Abstract

A novel copper doped graphitic carbon nitride (Cu-C3N4) was successfully synthesized and used as an effective Fenton-like catalyst. Cu-C3N4 was characterized by scanning electron microscopy, surface area analyzer, Fourier transform infrared spectroscopy, X-ray diffractometer, and X-ray photoelectron spectroscopy. Effect of process parameters including catalyst dosage, hydrogen peroxide (H2O2) concentration, solution pH, and initial methylene blue (MB) concentration was investigated to evaluate catalytic performance. The pseudo first-order kinetic model was used to describe the catalytic process. The enhancement of MB degradation is observed assisted by ultrasound. MB degradation of 96% is obtained within 30 min in Cu-C3N4/H2O2/ultrasound system, and the corresponding rate constant is 0.099 min−1. Effective MB degradation is obtained over a broad pH range (3.3–9.9). The catalytic mechanism is examined by ultraviolet-visible spectra, quenching test, and electron spin resonance determination. The dominant mechanism of MB degradation is ascribed to the ultrasonic H2O2 activation by Cu-C3N4 for hydroxyl radical generation. Cu-C3N4 has good reusability and is effective to degrade rhodamine B and acid orange 7. This work not only contributes to the field of wastewater treatment, but also provides insights into the synthesis of Fenton-like catalysts. The results manifest that Cu-C3N4 is a promising Fenton-like catalyst for dye degradation in the field of environmental pollution remediation.

HIGHLIGHTS

  • A novel Cu doped g-C3N4 is easily synthesized.

  • Cu species uniformly doped on porous g-C3N4 contribute to catalytic ability.

  • Ultrasound significantly improves MB degradation in the presence of Cu-C3N4/H2O2.

  • MB degradation is effectively obtained in a broad pH range.

  • Plausible mechanism of H2O2 activation for •OH generation is revealed for MB degradation.

Graphical Abstract

Graphical Abstract
Graphical Abstract

INTRODUCTION

With rapid industrial development, large amounts of persistent organic pollutants are discharged into the natural environment and cause serious environmental pollution. Approximately 80% of total global wastewater is released into the environment. The wastewater emission poses serious threats to ecosystem and public health (Mian & Liu 2018). Wastewater pollution has been a global challenge in the field of environmental remediation. Dyes are common organic pollutants in wastewater from textile, printing, leather tanning, and cosmetic industries, and they are toxic to ecosystems and the human body (Chanikya et al. 2021). There is special concern about removal of toxic dyes from wastewater, which is still challenging (Routoula & Patwardhan 2020).

Conventional techniques, such as chemical coagulation, adsorption, and biological oxidation, generally cause unsatisfactory results for wastewater treatment (Kothai et al. 2021). Advanced oxidation processes based on reactive radicals are promising for removing toxic dyes from wastewater due to high oxidizability, complete mineralization, and easy operation. The Fenton reaction is an effective way to generate hydroxyl radicals (•OH) with high oxidation ability (E = 2.80 V/NHE). Fenton processes are widely applied to wastewater treatment for removing organic pollutants. However, homogeneous Fenton processes suffer from the shortcomings of narrow pH range, producing large amounts of iron sludge, and impractical catalyst recovery (Ma et al. 2017). Iron-based Fenton catalysts confront the problem of narrow operation pH (Wang et al. 2021a). In this regard, extensive efforts have been conducted to develop effective Fenton-like catalysts operated over a wide pH range (Xin et al. 2021). Because of high abundance, low cost, and the striking reactivity towards hydrogen peroxide (H2O2), copper-based catalysts have received increasing global attention (Gawande et al. 2016). Copper ions are more efficient than iron ions for H2O2 activation to generate radical species. As shown in Equations (1)–(4), the rate constant of copper ions towards H2O2 is much higher than that of iron ions (Zhang et al. 2017; Wang et al. 2021a). In addition, H2O2 activation by cupric ion occurs over a broad pH range, which is highly attractive for the oxidation of organic pollutants in neutral conditions (Bokare & Choi 2014). Copper-based catalysts, such as copper/carbon composites (Wang et al. 2019a) and nanoscale zero-valent copper (Shah et al. 2020), have been investigated for Fenton-like degradation of organic pollutants in wastewater. Considering catalytic activity, synthesis process, and catalyst reusability, there is still big research gap for developing effective Fenton-like catalysts:
formula
(1)
formula
(2)
formula
(3)
formula
(4)

