Xuefeng Hua,∗, Huanhuan Ji a, Fei Chang b, Yongming Luoa,∗

a Key Laboratory of Coastal Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai, Shandong 264003, PR China

b School of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai 200093, PR China

a b s t r a c t

In this study, a rapid reduction of Cr(VI) and degradation of 2,4,6-trichlorophenol (2,4,6-TCP) in a simultaneous manner was reported through the catalysis of g-C3N4 under visible light ( > 420 nm) irradiation. The effects of initial concentration of reactants, dissolved O2 and pH value were investigated systematically. It indicated that, under the optimized concentration, the Cr(VI) reduction and 2,4,6-TCP oxidation could be accomplished in couple of hours in the presence of g-C3N4. And also, the O2 involvement and low pH value were able to signifificantly improve the removal rate of Cr(VI) and 2,4,6-TCP. In addition, the reaction mechanism was investigated through monitoring the reduced states of Cr(VI) and active oxygen intermediates formed during photoreaction by ESR and XPS, as well as determining the degradation products of 2,4,6-TCP by HPLC–MS. The results supported that the redox reactions of Cr(VI) and 2,4,6-TCP can be performed simultaneously via a synergistic oxidation–reduction mechanism in the presence of g-C3N4 under visible light irradiation. © 2013 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductor photocatalysis emerged as a promising technology for the purifification of world environmental pollution [1–4]. Various toxic organic and inorganic pollutants could be transformed into harmless species after photoreaction. A major factor complicated the cleanup of polluted sites is the co-occurrence of organic compounds and heavy metals. However, most of researches on remediation technologies have focused on either organic compounds or metal ions, few can solve the pollutants from both the organic and metals. Semiconductor photocatalysis showed the potential in decontamination of the mixed pollutants because it is favorable for the reduction of metal ions and the oxidation of organic species

2,4,6-TCP and chromium are common environmental pollutants, and classifified as priority pollutants by the U.S. Environmental Protection Agency (EPA) [5–8]. The mixture of chlorophenol and chromium can be easily found in many waste streams, for instance tannery effluents. Chromium commonly presented as Cr(VI) and Cr(III) in nature. Cr(VI) is highly toxic and carcinogenic, while Cr(III) can be immobilized and thus become less bioavailable, so the reduction of Cr(VI) to Cr(III) is highly desired. Semiconductor photocatalyst, like TiO2 could effectively reduce Cr(VI) to Cr(III) [9], and the reduction rate increased in the presence of organic compounds such as salicylic acid [10] and dye [11,12]. 2,4,6-TCP was decomposed to H2O, CO2 and Cl− after photocatalytic reaction [13]. Fu et al. [14] and Sun et al. [15] reported that 4-chlorophenol (4-CP) and Cr(VI) could be simultaneously degraded and reduced by TiO2 [14,15]. However, only less than 5% solar light can be used by TiO2 because of its wide band gap (3.0 eV for the rutile phase and 3.2 eV for the anatase phase).

Recently, thermal polycondensation of common organic monomers was conveniently used to synthesize carbon nitride polymers close to the graphitic sheet-like structure (g-C3N4) [16]. Compared with TiO2, g-C3N4 has an appropriate band gap (2.7 eV) for visible light absorption [17,18]. The capability of g-C3N4 in decomposing organic under visible light had attracted great attention, such as methyl orange (MO) [19], RhB [20], 4-CP [21]. Very recently, formate anion modified g-C3N4 was developed to photoreduce Cr(VI) [22], where the conduction band (CB) electrons were reported to govern the photoreaction of Cr(VI). Meanwhile, our previous work also demonstrated that the g-C3N4 was effective catalyst for 2,4,6-TCP oxidation and heavy metal (Cd2+, Cu2+, and Pb2+) reduction [23]. However, the study for one-pot reduction of Cr(VI) and oxidation of 2,4,6-TCP has not been reported so far.

