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persulfate system
Chemosphere 148 (2016) 34e40
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Improvement of phenol photodegradation efficiency by a combined gC3N4/Fe(III)/persulfate system Jian-Yang Hu, Ke Tian, Hong Jiang* CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
h i g h l i g h t s Unmodified g-C3N4 was employed to degrade refractory pollutant for the first time. The synergistic interaction of g-C3N4/Fe(III)/persulfate can improve the reactivity. An integrated radical and light Fenton mechanism was experimentally verified.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2015 Received in revised form 8 December 2015 Accepted 4 January 2016 Available online xxx
Graphite-like C3N4 (g-C3N4) is an efficient visible-light-driven photocatalyst commonly used in dye decolorization with very poor photocatalytic efficiency for degrading recalcitrant organic pollutants, such as phenol. In this study, we designed a g-C3N4/Fe(III)/persulfate system to significantly improve the phenol photodegradation efficacy by combining photocatalysis and light Fenton interaction. The phenol removal ratio and degradation rate of the g-C3N4/Fe(III)/persulfate system are 16.5- and 240-fold higher than those of individual g-C3N4 system. Sulfate radicals ðSO4 Þ and H2O2 are detected in the g-C3N4/ Fe(III)/persulfate system, suggesting that both radical decomposition and light Fenton interaction play important roles in phenol degradation. The efficient coupled photocatalytic system of g-C3N4 combined with Fe(III) and persulfate shows significant potential for application in large-scale degradation of environmental pollutants. © 2016 Elsevier Ltd. All rights reserved.
Handling Editor: Jun Huang Keywords: g-C3N4 Photocatalyst Persulfate Fenton Phenol degradation
1. Introduction Organic pollutants are widely existed in natural environment because of the development of industries. Phenol, a typical toxic organic pollutant, was produced from chemical, resin, pharmacy, and electronic industries (Jiang et al., 2003; Yang et al., 2006; Yotova et al., 2009; Liu et al., 2012), and inevitably released to environment. These phenolic compounds pose great risk to various creatures and human beings. Currently, the main methods to remove phenol from wastewater include biological oxidation ndez et al., 2015), adsorption onto the surface of porous (Ortega Me materials (Yousef et al., 2011), and advanced oxidation processes (AOPs) using strong oxidizing agents such as hydrogen peroxide (Cartaxo et al., 2012).
* Corresponding author. E-mail address: [email protected] (H. Jiang). http://dx.doi.org/10.1016/j.chemosphere.2016.01.002 0045-6535/© 2016 Elsevier Ltd. All rights reserved.
Photocatalytic technology using metal oxide semiconductors has recently attracted much attention as one of the energy-efficient techniques in wastewater treatment and surface water remediation (Hoffmann et al., 1995; Gnayem and Sasson, 2013). However, most metal oxide photocatalysts release toxic metal ions and can only utilize UV light to generate conduction band electrons (e) and valence band holes (hþ) for contaminant decomposition (Li et al., 2011; Ma-Hock et al., 2012). Graphitic carbon nitride (g-C3N4), which consists only of carbon and nitrogen, has been applied to different photocatalytic fields (Maeda, 2009; Dong and Zhang, 2012; Zhang et al., 2015a,b) under visible light to minimize environmental risk and increase the efficiency of solar utilization. This polymeric semiconductor is environment friendly, highly stable, inexpensive, and presents intrinsic visible light response (Wang et al., 2012). Despite its advantages, g-C3N4 exhibits low photoreactive efficiency because of the high recombination rate of its photogenerated charges (Zhang et al., 2010), and thus limit its practical
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application. Most previous studies just focused on the use of pure gC3N4 for decolorizing organic dyes (Dong et al., 2011; Xin and Meng, 2013). Several methods, such as fabrication of a porous structure (Wang et al., 2009), doping with metal or nonmetal elements (Liu et al., 2010), coupling with grapheme (Xiang et al., 2011), and combining with other semiconductors (Xu et al., 2011), have been developed to enhance the photocatalytic capability of g-C3N4. Zhang et al. reported that the introduction of different species of Ag or Fe, like Ag@AgCl, 1D Ag@AgVO3 nanowires, C þ Fe, or ZnFe2O4 nanoparticles, to the g-C3N4 by kinds of synthesis routes, all of them have a huge enhancement in photocatalysis activity (Zhang et al., 2013a, 2013b, 2015a, 2015b). Although these methods obviously improve g-C3N4 photocatalytic activity, the complex preparation process, high cost, and biological toxicity of the resultant photocatalysts reveal the need for further studies. A simple method combining photocatalysis and radicals, such as addition of H2O2 to a photocatalytic system to generate more hydroxyl radicals (HO), can significantly improve the efficiency of pollutant degradation in water (Xu, 2001). Liu et al. (2014) recently found that adding persulfate to a photodegradation system dramatically increases the degradation performance of this system, during which the sulfate radical ðSO4 Þ is produced by thermolytic cleavage of the peroxide bond. Persulfate can be oxidized and reduced, similar to the mechanism of H2O2 activity in a light Fenton system (Avetta et al., 2015). The presence of Fe(III) contributes to electron trapping at the semiconductor surface, facilitates efficient separation and utilization of the electronehole pairs, and finally promotes pollutant degradation. Thus, we assume that if persulfate and ferric ions are added to the g-C3N4 photodegradation system, an enhanced light Fenton system may be built to improve organic pollutant decomposition. The present study was performed to broaden the application of C3N4 in the degradation of recalcitrant organic pollutants and enhance the understanding of photochemical systems combined with SO4 and Fenton reagents. To this end, (1) photochemical degradation of phenol was investigated by adding persulfate and ferric ions to a visible-light-g-C3N4 system, and (2) an integrated light Fenton and radical photocatalytic system was verified through a series of photodegradation experiments and characterizations. This work proposed a new g-C3N4/Fe(III)/persulfate system which can significantly improve the phenol photodegradation efficacy, while deepened the understanding on the degradation mechanism of organic compounds in light Fenton and radical photocatalytic system.
