As a kind of refractory chlorinated hydrocarbon, 1,2-dichloroethane (1,2-DCA) has been frequently detected in contaminated groundwater, and it is hard for common technology to degrade it due to its stability. Moreover, the existence of 1,2-DCA can do harm to human organs. Hence, it is urgent to develop an effective technology for the remediation of 1,2-DCA-contaminated groundwater. In this study, a calcium peroxide (CaO2) system activated by Fe(III) was applied to the degradation of 1,2-DCA and 83.3% of 1,2-DCA could be effectively removed within 3 h when the molar ratio of CaO2/Fe(III)/1,2-DCA was 30/120/1. The results of probe experiments, electron paramagnetic resonance (EPR) detection, and scavenging tests demonstrated that both HO and O2−• were the key factors for 1,2-DCA degradation. The released amount of Cl (84.1%) revealed that most of the chlorine in 1,2-DCA could be dechlorinated. GC-MS was applied for the detection of intermediates during 1,2-DCA degradation and the possible degradation pathway was proposed that 1,2-DCA was first reduced to vinyl chloride (VC) and then oxidized to CO2 and H2O. Finally, 73.4% removal of 1,2-DCA could be achieved in actual groundwater when the molar ratio of CaO2/Fe(III)/1,2-DCA was 100/400/1, demonstrating that the CaO2/Fe(III) system has a remarkable prospect in 1,2-DCA-contaminated groundwater remediation.

  • Efficient degradation of 1,2-DCA in the CaO2/Fe(III) system was achieved.

  • HO and O2–• were the key factors for 1,2-DCA removal in the CaO2/Fe(III) system.

  • The possible degradation pathway of 1,2-DCA was proposed.

  • The effect of water matrices on 1,2-DCA degradation in the CaO2/Fe(III) system was investigated.

1,2-Dichloroethane (1,2-DCA) is widely used as the chlorinated organic solvent and raw material for insecticide production, resulting in ubiquitous contamination in soil and groundwater due to improper usage and disposal (Vilhunen et al. 2010; Lin et al. 2021; Rahim et al. 2022). The existence of 1,2-DCA in groundwater can do harm to human organs, thus, it has been listed as one of the priority pollutants by the United Kingdom (UK) and the United States Environmental Protection Agency (USEPA) due to the negative impact on the environment and human health (Morris 2000; Bejankiwar et al. 2005; Gwinn et al. 2011). Therefore, it is urgent to establish an efficient technology for the remediation of 1,2-DCA in contaminated groundwater. Various methods have been applied to the removal of 1,2-DCA up to now, such as adsorption, catalytic degradation, pyrolysis, and biodegradation (Dai et al. 2015). However, the limitation of catalyst deactivation resulted in the inefficient degradation of 1,2-DCA in actual groundwater (Huang et al. 2022). A high cost is required to achieve the desired removal by adsorption, while biodegradation is generally time-consuming and is seriously dependent on the underground environment (Bejankiwar et al. 2005). Furthermore, a study concerning advanced oxidation processes (AOPs) on 1,2-DCA removal has not been reported.

Advanced oxidation processes (AOPs) have attracted much attention and have been extensively employed to degrade refractory organic pollutants in contaminated sites due to their high efficiency and low cost (Martins et al. 2013; Santos-Juanes et al. 2017). Among various AOPs, the traditional Fenton process has the advantages of simple operation and effective performance and has been widely used in the remediation of contaminated soil and groundwater. The hydroxyl radical (HO) and superoxide radical (O2−•) will be generated in the Fenton process and they play leading roles in the degradation of pollutants (Seol et al. 2003). However, the traditional Fenton process has been gradually modified by researchers because of its shortcomings, such as fast reaction, short-term reaction time and difficulty in storage of H2O2. As a kind of solid H2O2 source, calcium peroxide (CaO2) can slowly release H2O2 in aqueous solution (Equations (1) and (2)) and degrade pollutants effectively and persistently (Zhang et al. 2015a), and hence has attracted much more interest from researchers recently.
(1)
(2)

The activation of CaO2 is commonly implemented by transition metals. Among numerous transition metals, Fe(III) has been widely applied for the activation of CaO2 due to its environmental friendliness and availability. Zhang et al. (2016) reported that 96% of trichloroethene was degraded within 180 min in actual groundwater in a CaO2/Fe(III)/citric acid (CA) system. As far as we know, the removal of 1,2-DCA in a CaO2 oxidation process activated by Fe(III) has not been reported yet. The mechanisms of reactive oxygen species (ROS) generated in a CaO2/Fe(III) system need to be clarified. In addition, the possible degradation pathways of 1,2-DCA need to be revealed.

