Abstract

The present study explored the influence of Cl, Br, CO32−, HCO3, PO43−, HPO42−, NO3, SO32− and natural organic matter (NOM) on the reaction kinetics and the formation of undesired degradation byproducts during phenol oxidation by heat-activated persulfate (PS). CO32− and PO43− promoted the phenol degradation, because the hydrolysis of CO32− and PO43− created basic pH conditions which were conducive to enhanced PS oxidation rate. Br promoted the reaction by reacting with sulfate radicals (SO4•−) to produce bromine radicals that can selectively react with electron-rich phenol. NOM scavenged reactive SO4•−, thus inhibiting the reaction. As a strong reducing agent, SO32− rapidly reduced PS, thus completely suppressing the reaction. HCO3, HPO42−, Cl, and NO3 had negligible impact on PS oxidation of phenol. Six intermediates were detected in the no anion control using gas chromatography–mass spectrometry (GC-MS). Various toxic halogenated phenols and halogenated hydroquinones were detected in the treatment containing Cl and Br. In contrast, in the treatment containing CO32−, HCO3, PO43−, HPO42−, and NO3, no new intermediates were identified except for the intermediates already detected in the control treatment. Based on intermediates identified, reaction pathways for PS oxidation of phenol without anions and in the presence of halides were proposed respectively.

NOMENCLATURE

     
  • PS

    Persulfate

  •  
  • PS-AOP

    Persulfate-based advanced oxidation process

  •  
  • SR-AOP

    Sulfate radical-based advanced oxidation process

  •  
  • NOM

    Natural organic matter

  •  
  • GC-MS

    gas chromatography–mass spectrometry

INTRODUCTION

As a novel and powerful oxidant, persulfate (S2O82−) (E0 = 2.01 V) has recently been increasingly used for soil and groundwater remediation and water treatment. The persulfate (PS) ion has a O–O bond with a bond distance of 1.453 Å and a bond energy of 140 kJ/mol (Zhang et al. 2015). After activation, the O–O bond would break to generate the sulfate radical (SO4•−), which is an even more powerful oxidant (E0 = 2.60 V) that can rapidly oxidize most refractory organic pollutants in water (Sun & Wang 2015). Keeping a high abundance of SO4•− is critical for the successful application of a persulfate-based advanced oxidation process (PS-AOP) (Hu & Long 2016). However, naturally occurring inorganic anions and natural organic matter (NOM) that are ubiquitous in waters may scavenge reactive radicals and reduce the efficiency of PS-AOP (Ma et al. 2018). Successful application of PS-AOP requires improved understanding of the influence of these scavenging matrix species.

Several studies explored the impacts of matrix species on the kinetics of PS-AOP (Bennedsen et al. 2012; Jiang et al. 2016; Luo et al. 2016; Ma et al. 2018). However, most of these studies only investigated three to five matrix species among which Cl, CO32−/HCO3, and NOM received the most attention. The chemical compositions of aquatic environments are very complex and may contain various types and abundance of inorganic and organic matrix constituents. A more inclusive study that investigated more matrix species would be of great value for a deeper and more comprehensive understanding of this problem. This knowledge would help practitioners to identify the conditions where PS-AOP may or may not be applicable and estimate the dosage requirement of the oxidant.

Phenolic compounds are a group of high production-volume chemicals that find broad applications in various industrial processes and are a ubiquitous class of environmental contaminants commonly detected in various aquatic and terrestrial environments (Anipsitakis et al. 2006). Phenols and their derivatives are corrosive to the eyes, skin, and respiratory tract and causes harmful effects on the central nervous system and heart, thereby producing dysrhythmia, seizures, and persistent vegetative state in affected persons (Anipsitakis et al. 2006). Eleven phenolic compounds have been classified as priority pollutants by USEPA (Anipsitakis et al. 2006). Moreover, phenols are the main configurations composed of humic substances, which are regarded as the important NOM in the environment. Therefore, phenol is often invoked as a model for phenolic moieties in NOM (Liu et al. 2015).

