The chlorobenzene (CB) degradation performances by various oxidants, including hydrogen peroxide (H2O2), nanoscale calcium peroxide (nCaO2) and sodium percarbonate (SPC), activated with ferrous iron (Fe(II)) were investigated and thoroughly compared. The results showed that all tested systems had strong abilities to degrade CB. The CB removal rate increased with increasing dosages of oxidants or Fe(II) because the generation of reactive oxygen species could be promoted with the chemical dosages' increase. Response surface and contour plots showed that CB could achieve a better removal performance at the same H2O2 and Fe(II) molar content, but the Fe(II) dosage was higher than that of oxidants in the nCaO2 and SPC systems. The optimal molar ratios of H2O2/Fe(II)/CB, nCaO2/Fe(II)/CB and SPC /Fe(II)/CB were 5.2/7.6/1, 8/8/1, and 4.5/8/1, respectively, in which 98.1%, 98%, and 96.4% CB removals could be obtained in 30 min reaction. The optimal pH condition was around 3, while CB removal rates were less than 20% in all three systems when the initial pH was adjusted to 9. The oxidative hydroxyl radicals (HO•) and singlet oxygen (1O2) had been detected by the electron paramagnetic resonance test. Based upon the results of liquid chromatograph-mass spectrometer analysis, the pathways of CB degradation were proposed, in which 1O2 roles were elaborated innovatively in the CB degradation mechanism. The CB degradation performance was significantly affected in actual groundwater, while increasing the molar ratio of oxidant/Fe(II)/CB was an effective way to overcome the adverse effects caused by the complex of actual groundwater matrix.

• Comparison of the H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems on chlorobenzene degradation.

• HO· and 1O2 of the H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems have been investigated.

• The pathways of chlorobenzene degradation were proposed involving HO· and 1O2.

• The influence of actual groundwater have been studied.

Chlorobenzene (CB) is widely used in printing, pesticides, plastics and pharmaceutical industries, and the contamination of groundwater with CB has become a serious environmental problem. CB has been identified as one of the priority pollutants by the US Environmental Protection Agency (EPA) and is also black-listed by the China National Environmental Monitoring Center (China CNEMC) because of its characteristics of carcinogenicity, teratogenicity and neurotoxicity (Zhang et al. 2011). CB poses a serious threat to human health due to bio-accumulation (Jiang et al. 2020). It was reported that the total hydrocarbons of 980.20 mg L−1 were detected in the groundwater of an abandoned petrochemical plant in Lanzhou, China, where C6-C9 alkanes containing CB make up 95.3% of the total hydrocarbons (Sun et al. 2017).

The current techniques for CB removal mainly involve adsorption, biological methods, and advanced oxidation processes (AOPs), as reported in literatures (Soto et al. 2011; Cheng et al. 2015; Han et al. 2020). AOPs, including the Fenton process, were developed to destroy contaminants with high efficiency. The AOPs could be divided by the following subsections: hydrogen peroxide (H2O2), ozone (O3), peroxone (the combination of H2O2 and O3), persulfate (S2O82−), peroxymonosulfate (HSO5) and hydrodynamic cavitation (Boczkaj & Fernandes 2017; Gagol et al. 2018). Sulfate radical (SO4) and HO• from peroxymonosulfate were the main reason for pollutant degradation. Novel catalysts such as asphaltenes, monolith catalysts with 3D hierarchical structures, and carbon black nano-spheres had been reported in the activation of peroxymonosulfate (Fedorov et al. 2020; Soltani et al. 2020; Yuan et al. 2020). Hydrodynamic cavitation with external oxidants was confirmed as much more effective in the degradation of effluents polluted by brilliant cresyl blue dye, sulfide ions, and organic sulfides (Gagol et al. 2019; Elvana et al. 2020). Gagol et al. (2020) reported that hydrochloric acids could improve in some manner the pollutant degradation effectiveness and nitrate ions could promote heterocyclic compounds degradation in the hydrodynamic cavitation system. On the other hand, H2O2 is a strong oxidant and has a high standard potential (1.8 V) (Wang & Xu 2012; Oturan & Aaron 2014). Calcium peroxide (CaO2) and sodium percarbonate (SPC) are considered alternative and environmentally friendly oxidants because they can release oxygen (O2) and H2O2 when dissolved in water (Qian et al. 2013; Danish et al. 2017). Since CaO2 and SPC have similar chemical properties and various advantages, they were used in many aspects instead of H2O2. In recent years, several types of catalysts, such as TiO2 and ZnO, have also been applied in the H2O2-based Fenton system (Fernandes et al. 2019). Fernandes et al. (2020) reported that a TiO2/UV/O3/H2O2 system could effectively reduce chemical oxygen demand and destroy volatile organic compounds in wastewater.

