Multi-component composite pollution systems, such as electroplating wastewater, sludge in polluted water treatment plants, and urban refuse, are widely distributed in nature due to the rapid development of industry and agriculture [1-3]. Since photocatalytic hydrogen production from water splitting was discovered in 1972, photocatalytic technology has been widely used to remove environmental pollution [4-6]. Photogenerated electrons can reduce highly toxic hexavalent chromium (Cr(Ⅵ)) in aqueous dichromate (Cr₂O₇^2-) to low-toxicity trivalent chromium (Cr(Ⅲ)) under acidic conditions [7]. Furthermore, aqueous organic pollutants can be degraded to carbon dioxide and water by photocatalytic oxidation processes [8]. However, in the current field of photocatalysis, most research is focused on developing novel photocatalysts and determining their photocatalytic activities in single-component photocatalytic systems, with scientific issues inherent to the photocatalytic reaction itself often ignored [9-11]. In fact, the application of photocatalytic technology to composite pollution systems has great practical significance. A few researchers have attempted to treat multi-component wastewater using photocatalytic processes [12, 13]. However, synergistic photocatalytic effects and mechanisms in composite pollution systems have not been clarified.

In recent years, graphitic carbon nitride (g-C₃N₄) has been widely applied as a metal-free semiconductor photocatalyst in the field of environmental photocatalysis due to its advantages, including visible-light response, environmental friendliness, and low cost [14-21]. However, the photocatalytic activity of g-C₃N₄ is poor in practical applications due to its bulk layered structure, which limits the surface migration of photogenerated carriers and increases mass transfer resistance in the photocatalytic reaction. Therefore, some researchers have improved the photocatalytic activity of g-C₃N₄ using morphology control strategies, such as constructing carbon nitride nanosheets, hollow spheres, and quantum dots [22-24]. In our previous study, porous g-C₃N₄ was successfully fabricated via a simple precursor pretreatment strategy using melamine co-pretreated with HCl and ethylene glycol (EG) as a raw material [25, 26]. The as-prepared porous g-C₃N₄ showed excellent photocatalytic activity owing to the construction of richly porous microstructures.

In this communication, we report a preliminarily study of the photocatalytic reduction of aqueous Cr(Ⅵ) to Cr(Ⅲ) using as-prepared g-C₃N₄ or porous g-C₃N₄ as photocatalysts under acidic conditions. We also study the synergistic photocatalytic effect in a composite pollution system of Cr(Ⅵ) and 4-chlorophenol (4-CP) using as-prepared porous g-C₃N₄ at different pH values. Compared with single-component photocatalytic systems, the reduction efficiency of Cr(Ⅵ) and degradation efficiency of 4-CP were simultaneously improved in the composite system, demonstrating a synergistic photocatalytic effect in the Cr(Ⅵ)/4-CP composite pollution system. Finally, we discuss the synergistic photocatalytic mechanism operating in the Cr(Ⅵ)/ 4-CP composite pollution system.

The photocatalysts were prepared according to our previously reported method [25, 26]. Specifically, porous g-C₃N₄ was fabricated via a precursor pretreatment strategy using HCl/EG co-pretreated melamine as a raw material, while g-C₃N₄ was prepared directly using non-pretreated melamine as a precursor. In a typical synthesis, melamine (3 g) was placed into a 100-mL beaker, followed by the addition of HCl (10 mL), EG (10 mL), and water (3 mL). After stirring the white viscous suspension for 1 h at room temperature, the HCl/EG co-pretreated melamine precursor was obtained by washing, centrifugation, and drying. Subsequently, the HCl/EG co-pretreated melamine precursor was transferred to a 10-mL alumina crucible with a cover. The crucible was heated from room temperature to 250 ℃ in a muffle furnace at a heating rate of 5 ℃ /min, and then further heated to 550 ℃ at a rate of 10 ℃ /min. After maintaining a temperature of 550 ℃ for 2 h, a sample of yellow porous g-C₃N₄ was obtained after natural cooling and denoted as pg-C₃N₄. For comparison, g-C₃N₄ was prepared by the same method, but using non-pretreated melamine as precursor.

The morphologies of as-prepared g-C₃N₄ and pg-C₃N₄ were characterized by transmission electron microscopy (TEM) using a JEM-2100F transmission electron microscope at an accelerating voltage of 200 kV. The phase structures of as-prepared g-C₃N₄ and pg-C₃N₄ were analyzed by X-ray diffraction (XRD) using a D8 ADVANCE diffractometer with Cu-K_α radiation. The photocatalytic quantum efficiencies of as-prepared g-C₃N₄ and pg-C₃N₄ were studied by photoluminescence (PL) measurements recorded using a HITACHI F-7000 fluorescence spectrophotometer.

