Microwave-assisted method to degrade phenol using persulfate or hydrogen peroxide catalyzed by Cu-bearing silicon carbide

Phenols are important chemical and pharmaceutical raw materials and function as intermediates in the synthesis of many high molecular weight aromatic hydrocarbons (Santos et al. 2002). Phenol is a major environmental pollutant, and its high biotoxicity makes it impossible for the biochemical methods to completely mineralize it. Advanced oxidation technologies for water treatment using sulfate and hydroxyl radicals have been developed for the remediation of contaminated groundwater and the degradation of wastewater containing dyes, chlorophenol, and phenol. Liou and Chen mineralized phenol in wastewater using a copper/activated carbon (Cu/AC) composite catalyst in the presence of H₂O₂; experiments have shown that the Cu/AC has a higher utilization rate of hydroxyl radicals than ordinary iron oxides and can degrade pollutants faster, like complete degradation of phenol within 30 min. Additionally, more than 90% of the chemical oxygen demand (COD) from the intermediate compounds was also reduced (Liou & Chen 2009).

Microwave (MW) radiation catalysis is a technology for the degradation of organic pollutants where traditional advanced oxidation technologies are combined with the microwave irradiation that converts microwave electromagnetic energy into heat. It has high selectivity between the heated materials due to the differences in dielectric loss (such as water and SiC). Chen et al. reported that the microwave heating leads to the formation of ‘hot spots’, locations where non-uniform heating occurs with the formation of high temperature regions that provide active sites for the reactions and more effective chemical reactions (Chen et al. 2017). In the catalytic processes, microwave radiation also has non-heating effects. For instance, Qiu et al. reported that the microwave radiation induced the formation of electron–hole pairs in the semiconductor catalysts and dramatically enhanced the catalytic capacity (Qiu et al. 2016).

Previous studies have shown that the catalysts combined with other reagents can greatly improve the removal efficiency of contaminants under microwave irradiation (Xia et al. 2019), and the descriptions of the mechanisms involved are mostly discussed at a certain point but not summarized. Herein, copper-based silicon carbide (Cu/SiC) catalysts were prepared by an impregnation method. Catalytic performance and repeatability of the as prepared catalyst was studied using phenol was as a model pollutant under microwave irradiation. Instrumental techniques like scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure and components of the catalyst along with the valence state analysis to study the formation of radicals. The intermediates corresponding to the phenol degradation were determined, and the formation and mineralization mechanisms of the different intermediates were proposed. By comparing the results, the activation modes of different oxidants under the same catalytic environment were discussed to determine the favorable catalytic conditions for the efficient degradation of pollutants.

Phenol, quinone, catechol and hydroquinone were purchased from Aladdin. H₂O₂ and Na₂S₂O₈ were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Methanol was purchased from TEDIA. Tert-butyl alcohol and ethanol were procured from Chengdu Kelong Chemical Reagent Factory. Cupric nitrate and nitric acid were purchased from Kemiou Chemical Reagent Co., Ltd (Tianjin, China) and were used for the catalyst synthesis. β-SiC was obtained from the Institute of China Building Materials Academy. All other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All the chemicals were of analytical grade and used without further purification. All the solutions were prepared with ultrapure water.

The Cu/SiC composites were prepared by an impregnation method using SiC as the catalyst support. Copper was loaded using an impregnation method. First, SiC was ultrasonically washed in 1% HNO₃ for 30 min, followed by washing with deionized water for five times and then dried at 105 °C for 8 h. Then, the obtained Cu/SiC was blended with 100 mL of 0.7 mol L⁻¹ or 1.5 mol L⁻¹ Cu(NO₃)₂ solution and then stirred for 2 h. Finally, the slurry was dried at 105 °C for 8 h and then the mixture was calcined at 300 °C in air for 4 h. The prepared materials Cu/SiC 2 and Cu/SiC 4 with respective copper mass fractions of 2.43% and 4.14% were used as catalysts.

