Catalytic wet peroxide oxidation (CWPO) of phenol over clay-based catalysts in the presence and absence of NaCl was investigated. Changes in the H2O2, Cl, and dissolved metal ion concentration, as well as solution pH during phenol oxidation, were also studied. Additionally, the intermediates formed during phenol oxidation were detected by liquid chromatography-mass spectroscopy and the chemical bonding information of the catalyst surfaces was analyzed by X-ray photoelectron spectroscopy (XPS). The results showed that the presence of Cl increased the oxidation rate of phenol to 155%, and this phenomenon was ubiquitous during the oxidation of phenolic compounds by H2O2 over clay-based catalysts. Cl-assisted oxidation of phenol was evidenced by several analytical techniques such as mass spectroscopy (MS) and XPS, and it was hypothesized that the rate-limiting step was accelerated in the presence of Cl. Based on the results of this study, the CWPO technology appears to be promising for applications in actual saline phenolic wastewater treatment.

INTRODUCTION

Over the last few years, catalytic wet peroxide oxidation (CWPO) has been regarded as one of the most promising technologies for the abatement of toxic organic compounds in wastewater. In this method, hydroxyl radicals () generated via the catalytic decomposition of H2O2 promote the degradation of organics in wastewater (Liotta et al. 2009; Garrido-Ramírez et al. 2010; Rokhina & Virkutyte 2011). Fe- and Cu-pillared clays (PILCs), in particular, have been widely used as heterogeneous catalysts for the CWPO reactions of phenolic substrates, in view of their outstanding advantages such as low cost, widespread availability, good rate of conversion, wide operating range of pH, and none or marginal leaching of active ions (Kurian & Sugunan 2006; Liotta et al. 2009; Rokhina & Virkutyte 2011; Galeano et al. 2014). A variety of highly-efficient PILCs have been prepared and the influences of various operating parameters, as well as the role of reactive oxidizing species in determining the activity of the catalysts, have been investigated. In addition, the phenol degradation path and the mechanisms of catalytic oxidation have been elucidated, and the potential for the use of CWPO in future has been evaluated (Kurian & Sugunan 2006; Liotta et al. 2009; Rokhina & Virkutyte 2011; Galeano et al. 2014). Despite the vast amount of work conducted so far, there are still significant knowledge gaps in the understanding of the kinetic processes and interaction mechanisms of the complicated Fenton-like oxidation reaction.

In a previous study (Zhou et al. 2011a), we found that the presence of inorganic chlorides significantly increased the oxidation rate of phenol over Cu-Ni-Al hydrotalcite. Micó et al. (2013) and Mahamuni & Pandit (2006) also observed similar phenomena during the photo-Fenton oxidation of pesticides and ultrasonic degradation of phenol at high concentration of NaCl solutions, respectively. It is a novel observation which deserved further attention, because it is usually thought that the presence of inorganic salts such as chlorides, sulphates and nitrates during the CWPO reaction inhibits the oxidation process by scavenging the transitory of (De Laat et al. 2004; De Laat & Le 2006). Vione et al. (2005) observed that phenol chlorination might occur in the presence of dissolved Fe(III), H2O2, and Cl which, in turn, may enhance the oxidation of phenolic compounds via the formation of chlorinated intermediates that act as electron shuttles (Chen & Pignatello 1997; Du et al. 2006). Furthermore, Micó et al. (2013) attributed the enhancement of Cl to the participation of in the reaction, whose reaction capacity is comparable with that of . However, Mahamuni & Pandit (2006) concluded that NaCl was responsible for only physically pushing the phenol towards the cavity/water interface, owing to the salting-out effect and was not taking part in any chemical reactions. It is obvious that Cl exerts a complex effect on the CWPO reaction of phenol, and its mechanism is still unclear.