Various studies have explored the effectiveness of PS-AOP by different activation methods in degrading phenols (Anipsitakis et al. 2006; Ahmad et al. 2013; Ma et al. 2017). However, the impacts of water matrix species on PS oxidation of phenols are not well understood. In addition, because of the electron donating capacity of the hydroxyl group, phenols are especially susceptible to electrophilic substitution reactions with electrophiles. Previous studies demonstrated that reactions between halide anions and SO4•− may generate reactive halogen radicals including X, X2•−, and XOH•−. These secondary radicals are electrophilic species that can react with phenols to form toxic halogenated byproducts (Anipsitakis et al. 2006; Liu et al. 2015). Besides halides, little is currently known about the influence of anions other than halides on the formation of undesired degradation byproducts during PS-AOP. It is not known whether the presence of anions such as CO32−/HCO3/NO3 may also lead to the formation of undesired degradation byproducts. A comprehensive study that investigates the impacts of anions other than halides would help to fill this important knowledge gap.

This is the first study that has a comprehensive investigation of the impacts of eight anions (CO32−, HCO3, Cl, Br, PO43−, HPO42−, NO3, and SO32−) and NOM on the reaction kinetics and the formation of undesired degradation byproducts during phenol oxidation by sulfate radical-based advanced oxidation process (SR-AOP). Formation of degradation byproducts was identified using gas chromatography–mass spectrometry (GC-MS).

MATERIALS AND METHODS

Chemicals

Phenol was purchased from Tianjin Yongda Chemical Regent Company (Tianjin, China). K3PO4·3H2O (>99%) was purchased from Tianjin Fuchen Chemical Reagent Works (Tianjin, China). H2SO4 (95–98%) and NaNO3 (>98%) were purchased from Beijing Chemical Reagent Works (Beijing, China). Na2S2O8 (>99%), NaCl (>99%), NaBr (>99%), KH2PO4 (>99.5%), NaHCO3 (>99.8%), Na2CO3 (>99.8%) and Na2SO3 (>98.0%) were purchased from Aladdin BioChem Technology (Shanghai, China). Methanol (>99.9%) and dichloromethane were purchased from Fisher Scientific. Humic acid (HA, ash ≤10%) was purchased from Sinopharm Chemical Reagent Beijing (Beijing, China). The reaction solution was prepared using ultrapure water (18.2 MΩ cm) produced by a Master-RUV ultrapure water system (Hitech Instruments, Shanghai, China).

Experimental procedure

In order to explore the influence of matrix species on phenol degradation, batch experiments were conducted in 50 mL solution that contained 10 mM PS, 0.1 mM phenol and one type of matrix species in each experiment. Eight anions (CO32−, HCO3, Cl, Br, PO43−, HPO42−, NO3, and SO32−) and NOM (humic acid) were investigated. Three anion concentrations (1 mM, 10 mM, and 100 mM) and four NOM concentrations (11.6, 23.1, 61.7, and 123.4 mg/L-TOC) were explored. No pH buffer was used in order to avoid potential interference from reactions among buffering species, PS, phenol and matrix species. All degradation experiments were conducted in triplicates at 50 °C in a constant-temperature water bath shaker (HWS-24, Shanghai Yiheng, China). The results were reported as the average value with standard deviation. Whether differences between treatments were statistically significant was determined using Student's t-test at the 95% confidence level.

Sample collection and analysis

At predetermined sampling time, 1 mL reaction solution was collected and quenched with 0.02 mL Na2SO3 (1 M) immediately. A high pressure liquid chromatography (Agilent 1260 Infinity, USA) equipped with a diode array detector (DAD) and a reversed-phase Poroshell 120 EC-C18 analytical column (100 mm × 4.6 mm × 2.7 μm) were used for phenol analysis. The detection wavelength of DAD was 270 nm. The injection volume was 10 μL. A mixture of 70% methanol and 30% water was used as the mobile phase at a flow rate of 1 mL/min. The temperature of the column was set at 30 °C. A pH meter (Rex DZS-708, Shanghai INESA Scientific Instrument, China) was used to measure the solution pH at the beginning and at the end of each degradation experiment.

