm{O}}_4^{\bullet -}$), superoxide radicals (${\mathrm{O}}_2^{\bullet -}$), and singlet oxygen (¹O₂) [23], which have the advantages of cleaning, thorough treatment, and easy operation, and have broad application and development prospects in the field of printing and dyeing wastewater treatment. Sulfate Radical-based Advanced Oxidation Processes (SR-AOPs) offer a range of advantages over radical-based AOPs, such as high reactivity over a wide pH range, selectivity for contaminants, and multiple methods to activate persulfate (PS, ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$) and peroxide monosulfate (PMS, ${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}$) to generate sulfate radicals (${\mathrm{S}\mathrm{O}}_4^{\bullet -}$) [24].

In this paper, a catalyst is found through experimental exploration, which can not only reduce energy consumption but can also reduce secondary pollution and recycle and efficiently treat printing and dyeing wastewater. As a new type of metal material, Pd has been widely used in the field of catalysis due to its multi-function, size, shape, and composition-dependent tunability, as well as excellent physical, chemical, and biological properties. However, there is a problem of catalyst loss during use. It is of great significance to accurately control the size, composition, morphology, and structure of the catalyst to improve the catalytic activity and stability of the Pd-based catalyst for the treatment of wastewater and pollutants. In this study, a PdCu spherical nanocatalyst was prepared by disproportionation reaction with spherical Cu₂O used as a template, and then methylene blue and rhodamine B were used as target pollutants in the treatment of printing and dyeing wastewater by the advanced oxidation technology of PdCu spherical nanocatalyst, which could activate persulfate.

2. Results and Discussion

2.1. Effect of the Ratio of Na₂PdCl₄ to Cu₂O

A discussion was conducted on the effects of different ratios of Na₂PdCl₄ to spherical Cu₂O on the catalytic performance of PdCu. In the process of catalyst preparation, the effect of the ratio of Na₂PdCl₄ to spherical Cu₂O on the catalytic degradation efficiency of MB and RhB was studied under the condition that the concentration of PVP and H₂SO₄ remained unchanged. As shown in Figure S1, the ratios of M(Cu₂O) to M(Na₂PdCl₄) at 10:1, 5:1, and 5:3 were indicated as PdCu-1, PdCu-2, and PdCu-3, respectively. When the ratio of M(Cu₂O) to M(Na₂PdCl₄) was 10:1, the removal rate of MB and RhB reached 99.9% after 20 min. When the ratio of M(Cu₂O) to M(Na₂PdCl₄) was 5:3, it was found that the catalytic rate of the catalyst was not significantly improved, which may be attributed to the reduction in the catalytic activity of the catalyst caused by the agglomeration of PdCu nanoparticles. Considering the cost of catalysts, the PdCu nanocatalyst prepared with the ratio of M(Cu₂O) to M(Na₂PdCl₄) at 10:1 was selected for the subsequent degradation experiment.

2.2. Characterizations of Catalyst

Scanning electron microscopy (SEM) was used to analyze and observe Cu₂O in detail, and the study was conducted on the morphology and micromorphology of Cu₂O at different magnifications in order to further explore the microstructure characteristics of Cu₂O. As shown in Figure S2, the morphology of Cu₂O presents a typical spherical structure with no obvious edges or irregular shapes, indicating that it is composed of the same component evenly and tightly packed.

During the detailed study of PdCu nanoparticles, transmission electron microscopy (TEM) revealed their unique microscopic morphology, which clearly showed that PdCu nanoparticles existed in a dispersed form, suggesting that they had extremely small sizes, as shown in Figure 1a,b. As can be observed from Figure 1c, the PdCu lattice space is 0.21 nm and 0.19 nm, determined by HR-TEM calculation, and the PdCu lattice size is significantly smaller than that of the pure Pd(111) plane (0.227 nm) and Pd(100) plane (0.194 nm). EDS spectroscopy analysis was carried out in order to fully understand the elemental composition of PdCu, and it can be observed from Figure 1f that PdCu contains two elements, Pd and Cu, which indicates that it is a bimetallic compound, and the weight percentage of palladium is 0.79%, which reflects the relative proportion of palladium to other elements in PdCu.

The nitrogen adsorption–desorption isotherms showed that Cu₂O and PdCu belonged to typical IV isotherms, accompanied by type H3 hysteresis (Figure S3), indicating a mesoporous structure [25]. The BET surface area of PdCu (42.3451 m²/g) was substantially greater than that of Cu₂O (27.7512 m²/g).

Further analysis was conducted with X-ray diffraction (XRD) to accurately reveal the crystal structure of the synthesized PdCu nanomaterials. In Figure 2, it can be seen that the characteristic peaks of the prepared PdCu are observed at 2θ diffraction angles of 42.0°, 47.0°, and 68.6°, which are generated by the diffraction of (111), (200), and (220) crystal planes of PdCu oxide, respectively. The diffraction peaks of (111), (200), and (220) crystal planes are located between the pure Pd (JCPDSNo.46-1043) and Cu (JCPDSNo.04-0836) crystal phases. The characteristic peaks of Cu and its oxides were not detected, which confirmed the synthesis of PdCu nanocatalysts. The XRD of the PdCu before and after the reaction is shown in Figure S4, and it is observed that there are still characteristic peaks after the reaction, but the characteristic peaks are slightly weakened, indicating that the degradation reaction does not have much effect on the structure of the catalyst.