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microscope (TEM) that was operated at a 200-kV accelerating voltage. 2.3. Photodegradation experiment Photocatalytic degradation of phenol was performed in a 125 mL magnetically stirred cylindrical reactor top-irradiated by a 350 W Xenon lamp (XD-300, Nanjing Yanan Special Lighting Co., Ltd.) at room temperature (26 C). In a typical run, 80 mL of 10 mg L1 phenol aqueous solution was added to the reactor. The pH of the phenol solution was kept at the initial state. A 150 mg portion of the photocatalyst was added to the reactor, and the system was stirred in the dark for 30 min to achieve adsorption equilibrium. This degradation process is similar to that described in our previous work (Zhang et al., 2014). The samples were centrifuged to sediment the photocatalyst, and the phenol concentration in the supernatants was monitored by measuring solution absorbance using a UV-2450 spectrophotometer (Shimadzu Inc., Japan) at 510 nm wavelength. Aromatic intermediates were identified by LC-100 high-performance liquid chromatograph (HPLC; Wufeng Inc., China) using 1.2 mL min1 of acetonitrile (25 vol.%) and water (75 vol.%) as the mobile phase at 280 nm. The reactor was left open to air over the entire process to ensure aerobic conditions. Based on the initial experimental conditions above, Fe(NO3)3 (350 mg L1) and K2S2O8 (300 mg L1) were added to the reaction system to investigate the effects of Fe(III) and sulfate on phenol degradation. Several comparative experiments were performed by changing the dosages of reagents. Most conditions were tested at least in triplicate. In a ultrasound treatment experiment, we put the system in ultrasound for 15 min before the dark absorption and light irradiation, other conditions are the same as other experiments. The photocatalytic degradation process accords with the pseudo first-order kinetics (Eq. (1)) by data fitting, thus the apparent kinetic constant (kapp) can be used to compare the photocatalytic activity of each photocatalytic system quantitatively (Yan et al., 2010; Dong et al., 2012):
lnðC=C0 Þ ¼ kapp t
(1)
3. Results and discussion 3.1. Characterization
2. Materials and methods 2.1. Preparation The g-C3N4 photocatalyst was prepared by directly heating melamine in a semiclosed system to prevent melamine sublimation. Then, 10 g of melamine powder was placed in an alumina crucible with a cover and then heated to 600 C in a muffle furnace for 8 h at a heating rate of 4.17 C/min. The resultant yellow powder was collected and used without further treatment. 2.2. Characterization Fourier Transform infrared (FTIR) spectra in the range of 4000e400 cm1 were collected on a VERTEX 70 FT-IR spectrometer (Bruker Inc., Germany) by the KBr pellet method. Each spectrum represents the average of 64 scans at a spectral resolution of 4 cm1. The morphologies of g-C3N4 were observed on a SIRION-200 scanning electron microscope (SEM) that was operated at a 30-kV accelerating voltage and a JEOL-2100F transmission electron
The FTIR spectra (Fig. 1a) show little difference between the gC3N4 samples before and after light irradiation. Both molecules possess characteristic functional groups on the carbon nitride polymer. The broad peak around 3174 cm1 is attributed to primary and secondary amino groups, and the presence of the peaks indicates that thermo-condensation during the synthesis of g-C3N4 is incomplete (Thomas et al., 2008). For example, some eNH2 or eNH groups existed in the side of the layer after the thermal treatment (Zhang et al., 2014). Strong absorption bands ranging from 1200 to 1600 cm1 correspond to the skeletal stretching of the tri-s-triazine heterocycle motif, and the peak at 801 cm1 shows the typical ringbreathing vibration of the tri-s-triazine heterocycle, which indicates that degradation experiment does not destroy the in-plane tri-s-triazine structural motif of g-C3N4. Moreover, there was no obvious characteristic peak of phenol in the curve, suggesting the negligible adsorption of phenol on g-C3N4 in the dark. Fig. 1b shows the SEM image of g-C3N4 samples. The morphology of sample is highly ordered and dominantly comprises aggregated layers with a size of several micrometers.
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Fig. 1. a) FTIR patterns for g-C3N4 samples before and after irradiation. b) SEM image for g-C3N4 sample.
system is higher than that in systems with similar Fe(III)/persulfate concentrations but without photocatalysts (Liang and Su, 2009). The photocatalytic experiments indicate that the presence of C3N4, Fe(III), and persulfate exert synergistic effects on the degradation of organic compounds. Although previous reports have described persulfate activation by metallic minerals, metal-containing oxides, metal-salts, zero-valent metals, and calefaction (Liang and Lai, 2008), reports on the influence of the aforementioned systems after photocatalyst addition are scarce. The degraded products of phenol were determined by HPLC (Fig. 2c), and quinol, catechol, resorcinol, and benzoquinone are observed. These findings indicate that an oxidation mechanism dominates the photodegradation of organic pollutants in the g-C3N4/Fe(III)/persulfate system (Mijangos et al., 2006). The influence of different conditions, such as pH of the solution (the pH of the phenol solution was adjusted to a set value of 4, 7, and 10 using 1 M HClO4 and NaOH solutions), concentration of Fe(III)/persulfate (the concentration of Fe(III)/persulfate was doubled), and external ultrasound application (the system was put in ultrasound for 15 min before the irradiation), on the g-C3N4/ Fe(III)/persulfate system was investigated, and relevant results are shown in Fig. 3. Almost no difference in the phenol degradation efficiency is observed at different pH values (Liang et al., 2007). While SO4 is the dominant species under acidic pH conditions, relatively more HO are generated at higher pH levels (Liang et al., 2007); these radicals are robust oxidizers of phenol. The rate of phenol degradation increases as the Fe(III)/persulfate concentration increases within a certain range; this finding is similar to a phenomenon observed by Xu and Li (2010). Numerous studies reported that ultrasonic dispersion can increase the specific surface area of some photocatalysts and consequently improve the degradation effects (Yu et al., 2001). The present results show that ultrasonic treatment reduces phenol degradation, likely because of destruction of the sheet structure of g-C3N4 during degradation, which leads to declines in photocatalytic activity. This assumption is partly confirmed by some reports discussing other ultrasoundtreated layered materials (Quintana et al., 2012).