For the photocatalytic tests, a PLS-SXE 300 Xe lamp (300 W, Beijing PerfectLight Co. Ltd., China) with an output wavelength of λ > 320 nm served as the simulated sunlight source. Photocatalyst (100 mg) and a reaction solution (100 mL) containing K₂Cr₂O₇ (5 mg/L) and 4-CP (20 mg/L) were poured into a 250-mL beaker with a quartz cover. The suspension was ultrasonicated for 10 min and stirred in the dark until adsorption-desorption equilibrium was reached. The light source was then switched on and fixed aliquots of the reaction solution were extracted at predetermined time intervals during irradiation. Changes in Cr(Ⅵ) concentration were analyzed using a diphenylcarbazide spectrophotometric method on a Lambda 750S UV/Vis/NIR spectrometer at λ = 540 nm. The total Cr concentration was determined using a ContrAA700 graphite furnace atom adsorption spectrometer. The Cr(Ⅲ) concentration was calculated by subtracting the Cr(Ⅵ) content from the total Cr content. Changes in 4-CP concentration were analyzed using an Agilent 1100 series high-performance liquid chromatography system with a C₁₈ column and a UV detector (λ = 277 nm). Acetonitrile/water (60/40, v/v) was used as the mobile phase at a flow rate of 1.0 mL/min. Variations in the UV-vis adsorption spectra of Cr(Ⅵ) and 4-CP under simulated sunlight irradiation were detected using a Lambda 750S UV/Vis/NIR spectrometer. For comparison, photocatalytic tests were also performed on a single-component photocatalytic systems containing either K₂Cr₂O₇ (5 mg/L) or 4-CP (20 mg/L).

The morphologies of as-prepared g-C₃N₄ and pg-C₃N₄ were studied using TEM (Fig. 1). As shown in Fig. 1(a), g-C₃N₄ exhibited a bulk layered graphite-like structure originating from melamine polycondensation under high temperature. As shown in Fig. 1(b), pg-C₃N₄ with a three-dimensional pore geometry was successfully fabricated using HCl/EG co-pretreated melamine as the precursor. Phase structures of as-prepared g-C₃N₄ and pg-C₃N₄ were obtained by XRD analysis (Fig. 2). The results showed that g-C₃N₄ exhibited a typical (002) interlayer stacking peak at 27.5°, corresponding to an interlayer distance of d = 0.33 nm, while the (100) peak at 12.9° represented an in-plane structural packing motif with a period of 0.675 nm. The weaker diffraction intensity of the (002) peak in pg-C₃N₄ compared with that in g-C₃N₄ was attributed to the reduced amount of layered structure resulting from the construction of a porous microstructure. The photocatalytic quantum efficiencies of as-prepared g-C₃N₄ and pg-C₃N₄ were studied using PL measurements (Fig. 3). As shown in Fig. 3, g-C₃N₄ exhibited a broad fluorescence emission peak in the range 400-600 nm with an excitation wavelength of 330 nm and an operating voltage of 400 V. This finding suggested that photoinduced electron-hole (e^--h⁺) pairs were generated and recombined within g-C₃N₄. Compared with g-C₃N₄, the decreased PL intensity of pg-C₃N₄ indicated that efficient separation and transportation of photogenerated carriers were realized after formation of the porous microstructure.

The photocatalytic reduction of aqueous Cr(Ⅵ) to Cr(Ⅲ) was studied preliminarily using as-prepared g-C₃N₄ and pg-C₃N₄ under simulated sunlight irradiation. The reaction solution pH was adjusted to 3 with dilute H₂SO₄. As shown in Fig. 4(a), the blank test result showed that aqueous Cr(Ⅵ) could not be reduced to Cr(Ⅲ) in the absence of photogenerated electrons. As clearly shown in Fig. 4(a), the reduction rate of Cr(Ⅵ) and generation rate of Cr(Ⅲ) were simultaneously increased using pg-C₃N₄ compared with using g-C₃N₄. This result indicated that the photocatalytic activity of pg-C₃N₄ was better than that of g-C₃N₄ because the construction of porous microstructures facilitated the surface migration of photogenerated carriers and decreased mass transfer resistance in the photocatalytic reaction. Fig. 4(b) shows variations in the characteristic absorption of Cr(Ⅵ) under simulated sunlight irradiation using pg-C₃N₄ as a photocatalyst. The characteristic absorption of Cr(Ⅵ) gradually decreased with increasing irradiation time, which was consistent with the photocatalytic activity curve of pg-C₃N₄, as shown in Fig. 4(a).

