Enhanced oxidative degradation of tetracycline by visible light-promoted g-C₃N₄ modified Cu₃(OH)₄SO₄/Cu₇S₄ composites under an air atmosphere

Abstract

Herein, a simple solvothermal method was used to prepare a CuS_x/g-C₃N₄ (CSG) heterojunction as an efficient oxygen activator containing sulfur vacancies (SVs). The as-prepared catalyst can activate oxygen to effectively degrade tetracycline (TC). The degradation rate of TC (20 mg L⁻¹) reached 98.9% in 90 min over CSG (0.02 g L⁻¹) under an air atmosphere, which maintained good performance within a wide pH range of 6–10. It was almost unaffected by any light source and water quality, indicating a highly efficient oxygen activator and good stability. Moreover, the trapping experiments and EPR results revealed that the main reactive oxygen species (ROSs) were singlet oxygen (¹O₂) and hydroxyl radicals (˙OH). CSG (4 : 1) enabled the activation of oxygen-produced ROS for removing TC even in the dark, which was in accordance with the EPR results. In addition, the CSG catalyst exhibited a good removal performance in practical wastewater treatment. Our study demonstrated that the CSG catalyst had the potential to remove organic pollutants from wastewater.

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Introduction

Tetracycline (TC), as a broad-spectrum antibiotic, has been applied in various fields including agriculture and livestock products and bacterial infections because of its low production cost and broad-spectrum antibacterial activity.¹ However, it is reported that microbial resistance to antibiotics can result from the long-term enrichment of TC, which produces direct toxicity to native microorganisms at the same time.² Thus, the excessive use of antibiotics causes significant risks to eco-environments and human health, and has become a global problem in environmental governance.³ Various international health organizations have set maximum residue levels of 100 ppb for TC in fish and milk.³ Unfortunately, the diversified receiving aquatic environments including drinking water can determine the residues of TC.^4,5 Given these, it is urgent to develop efficient methods for the removal of TC from aquatic environments.

Up to now, many techniques have been proposed to remove TC, such as adsorption, membrane filtration, ion exchange and advanced oxidation processes (AOPs).^1,3,6,7 Among these methods, AOPs (such as electrochemical oxidation, ultrasonic degradation, photocatalysis, microwave irradiation, Fenton/Fenton-like oxidation and so on) have proven a good application for the treatment of refractory organic pollutants in the environment.^8–15 Currently, the conventional peroxymonosulfate (PMS) and H₂O₂ activation processes based on AOPs by transition metals or UV irradiation are gaining researchers' attention.^16,17 However, using dissolved oxygen (O₂) as an environmentally safe oxidant to generate radical or anionic reactive oxygen species (ROS: ˙OH, ˙O₂⁻, and ¹O₂) has been rarely reported to remove refractory organics in wastewater or groundwater.¹⁸

Owing to their similar redox properties to Fe, Cu-based Fenton-like catalysts also have excellent catalytic performance for the degradation of organic pollutants, such as Cu₂(OH)PO₄/g-C₃N₄, Cu₂O@g-C₃N₄, Cu-modified alkalinized g-C₃N₄ and CoS_x–CuS_x/copper foam.^19–22 Among them, copper sulfide (CuS_x), a p-type indirect band-gap semiconductor material, has attracted extensive interest due to its nontoxicity, variable valence state, resource abundance, relative eco-friendliness and variable stoichiometries with different band-gap energies and electronic properties.²³ Nevertheless, the insufficient active sites of CuS have resulted in unsatisfactory degradation efficiency. In addition, Cu₃(OH)₄SO₄ has been proven to be photocatalytically active under NIR irradiation because of its unique structure.²⁴ Two different Cu²⁺ environments in Cu₃(OH)₄SO₄ forming CuO_m and Cu′O_n polyhedra can be joined by Cu–O–Cu′ bridges. Moreover, SO₄ as an acceptor ligand provides empty orbitals (namely, the σ* orbital of the S–O bonds) for extra electrons, so that the empty states of Cu₃(OH)₄SO₄ around its gap band minimum appear as the orbital character of SO₄. Accordingly, the “siphoning” of photogenerated electrons into SO₄ would boost the separation of photogenerated electron–hole pairs in Cu₃(OH)₄SO₄.²⁴

Graphite-like carbon nitride (g-C₃N₄) is a kind of metal-free polymeric semiconductor, which is facilely prepared by thermal polycondensation. In recent years, g-C₃N₄ has exhibited good performance in environmental pollutant removal and photocatalytic hydrogen generation because of its excellent physical and chemical stability, low cost, unique electronic structure and narrow band gap (2.7 eV).^25,26 For example, g-C₃N₄ as a visible light photocatalyst was employed in the degradation of rhodamine B (RhB) by activating H₂O₂.²⁷ However, the inevitable shortcomings of low utilization efficiency and rapid recombination of photoinduced electron–hole pairs would limit the photocatalytic activity of g-C₃N₄.²⁶ Surprisingly, the formation of a heterojunction by combining g-C₃N₄ has been proven to be an effective strategy to improve its performance. For example, Cai et al. designed g-C₃N₄/CuS heterojunctions for the degradation of RhB and MB.²⁶ Fe and Cu doped g-C₃N₄ catalysts were employed to activate PMS for degrading ofloxacin with excellent catalytic activity.²⁸

In this regard, a novel CuS_x/g-C₃N₄ (CSG) heterojunction with sulfur vacancies (SVs) was successfully synthesized by the formation of Cu₃(OH)₄SO₄ and Cu₇S₄ coupled with g-C₃N₄ for efficient antibiotic removal. The obtained CSG was characterized by SEM, TEM, XRD, BET and XPS measurements. The experimental results show that the CSG heterojunction exhibits a high performance in activating O₂ for the removal of TC even in the dark, owing to sufficient SV contributions. In addition, it was found that structural stability and more metallic active sites were guaranteed by g-C₃N₄. This proposed study of CSG heterojunctions provides a new prospect for the exploitation of efficient materials to destroy organic pollutants.