Hence, in this work, 1,2-DCA removal in a CaO2/Fe(III) system was investigated. The objectives of this work are to: (1) evaluate the performance of 1,2-DCA removal in the various systems and explore the effect of experimental parameters on 1,2-DCA degradation; (2) identify the dominant ROS for 1,2-DCA removal in the CaO2/Fe(III) system; (3) determine the effect of the water matrix on 1,2-DCA degradation; (4) illuminate the dechlorination performance of 1,2-DCA and propose the possible degradation pathways of 1,2-DCA in the CaO2/Fe(III) system; and finally (5) assess the effectiveness of the CaO2/Fe(III) system for 1,2-DCA removal in actual groundwater. It is expected that the outcomes of this research will provide fundamental support for the practical implementation of the CaO2/Fe(III) system in the remediation of 1,2-DCA-contaminated groundwater.

Materials and chemicals

All the materials and equipment used in this work are provided in Tables 1 and 2.

Table 1

Materials and chemicals

ReagentPurityManufacturer (Shanghai, China)
1,2-Dichloroethane (C2H4Cl299.00% Aladdin Reagent Co. Ltd 
Iron sulphate heptahydrate (FeSO4·7H2O) 99.50% Aladdin Reagent Co. Ltd 
Iron sulfate hydrate (Fe2(SO4)3·xH2O) AR Aladdin Reagent Co. Ltd 
n-Hexane (C6H1497.00% Aladdin Reagent Co. Ltd 
Tertiary butyl alcohol ((CH3)3OH) ≥99.0% Aladdin Reagent Co. Ltd 
5,5-Dimethyl-1-pyrroline N-oxide (C6H11NO) 97.00% Aladdin Reagent Co. Ltd 
Phosphoric acid (H3PO4HPLC,85% ∼ 90% Aladdin Reagent Co. Ltd 
Benzoic acid (C7H6O299.50% Aladdin Reagent Co. Ltd 
Potassium peroxymonosulfate sulfate (KHSO5≥99.0% Shanghai Lingfeng Chemical Reagent Co. Ltd 
Titanium (IV) oxysulfate–sulfuric acid hydrate (TiOSO4·xH2SO4·xH2O) 93.00% Shanghai Lingfeng Chemical Reagent Co. Ltd 
Sulfuric acid (H2SO498.00% Shanghai Lingfeng Chemical Reagent Co. Ltd 
Carbon tetrachloride (CCl499.50% Shanghai Lingfeng Chemical Reagent Co. Ltd 
Calcium peroxide (CaO2≥75% Shanghai Titan Scientific Co. Ltd 
Sodium chloride (NaCl) 96.00% Shanghai Titan Scientific Co. Ltd 
Sodium bicarbonate (NaHCO3≥99.5% Shanghai Titan Scientific Co. Ltd 
Sodium hydroxide (NaOH) ≥96.0% Shanghai Titan Scientific Co. Ltd 
Anhydrous sodium sulfate (Na2SO4≥99.0% Sinopharm Chemical Reagent Co. Ltd 
Sodium nitrate (NaNO399.50% Sinopharm Chemical Reagent Co. Ltd 
Chloroform (CHCl399.00% Sinopharm Chemical Reagent Co. Ltd 
Nitrobenzene (NB) 99.00% Shanghai Chemical Reagent Co. Ltd 
Groundwater – 15 m deep below the surface (Minhang, Shanghai, China) 
ReagentPurityManufacturer (Shanghai, China)
1,2-Dichloroethane (C2H4Cl299.00% Aladdin Reagent Co. Ltd 
Iron sulphate heptahydrate (FeSO4·7H2O) 99.50% Aladdin Reagent Co. Ltd 
Iron sulfate hydrate (Fe2(SO4)3·xH2O) AR Aladdin Reagent Co. Ltd 
n-Hexane (C6H1497.00% Aladdin Reagent Co. Ltd 
Tertiary butyl alcohol ((CH3)3OH) ≥99.0% Aladdin Reagent Co. Ltd 
5,5-Dimethyl-1-pyrroline N-oxide (C6H11NO) 97.00% Aladdin Reagent Co. Ltd 
Phosphoric acid (H3PO4HPLC,85% ∼ 90% Aladdin Reagent Co. Ltd 
Benzoic acid (C7H6O299.50% Aladdin Reagent Co. Ltd 
Potassium peroxymonosulfate sulfate (KHSO5≥99.0% Shanghai Lingfeng Chemical Reagent Co. Ltd 
Titanium (IV) oxysulfate–sulfuric acid hydrate (TiOSO4·xH2SO4·xH2O) 93.00% Shanghai Lingfeng Chemical Reagent Co. Ltd 
Sulfuric acid (H2SO498.00% Shanghai Lingfeng Chemical Reagent Co. Ltd 
Carbon tetrachloride (CCl499.50% Shanghai Lingfeng Chemical Reagent Co. Ltd 
Calcium peroxide (CaO2≥75% Shanghai Titan Scientific Co. Ltd 
Sodium chloride (NaCl) 96.00% Shanghai Titan Scientific Co. Ltd 
Sodium bicarbonate (NaHCO3≥99.5% Shanghai Titan Scientific Co. Ltd 
Sodium hydroxide (NaOH) ≥96.0% Shanghai Titan Scientific Co. Ltd 
Anhydrous sodium sulfate (Na2SO4≥99.0% Sinopharm Chemical Reagent Co. Ltd 
Sodium nitrate (NaNO399.50% Sinopharm Chemical Reagent Co. Ltd 
Chloroform (CHCl399.00% Sinopharm Chemical Reagent Co. Ltd 
Nitrobenzene (NB) 99.00% Shanghai Chemical Reagent Co. Ltd 
Groundwater – 15 m deep below the surface (Minhang, Shanghai, China) 
Table 2