In our previous study, nanoscale CaO2 particles (nCaO2) were synthesized for remediating BTEX and achieved better results than commercial CaO2 (Sun et al. 2019a, 2019b). SPC can also be activated by transition metals (e.g. iron) and produce reactive oxygen species (ROSs) to degrade hydrocarbons in groundwater (Fu et al. 2018). It has been widely reported that HO• was the dominant active species in the degradation of most contaminants in H2O2, CaO2, or SPC systems (Neyens & Baeyens 2003; Xue et al. 2016; Danish et al. 2017). However, 1O2 is the lowest excited state of molecular oxygen that can be generated from H2O2, CaO2, or SPC as well (Maetzke & Jensen 2006; Zhang et al. 2015a; Tian et al. 2017), and it is reported that 1O2 is involved in the efficient degradation of aqueous organic contaminants through the oxidation reaction (Alberti & Orfanopoulos 2010; Liu et al. 2018; Zheng et al. 2019; Huang et al. 2020; Tang et al. 2020). Though different radicals were confirmed and detected in these Fenton processes, HO• was commonly deemed to be the dominant radical and other ROSs took part in partial degradation of pollutants (He et al. 2016).

H2O2, CaO2, and SPC are highly efficient oxidizers that can oxidize and degrade pollutants in groundwater due to their similar oxidation properties. However, their performances in pollutant degradation in groundwater remediation are rarely compared, particularly how 1O2 takes part in CB degradation is still unclear yet. Therefore, in this study, the H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems were applied in the remediation of contaminated groundwater and CB was selected as the target pollutant. The purposes of this research are (1) to compare the performance of CB degradation in the H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems, (2) to explore the ROSs responsible for CB degradation in the above three systems, (3) to specify the degradation pathways of CB, and (4) to evaluate the application potential of these systems for CB removal in the actual groundwater remediation implementation.

### Materials

Chlorobenzene (C6H5Cl, 99.5%), hydrogen peroxide (H2O2, 30%wt), ferrous sulfate heptahydrate (FeSO4•7H2O, 99.0%), calcium chloride (CaCl2, 96%), polyethylene glycol 200 (PEG 200, 99.0%), ammonia (NH3•H2O, 28%), sodium percarbonate (2Na2CO3•3H2O2, 99%), sulfuric acid (H2SO4, 98%) and sodium hydroxide (NaOH, 96.0%) were purchased from Aladdin Reagent Co. Ltd (Shanghai, China). Potassium biphthalate (C8H5KO4, 99.8%), o-phenanthroline (C12H8N2, 97%), ethyl acetate (C4H8O2, 99.8%), 5,5-dimethyl-1-pyrroline N-oxide (C6H11NO, DMPO, 99.0%), 2,2,6,6-tetramethylpiperidine (C9H19N, TEMP, 99.0%) were purchased from Shanghai Lingfeng Reagent Co. Ltd (Shanghai, China). The ultrapure water was provided by a Milli-Q water purification system (Classic DI; ELGA, Marlow, UK). The actual groundwater from a well approximately 15 m deep below the surface (Songjiang, Shanghai, China) was used for preparing the actual CB contaminated solutions.

### Preparation of nCaO2

The preparation method of nCaO2 can be found in our previous paper (Sun et al. 2019a, 2019b). Briefly, CaCl2 was as the precursor and H2O2 was added drop-by-drop in the presence of the surface stabilizer PEG 200 (Supplementary Materials Text S1).