The synthesis of nanoscale catalysts is an attractive strategy for improving catalytic activity. Metals supported on carbon matrix such as activated carbon, biochar, and graphene have been reported (Wu et al. 2020; Wang et al. 2021b). Graphitic carbon nitride (g-C3N4), a layered material with a 2D π-conjugated polymeric structure, is a promising matrix for loading active metals. The unique physical structure, excellent chemical stability and tunable electronic properties support the photocatalytic activity in environmental applications (Yang et al. 2020; Hasija et al. 2021). g-C3N4 can be easily synthesized through the thermal polymerization of low-cost N-rich precursors such as urea, melamine, dicyandiamide, and thiourea (Jourshabani et al. 2020; Nguyen et al. 2021). Various modification strategies, such as elemental doping, heterojunction, defects engineering and structural regulation, have been reported to improve the properties for potential applications (Chen et al. 2020a). Due to the chemical stability and ligand-field effects, metal doped g-C3N4 can be promising candidates for Fenton-like catalysts (Zhu et al. 2019).

To solve the problem of developing effective Fenton-like catalysts, a novel copper doped g-C3N4 was designed and facilely synthesized. Methylene blue (MB) was chosen as a target pollutant. Ultrasound-assisted MB degradation was evaluated in the presence of H2O2. The objectives of this study are (1) to characterize the catalyst, (2) to determine the catalytic performance, and (3) to discuss the plausible catalytic mechanism.

METHODS

Materials

All used chemicals were analytically pure, including hydrogen peroxide (H2O2; Sinopharm Chemical Reagent Co., Ltd), copper chloride (CuCl2·2H2O; Sinopharm Chemical Reagent Co., Ltd), melamine (C3H6N6; Macklin Reagent Co., Ltd), nitric acid (HNO3; Sinopharm Chemical Reagent Co., Ltd), sodium hydroxide (NaOH; Sinopharm Chemical Reagent Co., Ltd), isopropyl alcohol (IPA; C3H8O; ALADDIN Reagent (Shanghai) Co., Ltd), methylene blue (C16H18ClN3S; Tianjin Kemiou Chemicals Co., Ltd), rhodamine B (C28H31ClN2O3; Tianjin Guangfu Fine Chemical Research Institute), and acid orange 7 (C16H11N2NaO4S; ALADDIN Reagent (Shanghai) Co., Ltd). Ultrapure water from a Millipore Milli-Q water purification system was used in experiments.

Catalyst synthesis

The synthesis of catalysts was conducted by metal ion blending and carbonization as shown in Fig. S1 (Supporting Information). First, melamine (2 g) and copper chloride (1.62 g) were dissolved in 10 mL methanol to obtain a uniform solution. Methanol was the chosen solvent because both melamine and copper chloride can be dissolved in it. After removing methanol by stirring at 60 °C, the solid product with uniformly mixed melamine and copper was obtained. Finally, the solid product was calcined in a muffle furnace with limited oxygen. The temperature was elevated to 550 °C with a ramping rate of 5 °C/min and maintained at 550 °C for 120 min. The calcined product was rinsed using ethanol solution (50%) to remove residual substances or by-products of carbonization. After drying at 100 °C for 8 h, the obtained copper doped carbon nitride (Cu-C3N4) was used as a catalyst.

Characterizations

A scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS; FEI Quanta FEG 250) was used to analyze the morphology and surface elements. The N2 adsorption–desorption isotherm was determined at 77 K to analyze the surface area using a surface area analyzer (Micromeritics, ASAP 2020, USA). Fourier transform infrared spectroscopy (FTIR; Frontier, PerkinElmer, USA) was used to record the FTIR spectrum. X-ray diffractometer (XRD; Ultimate IV, Rigaku Co., Japan) was employed to determine the chemical phases. X-ray photoelectron spectroscopy (XPS; Thermo ESCALAB 250XI, Thermo Scientific, USA) was used to examine the surface elements. An ultraviolet–visible spectrophotometer (UV–vis; UV-2600, Shimadzu, Japan) was employed to measure the absorbance of characteristic peak and UV–vis spectra of dye solution. A JES FA200 spectrometer was used to detect the electron spin resonance (ESR) signals using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the radical-capture reagent.

Catalytic experiments

The research design of this work is schematically shown in Fig. S2. Batch experiments were conducted in 150 mL conical flask at room temperature. The conical flask containing 50 mL MB solution was put in an ultrasonic cleaner (40 kHz, 150 W; XM-3200UVF, Xiaomei Instrument Limited Company, China). Experimental parameters were investigated including catalyst (0–0.6 g/L), H2O2 (0–40 mM), pH (3.3–9.9), and MB concentration (10–30 mg/L). Quenching experiments were conducted using IPA as radical scavenger. The reusability of the catalyst was studied for MB degradation through five cycles. Catalytic degradation of different dyes was also evaluated. During the experiments, a solution sample was taken out, centrifugally separated, and immediately determined using an UV–vis spectrophotometer. The calculation of MB removal is described in Supporting Information. The pseudo first-order model has been widely used for kinetic analysis. Reaction rate was examined by the kinetic model (described in Supporting Information).