To evaluate the effect of catalyst adsorption on the degradation process, Cu/SiC (1 g) was mixed with phenol-containing solution (200 mg L⁻¹, 100 mL) using a shaking-table for desired time interval. Microwave catalytic experiments were conducted in aqueous solution using a modified domestic microwave (microwave power: 0–900 W) (Chen et al. 2017). The catalytic performance of Cu/SiC in the oxidation of phenol by H₂O₂ or Na₂S₂O₈ was evaluated in a 100 mL batch reactor system. The optimum quantity of H₂O₂ or Na₂S₂O₈ was determined under the same conditions at 98 °C, and the temperature of the solution and catalyst during the catalytic process was recorded with a K-type thermocouple probe. After mixing phenol and the catalyst, volatile organic compounds were formed as a result of phenol decomposition which was refluxed into a quartz reactor through a condensing unit, under microwave irradiation (600 W). Samples (1 mL) were withdrawn from the reactor at set intervals and immediately filtered through a 0.22 μm filter. Reusability of the catalyst was verified by using the catalyst in successive catalytic experiments under the same optimized conditions. After each cycle, the catalysts were collected and rinsed three times with deionized water, dried, and then were subjected to another catalytic cycle. After each catalytic cycle, XPS was used to characterize the chemical composition and valence states of the elements on the catalyst surfaces. The copper content in the recovered catalysts was determined to estimate the stability of the catalysts.

The active species present in the peroxide catalytic system were also studied. Ethanol, tert-butanol (·OH and scavenger), and p-benzoquinone (⁠ scavenger) were added to the catalytic systems, and then the phenol concentrations in the solution were measured (Wang et al. 2018).

After microwave treatment, the phenol and degradation products were analyzed using reverse-phase high performance liquid chromatography (HPLC, Agilent 1200) equipped with an Agilent 5 TC-C18 column (5 μm, 250 × 4.6 mm) at a wavelength of 254 nm. Methanol and pure water in a ratio of 40:60 were used as the mobile phase. The standard solutions of phenol, catechol, hydroquinone, resorcinol and benzoquinone were prepared to obtain the standard curves and the retention time for individual component was 8.869 min, 4.254 min, 3.181 min, 3.562 min, and 3.962 min, respectively. The solution pH was measured by a pH meter (Mettler Toledo, FE28), the TOC was measured using a Multi TOC Analyzer (2100, Analytik Jena, Germany).

The surface morphology, size distribution of the prepared and reacted copper oxides and the elemental composition of the catalysts were followed by a scanning electron microscope (SEM, S-4800, Hitachi Corporation) equipped with an EDS. The Cu contents were detected by an inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin Elmer 8300, PerkinElmer Corporation). The surface chemical composition was further analyzed by X-ray photoelectron spectroscopy (XPS, AXIS Supra, Kratos Corporation).

The distribution of copper oxides and copper on the surface of the prepared and reacted Cu/SiC were investigated by SEM-EDS. The SEM images in Figure 1 indicate that the calcination process did not destroy the honeycomb structure of the SiC. The highly-dispersed Cu species increased the number of exposed active sites which improved the catalytic performance. The XPS spectra of the catalyst surface in Figure S1 (Supplementary Information) shows that the oxygen was contributed mainly by two species: hydroxyls/carbonates (532 eV) and a mixture of copper oxides (529.9 eV) (Sutthiumporn & Kawi 2011; Rat'ko et al. 2012). The XPS spectrum in Figure 2 shows the chemical states of the metal oxides in the prepared catalyst samples. The Cu 2p_3/2 characteristic peak at 934.5 eV along with the presence of the characteristic shake-up satellite peaks at 941.8 eV and 944.1 eV demonstrated the existence of Cu(II) (Velu et al. 2005; Wadekar et al. 2005; Wang et al. 2018). The presence of a peak at 933.3 eV in the H₂O₂ reaction system after five consecutive catalytic experiments indicates the appearance of Cu(I) (Figure 2(b)), but presence of Cu(I) was not detected on the catalyst surface for the system (Figure 2(c)).

The surface area of the catalyst was determined to be 230 m² g⁻¹. The ICP-OES analysis showed that the amount of copper increased with increasing copper nitrate concentration. After five successive catalytic cycles in the presence of H₂O, the weight ratio of copper in the reacted catalyst was nearly the same as to that of the fresh catalyst (2.4 − 2.43 wt%). However, in the Na₂S₂O₈ system, the weight ratio of copper was reduced to 1.8 wt%.

