Rapid degradation of malachite green by CoFe2O4–SiC foam under microwave radiation

This study demonstrated rapid degradation of malachite green (MG) by a microwave (MW)-induced enhanced catalytic process with CoFe2O4–SiC foam. The catalyst was synthesized from CoFe2O4 particles and SiC foam by the hydrothermal method. X-ray diffraction and scanning electron microscopy techniques were used to confirm that CoFe2O4 particles were settled on the surface of SiC foam. In this experiment, a novel fixed-bed reactor was set up with this catalyst for a continuous flow process in a MW oven. The different parameters that affect the MW-induced degradation rate of MG were explored. The MW irradiation leads to the effective catalytic degradation of MG, achieving 95.01% degradation within 5 min at pH 8.5. At the same time, the good stability and applicability of CoFe2O4–SiC foam for the degradation process were also discussed, as well as the underlying mechanism. In brief, these findings make the CoFe2O4–SiC foam an excellent catalyst that could be used in practical rapid degradation of MG.


Introduction
Rapid industrial and economic development could increase the risk of environmental pollution. In particular, industries such as paper mills, pharmaceutical production, textile printing and dyeing industry can produce persistent pollutants in waste water that are difficult to degrade after discharge. These persistent pollutants are very harmful to wildlife and humans. As a representative organic dye, malachite green (MG) is usually used in the tinting of silk, leather and  efficiency and activity of the prepared catalysts were investigated. The stability and applicability of CoFe 2 O 4 -SiC for use in the degradation process were also discussed, and a possible degradation mechanism was proposed. passed through the gas absorbing bottles and collected using a gas collection bottle. At the other end of the reactor, a condenser tube was connected with the reactor to cool the solution heated by the MW irradiation. The solution after cooling was collected, and the residual MG and the degradation products were measured using the spectrophotometry method and high-performance liquid chromatography, respectively. The degradation rate (R) by the MW catalytic reaction was calculated by the following equation:

Preparation of catalysts
where C 0 is the original concentration of MG and C is the concentration of MG after the reaction. The experiment parameters were varied in the range of 2.5-10.5 for pH, 10-25 ml min −1 for flow rate, 90-900 W for MW power and 20-50 mg l −1 for MG initial concentration.

Instrumentation
The morphology and surface characteristics of SiC and CoFe 2 O 4 -SiC were observed with a Shimadzu EPMA-1600 Electron Probe Microanalyzer. The powder X-ray diffraction (XRD) spectra of the catalyst were obtained (LabX XRD-6000, Shimadzu, Japan) with Cu KR radiation. The pH values of the MG solutions were adjusted using a PHS-3C pH meter (Shanghai, China) combined with a glass electrode. The concentration of MG was measured by an ultraviolet spectrophotometer (TU-1901, Persee, Beijing, China). The degradation product from the reactor was measured by liquid chromatography-mass spectrometry (LC-MS; Agilent-6100, Germany).

Characterization of CoFe 2 O 4 -SiC
XRD patterns, scanning electron microscopy (SEM) images and X-ray photoelectron spectral features were used to study the crystal structure and morphological feature of the CoFe 2 O 4 -SiC foam. Figure 2a,b shows photographic images of the SiC and CoFe 2 O 4 -SiC foams, respectively. As shown in figure 2c, before loading the catalyst, the SEM image indicates that the surface of SiC was porous and smooth, and the porous structure could facilitate the adsorption of CoFe 2 O 4 . Figure 2d shows the SEM image after loading the CoFe 2 O 4 particles (shown as spheres attached to the apertures). These results suggest the successful deposition of catalyst microparticles. As shown in figure 2f, the prepared CoFe 2 O 4 -SiC has sharp XRD peaks, showing a CoFe 2 O 4 , crystal pattern which is in agreement with previous reports [21].
The chemical composition and electronic structure of the synthesized CoFe 2 O 4 -SiC foam are given. As observed in figure

Activity of CoFe 2 O 4 in microwave-assisted catalytic reaction
The activity of CoFe 2 O 4 was examined both in the absence and presence of MW irradiation. As shown in figure 4, with MW and without the catalyst, the solution passing through the reactor maintained the initial MG concentration (20 mg l −1 ). The same was found with the catalyst and without MW (final MG concentration: 19.625 mg l −1 ); it can be seen that SiC foam had no effect on the adsorption of MG solution. However, when both the MW radiation (900 W) and the catalyst were present, the MG concentration was dramatically reduced to 2.627 mg l −1 . Therefore, both MW irradiation and CoFe 2 O 4 -SiC foam are required to effectively remove MG from the water.