3.2. Phenol degradation 3.3. Mechanism To broaden the practical application of this novel photocatalyst, g-C3N4 was used to photodegrade phenol. Fig. 2 shows the adsorption properties, photocatalytic activities, and apparent kinetic constants of various catalytic systems for phenol degradation under visible light irradiation. The adsorption of phenol on C3N4 in the dark is negligible (Fig. 2a), consistent with the FTIR spectra and the published data (Zhang et al., 2013a,b). Pure C3N4 under visible irradiation displays very weak photocatalytic activity; here, only 2% of the phenol in solution is degraded. Fe(III) and persulfate have been reported to significantly enhance degradation of organic compounds in aqueous solution (Fang et al., 2013). However, addition of Fe(III) or persulfate only cannot obviously improve the efficiency of refractory phenol degradation. Even simultaneous addition of relevant concentrations of Fe(III) and persulfate show no evident increases in the efficiency of phenol degradation (14%). Under visible light irradiation, over 33% of the phenol in solution can be degraded within 90 min in the presence of g-C3N4/Fe(III)/ persulfate, which is 16.5- and 2.3-fold faster than that in individual g-C3N4 (2%) and Fe(III)/persulfate (14%) systems, respectively. The reactive rate constant k in different system was calculated, and the results further show that the degradation rate of phenol in the gC3N4/Fe(III)/persulfate system is 4.8 103 min1, which is 240and 2.7-fold faster than those observed in individual g-C3N4 (2 105 min1) and Fe(III)/persulfate (1.8 103 min1) system, respectively (Fig. 2b). The rate of phenol degradation in the present
The mechanism of phenol degradation via the g-C3N4/Fe(III)/ persulfate system under visible irradiation has seldom been investigated. A series of experiments were conducted to elucidate the mechanism of the complex redox-photocatalytic system. First, radical scavengers were added into the solution to verify the role of free radicals in the g-C3N4/Fe(III)/persulfate system. Tert-butyl alcohol (TBA) captures HO, while ethanol (EtOH) captures both HO and SO4 (Anipsitakis and Dionysiou, 2004). Fig. 4a shows the effects of TBA and EtOH addition on phenol degradation by the gC3N4/Fe(III)/persulfate system under visible light. No difference in phenol degradation efficiency is observed upon addition of TBA, which suggests that HO performs a trivial function in phenol degradation. By contrast, phenol degradation efficiency is significantly reduced by addition of EtOH, which indicates that SO4 is an important factor contributing to phenol degradation. Phenol degradation probably primarily occurs in solution because of the weak phenol adsorption ability of g-C3N4. Nevertheless, HO is mostly produced on the surface of g-C3N4 under visible light irradiation and thus rarely participates in phenol degradation. SO4 is generated by persulfate activation and is nearly always present in bulk solutions (Criquet and Leitner, 2009). Consequently, addition of EtOH consumes SO4 and remarkably affects phenol degradation. SO4 is produced by the reaction of persulfate and Fe(III) as described in Eq. (2) (Avetta et al., 2015):
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Fig. 2. a) Adsorption properties, photocatalytic activities and b) apparent kinetic constants in different catalytic systems for degradation of phenol under visible light; c) Intermediate products identified by HPLC chromatography for degradation of phenol under visible light.
Fig. 3. Adsorption property and photocatalytic activities indifferent reaction environment based on the g-C3N4/Fe(III)/persulfate system for degradation of phenol under visible light. Initial: as mentioned in photodegradation experiment; ultrasound: put the system in ultrasound for 15 min before the irradiation; concentration: double the concentration of Fe(III)/persulfate; acidic: adjust the system pH to 4; neutral: adjust the system pH to 7; alkaline: adjust the system pH to 10.