In this work, simultaneous reactions of Cr(VI) reduction and 2,4,6-TCP oxidation were studied systematically over bulk gC3N4 under visible light irradiation. Control experiments were performed to reveal the role of Cr(VI), 2,4,6-TCP and g-C3N4 on the oxidation and reduction processes. The effects of initial concentration, pH, and dissolved oxygen were also studied. A synergistic reduction–oxidation mechanism was fifinally proposed base on the experimental results.

2. Experimental

2.1. Reagents and solutions

Dicyandiamide (>98.0%) was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). 2,4,6-Trichlorophenol (2,4,6-TCP) (98%) was purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China).K2Cr2O7 (>99.8%) waspurchasedfromTianjinTiandaChemical Experiment Factory (Tianjin China). All reagents were used as received without further purification. Freshly deionized water (18.2 M cm specifific resistance) generated by a Pall Cascada laboratory water system was used to prepare all solutions. 2,4,6-TCP (10−3 M) and Cr(VI) (10−2 M) stock solutions were prepared by dissolving 2,4,6-TCP (98%) or K2Cr2O7 (A.R.) into deionized water.

2.2. Material preparation and characterization

In this study, g-C3N4 catalyst was prepared using thermal condensation of dicyandiamide method [24] and strictly according to our previous report [23]. A 50 mL ceramic crucible containing 2 g dicyandiamide was introduced into a muffle furnace. Within 4 h, the temperature of the furnace raised to 550 ◦C from room temperature in air conditions and kept at this temperature for 4 h. The obtained yellow product was collected and ground into powder prior to use.

The crystal structure of the sample was investigated using X-ray diffraction (XRD; Rigaka D/max 2500 X-ray diffractometer) with Cu K 1 radiation,  = 1.54056 ˚A. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300W Al K radiation. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. Elemental analysis (EA) was carried out on an Elemental Vario III elemental analyzer. The Fourier transform infrared (FT-IR) spectrum of the sample was recorded on a Nicolet iS 10 FT-IR spectrometer. The electron spin resonance (ESR) technique was used to detect Cr transients and hydroxyl radicals on a Bruker (ESP 300E) spectrometer equipped with a 532 nm laser. Before hydroxyl radicals measurement, DMPO was added to the catalyst aqueous suspension. The EPR settings were modulation amplitude 1.94 G and microwave frequency 9.750 GHz.

2.3. Photocatalytic experiments

Cr(VI)/2,4,6-TCP reaction solution was prepared by diluting certain volume of 2,4,6-TCP and Cr(VI) stock solution to 30 mL with deionized water in a 50 mL serum bottle. The finally concentration of Cr(VI) and 2,4,6-TCP were respectively 2 × 10−4 M and 1 × 10−4 M unless otherwise noted. Before irradiation, 30 mg gC3N4 was added to the solution and magnetically stirred for 0.5 h in dark to obtain adsorption/desorption equilibrium. The light irradiation system was equipped with a 300W Xe lamp (CEL-HXF300, Beijing Aulight Co., Ltd.) and a cut-off filter to ensure the wavelength of irradiation light is above 420 nm. The light intensity impinging on the suspension was 150 mW/cm2 as measured with a radiometer (CEL-NP2000, Beijing Aulight Co., Ltd.). During photoreaction, 3 mL aliquots were sampled at every 0.5 h interval for subsequent analysis after centrifugation and fifiltration.

2.4. Analysis

The concentration of 2,4,6-TCP was measured by a HPLC (waters 2695–2998) system. LC–MS information was given by coupling HPLC with a LCQ Fleet ion-trap mass spectrometer (Thermo Fisher Scientific, USA)installed with aXcalibur software. The diphenylcarbazide photometric method (Chinese National Standard Procedure, GB7467-87) was used to analyze Cr(VI) concentration at 540 nm by a spectrophotometer (Beckman coulter DU800). Standard curve was plotted (Fig. S1) and the Cr(VI) concentration was estimated based on the standard curve. For total Cr examination, KMnO4 was used to oxidize lower valent Cr to Cr(VI). For H2O2 concentration analysis, 1 mL sample, 1 mL pH 6.0 buffer, 50  L DPD and 50  L POD were mixed in a 1 cm cuvette. The absorption spectrum of DPD oxidation product (DPD+) at 550 nm was measured after 45 s reaction by a spectrophotometer (Beckman coulter DU800). However, the determination of H2O2 was disturbed due to Cr(VI) can oxidize DPD directly. For this reason, the absorption of DPD+ in the absence of POD was measured as a background to eliminate the disturbance of Cr(VI). Preparation of pH buffer, DPD and POD solution, as well as the calculation of H2O2 concentration were performed according to literature [25]. The released chloride ions were monitored using an ion chromatograph (Dionex ICS3000). The buffer solution was 4.5 mM Na2CO3/0.8 mM NaHCO3, and a Dionex AS18 column was used.