Herein, the role of Cl in the mediator-assisted CWPO reaction of phenol over clay-based catalysts has been investigated. The aim of this study was two-fold: (1) to present evidence confirming that the promotion of phenol oxidation by Cl is a ubiquitous phenomenon which still occurs even in high salt (0.17 M NaCl solution); and (2) to reveal the mechanism involved in Cl-assisted phenol oxidation, which would help extend the applications of the CWPO technology to the treatment of actual saline phenol-containing wastewater.

EXPERIMENTAL

Clay-based catalysts

Using common copolymerization procedures, a series of clay materials (Cu4.8Ni1.2Al2-hydrotalcite, Cu6Al2-hydrotalcite/clay composite and Al5-Fe0.5-Cu0.5-pillared clay) were synthesized and characterized in our previous studies (Zhou et al. 2011a, b, 2014). These materials, which exhibit both high catalytic activity and high stability, are capable of promoting complete oxidation of phenol or 4-chlorophenol (4-CP) within 2 h at 30–40 °C. Therefore, these materials were used in this work, as high-efficiency heterogeneous catalysts.

Catalytic oxidation of phenol by H2O2

Following the method described in the previous studies (Zhou et al. 2011a, b, 2014), the catalytic oxidation reaction was carried out in a 500-mL three-neck glass flask fitted with a reflux condenser, magnetic stirrer and thermostated water bath. For a typical run, 250 mL of phenol (2.66 mM) and 0.25 g of catalyst in powder-form were loaded into the flask. The reaction mixture was magnetically stirred and heated to 30 °C, following which H2O2 (10 mmol) was added immediately, to initiate the catalytic oxidation reaction. During the oxidation reaction, 10 mL aliquots were withdrawn at certain time intervals, and divided into two 5 mL portions. One of these portions was mixed with 0.1 g MnO2 in order to eliminate residual H2O2 (Liou & Chen 2009), then filtered and the concentrations of phenol, total organic carbon (TOC), Cl and dissolved metals (Fe and Cu) were analyzed; whereas the other portion was directly filtered to determine the residual amount of H2O2. After reaction, the catalyst was separated from the reaction mixture by centrifugation and air-dried prior to X-ray photoelectron spectroscopy (XPS) analysis.

Analytical methods

Prior to gas chromatography (GC) analysis, phenol in aqueous solution was extracted by CH2Cl2, and then its concentration was determined using an Agilent 7890A GC, following the previous method (Zhou et al. 2011a). Another analysis in aqueous solution was done, where TOC content was measured using a Shimadzu TOC-5000 Analyzer; dissolved metals were determined by inductively coupled plasma-mass spectroscopy (ICP-MS; ELAN DRC II); the Cl was measured by ion chromatography (Dionex ICS3000); H2O2 content was analyzed by a spectrophotometric method using ammonium metavanadate (Nogueira et al. 2005). Additionally, solution pH was monitored every 2 s during the reaction process with a Metrohm 888 Titrando. The phenol conversion process and the appearance of intermediates were detected by ion-trap liquid chromatography-mass spectroscopy (LC-MS; LCQ Fleet). Atomic concentration and chemical bonding information were obtained from the catalyst surfaces before and after the CWPO reaction by XPS (Thermo K-Alpha XPS).