### Experimental set-up

The concentration of CB solution in all tests was set at 1.0 mM. CB and ultrapure water or actual groundwater were stirred over 8 h at room temperature (20 °C) to mix the solution homogenously before tests. The batch experiments were conducted by adding the predetermined dosages of chemicals in a glass reactor having a volume of 250 mL with magnetic stirring in three systems (H2O2/Fe(II), nCaO2/Fe(II), and SPC/Fe(II)). The initial pH of the solution was adjusted after the dissolution of Fe(II) but before adding oxidant using 1.0 M H2SO4 or NaOH. At the desired intervals, 10 mL samples were transferred rapidly into the headspace vials containing 3.0 g dry NaCl. Then the vials were sealed quickly for analysis by gas chromatography (Agilent, China) coupling with a headspace auto-sampler, a flame ionization detector (FID), and a HP-5 column (30 m × 0.32 mm × 0.25 μm) as described in Supplementary Materials (Text S2). Total organic carbon (TOC) was determined by using a TOC analyzer (LiquiTOC, Germany).

The samples were extracted at 3 min after the start of the reaction for the analysis of intermediates during CB degradation. 150 mL ethyl acetate was mixed with 150 mL of the reaction solution and rested for 5 min. The top layer ethyl acetate mixed solution was condensed to 1.5 mL by a Rotary Evaporator (N-1300D; Eyela, Japan). The condensed solution was filtered by an organic-phase filter and tested by an LC-MS (Q-Exactive plus; ThermoFisher, China) (Supplementary Materials Text S2). At 3 min after the start of the reaction, 1.0 mL samples were withdrawn from the reactor and mixed with 1.0 mL DMPO (20.0 mM) or TEMP (20.0 mM) for 1 min, then the mixed samples were analyzed by EPR (EMX-8/2.7, Bruker, U.S.) for the detection of ROSs. The DMPO-OH and TEMP-1O2 were monitored at the setting for the EPR spectrometer of center field (3,510.00 G), microwave frequency (9.79 GHz), and power (5.05 mW). The CB degradation test in the actual groundwater was carried out by replacing ultrapure water with actual groundwater. The middle-flow groundwater was collected from a deep well and saved in acid-washed clean polyethylene bottles and groundwater was immediately characterized in the lab.

### Analytical methods

The CB contaminant removal rate was computed using the following formula (Equation (1)):
(1)
where Ci and C0 (mg L−1) represent the target contaminant concentration at a certain time and initial concentration, respectively.
Based on the CB removal pattern, CB degradation is well fixed to a second-order reaction, which was described as Equations (2) and (3) (Xue et al. 2016):
(2)
(3)
where kobs (M−1 min−1) and t (min) indicated the rate constant and the degradation time, respectively (Tang et al. 2020).

A central composite design (CCD) based on the response surface methodology (RSM) was used for CB degradation analysis (Dong et al. 2016). The dosage of oxidants (H2O2, nCaO2, or SPC) and the dosage of Fe(II) were considered as the X1 and X2. The degradation time was considered as the X3. 51 groups of experiments were tested and the ranges of dosages are displayed in Table 1.

Table 1

Ranges of dosages used in the CB degradation experiment

VariablesCodeLowHighUnits
H2O2 X1 1.0 8.0 mM
nCP X1 1.0 8.0 mM
SPC X1 1.0 8.0 mM
Fe(II) X2 1.0 8.0 mM
Time X3 30 min
VariablesCodeLowHighUnits
H2O2 X1 1.0 8.0 mM
nCP X1 1.0 8.0 mM
SPC X1 1.0 8.0 mM
Fe(II) X2 1.0 8.0 mM
Time X3 30 min