Reaction intermediates and pathways

In order to identify the degradation byproducts during PS oxidation of phenol, batch experiments were conducted in 50 mL solution that contained 5 mM phenol, 20 mM PS, and 200 mM selected anions (CO32−, HCO3, Cl, Br, PO43−, HPO42−, and NO3) as well as in the absence of anions (control treatment). At the desired sampling times during the experiment, a 10 ml water sample was collected and adjusted to pH < 1 with sulfuric acid. Then the water sample was extracted with 1 mL dichloromethane. The dichloromethane extracts collected at different sampling times were combined and then analyzed by GC-MS (Agilent 7890-5977B, USA) equipped with an HP-5 fused silica capillary column (30 m × 0.53 mm × 1.5 μm). The carrier gas was helium (99.999%) with a flow rate of 1.5 mL/min. The temperature program was as follows: initial temperature of 40 °C held for 10 min, then increased at a rate of 10 °C/min to 300 °C, and held for 10 min. The injector temperature was 200 °C. The transfer line temperature was 250 °C. The mass detector was set at the following conditions: operated in the standard electron ionization (EI) mode of 70 eV in the 50–400 amu scan range; the ion source temperature was 230 °C; and the quadrupole temperature was 150 °C.

RESULTS AND DISCUSSION

Impacts of Cl on reaction kinetics

Cl did not affect the PS oxidation of phenol. The phenol degradation plots of 1 mM, 10 mM, and 100 mM of Cl almost overlapped with the plot of the no anion control (Figure 1(a)). The phenol concentration data can be fitted by the first-order kinetic model to calculate the pseudo-first-order degradation rate constant (kobs). The kobs of 1–100 mM of Cl had no statistically significant difference (p > 0.05) from that of the control (Figure 2(a)). The phenol removal efficiency data support the kobs data (Figure 2(b)). Reactions of Cl with SO4•− produce chlorine radicals (Cl and Cl2•− and ClOH•− in Equations (1)–(4)). Chlorine radicals are very reactive with electron rich compounds such as phenol. For example, the reaction rate of Cl2•− with phenol is as high as 4.0 × 108 M−1·s−1. The high reactivity of secondary chlorine radicals with phenol offset the inhibitory effects due to SO4•− scavenging by Cl, therefore phenol degradation was not affected by the presence of up to 100 mM of Cl.
formula
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Impacts of Br on reaction kinetics

Br of 100 mM significantly (p < 0.05) promoted the phenol degradation while the promoting effects was insignificant (p > 0.05) for 1–10 mM of Br (Figure 1(b)). The influence of Br on the kinetics of SR-AOP is rarely studied. A few available studies reported conflicting results. Wang et al. found that 500 mM of Br promoted the bleaching of azo dye Orange II by Co2+/PMS (Wang et al. 2011). De Luca et al. found that Br promoted the degradation and mineralization of nitrobenzene by UV/PS/Fe2+ while inhibiting the degradation of benzophenone-4, nitrobenzoic acid, atrazine, and ampicillin (De Luca et al. 2017). Yang et al. investigated UV/PS degradation of a mixture of benzoic acid (BA), 3-cyclohexene-1-carboxylic acid (3CCA) and cyclohexanecarboxylic acid (CCA) and found that Br inhibited the degradation of BA and CCA but did not affect the degradation of 3CCA (Yang et al. 2014). In our previous study, we investigated impacts of Br on the oxidation of a mixture of benzene, toluene, ethylbenzene, and xylenes (BTEX) by heat-activated PS (Ma et al. 2018). It was found that Br completely suppressed the degradation of benzene while 500 mM of Br strongly promoted the degradation of three xylene isomers. Br scavenges SO4•− to produce Br, which further reacts with Br and OH to produce bromine radicals (Br, Br2•−, and BrOH) (Equations (5)–(7)). Although these reactions consume reactive SO4•−, the produced bromine radicals can selectively react with electron-rich phenol, thus the overall degradation rate was increased.
formula
(5)
formula
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Figure 1

Impacts of Cl and Br on phenol oxidation by TAP. Experimental conditions: [PS]0 = 10 mM, | [Phenol] = 0.1 mM, and T = 50 °C.

Figure 1

Impacts of Cl and Br on phenol oxidation by TAP. Experimental conditions: [PS]0 = 10 mM, | [Phenol] = 0.1 mM, and T = 50 °C.