3. Results and discussion

3.1. Structural features of the g-C3N4

The XRD pattern (Fig. S2) of the as-prepared g-C3N4 are dominated by the characteristic (0 0 2) peak at 27.42 of a interlayer stacking peak of aromatic systems. The relatively weak peak at 13.14, which is indexed as (1 0 0) plane, is associated with an in-plane structural packing motif. The elemental analysis ofthe catalyst provided an average C/N molar ratio value of 0.67 (theoretical value:0.75 for C3N4). The additional small amount of hydrogen we found (2.3%) were attributed to the uncondensed amino functions, adsorbed water and structural defects on the surface [26].

The Fourier transform infrared (FT-IR) result proves that the existence of a graphite-like structure of carbon nitride again as shown in Fig. S3. Several strong bands in the 1200–1650 cm−1 region correspond to the typical stretching modes of CN heterocycles. Additionally, the characteristic ring breath of the triazine units is found at 809 cm−1 [23].

3.2. Synergistic effect of Cr(VI) reduction and 2,4,6-TCP degradation over g-C3N4

The feasibility of photocatalytic decontamination of the mixture containing Cr(VI) and 2,4,6-TCP over g-C3N4 was investigated fifirstly. As illustrated in Fig. 1A and B, 2 × 10−4 M Cr(VI) was completely reduced over g-C3N4 after 3 h visible light irradiation. Meanwhile, 10−4 M 2,4,6-TCP co-existed in the reaction system was also consumed after 2 h irradiation. Simultaneous photoreduction of Cr(VI) and photodegradation of 2,4,6-TCP were achieved successfully.

To make clear the role of each substrate, control experiments were conducted. The reduction of Cr(VI)is negligible in the absence of 2,4,6-TCP or g-C3N4, as shown in Fig. 1A. Similarly, the very low reaction rate between Cr(VI) and chlorophenol under visible light irradiation was also reported [15]. It is general that g-C3N4 can created electrons and holes under visible light, and the generated electrons can be used for the reduction of Cr(VI). However, the photogenerated holes of g-C3N4 are incapable of oxidizing the 

surfacehydroxyl groupsdirectly because ofits low oxidationpotential (1.53V) [27]. It is difficult to achieve effective electron–hole pairs separation and then reduce Cr(VI) in the absence of electron donor, 2,4,6-TCP. As seen from Fig. 1B, only approximately 15% 2,4,6-TCP was degraded within three hours in the 2,4,6-TCP/Cr(VI) system, which is consistent with the low reduction rate of Cr(VI) in Fig. 1A. g-C3N4 could degrade 2,4,6-TCP alone, but it took total 3 h to remove 2,4,6-TCP (10−4 M) completely in the 2,4,6-TCP/gC3N4 system. However, in the 2,4,6-TCP/Cr(VI)/g-C3N4 system, the complete removal of 2,4,6-TCP (10−4 M) was accomplished merely after 2 h. These results confirmed the synergistic effect of Cr(VI) reduction and 2,4,6-TCP oxidation.