### Performance of CB degradation in the H2O2, nCaO2 or SPC oxidant system

The control tests were carried out under the same condition with CB degradation test without oxidants or Fe(II), and the results showed less than 5% CB escaped by volatilization from the entire experimental procedure. The CB degradation performances in H2O2/Fe(II), nCaO2/Fe(II), and SPC/Fe(II) systems at the oxidant/Fe(II)/CB molar ratios ranging from 1/1/1 to 8/8/1 with the initial CB concentration of 1.0 mM, were compared and the results are presented in Figure 1(a). It was obvious that H2O2, nCaO2, and SPC showed a strong ability to degrade CB in the experimental conditions, and CB was almost completely degraded (99.4%, 98.5%, and 99.8%, respectively) at the oxidant/Fe(II)/CB molar ratio of 8/8/1 in 30 min. It was noteworthy that CB degradation by H2O2 was better than SPC and nCaO2 at the same oxidant/Fe(II)/CB molar ratios and nCaO2 showed a slightly lower CB removal rate. This was most probably due to the slow release of O2 from nCaO2 upon dissolution in water, causing nCaO2 consumption (Wang et al. 2016). Moreover, Ca(OH)2, the final product of CaO2 after reaction with water, increased the pH of the reaction solution to restrain the effect of the Fenton reaction, though the release of H2O2 can be regulated by adjusting pH (Pan et al. 2018). The effect of initial pH on CB removal is discussed in the following sections. SPC releases H2O2 and Na2CO3 at the same time and similarly creates an alkaline environment under aqueous conditions (Li et al. 2019). Zhang et al. (2017b) reported that CB was completely removed at the molar ratio 8/8/1 of SPC/Fe(II)/CB. It also has been reported that H2O2/Fe0, Fe0/H2O2/Fe3+, and Cu2+/Fe0/H2O2 systems are effective for degrading CB (Pagano et al. 2011). TOC test results showed that TOC removal could achieve 83.4%, 74.5% and 76.0% at the oxidant/Fe(II)/CB molar ratio of 8/8/1 in H2O2/Fe(II)/CB, nCaO2/Fe(II)/CB, and SPC/Fe(II)/CB systems, respectively (Supplementary Materials Figure S1). Hence, Fenton or nCaO2 and SPC based Fenton-like processes were available for the degradation of CB under aqueous conditions.

Figure 1

CB removal performance in H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems at different oxidants/Fe(II)/CB molar ratios ([chlorobenzene] = 1.0 mM; time = 30 min).

Figure 1

CB removal performance in H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems at different oxidants/Fe(II)/CB molar ratios ([chlorobenzene] = 1.0 mM; time = 30 min).

Close modal

In order to investigate the effect of H2O2, nCaO2, SPC dosage on CB degradation performance, the experiments were further carried out in the different systems and the results are evaluated in Figure 1(b) and 1(c). Notably, CB degradation was increased with the increase in dosage of H2O2, nCaO2, and SPC, while keeping iron ions constant. Increasing the dosage of nCaO2 and SPC could increase the H2O2 concentration in the reaction solution and promote the occurrence and propagation of free-radical reactions. Besides, a high concentration of oxidants could not only increase the yield of HO•, but also increase the generation of O2 and HO2, and therefore provide a variety of ways of CB removal (Honetschlagerova et al. 2019). The removal rate of CB increased from 12, 55 and 32% to 100, 98 and 99% at molar ratios ranging from 1/8/1 to 8/8/1 in the H2O2/Fe(II)/CB, nCaO2/Fe(II)/CB and SPC/Fe(II)/CB systems, respectively. Under low oxidant concentrations, the yield of HO• was inadequate to completely degrade CB. Meanwhile, excess iron consumed HO• in the systems (Venny et al. 2012). However, at a fixed oxidant amount, CB removal rates were increased with an increase in Fe(II) concentration (oxidants/Fe(II)/CB molar ratio from 8/1/1 to 8/8/1) in all three systems. When the oxidants/Fe(II)/CB molar ratio was 8/1/1, CB removal rates were 81.2%, 4.7%, and 33.6% in H2O2/Fe(II), nCaO2/Fe(II), and SPC/Fe(II) systems respectively. Hence, increasing the concentration of Fe(II) could effectively improve the utilization of H2O2, nCaO2, and SPC, and these conclusions were consistent with previous studies (Li et al. 2017; Zhang et al. 2017a; Huang et al. 2020). It was noteworthy that changing the iron concentration showed the most serious impact on CB removal in the nCaO2/Fe(II) system. The possible reason was that the OH produced from Ca(OH)2 precipitated some iron and reduced the catalytic capacity of the system accordingly.

Figure 2 displays the CB removal rate constants in different chemical dosages in H2O2/Fe(II) and nCaO2/Fe(II) systems. Based on the calculation, the removal of CB followed the second-order kinetics reaction (Xue et al. 2018; Li et al. 2020). The kobs was the second-order reaction rate constant in different systems that represented the overall rate of CB degradation by a variety of oxidizing species generated. The value of kobs in the H2O2/Fe(II) system was higher than that in the nCaO2/Fe(II) system at the same nCaO2/Fe(II) molar ratio (1/1), suggesting that CB had a faster degradation rate in the H2O2/Fe(II) system than in the nCaO2/Fe(II) system. It was also observed that kobs increased from 0.0336 to 1.3408 M−1 min−1 in the nCaO2/Fe(II) system with the increase of nCaO2/Fe(II)/CB molar ratio, mainly due to the fast generation of reactive species at the high dosage of oxidants and catalysis.