Figure 2

First-order degradation rate constant of phenol (kobs) and phenol removal efficiency after 140 min of reaction in the presence of different matrix species.

Figure 2

First-order degradation rate constant of phenol (kobs) and phenol removal efficiency after 140 min of reaction in the presence of different matrix species.

Impacts of NO3 on reaction kinetics

NO3 did not affect the phenol oxidation by PS. The phenol degradation plots of 1 mM, 10 mM, and 100 mM of NO3 overlapped with that of the no anion control (Figure 3(a)). The negligible impacts of NO3 on SR-AOP have also been reported by other studies (Yang et al. 2010; He et al. 2014; Ma et al. 2018). Theoretically, NO3 can scavenge SO4•− to produce less reactive NO3 (Equation (8)). However, this reaction is very slow with a rate constant of 5.5 × 104 M−1s−1. Therefore, the scavenging effect of NO3 can be neglected.
formula
(8)
Figure 3

Impacts of NO3 and SO32− on phenol oxidation by TAP. Experimental conditions: [PS]0 = 10 mM, [Phenol] = 0.1 mM, and T = 50 °C.

Figure 3

Impacts of NO3 and SO32− on phenol oxidation by TAP. Experimental conditions: [PS]0 = 10 mM, [Phenol] = 0.1 mM, and T = 50 °C.

Impacts of SO32− on reaction kinetics

SO32− significantly inhibited the PS oxidation of phenol. Phenol degradation was significantly delayed in the presence of 1 mM SO32−, and 10–100 mM of SO32− completely suppressed phenol degradation (Figure 3(b)). As a strong reducing agent, SO32− rapidly quenches PS to form SO42−. Based on the reaction stoichiometry, 1 mole SO32− can consume 1 mole PS. Therefore, theoretically, 1 mM SO32− could only consume 10% of 10 mM PS. In the treatment containing 1 mM SO32−, the phenol concentration did not change during the first 40 min of reaction (Figure 3(b)). After 40 min, SO32− was depleted by PS, and excessive PS can continue to oxidize phenol. Therefore, phenol concentration began decreasing after 40 min. In contrast, phenol degradation was completely suppressed in the presence of ≥10 mM SO32−. In fact, sodium sulfite is routinely used as a quenching agent to terminate the reaction between SO4•− and organic compounds in laboratory studies (Ma et al. 2018).
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Impacts of CO32− on reaction kinetics

CO32− promoted the PS oxidation of phenol, and the promoting effect was enhanced with increasing CO32− concentration (Figure 4(a)). The kobs of 1 mM, 10 mM and 100 mM of CO32− were 1.5, 1.9 and 2.7 times higher than that of the no anion control (Figure 2(a)). After 140 min of oxidation, the phenol removal efficiencies of 1 mM, 10 mM and 100 mM of CO32− were 79.8± 5.1%, 86.0 ± 0.3% and 93.5 ± 0.2% which were all significantly higher (p < 0.05) than that of the control (68.8 ± 5.8%) (Figure 2(b)).

Figure 4

Impacts of CO32− and HCO3 on phenol oxidation by TAP. Experimental conditions: [PS]0 = 10 mM, [Phenol] = 0.1 mM, and T = 50 °C.

Figure 4

Impacts of CO32− and HCO3 on phenol oxidation by TAP. Experimental conditions: [PS]0 = 10 mM, [Phenol] = 0.1 mM, and T = 50 °C.