3.3. Effect of TCP and Cr(VI) concentration

As discussed above, 2,4,6-TCP and Cr(VI) enhanced each other’s removal in the g-C3N4 photocatalytic reaction system. Various 2,4,6-TCP and Cr(VI) initial concentrations were used to further investigate their interaction during photoreaction. 2,4,6-TCP, as an electron donor, reactirreversibly with the photogeneratedVB holes can enhance the photocatalytic electron–hole separation, which results in much more CB electrons for Cr(VI) reduction. As shown in Fig. 2A, low concentration (5 × 10−5) of 2,4,6-TCP cannot capture the VB holes effificiently and then the reduction rate of Cr(VI) was low. 10−4 M 2,4,6-TCP is optimal for 2 × 10−4 M Cr(VI) reduction in our reaction condition. However, further increase of the 2,4,6-TCP concentration suppressed Cr(VI) reduction. Fig. 2B shows 2,4,6-TCP degradation rates increased along with the initial concentration of Cr(VI) from 0 to 5 × 10−4 M. There was no appreciable increase of 2,4,6-TCP degradation rate with further increasing the Cr(VI) concentration to 10−3 M.

3.4. Effect of pH

In this section,the effect of solution pH was investigated and the results were plotted in Fig. 3. Compared to the alkaline and neutral conditions, Cr(VI) reduction and 2,4,6-TCP degradation both take place much quickly in acid conditions. Generally, the reaction rates increased with the decrease of pH value, especially for the Cr(VI) reduction. Under neutral and alkaline conditions, the Cr(VI) reduction is negligible, and the 2,4,6-TCP degradations were very slow. The results may be caused by the following three reasons. (1) The reduction potential of Cr(VI)/Cr(III) shift 138 mV per pH unit to more cathodic potentials, whereas the conduction band of the semiconductor shifts 59 mV per pH [28]. Consequently, the thermodynamic driving force for the reduction of Cr(VI) decreased by 79 mV with an increase of pH by one unit. (2) From the simulation of the Cr(VI) speciation with Visual MINTEQ, HCrO4− was the major Cr(VI) species below pH 5, while CrO42− was the major species above pH 7. This results suggested that the reduction of Cr(VI) could be described by Eq. (1) when the pH value is below 5, while its reduction could be described by Eq. (2) when the pH value is above 7. The increasing reaction rate with an increase in acidity was interpreted in terms of higher susceptibility of HCrO4 than CrO42− to undergo reduction [29]. (3) In the neutral and alkaline conditions, Cr(III) could precipitate on the surface of g-C3N4 in the form of Cr(OH)3, suppressing the activity of catalyst.

3.5. Effect of dissolved oxygen

Fig. 4 shows that Cr(VI) reduction and 2,4,6-TCP degradation were both suppressed in N2 ambient. The suppression effect was more evident for Cr(VI) reduction, it is just ca. 40% Cr(VI) was reduced in N2 gas ambient compared with 100% reduction in air after 3 h photoreaction. The phenomenon illustrated that O2 played an important role in the photocatalytic reaction. In aerated systems, the conduction band electrons, formed by photoexcition of catalyst, are usually scavenged by O2 to yield superoxide radical O2 •−/•OOH (Eqs. (3) and (4)) or by other electron acceptor [2]. O2 •− and its further reduction/disproportionation products (H2O2, •OH) (Eqs.(5)–(7)) are important active oxidative species in the degradation of organic pollutants [30]. It had been reported that O2 •− react with Cr(VI) to generate Cr(V) (Eq. (8)) [31]. Then O2 •− may also contribute to the Cr(VI) reduction during photoreaction. For these reasons, the rate of 2,4,6-TCP degradation and Cr(VI) reduction is higher in air than that in N2 atmosphere.

3.6. Pathways of Cr(VI) reduction

ReductionofCr(VI)toCr(III) was a globalthree-electron-transfer reaction. It had been reported that Cr(VI) reduction occurred via sequential one-electron-transfer steps (Eq. (9)) in a TiO2 photocatalyzed system [32]. Whether Cr(VI) was reduced through one-electron-transfer steps or by reacting with O2 •−, Cr(V) was the intermediate. ESR spectroscopy is deemed as a powerful tool for studying Cr(V) transients generated during the reduction of Cr(VI). From the ESR spectra in Fig. 5, a g value of 1.9527 was determined, which is assigned to Cr(V) [32].