RESULTS AND DISCUSSION

Characterizations

The morphology and elemental distribution of Cu-C3N4 are characterized by SEM-EDS. The g-C3N4 is a layered massive particle (Hu et al. 2019a). As shown in Figure 1, Cu-C3N4 shows a sheet-like structure similar to that of g-C3N4. Aggregated nanosized particles without well-defined morphology were observed, which may be the doped copper oxides. Cu-C3N4 also manifests a porous morphology with intersect channels, which resulted from the release of gaseous products in the heating process (Ma et al. 2017; Qin et al. 2019). Fig. S3 manifests the major elements of Cu, C, N, and O. Elemental mapping displays the uniform distribution of C, N, O, and Cu in the material, confirming that Cu is doped on g-C3N4 (Hu et al. 2019a).

Figure 1

(a) and (b) Micro-morphology of Cu-C3N4, and elemental distribution: (c) C, (d) N, (e) O, and (f) Cu.

Figure 1

(a) and (b) Micro-morphology of Cu-C3N4, and elemental distribution: (c) C, (d) N, (e) O, and (f) Cu.

As shown in Figure 2(a), the N2 adsorption–desorption isotherm of Cu-C3N4 shows a type IV isotherm with an H3 hysteresis loop according to the IUPAC classification, which suggests the slit-shaped pores resulted from aggregated particles (Ma et al. 2017; Jiang et al. 2019). The surface area and pore parameters are listed in Table S1 (Supporting Information). The BET surface area is 7.18 m2/g. The pore size distribution plotted by the BJH method displays a bimodal pore distribution ranging from 2 to 10 nm. The average pore diameter is 12 nm.

Figure 2

(a) N2 adsorption–desorption isotherm, (b) FTIR spectrum, (c) XRD pattern, (d) full XPS spectra, (e) high-resolution C 1s spectra, (f) high-resolution N 1s spectra, (g) high-resolution O 1s spectra, and (h) high-resolution Cu 2p spectra of Cu-C3N4.

Figure 2

(a) N2 adsorption–desorption isotherm, (b) FTIR spectrum, (c) XRD pattern, (d) full XPS spectra, (e) high-resolution C 1s spectra, (f) high-resolution N 1s spectra, (g) high-resolution O 1s spectra, and (h) high-resolution Cu 2p spectra of Cu-C3N4.

Figure 2(b) shows the FTIR spectrum of Cu-C3N4. The peaks at 3,508–3,423 cm−1 are related to the stretching vibration of the N-H bond and -OH bond (Wang et al. 2019b). The characteristic peaks in the region ranging from 1,132 to 1,624 cm−1 (1,132, 1,292, 1,403, and 1,624 cm−1) corresponded to the trigonal C-N(-C)-C or the bridging C-NH-C units (Yao et al. 2015). The peak associated with the breathing vibration of the s-triazine units appears at 781 cm−1, lower that of g-C3N4 (at around 807 cm−1) (Muniandy et al. 2017). The peak at 549 cm−1 may be ascribed to the stretching vibration of the Cu-N bond, implying that copper species are chemically coordinated to g-C3N4 (Dong et al. 2017).

The crystallinity of Cu-C3N4 was examined by XRD analysis (Figure 2(c)). The diffraction peak at 2-theta of 27.7° is ascribed to the (002) plane for graphitic materials (PDF#87-1526), suggesting the presence of g-C3N4 structure (Wang et al. 2019b). The diffraction peaks at 2-theta of 32.4°, 35.4°, 38.6°, 48.8°, 53.3°, 58.2°, 61.5°, 66.2°, and 67.9°, which are indexed to the (1 1 0), (1 1 −1), (1 1 1), (2 0 −2), (0 2 0), (2 0 2), (1 1 −3), (3 1 −1), and (1 1 3) crystal planes of CuO (PDF#48-1548) (Feng et al. 2019). The peaks at 2-theta of 36.4° and 41.2° are associated with the (1 1 1) and (2 0 0) crystal planes of Cu2O (PDF#65-3288) (Omrani & Nezamzadeh-Ejhieh 2020). The diffraction peaks at 2-theta of 43.2°, 50.4°, and 74.8° are attributed to the (1 1 1), (2 0 0), and (2 2 0) crystal planes of Cu (PDF#65-9026) (Zhang et al. 2020a).