CO32− are usually considered to be SO4•− scavengers that transform SO4•− to less reactive carbonate radical (CO3•−) through Equation (10). However, several studies show that CO3•− can selectively participate in the oxidation of the electron rich compounds (Yang et al. 2014). Phenol is a typical electron rich compound, thus CO3•− can effectively oxidize phenol. Another positive effect of CO32− is that the hydrolysis of CO32− creates a basic-pH condition (Table 1). Phenol degradation by PS was significantly promoted by the basic pH, which can be attributed to three reasons (Ma et al. 2017). The first one is that the base activates PS and increases its oxidation power (Furman et al. 2010). The second reason is that basic pH enhanced the reactivity of phenol. Phenol exists in the anionic form (called phenoxide) when the solution pH is higher than 10. Compared to phenol, phenoxide is one to seven orders of magnitude more reactive with common oxidizing radicals (Neta et al. 1988). Therefore, basic pH transforms less reactive phenol to more reactive phenoxide, thus increasing the overall reaction rate. The third reason is that phenoxide can activate PS while phenol can not. Ahmad et al. reported that phenol (pKa of 10.0) can activate persulfate at pH 12 but not at pH 8, since activation occurred only via the phenoxide form (Ahmad et al. 2013). Overall, CO32− has both negative effects (scavenging reactive SO4•−) and positive effects (creating basic pH conditions and selectively reacting with electron-rich phenol) on PS degradation of phenol. Our results suggest that the positive effects dominated over the negative ones and thus the phenol degradation was significantly promoted by increasing CO32− concentrations.
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Table 1

Changes in pH at the beginning and at the end of each degradation experiment

Inorganic anions
 1 mM
10 mM
100 mM
pHinitialpHfinalpHinitialpHfinalpHinitialpHfinal
Na2CO3 10.3 ± 0.2 9.6 ± 0.3 10.9 ± 0.3 10.7 ± 0.3 11.5 ± 0.2 11.3 ± 0.2   
NaHCO3 9.0 ± 0.1 7.4 ± 0.2 9.5 ± 0.2 9.2 ± 0.3 9.8 ± 0.2 9.5 ± 0.2   
NaNO3 4.1 ± 0.3 3.6 ± 0.2 4.2 ± 0.1 3.6 ± 0.2 6.1 ± 0.2 3.9 ± 0.1   
KCl 4.4 ± 0.2 3.6 ± 0.1 4.3 ± 0.2 3.6 ± 0.1 3.9 ± 0.1 3.5 ± 0.1   
NaBr 4.3 ± 0.1 3.6 ± 0.0 4.5 ± 0.2 3.6 ± 0.1 6.3 ± 0.3 4.2 ± 0.2   
K3PO4 10.8 ± 0.3 10.0 ± 0.3 11.7 ± 0.4 11.6 ± 0.3 12.3 ± 0.4 12.2 ± 0.2   
K2HPO4 7.8 ± 0.3 7.2 ± 0.2 8.7 ± 0.2 8.2 ± 0.2 9.1 ± 0.2 8.7 ± 0.2   
Na2SO3 7.9 ± 0.2 3.3 ± 0.1 8.6 ± 0.1 2.7 ± 0.1 8.6 ± 0.1 7.1 ± 0.3   
NOM
11.6 mg/L-TOC
23.1 mg/L-TOC
61.7 mg/L-TOC
123.4 mg/L-TOC
pHinitialpHfinalpHinitialpHfinalpHinitialpHfinalpHinitialpHfinal
NOM 3.6 ± 0.2 2.7 ± 0.2 3.7 ± 0.3 2.7 ± 0.1 4.0 ± 0.4 2.9 ± 0.2 4.3 ± 0.2 3.0 ± 0.1 
Inorganic anions
 1 mM
10 mM
100 mM
pHinitialpHfinalpHinitialpHfinalpHinitialpHfinal
Na2CO3 10.3 ± 0.2 9.6 ± 0.3 10.9 ± 0.3 10.7 ± 0.3 11.5 ± 0.2 11.3 ± 0.2   
NaHCO3 9.0 ± 0.1 7.4 ± 0.2 9.5 ± 0.2 9.2 ± 0.3 9.8 ± 0.2 9.5 ± 0.2   
NaNO3 4.1 ± 0.3 3.6 ± 0.2 4.2 ± 0.1 3.6 ± 0.2 6.1 ± 0.2 3.9 ± 0.1   
KCl 4.4 ± 0.2 3.6 ± 0.1 4.3 ± 0.2 3.6 ± 0.1 3.9 ± 0.1 3.5 ± 0.1   
NaBr 4.3 ± 0.1 3.6 ± 0.0 4.5 ± 0.2 3.6 ± 0.1 6.3 ± 0.3 4.2 ± 0.2   
K3PO4 10.8 ± 0.3 10.0 ± 0.3 11.7 ± 0.4 11.6 ± 0.3 12.3 ± 0.4 12.2 ± 0.2   
K2HPO4 7.8 ± 0.3 7.2 ± 0.2 8.7 ± 0.2 8.2 ± 0.2 9.1 ± 0.2 8.7 ± 0.2   
Na2SO3 7.9 ± 0.2 3.3 ± 0.1 8.6 ± 0.1 2.7 ± 0.1 8.6 ± 0.1 7.1 ± 0.3   
NOM
11.6 mg/L-TOC
23.1 mg/L-TOC
61.7 mg/L-TOC
123.4 mg/L-TOC
pHinitialpHfinalpHinitialpHfinalpHinitialpHfinalpHinitialpHfinal
NOM 3.6 ± 0.2 2.7 ± 0.2 3.7 ± 0.3 2.7 ± 0.1 4.0 ± 0.4 2.9 ± 0.2 4.3 ± 0.2 3.0 ± 0.1 