Experimental section

Materials and reagents

All chemicals and solvents used in the experiments were of analytical grade and used directly without additional treatments. Tetracycline (TC), copper acetate monohydrate (Cu(OAc)₂·H₂O), thiourea (CH₄N₂S), cetyltrimethylammonium bromide (CTAB), ethanol, urea, cupric chloride (CuCl₂) and ammonium hydroxide (NH₃·H₂O) were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Deionized water was prepared in the laboratory and used throughout the experiments. Ethylenediamine was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. Copper(II) sulfate pentahydrate (CuSO₄·5H₂O) was purchased from Sigma-Aldrich (Finland).

Preparation of the CSG heterojunction

In this study, the CSG heterojunction was synthesized by a solvothermal method according to a previously reported procedure with some modifications.²⁹ In brief, 10 g of urea was placed in a muffle furnace and heated at 550 °C for 3 h with a heating rate of 10 °C min⁻¹. After cooling to room temperature, a pale yellow solid powder was obtained (g-C₃N₄) for further use. Then, 0.1 g of the as-synthesized g-C₃N₄, 0.182 g of CTAB, 0.3433 g of Cu(OAc)₂·H₂O and 0.4933 g of CH₄N₂S were dispersed in 30 mL of ethanol with ultrasonication. After that, the solution was transferred to a three-neck round bottom flask and heated at 65 °C for 3 h. The cyan-grey solid product was collected by centrifugation, washed three times with ethanol and dried in an oven at 60 °C for 12 h. Finally, the material was ground and calcined in a tube furnace at 400 °C for 4 h, which was named as CSG (4 : 1) according to the mass ratio of Cu(OAc)₂·H₂O and g-C₃N₄.

For comparison, the samples were obtained under the same conditions mentioned above with different Cu amounts, and were marked as CSG (1 : 4), CSG (1 : 1) and CSG (9 : 1). In addition, the sample was prepared without g-C₃N₄ and named as CS. We synthesized Cu₃(OH)₄SO₄, Cu₇S₄, Cu₃(OH)₄SO₄/g-C₃N₄ and Cu₇S₄/g-C₃N₄ following the previous reports.^24,30 For Cu₃(OH)₄SO₄, 0.02 mol of CuSO₄·5H₂O was dissolved in 80 mL of deionized water containing 1 mL of NH₃·H₂O. After stirring for 30 min, the suspension was transferred to a 120 mL sealed PTFE-lined automated crucible and hydrothermally treated at 150 °C for 12 h. Finally, it was separated by centrifugation with ethanol and dried in an oven overnight. For the Cu₃(OH)₄SO₄/g-C₃N₄ heterojunction, the other operations were the same as those for Cu₃(OH)₄SO₄, except for the addition of 1.4 g of g-C₃N₄ powder. For Cu₇S₄, 0.618 g of CuCl₂ was added to 50 mL of ethylenediamine with constant stirring. When the colour of the solution changed from clear to dark blue, 0.182 g of CH₄N₂S was added to the above solution. After stirring for 10 min, the solutions were transferred to a Teflon-lined stainless steel autoclave, sealed and kept at 150 °C for 12 h. For Cu₇S₄/g-C₃N₄, the other operations were the same as those for Cu₇S₄, except for the addition of g-C₃N₄ powder. In addition, Cu₃(OH)₄SO₄/Cu₇S₄ (1 : 1) was prepared by mixing Cu₃(OH)₄SO₄ and Cu₇S₄ in a mass ratio of 1 : 1.

Material characterization

The morphologies and surface structures of the as-prepared samples were determined by scanning electron microscopy (FESEM, Fei Talos F200x G2) and transmission electron microscopy (TEM, Fei Talos-F200s). The structures and elemental compositions of the samples were confirmed by powder X-ray diffraction (XRD, MiniFlex600), electron paramagnetic resonance spectroscopy (ESR, Bruker EMXplus-6/1) and X-ray photoelectron spectroscopy (XPS, Axis supra). Absorbance measurements were performed using a UV-Vis-NIR spectrophotometer (LAMBDA950, PerkinElmer). The Brunauer–Emmett–Teller (BET) surface area was calculated by monitoring the N₂ adsorption/desorption using a NOVA 2000 surface area analyzer (Quantachrome) at 77 K.

General procedure for the removal of TC

First, we chose 20 mg L⁻¹ TC in the comparative photocatalytic experiment. 20 mg of the CSG catalyst was selected and added to a 20 mL TC aqueous solution. The process lasted for 1.5 h under visible light irradiation. A visible light source (780 nm ≥ λ ≥ 420 nm) was supplied by a daylight lamp (30 W). The absorbance of the solution was measured using a UV-vis spectrophotometer. In addition, to test the durability and recyclability of the material, recycling experiments were carried out. After each cycle, the solid catalyst was filtered, washed with deionized water to remove residual contaminants, dried overnight at 60 °C, and ready for reuse.