RESULTS AND DISCUSSION

Oxidation of phenol promoted by Cl

In a previous study (Zhou et al. 2011a), we found that the presence of NaNO3 markedly decreased the oxidation rate of phenol over Cu-Ni-Al hydrotalcite, whereas the presence of Cl significantly increased the oxidation rate. Table 1, which shows that the rates of oxidation of phenol over three types of clay-based catalysts increased markedly in the presence of Cl, further supports the results from the previous study. The maximum increase in the rate of oxidation of phenol was 155% in the presence of 10 g L−1 (0.17 M) of NaCl, whereas the oxidation rate decreased greatly in the presence of NaBr and NaNO3. These results have significant implications for the possible application of the CWPO technology in phenolic wastewater treatment, considering the high Cl concentration in phenolic wastewater. In other words, the results suggest that the CWPO technology can be extended to actual saline phenol-containing wastewater treatment. In addition, mono-, di-, and tri-chlorophenols (MCPs, DCPs and TCPs, respectively) were found to undergo Cl-assisted oxidation over the clay-based catalyst considered in this study (Table 2). Cl-assisted oxidation appears to be a novel and ubiquitous phenomenon, in particular Cl-assisted oxidation of CPs has never been observed previously, because it is widely accepted that Cl inhibits Fenton-like processes by scavenging or by competing with H2O2 for the complexation of Fe(III) (De Laat & Le 2006; Pignatello et al. 2006), and even the inhibitory effect is noticeable only at Cl concentrations higher than 0.01 M (Pignatello et al. 2006; Machulek et al. 2007). However, this study shows that even in the presence of 10 g L−1 of NaCl (0.17 M Cl), which is a much higher concentration, phenol oxidation is in fact promoted, and moreover, the rates of phenol oxidation increased obviously with increasing Cl concentration (Table 1). Therefore, Cl plays an important role in promotion of the oxidation reactions of phenolic compounds, and this novel finding deserves attention.

Table 1

Influence of inorganic salts on phenol oxidation by H2O2 over different heterogeneous catalysts (reaction conditions: [phenol] = 2.66 mM; n (H2O2)/n (phenol) = 15; [catalyst] = 1 g L−1; temperature = 30 °C)

Salt
CatalystaTypeConcentration (mM)Time (min)Conversion (%)Average rate (mM min−1)Increase rate (%)
Control 90 100 2.95 × 10−2 – 
NaCl 17.11 60 100 4.42 × 10−2 49.8 
NaCl 102.66 40 100 6.64 × 10−2 125.1 
KCl 13.41 60 94.4 4.18 × 10−2 41.7 
NH4Cl 18.69 90 100 2.95 × 10−2 
CaCl2 9.01 60 100 4.42 × 10−2 49.8 
MgCl2 10.50 60 98.9 4.38 × 10−2 48.5 
NaBr 9.72 120 19.8 4.39 × 10−3 −85.1 
NaNO3 11.77 120 41.3 9.14 × 10−3 −69.0 
Control 90 99.2 2.93 × 10−2 – 
NaCl 8.56 60 91.4 4.05 × 10−2 38.2 
NaCl 17.11 60 97.8 4.33 × 10−2 47.8 
KCl 13.41 60 93.8 4.16 × 10−2 41.9 
NH4Cl 18.69 90 99.0 2.93 × 10−2 
NH4Cl 28.04 60 95.0 4.21 × 10−2 43.7 
CaCl2 9.01 60 100 4.42 × 10−2 50.9 
MgCl2 10.50 60 75.8 3.36 × 10−2 14.6 
NaBr 9.72 120 18.8 4.16 × 10−3 −85.8 
Control 55 27.0 1.31 × 10−2 – 
NaCl 17.11 55 34.7 1.68 × 10−2 28.2 
NaCl 171.11 55 69.1 3.34 × 10−2 155.0 
NaBr 9.72 120 18.1 4.01 × 10−3 −69.3 
Salt
CatalystaTypeConcentration (mM)Time (min)Conversion (%)Average rate (mM min−1)Increase rate (%)
Control 90 100 2.95 × 10−2 – 
NaCl 17.11 60 100 4.42 × 10−2 49.8 
NaCl 102.66 40 100 6.64 × 10−2 125.1 
KCl 13.41 60 94.4 4.18 × 10−2 41.7 
NH4Cl 18.69 90 100 2.95 × 10−2 
CaCl2 9.01 60 100 4.42 × 10−2 49.8 
MgCl2 10.50 60 98.9 4.38 × 10−2 48.5 
NaBr 9.72 120 19.8 4.39 × 10−3 −85.1 
NaNO3 11.77 120 41.3 9.14 × 10−3 −69.0 
Control 90 99.2 2.93 × 10−2 – 
NaCl 8.56 60 91.4 4.05 × 10−2 38.2 
NaCl 17.11 60 97.8 4.33 × 10−2 47.8 
KCl 13.41 60 93.8 4.16 × 10−2 41.9 
NH4Cl 18.69 90 99.0 2.93 × 10−2 
NH4Cl 28.04 60 95.0 4.21 × 10−2 43.7 
CaCl2 9.01 60 100 4.42 × 10−2 50.9 
MgCl2 10.50 60 75.8 3.36 × 10−2 14.6 
NaBr 9.72 120 18.8 4.16 × 10−3 −85.8 
Control 55 27.0 1.31 × 10−2 – 
NaCl 17.11 55 34.7 1.68 × 10−2 28.2 
NaCl 171.11 55 69.1 3.34 × 10−2 155.0 
NaBr 9.72 120 18.1 4.01 × 10−3 −69.3 