Impacts of HCO3 on reaction kinetics

HCO3 did not affect the PS oxidation of phenol. The phenol degradation plots in the presence of 1 mM, 10 mM and 100 mM of HCO3 almost overlapped with that of the no anion control (Figure 4(b)). Conflicting results are reported on the influence of HCO3 on SR-AOP. Many studies report that HCO3 reduces the contaminant oxidation rate by reacting with reactive SO4•− to form less reactive bicarbonate radicals (HCO3) (Equation (11)) (Nie et al. 2014; Luo et al. 2016; Yang et al. 2017). However, several recent studies demonstrate that HCO3 selectively oxidizes electron rich compounds with relatively fast rate (Yang et al. 2014). Since phenol is an electron rich compound, the high reaction rate of HCO3 with phenol may offset the negative effects due to SO4•− scavenging, therefore HCO3 does not affect phenol oxidation.
formula
(11)

Impacts of PO43− on reaction kinetics

PO43− significantly (p < 0.05) promoted phenol degradation and the promoting effect was enhanced with increasing PO43− concentrations (Figure 5(a)). The kobs of 1 mM, 10 mM and 100 mM of PO43− were 1.6, 2.0 and 3.9, times higher than that of the control (Figure 2(a)). The phenol removal efficiency data corroborated the kobs data (Figure 2(b)). Similar to CO32−, the hydrolysis of PO43− created a strong basic condition (Table 1). As discussed previously, basic pH significantly enhanced the rate of phenol degradation by PS. Higher PO43− concentration resulted in higher solution pH and thus faster phenol removal. Although PO43− scavenges SO4•− to form phosphate radical (PO4•2−) (Equation (12)), the rate of the reaction of PO4•2− with phenoxide is relatively high (5.9 × 108 M−1·s−1, Equation (13)) (Neta et al. 1988), which significantly offset negative effects of SO4•− scavenging. This also contributed to the increases in phenol degradation rate in the presence of PO43−.
formula
(12)
formula
(13)
Figure 5

Impacts of PO43− (a), HPO42− (b) and NOM (c) on phenol oxidation by TAP. Experimental conditions: [PS]0 = 10 mM, [Phenol] = 0.1 mM, and T = 50 °C.

Figure 5

Impacts of PO43− (a), HPO42− (b) and NOM (c) on phenol oxidation by TAP. Experimental conditions: [PS]0 = 10 mM, [Phenol] = 0.1 mM, and T = 50 °C.

Impacts of HPO42− on reaction kinetics

HPO42− did not affect the PS oxidation of phenol. The phenol degradation plots 1–100 mM of HPO42− almost overlapped with the plot of the no anion control (Figure 5(b)). Both kobs data and the phenol removal efficiency data of 1–100 mM of HPO42− had no statistically significant difference (p > 0.05) with those of the control (Figure 2). Several studies reported negligible influence of HPO42− on PS-AOP (Yang et al. 2011; Nie et al. 2014). SO4•− can be scavenged by HPO42− to produce HPO4•− (Equation (14)). The negligible influence of HPO42− may be due to the relatively high reactivity of HPO4•− with phenol.
formula
(14)