Main experimental equipment

EquipmentModelManufacturer
Gas chromatograph (GC) Agilent 7890A Agilent Technologies Co. Ltd (USA) 
High performance liquid chromatography (HPLC) LC-20AT Shimadzu Co. Ltd (Japan) 
Thermostatic magnetic stirrer Feb-85 Shanghai Minhang Hongpu Instrument Factory (China) 
Electron sequential resonator (EPR) EMX-8/2.7C Burker Co. Ltd (Germany) 
UV–visible spectrophotometer DR-6000 Hach Co. Ltd (China) 
Ion chromatography (IC) ICS-1000 Dionex Co. Ltd (USA) 
pH meter P8–10 Sartorius Co. Ltd (Germany) 
Electronic analytical balance AL204 Mettler Toledo Co. Ltd (Switzerland) 
Eddy current shaker XW-80A Shanghai Qingpu Huxi Instrument Factory (China) 
Ultrapure water machine CLASSIC UV MK2 ELGA Co. Ltd (England) 
EquipmentModelManufacturer
Gas chromatograph (GC) Agilent 7890A Agilent Technologies Co. Ltd (USA) 
High performance liquid chromatography (HPLC) LC-20AT Shimadzu Co. Ltd (Japan) 
Thermostatic magnetic stirrer Feb-85 Shanghai Minhang Hongpu Instrument Factory (China) 
Electron sequential resonator (EPR) EMX-8/2.7C Burker Co. Ltd (Germany) 
UV–visible spectrophotometer DR-6000 Hach Co. Ltd (China) 
Ion chromatography (IC) ICS-1000 Dionex Co. Ltd (USA) 
pH meter P8–10 Sartorius Co. Ltd (Germany) 
Electronic analytical balance AL204 Mettler Toledo Co. Ltd (Switzerland) 
Eddy current shaker XW-80A Shanghai Qingpu Huxi Instrument Factory (China) 
Ultrapure water machine CLASSIC UV MK2 ELGA Co. Ltd (England) 

Experimental procedures

In the 1,2-DCA degradation experiments, the reactor (250 mL) was placed on a magnetic stirrer at a constant speed of 600 rpm and the reaction temperature was controlled at 20 ± 0.5 °C. The predetermined dosage of Fe(II) or Fe(III) was added into the reactor containing 1,2-DCA (0.2 mM) solution and then the reaction began immediately after CaO2 addition. At a given time, a 1.0 mL sample was taken out to a brown vial pre-filled with 1.0 mL n-hexane to terminate the reaction and extract the remaining 1,2-DCA. The concentration of 1,2-DCA in the extracted samples was analyzed by gas chromatograph (GC, 7890A, Agilent, USA). In order to investigate the influence of anions, various anions were added before reaction began.