FeðIIIÞ þ S2 O8 2 /FeðIIÞ þ S2 O8
(2)
Fe(II) is oxidized rapidly by persulfate to regenerate Fe(III) and produce SO4 and sulfate (Eq. (3)):
FeðIIÞ þ S2 O8 2 /FeðIIIÞ þ SO4 2 þ SO4
(3)
Fe(II) performs an important function in phenol degradation. To confirm the production of Fe(II), 1,10-phenanthroline spectrophotometry was adopted to quantitatively measure the Fe(II) content in the reaction system. The results of the different catalytic systems are shown in Fig. 4b. Fig. 4b shows that the concentrations of Fe(II) in the bulk solution of the Fe(II)/phenol system before and after 2 h of visible light irradiation are nearly constant, which indicates that Fe(II) can hardly be oxidized into Fe(III) by oxygen in air. This phenomenon implies that production of Fe(II) in the solution is closely related to persulfate and g-C3N4 rather than dissolved oxygen. In the Fe(II)/g-C3N4/phenol system, the concentration of Fe(II) in the bulk solution decreases slightly after reaction, which indicates that very few Fe(II) ions are adsorbed on the g-C3N4 surface. Comparative results show that the concentration of Fe(II) in the bulk solution, which was determined by adding hydroxylamine hydrochloride to convert Fe(III) into Fe(II) (Che et al., 1995), obviously decreases in the Fe(III)/g-C3N4/phenol system. This result indicates that Fe(III) can be adsorbed by g-C3N4, which is advantageous to electron trapping at the semiconductor surface (Vamathevan et al., 2001). Fig. 4c shows that the g-C3N4/Fe(III) system has the highest concentration of Fe(II). Fe(II) is re-oxidized in situ by holes or other reactive species generated in the system (Cheng et al., 2004). The Fe(II) concentration in the g-C3N4/Fe(III)/persulfate system is even lower than that in the Fe(III)/persulfate system, probably because more drastic reactions of the Fe(III)/Fe(II) cycle occur in the g-C3N4/ Fe(III)/persulfate system than that in the Fe(III)/persulfate system. Further trials are in progress to verify this assumption. Moreover, Fe(II) concentrations in the different catalytic systems are relatively low, which suggests the fast redox rates of Fe species. Photogenerated electrons react with O2 on the surface of g-C3N4
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Fig. 4. a) Effects of tert-butyl alcohol (TBA) and ethanol (EtOH) addition on phenol degradation by the g-C3N4/Fe(III)/persulfate system under visible light. Initial: as mentioned in photodegradation experiment; þTBA: remove hydroxyl radical (HO) in the system; þEtOH: remove both hydroxyl radical (HO) and sulfate radical ðSO4 Þ in the system; Concentration of Fe(II) in different catalytic systems for degradation of phenol under visible light when b) adding hydroxylamine hydrochloride in the g-C3N4/Fe(III) system to convert Fe(III) to Fe(II) and c) without adding hydroxylamine hydrochloride in any systems; d) Concentration of H2O2 in different catalytic systems for degradation of phenol under visible light.
originating from persulfate decomposition to generate the superoxide anion radical ðO2 Þ under visible light (Jiang et al., 2013), which subsequently undergoes iron-catalyzed dismutation in the presence of Fe(III) (Eqs. (4)e(6)):
e CB þ O2 /O2
(4)
FeðIIIÞ þ O2 /FeðIIÞ þ O2
(5)
FeðIIÞ þ O2 þ 2Hþ /FeðIIIÞ þ H2 O2
(6)
We infer that H2O2 produced by superoxide dismutation reacts with Fe(II)/Fe(III) through Fenton-like reactions in the solution. Considering the possible existence of the light Fenton system, titanium potassium oxalates spectrophotometry was used to verify the existence of H2O2. The concentrations of H2O2 in the different catalytic systems are shown in Fig. 4d. H2O2 exists in the g-C3N4/ Fe(III)/persulfate system but not in the Fe(III)/persulfate system, which indicates the presence of light Fenton interaction in the gC3N4/Fe(III)/persulfate system. Thus, more violent reactions of the Fe(III)/Fe(II) cycle may occur because of light Fenton reactions, in accordance with the aforementioned hypothesis. The enhanced degradation process can be described in two steps with synergistic interaction: activation of persulfate and establishment of light Fenton system (Fig. 5). First, persulfate is activated to produce reactive species of SO4 in the presence of Fe(III) under illumination, which is extremely beneficial to phenol degradation. Meanwhile, electronic transition from the valence band to the conduction band of g-C3N4 causes formation of electronehole pairs. Fe(III) is adsorbed to the surface of g-C3N4 continuously and
traps photogenerated electrons to convert into Fe(II), which accelerates the circulation of Fe(II)/Fe(III) and the production of SO4 . In addition, another degradation path is activated when O2 adsorbs photogenerated electrons on the surface of g-C3N4 and traps electrons to produce reactive species of O2 under intensive stirring; these radical species then undergoes iron-catalyzed dismutation in the presence of Fe(III) to form H2O2. In this case, Fe(II) and H2O2 in the solution constitute a light Fenton system and significantly enhance degradation effects.
4. Conclusion An integrated light Fenton and radical photocatalytic system by adding Fe(III) and persulfate into the degradation system can significantly improve the phenol degradation efficiency. Although the effect of the present coupled system, which involves promotion of free radical generation and acceleration of the Fe(III)/Fe(II) cycle, was much better than those observed in separate degradation mechanisms, the efficiency of phenol degradation was unsatisfactory. However, improving the degradation efficiency of g-C3N4 systems remains worthy of investigation. At present, most photocatalytic studies focus on dye decolorization, which has high efficiency but limited application. By contrast, phenol degradation presents a low degradation rate but much more environmental significance in controlling water pollution. Compared with wellknown photocatalysts that can only utilize UV light, g-C3N4 is one of the few non-doped photocatalysts that use visible light; this characteristic is its unique advantage.
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Fig. 5. Illustrative representation of the g-C3N4/Fe(III)/persulfate system under visible light.