The elements were analyzed with X-ray photoelectron spectroscopy (XPS) to better understand the elemental composition and chemical valence state of PdCu nanomaterials. As shown in Figure 3, three characteristic peaks are observed in the XPS full-spectrum scan of PdCu, representing the valence information of three different elements, i.e., Pd 3d, C1s, and Cu 2p.

The Pd 3D spectra of PdCu are shown in Figure 4a, with Pd 3D peaks of 340.6 eV (Pd 3p3/2) and 335.2 eV (Pd 3d/2), corresponding to the element Pd(0), which indicates that divalent palladium has been reduced to palladium nanoparticles. The Cu 2p spectra of PdCu are shown in Figure 4b. The Cu 2p peaks are 931.9 eV (Cu 2p3/2) and 951.9 eV (Cu 2p1/2), respectively, corresponding to the element Cu(0) [26].

Fourier transform infrared spectroscopy (FT-IR) analysis was performed to identify the functional groups on the surface of the prepared catalyst. As shown in Figure 5, there is an absorption peak between 500–750 cm⁻¹, and the absorption peak at 630 cm⁻¹ corresponds to the Cu-O stretching vibration. The characteristic peaks of Cu₂O occur at around 1571 cm⁻¹ and 3410 cm⁻¹. After the introduction of Pd metal species, it is observed that intensity of Cu₂O absorption peaks decreases slightly, indicating that some Cu-O bonds may be replaced by other metal elements. Since palladium metal itself does not exhibit specific characteristic peaks in the infrared spectrum, the corresponding absorption peaks of PdCu remain basically unchanged.

2.3. Degradation of Methylene Blue by Activated PMS

2.3.1. Catalytic Performance of Different Systems

As shown in Figure 6a, a high absorption peak at 664 nm could be observed for MB, which was the characteristic absorption peak of MB. With the progression of reaction, the intensity of the characteristic absorption peak of MB gradually decreased at 664 nm, which was the result of the gradual decomposition and degradation of MB under the action of PdCu catalyst, and the solution color of MB gradually changed from blue to colorless, which further confirmed that PdCu contributed to effective decolorization treatment of MB.

As can be seen from Figure 6b, when we explored first the adsorption effect of MB on the PdCu catalyst in the organic pollutant solution containing only the catalyst, we found that the degradation efficiency of the specific organic pollutant was only 10.3%, indicating that the adsorption capacity of PdCu for MB was almost insignificant. Similarly, in the system containing only PMS, it was found that the removal rate of MB within 30 min was only 37.6%. This data indicated that PMS, as a conventional water treatment technology, had a very limited ability to degrade MB without the addition of catalysts. When PdCu and PMS were applied together to the same system, MB could be completely removed within 25 min, indicating that the PdCu/PMS system had superior catalytic degradation effect. As can be seen from Figure 6c, the k_obs of PdCu/PMS reaches 0.1722 min⁻¹. Experiments with different catalytic systems were carried out to clarify and evaluate the actual role of PdCu and PMS in catalytic systems.

The degradation rate constant (k) of the target pollutant was calculated by the first-order dynamic model (1):

$$\mathrm{l}\mathrm{n}\left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}}\right)=-\mathrm{k}\mathrm{t},$$

(1)

where C₀ and C_t are pollutant concentrations at time 0 and pollutant concentrations at time t, respectively (mg/L), and k is the kinetic rate constant (min⁻¹).

2.3.2. Factors Influencing the Degradation of Methylene Blue

In the in-depth study and application of PMS technology, researchers have found that when the catalyst is activated and reacts with oxygen, a large number of reactive oxygen species (ROSs) are produced. These reactive substances play a crucial role in the oxidative degradation of organic compounds, making the removal process of organic matter efficient and thorough. Figure 7a shows that the amount of PMS added has a significant effect on the generation of ROSs. It was found that the removal efficiency of MB could be as high as 93.2% after the treatment with 0.2 g/L PMS for 30 min, which clearly indicated that insufficient PMS input was one of the main reasons for the unsatisfactory degradation efficiency. Therefore, by adjusting the amount of PMS, the degradation efficiency of the target organic compounds can be significantly improved. Increasing the concentration of PMS from 0.6 g/L to 1.0 g/L resulted in a complete degradation of MB and a K-value increase from 0.069 min⁻¹ to 1.98 times. Such a performance leap clearly benefited from the fuller contact between the catalyst and the oxidant and the subsequent production of more ROSs due to excessive PMS use. However, when the concentration of PMS increased to 1.6 g/L, the k value only increased from 0.176 min⁻¹ to 0.309 min⁻¹. This inferior increase in k value was mainly attributed to the self-quenching reaction of PMS in the advanced oxidation system at high PMS concentrations, during which ${\mathrm{S}\mathrm{O}}_4^{\bullet -}$ was used to produce low oxidation ${\mathrm{S}\mathrm{O}}_5^{\bullet -}$ [27].