Result and discussion

As shown in Fig. 1, XRD spectra are employed to verify the composite compositions and crystalline properties of the prepared pure g-C₃N₄, CSG (1 : 4), CSG (1 : 1), CSG (4 : 1), and CSG (9 : 1) heterojunctions, as well as Cu₃(OH)₄SO₄, Cu₇S₄, Cu₃(OH)₄SO₄/g-C₃N₄ and Cu₇S₄/g-C₃N₄. For g-C₃N₄, the characteristic peaks at 12.8° and 27.3° correspond to (100) and (002) facets, respectively, suggesting the successful preparation of g-C₃N₄. In addition, the (100) and (002) facets in the g-C₃N₄ phase are associated with the in-plane packing of the tri-s-triazine facet and the interlayer stacking of the conjugated aromatic rings, respectively.²⁸ For CS, the characteristic peaks at 27.6°, 29.3°, 31.6°, 32.9°, 47.9°, 52.6° and 59.3° correspond to the (101), (102), (103), (006), (110), (108) and (116) planes of CuS (JCPDS NO. 06-0464), and other peaks at 27.6°, 31.6°, 46.1° and 54.6° correspond to the (111), (200), (220) and (311) planes of Cu_7.2S₄ (JCPDS NO. 24-0061). Moreover, six peaks of CSG (4 : 1) and CSG (9 : 1) heterojunctions at 14.4°, 16.1°, 17.9°, 24.3°, 32.9° and 34.6° were obviously observed, which match well with those of synthetic antlerite Cu₃(OH)₄SO₄ (JCPDS card No. 07-0407).³¹ Meanwhile, another four peaks of CSG (4 : 1) and CSG (9 : 1) heterojunctions at 32.0°, 35.3°, 42.1° and 46.2° correspond to the diffraction plane of (220), (031), (124) and (224) of Cu₇S₄ (JCPDS card No. 33-0489), respectively. And the intensity of g-C₃N₄ peaks gradually becomes weaker. However, CSG (1 : 4) and CSG (1 : 1) heterojunctions exhibited only g-C₃N₄ peaks. This indicates that Cu-based materials (Cu₃(OH)₄SO₄ and Cu₇S₄ combined as CuS_x in this study) were successfully introduced into g-C₃N₄via our synthetic strategy when a certain amount of Cu(OAc)₂·H₂O was added. The XRD patterns of Cu₃(OH)₄SO₄, Cu₇S₄, Cu₃(OH)₄SO₄/g-C₃N₄ and Cu₇S₄/g-C₃N₄ are shown in Fig. 1B. The diffraction peaks matched well with those in Fig. 1A, suggesting the successful synthesis of the samples.

Fig. 1 (A) XRD patterns of g-C₃N₄, CSG (1 : 4), CSG (1 : 1), CSG (4 : 1), CSG (9 : 1) heterojunctions and CS. (B) XRD patterns of Cu₃(OH)₄SO₄, Cu₇S₄, Cu₃(OH)₄SO₄/g-C₃N₄ and Cu₇S₄/g-C₃N₄. (C) XRD patterns of CSG (4 : 1) heterojunctions treated at different calcination temperatures.

SEM and TEM were utilized to reveal the surface morphologies of the as-prepared samples. Fig. 2 shows the SEM and TEM images of pure g-C₃N₄ and CSG (4 : 1) heterojunctions. As shown in Fig. 2A, the obtained g-C₃N₄ consists of layered structures and irregular granules with many pores because ammonia and CO₂ gases are released during urea pyrolysis.³² The SEM image of CS displayed nanoparticle morphology (Fig. S1, ESI†). The SEM and TEM images of the CSG (4 : 1) heterojunctions display similar porous-layered structures, suggesting that the introduction of Cu₃(OH)₄SO₄ and Cu₇S₄ does not change the skeleton structure of g-C₃N₄ (Fig. 2B–E). In addition, the components and structure were assessed using TEM elemental mapping, which indicated a uniform distribution of Cu, S, C and N in the CSG (4 : 1) heterojunctions (Fig. 2F). As shown in Fig. S2A (ESI†), the lattice stripe spacing of g-C₃N₄ cannot be observed in the HRTEM image, which may be derived from a low degree of order in the layers of g-C₃N₄.³³ As seen from the HRTEM image of CSG (4 : 1) in Fig. S2B (ESI†), the observed lattice fringe spacing of 0.280 nm indicated the (220) planes of Cu₇S₄, and a spacing of 0.267 nm corresponded to the (310) planes of Cu₃(OH)₄SO₄.

Fig. 2 SEM images of (A) pure g-C₃N₄ and (B) and (C) CSG (4 : 1) heterojunctions. TEM images (D) and (E) and TEM elemental mapping (F) of CSG (4 : 1) heterojunctions.

The N₂ adsorption–desorption experiments were performed to investigate the BET specific surface areas (S_BET) of pure CS, pure g-C₃N₄ and CSG (4 : 1) heterojunctions. Fig. S3 (ESI†) reveals that all samples exhibit type IV isotherms, which proves the existence of mesoporous structures. CSG (4 : 1) (29.24 m² g⁻¹) has a BET surface area between that of pure g-C₃N₄ (54.85 m² g⁻¹) and CS (14.33 m² g⁻¹). This phenomenon may be because the deposited CS nanoparticles obstructed some mesopores in g-C₃N₄. Thus, the CSG (4 : 1) heterojunctions with a large surface area can be expected to promote the activity in the adsorption and photocatalytic degradation because more reactive sites and stronger adsorption ability may derive from a high surface area.³⁴