Influence of initial pH
The effect of initial pH on the degradation rate of MG is shown in figure 5. The pH of the original MG solution was 4.5, and it was then adjusted to various pH values between 2.5 and 10.5 by using HCl and NaOH. In general, MG removal rate was enhanced as the pH increased from 2.5 to 8.5, and then declined slightly between pH 8.5 and 10.5. In the range of 2.5-8.5, the degradation rate of MG increased from

Influence of microwave power
The power of MW irradiation is an important operating parameter in MW-assisted catalytic reactions. In this experiment, five different power settings of the microwave oven were used (90, 270, 500, 720 and 900 W), while other conditions were fixed (pH: 4.5, flow rate: 10 ml min −1 and initial MG concentration: 20 mg l −1 ). As shown in figure 6, the degradation rate of MG by CoFe 2 O 4 -SiC increases monotonically with the MW power in the range of 90-900 W. There are also two turning points at 270 and 720 W, between which the MG degradation changes the fastest with MW power (i.e. steepest slope). When only SiC foam was used, on the other hand, the MG degradation ratio changes at the same rate with increasing MW power. There are two potential mechanisms for the enhanced MG degradation. First of all, a higher MW power leads to a higher temperature, which inevitably accelerates the degradation reaction. Secondly, more hotspots are generated as more MW energy is absorbed by CoFe 2 O 4 , thereby producing more catalytically active sites on the CoFe 2 O 4 particle surface. At the same time, the SiC foam can effectively absorb the MW irradiation, and the increased degradation of MG on the surface of SiC foam alone is mainly attributed to the temperature change. Therefore, in subsequent experiments we used the MW power of 900 W and the CoFe 2 O 4 -SiC catalyst, unless noted otherwise.

Influence of initial malachite green concentration
The effect of initial MG concentration in the range of 20-50 mg l −1 on the degradation rate was investigated. As shown in figure 7, the degradation rate of MG decreased with its initial concentration. In this experiment, the MG solution was moving through the reactor while being irradiated by MW. The MG then made contact with the catalyst on the surface of the CoFe 2 O 4 -SiC foam. When the MG concentration was high, the number of hotspots on the surface was insufficient for degrading the MG in time, hence the reduced degradation rate. Therefore, the concentration of MG in the solution is another important parameter of the reaction.

Influence of flow speed
Unlike previous studies, the reactor here was designed to allow the MG solution to flow through at a rate (10-25 ml min −1 ) controlled by a peristaltic pump. As shown in figure 8, the degradation rate of MG decreased with rising current speed. Particularly, when only SiC foam was used, the degradation rate dropped sharply at 15 ml min −1 . As the MW radiation heats water in the reactor, a higher flow rate means less temperature rise in the reactor, which could have a huge impact on the reaction. The CoFe 2 O 4 particles loaded on the SiC foam could absorb more MW radiation and raise the temperature even further. This could offset the reduced temperature rise and prevent the MG degradation rate from dropping rapidly. The effect of current speed indicates that, in a continuous-mode reaction, a slower speed of solution flow is beneficial for the reaction.

Influence of CoFe 2 O 4 loading
The degradation rate was enhanced with more CoFe 2 O 4 catalyst, as it provides more active sites on the surface of SiC foam in contact with MG during the MW-assisted catalytic reaction (figures 5-8). However, as the amount of CoFe 2 O 4 particles increased beyond a certain threshold, the particles start to overlap, causing no obvious increase of effective contact with MG. Considering that the power of MW irradiation and the current speed both affect the reaction, the amount of CoFe 2 O 4 should be adjusted accordingly.

Catalytic stability of CoFe 2 O 4 -SiC
The stability and recyclability of a catalyst are very important in practical applications. In this study, the used CoFe 2 O 4 -SiC foam was taken out, washed several times with ultrapure water, dried at 80°C for 4 h and then reused under MW irradiation. As shown in figure 9, after 10 consecutive runs, the degradation rate remained as high as 81.53%, indicating that the CoFe 2 O 4 -SiC possesses good stability in the MWassisted catalytic degradation reaction. Zhang [20] synthesized CFA/CFO composites, but the composite must be regenerated by the thermal method. However, the stability of the CoFe 2 O 4 -SiC foam was very good and did not need the regenerative process. Compared with the study of Gao et al. [16], the stability of CoFe 2 O 4 -SiC foam was better than that of MgFe 2 O 4 -SiC.