With respect to the mechanistic aspect of phenol degradation, it can be divided into two steps; the generation of radicals and the oxidation of phenol by the generated radicals. The reaction rates of and ·OH with phenol were reported to be 8.8 × 10⁹ M·s⁻¹ and 6.6 × 10⁹ M·s⁻¹, respectively (Du et al. 2017), so the rate determination step for the degradation of phenol was the production of radicals, and the action of microwaves was considered to enhance the rate of radical production. To verify the effect of microwaves on the catalytic activity in this experiment, kinetics of degradation was studied. Figure 4 shows the phenol removal rate with time along with the fitting of kinetic model from the experiments carried out in a water bath containing aqueous solution (SiC, 98 °C) and in presence of MW (SiC or Cu/SiC 2); Table 1 presents a comparison of the kinetic curves and rate constants for the phenol removal under three different conditions studied. The phenol degradation in this study followed a pseudo-first-order kinetics model, and the kinetic coefficients of phenol degradation for SiC in water bath, SiC in MW and Cu/SiC in MW were 0.0493 min⁻¹, 0.1538 min⁻¹, and 0.3762 min⁻¹, respectively. Among them lower R² ≥ 0.9595 under microwave radiation was due to the influence of temperature during the initial duration of 2 min and as the temperature in the solution reaches 98 °C, phenol achieves a steady increase in mineralization efficiency; the rate of reaction catalyzed by SiC in the presence of microwave was about 3.12 times higher than that of the water bath heating (Table 1). There are some plausible mechanisms for this enhancement under microwave irradiation. First, the rate of temperature rise under microwave irradiation varies in different media, and the heterogeneous reaction system presents a thermodynamic non-equilibrium state. In particular, the surface micro-regions of the solid-phase catalysts had a steep thermal gradient (hot spots) due to the interface polarization. The molecules were activated easily in the high-temperature micro-regions. Secondly, it has been reported that the microwave could reduce or increase the activation energy (Costa et al. 2009), which could be explained based on the temperature gradient. Based on the Arrhenius reaction rate equation, the activation energy is related to the localized microscopic high temperatures, rather than the overall temperature. In fact, only the bulk temperature was measured and the microscopic temperature could not be assessed and if the temperature range was large, an error will occur in the calculation of the activation energy.

The presence of Cu(I) in the Cu/SiC samples recovered after five catalytic cycles was confirmed by XPS (Figure 2(b)), which showed that a copper-redox cycle occurred during the microwave assisted catalysis. Theoretically, microwave can speed up the separation of holes and electrons by SiC, and the transferred electron could induce the redox reaction of Cu(II) (Chen et al. 2017). Although Cu(I) was not detected in the persulfate system, the occurrence of electron transfer was not excluded due to its strong oxidation behavior. In addition, Cu/SiC 2 exhibited a higher catalytic activity towards phenol than Cu/SiC 4, indicating that the dispersion of active Cu species also significantly affected the catalytic activity. Hence, smaller quantity of Cu displayed superior catalytic effect. In contrast, aggregation of the active copper species occurring in the cavities of the carbon sphere, decreases the number of exposed active sites and thus reducing its utilization rate at higher quantity (Figure 1).

The reactions of sulfate radicals with organic matter mainly include hydrogen extraction (Equation (2)), electron mobility (Equation (3)), and addition mechanisms (Equation (4)). Catechol and benzoquinone were detected in the Na₂S₂O₈-Cu/SiC 2 reaction system within 3 and 4 min, respectively (Figure 5(b)), and the addition mechanism was verified by the determination of the product catechol. First, ·O- was generated at the beginning due to the interactions between · and organic matter. Then the O side (containing a single electron) was added to the benzene ring and separated a H. Then one O-S bond was broken and the formed -O· combined with the H atom to form a new hydroxyl group (-OH), resulting in the formation of catechol. The subsequent degradation was similar to the degradation of the organic compounds in presence of the hydroxyl radicals. In order to comprehensively evaluate the degradation effect of phenol, TOC degradation experiment was carried out further (Figure S2, Supplementary Information). The remaining amount of TOC was maintained at 20–35%, phenol did not completely degrade into carbon dioxide and water, but instead generated corresponding organic acids. This result validates the viewpoint of M. Sillanpää et al. that the AOPs oxidize organic matter to low molecular weight organic matter in practical applications (Sillanpää et al. 2018).

Ethanol (EtOH) and tert-butyl alcohol (TBA) were used as the scavengers and their rate constants are shown in Table S1 (Supplementary Information). Both the EtOH and TBA captured ·OH and ⁠, while TBA scavenged ·OH approximately 1000-times faster than that of ⁠. Thus, these compounds can be used to distinguish from ·OH. According to the results, ·OH and were the dominant oxidizing species in the H₂O₂ and Na₂S₂O₈ systems, respectively (Figure 6). Meanwhile, p-benzoquinone (BQ), acted as an scavenger, and inhibited the degradation of phenol, suggesting that was present in both the reaction systems. Therefore, it was presumed that the generation of occurred due to the redox reaction of H₂O₂ and Cu(II), and the resulting Cu(I) reacted with the to produce (Equations (5), (6), (7), (9)). Fig. S3 shows that in the presence of ⁠, the pH was decreased to reach a minimum value of 2.0 after 8 min and then remained in a relative stable state until the end of the reaction, which was directly related to the formation of (Equation (6)). Referring to previous reports, was the predominant radical species at pH 5, while and ·OH were the radical species in the system at pH 9 (Li et al. 2018) and the conversion of to ·OH was lesser under acidic conditions (Equation (8)). The results suggest that Cu(II) and Cu(I) cycles were formed and were continuously generated.