Figure 2

Rate constants of CB degradation under different dosages in H2O2/Fe(II) and nCaO2/Fe(II) systems ([chlorobenzene] = 1.0 mM; temperature = 25 °C).

Figure 2

Rate constants of CB degradation under different dosages in H2O2/Fe(II) and nCaO2/Fe(II) systems ([chlorobenzene] = 1.0 mM; temperature = 25 °C).

Close modal

### Response surface and contour plots

Figure 3 illustrates the response and contour plots of CB removal with different dosages of oxidant, Fe(II) and CB. Overall, the degradation of CB increased along with the increase of Fe(II) concentration and oxidant dosage. But, in H2O2/Fe(II) system, the influence on CB degradation of two factors (X1 and X2) was the same, revealing that CB had a better removal rate at the same H2O2 and Fe(II) content, and increasing H2O2 or Fe(II) alone could not always be efficient in achieving a higher CB removal. However, in nCaO2/Fe(II) and SPC/Fe(II) systems, the improvement of CB degradation rate did not follow the above rules. The effect of iron dosage on CB degradation was higher than that of oxidants.

Figure 3

Response surface (a) and contour plot (b) for H2O2 (X1) and Fe(II) (X2) interaction; response surface (c) and contour plot (d) for nCaO2 (X1) and Fe(II) (X2) interaction; response surface (e) and contour plot (f) for SPC (X1) and Fe(II) (X2) interaction (time = 30 min).

Figure 3

Response surface (a) and contour plot (b) for H2O2 (X1) and Fe(II) (X2) interaction; response surface (c) and contour plot (d) for nCaO2 (X1) and Fe(II) (X2) interaction; response surface (e) and contour plot (f) for SPC (X1) and Fe(II) (X2) interaction (time = 30 min).

Close modal

Fe(II) was an essential condition for generating ROSs. Inadequate Fe(II) made the Fenton reaction be restricted in generating ROSs and the removal of CB was inhibited consequently. When there was excess Fe(II) in the system, Fe(II) could rapidly catalyze H2O2 and transform into Fe(III) (Zazo et al. 2016; Christoforidis et al. 2018). A large number and instantaneous generation of HO• may be lost due to collision with each other instead of reaction with CB. In the nCaO2/Fe(II) and SPC/Fe(II) systems, the effect of iron on the degradation of CB was more significant than that of oxidants, indicating that the degradation of the same amount of CB required more iron under the same oxidant dosage. OH produced in these two systems was the reason why iron was consumed in large quantities (Fu et al. 2017; Xue et al. 2019). The optimal molar ratios of H2O2/Fe(II)/CB, nCaO2/Fe(II)/CB and SPC/Fe(II)/CB were 5.2/7.6/1, 8/8/1, and 4.5/8/1 with 98.1%, 98%, and 96.4% CB removals in 30 min, respectively, based on the calculation by RSM.

### Effect of initial solution pH on CB degradation

The experiments were carried out with adjusted initial solution pH of 3, 6 and 9 by keeping the oxidants/Fe(II)/CB molar ratio at 8/8/1 and the initial concentration of CB at 1.0 mM. The solution pH was adjusted after adding Fe(II) but before oxidants by 0.1 M H2SO4 or NaOH. CB removal rates in 30 min and the final solution pH after reaction are presented in Figure 4. Overall, H2O2, nCaO2, and SPC showed an excellent removal effect on CB in a wide pH range (3–6). However, CB removal rates were less than 20% in all three systems when the initial pH was adjusted to 9. The results showed that low pH was favorable for CB degradation, and the removal efficiency of CB was significantly affected by high initial pH in the H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems. These results were consistent with the Fenton or Fenton-like reaction. Fenton reaction is affected by solution pH due to the nature of H2O2 and Fe(II), while the optimum pH condition was 3 (Wang 2008; Sarmento et al. 2016).