The signals of C 1s, N 1s, O 1s, and Cu 2p are observed at around 285 eV, 399 eV, 530 eV, and 932 eV in the XPS spectra of Cu-C3N4 (Figure 2(d)). As shown in Figure 2(e), the high-resolution XPS spectra of C 1s manifest the peaks at 284.8 eV (C-C), 286.2 eV (C-O), and 288.3 eV (N-C = N) (Zhang et al. 2020b; Song et al. 2021). Figure 2(f) displays the high-resolution N 1s spectra. The peaks at 398.9 eV, 399.6 eV and 400.7 eV are assigned to the sp2-hybridized pyridine nitrogen (C-N = C), tertiary pyrrolic nitrogen (N-C3), and C-N-H groups (Li et al. 2018a). High-resolution O 1s spectra show peaks at 529.9 eV and 531.5 eV (Figure 2(g)), which are associated to the oxygen coordinated with copper (Cu-O) and the hydroxyl groups (-OH) (Kumar et al. 2014). Figure 2(h) shows the high-resolution XPS spectra of Cu 2p. The binding energy peaks at 932.9 eV and 952.8 eV correspond to Cu+ or Cu0 (Song et al. 2021). The peaks at 935.1 eV and 954.9 eV are attributed to the presence of Cu2+ (Liu & Hensen 2013). The peak at 570.1 eV in the Cu LMM spectrum confirms the presence of Cu+. In addition, the shakeup satellite peaks are observed at 942.5 eV and 962.5 eV (Liu & Hensen 2013). The peak fittings of high-resolution Cu 2p spectra reveal the existence of Cu2+, Cu+, and Cu0 in Cu-C3N4, which agrees with XRD analysis. Combining the FTIR, XRD, and XPS analysis, it can be concluded that copper is chemically incorporated into the g-C3N4 structure, probably embedded in the cavities of g-C3N4, or formed copper porphyrins and phthalocyanines (Dong et al. 2017).

Effect of catalyst dosage

It is reported that ultrasound is effective to enhance Fenton-like reactions (Wang et al. 2020a). Fig. S4 shows the comparison of MB degradation under ultrasound and oscillation. MB removal increases progressively with increasing time. The removal is approximately 80% at 90 min under oscillation, while it reaches above 90% at 40 min assisted by ultrasound. The pseudo first-order model has been extensively reported for studying the kinetics of Fenton-like processes (Wang et al. 2021b). Linear fitting of the model offers the apparent rate constant of pseudo first-order kinetics. The rate constant is significantly improved from 0.019 to 0.048 min−1 with the assistance of ultrasound (Fig. S5). The catalytic system of Cu-C3N4/H2O2/ultrasound was used for subsequent degradation experiments.

The proper dosage of catalyst needs to be determined since it is closely related to operation cost for application. Figure 3(a) shows MB removal in the presence of different Cu-C3N4 dosage. MB removal is notably improved when increasing Cu-C3N4 from 0 to 0.1 g/L. When Cu-C3N4 is increased from 0.1 to 0.4 g/L, the removal is slightly enhanced. MB removal is nearly unchanged with further increasing Cu-C3N4 dosage. The apparent rate constant is 0.004 min−1, 0.029 min−1, 0.042 min−1, 0.081 min−1, and 0.087 min−1, respectively (Figure 3(b)). The improved MB degradation associated with catalyst dosage was reported by Qin et al. (2018). The improvement of MB removal with increasing Cu-C3N4 is due to introducing more active sites, benefiting the generation of radical species that promote the degradation reactions (Wang et al. 2021b). MB removal is not further enhanced with excessive catalyst, probably because of particle agglomeration of the catalyst and undesirable scavenging radicals (Qin et al. 2018). 0.4 g/L of Cu-C3N4 was selected as optimal dosage for MB removal.

Figure 3

(a) Effect of Cu-C3N4 dosage on MB removal, (b) the apparent rate constant. (Experimental conditions: H2O2 20 mM, solution pH 6.7, MB 10 mg/L, and ultrasound.)

Figure 3

(a) Effect of Cu-C3N4 dosage on MB removal, (b) the apparent rate constant. (Experimental conditions: H2O2 20 mM, solution pH 6.7, MB 10 mg/L, and ultrasound.)