aA: Cu4.8Ni1.2Al2-hydrotalcite; B: Cu6Al2-hydrotalcite/clay composite; C: Al5-Fe0.5-Cu0.5-pillared clay (500).

Table 2

The oxidation of chlorophenols (CPs) by H2O2 over Cu6Al2-hydrotalcite/clay composite in the absence and presence of NaCl (reaction conditions: [CP] = 2 mM; n (H2O2)/n (CP) = 20; [catalyst] = 1.0 g L−1; [NaCl] = 1 g L−1; temperature = 40 °C)

Chlorophenol (CP)NaClTime (min)Average rate (mM min−1)Increase rate (%)
3-CP Without 10 1.02 × 10−1 – 
3-CP With 10 1.64 × 10−1 60.8 
2-CP Without 20 6.53 × 10−2 – 
2-CP With 20 8.83 × 10−2 35.2 
4-CP Without 30 5.07 × 10−2 – 
4-CP With 30 5.91 × 10−2 16.6 
3,4-DCP Without 30 4.39 × 10−2 – 
3,4-DCP With 30 6.23 × 10−2 41.9 
3,5-DCP Without 20 4.86 × 10−2 – 
3,5-DCP With 20 8.86 × 10−2 82.3 
2,5-DCP Without 30 2.56 × 10−2 – 
2,5-DCP With 30 5.41 × 10−2 111.3 
2,4-DCP Without 90 1.74 × 10−2 – 
2,4-DCP With 90 2.07 × 10−2 19.0 
2,6-DCP Without 180 8.98 × 10−3 – 
2,6-DCP With 180 9.05 × 10−3 0.8 
2,4,6-TCP Without 300 5.58 × 10−3 – 
2,4,6-TCP With 300 5.95 × 10−3 6.6 
Chlorophenol (CP)NaClTime (min)Average rate (mM min−1)Increase rate (%)
3-CP Without 10 1.02 × 10−1 – 
3-CP With 10 1.64 × 10−1 60.8 
2-CP Without 20 6.53 × 10−2 – 
2-CP With 20 8.83 × 10−2 35.2 
4-CP Without 30 5.07 × 10−2 – 
4-CP With 30 5.91 × 10−2 16.6 
3,4-DCP Without 30 4.39 × 10−2 – 
3,4-DCP With 30 6.23 × 10−2 41.9 
3,5-DCP Without 20 4.86 × 10−2 – 
3,5-DCP With 20 8.86 × 10−2 82.3 
2,5-DCP Without 30 2.56 × 10−2 – 
2,5-DCP With 30 5.41 × 10−2 111.3 
2,4-DCP Without 90 1.74 × 10−2 – 
2,4-DCP With 90 2.07 × 10−2 19.0 
2,6-DCP Without 180 8.98 × 10−3 – 
2,6-DCP With 180 9.05 × 10−3 0.8 
2,4,6-TCP Without 300 5.58 × 10−3 – 
2,4,6-TCP With 300 5.95 × 10−3 6.6 