Figure 4

Effect of initial solution pH on CB degradation in H2O2/Fe(II) system (a), nCaO2/Fe(II) system (b) and SPC/Fe(II) system (c). ([CB] = 1.0 mM).

Figure 4

Effect of initial solution pH on CB degradation in H2O2/Fe(II) system (a), nCaO2/Fe(II) system (b) and SPC/Fe(II) system (c). ([CB] = 1.0 mM).

Close modal

The species of Fe(II) in the Fenton reaction was strongly influenced by pH of the solution (Zhang et al. 2015a, 2015b). A high pH was not conducive to the presence of dissolved Fe ions in solution, which led to a reduction in the catalytic capacity of the system. Appropriate measures could be considered to avoid Fe(OH)3 precipitation in order to improve the removal efficiency of pollutants. Moreover, under high pH conditions, H2O2 was easily decomposed into H2O and O2, which was not conducive to the generation of HO• in the H2O2/Fe(II) system (Chu 2001). CaO2 reacted with H2O to produce O2 and Ca(OH)2 under high pH conditions, which led to limited H2O2 production in the nCaO2/Fe(II) system (Zhang et al. 2015a, 2015b). Similarly, precipitation in the form of Fe(OH)3 also reduced the yield of HO• in the SPC/Fe(II) system (Cui et al. 2016).

### Possible pathways and intermediates during CB degradation

HO• is a relatively indiscriminate oxidant with a high oxidation potential (2.76 V) which can be trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) with the electron paramagnetic resonance (EPR) spin-trapping method (Li et al. 2007; Che et al. 2011). Compared with HO•, 1O2 has a lower oxidation potential (0.65 V) and a short lifetime in pure water (4 μs) (Fernandez-Castro et al. 2015), and 1O2 could be trapped by 2,2,6,6-tetramethylpiperidine (TEMP) to generate TEMP-1O2 adduct, which can be detected using an ESR instrument (Chen et al. 2015).

EPR has been designed for the detection of reactive species using DMPO and TEMP as the spin-trapping agent to identify HO• and 1O2 respectively, and the results are presented in Figure 5. As illustrated in Figure 5, both HO• and 1O2 could be generated in H2O2/Fe(II), nCaO2/Fe(II), and SPC/Fe(II) systems with characteristic intensities of 1:2:2:1 and 1:1:1. The EPR results indicated that nCaO2 and SPC are effective alternatives for Fenton-like processes. Previous studies confirmed that HO• and 1O2 could be generated in all three systems and HO• was the dominant active species (Yang et al. 2013; Bokare & Choi 2014; Fu et al. 2015; Zhang et al. 2015a, 2015b). The presence of HO• and 1O2 in these systems indicated that HO• and 1O2 an may contribute to CB degradation. Studies have confirmed that a portion of generated HO• was converted to 1O2 and the remainder radicals participated in CB degradation (Zhang et al. 2017a, 2017b; Yang et al. 2019). 1O2, an energy-rich active oxygen species, is an excited-state species of oxygen that has high selectivity and unique reactivity with organic pollution such as oledefins, conjugated dienes, and phenols (Catir 2017). The 1O2 encounters the phenols in its addition to form hydroperoxide ketones (Equation (4)), which makes it play an important role in the degradation of aromatic compounds (Canonica & Tratnyek 2003; Batiha et al. 2012).

Figure 5

ESR spectra recorded about (a) DMPO-OH and (b) TEMP-1O2 in H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems at 3 min at the oxidant/Fe(II)/CB molar ratio of 4/4/1. ([CB] = 1.0 mM).

Figure 5

ESR spectra recorded about (a) DMPO-OH and (b) TEMP-1O2 in H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems at 3 min at the oxidant/Fe(II)/CB molar ratio of 4/4/1. ([CB] = 1.0 mM).