Effect of H2O2 concentration

Figure 4(a) displays MB removal in the presence of different H2O2 concentration. MB removal is ineffective in the absence of H2O2 (Fig. S6), implying the poor adsorption ability of Cu-C3N4 and minor effect of ultrasound (Wang et al. 2020a, 2021b). MB removal is sharply promoted in the presence of 10 mM H2O2. The removal is further improved by adding 20 mM H2O2. Further increasing H2O2 levels results in slight improvement in MB removal. The effect of H2O2 concentration can be verified by the apparent rate constant (Figure 4(b)). Similar results have been reported by Chen et al. (2020b) and Xin et al. (2021). The positive effect of H2O2 is related to the accelerated generation of reactive radicals that are responsible for MB degradation. In the presence of excessive H2O2, MB removal is minorly improved. This can be attributed to the undesirable scavenging reactions (Equations (5) and (6)) (Liu et al. 2020b; Wang et al. 2021b). Additionally, excessive H2O2 may cause the self-decomposition into H2O and O2 (Equation (7)), and the radical–radical reaction (Equation (8)) causes additional consumption of hydroxyl radical (Yuan et al. 2019; Wang et al. 2020b). 20 mM of H2O2 was selected for MB removal:
formula
(5)
formula
(6)
formula
(7)
formula
(8)
Figure 4

(a) Effect of H2O2 concentration on MB removal, (b) the apparent rate constant. (Experimental conditions: Cu-C3N4 0.4 g/L, solution pH 6.7, MB 10 mg/L, and ultrasound.)

Figure 4

(a) Effect of H2O2 concentration on MB removal, (b) the apparent rate constant. (Experimental conditions: Cu-C3N4 0.4 g/L, solution pH 6.7, MB 10 mg/L, and ultrasound.)

Effect of solution pH

Solution pH is a key parameter for the Fenton-like process, and it influences the oxidation potential of reactive radicals, the stability of H2O2, and the reactivity of catalysts (Sun et al. 2021). The effect of solution pH on MB removal is shown in Figure 5(a). MB degradation remains almost the same over broad pH regions of 3.3–9.9. Although there exists a small difference in the apparent rate constant (Figure 5(b)), effective MB removal is achieved within 40 min under different pH value. This can be ascribed to the wide operation pH of copper ion. Generally, iron-based catalysts suffer from the narrow operation pH (Yin et al. 2020; de Melo Costa-Serge et al. 2021). Dye degradation at neutral pH has been highlighted by many researchers (Šuligoj et al. 2020; Wang et al. 2020b). Effective MB degradation obtained in a broad pH range is superior to the conventional Fenton process and iron-based catalysts.

Figure 5

(a) Effect of solution pH on MB removal, (b) the apparent rate constant. (Experimental conditions: Cu-C3N4 0.4 g/L, H2O2 20 mM, MB 10 mg/L, and ultrasound.)

Figure 5

(a) Effect of solution pH on MB removal, (b) the apparent rate constant. (Experimental conditions: Cu-C3N4 0.4 g/L, H2O2 20 mM, MB 10 mg/L, and ultrasound.)

Effect of MB concentration

The dye concentration in real wastewater varies with time and sources, and thus it is required to evaluate the effect of initial dye concentration. Figure 6(a) displays MB degradation with different initial MB concentration. MB removal increases with increasing reaction time. Effective degradation of MB solution with higher concentration can be obtained by extending reaction time, suggesting the feasibility of MB removal with different concentration. As observed in Figure 6(b), the apparent rate constant is reduced from 0.0810 to 0.0225 min−1 when the initial MB concentration increases from 10 to 30 mg/L. The removal is decreased with an increase in MB concentration. This can be ascribed to the lack of enough reactive radicals (Wang et al. 2020b). Additionally, high MB concentration generates more degradation intermediates, which potentially poison catalyst surface (Wang et al. 2019c).

Figure 6

(a) Effect of initial MB concentration on MB removal, (b) the apparent rate constant. (Experimental conditions: Cu-C3N4 0.4 g/L, H2O2 20 mM, solution pH 6.7, and ultrasound.)

Figure 6

(a) Effect of initial MB concentration on MB removal, (b) the apparent rate constant. (Experimental conditions: Cu-C3N4 0.4 g/L, H2O2 20 mM, solution pH 6.7, and ultrasound.)

Reusability of catalyst

To evaluate the practicability of real application, the reusability of Cu-C3N4 was determined in the catalytic system. MB removal is slightly decreased (approximate 7%) for four runs (Figure 7). Obvious decline in MB removal is seen for fifth run, but effective MB removal can be obtained by prolonging reaction time (60 min). The results suggest the good reusability for application. Cu-C3N4 as Fenton-like catalyst has outstanding advantages such as high activity, broad operation pH, and good reusability. The process is promising for potential application to wastewater treatment.

Figure 7

The reusability of catalyst in Cu-C3N4/H2O2/ultrasound system. (Experimental conditions: Cu-C3N4 0.4 g/L, H2O2 20 mM, solution pH 6.7, MB 10 mg/L, and ultrasound.)