Hot spots
Unlike conventional heating methods, MW radiation permeates and heats materials at the molecular level. Specifically, the dipole molecules under MW radiation can never rotate fast enough to follow the oscillating electric field. This phenomenon leads to the thermal effect, as a large amount of energy is consumed by this type of delay [22,23]. Some particles on the catalyst surface can absorb MW radiation more strongly than other parts, reaching a temperature of 1000°C or higher [24]. These overheated particles are referred to as 'hot spots' [25], and they have been proved to promote the degradation of organic pollutants due to the high temperature [26]. In this case, the CoFe 2 O 4 particles (figure 2d) loaded on SiC foam can become much hotter than other parts. Additionally, the SiC foam is also effective in absorbing MW radiation. Therefore, when the waste water flowed into the reactor, the high temperature caused the water to vaporize, leaving MG on the catalyst surface to be degraded by CoFe 2 O 4 particles under MW irradiation. At the same time, because the whole reaction was exposed to air, the large organic molecules were further oxidized into smaller ones.

Hydroxyl radical as oxidation agent
The Fenton process is a typical advanced oxidation process [27], in which the main reactions can be expressed according to the following equations: The ·OH (figure 10) radicals produced by the Fenton process are strongly oxidizing, and therefore could be used to degrade organics. When Fe 2+ is replaced with other species (denoted as M n+1 ), ·OH still can be generated from catalysing H 2 O 2 through a slightly different Fenton-like process, as shown in the following equations [28,29]: The Fenton/Fenton-like processes are the main mechanisms for generating ·OH. When combined with MW irradiation, the process is accelerated to produce ·OH more rapidly [30]. Gao et al. [16] confirmed that in the MW-assisted catalytic reaction with MgFe 2 O 4 -SiC, ·OH could still be generated  With the generated ·OH and H + , the organics in the waste water could be oxidized into small molecules [31]. Hence, there are good reasons to believe that ·OH and H + are the main active species in the MW-assisted catalytic degradation of MG [32].

Degradation products
A variety of degradation products were identified from the collected solution after the reaction by LC-MS analysis. Electronic supplementary material, figure SF-2(a), shows the liquid chromatogram of the solution before the reaction, showing a single peak at 3.5 min. The corresponding MS peak in electronic supplementary material, figure SF-2(a), is located at m/z = 329, confirming that MG was the only organic matter in the original solution. In the chromatogram after the reaction (electronic supplementary material, figure SF-2(b)), in addition to the MG peak at 3.5 min, two additional peaks appeared at 6.7 and 6.8 min. The former has a formula weight of 273 according to the MS, and the intensity of the latter was very weak and could be ignored. Liu et al. [33] investigated the reaction intermediate in the photocatalytic oxidation of MG. Based on that study, the MG degradation process can be described by the reactions in electronic supplementary material, figure SF-3. First, MG is oxidized to dimethylnitrosamine and quinones. Indeed, a small amount of quinones was identified in electronic supplementary material, figure SF-2(b). Considering that the boiling points of the quinones are low and the temperature in the reactor was very high, some of the quinones could have volatilized into air during the experiment and were not detected by LC-MS. At the same time, most quinones were oxidized to carbon dioxide and water by oxygen.
Compared with traditional adsorption treatment of organic matter in aqueous medium, the process of MW catalytic degradation was a chemical process, and MG in aqueous medium was degraded into small molecules. Furthermore, the efficiency of MW degradation and adsorption were both very high, hence the MW catalytic degradation system could be used in practical application.

Conclusion
In this work, CoFe 2 O 4 loaded on a SiC foam support was successfully synthesized by a hydrothermal method. The SiC skeleton effectively improved the ability to absorb MW radiation. The prepared CoFe 2 O 4 -SiC foam was shown to be a novel and efficient catalyst during the MW-assisted catalytic degradation of MG. The degradation rate reached 95.01% in less than 5 min. In addition, the effects of reaction conditions (MW power, pH, flow speed, amount of catalyst and initial MG concentration) on the degradation rate were examined. Moreover, a possible mechanism for the MW-assisted catalytic process was proposed. The temperature rise from heating by MW irradiation and the generated ·OH radicals are key for the reaction. The overall process was continuous and highly efficient, and the catalyst had good recyclability. Therefore, this new MW catalyst exhibited high degradation efficiency, broad pH range and excellent stability. It is expected to have great potential in applications of waste water treatment.