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Xu, Y., 2001. Comparative studies of the Fe3þ/2þ-UV, H2O2-UV, TiO2-UV/vis systems for the decolourization of a textile dye X-3B in water. Chemosphere 43, 1103e1107. Yan, S.C., Li, Z.S., Zou, Z.G., 2010. Photodegradation of rhodamine B and methyl orange over boron-doped g-C3N4 under visible light irradiation. Langmuir 26, 3894e3901. Yang, C., Qian, Y., Zhang, L., Feng, J., 2006. Solvent extraction process development and on-site trial-plant for phenol removal from industrial coal-gasification wastewater. Chem. Eng. J. 117, 179e185. Yotova, L., Tzibranska, I., Tileva, F., Markx, G.H., Georgieva, N., 2009. Kinetics of the biodegradation of phenol in wastewaters from the chemical industry by covalently immobilized Trichosporon cutaneum cells. J. Ind. Microbiol. Biot. 36, 367e372. Yousef, R.I., El-Eswed, B., Al-Muhtaseb, A.H., 2011. Adsorption characteristics of natural zeolites as solid adsorbents for phenol removal from aqueous solutions: kinetics, mechanism, and thermodynamics studies. Chem. Eng. J. 171, 1143e1149. Yu, J.C., Yu, J., Ho, W., Zhang, L., 2001. Preparation of highly photocatalytic active nano-sized TiO2 particles via ultrasonic irradiation. Chem. Commun. 19, 1942e1943.
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Improvement of phenol photodegradation efficiency by a combined gC3N4/Fe(III)/persulfate system Jian-Yang Hu, Ke Tian, Hong Jiang* CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
h i g h l i g h t s Unmodified g-C3N4 was employed to degrade refractory pollutant for the first time. The synergistic interaction of g-C3N4/Fe(III)/persulfate can improve the reactivity. An integrated radical and light Fenton mechanism was experimentally verified.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2015 Received in revised form 8 December 2015 Accepted 4 January 2016 Available online xxx
Graphite-like C3N4 (g-C3N4) is an efficient visible-light-driven photocatalyst commonly used in dye decolorization with very poor photocatalytic efficiency for degrading recalcitrant organic pollutants, such as phenol. In this study, we designed a g-C3N4/Fe(III)/persulfate system to significantly improve the phenol photodegradation efficacy by combining photocatalysis and light Fenton interaction. The phenol removal ratio and degradation rate of the g-C3N4/Fe(III)/persulfate system are 16.5- and 240-fold higher than those of individual g-C3N4 system. Sulfate radicals ðSO4 Þ and H2O2 are detected in the g-C3N4/ Fe(III)/persulfate system, suggesting that both radical decomposition and light Fenton interaction play important roles in phenol degradation. The efficient coupled photocatalytic system of g-C3N4 combined with Fe(III) and persulfate shows significant potential for application in large-scale degradation of environmental pollutants. © 2016 Elsevier Ltd. All rights reserved.
Handling Editor: Jun Huang Keywords: g-C3N4 Photocatalyst Persulfate Fenton Phenol degradation
1. Introduction Organic pollutants are widely existed in natural environment because of the development of industries. Phenol, a typical toxic organic pollutant, was produced from chemical, resin, pharmacy, and electronic industries (Jiang et al., 2003; Yang et al., 2006; Yotova et al., 2009; Liu et al., 2012), and inevitably released to environment. These phenolic compounds pose great risk to various creatures and human beings. Currently, the main methods to remove phenol from wastewater include biological oxidation ndez et al., 2015), adsorption onto the surface of porous (Ortega Me materials (Yousef et al., 2011), and advanced oxidation processes (AOPs) using strong oxidizing agents such as hydrogen peroxide (Cartaxo et al., 2012).
* Corresponding author. E-mail address: [email protected] (H. Jiang). http://dx.doi.org/10.1016/j.chemosphere.2016.01.002 0045-6535/© 2016 Elsevier Ltd. All rights reserved.
Photocatalytic technology using metal oxide semiconductors has recently attracted much attention as one of the energy-efficient techniques in wastewater treatment and surface water remediation (Hoffmann et al., 1995; Gnayem and Sasson, 2013). However, most metal oxide photocatalysts release toxic metal ions and can only utilize UV light to generate conduction band electrons (e) and valence band holes (hþ) for contaminant decomposition (Li et al., 2011; Ma-Hock et al., 2012). Graphitic carbon nitride (g-C3N4), which consists only of carbon and nitrogen, has been applied to different photocatalytic fields (Maeda, 2009; Dong and Zhang, 2012; Zhang et al., 2015a,b) under visible light to minimize environmental risk and increase the efficiency of solar utilization. This polymeric semiconductor is environment friendly, highly stable, inexpensive, and presents intrinsic visible light response (Wang et al., 2012). Despite its advantages, g-C3N4 exhibits low photoreactive efficiency because of the high recombination rate of its photogenerated charges (Zhang et al., 2010), and thus limit its practical
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application. Most previous studies just focused on the use of pure gC3N4 for decolorizing organic dyes (Dong et al., 2011; Xin and Meng, 2013). Several methods, such as fabrication of a porous structure (Wang et al., 2009), doping with metal or nonmetal elements (Liu et al., 2010), coupling with grapheme (Xiang et al., 2011), and combining with other semiconductors (Xu et al., 2011), have been developed to enhance the photocatalytic capability of g-C3N4. Zhang et al. reported that the introduction of different species of Ag or Fe, like Ag@AgCl, 1D Ag@AgVO3 nanowires, C þ Fe, or ZnFe2O4 nanoparticles, to the g-C3N4 by kinds of synthesis routes, all of them have a huge enhancement in photocatalysis activity (Zhang et al., 2013a, 2013b, 2015a, 2015b). Although these methods obviously improve g-C3N4 photocatalytic activity, the complex preparation process, high cost, and biological toxicity of the resultant photocatalysts reveal the need for further studies. A simple method combining photocatalysis and radicals, such as addition of H2O2 to a photocatalytic system to generate more hydroxyl radicals (HO), can significantly improve the efficiency of pollutant degradation in water (Xu, 2001). Liu et al. (2014) recently found that adding persulfate to a photodegradation system dramatically increases the degradation performance of this system, during which the sulfate radical ðSO4 Þ is produced by thermolytic cleavage of the peroxide bond. Persulfate can be oxidized and reduced, similar to the mechanism of H2O2 activity in a light Fenton system (Avetta et al., 2015). The presence of Fe(III) contributes to electron trapping at the semiconductor surface, facilitates efficient separation and utilization of the electronehole pairs, and finally promotes pollutant degradation. Thus, we assume that if persulfate and ferric ions are added to the g-C3N4 photodegradation system, an enhanced light Fenton system may be built to improve organic pollutant decomposition. The present study was performed to broaden the application of C3N4 in the degradation of recalcitrant organic pollutants and enhance the understanding of photochemical systems combined with SO4 and Fenton reagents. To this end, (1) photochemical degradation of phenol was investigated by adding persulfate and ferric ions to a visible-light-g-C3N4 system, and (2) an integrated light Fenton and radical photocatalytic system was verified through a series of photodegradation experiments and characterizations. This work proposed a new g-C3N4/Fe(III)/persulfate system which can significantly improve the phenol photodegradation efficacy, while deepened the understanding on the degradation mechanism of organic compounds in light Fenton and radical photocatalytic system.