The valence states and bond configurations of the as-synthesized heterojunctions were evaluated in detail by XPS. As shown in Fig. 3A, the survey spectrum indicates the presence of C, N, S and Cu elements, which is consistent with the XRD analysis. The signal peak of C 1s at 284.8 eV is attributed to the sp² C–C from carbon-containing contaminations, while the other peak at 288.0 eV is from the sp² N–C=N bond in the graphitic structure (Fig. 3B).³⁴ The spectrum of N 1s is fitted to four regions at around 398.7 eV, 399.2 eV, 400.7 eV and 404.7 eV, which is attributed to the bonding of C–N=C, N(N–(C)₃), N–H and π excitations, respectively (Fig. 3C).^29,31 The high-resolution XPS spectrum of Cu 2p shows two binding energy peaks located at 932.4 eV and 952.5 eV, which are assigned to Cu⁺ 2p_3/2 and Cu⁺ 2p_1/2, respectively (Fig. 3D). And the other two peaks centered at 933.1 eV and 953.3 eV are ascribed to Cu²⁺ 2p_3/2 and Cu²⁺ 2p_1/2, respectively. For the S 2p spectra (Fig. 3E), the characteristic peaks of S 2p_3/2 and S 2p_1/2 appear at 161.8 eV and 164.1 eV, which represent metal–sulfur bonds and sulfur with low coordination on the surface, respectively. It has been previously reported that the S 2p_1/2 core level is generally connected to the SVs in the structure.³⁵ In addition, the XPS curves of pure g-C₃N₄ and CS are shown in Fig. 3. The chemical state of N 1s in CSG (4 : 1) was similar to that of g-C₃N₄, and a slight shift in the binding energy revealed that the heterojunction was formed between g-C₃N₄ and CS (Fig. 3C). The high-resolution Cu 2p spectra of CS contain shakeup satellites, indicating the presence of a d⁹ configuration in Cu²⁺ (Fig. 3D).³⁶ However, the high-solution Cu 2p spectra of CSG (4 : 1) had no shakeup satellites, suggesting a decrease in the content of Cu²⁺. More valence electrons of CSG (4 : 1) were transferred to Cu²⁺ so that the binding energy was reduced.

Fig. 3 XPS of g-C₃N₄, CS and CSG (4 : 1) heterojunctions: (A) survey spectra, (B) C 1s, (C) N 1s, (D) Cu 2p and (E) S 2p high-resolution spectra.

To investigate the catalytic effect of CSG (4 : 1) heterojunctions on TC abatement under an air atmosphere, the control experiments were performed. The effects of light irradiation and dark conditions on TC removal are shown in Fig. S4 (ESI†). The removal efficiency of TC was visibly lowered without being exposed to light, suggesting that CSG (4 : 1) enabled the activation of oxygen-produced ROS for removing TC even without light irradiation.³⁷ The initial TC concentration and the catalyst amount were 20 mg L⁻¹ and 1.0 mg mL⁻¹, respectively. Fig. 4A shows the effect of Cu on TC removal efficiency over catalysts with different Cu loading amounts. For pure g-C₃N₄, TC decreased by only 8.9% after 90 min. In addition, the removal efficiency of TC over the CS sample without g-C₃N₄ was lower than that of CSG (1 : 1) and CSG (4 : 1), suggesting the importance of g-C₃N₄. Among the g-C₃N₄, CS, CSG (1 : 4), CSG (1 : 1), CSG (4 : 1) and CSG (9 : 1) catalysts, CSG (4 : 1) achieved the highest removal of TC, indicating that a moderate Cu amount may elevate the active site density on the surface of g-C₃N₄.³⁸ Thereby, the CSG (4 : 1) heterojunctions were chosen for further investigation. In addition, the removal efficiencies of TC over CSG (4 : 1) samples obtained at different calcination temperatures (300–600 °C) are illustrated in Fig. 4B. Optimum catalytic activity of the CSG (4 : 1) was observed after calcination treatment at 400 °C, which brought about 96.3% of TC abatement at 90 min. As shown in Fig. 1C, the XRD patterns of the samples treated at 500 °C and 600 °C agreed with those treated at 400 °C, which suggested that the structures of the catalysts did not change with temperature. CSG (4 : 1) is considered as the product after the heat treatment at 400 °C. As shown in Fig. 4C, according to the Langmuir–Hinshelwood model, the degradation rate constant of TC over the CSG (4 : 1) was calculated following the equation ln(C₀/C) = kt, where C₀ and C represent the initial concentration and the concentration over time of the TC solution, respectively, and the slope k is the apparent reaction rate constant (min⁻¹).³⁴ As shown in Fig. 4D, the CSG (4 : 1) at 400 °C provides the highest slope k value (0.049 min⁻¹) for TC degradation.

Fig. 4 (A) Effect of the Cu(OAc)₂ mass ratio on TC abatement over different catalysts. (B) Effect of calcination temperature on TC abatement over CSG (4 : 1) heterojunctions. (C) Pseudo first-order kinetic plots and (D) corresponding rate constants of TC abatement over CSG (4 : 1) heterojunctions at different calcination temperatures.

UV-vis absorption spectra were utilized to characterize the optical absorption properties of g-C₃N₄, CS and CSG (4 : 1). As shown in Fig. 5A, g-C₃N₄ has a steep absorption edge at around 450 nm with an absorption band of less than 450 nm, which was ascribed to the reported optical band gap of ∼2.7 eV, suggesting a rather low photoconversion efficiency.²⁶ With the formation of a heterojunction, the absorption intensity of CSG (4 : 1) increased, indicating enhanced visible light absorption. PL spectra and photoelectrochemical measurements were employed to study the separation and migration of photogenerated charges.⁸ The diagrams of (αhν)²vs. (hν) obtained using Tauc's method are shown in Fig. 5B. The E_g of pure g-C₃N₄ = 2.74 eV in accordance with the previous report.²⁶ In addition, the E_g of CS was calculated to be 1.96 eV, while for CSG (4 : 1), the E_g was 1.93 eV. Within a certain range, the reduced E_g can facilitate the transfer of photoelectrons from the valence band to the conduction band.

Fig. 5 (A) UV-vis absorption spectra and (B) bandgap fitting by the Kubelka–Munk function of g-C₃N₄, CS and CSG (4 : 1).