Figure 7

The reusability of catalyst in Cu-C3N4/H2O2/ultrasound system. (Experimental conditions: Cu-C3N4 0.4 g/L, H2O2 20 mM, solution pH 6.7, MB 10 mg/L, and ultrasound.)

Mechanism of catalytic reactions

Figure 8(a) displays the UV–vis spectra of MB solution. The peak at 664 nm is assigned to the chromophore groups (C = S and C = N bonds) in MB molecules (Fig. S7) (Wang et al. 2020b). The rapid decrease of the peak at 664 nm is observed with slight shift. This indicates that MB degradation is predominated by chromophore cleavage in the early stage under experimental conditions (He et al. 2009). The hypsochromic shift of the characteristic peak at later stage suggests the possible pathway of N-de-ethylation due to the auxochromic property of the N-ethyl group (Mitoraj et al. 2018). The degradation intermediates may undergo other reactions including hydroxylation, aromatic ring opening and mineralization (Zhou et al. 2015). MB degradation can be verified by the color change of MB solution (Fig. S8). The dark blue color becomes gradually colorless, and complete discoloration is achieved within 40 min. The color change of MB solution is consistent to the time-dependent UV–vis spectra.

Figure 8

(a) UV–vis spectra of MB solution associated with different reaction time, (b) ESR spectrum of DMPO-trapped •OH.

Figure 8

(a) UV–vis spectra of MB solution associated with different reaction time, (b) ESR spectrum of DMPO-trapped •OH.

MB removal in the presence of Cu-C3N4 is negligible within 120 min. The result implies the minor role of adsorption in MB removal, probably due to the small surface area of Cu-C3N4. Fenton-like reactions play dominated role in MB removal. Copper species in Cu-C3N4 provide active sites to activate H2O2 for the generation of reactive radicals (Liu et al. 2021). Cu0 can be converted into Cu+ (Equation (9)). Cu+ reacts with H2O2 to generate •OH. Cu2+ can be reduced to Cu+ by reaction with H2O2, realizing the Cu2+/Cu+ cycle. In the catalytic system, more radical species are generated assisted by ultrasound. Ultrasound also enables H2O2 activation into •OH radicals (Equations (10)–(12)) (Prakash et al. 2020; Wang et al. 2020a):
formula
(9)
formula
(10)
formula
(11)
formula
(12)

The radicals are responsible for MB degradation. As reported by previous studies, •OH is the dominant oxidant in Fenton-like processes (Wang et al. 2019a; Xin et al. 2021). To verify the role of •OH in the Cu-C3N4/H2O2/ultrasound system, IPA is used as radical scavenger. Fig. S9 displays MB removal, which is lowered in the presence of IPA, and which is attributed to the scavenging effect. The existence of •OH is further confirmed by ESR spectrum using DMPO as a probe. A strong 4-fold characteristic peak of typical DMPO-trapped •OH adduct with an intensity ratio of 1:2:2:1 is observed in Figure 8(b) (Xin et al. 2021). It should be noted that besides •OH, other radicals cannot be ruled out for MB degradation. Based on above analysis, the plausible catalytic mechanism in Cu-C3N4/H2O2/ultrasound system is schematically shown in Figure 9.

Figure 9

Schematical scheme of plausible catalytic mechanism in Cu-C3N4/H2O2/ultrasound system.

Figure 9

Schematical scheme of plausible catalytic mechanism in Cu-C3N4/H2O2/ultrasound system.

Degradation of different dyes including azo dye (AO7) and triphenylmethane dye (RhB) were further conducted. Figure 10(a) manifests the UV–vis spectra of RhB. The maximum peak at 554 nm is ascribed to the N-ethyl group of RhB (Wang et al. 2021b). Effective degradation of RhB is obtained within 30 min as verified by the sharp decline of peak at 554 nm. The pathway of RhB degradation involves chromophore groups and N-de-ethylation (Wang et al. 2020a). Similar to MB degradation, the degradation of RhB is dominated by chromophore cleavage, while N-de-ethylation succeeded. Figure 10(b) shows that the typical peak at 484 nm of AO7 significantly declined within 40 min, validating the effective degradation of AO7 in the catalytic system (Wang et al. 2019c). Unlike MB degradation, obvious shift of the characteristic peak occurs in the initial stage, suggesting that degradation is not controlled by chromophore cleavage. The results verify that different dyes can be effectively degraded in the Cu-C3N4/H2O2/ultrasound system.

Figure 10

Time-dependent UV–vis spectra of (a) RhB and (b) AO7 solution.

Figure 10

Time-dependent UV–vis spectra of (a) RhB and (b) AO7 solution.