Moreover, in the present study, the presence of Cl accelerated the conversion of phenol and its main intermediate, p-benzoquinone, into low-molecular weight organic acids. The rate of conversion of phenol increased with increase in the concentration of NaCl (Figure 1). Furthermore, it is observed from Figure 1 that concentration of Cl alters the composition of the dominant organic acids formed. For example, acetic acid (RT = 2.99 min) and acrylic acid (RT = 1.05 min) were the predominant products after reaction for 120 min in the absence of NaCl, whereas in the presence of 10 g L−1 of NaCl, oxalic acid (RT = 1.47 min), rather than acrylic acid, became the second most dominant organic acid. This implies that faster and more complete oxidation of phenol occurred in the presence of NaCl, since oxalic acid is more stable compared with acrylic acid. However, no difference was observed in the final TOC conversion (%) in the presence and absence of NaCl (Supplementary Material, Figure S1, available with the online version of this paper), suggesting that Cl only played a major role in increasing the oxidation rate of phenol (conversion), and did not significantly influence its deep oxidation (mineralization).
Figure 1

Chromatograms of LC-MS for phenol oxidation by H2O2 over Al5-Fe0.5-Cu0.5-PILC (500) under different NaCl addition after 60 min (left), 90 min (middle) and 120 min (right) (reaction conditions: [phenol] = 2.66 mM; [H2O2] = 40 mM; [catalyst] = 1 g L−1; temperature = 30 °C).

Figure 1

Chromatograms of LC-MS for phenol oxidation by H2O2 over Al5-Fe0.5-Cu0.5-PILC (500) under different NaCl addition after 60 min (left), 90 min (middle) and 120 min (right) (reaction conditions: [phenol] = 2.66 mM; [H2O2] = 40 mM; [catalyst] = 1 g L−1; temperature = 30 °C).

The above discussion on the role of Cl suggests that the presence of Cl could be expected to increase the oxidation rate of toxic organic compounds such as phenol, chlorophenols, bisphenol A and pesticides in CWPO or other advanced oxidation processes, in particular photo-degradation due to the formation of high active chlorine and radicals besides (Vione et al. 2005; Micó et al. 2013; Xiao et al. 2015; Zhou et al. 2015). In other words, the heterogeneous catalytic oxidation technologies were expected to extend to actual saline wastewater. However, Table 1 showed that NH4Cl results are not closer with respect to the rest, the reason for which is not clear. What is more important is that NH4+ and NO3 are also present in actual saline wastewater, which would undoubtedly weaken the effect of Cl. Maybe this technology should be tried first to apply to organic wastewater with NaCl as dominant salt components.

In fact, Micó et al. (2013) also observed a similar phenomenon during the photo-Fenton oxidation of pesticides at high salinities. However, they attributed the enhancement in pesticide depletion to the participation of in the reaction, since the rate constants for the reactions of organic compounds with were comparable with those with . However, it was almost impossible to generate in reaction systems near neutral pH in view of reaction (1) (Pignatello et al. 2006), which involves not only the consumption of , but also the formation of less reactive .