As can be seen from Figure 7b, the removal rate and k value of MB increased significantly as the amount of PdCu added increased from 20 mg/L to 160 mg/L, indicating that PdCu played a positive role as a catalyst in the removal of MB. The number of catalysts was very important to the catalytic performance of PMS because it was related to the activity efficiency of the whole catalytic reaction system. The results showed that when the number of catalysts increased, the contact area between PMS and the catalyst surface also increased. As a direct consequence of this change, the PMS was able to access more catalytically active sites during the reaction. In other words, more active centers were activated and more active functional groups were produced, both of which were important factors in accelerating the reaction rate and increasing the degradation effect. In this study, it was finally confirmed that the 60 mg/L PdCu catalyst exhibited the best degradation performance in the treatment of MB.

As can be seen from Figure 7c, MB can be effectively removed in a wide pH range between 5.0 and 7.0, which indicated that the PdCu/PMS catalyst can still maintain good catalytic performance at a high pH level. In addition, the calculated k values were 0.139 and 0.171 min⁻¹, respectively, which provided us with a basis for quantitative analysis. When the pH of the reaction solution was reduced to 3, the MB removal efficiency was significantly reduced. Particularly, the removal rate of MB decreased to 82.2% only within 30 min. The reason for this phenomenon may be that under acidic conditions, a large amount of H⁺ would occupy the adsorption point of HSO⁵⁻ on the catalyst, resulting in protonation of PMS and reduction in the generation of some ROSs [28].

In a catalyst system, the presence and concentration of contaminants are undoubtedly the key factors that determine their degradation efficiency. We can observe that during the treatment of catalysts, the concentration changes in various pollutants can greatly affect their removal effect. As shown in Figure 7d, when the MB concentration was in the range of 10 mg/L to 30 mg/L, the removal rate of MB (i.e., rate constant k) showed a significant downward trend, from 0.176 min⁻¹ to 0.061 min⁻¹. The main reason for this phenomenon was that the number of ROSs produced in the catalytic degradation system was not enough to completely remove the excess MB, and the high concentration of MB may also coat the catalyst surface, thus preventing the adsorption of PMS on the catalyst surface.

It can be seen from the data in Figure 7e that the MB removal rate was 97.5% at 10 °C, suggesting that low temperature may slow down the migration and transformation process of MB. Furthermore, when the temperature rose to 40 °C, it only took 10 min for the MB to be completely removed. This phenomenon indicated that the removal rate of MB would be significantly accelerated with the increase in reaction temperature. As it can be clearly seen from Figure 7f, the degradation rate of MB in the PdCu/PMS composite system was consistent with the first-order kinetic equation. This equation not only accurately described the dynamic changes in MB degradation but also revealed that the k value (reaction rate constant) showed a corresponding increasing trend with increase in temperature. This meant that within a certain range, the rate of catalytic reaction increased as the temperature increased. The curve of the MB solution with only PdCu as a function of temperature is shown in Figure S8a. According to the first-order kinetic reaction equation, the reaction is endothermic, and the reaction rate accelerates as the temperature increases.

In this study, analysis and evaluation were conducted on the potential effects of various inorganic anions and organic substances in dye wastewater on the catalytic degradation of MB, during which special attention was paid to the effects of ${\mathrm{S}\mathrm{O}}_4^{2-}$, Cl⁻, ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$, and humic acid (HA) on the catalytic system. As shown in Figure 7g, we found that these inorganic anions and species did not exhibit significant negative effects under the operating conditions of the catalytic system. When ${\mathrm{S}\mathrm{O}}_4^{2-}$, ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$, and HA were present, the catalytic degradation system was less affected, and the degradation trend of MB was consistent with the trend when it did not contain anions, which indicated that the system has strong stability and adaptability and could effectively cope with the challenges of these common inorganic ions and organic substances in the environment. Approximately 80.7% of the MB was degraded within 30 min after 10 mM of Cl⁻ was added to the system, resulting in a significant inhibitory effect because Cl^- preferentially depleted free radical radicals (${\mathrm{S}\mathrm{O}}_4^{\bullet -}$) or ${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}$, thus leading to the production of low-activity Cl^• (Equations (1) and (2)) [29]. However, when the concentration of Cl⁻ was further increased, the degradation efficiency of MB was slightly improved (Figure 7h), mainly due to the production of ClOH or Cl₂ by Cl^- and some PMS (Equations (4) and (5)), which could directly oxidize MB [30].

$${\mathrm{S}\mathrm{O}}_4^{\bullet -}+{\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{S}\mathrm{O}}_4^{2-}+{\mathrm{C}\mathrm{l}}^{\bullet }$$

(2)

$$\mathrm{C}\mathrm{l}+{\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{C}\mathrm{l}}_2^{\bullet -}$$

(3)

$${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}+{\mathrm{C}\mathrm{l}}^{-}\to {\mathrm{S}\mathrm{O}}_4^{2-}+\mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}$$