Compared with g-C₃N₄ and CS (Fig. 6A), the CSG heterojunction exhibited the highest photocurrent intensity, suggesting that the CSG heterojunction can enhance charge separation.^39,40Fig. 6B shows representative Nyquist plots of the impedance response of the samples. As expected, the CSG heterojunction presented the lowest charge transport resistance, revealing that the formation of heterojunctions can boost the charge mobility and photogenic electron–hole separation efficiency.^41–43 These results are consistent with the transient photocurrent performance. Besides, Fig. 6C depicts the PL spectra of g-C₃N₄, CS, CSG (4 : 1), Cu₃(OH)₄SO₄/g-C₃N₄, Cu₇S₄/g-C₃N₄ and Cu₃(OH)₄SO₄/Cu₇S₄, and g-C₃N₄ presented the highest PL emission intensity. However, the PL peak intensity of the CSG heterojunction was the lowest, demonstrating that the combination of g-C₃N₄ and CS can restrain the recombination of photogenerated electron–hole pairs.⁴⁴ The time-resolved photoluminescence (TRPL) test was employed to characterize the fluorescence lifetime of the carrier.⁴⁵ As shown in Fig. 6D, the average fluorescence lifetimes of pure g-C₃N₄, CS and CSG (4 : 1) were 5.62, 4.00 and 3.64 ns, respectively. Among the three, the shortest fluorescence lifetime of CSG (4 : 1) indicated the most rapid electron transfer rate through the interface.^46,47

Fig. 6 (A) Transient photocurrent performance, (B) electrochemical impedance spectra and (C) PL spectra of g-C₃N₄, CS, CSG (4 : 1), Cu₃(OH)₄SO₄/g-C₃N₄, Cu₇S₄/g-C₃N₄ and Cu₃(OH)₄SO₄/Cu₇S₄. (D) Time-resolved PL decays of g-C₃N₄, CS and CSG (4 : 1).

In the next set of experiments, the effects of various parameters including the pH value, catalyst dosage, TC concentration and temperature in the removal process over CSG (4 : 1) heterojunctions were systematically investigated. Fig. 7A shows the influence of pH on the removal efficiency of TC. The highest TC removal efficiency was obtained at pH 6.0, and a profitable environment for TC abatement was achieved under weakly acidic, neutral and alkaline conditions. This result was consistent with that of the previous report, suggesting that the peracidity was bad for the degradation of TC molecules.⁴⁸ Significantly, TC molecules can easily attract ˙OH radicals for self-oxidation due to their negative charges and high electric density in neutral alkaline conditions.⁴⁹ The effect of CSG (4 : 1) dosage is illustrated in Fig. 7B. The removal efficiency of TC increased with the increase of the catalyst dosage from 0.5 to 1.0 mg mL⁻¹, and the efficiency remained roughly the same as the CSG (4 : 1) dosage increased from 1.0 to 2.0 mg mL⁻¹. Therefore, subsequent experiments in this study were performed with 1.0 mg mL⁻¹ owing to the principle of saving dosage. As shown in Fig. 7C, it is apparent that the removal efficiency first improved as the TC concentration increased from 10 to 20 mg L⁻¹. However, the removal efficiency decreased with a continuous increase of the TC concentration from 20 to 60 mg L⁻¹. Therefore, the TC concentration of 20 mg L⁻¹ was selected for further study. As shown in Fig. 7D, the temperature had almost no effect on TC removal efficiency after 90 min. Thus, 25 °C was chosen based on mild conditions. The optimal pH, catalyst dosage, TC concentration and temperature were 6.0, 1.0 mg mL⁻¹, 20 mg L⁻¹ and 25 °C, respectively.

Fig. 7 Effects of various parameters including pH (A), catalyst dosage (B), TC concentration (C) and temperature (D) on the removal efficiency of TC over CSG (4 : 1) heterojunctions.

Then, we evaluated the photoactivity of the samples under optimal conditions. As shown in Fig. 8, the removal efficiencies of TC over Cu₃(OH)₄SO₄, Cu₇S₄, Cu₃(OH)₄SO₄/g-C₃N₄, Cu₇S₄/g-C₃N₄ and Cu₃(OH)₄SO₄/Cu₇S₄ (1 : 1) after 90 min were 68.0%, 62.4%, 67.9%, 32.8% and 70.5%, respectively. It follows that whether it is a single material or a pairwise composite material, the performance was better than that of pure g-C₃N₄. However, as expected, all data showed that the synthesized CSG (4 : 1) heterojunction still performed the best (96.3%). The preparation processes of the obtained Cu₇S₄/Cu₃(OH)₄SO₄, Cu₃(OH)₄SO₄/g-C₃N₄, and Cu₇S₄/g-C₃N₄ were different from that of our CSG (4 : 1). As a result, the number of components in a material cannot be regulated in the same way. There was no synergistic effect in Cu₇S₄/Cu₃(OH)₄SO₄, Cu₃(OH)₄SO₄/g-C₃N₄, and Cu₇S₄/g-C₃N₄, possibly because excess g-C₃N₄ covered the material to reflect the synergistic effect, thus reducing the catalytic efficiency. The enhancement of the removal efficiency of the CSG (4 : 1) heterojunction may be due to the synergistic effect of Cu₃(OH)₄SO₄, Cu₇S₄, and g-C₃N₄, which can produce effective heterojunctions and reduce the recombination efficiency of electron–hole pairs.^50,51 However, the synergistic effect of Cu₇S₄/Cu₃(OH)₄SO₄/g-C₃N₄ needs to be further explored. A synergy coefficient may be used for quantifying the synergistic effect of Cu₇S₄/Cu₃(OH)₄SO₄/g-C₃N₄ on the reaction process.⁵² Also, density functional theory calculations may be utilized to understand the enhanced TC removal activity.⁵³ Under the optimized conditions, the relative degradation activity of the CSG (4 : 1) heterojunction maintained 91.1% after 8 cycles and 68.2% after 10 cycles (Fig. S5, ESI†), indicating that the CSG (4 : 1) heterojunction was a durable and recyclable catalyst for TC removal. As shown in Fig. S6 (ESI†), the SEM and TEM images of CSG (4 : 1) after the reaction still showed porous-layered structures as before. In addition, we investigated the surface state of CSG (4 : 1) after the reaction by XPS. As shown in Fig. S7 (ESI†), compared to that before the reaction (Fig. 3), there was no obvious difference in the XPS spectra in the C 1s, N 1s, Cu 2p and S 2p regions. This indicated that CSG (4 : 1) after the reaction still maintained its composition and the chemical state of all elements.