In the Cr(Ⅵ)/4-CP composite pollution system with pH adjusted to 3 using dilute H₂SO₄, Cr(Ⅵ) reduction and 4-CP degradation were simultaneously carried out under simulated sunlight irradiation and the system demonstrated a synergistic photocatalytic effect. As shown in Fig. 5(a), the blank test result showed that both Cr(Ⅵ) and 4-CP concentrations hardly decreased in the absence of photocatalyst. This result indicated that the redox reaction between Cr(Ⅵ) and 4-CP was difficult to carry out in dilute solution. As clearly shown in Fig. 5(a), in comparison with single-component photocatalytic systems adjusted to pH 3 using dilute H₂SO₄, both the Cr(Ⅵ) reduction efficiency and 4-CP degradation efficiency were significantly improved in the Cr(Ⅵ)/4-CP composite pollution system. This result implied that a new photocatalytic reaction was occurring in addition to the individual photocatalytic reduction of Cr(Ⅵ) and photocatalytic oxidation of 4-CP that generated a synergistic photocatalytic effect in the Cr(Ⅵ)/4-CP composite pollution system. Fig. 5(b) shows variations in the characteristic absorption of Cr(Ⅵ) and 4-CP under simulated sunlight irradiation using pg-C₃N₄ as a photocatalyst. The characteristic absorptions of Cr(Ⅵ) and 4-CP were reduced by prolonging the irradiation time to 30 min, and the characteristic absorptions of Cr(Ⅵ) and 4-CP were hardly observed after 60 min of simulated sunlight irradiation. This result was consistent with the photocatalytic activity curves of the Cr(Ⅵ)/4-CP composite pollution system, as shown in Fig. 5(a).

In the Cr(Ⅵ)/4-CP composite pollution system without adjusted pH, Cr(Ⅵ) reduction and 4-CP degradation were also performed simultaneously under simulated sunlight irradiation, with a synergistic photocatalytic effect identified in the system. As shown in Fig. 6(a), almost no redox reaction occurred between Cr(Ⅵ) and 4-CP in the blank test without photocatalyst under simulated sunlight irradiation. In the Cr(Ⅵ) reduction single-component photocatalytic system without adjusted pH, aqueous Cr(Ⅵ) could not be reduced by photogenerated electrons under neutral conditions due to the high electron cloud density of the oxygen atoms. In the 4-CP degradation single-component photocatalytic system without adjusted pH, aqueous 4-CP was degraded smoothly by the photocatalytic oxidation process. In the Cr(Ⅵ)/4-CP composite pollution system without adjusted pH, both the Cr(Ⅵ) reduction efficiency and 4-CP degradation efficiency were improved compared with the respective single-component photocatalytic systems, further demonstrating that a synergistic photocatalytic effect existed in the Cr(Ⅵ)/4-CP composite pollution system. Fig. 6(b) shows variations in the characteristic absorption of Cr(Ⅵ) and 4-CP under simulated sunlight irradiation using pg-C₃N₄ as a photocatalyst. The characteristic absorptions of Cr(Ⅵ) and 4-CP gradually decreased with prolonged irradiation time, which was consistent with the photocatalytic activity curves of the Cr(Ⅵ)/4-CP composite pollution system, as shown in Fig. 6(a).

The above simultaneous Cr(Ⅵ) reduction with 4-CP degradation reaction was repeated four times to evaluate the stability of the as-prepared pg-C₃N₄ catalyst in the Cr(Ⅵ)/4-CP composite pollution system. After the first catalytic run, the catalyst was recovered by centrifugation and washed with water at room temperature. The recovered catalyst was used for subsequent catalytic runs under the same experimental conditions. As shown in Fig. 7(a) and (b), as-prepared pg-C₃N₄ exhibited high stability in the Cr(Ⅵ)/4-CP composite pollution system and maintained similar reactivity after four catalytic cycles. The gradual decrease in photocatalytic activity could be attributed to loss of photocatalyst during the recovery process.