Close modal

(4)

To further confirm HO• and 1O2 participation in CB degradation and to clarify the possible degradation pathway of CB in the H2O2/Fe(II), nCaO2/Fe(II), and SPC/Fe(II) systems, several intermediate byproduct compounds along with CB degradation were detected by LC-MS. The results are described in Supplementary Materials (Figure S2). The major intermediate byproduct compounds detected in the H2O2/Fe(II) system, nCaO2/Fe(II) system, and SPC/Fe(II) system were same. Several intermediates were detected including phenol (C6H6O), hydroquinone (C6H6O2), 1,2,4-benzenetriol (C6H6O3), chlorophenol (C6H5OCl), 2-chlorohydroquinone (C6H5O2Cl), benzoquinone (C6H4O2), o-benzoquinone (C6H4O2), hydroperoxide ketones (C6H5O3), allylic hydroperoxides (C6H4O3Cl) and their isomers. The difference is that more short-chain alkanes and alkenes, such as decane (C10H22) and hexene (C6H12), were detected in the H2O2/Fe(II) system. The reasons for this difference may be the different reaction rates and different levels of mineralization.

Base on the LC-MS results and the information previously reported (Jiang et al. 2002; Li et al. 2005; Liu & Jiang 2008; Wu et al. 2009; Zazou et al. 2016), the CB degradation mechanism in these three systems was postulated (Figure 6). First, a portion of generated HO• attacked the benzene ring to form chlorophenol and isomers. Then chlorophenol continued to be attacked by HO• to form chlorohydroquinone and isomers. Part generated HO• was transformed to 1O2 at the same time. The dechlorination of CB occurred in the form of phenol, hydroquinone, and pyrogallol before the ring-opening reactions. Then, phenol and hydroquinone were converted into benzoquinone and o-benzoquinone, which were prone to ring-opening reaction and were further oxidized to aliphatic compounds such as hexendioic acid and succinic acid. These aliphatic compounds were mineralized into H2O and CO2 finally.

Figure 6

Proposed CB degradation and transformation pathways in H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems.

Figure 6

Proposed CB degradation and transformation pathways in H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems.

Close modal

Both HO• and 1O2 participated in the process simultaneously. Phenol was attacked by 1O2 and converted to hydroperoxide ketones. 1O2 attacked the chlorophenol, leading to the formation of four types of unsaturated compounds, namely allylic hydroperoxides, 1,4-peroxides, 1,2-peroxides, and hydroperoxide ketones. These unsaturated compounds were also mineralized into H2O and CO2 finally. It has also been reported that the H2O2/Fe(II) system and thermally-activated persulfate system were effective in the degradation of CB in aqueous solution (Pagano et al. 2011; Luo 2014). CB degradation in these processes was initially oxidized to chlorophenol, and then to other intermediate byproducts such as p-benzoquinone, hydroquinone, phthalic acid, acetic acid and dicarboxyl (Ouyang et al. 2019), and the intermediates could be decomposed further to CO2 and H2O and Cl finally (Ye et al. 2018). These studies were consistent with the results of this research. In this study, most of the intermediates had similar structures and belonged to aromatic compounds. The fate in the CB degradation and toxicity needs to be paid attention and the related study will be investigated in our future research.