In the test of HO quantification, 1,2-DCA was replaced by BA and then a 1.0 mL sample was mixed with 1.0 mL NaOH (0.1 mM) and 0.1 mL methanol (Xue et al. 2018b). The mixture was filtered into a vial containing 0.5 mL H2SO4 (1.0 mM) to adjust pH. The concentration of p-hydroxybenzoic acid (p-HBA) in the sample was analyzed by high performance liquid chromatography (HPLC, LC-20AT, Shimadzu, Japan).

In the scavenging experiments, tertiary butyl alcohol (TBA) and trichloromethane (CF) were selected as the scavengers of HO and O2−•, respectively (Wang et al. 2016). In the probe tests, nitrobenzene (NB) and tetrachloride (CT), as the probe compounds of HO and O2−•, respectively, were used to replace 1,2-DCA (Buxton et al. 1988; Teel & Watts 2002).

In the test of dechlorination investigation, a 5.0 mL sample was taken out in the given time and mixed with 1.0 mL methanol to stop the reaction in an exposed bottle. Then the sample was stood still for 12 h and the concentration of Cl in the sample was analyzed by ion chromatography (IC, Dionex ICS-1000, Sunnyvale, USA). To ensure the accuracy of the experiments, all tests were conducted at least three times.

Analytical methods

The concentration of 1,2-DCA was analyzed by GC at the oven, injector and detector temperatures of 90, 220, and 240 °C, respectively. The ROS generated in aqueous solution were determined by electron paramagnetic resonance (EPR). The concentrations of BA and p-HBA were determined by HPLC coupled with a UV–vis detector (SPD-20A) and an auto-sampler (SIL-20A) at the detection wavelengths of 255 and 225 nm, respectively. The mobile phase was a mixture of 0.1% phosphoric acid and methanol (58/42 (v/v)), the oven temperature was 35 °C and the injection value was 10 μL. The Fe(II) concentrations in aqueous solution were analyzed by o-phenanthroline spectrophotometry at the detection wavelength of 512 nm (Tamura et al. 1974). The H2O2 concentrations were analyzed by the TiSO4 method at the detection wavelength of 400 nm (Vega et al. 2022). The concentrations of Cl were determined by IC. The pH meter (Mettler-Toledo DELTA 320, Greifensee, Switzerland) was used to measure the solution pH. CT was analyzed by GC at a split ratio of 20:1, and the temperatures of the oven, injector, and detector were 75, 240, and 260 °C, respectively. NB was analyzed by the HPLC at the temperature of 35 °C. The mobile phase was a mixture of 40% water (solvent A) and 60% methanol (solvent B) at a total flow rate of 1.0 mL min−1. The injection volume was 10 μL and the wavelength of the UV detector was fixed at 254 nm.

The performance of 1,2-DCA degradation in various systems

The 1,2-DCA degradation performance in different systems is shown in Figure 1, in which the initial concentrations of 1,2-DCA, CaO2, Fe(II), and Fe(III) were set as 0.2, 6.0, 24, and 24 mM, respectively. Without addition of any chemicals, the volatilization loss of 1,2-DCA was less than 1% at 20 ± 0.5 °C. When only CaO2, Fe(II), and Fe(III) was added, the removal of 1,2-DCA in aqueous solution was 1.62%, 0.39%, and 1.95% within 180 min, respectively. Moreover, only 2.4% of 1,2-DCA was removed in the CaO2/Fe(II) system. It is reported that excessive Fe(II) could not only scavenge the generated HO, but also consume the O2−•, resulting in the low removal of 1,2-DCA (Equation (3)) (Yamazaki & Piette 1990; Zhang et al. 2015b; Wang et al. 2019). However, 83.3% of 1,2-DCA removal was obtained within 180 min in the CaO2/Fe(III) system, indicating that CaO2 could be effectively activated by Fe(III) and produce ROS to remove 1,2-DCA. On the one hand, Fe(III) could be slowly converted to Fe(II) and the regenerated Fe(II) could persistently catalyze H2O2 released by CaO2 to generate ROS in the CaO2/Fe(III) system (Equations (4)–(6)) (Walling 1975; Xue et al. 2019). On the other hand, CaO2 can be directly activated by Fe(III) and generate numerous ROS.
(3)
(4)
(5)
(6)
Figure 1

The performance of 1,2-DCA removal in various systems ([CaO2]0 = 6.0 mM, [Fe(II)]0 = [Fe(III)]0 = 24 mM, 1,2-DCA = 0.2 mM).

Figure 1

The performance of 1,2-DCA removal in various systems ([CaO2]0 = 6.0 mM, [Fe(II)]0 = [Fe(III)]0 = 24 mM, 1,2-DCA = 0.2 mM).