Impacts of NOM on reaction kinetics

The treatment containing higher concentrations of NOM had significantly lower (p < 0.05) phenol degradation rate and significantly lower (p < 0.05) phenol removal efficiency (Figures 2 and 5(c)). Electrophilic radicals such as SO4•− can easily attack the electron-rich moieties within NOM molecular structure (Equation (15)). Such reaction consumes reactive SO4•− and reduces the overall oxidation potential. Higher concentration of NOM leads to more SO4•− scavenged and lower PS oxidation rate. The inhibitory impacts of NOM on SR-AOP have also been reported by many studies (Nie et al. 2014; Jiang et al. 2016; Luo et al. 2016; Oliveira et al. 2016; Ferreira et al. 2017; Ma et al. 2018).
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(15)

Formation of undesired degradation byproducts and the proposed reaction pathways

In the control treatment without anions, six intermediates were identified: p-benzoquinone, 2-phenoxy-phenol, 4-phenoxy-phenol, 2,2′-dihydroxybiphenyl, 3,3′-dihydroxybiphenyl, and 4,4′-dihydroxybiphenyl (Table 2). Based on the above intermediates identified and the data from previous studies (Anipsitakis et al. 2006), the possible reaction pathway of PS oxidation of phenol is proposed as follows. The first step is the addition of a SO4•− to the aromatic ring, which results in an unstable form (P1) (Figure 6(a)). Since sulfate is an excellent leaving group, the hydroxycyclohexadienyl radical (P2) is formed via elimination of the sulfate group. Hydrolysis of the hydroxycyclohexadienyl radical (P2) leads to the formation of hydroxylated radical products, which further react with O2 to form the stable intermediate hydroquinone (P3) (Anipsitakis et al. 2006). Quinone (P4) is then generated via hydrogen abstraction from hydroquinone (P3). Two hydroxycyclohexadienyl radicals (P2) may also combine with each other to generate dihydroxybiphenyls (P5, P6, and P7). An alternative pathway for the first step is that SO4•− reacts with phenol to form hydroxylated radicals (P8) via H abstraction. A hydroxylated radical may react with a hydroxycyclohexadienyl radical to form phenoxy-phenols (P9 and P10).

Table 2

Degradation intermediates identified in the control without anions and in the treatments containing halidesa

PS + phenolPS + phenol + ClPS + phenol + Br
   
PS + phenolPS + phenol + ClPS + phenol + Br
   

aThe GC-MS spectrum for each halogenated intermediate can be found in the supplementary materials (available with the online version of this paper).

Figure 6

Reaction pathway in the control treatment without anions (a) and in the treatment containing halides (b).

Figure 6

Reaction pathway in the control treatment without anions (a) and in the treatment containing halides (b).

In the treatment containing Cl, GC-MS detected three chlorinated intermediates (2-chlorophenol, 4-chlorophenol, and 2, 4-dichlorophenol) and three non-halogen intermediates (p-benzoquinone, 2-phenoxy-phenol and 2,2′-dihydroxybiphenyl) (Table 2). In the treatment containing Br, GC-MS detected eight brominated intermediates (2-bromophenol, 4-bromophenol 2,4-dibromophenol, 2,6-dibromophenol, 2,4,6-tribromophenol, 2,6-dibromohydroquinone, 2,4,6-tribromo-1,3-benzenediol, and tetrabromocatechol) and the same three non-halogen intermediates as in the treatment containing Cl (Table 2). As discussed previously, Cl and Br can be oxidized by SO4•− through a series of chain reactions to generate reactive halogen radicals (e.g., X, X2•−, and XOH•−). Given the relatively high concentration of Cl and Br, a significant fraction of SO4•− in the reaction solution was expected to be transformed to halogen radicals. It was reported that the concentration of generated halogen radicals could exceed that of SO4•− by several orders of magnitude (Yang et al. 2014). Halogen radicals are selective oxidants that can be involved in H abstraction, electron transfer, or addition reactions with organic compounds. It is generally accepted that H abstraction and electron transfer contribute to the mineralization of contaminants, whereas addition reaction leads to the formation of halogenated compounds. As previously discussed, SO4•− can attack the aromatic ring of phenol to form an unstable intermediate P1, which is then transformed to a hydroxycyclohexadienyl radical (P2) via elimination of the sulfate group (Figure 6(b)). The combination of a halogen radical with a hydroxycyclohexadienyl radical leads to the formation of monohalogenated byproducts (P11, P12 and P13). Further halogenation leads to the formation of dihalogenated byproducts (P14 and P15) and trihalogenated byproducts (P16) (Anipsitakis et al. 2006; Liu et al. 2015). An alternative pathway is that the hydrolysis of the hydroxycyclohexadienyl radical (P2) leads to the formation of hydroxylated radical products which further react with O2 to form the stable intermediate hydroquinone (P3). Hydroquinone (P3) could react with halogen radicals to form halogenated hydroquinones (P17, P18, and P19). Formation of halogenated intermediates during SR-AOP oxidation of organic contaminants other than phenol have also been reported by other studies (Wang et al. 2014; Liu et al. 2015; Lu et al. 2015).