### CB degradation performance in the actual groundwater

The chemical composition of the actual groundwater was complex (Table 2). The degradation of CB in the actual groundwater in these three systems was compared to their degradation performance in ultrapure water (Figure 7). In actual groundwater, the removal rates of CB in H2O2/Fe(II), nCaO2/Fe(II) and SPC/Fe(II) systems descended to 98, 70, and 36%, respectively at the oxidant/Fe(II)/CB molar ratio of 8/8/1, indicating that the degradation of CB in these systems was significantly affected by the complexity of the actual groundwater matrix. It is estimated that the chemical cost of $4.03/m3 was incurred by the H2O2/Fe(II) system at the molar ratio of 8/8/1. One possible reason was that the pH of the actual groundwater was 7.28 and the strong buffering capacity could reduce the catalytic capacity of the systems. Some anions and natural organic matter (NOM) in groundwater also had a negative effect on CB degradation (Zhang et al. 2015a, 2015b). Increasing the molar ratio of oxidant/Fe(II)/CB was an effective way to overcome the adverse effects of actual groundwater on Fe(II) catalytic H2O2/nCaO2/SPC degradation of CB. CB could be completely degraded in the actual groundwater when the molar ratio of nCaO2/Fe(II)/CB was increased to 20/20/1 with the chemical cost of$26.50/m3. The SPC/Fe(II) system analogously showed a strong ability to degrade CB at the condition of 20/20/1 molar ratio in actual groundwater and estimated the chemical cost as $100.14/m3. Hence, all three systems, at an optimum molar ratio of chemicals, have broad application prospects in the actual groundwater environment in which the H2O2/Fe(II) system is the cheapest option. Table 2 Characteristics of the actual groundwater used in experiments ParameterUnitsValue pH 7.28 ± 0.2 Total organic carbon (TOC) mg L−1 11.35 ± 0.5 Cl concentration mg L−1 42.5 ± 1.2 HCO3 concentration mg L−1 122.0 ± 2.0 NO3 concentration mg L−1 7.7 ± 0.1 SO42− concentration mg L−1 61.9 ± 3.5 Ca2+ concentration mg L−1 120.0 ± 6.0 Mg2+ concentration mg L−1 34.0 ± 2.6 CB concentration mg L−1 Not detected ParameterUnitsValue pH 7.28 ± 0.2 Total organic carbon (TOC) mg L−1 11.35 ± 0.5 Cl concentration mg L−1 42.5 ± 1.2 HCO3 concentration mg L−1 122.0 ± 2.0 NO3 concentration mg L−1 7.7 ± 0.1 SO42− concentration mg L−1 61.9 ± 3.5 Ca2+ concentration mg L−1 120.0 ± 6.0 Mg2+ concentration mg L−1 34.0 ± 2.6 CB concentration mg L−1 Not detected Figure 7 CB removal performances in (a) H2O2/Fe(II) system, (b) nCaO2/Fe(II) system and (c) SPC/Fe(II) system in ultrapure water and actual groundwater at 30 min. ([CB] = 1.0 mM) (uw represents ultrapure water, gw represents actual groundwater). Figure 7 CB removal performances in (a) H2O2/Fe(II) system, (b) nCaO2/Fe(II) system and (c) SPC/Fe(II) system in ultrapure water and actual groundwater at 30 min. ([CB] = 1.0 mM) (uw represents ultrapure water, gw represents actual groundwater). Close modal In this study, CB degradation in various oxic environments (H2O2, nCaO2 or SPC) activated with Fe(II) was investigated and compared. The H2O2/Fe(II) system, nCaO2/Fe(II) system or SPC/Fe(II) system showed strong abilities to degrade CB in the conditions of the aqueous solution and CB removal rates were 99.4%, 98.5%, and 99.8% in 30 min at the oxidant/Fe(II)/CB molar ratio of 8/8/1, respectively. CB had a better removal rate at the same H2O2 and Fe(II) content and the effect of iron dosage was significant compared to that of oxidants measurement in nCaO2 and SPC systems. The optimal oxidant/Fe(II)/CB molar ratios of 5.2/7.6/1, 8/8/1 and 4.5/8/1 with 98.1%, 98%, and 96.4% CB removals in 30 min were obtained in the H2O2/Fe(II)/CB, nCaO2/Fe(II)/CB and SPC/Fe(II)/CB systems, respectively. 83.4%, 74.5%, and 76.0% TOC removals were obtained at the molar ratio of 8/8/1 in H2O2/Fe(II)/CB, nCaO2/Fe(II)/CB and SPC/Fe(II)/CB systems, respectively. The biotoxicity of the solution was decreasing significantly, therefore suggesting that Fenton or nCaO2 and SPC based Fenton-like processes were available for the degradation of CB in aqueous solution. The initial pH of the solution was an important factor affecting CB degradation and CB removal rates were less than 20% in all three systems when the initial pH was adjusted to 9. HO and 1O2 had been detected by the EPR test. Uniting the results of the LC-MS test, the pathways of CB degradation were proposed. 1O2 roles were elaborated innovatively in the CB degradation mechanism. The degradation of CB in systems was significantly affected by the actual groundwater and increasing molar ratio of chemicals was an effective way to overcome the adverse effects, suggesting a broad application prospect for the tested three systems in an actual groundwater environment. The H2O2/Fe(II) system is the cheapest option (estimated chemical cost of$4.03/m3) with an excellent CB removal performance.

This study was financially supported by ‘One Belt and One Road’ international academic cooperation and exchange program of Shanghai Science and Technology Committee (No. 19230742200) and the National Natural Science Foundation of China (No. 41977164).

All relevant data are included in the paper or its Supplementary Information.

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