Fig. 8 Removal efficiencies of TC over Cu₃(OH)₄SO₄, Cu₇S₄, Cu₃(OH)₄SO₄/g-C₃N₄, Cu₇S₄/g-C₃N₄ and Cu₃(OH)₄SO₄/Cu₇S₄ (1 : 1).

Free-radical quenching experiments were carried out by using specific free radical scavengers. As shown in Fig. 9A, the removal efficiency of TC is 98.6% without quenchers. The addition of tert-butanol (TBA) and ethylene diamine tetraacetic acid (EDTA) to the CSG (4 : 1) heterojunctions/TC/air system, except for p-benzoquinone (BQ), demonstrated that the main active species were ˙OH and vacancies in the removal process of TC. Additionally, electron spin resonance spectroscopy (ESR) was used to confirm the existence of reactive oxygen species (ROSs) in the removal process (Fig. 9).^54,55 As expected, the ˙OH was found using a DMPO trapping agent with four characteristic peaks of DMPO-˙OH with a relative intensity of 1 : 2 : 2 : 1 (Fig. 9B). Meanwhile, ¹O₂ was monitored using a TEMP trapping agent with TEMP-¹O₂ signals of 1 : 1 : 1 triplet signal intensity (Fig. 9C). Besides, all signals intensities were basically same in the dark and light, which suggested that the CSG (4 : 1) heterojunctions generated reactive species ˙OH and ¹O₂ without an additional light source. As shown in Fig. 9A, the removal efficiency of TC decreased to 56.1% after the addition of L-histidine (L-His). When BQ was added, the removal efficiency of TC also decreased, suggesting the presence of ˙O₂⁻ during ¹O₂ generation. In addition, we have performed the elimination of TC under different atmospheres. Fig. S8 (ESI†) shows that the N₂ atmosphere led to a reduction in the removal efficiency of TC, which demonstrated that ¹O₂ was derived from dissolved oxygen.⁵⁶ Therefore, some ˙O₂⁻ could react with water to generate ¹O₂.^22,57 As shown in Fig. 8D, the ESR spectrum of the CSG (4 : 1) heterojunctions displays a nearly isotropic signal at g of 2.005, which is ascribed to the free electrons trapped by the SVs in CSG (4 : 1) heterojunctions.⁵⁸ Significantly, the SVs can serve as the active sites for oxygen atoms to interact with the catalyst surface, which is beneficial to lower adsorption energy and promote the generation of ROSs.

Fig. 9 (A) Effect of scavenges on the removal activity of TC. ESR spectra of (B) DMPO-˙OH and (C) TEMP-¹O₂. (D) EPR spectra of the CSG (4 : 1) heterojunctions.

Based on the above analysis, we proposed a possible mechanism for TC removal on a CSG (4 : 1) heterojunction. The ˙O₂⁻ intermediates were generated from O₂ adsorbed to SVs by delocalized electrons and then transferred to ¹O₂.^58,59 With respect to the active species of Cu⁺ in the CSG (4 : 1) heterojunction, it can catalyze H₂O₂ to ˙OH in the system.^19,60Eqn (1)–(4) show the corresponding illustration.

The mineralization efficiency of TC over g-C₃N₄ and CSG (4 : 1) was explored and the TOC values were measured to be 5.58% and 9.21%, respectively, which were lower than the literature data.⁶¹ The main reasons might be the following: (1) the main active species were ˙OH and ¹O₂ in this work, in which ¹O₂ is reported not to be a competent ROS for organic pollutant control.⁴⁴ (2) TC would produce intermediates other than CO₂ and H₂O by breaking different bonds, and it is reported that high TC removal rates were normally accompanied by poor mineralization efficiencies.^61,62 (3) The short reaction time made it inefficient for TC to be mineralized. To identify the intermediates produced after TC degradation, LC-MS and ESI-MS tests were performed, as shown in Fig. S9 (ESI†). Fig. S9A and B (ESI†) show the LC-MS analysis of the TC solution before and after degradation. It can be seen that the TC in the solution became very low after 90 min of degradation. The three intermediates with m/z values of 301.1403, 407.1340 and 423.1658 (Fig. S9C, ESI†) might correspond to the dehydroxylation and dehydration products of TC molecules, displaying a low mineralization efficiency.⁶³ The degradation ability of CSG (4 : 1) on TC in different water environments (deionized water, tap water, and rainwater) was investigated using different water qualities. Fig. S10 (ESI†) shows that there was a 10% difference in the degradation efficiency between tap water and ultrapure water, and it may be due to some interfering substances in tap water. Thus, the degradation level in the actual water samples was maintained at a very high level, which indicated that the material was basically adapted to all aqueous environments.

Conclusions

In summary, we successfully synthesized a new type of CSG heterojunction with SVs for efficient removal of TC. The obtained CSG composites were characterized by SEM, TEM, XRD, BET and XPS. Such heterostructured structures of the CSG (4 : 1) composite can activate oxygen to the ¹O₂ radical, leading to considerable enhancement in TC removal under visible light irradiation. Experimental studies by radical trapping and EPR revealed that the main ROS are ¹O₂ and˙OH, and the presence of SVs was confirmed. Besides, CSG (4 : 1) enabled the activation of oxygen-produced ROS for removing TC even in the dark. ¹O₂ was generated by direct oxidation or conversion of oxygen by electrons provided by SVs in solution, while ˙OH was generated by the intermediate H₂O₂ produced in the experiment activated by Cu⁺ ions. Thus far, the proposed CSG composites can remove TC under mild conditions without additional chemical oxidants or energy inputs. Furthermore, the method is widely used, pH-adaptable, highly reusable and environmentally friendly.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the project fund of the National Natural Science Foundation of China (21575064) and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (22KJB150025).