In the H₂O₂ reaction, reduction of Cu(II) species was known to be the dominant process as per the XPS analysis and all the reactions involving and ·OH occur (Equations (9)–(12)). But, at these high concentrations of H₂O₂, was insignificant oxidant of Cu(I) with H₂O₂-mediated oxidation-reduction being the dominant, particularly in the early stage of the reaction process. Cu(II) first react with hydrogen peroxide to produce Cu(I), reflecting the reducibility of hydrogen peroxide and then reacted with hydrogen peroxide to regenerate Cu(II) (Equations (9) and (10)). The radical generation and phenol degradation mechanisms are illustrated in Figure 7.

As shown in Figure S4, the catalysts showed good reusability and stability over five successive experiments. The degradation efficiency of phenol in the H₂O₂-Cu/SiC 2 system was stable during each cycle, remaining almost as high as the fresh catalyst. The dephenolation efficiency of the Na₂S₂O₈ system in the fourth and fifth cycles decreased to 87% and 86%, respectively (Figure 8). Some studies have shown that the catalytic reaction rate increases with the increase of the mobility of metal ions (Ivanets et al. 2019), and the copper ion dissolved from the CuO under strongly acidic conditions (pH<3.0) promotes the catalytic reaction in the persulfate system. However, with the increasing reaction time, there was gradual decrease in the release of Cu ions with the corresponding decrease in the degradation efficiency of phenol, which explains why the degradation rate of phenol decreased in repeated degradation experiments (Figure 6). According to the ICP-OES analysis after five catalytic cycles, the mass loss of copper (0.6 wt%) was most likely responsible for the decreased catalytic activity. Previous experiments had demonstrated that the presence of copper ions enhanced the degradation rate of phenol by persulfate (2.45 times, Table 1), but the results of XPS showed insignificant valence changes of Cu on the catalyst surface after five catalytic cycles (Figure 2(c)). The analysis concluded that a portion of copper ions diffused to the Na₂S₂O₈ solution and participated in the homogeneous reaction. In addition, the Cu(I) on the surface of the catalysts was oxidized by strong oxidizing agents and no valence changes compared with the reaction of copper ions and hydrogen peroxide. The release of copper ion leads to the acceleration of the reaction rate, but at the same time reduces the life of catalyst. It is worth noting that the process of reloading Cu ion on SiC is simple. In addition, how to improve the combination of Cu ion and SiC is an important direction to improve the economic benefits.

From the current research results, persulfate benefits from the high rate of free radical production, which shows higher degradation efficiency than hydrogen peroxide and has strong application potential. This work provided evidence that ·OH and were the main active species during the degradation of phenol, which predominantly occurred at the ortho-position. The decomposition of persulfate depends on the reaction temperature, and the efficient degradation caused by the microwave irradiation was attributed to the hot spots caused by a local thermal imbalance. The valence analysis and kinetic experiments were conducted to demonstrate that the microwave radiation acts on the degradation process in two ways: electron transfer and thermal activation. In addition, due to the thermal effect of microwave, the process of temperature rise will inevitably bring great energy loss in this experimental process. It is a feasible way to increase the temperature of the wastewater containing phenol entering the reactor through the high temperature degradation wastewater heat exchange. From the perspective of the economic benefits of degradation, hydrogen peroxide has a low cost, does not introduce new sulfate ions and does not require subsequent treatment, so it is an environmentally friendly oxidant. However, its insensitivity to temperature makes it limited in microwave radiation. From the perspective of the degradation efficiency, persulfate was more suitable for the degradation of phenol under microwave environment. However, preliminary experiments showed that Cu/SiC was unstable in the presence of persulfates, suggesting a limitation in this regard. Nevertheless, this scheme may prove useful in applications where sulfates are highly used, and further experiments are required to improve the binding of copper to improve the efficiency of continuous treatments. The stability and regeneration of the catalyst is also a major way to improve the economic efficiency.

The research project was supported by State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau Foundation (A314021402-1605).

All relevant data are included in the paper or its Supplementary Information.