(4)

$${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}+{2\mathrm{C}\mathrm{l}}^{-}+{\mathrm{H}}^{-}\to {\mathrm{S}\mathrm{O}}_4^{2-}+{\mathrm{C}\mathrm{l}}_2+{\mathrm{H}}_2\mathrm{O}$$

(5)

2.3.3. Stability Analysis of Catalysts

The catalytic degradation efficiency of the PdCu catalyst within 30 min was carefully observed and analyzed to evaluate the stability of the PdCu catalyst. It can be clearly seen from the data shown in Figure S5 that the performance of the PdCu catalyst could still be maintained at more than 90.0% even after three cycles, which not only indicated its excellent catalytic activity but also verified the material’s high stability and reusability. These results were of great significance for the promotion and application of PdCu catalysts in various environmental pollution control fields, especially in the occasions where fast reaction was required and the catalyst needed to operate stably for a long time. In addition, this research provided valuable reference information for further optimization of catalyst formulation, which is expected to lead to more efficient and cost-effective contamination control solutions in the future.

2.4. Degradation of Rhodamine B by Activated PMS

2.4.1. Catalytic Performance of Different Systems

Extensive studies had been made on the degradation of printing and dyeing wastewater by catalyst-activated PMS, in which UV-vis was used to determine the concentration change in RhB during the reaction. As shown in Figure 8a, the absorbance at 554 nm disappeared completely after 20 min of the reaction, indicating that RhB was almost completely degraded and the solution color of RhB gradually became colorless.

It was evident from the analysis results in Figure 8b that when we explored the adsorption effect of RhB, a specific organic pollutant, on the PdCu catalyst in the solution of organic pollutants containing only the catalyst, the degradation efficiency was only 8%, indicating that the adsorption capacity of PdCu for RhB was almost insignificant. This further confirmed that the selection and use of catalysts was crucial in the contaminant removal process, as it was directly related to the final contaminant removal effect. Similarly, it was found that when the system contained only PMS, the removal rate of RhB within 20 min was only 14%. This data reflected that PMS, as a traditional water treatment technology, had very limited ability to degrade RhB without the addition of catalysts. However, when both PdCu and PMS were applied to the same system, the results we observe were particularly encouraging. RhB was almost completely degraded after 20 min. As can be seen from Figure 8c, the k_obs of PdCu/PMS reaches 0.2341 min⁻¹. This data showed that the PdCu/PMS system exhibited significant advantages, which could not only improve the efficiency of pollutant removal but also showed a higher efficiency in treating organic pollutants. This synergistic mechanism provides new ideas and strategies to solve the problems in industrial wastewater treatment, especially for pollutants that are difficult to treat by a single technology or a simple combination.

2.4.2. Factors Influencing the Degradation of Rhodamine B

When PMS is activated and released, it produces a large number of reactive oxygen species (ROSs) that play a vital role in biochemical reactions by exhibiting strong catalytic oxidation capabilities for organic matter. Multiple concentration gradients were set up to evaluate the effect of PMS on the removal of RhB at different concentrations in order to further explore the effect of PMS added on the formation of ROSs. The experimental results are shown in Figure 9a, and it can be clearly seen that the removal rate of RhB had increased significantly with the increase in PMS. When the concentration of PMS was 0.2 g/L, the removal rate of RhB was only 80.8% after 20 min of treatment, which indicated that the system did not have a good effect on the removal of RhB. Further analysis showed that the reason for this phenomenon was that the initial amount of PMS was too small, resulting in too low sulfate concentration in the system, which could not effectively promote the oxidative degradation of RhB. When the amount of PMS was increased to 0.6 g/L, the results showed that RhB could be completely degraded within 20 min. This significant degradation effect was a qualitative leap compared with the previous one, which fully proves that appropriately increasing the concentration of PMS could effectively enhance the removal efficiency of RhB. When the concentration of PMS reached 1.0 g/L, RhB could be completely degraded in a short time. When the concentration of PMS was increased to 1.6 g/L, the degradation efficiency of RhB did not change greatly because the number of catalysts was limited and the activation site was insufficient, and in such a case, some PMS could not be activated in time and the production of sulfate radicals in the final system was limited [31].

As can be seen from Figure 9b, the removal efficiency of RhB could reach 98.9% within 20 min when the concentration of the PdCu catalyst was 20 mg/L, which indicated that the reaction effect was slightly worse at this time. However, when the catalyst concentration was increased to 60 mg/L, 100 mg/L, and 160 mg/L, the removal rates of RhB reached 100%, 97.9%, and 96.2%, respectively, at 12 min, 12 min, and 8 min. The results show that with the continuous increase in catalyst addition, the activity of PMS was enhanced, which, in return, allowed RhB to be degraded more quickly. Firstly, the contact area between the PMS and the catalyst increased with the number of catalysts. As the contact area increased, more catalyst active sites could bind more closely to the PMS, activating the PMS and triggering the production of more active groups that were responsible for attacking and oxidizing pollutant molecules. Secondly, an excess of catalyst may lead to an increase in the leaching rate of metal ions in solution, which often affected the stability and reusability of the catalyst. In addition, there was a limit to the amount of oxidant PMS that could be added, and if it was excessive, it may cause unstable degradation or destroy environment-friendly operating conditions due to overly drastic chemical reactions.