35
microscope (TEM) that was operated at a 200-kV accelerating voltage. 2.3. Photodegradation experiment Photocatalytic degradation of phenol was performed in a 125 mL magnetically stirred cylindrical reactor top-irradiated by a 350 W Xenon lamp (XD-300, Nanjing Yanan Special Lighting Co., Ltd.) at room temperature (26 C). In a typical run, 80 mL of 10 mg L1 phenol aqueous solution was added to the reactor. The pH of the phenol solution was kept at the initial state. A 150 mg portion of the photocatalyst was added to the reactor, and the system was stirred in the dark for 30 min to achieve adsorption equilibrium. This degradation process is similar to that described in our previous work (Zhang et al., 2014). The samples were centrifuged to sediment the photocatalyst, and the phenol concentration in the supernatants was monitored by measuring solution absorbance using a UV-2450 spectrophotometer (Shimadzu Inc., Japan) at 510 nm wavelength. Aromatic intermediates were identified by LC-100 high-performance liquid chromatograph (HPLC; Wufeng Inc., China) using 1.2 mL min1 of acetonitrile (25 vol.%) and water (75 vol.%) as the mobile phase at 280 nm. The reactor was left open to air over the entire process to ensure aerobic conditions. Based on the initial experimental conditions above, Fe(NO3)3 (350 mg L1) and K2S2O8 (300 mg L1) were added to the reaction system to investigate the effects of Fe(III) and sulfate on phenol degradation. Several comparative experiments were performed by changing the dosages of reagents. Most conditions were tested at least in triplicate. In a ultrasound treatment experiment, we put the system in ultrasound for 15 min before the dark absorption and light irradiation, other conditions are the same as other experiments. The photocatalytic degradation process accords with the pseudo first-order kinetics (Eq. (1)) by data fitting, thus the apparent kinetic constant (kapp) can be used to compare the photocatalytic activity of each photocatalytic system quantitatively (Yan et al., 2010; Dong et al., 2012):
lnðC=C0 Þ ¼ kapp t
(1)
3. Results and discussion 3.1. Characterization
2. Materials and methods 2.1. Preparation The g-C3N4 photocatalyst was prepared by directly heating melamine in a semiclosed system to prevent melamine sublimation. Then, 10 g of melamine powder was placed in an alumina crucible with a cover and then heated to 600 C in a muffle furnace for 8 h at a heating rate of 4.17 C/min. The resultant yellow powder was collected and used without further treatment. 2.2. Characterization Fourier Transform infrared (FTIR) spectra in the range of 4000e400 cm1 were collected on a VERTEX 70 FT-IR spectrometer (Bruker Inc., Germany) by the KBr pellet method. Each spectrum represents the average of 64 scans at a spectral resolution of 4 cm1. The morphologies of g-C3N4 were observed on a SIRION-200 scanning electron microscope (SEM) that was operated at a 30-kV accelerating voltage and a JEOL-2100F transmission electron
The FTIR spectra (Fig. 1a) show little difference between the gC3N4 samples before and after light irradiation. Both molecules possess characteristic functional groups on the carbon nitride polymer. The broad peak around 3174 cm1 is attributed to primary and secondary amino groups, and the presence of the peaks indicates that thermo-condensation during the synthesis of g-C3N4 is incomplete (Thomas et al., 2008). For example, some eNH2 or eNH groups existed in the side of the layer after the thermal treatment (Zhang et al., 2014). Strong absorption bands ranging from 1200 to 1600 cm1 correspond to the skeletal stretching of the tri-s-triazine heterocycle motif, and the peak at 801 cm1 shows the typical ringbreathing vibration of the tri-s-triazine heterocycle, which indicates that degradation experiment does not destroy the in-plane tri-s-triazine structural motif of g-C3N4. Moreover, there was no obvious characteristic peak of phenol in the curve, suggesting the negligible adsorption of phenol on g-C3N4 in the dark. Fig. 1b shows the SEM image of g-C3N4 samples. The morphology of sample is highly ordered and dominantly comprises aggregated layers with a size of several micrometers.