Close modal

The concentration of Fe(II) is generally a key factor affecting the production of ROS in AOPs and the variation of Fe(II) in different systems in this work is shown in Figure 2(a). The initial concentrations of CaO2, Fe(III), 1,2-DCA, and CF were 6.0, 24, 0.2, and 50 mM, respectively. In the CaO2/Fe(III) system, the fluctuation of Fe(II) was small and the Fe(II) concentration was maintained at 0.2 mM. The concentration of Fe(II) in the CaO2/Fe(III)/1,2-DCA system was higher than that in the CaO2/Fe(III) system in this work, indicating that reductive intermediates were produced during the reaction. Similarly, Xue et al. (2019) compared CaO2/Fe(II) systems with and without benzene, and they also found that the concentration of Fe(II) increased in the CaO2/Fe(II)/benzene system. In addition, the same trend of Fe(II) concentration variation in CaO2/Fe(III)/1,2-DCA and CaO2/Fe(III)/1,2-DCA/CF systems was observed, but the concentration of Fe(II) in the CaO2/Fe(III)/1,2-DCA system was higher than that in the CaO2/Fe(III)/1,2-DCA/CF system, suggesting that the addition of CF could hinder Fe(II) recovery by scavenging O2−• (Equation (6)) (Neyens & Baeyens 2003).

Figure 2

(a) The concentrations of Fe(II) and (b) the released concentrations of H2O2 in various systems ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM, [CF]0 = 50 mM).

Figure 2

(a) The concentrations of Fe(II) and (b) the released concentrations of H2O2 in various systems ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM, [CF]0 = 50 mM).

Close modal

The concentration of released H2O2 is an important factor concerning the degradation performance of contaminants in Fenton-like systems. As shown in Figure 2(b), the released concentrations of H2O2 in CaO2/Fe(III), CaO2/Fe(III)/1,2-DCA, and CaO2/Fe(III)/1,2-DCA/CF systems were investigated. The initial concentrations of 1,2-DCA, CaO2, Fe(III), and CF were 0.2, 6.0, 24, and 50 mM, respectively. The maximum released concentrations of H2O2 were 4.49, 4.20, and 3.34 mM in CaO2/Fe(III), CaO2/Fe(III)/1,2-DCA, and CaO2/Fe(III)/1,2-DCA/CF systems, respectively. These results indicated that 1,2-DCA could consume H2O2 released by CaO2 in aqueous solution. Compared with the CaO2/Fe(III)/1,2-DCA system, the concentration of H2O2 in CaO2/Fe(III)/1,2-DCA/CF was maintained at a relatively higher concentration, showing that the existence of CF could inhibit the reaction between 1,2-DCA and H2O2, resulting in the decreased consumption of H2O2.

The amount of HO produced in the CaO2/Fe(III) system was further determined to illustrate the relationship between the 1,2-DCA degradation rate and the generation of HO when the CaO2/Fe(III) molar ratio was controlled at 1/4 (Figure 3). The generation of HO was divided into two stages. The generated amount of HO increased to 1.58 mM within 60 min, while it rapidly decreased to 0.01 mM after 60 min when CaO2 was 6.0 mM, which was consistent with the degradation performance of 1,2-DCA. The amount of generated HO increased from 0.12 mM to 1.59 mM with the CaO2 dosage increasing from 4 to 6 mM, suggesting that increasing the CaO2 dosage would significantly enhance the generated HO amount as well as 1,2-DCA removal. However, when the CaO2 dosage further increased to 12 mM, the generated HO concentration increased only a little bit, because the numerous generated HO could be consumed by itself.

Figure 3

HO generation in CaO2/Fe(III) system (the molar ratio of [CaO2]0/[Fe(III)]0 = 1/4).

Figure 3

HO generation in CaO2/Fe(III) system (the molar ratio of [CaO2]0/[Fe(III)]0 = 1/4).

Close modal

Effects of experimental parameters on 1,2-DCA degradation

The influence of Fe(III) dosage on 1,2-DCA degradation in the CaO2/Fe(III) system was investigated and the initial concentrations of CaO2 and 1,2-DCA were fixed at 6.0 and 0.2 mM, respectively. As shown in Figure 4(a), with the concentration of Fe(III) increasing from 6.0 to 24 mM, the 1,2-DCA removal increased from 2.5% to 83.3%, indicating that the degradation efficiency of 1,2-DCA could be enhanced by the augmentation of Fe(III) dosage. On one hand, the addition of Fe(III) could lower the pH, which prevented the formation of iron precipitation and maintained the soluble iron ions in solution. In addition, massive Fe(III) could promote the regeneration of Fe(II) and further facilitate the activation of CaO2 (Equations (4) and (6)). However, when the dosage of Fe(III) increased from 24 to 36 mM, the removal of 1,2-DCA only increased by 6.2%, thus the Fe(III) concentration was controlled at 24 mM in the subsequent experiments.