Notes and references

1. D. Wang, F. Jia, H. Wang, F. Chen, Y. Fang, W. Dong, G. Zeng, X. Li, Q. Yang and X. Yuan, J. Colloid Interface Sci., 2018, 519, 273–284.
2. J. Jin, Z. Yang, W. Xiong, Z. Zhou, R. Xu, Y. Zhang, J. Cao, X. Li and C. Zhou, Sci. Total Environ., 2019, 650, 408–418.
3. W. Zhang, L. Wang, Y. Su, Z. Liu and C. Du, Appl. Surf. Sci., 2021, 566, 150708.
4. N. Kemper, Ecol. Indic., 2008, 8, 1–13.
5. Y. Valcarcel, S. Gonzalez Alonso, J. L. Rodriguez-Gil, A. Gil and M. Catala, Chemosphere, 2011, 84, 1336–1348.
6. N. Qi, P. Wang, C. Wang and Y. Ao, J. Hazard. Mater., 2017, 341, 187–197.
7. H. Li, J. Hu, Y. Meng, J. Su and X. Wang, Sci. Total Environ., 2017, 603–604, 39–48.
8. Y. Shi, C. Zhang, Z. Yang, X. Liu, X. Zhang, C. Ling, J. Cheng, C. Liang, C. Mao and L. Zhang, J. Phys. Chem. C, 2022, 126, 21847–21856.
9. R. Nazari, L. Rajic, A. Ciblak, S. Hernández, I. E. Mousa, W. Zhou, D. Bhattacharyya and A. N. Alshawabkeh, Chemosphere, 2019, 216, 556–563.
10. J. Dewulf, H. Van Langenhove, A. De Visscher and S. Sabbe, Ultrason. Sonochem., 2001, 8, 143–150.
11. X. Zhang, K. Yue, R. Rao, J. Chen, Q. Liu, Y. Yang, F. Bi, Y. Wang, J. Xu and N. Liu, Appl. Catal., B, 2022, 310, 121300.
12. C.-L. Lee, H.-Y. Lee, K.-H. Tseng, P. K. Andy Hong and C.-J. G. Jou, Environ. Chem. Lett., 2011, 9, 355–359.
13. R. Xie, J. Cao, X. Xie, D. Lei, K. Guo, H. Liu, Y. Zeng and H. Huang, Chem. Eng. J., 2020, 401, 126077.
14. H. Shen, X. Zhan, S. Hong, L. Xu, C. Yang, A. W. Robertson, L. Hao, F. Fu and Z. Sun, Nano Res., 2023, 16, 10713–10723.
15. H. Shen, C. Yang, W. Xue, L. Hao, D. Wang, F. Fu and Z. Sun, Chem. – Eur. J., 2023, 29, e202300748.
16. Y. Zhang, X. Ren, L. Yang and Z. Chen, J. Mater. Sci., 2021, 56, 17556–17567.
17. L. Li, C. Hu, L. Zhang and B. Shi, J. Hazard. Mater., 2021, 406, 124739.
18. D. Cheng, A. Neumann, S. Yuan, W. Liao and A. Qian, Environ. Sci. Technol., 2020, 54, 4091–4101.
19. Q. Dong, Y. Chen, L. Wang, S. Ai and H. Ding, Appl. Surf. Sci., 2017, 426, 1133–1140.
20. C. Chen, Y. Zhou, N. Wang, L. Cheng and H. Ding, RSC Adv., 2015, 5, 95523–95531.
21. L. Liu, Y. Qi, J. Hu, Y. Liang and W. Cui, Appl. Surf. Sci., 2015, 351, 1146–1154.
22. A. Du, H. Fu, P. Wang, C. Zhao and C.-C. Wang, J. Hazard. Mater., 2022, 426, 128134.
23. M. P. Ravele, O. A. Oyewo, S. Ramaila, L. Mavuru and D. C. Onwudiwe, Catalysts, 2021, 11, 1238.
24. G. Wang, B. Huang, Z. Li, Z. Wang, X. Qin, X. Zhang, Y. Dai and M.-H. Whangbo, Chem. – Eur. J., 2015, 21, 13583–13587.
25. X. Liu, A. Jin, Y. Jia, T. Xia, C. Deng, M. Zhu, C. Chen and X. Chen, Appl. Surf. Sci., 2017, 405, 359–371.
26. Z. Cai, Y. Zhou, S. Ma, S. Li, H. Yang, S. Zhao, X. Zhong and W. Wu, J. Photochem. Photobiol., A, 2017, 348, 168–178.
27. Y. Cui, Z. Ding, P. Liu, M. Antonietti, X. Fu and X. Wang, Phys. Chem. Chem. Phys., 2012, 14, 1455–1462.
28. Y. Tian, Q. Li, M. Zhang, Y. Nie, X. Tian, C. Yang and Y. Li, J. Cleaner Prod., 2021, 315, 128207.
29. X. Li, Y. Guo, L. Yan, T. Yan, W. Song, R. Feng and Y. Zhao, Chem. Eng. J., 2022, 429, 132234.
30. X. Cao, Q. Lu, X. Xu, J. Yan and H. Zeng, Mater. Lett., 2008, 62, 2567–2570.
31. N. Koga, A. Mako, T. Kimizu and Y. Tanaka, Thermochim. Acta, 2018, 467, 11–19.
32. M. Arumugam, M. Tahir and P. Praserthdam, Chemosphere, 2022, 286, 131765.
33. M. T. Uddin, Y. Nicolas, C. Olivier, T. Toupance, L. Servant, M. M. Müller, H.-J. Kleebe, J. Ziegler and W. Jaegermann, Inorg. Chem., 2012, 51, 7764–7773.