Mechanism of the enhancement of phenol oxidation rate by Cl

Figure 2 illustrates the interdependence among H2O2 decomposition, phenol oxidation, metallic dissolution, and pH change. Obviously, active metal dissolution was not the reason for the initiation of the oxidation of phenol, as suggested by Belaroui & Bengueddach (2012). The results shown in Figure 2 suggest that the decrease in pH caused by the surface-catalyzed reaction is responsible for initiating the oxidation of phenol, because phenol was almost oxidized completely (90.3%), whereas metal dissolution rate was very low (only 0.7 mg L−1) in 90 min. Thus, the induction period that is often observed should be attributed to the activation of the catalyst surface, as described in the following reactions ((2)–(4)) (Kwan & Voelker 2003; Huang & Huang 2008):
formula
2
formula
3
formula
4
Timofeeva et al. (2005) confirmed the formation of peroxide-Fe3+ complexes upon the interaction of H2O2 and Al-Fe-PILC, by electron spin resonance (ESR) and diffuse reflectance UV-visible (DR-UV-Vis) spectra. It was also assumed that reaction (3), where the pH is lower as a result of the reduction of Fe(III) to Fe(II), is the rate-limiting step (Kwan & Voelker 2003). Therefore, the presence of Fe(II) and a sufficiently acidic catalyst surface are important and necessary for catalyzing the decomposition of H2O2 into (reaction (4)), and for initiating the oxidation of phenol. Tatibouët et al. (2005) also found that the pH of the reaction played an important role in the generation of , which reached a maximum at pH values around 3.7. In view of the above results, the induction period could be postulated as an activation process involving the complexation of Fe(III) with H2O2 and the subsequent reduction-decomposition of the complexes. In addition, the lower pH, the shorter the induction time would be. In short, at the initial stage of the catalytic reaction (induction period), the pH decreased monotonously during the adsorption and subsequent decomposition of H2O2, with little formation and therefore, only slight oxidation of phenol. On the other hand, after the induction period, a large number of radicals were generated, resulting in the fast and complete oxidation of phenol.
Figure 2

Kinetic profiles of H2O2 decomposition, metallic dissolution and pH changes during catalytic reaction of phenol over Al5-Fe0.5-Cu0.5-PILC (500) (reaction conditions: [phenol] = 2.66 mM; [H2O2] = 40 mM; [catalyst] = 1 g L−1; temperature = 30 °C).

Figure 2

Kinetic profiles of H2O2 decomposition, metallic dissolution and pH changes during catalytic reaction of phenol over Al5-Fe0.5-Cu0.5-PILC (500) (reaction conditions: [phenol] = 2.66 mM; [H2O2] = 40 mM; [catalyst] = 1 g L−1; temperature = 30 °C).

Phenol oxidation, H2O2 decomposition, and pH decrease were observed to exhibit a highly consistent relationship in the presence and absence of NaCl (Figure 3). In other words, compared with the control sample (without NaCl), faster decomposition of H2O2 and thus faster decrease in pH were observed in the presence of NaCl, resulting in the faster oxidation of phenol. Thus, it was considered that the presence of Cl promoted the rate of oxidation of phenol by accelerating the rate-limiting step of the surface-catalyzed decomposition of H2O2 (reaction (3)).
Figure 3

Profiles of phenol oxidation, H2O2 decomposition and pH change during catalytic reaction of phenol over Al5-Fe0.5-Cu0.5-PILC (500) under different NaCl addition (reaction conditions: [phenol] = 2.66 mM; [H2O2] = 40 mM; [catalyst] = 1 g L−1; temperature = 30 °C).

Figure 3

Profiles of phenol oxidation, H2O2 decomposition and pH change during catalytic reaction of phenol over Al5-Fe0.5-Cu0.5-PILC (500) under different NaCl addition (reaction conditions: [phenol] = 2.66 mM; [H2O2] = 40 mM; [catalyst] = 1 g L−1; temperature = 30 °C).

Further, the O/Fe atomic ratio decreased markedly by 13.5 in the presence of 10 g L−1 of NaCl compared with the control (Table 3), indicating a more reductive environment for the degradation of phenol. Additionally, in the XPS spectra of the catalyst surface, apparent chemical shifts to lower binding energies were observed in Figure 4 in the presence of NaCl. Chemical shifts of −1.46 eV and −2.50 eV for Fe2p2/3 and Cu2p2/3, respectively, were observed at 10 g L−1 NaCl, compared with the control, suggesting that chemical reduction had occurred on the catalyst surface (De Los Arcos et al. 2002). To sum up, XPS analysis strongly supported the hypothesis that the presence of Cl promoted the rate of oxidation of phenol by accelerating the rate-limiting step of the decomposition of H2O2 (the process of activation via the reduction of active metal species on the catalyst surfaces).
Table 3