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Fig. 1. a) FTIR patterns for g-C3N4 samples before and after irradiation. b) SEM image for g-C3N4 sample.
system is higher than that in systems with similar Fe(III)/persulfate concentrations but without photocatalysts (Liang and Su, 2009). The photocatalytic experiments indicate that the presence of C3N4, Fe(III), and persulfate exert synergistic effects on the degradation of organic compounds. Although previous reports have described persulfate activation by metallic minerals, metal-containing oxides, metal-salts, zero-valent metals, and calefaction (Liang and Lai, 2008), reports on the influence of the aforementioned systems after photocatalyst addition are scarce. The degraded products of phenol were determined by HPLC (Fig. 2c), and quinol, catechol, resorcinol, and benzoquinone are observed. These findings indicate that an oxidation mechanism dominates the photodegradation of organic pollutants in the g-C3N4/Fe(III)/persulfate system (Mijangos et al., 2006). The influence of different conditions, such as pH of the solution (the pH of the phenol solution was adjusted to a set value of 4, 7, and 10 using 1 M HClO4 and NaOH solutions), concentration of Fe(III)/persulfate (the concentration of Fe(III)/persulfate was doubled), and external ultrasound application (the system was put in ultrasound for 15 min before the irradiation), on the g-C3N4/ Fe(III)/persulfate system was investigated, and relevant results are shown in Fig. 3. Almost no difference in the phenol degradation efficiency is observed at different pH values (Liang et al., 2007). While SO4 is the dominant species under acidic pH conditions, relatively more HO are generated at higher pH levels (Liang et al., 2007); these radicals are robust oxidizers of phenol. The rate of phenol degradation increases as the Fe(III)/persulfate concentration increases within a certain range; this finding is similar to a phenomenon observed by Xu and Li (2010). Numerous studies reported that ultrasonic dispersion can increase the specific surface area of some photocatalysts and consequently improve the degradation effects (Yu et al., 2001). The present results show that ultrasonic treatment reduces phenol degradation, likely because of destruction of the sheet structure of g-C3N4 during degradation, which leads to declines in photocatalytic activity. This assumption is partly confirmed by some reports discussing other ultrasoundtreated layered materials (Quintana et al., 2012).
3.2. Phenol degradation 3.3. Mechanism To broaden the practical application of this novel photocatalyst, g-C3N4 was used to photodegrade phenol. Fig. 2 shows the adsorption properties, photocatalytic activities, and apparent kinetic constants of various catalytic systems for phenol degradation under visible light irradiation. The adsorption of phenol on C3N4 in the dark is negligible (Fig. 2a), consistent with the FTIR spectra and the published data (Zhang et al., 2013a,b). Pure C3N4 under visible irradiation displays very weak photocatalytic activity; here, only 2% of the phenol in solution is degraded. Fe(III) and persulfate have been reported to significantly enhance degradation of organic compounds in aqueous solution (Fang et al., 2013). However, addition of Fe(III) or persulfate only cannot obviously improve the efficiency of refractory phenol degradation. Even simultaneous addition of relevant concentrations of Fe(III) and persulfate show no evident increases in the efficiency of phenol degradation (14%). Under visible light irradiation, over 33% of the phenol in solution can be degraded within 90 min in the presence of g-C3N4/Fe(III)/ persulfate, which is 16.5- and 2.3-fold faster than that in individual g-C3N4 (2%) and Fe(III)/persulfate (14%) systems, respectively. The reactive rate constant k in different system was calculated, and the results further show that the degradation rate of phenol in the gC3N4/Fe(III)/persulfate system is 4.8 103 min1, which is 240and 2.7-fold faster than those observed in individual g-C3N4 (2 105 min1) and Fe(III)/persulfate (1.8 103 min1) system, respectively (Fig. 2b). The rate of phenol degradation in the present
The mechanism of phenol degradation via the g-C3N4/Fe(III)/ persulfate system under visible irradiation has seldom been investigated. A series of experiments were conducted to elucidate the mechanism of the complex redox-photocatalytic system. First, radical scavengers were added into the solution to verify the role of free radicals in the g-C3N4/Fe(III)/persulfate system. Tert-butyl alcohol (TBA) captures HO, while ethanol (EtOH) captures both HO and SO4 (Anipsitakis and Dionysiou, 2004). Fig. 4a shows the effects of TBA and EtOH addition on phenol degradation by the gC3N4/Fe(III)/persulfate system under visible light. No difference in phenol degradation efficiency is observed upon addition of TBA, which suggests that HO performs a trivial function in phenol degradation. By contrast, phenol degradation efficiency is significantly reduced by addition of EtOH, which indicates that SO4 is an important factor contributing to phenol degradation. Phenol degradation probably primarily occurs in solution because of the weak phenol adsorption ability of g-C3N4. Nevertheless, HO is mostly produced on the surface of g-C3N4 under visible light irradiation and thus rarely participates in phenol degradation. SO4 is generated by persulfate activation and is nearly always present in bulk solutions (Criquet and Leitner, 2009). Consequently, addition of EtOH consumes SO4 and remarkably affects phenol degradation. SO4 is produced by the reaction of persulfate and Fe(III) as described in Eq. (2) (Avetta et al., 2015):
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Fig. 2. a) Adsorption properties, photocatalytic activities and b) apparent kinetic constants in different catalytic systems for degradation of phenol under visible light; c) Intermediate products identified by HPLC chromatography for degradation of phenol under visible light.
Fig. 3. Adsorption property and photocatalytic activities indifferent reaction environment based on the g-C3N4/Fe(III)/persulfate system for degradation of phenol under visible light. Initial: as mentioned in photodegradation experiment; ultrasound: put the system in ultrasound for 15 min before the irradiation; concentration: double the concentration of Fe(III)/persulfate; acidic: adjust the system pH to 4; neutral: adjust the system pH to 7; alkaline: adjust the system pH to 10.