34. P. Gao, X. Chen, M. Hao, F. Xiao and S. Yang, Chemosphere, 2019, 228, 521–527.
35. C. Han, T. Zhang, J. Li, B. Li and Z. Lin, Nano Energy, 2020, 77, 105165.
36. C. K. Wu, M. Yin, S. O’ Brien and T. Koberstein, Chem. Mater., 2006, 18, 6054–6058.
37. T. D. Trang, E. Kwon, J.-C. Wen, N. N. Huy, V. S. Munagapati, S. Ghotekar, K.-P. Yu and K.-Y. A. Lin, J. Mol. Liq., 2023, 389, 122832.
38. F. Guo, W. Shi, H. Wang, H. Han, H. Li, H. Huang, Y. Liu and Z. Kang, Catal. Sci. Technol., 2017, 7, 3325–3331.
39. C. Zhao, X. Li, L. Yue, S. Yuan, X. Ren, Z. Zeng, X. Hu, Y. Wu and Y. He, J. Alloys Compd., 2023, 968, 171956.
40. Z. Feng, L. Zeng, Q. Zhang, S. Ge, X. Zhao, H. Lin and Y. He, J. Environ. Sci., 2020, 87, 149–162.
41. K. Wang, Z. Guan, X. Liang, S. Song, P. Lu, C. Zhao, L. Yue, Z. Zeng, Y. Wu and Y. He, Ultrason. Sonochem., 2023, 100, 106616.
42. C. Zhao, X. Li, L. Yue, X. Ren, S. Yuan, Z. Zeng, X. Hu, Y. Wu and Y. He, ACS Appl. Nano Mater., 2023, 6, 15709–15720.
43. Y. Shi, X. Wang, X. Liu, C. Ling, W. Shen and L. Zhang, Appl. Catal., B, 2020, 277, 119229.
44. Y. Shi, Z. Yang, L. Shi, H. Li, X. Liu, X. Zhang, J. Cheng, C. Liang, S. Cao, F. Guo, X. Liu, Z. Ai and L. Zhang, Environ. Sci. Technol., 2022, 56, 14478–14486.
45. R. Du, C. Wang, L. Guo, R. A. Soomro, B. Xu, C. Yang, F. Fu and D. Wang, Small, 2023, 19, 2302330.
46. Y.-L. Li, Q. Zhao, X.-J. Liu, Y. Liu, Y.-J. Hao, X.-J. Wang, X.-Y. Liu, D. Hildebrandt, F.-Y. Li and F.-T. Li, Small Struct., 2023, 2300177.
47. H. Bhatt, T. Goswami, D. K. Yadav, N. Ghorai, A. Shukla, G. Kaur, A. Kaur and H. N. Ghosh, J. Phys. Chem. Lett., 2021, 12, 11865.
48. L. Wang, W. Zhang, Y. Su, Z. Liu and C. Du, Appl. Clay Sci., 2021, 213, 106238.
49. H. Dong, X. Guo, C. Yang and Z. Ouyang, Appl. Catal., B, 2018, 230, 65–76.
50. D. Masih, Y. Ma and S. Rohani, Appl. Catal., B, 2017, 206, 556–588.
51. F. Dai, Y. Wang, R. Zhao, X. Zhou, J. Han and L. Wang, Int. J. Hydrogen Energy, 2020, 45, 28783–28791.
52. S. Jing, X. Yan, Y. Xiong, T. Li, J. Xiong, T. Hu, Z. Wang, L. Lou and X. Ge, J. Energy Chem., 2023, 84, 34–40.
53. Z. Jiang, W. Wan, H. Li, S. Yuan, H. Zhao and P. K. Wong, Adv. Mater., 2018, 30, 1706108.
54. T. Ma, C. Yang, L. Guo, R. A. Soomro, D. Wang, B. Xu and F. Fu, Appl. Catal., B, 2023, 330, 122643.
55. C. Yang, Y. Zhang, F. Yue, R. Du, T. Ma, Y. Bian, R. Li, L. Guo, D. Wang and F. Fu, Appl. Catal., B, 2023, 338, 123057.
56. C. Zhao, L. Meng, H. Chu, J.-F. Wang, T. Wang, Y. Ma and C.-C. Wang, Appl. Catal., B, 2023, 321, 122034.
57. H. Zeng, L. Deng, H. Zhang, C. Zhou and Z. Shi, J. Hazard. Mater., 2020, 400, 123297.
58. L. Wu, P. Guo, X. Wang, H. Li, X. Zhang, K. Chen and P. Zhou, Chem. Eng. J., 2022, 446, 136759.
59. S. Wang, S. Gao, J. Tian, Q. Wang, T. Wang, X. Hao and F. Cui, J. Hazard. Mater., 2020, 387, 121995.
60. J. Long, L. Xu, L. Zhao, H. Chu, Y. Mao and D. Wu, Chemosphere, 2020, 241, 125034.
61. Y. Ma, H. Xiong, Z. Zhao, Y. Yu, D. Zhou and S. Dong, Chem. Eng. J., 2018, 351, 967–975.
62. S. Dong, S. Dong, X. Tian, Z. Xu, D. Ma, B. Cui, N. Ren and B. E. Rittmann, J. Hazard. Mater., 2016, 302, 386–394.
63. T. Jiang, Z. Peng, M. Xie, X. Fang, Y. Hong, Z. Huang, W. Gao, Z. Zhou, L. Li and Z. Zhu, Anal. Methods, 2020, 12, 535–543.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nj04276f

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