CONCLUSIONS

As verified by SEM, XRD, FTIR, and XPS, Cu-C3N4 contains copper species of Cu, Cu2O and CuO, which are uniformly doped on the surface of porous g-C3N4. Enhanced MB degradation is observed by ultrasound. MB degradation in the Cu-C3N4/H2O2/ultrasound system is described by pseudo first-order kinetics. MB degradation is influenced by catalyst dosage, H2O2 concentration, solution pH, and initial MB concentration. Under conditions of Cu-C3N4 0.4 g/L, H2O2 20 mM, solution pH 6.7, MB 10 mg/L, and ultrasound, MB degradation of 96% is obtained within 30 min, and the corresponding rate constant is 0.099 min−1. Effective MB degradation over a broad pH range (3.3–9.9) is superior to the conventional Fenton process or iron-based catalysts. Cu-C3N4 has good reusability as Fenton-like catalysts. Quenching test and ESR spectrum confirmed the existence of •OH in the catalytic system. Ultrasonically improved H2O2 activation by Cu-C3N4 for the generation of •OH radical is the dominant mechanism for MB degradation. The degradation of RhB and AO7 is also effectively achieved. The commercial application of the catalytic system for wastewater treatment requires more efforts. Nevertheless, this work provides insights into designing Fenton-like catalysts and wastewater treatment.

ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (51804276); and the Excellent Youth Scientist of China Association for Science and Technology.

COMPETING INTERESTS STATEMENT

The authors have no competing interests to declare.

DATA AVAILABILITY STATEMENT

Data cannot be made publicly available; readers should contact the corresponding author for details.