Figure 4

Effects of (a) Fe(III) dosage ([CaO2]0 = 6.0 mM, [1,2-DCA]0 = 0.2 mM) and (b) CaO2 dosage ([Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM) on 1,2-DCA degradation in the CaO2/Fe(III) system.

Figure 4

Effects of (a) Fe(III) dosage ([CaO2]0 = 6.0 mM, [1,2-DCA]0 = 0.2 mM) and (b) CaO2 dosage ([Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM) on 1,2-DCA degradation in the CaO2/Fe(III) system.

Close modal

The amounts of Fe(III) and 1,2-DCA were fixed at 24 mM and 0.2 mM to further investigate the influence of the CaO2 amount on 1,2-DCA degradation. As illustrated in Figure 4(b), the removal of 1,2-DCA increased from 54.7% to 83.3% within 180 min when the concentration of CaO2 increased from 2 to 6 mM. This consequence indicated that appropriately increasing the CaO2 dosage could produce more ROS and promote the degradation of 1,2-DCA in the CaO2/Fe(III) system. With the CaO2 concentration further increasing from 6 to 10 mM, 1,2-DCA removal increased just from 83.3% to 85.2%. When the dosage of CaO2 further increased to 12 mM, the degradation of 1,2-DCA was slightly inhibited. Over-concentrated CaO2 in aqueous solution could increase the pH, which in turn accelerated the formation of iron precipitation and was not conducive to 1,2-DCA removal. Similarly, Xue et al. (2018a) reported that BTEX removal was suppressed in the CaO2/Fe(II) system when the concentration of CaO2 increased from 2.5 to 20 mM. In consideration of the practical application and the chemical cost-saving, the optimal concentration of CaO2 was set as 6.0 mM.

Mechanisms of 1,2-DCA degradation

Probe compounds and EPR detection were used to confirm the ROS generated in the 1,2-DCA degradation process. In the probe experiments, NB and CT were used to detect the existence of HO and O2−•, and the initial concentrations of CaO2, Fe(III), NB and CT were 6.0, 24, 0.2 and 0.2 mM, respectively. As shown in Figure 5, NB was completely removed within 10 min, demonstrating that a large amount of HO was generated in the CaO2/Fe(III) system, and 32.3% of CT was degraded in the CaO2/Fe(III) system, accounting for the presence of O2−•. Moreover, the signal of DMPO-HO peaks was detected in the CaO2/Fe(III) system, and the result proved the generation and presence of HO (Figure 6). Unfortunately, no DMPO-O2−• peak was found in the CaO2/Fe(III) system due to the instability of O2−•.

Figure 5

Degradation performance of probe compounds in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = [NB]0 = [CT]0 = 0.2 mM).

Figure 5

Degradation performance of probe compounds in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = [NB]0 = [CT]0 = 0.2 mM).

Close modal
Figure 6

EPR spectra at the reaction time of 60 min in the CaO2/Fe(III) system (DMPO as radical trap).

Figure 6

EPR spectra at the reaction time of 60 min in the CaO2/Fe(III) system (DMPO as radical trap).

Close modal

To confirm the generated ROS and their contribution to 1,2-DCA degradation in the CaO2/Fe(III) system, scavenging tests were implemented. TBA and CF were used as HO and O2−• scavengers, respectively, and the initial concentrations of CaO2, Fe(III), TBA, and CF were 6.0, 24, 100, and 50 mM, respectively. As shown in Figure 7, the removal of 1,2-DCA decreased from 83.3% to 3.3% with the addition of 100 mM TBA, which revealed that HO played a crucial role in 1,2-DCA degradation. Notably, 1,2-DCA removal decreased from 83.3% to 3.5% when 50 mM CF was added before the reaction started, confirming that O2−• also played an important role in 1,2-DCA degradation. Differently, Zhang et al. (2016) reported the degradation performance of trichloroethene (TCE) in a CaO2/Fe(III)/citric acid system and they found that HO was the dominant ROS for TCE degradation, while the contribution of O2−• to TCE removal could be negligible. Hence, the contribution of various ROS could be different due to the properties and structures of target contaminants. Experiments of adding CF at different reaction times were carried out to further investigate the contribution of ROS to 1,2-DCA removal. The 1,2-DCA removal decreased to 3.5%, 30.8%, 43.4%, and 62.7% when CF was added at 0, 10, 30, and 60 min, respectively (Figure 7). The result indicated that 1,2-DCA degradation was significantly inhibited in the presence of CF and O2−• played an important role in 1,2-DCA degradation.