The content (atom %) of main elements and atomic ratios (O/Fe, C/Fe and Cl/Fe) of Al5-Fe0.5-Cu0.5-PILC (500) measured by XPS before and after catalytic oxidation of phenol by H2O2 with and without NaCl

SystemO1sSi2pAl2pC1sFe2pCl2pNa1sO/FeC/FeCl/Fe
Control (before) 53.41 18.14 12.36 9.99 1.37 1.18 – 38.88 7.27 0.86 
Control (after) 52.90 16.95 13.27 13.02 1.40 0.88 – 37.80 9.30 0.63 
1 g L−1 NaCl 49.86 17.24 11.54 12.82 1.48 1.42 – 33.67 8.66 0.96 
10 g L−1 NaCl 35.79 10.60 6.95 25.90 1.47 4.19 4.34 24.32 17.60 2.85 
SystemO1sSi2pAl2pC1sFe2pCl2pNa1sO/FeC/FeCl/Fe
Control (before) 53.41 18.14 12.36 9.99 1.37 1.18 – 38.88 7.27 0.86 
Control (after) 52.90 16.95 13.27 13.02 1.40 0.88 – 37.80 9.30 0.63 
1 g L−1 NaCl 49.86 17.24 11.54 12.82 1.48 1.42 – 33.67 8.66 0.96 
10 g L−1 NaCl 35.79 10.60 6.95 25.90 1.47 4.19 4.34 24.32 17.60 2.85 
Figure 4

XPS spectra of Cu2p, Fe2p, O1s, C1s and Cl2p spectral regions of Al5-Fe0.5-Cu0.5-PILC (500) before and after catalytic oxidation of phenol by H2O2 with and without NaCl: (a) control before reaction; (b) control after reaction; (c) 1 g L−1 NaCl; and (d) 10 g L−1 NaCl.

Figure 4

XPS spectra of Cu2p, Fe2p, O1s, C1s and Cl2p spectral regions of Al5-Fe0.5-Cu0.5-PILC (500) before and after catalytic oxidation of phenol by H2O2 with and without NaCl: (a) control before reaction; (b) control after reaction; (c) 1 g L−1 NaCl; and (d) 10 g L−1 NaCl.

Although the changes in the Cl concentration (Supplementary Material, Figure S2) and the mass spectra (Supplementary Material, Figure S3) (Supplementary Material is available with the online version of this paper) showed that trace amounts of Cl directly participated in the oxidation of phenol resulting in chlorination (the formation of 4-CP and other chlorinated intermediates), the extent of chlorination reaction was negligible. Thus, the enhancement in the oxidation rate of phenol by Cl was not due to chlorination, as proposed by some researchers (Chen & Pignatello 1997; Vione et al. 2005; Du et al. 2006), but due to the promotion of the reduction-decomposition reaction (reaction (3)) mentioned above.

CONCLUSIONS

The oxidation of phenol by H2O2 over clay-based catalysts with and without NaCl was studied. The results show that the presence of Cl promoted the oxidation rate of phenol significantly (up to a maximum of 155%) and altered the composition of the dominant organic acids formed during the reaction. However, the presence of Cl did not significantly influence the deep oxidation of phenol (mineralization). Multiple indicators including such as Cl concentration, extent of metal leaching, H2O2 decomposition, and solution pH, in addition to MS and XPS analyses, showed that the presence of Cl effectively accelerated phenol oxidation and shortened the induction period by promoting the rate-limiting step of H2O2 decomposition. The results of this study suggest that the CWPO technology is promising for applications in actual saline phenolic wastewater treatment.

ACKNOWLEDGEMENTS

This work was financially supported by the National Natural Science Foundation of China (No. 41271254). The authors thank Dr Ying Liu for LC-MS analysis, Dr Chengli Qu for ICP-MS and IC analysis and Dr Yang Tan for TOC analysis.

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Supplementary data