As discussed previously, reaction of SO4•− with anions other than halides may also generate secondary radicals such as NO3, CO3•−, HCO3, PO4•2−, and HPO4•−. Theoretically, these secondary radicals may also be involved in the radical chain propagation, thus influencing the byproduct distribution (Ji et al. 2017). Ji et al. found that nitrobenzene oxidation by heat-activated PS generated a series of byproducts including mononitrophenols, dinitrophenols, trinitrophenols, and coupling products (Ji et al. 2017). Formation of nitrated byproducts indicates that both denitration and renitration processes occur during PS oxidation of nitrobenzene. However, in this study, no new intermediates were identified in the treatment containing CO32−, HCO3, PO43−, HPO42−, and NO3 except for those intermediates detected in the no anion control. We acknowledge that absence of evidence is not evidence of absence. Therefore, further studies using more advanced analytical tools (e.g., high resolution mass spectrometer) are needed to have a better understanding of this problem.

CONCLUSION

This is the first study that has a comprehensive investigation on the impacts of eight anions (CO32−, HCO3, Cl, Br, PO43−, HPO42−, NO3, and SO32−) and NOM on the reaction kinetics and the formation of undesired degradation byproducts during phenol oxidation by SR-AOP. We found that different matrix species have different impacts on the kinetics of phenol oxidation by heat-activated PS. CO32−, PO43−, and Br promoted the reaction and the promoting effects were enhanced with increasing anion concentrations. NOM inhibited the reaction and the inhibiting effect was enhanced with increasing NOM concentrations. SO32− with concentrations higher than 10 mM completely suppressed the reaction. HCO3, Cl, NO3 and HPO42− with the concentrations of 1–100 mM did not affect the reaction. In addition to the kinetic study, formation of degradation byproducts in the no anion control treatment and in the treatment containing CO32−, HCO3, Cl, Br, PO43−, HPO42−, and NO3 were screened by GC-MS. Six intermediates were detected in the no anion control (p-benzoquinone, 2-phenoxy-phenol, 4-phenoxy-phenol, 2,2′-dihydroxybiphenyl, 3,3′-dihydroxybiphenyl, and 4,4′-dihydroxybiphenyl). A variety of toxic halogenated phenols and hydroquinones were detected in the treatment containing Cl and Br. However, in the treatment containing CO32−, HCO3, PO43−, HPO42−, and NO3, no new intermediates were identified except for the intermediates already detected in the no anion control. Based on the intermediates identified, reaction pathways for PS oxidation of phenol without anions and in the presence of halides were proposed respectively. Overall, we found that phenol can be effectively removed by heat-activated persulfate, but water matrix species significantly affected the reaction kinetics and may even produce halogenated byproducts. Because many halogenated organic compounds are toxic and/or refractory, great attention should be paid when SR-AOPs are used for wastewater treatment or soil and groundwater remediation when there are high levels of halides.

ACKNOWLEDGEMENTS

This study was financially supported by Beijing NOVA program (Grant No. Z181100006218088), Science Foundation of China University of Petroleum-Beijing (Grant No. 2462018BJC003), and the Independent Project Program of State Key Laboratory of Petroleum Pollution Control (Grant No. PPCIP2017004), CNPC Research Institute of Safety & Environment Technology.

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Supplementary data