In our previous study, we showed that photocatalytic reactions involved in degradation, hydrogen evolution, and simultaneous hydrogen evolution/organic pollutant degradation systems are redox processes, with the photocatalyst acting as an electron transfer medium [27, 28]. Specifically, oxygen acts as an oxidizing agent and organic pollutants act reducing agents in photocatalytic degradation systems, water acts as an oxidizing agent and electron donors act as reducing agents in photocatalytic hydrogen evolution systems, and water acts as an oxidizing agent and organic pollutants act as reducing agents in simultaneous hydrogen evolution/organic pollutant degradation photocatalytic systems. In the current Cr(Ⅵ) reduction single-component photocatalytic system, the electron cloud density of the oxygen atoms in Cr₂O₇^2- was decreased under acidic conditions, and Cr₂O₇^2- itself underwent redox reactions by electron transfer with the photocatalyst (Scheme 1). Therefore, Cr(Ⅵ) reduction by photogenerated electrons proceeded smoothly under acidic conditions (Fig. 4(a)). Conversely, Cr(Ⅵ) was not reduced by photogenerated electrons under neutral conditions due to the high electron cloud density of oxygen atoms in Cr₂O₇^2- (Scheme 1 and Fig. 6(a)). In the 4-CP degradation single-component photocatalytic system, the redox reaction between 4-CP and dissolved oxygen occurred by electron transfer with the photocatalyst (Scheme 1). Therefore, hydrogen ions had little effect on the 4-CP degradation efficiency (Figs. 5(a) and 6(a)). Furthermore, the redox reaction between Cr₂O₇^2- and 4-CP was switched on by electron transfer with the photocatalyst in the Cr(Ⅵ)/4-CP composite pollution system (Scheme 1). Therefore, compared with the single-component photocatalytic systems, both the Cr(Ⅵ) reduction efficiency and 4-CP degradation efficiency were improved in the Cr(Ⅵ)/4-CP composite pollution system (Figs. 5(a) and 6(a)). This showed that a synergistic photocatalytic effect existed in the Cr(Ⅵ)/4-CP composite pollution system. In general, Cr₂O₇^2- showed a strong oxidizing ability under acidic conditions because the electron cloud density of oxygen atoms in Cr₂O₇^2- was decreased by hydrogen ions. However, Cr₂O₇^2- hardly exhibited oxidative properties under neutral conditions due to the high electron cloud density of oxygen atoms in Cr₂O₇^2- [29, 30]. In the Cr(Ⅵ)/4-CP composite pollution system with pH adjusted to 3 using dilute H₂SO₄, the redox reaction between Cr₂O₇^2- and 4-CP was easily carried out by electron transfer with pg-C₃N₄ in the presence of a large amount of hydrogen ions owing to the low electron cloud density of oxygen atoms in the Cr₂O₇^2-. Therefore, an obvious synergistic photocatalytic effect was observed in the Cr(Ⅵ)/4-CP composite pollution system with pH adjusted to 3 by dilute H₂SO₄ (Fig. 5(a)). However, in the Cr(Ⅵ)/4-CP composite pollution system without adjusted pH, the electron cloud density of oxygen atoms in Cr₂O₇^2- was only be reduced by hydrogen ions derived from 4-CP. Therefore, the redox reaction between Cr₂O₇^2- and 4-CP was inhibited in the Cr(Ⅵ)/4-CP composite pollution system without adjusting the pH (Fig. 6(a)).

Compared with g-C₃N₄, the photocatalytic activity of pg-C₃N₄ was significantly increased in the reduction of Cr(Ⅵ) owing to the construction of porous microstructures. In the Cr(Ⅵ)/4-CP composite pollution system, both the Cr(Ⅵ) reduction efficiency and 4-CP degradation efficiency were improved with respect to the single-component photocatalytic systems regardless of whether the pH was adjusted, demonstrating that a synergistic photocatalytic effect existed in the Cr(Ⅵ)/4-CP composite pollution system. The synergistic photocatalytic effect in the Cr(Ⅵ)/4-CP composite pollution system was attributed to the accelerated redox reaction between Cr₂O₇^2- and 4-CP via electron transfer with pg-C₃N₄. In the Cr(Ⅵ)/4-CP composite pollution system adjusted to pH = 3 with dilute H₂SO₄, the redox reaction between Cr₂O₇^2- and 4-CP was easily performed via electron transfer with pg-C₃N₄ because of the low electron cloud density of oxygen atoms in Cr₂O₇^2-. Therefore, an obvious synergistic photocatalytic effect was observed in the Cr(Ⅵ)/4-CP composite pollution system when adjusted to pH = 3.