As it can be seen from Figure 9c, when the initial concentration of RhB was 10 mg/L, RhB could be completely removed within 16 min in the PdCu/PMS system, showing a good degradation effect. This phenomenon demonstrated the effective degradation capacity of the catalyst for RhB at low concentrations. However, as the concentration of the initial RhB solution was increased further, the removal efficiency decreased significantly. When the RhB concentration was increased to 15, 20, or even 30 mg/L, the removal rates were only 95.6%, 90.6% and 75.5%, respectively, even at the exact same reaction time. The reason was that overly high RhB concentration could hinder or even inhibit the catalytic reaction process, resulting in a decrease in degradation efficiency. The reasons behind this were complex and involved the interaction of catalyst mechanisms as well as free radical generation mechanisms. When the concentration of RhB in solution was increased, more sulfate radicals needed to be produced to react with the contaminants. However, the catalyst played a crucial role in the overall system, and the number of catalysts and PMS added was fixed. If the amount of sulfate radicals generated was not sufficient to cover the entire solution of RhB molecules, then the degradation efficiency of RhB was consequently limited. Therefore, as the concentration of RhB solution was increased, the contact area between the catalyst and PMS decreased, thus the production of sulfate radicals was also reduced.

As it can be seen from Figure 9d, the removal efficiency of RhB showed significant differences under different pH conditions. When pH was between 5 and 9, the removal efficiency of RhB remained at a high level, indicating strong tolerance and degradation activity. When pH was 3, the degradation efficiency of RhB could only reach 88.1% within 20 min, possibly because excess H⁺ bound to ${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}$, leading to reduction in active groups produced in the system. Another possible reason may be that the carboxyl group (-COOH) of RhB ionized to form a negatively charged -COO⁻. As a result, the anionic sulfate radical and the negatively charged RhB were electrostatically repulsed, thus hindering the degradation of RhB [32]. When pH was 5 and 7, the removal rates of RhB reached 96.3% and 96.6%, respectively, at 20 min. Without adjusting the pH of the solution, RhB could be completely removed in less than 20 min. However, with the continuous increase in pH value to 11, the degradation rate of RhB decreased to 92.3%, indicating that high pH value would have an adverse effect on the activation of persulfate by PdCu. This was mainly due to the fact that the pKa value of the PMS molecule was as high as 9.4, which is highly susceptible to hydrolysis to form ${\mathrm{S}\mathrm{O}}_5^{2-}$ in a strong alkaline environment. In addition, a large amount of OH− would be attached to the surface of the catalyst under the condition of alkali, which inhibited the contact between the catalyst and PMS, hindered the generation of ${\mathrm{S}\mathrm{O}}_4^{\bullet -}$, and led to the reduction in RhB degradation efficiency [33].

As it can be seen from Figure 9e, when the temperature dropped to 10 °C, the removal rate of RhB was only 94.7% at 20 min, which indicated that the degradation effect of RhB in the system was slightly worse when the temperature was too low. However, when the temperature rose from 10 °C to 20 °C, the slow rate of degradation was significantly accelerated. Furthermore, RhB could be completely degraded at 30 °C and 40 °C, and it only took 12 min and 8 min to realize complete degradation, respectively, which indicated that the promotion effect of temperature on degradation efficiency was extremely significant. As can be seen from Figure 9f, there was a close relationship between the first-order kinetic constant and the temperature rise. This constant represented the regularity of the chemical reaction rate with temperature, and its magnitude reflected the speed of the reaction rate. The results showed that the reaction rate constant increased rapidly from 0.1468 min⁻¹ to 0.3286 min⁻¹ as the temperature rose from 10 °C to 20 °C, increased to 0.4467 min⁻¹ when the temperature rose to 35 °C, and finally reached 1.373 min⁻¹ at 40 °C. The curve of the RHB solution with only PdCu as a function of temperature is shown in Figure S8b, and there is little change in temperature.

As shown in Figure 9g, when ${\mathrm{S}\mathrm{O}}_4^{2-}$ was present in the RhB solution, the degradation efficiency of RhB was 97.3% at 20 min, which indicated that ${\mathrm{S}\mathrm{O}}_4^{2-}$ had little effect on RhB degradation, and the degradation trend of RhB was consistent with that of RhB without anion. When the RhB solution contained 20 mg/L humic acid, the degradation efficiency of RhB could reach 95.5% at 20 min. The removal effect of this slight inhibition may possibly be explained by the mechanisms that (i) HA and PMS competed for active sites on the catalyst surface, thus limiting the activation of PMS and (ii) when HA competed with RhB for ROSs, the utilization of ROSs decreased [34].

As can be seen from Figure 9h, the carbonates that were ubiquitous in water had a significant inhibitory effect on the degradation process of RhB. The root cause of this phenomenon was that sodium bicarbonate (${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$) could react chemically with RhB, resulting in the decrease in its degradation rate to 87.6% in just 20 min. This rate of degradation was mainly due to the fact that ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$ was involved in the reaction process. Under this mechanism, it was able to react to generate some relatively low-activity free radicals, which further reacted with other molecules, thereby accelerating the entire degradation process [35].