REFERENCES

Chanikya
P.
Nidheesh
P. V.
Babu
D. S.
Gopinath
A.
Kumar
M. S.
2021
Treatment of dyeing wastewater by combined sulfate radical based electrochemical advanced oxidation and electrocoagulation processes
.
Separation and Purification Technology
254
,
117570
.
Chen
Z.
Zhang
S.
Liu
Y.
Alharbi
N. S.
Rabah
S. O.
Wang
S.
Wang
X.
2020a
Synthesis and fabrication of g-C3N4-based materials and their application in elimination of pollutants
.
Science of the Total Environment
731
,
139054
.
Chen
X. L.
Li
F.
Chen
H.
Wang
H.
Li
G.
2020b
Fe2O3/TiO2 functionalized biochar as a heterogeneous catalyst for dyes degradation in water under Fenton processes
.
Journal of Environmental Chemical Engineering
8
(
4
),
103905
.
de Melo Costa-Serge
N.
Gonçalves
R. G. L.
Rojas-Mantilla
H. D.
Santilli
C. V.
Hammer
P.
Nogueira
R. F. P.
2021
Fenton-like degradation of sulfathiazole using copper-modified MgFe-CO3 layered double hydroxide
.
Journal of Hazardous Materials
413
,
125388
.
Dong
Q.
Chen
Y.
Wang
L.
Ai
S.
Ding
H.
2017
Cu-modified alkalinized g-C3N4 as photocatalytically assisted heterogeneous Fenton-like catalyst
.
Applied Surface Science
426
,
1133
1140
.
Feng
L.
Wang
R.
Zhang
Y.
Ji
S.
Chuan
Y.
Zhang
W.
Du
C.
2019
In situ XRD observation of CuO anode phase conversion in lithium-ion batteries
.
Journal of Materials Science
54
(
2
),
1520
1528
.
Gawande
M. B.
Goswami
A.
Felpin
F. X.
Asefa
T.
Huang
X.
Silva
R.
Varma
R. S.
2016
Cu and Cu-based nanoparticles: synthesis and applications in catalysis
.
Chemical Reviews
116
(
6
),
3722
3811
.
Hasija
V.
Nguyen
V. H.
Kumar
A.
Raizada
P.
Krishnan
V.
Khan
A. A. P.
Huong
P. T.
2021
Advanced activation of persulfate by polymeric g-C3N4 based photocatalysts for environmental remediation: a review
.
Journal of Hazardous Materials
413
,
125324
.
He
Z.
Sun
C.
Yang
S.
Ding
Y.
He
H.
Wang
Z.
2009
Photocatalytic degradation of rhodamine B by Bi2WO6 with electron accepting agent under microwave irradiation: mechanism and pathway
.
Journal of Hazardous Materials
162
(
2–3
),
1477
1486
.
Jiang
Y.
Xie
Q.
Zhang
Y.
Geng
C.
Yu
B.
Chi
J.
2019
Preparation of magnetically separable mesoporous activated carbons from brown coal with Fe3O4
.
International Journal of Mining Science and Technology
29
(
3
),
513
519
.
Kothai
A.
Sathishkumar
C.
Muthupriya
R.
Dharchana
R.
2021
Experimental investigation of textile dyeing wastewater treatment using aluminium in electro coagulation process and Fenton's reagent in advanced oxidation process
.
Materials Today: Proceedings
45
,
1411
1416
.
Li
X.
Huang
X.
Xi
S.
Miao
S.
Ding
J.
Cai
W.
Liu
S.
Yang
X.
Yang
H.
Gao
J.
Wang
J.
Huang
Y.
Liu
B.
2018a
Single cobalt atoms anchored on porous N-doped graphene with dual reaction sites for efficient Fenton-like catalysis
.
Journal of the American Chemical Society
140
(
39
),
12469
12475
.
Liu
P.
Hensen
E. J.
2013
Highly efficient and robust Au/MgCuCr2O4 catalyst for gas-phase oxidation of ethanol to acetaldehyde
.
Journal of the American Chemical Society
135
(
38
),
14032
14035
.
Liu
X.
Ma
R.
Zhuang
L.
Hu
B.
Chen
J.
Liu
X.
Wang
X.
2020b
Recent developments of doped g-C3N4 photocatalysts for the degradation of organic pollutants
.
Critical Reviews in Environmental Science and Technology
51
(
8
),
1
40
.
Mitoraj
D.
Lamdab
U.
Kangwansupamonkon
W.
Pacia
M.
Macyk
W.
Wetchakun
N.
Beranek
R.
2018
Revisiting the problem of using methylene blue as a model pollutant in photocatalysis: the case of InVO4/BiVO4 composites
.
Journal of Photochemistry and Photobiology A: Chemistry
366
,
103
110
.
Prakash
L. V.
Gopinath
A.
Gandhimathi
R.
Velmathi
S.
Ramesh
S. T.
Nidheesh
P. V.
2020
Ultrasound aided heterogeneous Fenton degradation of Acid Blue 15 over green synthesized magnetite nanoparticles
.
Separation and Purification Technology
266
,
118230
.
Qin
Z.
Chang
P.
Ma
L.
Bu
L.
Song
Z.
2019
Preparation and modulation of a novel thin-walled carbon foam
.
International Journal of Mining Science and Technology
29
(
2
),
281
287
.
Shah
N. S.
Khan
J. A.
Sayed
M.
Iqbal
J.
Khan
Z. U. H.
Muhammad
N.
Nazzal
Y.
2020
Nano-zerovalent copper as a Fenton-like catalyst for the degradation of ciprofloxacin in aqueous solution
.
Journal of Water Process Engineering
37
,
101325
.
Šuligoj
A.
Ristić
A.
Dražić
G.
Pintar
A.
Logar
N. Z.
Tušar
N. N.
2020
Bimetal Cu-Mn porous silica-supported catalyst for Fenton-like degradation of organic dyes in wastewater at neutral pH
.
Catalysis Today
358
,
270
277
.
Wu
Q.
Wang
J.
Wang
Z.
Xu
Y.
Xing
Z.
Zhang
X.
Li
X.
2020
High-loaded single Cu atoms decorated on N-doped graphene for boosting Fenton-like catalysis under neutral pH
.
Journal of Materials Chemistry A
8
(
27
),
13685
13693
.
Yao
Y.
Lu
F.
Zhu
Y.
Wei
F.
Liu
X.
Lian
C.
Wang
S.
2015
Magnetic core–shell CuFe2O4@C3N4 hybrids for visible light photocatalysis of Orange II
.
Journal of Hazardous Materials
297
,
224
233
.
Yin
Y.
Li
W.
Xu
C.
Shi
L.
Zhang
L. C.
Ao
Z.
Liu
M.
Lu
M.
Duan
X.
Wang
S.
Liu
S.
Sun
H.
2020
Ultrafine copper nanoclusters and single sites for Fenton-like reactions with high atom utilities
.
Environmental Science: Nano
7
(
9
),
2595
2606
.
Zhang
L.
Xu
D.
Hu
C.
Shi
Y.
2017
Framework Cu-doped AlPO4 as an effective Fenton-like catalyst for bisphenol A degradation
.
Applied Catalysis B: Environmental
207
,
9
16
.
Zhu
J. N.
Zhu
X. Q.
Cheng
F. F.
Li
P.
Wang
F.
Xiao
Y. W.
Xiong
W. W.
2019
Preparing copper doped carbon nitride from melamine templated crystalline copper chloride for Fenton-like catalysis
.
Applied Catalysis B: Environmental
256
,
117830
.
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