FeðIIIÞ þ S2 O8 2 /FeðIIÞ þ S2 O8
(2)
Fe(II) is oxidized rapidly by persulfate to regenerate Fe(III) and produce SO4 and sulfate (Eq. (3)):
FeðIIÞ þ S2 O8 2 /FeðIIIÞ þ SO4 2 þ SO4
(3)
Fe(II) performs an important function in phenol degradation. To confirm the production of Fe(II), 1,10-phenanthroline spectrophotometry was adopted to quantitatively measure the Fe(II) content in the reaction system. The results of the different catalytic systems are shown in Fig. 4b. Fig. 4b shows that the concentrations of Fe(II) in the bulk solution of the Fe(II)/phenol system before and after 2 h of visible light irradiation are nearly constant, which indicates that Fe(II) can hardly be oxidized into Fe(III) by oxygen in air. This phenomenon implies that production of Fe(II) in the solution is closely related to persulfate and g-C3N4 rather than dissolved oxygen. In the Fe(II)/g-C3N4/phenol system, the concentration of Fe(II) in the bulk solution decreases slightly after reaction, which indicates that very few Fe(II) ions are adsorbed on the g-C3N4 surface. Comparative results show that the concentration of Fe(II) in the bulk solution, which was determined by adding hydroxylamine hydrochloride to convert Fe(III) into Fe(II) (Che et al., 1995), obviously decreases in the Fe(III)/g-C3N4/phenol system. This result indicates that Fe(III) can be adsorbed by g-C3N4, which is advantageous to electron trapping at the semiconductor surface (Vamathevan et al., 2001). Fig. 4c shows that the g-C3N4/Fe(III) system has the highest concentration of Fe(II). Fe(II) is re-oxidized in situ by holes or other reactive species generated in the system (Cheng et al., 2004). The Fe(II) concentration in the g-C3N4/Fe(III)/persulfate system is even lower than that in the Fe(III)/persulfate system, probably because more drastic reactions of the Fe(III)/Fe(II) cycle occur in the g-C3N4/ Fe(III)/persulfate system than that in the Fe(III)/persulfate system. Further trials are in progress to verify this assumption. Moreover, Fe(II) concentrations in the different catalytic systems are relatively low, which suggests the fast redox rates of Fe species. Photogenerated electrons react with O2 on the surface of g-C3N4
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Fig. 4. a) Effects of tert-butyl alcohol (TBA) and ethanol (EtOH) addition on phenol degradation by the g-C3N4/Fe(III)/persulfate system under visible light. Initial: as mentioned in photodegradation experiment; þTBA: remove hydroxyl radical (HO) in the system; þEtOH: remove both hydroxyl radical (HO) and sulfate radical ðSO4 Þ in the system; Concentration of Fe(II) in different catalytic systems for degradation of phenol under visible light when b) adding hydroxylamine hydrochloride in the g-C3N4/Fe(III) system to convert Fe(III) to Fe(II) and c) without adding hydroxylamine hydrochloride in any systems; d) Concentration of H2O2 in different catalytic systems for degradation of phenol under visible light.
originating from persulfate decomposition to generate the superoxide anion radical ðO2 Þ under visible light (Jiang et al., 2013), which subsequently undergoes iron-catalyzed dismutation in the presence of Fe(III) (Eqs. (4)e(6)):
e CB þ O2 /O2
(4)
FeðIIIÞ þ O2 /FeðIIÞ þ O2
(5)
FeðIIÞ þ O2 þ 2Hþ /FeðIIIÞ þ H2 O2
(6)
We infer that H2O2 produced by superoxide dismutation reacts with Fe(II)/Fe(III) through Fenton-like reactions in the solution. Considering the possible existence of the light Fenton system, titanium potassium oxalates spectrophotometry was used to verify the existence of H2O2. The concentrations of H2O2 in the different catalytic systems are shown in Fig. 4d. H2O2 exists in the g-C3N4/ Fe(III)/persulfate system but not in the Fe(III)/persulfate system, which indicates the presence of light Fenton interaction in the gC3N4/Fe(III)/persulfate system. Thus, more violent reactions of the Fe(III)/Fe(II) cycle may occur because of light Fenton reactions, in accordance with the aforementioned hypothesis. The enhanced degradation process can be described in two steps with synergistic interaction: activation of persulfate and establishment of light Fenton system (Fig. 5). First, persulfate is activated to produce reactive species of SO4 in the presence of Fe(III) under illumination, which is extremely beneficial to phenol degradation. Meanwhile, electronic transition from the valence band to the conduction band of g-C3N4 causes formation of electronehole pairs. Fe(III) is adsorbed to the surface of g-C3N4 continuously and
traps photogenerated electrons to convert into Fe(II), which accelerates the circulation of Fe(II)/Fe(III) and the production of SO4 . In addition, another degradation path is activated when O2 adsorbs photogenerated electrons on the surface of g-C3N4 and traps electrons to produce reactive species of O2 under intensive stirring; these radical species then undergoes iron-catalyzed dismutation in the presence of Fe(III) to form H2O2. In this case, Fe(II) and H2O2 in the solution constitute a light Fenton system and significantly enhance degradation effects.
4. Conclusion An integrated light Fenton and radical photocatalytic system by adding Fe(III) and persulfate into the degradation system can significantly improve the phenol degradation efficiency. Although the effect of the present coupled system, which involves promotion of free radical generation and acceleration of the Fe(III)/Fe(II) cycle, was much better than those observed in separate degradation mechanisms, the efficiency of phenol degradation was unsatisfactory. However, improving the degradation efficiency of g-C3N4 systems remains worthy of investigation. At present, most photocatalytic studies focus on dye decolorization, which has high efficiency but limited application. By contrast, phenol degradation presents a low degradation rate but much more environmental significance in controlling water pollution. Compared with wellknown photocatalysts that can only utilize UV light, g-C3N4 is one of the few non-doped photocatalysts that use visible light; this characteristic is its unique advantage.
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Fig. 5. Illustrative representation of the g-C3N4/Fe(III)/persulfate system under visible light.
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