2.4.3. Stability Analysis of Catalysts

In order to evaluate the stability of the PdCu catalyst, the catalytic degradation efficiency of the PdCu catalyst within 20 min was carefully observed and analyzed. From the data shown in Figure S6, we can clearly see that the performance of the PdCu catalyst can still be maintained at more than 90.0% even after three cycles, which indicates that the PdCu has good reusability and stability. In addition, there is no significant change in the characteristic peaks of PdCu, which further proves its excellent structural stability.

2.5. Degradation Mechanism Studies

2.5.1. Quenching Experiments

A unique and precise method of quenching experiment was adopted in order to further explore the dynamic changes in reactive oxygen radicals and non-free radical species involved in the degradation reaction of RhB (MB). The activation mechanism of PMS could be categorized into two types, i.e., reduction through radical species (${\mathrm{S}\mathrm{O}}_4^{\bullet -}$, $\bullet \mathrm{O}\mathrm{H}$) and oxidation via nonradical processes (electron transfer, ¹O₂) [36]. At 20 °C, 60 mg/L PdCu catalyst and 0.6 g/L PMS were added to the RhB solution at an initial concentration of 10 mg/L to ensure that all reactions proceeded smoothly. Subsequently, a quencher was slowly added to the system to make the reactive oxygen species effectively quenched so that non-free radical species could be revealed. The key to this process was to select the appropriate quencher to minimize interference with the results by other non-target species. The quencher consisted of methanol (MA), 6 mol/L, for quenching $\bullet \mathrm{O}\mathrm{H}$ and ${\mathrm{S}\mathrm{O}}_4^{\bullet -}$ [37], tert-butanol (TBA), 6 mol/L, for quenching $\bullet \mathrm{O}\mathrm{H}$ [38], and P-benzoquinone (p-BQ), 0.1 mol, for quenching ${\mathrm{O}}_2^{\bullet -}$ [39].

As shown in Figure S7a, the degradation rate of RhB dye was 33.7% after 20 min of treatment time after adding an appropriate amount of methanol (MA) to the system. This result clearly showed that the active groups, ${\mathrm{S}\mathrm{O}}_4^{\bullet -}$ and $\bullet \mathrm{O}\mathrm{H}$, were involved in the reaction as oxidizing agents. However, when TBA was added to the catalytic system, the degradation rate of RhB increased significantly, reaching 71.7%. This phenomenon indicated that the oxidative degradation process of RhB by OH in the PdCu/PMS system was not enhanced as expected. In addition, the degradation rate of RhB was 57.7% after the addition of p-BQ, which revealed that ${\mathrm{O}}_2^{\bullet -}$ may be produced in this system. As shown in Figure S7b, in the PdCu/PMS system, one major reactive oxygen species, ${\mathrm{S}\mathrm{O}}_4^{\bullet -}$, played a decisive role, and other secondary reactive oxygen species, ${\mathrm{O}}_2^{\bullet -}$ and $\bullet \mathrm{O}\mathrm{H}$, also play a role that cannot be ignored.

2.5.2. Electron Spin Resonance Resonance (EPR) Experiments

In this study, electron paramagnetic resonance (EPR) was used to detect ROSs present in the PdCu/PMS system, in which 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and 2,2,6,6-tetramethyl-4-piperidinone (TEMP) were selected as spin trappers for the capture of free radicals and ¹O₂ required for the experiment. The active substances in PMS and PdCu/PMS systems were analyzed experimentally. As it can be seen in Figure 10, no significant reaction signal was detected in the samples containing only PMS and contaminants, either in the DMPO/PMS or TEMP/PMS systems, clearly revealing the key fact that PMS cannot independently produce ROSs without application of a catalyst.

As shown in Figure 10a, two significant characteristic peaks could be observed in the PdCu/PMS-containing aqueous system after the addition of 80 mM DMPO, which represented the characteristic peaks of DMPO-$\bullet \mathrm{O}\mathrm{H}$ and DMPO-${\mathrm{S}\mathrm{O}}_4^{\bullet -}$, respectively, indicating the presence of $\bullet \mathrm{O}\mathrm{H}$ and ${\mathrm{S}\mathrm{O}}_4^{\bullet -}$ in the catalytic degradation system. Figure 10b illustrated the effect of DMPO added in an appropriate amount to the PdCu/PMS methanol system. The results showed that, with the addition of DMPO, a multi-peak group composed of 6 peaks, i.e., DMPO-${\mathrm{O}}_2^{\bullet -}$ characteristic peaks, was formed and the peak group increased with the extension of reaction time. In Figure 10c, it can be clearly seen that after the addition of TEMP to the PdCu/PMS system, three obvious absorption peaks were observed, and their intensity distribution presented a ratio of 1:1:1. These three peaks represent the characteristic peaks of TEMP-¹O₂. These results indicated that there was a specific chemical reaction in the PdCu/PMS system, which generated ¹O₂, thus promoting the degradation of pollutants. The results showed that the PdCu/PMS system contained not only active free radicals ($\bullet \mathrm{O}\mathrm{H}$, ${\mathrm{S}\mathrm{O}}_4^{\bullet -}$ and ${\mathrm{O}}_2^{\bullet -}$) but also non-free radicals (¹O₂), which jointly promoted the removal of RhB. Among them, ${\mathrm{S}\mathrm{O}}_4^{\bullet -}$ was the main active species (ROSs) for the removal of RhB in the PdCu/PMS system, which played a crucial role in the degradation of RhB.