Figure 7

Effect of scavengers on 1,2-DCA degradation in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM, [TBA]0 = 100 mM, [CF]0 = 50 mM).

Figure 7

Effect of scavengers on 1,2-DCA degradation in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM, [TBA]0 = 100 mM, [CF]0 = 50 mM).

Close modal

Effects of water matrices on 1,2-DCA degradation

To evaluate the application prospect of the CaO2/Fe(III) system in actual groundwater remediation, the effects of complex water matrices (pH, Cl, HCO3, NO3, and SO42−) on 1,2-DCA degradation were investigated. The initial concentrations of CaO2, Fe(III), and 1,2-DCA were 6.0, 24, 0.2 mM, respectively. As shown in Figure 8, 89.6%, 87.2%, 85.3%, 84.2%, and 80.3% of 1,2-DCA were degraded at the initial solution pH of 3.0, 5.0, 7.0, 9.0, and 11.0, respectively. It is worth noting that 1,2-DCA could be removed effectively in the CaO2/Fe(III) system in the pH range of 3–11 and acidic solution was conducive for 1,2-DCA removal. Northup & Cassidy (2008) reported that the dissolution of CaO2 and the release of H2O2 were promoted in an acidic environment, which could enhance pollutant degradation. The CaO2/Fe(III) system was still effective for 1,2-DCA removal in alkaline conditions because a large amount of Fe(III) was hydrolyzed, resulting in an acidic condition (Table 3).

Table 3

Parameter values under various experimental conditions

Experimental conditions1,2-DCA removal (%)pH (initial/final)
pH = 3a 89.6 3.02/2.62 
pH = 5a 87.1 5.01/2.68 
pH = 7a 85.3 7.05/2.73 
pH = 9a 84.2 9.00/2.75 
pH = 11a 80.4 10.98/2.76 
CaO2/Fe(III)/1,2-DCA = 10/40/1 29.3 2.56/2.72 
CaO2/Fe(III)/1,2-DCA = 20/80/1 63.4 2.39/2.42 
CaO2/Fe(III)/1,2-DCA = 30/120/1 83.3 2.26/2.35 
CaO2/Fe(III)/1,2-DCA = 40/160/1 89.9 2.16/2.36 
CaO2/Fe(III)/1,2-DCA = 60/240/1 93.3 2.06/2.21 
[Cl] = 1 mM 78.2 2.21/2.44 
[Cl] = 10 mM 36.4 2.22/2.42 
[Cl] = 100 mM 37.4 2.23/2.44 
[HCO3] = 1 mM 82.4 2.22/2.43 
[HCO3] = 10 mM 82.2 2.58/2.50 
[HCO3] = 100 mM 4.3 6.58/7.05 
[NO3] = 1 mM 88.5 2.24/2.42 
[NO3] = 10 mM 86.7 2.24/2.43 
[NO3] = 100 mM 87.9 2.19/2.35 
[SO42−] = 1 mM 87.0 2.22/2.43 
[SO42−] = 10 mM 84.4 2.29/2.50 
[SO42−] = 100 mM 73.0 2.60/2.65 
CaO2/Fe(III)/1,2-DCA = 20/80/1b 32.1 2.68/2.53 
CaO2/Fe(III)/1,2-DCA = 30/120/1b 42.1 2.49/2.40 
CaO2/Fe(III)/1,2-DCA = 40/160/1b 47.6 2.31/2.33 
CaO2/Fe(III)/1,2-DCA = 60/240/1b 63.1 2.17/2.28 
CaO2/Fe(III)/1,2-DCA = 100/400/1b 73.4 2.01/2.23 
Experimental conditions1,2-DCA removal (%)pH (initial/final)
pH = 3a 89.6 3.02/2.62 
pH = 5a 87.1 5.01/2.68 
pH = 7a 85.3 7.05/2.73 
pH = 9a 84.2 9.00/2.75 
pH = 11a 80.4 10.98/2.76 
CaO2/Fe(III)/1,2-DCA = 10/40/1 29.3 2.56/2.72 
CaO2/Fe(III)/1,2-DCA = 20/80/1 63.4 2.39/2.42 
CaO2/Fe(III)/1,2-DCA = 30/120/1 83.3 2.26/2.35 
CaO2/Fe(III)/1,2-DCA = 40/160/1 89.9 2.16/2.36 
CaO2/Fe(III)/1,2-DCA = 60/240/1 93.3 2.06/2.21 
[Cl] = 1 mM 78.2 2.21/2.44 
[Cl] = 10 mM 36.4 2.22/2.42 
[Cl] = 100 mM 37.4 2.23/2.44 
[HCO3] = 1 mM 82.4 2.22/2.43 
[HCO3] = 10 mM 82.2 2.58/2.50 
[HCO3] = 100 mM 4.3 6.58/7.05 
[NO3] = 1 mM 88.5 2.24/2.42 
[NO3] = 10 mM 86.7 2.24/2.43 
[NO3] = 100 mM 87.9 2.19/2.35 
[SO42−] = 1 mM 87.0 2.22/2.43 
[SO42−] = 10 mM 84.4 2.29/2.50 
[SO42−] = 100 mM 73.0 2.60/2.65 
CaO2/Fe(III)/1,2-DCA = 20/80/1b 32.1 2.68/2.53 
CaO2/Fe(III)/1,2-DCA = 30/120/1b 42.1 2.49/2.40 
CaO2/Fe(III)/1,2-DCA = 40/160/1b 47.6 2.31/2.33 
CaO2/Fe(III)/1,2-DCA = 60/240/1b 63.1 2.17/2.28 
CaO2/Fe(III)/1,2-DCA = 100/400/1b 73.4 2.01/2.23 