2.5.3. Degradation Mechanism

It was critically concluded through reactive oxygen species’ quenching experiments. in combination with EPR technology. that ${\mathrm{S}\mathrm{O}}_4^{\bullet -}$ was one of the main active species (ROSs) that removed RhB in PdCu/PMS systems. In addition to ${\mathrm{S}\mathrm{O}}_4^{\bullet -}$, free radicals including $\bullet \mathrm{O}\mathrm{H}$, O₂, and ${\mathrm{O}}_2^{\bullet -}$ also played a secondary role. This discovery provided an important clue for understanding the mechanism of RhB degradation in PdCu-activated PMS systems. A systematic demonstration is provided in Figure 11 in order to further illustrate this mechanism. The diagram clearly illustrates the reaction of different forms of free radicals with PdCu, which not only revealed the inner workings of the PdCu/PMS system, but also showed the challenges that may be encountered in practical applications and their solutions. The lower valence states of $\equiv$Cu⁺ and $\equiv$Pd²⁺ were swiftly oxidized by ${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}$ to $\equiv$Cu²⁺ and $\equiv$Pd³⁺, respectively, accompanied by the generation of ${\mathrm{S}\mathrm{O}}_4^{\bullet -}$ and OH⁻ (Equations (7) and (8)) [40]. Through this analysis, researchers can better understand and optimize the application of the system in the field of industrial wastewater treatment to achieve the goals of more efficient and economical wastewater treatment.

$$\equiv {\mathrm{C}\mathrm{u}}^{2+}-{\mathrm{O}\mathrm{H}}^{-}+{\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}\to \equiv {\mathrm{C}\mathrm{u}\mathrm{O}}^{+}+{\mathrm{S}\mathrm{O}}_4^{\bullet -}+{\mathrm{H}}_2\mathrm{O}$$

(6)

$$\mathrm{C}\mathrm{l}\equiv {\mathrm{C}\mathrm{u}}^{+}+{\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}\to \equiv {\mathrm{C}\mathrm{u}}^{2+}+{\mathrm{S}\mathrm{O}}_4^{\bullet -}+{\mathrm{O}\mathrm{H}}^{-}$$

(7)

$${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}+{\mathrm{P}\mathrm{d}}^{2+}\to {\mathrm{P}\mathrm{d}}^{3+}+{\mathrm{S}\mathrm{O}}_4^{2-}+\bullet \mathrm{O}\mathrm{H}$$

(8)

$${\mathrm{H}\mathrm{S}\mathrm{O}}_5^{-}+\bullet \mathrm{O}\mathrm{H}\to {\mathrm{S}\mathrm{O}}_4^{\bullet -}+{\mathrm{H}}_2\mathrm{O}$$

(9)

3. Materials and Methods

3.1. Chemicals

Copper acetate (Cu(CH₃COO)·H₂O), N,N-dimethylformamide (C₃H₇NO), polyvinyl pyrrolidone ((C₆H₉NO)n), glucose (C₆H₁₂O₆), rhodamine B (C₂₈H₃₁ClN₂O₃), methylene blue (C₁₆H₁₈ClN₃S), methanol (CH₃OH), and ethanol (C₂H₅OH) were all acquired from Aladdin Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Sulphuric acid (H₂SO₄), potassium persulfate (2KHSO₅·KHSO₄·K₂SO₄), and hydrochloric acid (HCl) were purchased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). Sodium tetrachloropladate (Na₂PdCl₄) was purchased from Shanghai Maclin Biochemical Technology Co., Ltd. (Shanghai, China). Deionized Mini-Q water (18 MΩ·cm) was used throughout the experiments.

3.2. Preparation of Nanocatalysts

3.2.1. Synthesis of Cu₂O Spherical Particles

A solution was produced by dissolving 1.3 g of copper acetate, 0.5 g of polyvinyl pyrrolidone (K = 30), and 1.27 g of glucose into 100 mL DMF. After the solution was stirred magnetically at a constant temperature for about 1.5 h, the temperature was increased to 85 °C and then the solution was stirred continuously for 10 min. The final product was washed three times with alcohol and water and then freeze-dried.

3.2.2. Preparation of PdCu Nanocatalysts

First, 0.0882 g of Na₂PdCl₄ was completely dissolved into 30 mL of 20 mM HCl and sonicated for 30 min to prepare a 10 mM Na₂PdCl₄ solution. Then, 7.2 mg of spherical Cu₂O powder and 0.3 g of PVP were dispersed in 20 mL of H₂O by sonication, and 0.5 mL of 10 mM Na₂PdCl₄ was added to enable the reaction for 20 min at room temperature and pressure. In the end, 0.1 mL of 18 mM H₂SO₄ was added, and after 40 min of reaction at room temperature, the product was collected by centrifugation at 10,000 rpm for 5 min, which was then washed with deionized water and ethanol for several times. The PdCu hollow particles of spherical mesoporous nanoshells were freeze-dried.

3.3. Characterization of Nanocatalysts

Scanning electron microscopy (SEM, SIGMA 300, Saint Louis, MO, USA) and transmission electron microscopy (TEM, FEI Tecnai F20, Hillsboro, OR, USA) were used to characterize the surface morphology and structural morphology of the catalysts. X-ray diffractometry (XRD, D/max 2200, Rigaku Corporation, Tokyo, Japan) was used to characterize the crystal structure of nanocomposites. X-ray photoelectron spectroscopy (XPS) (Thermo Scientific K-Alpha, Waltham, MA, USA) was used to determine the elemental valence and relative content of elemental composition on the catalyst surface. Fourier transform infrared spectrometer (FT-IR, Tensor 27, Bruker Corporation, Berlin, Germany) was used to measure the infrared absorption spectrum of the sample and analyze its chemical bond and structure.

3.4. Catalytic Performance of Nanocatalysts

3.4.1. Catalytic Degradation Experiments

In the catalytic degradation experiment of printing and dyeing wastewater, 250 mL of the target pollutant solution with an initial concentration of 100 mg/L was prepared first with ultrapure water. Then, a certain amount of the pollutant solution with an initial concentration of 100 mg/L was taken into a 100 mL beaker and diluted into the corresponding concentration. After the solution was stirred in a magnetic stirrer (HJ-6 GuaHua Laboratory Instrument, Guohua, China) for 5 min, PMS with a certain concentration was added to initiate the degradation reaction. After the start of each reaction, 2 mL of the solution was taken at specific intervals and filtered through a 0.45 μm filter, then 1 mL of methanol was added for quenching. Finally, the concentration of the remaining target contaminants was determined using a UV spectrophotometer (UV-2800, MAPADA, Shanghai, China).

3.4.2. Reuse Experiments

The catalytic material that completed the reaction was removed from the reaction vessel and recovered, and the collected catalyst was washed several times using deionized water to remove any residues or impurities that may be present. It was then further cleaned with absolute ethanol to achieve the best washing results and ensure that the catalyst surface was clean and residue-free. Finally, the cleaned catalyst was dried in an oven (DHG-9013, HeHeng Laboratory Instrument, Shanghai, China) at 60 °C and then weighed after drying to accurately record its weight.

4. Conclusions

Spherical PdCu nanocatalysts were synthesized by disproportionation with a spherical Cu₂O used as template, and methylene blue (MB) and rhodamine B (RhB) were selected as target pollutants to study the performance and degradation mechanism of the catalysts. Spherical Cu₂O nanoparticles were obtained by reducing copper acetate in an N,N-dimethylformamide (DMF) system using glucose as the reducing agent and polyvinylpyrrolidone (PVP) as the surfactant, with which spherical PdCu nanoparticles were synthesized by disproportionation after the ratio of Na₂PdCl₄ to Cu₂O was adjusted. The results of TEM characterization showed that the prepared PdCu nanoparticles were uniformly distributed and had good crystal form, and the analysis results of XRD, XPS, and FT-IR showed that PdCu had a good crystal structure.

The catalytic performance of PdCu nanocatalysts under different operating conditions had been conducted, in which methylene blue was used as the target contaminant. The results showed that when 60 mg/L PdCu and 1.0 g/L PMS were used, the removal rate of 10 mg/L MB was 99.9% at 20 °C at 25 min, and the first-order kinetic rate constant k reached 0.176 min⁻¹. When Cl⁻ and HA were present in the water, the degradation efficiency of the catalyst was significantly reduced. When the pH was in the range of 5–11, the MB degradation efficiency was above 90%. The catalytic performance of PdCu nanocatalysts under different operating conditions was studied with RhB as the target contaminant. The results showed that when 60 mg/L PdCu and 0.6 g/L PMS were used, the removal rate of 10 mg/L RhB reached 99.9% at 20 °C at the 16th min and the first-order kinetic rate constant k reached 0.329 min⁻¹. In the presence of inorganic ions and natural organic compounds in water, the degradation efficiency of RhB was significantly reduced. When the pH was in the range of 5–11, the degradation efficiency of RhB was above 90%.

Based on the results of the free radical quenching experiment and electron paramagnetic resonance experiment, it was found, through analysis, that ¹O₂, ${\mathrm{S}\mathrm{O}}_4^{\bullet -},\bullet \mathrm{O}\mathrm{H},$ and ${\mathrm{O}}_2^{\bullet -}$ together promoted the removal of RhB. After three cycles, the removal rate of MB and RhB could still reach more than 90%, which proved the excellence of the system in reusability and provided strong theoretical support and practical guidance for future applications of the system in the field of environmental pollution control.