aAdjusting pH before adding other agents.

bUsing actual groundwater instead of Milli-Q water.

Figure 8

Effect of initial solution pH on 1,2-DCA degradation in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM).

Figure 8

Effect of initial solution pH on 1,2-DCA degradation in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM).

Close modal
The influences of inorganic anions (Cl, HCO3, NO3, and SO42−) on 1,2-DCA degradation were further investigated. As illustrated in Figure 9(a), 1,2-DCA removal decreased from 83.3% to 78.2%, 36.4%, and 37.4% when the Cl concentration increased from 0 to 1, 10, and 100 mM. The inhibition of 1,2-DCA removal caused by the presence of Cl could be attributed to the scavenging effects of HO and O2−• through Equations (7) and (8) (Walling 1975; Xu et al. 2017):
(7)
(8)
Figure 9

The influences of (a) Cl and HCO3, (b) NO3 and SO42− on 1,2-DCA degradation in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM).

Figure 9

The influences of (a) Cl and HCO3, (b) NO3 and SO42− on 1,2-DCA degradation in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM).

Close modal
The removal of 1,2-DCA decreased from 83.3% to 82.4%, 82.2%, and 4.3% with the concentration of HCO3 increasing from 0 mM to 1, 10, and 100 mM, respectively. The inhibition effect of HCO3 could be negligible when the concentration of HCO3 was 1 or 10 mM, but it was significant when the concentration of HCO3 was 100 mM. This is because the initial pH increased from 2.22 to 6.58 with the concentration of HCO3 increasing from 1 to 100 mM (Table 3), which was not conducive for 1,2-DCA removal. Yang et al. (2021) also found that the presence of HCO3 had an obvious inhibition on naphthalene degradation in an H2O2/Fe(II) system when the concentration of HCO3 was 100 mM. Moreover, the presence of HCO3 could scavenge the generated O2−• through Equations (9) and (10) (Schmidt 1972). Furthermore, the effect of NO3 addition on 1,2-DCA degradation was negligible (Figure 9(b)). The addition of 1 and 10 mM SO42− had little effect on the degradation of 1,2-DCA, but when the concentration of SO42− increased to 100 mM, the 1,2-DCA removal decreased from 83.3% to 73.0%, which could be attributed to the quenching of HO by SO42− (Tang et al. 2020).
(9)
(10)

To further assess the applicability of the CaO2/Fe(III) system in practice, actual groundwater was used to replace the ultrapure water in preparing the 0.20 mM 1,2-DCA aqueous solution. The main parameters of the actual groundwater used in this work are shown in Table 4. As shown in Figure 10, when the concentrations of CaO2 and Fe(III) were 4 and 16 mM, respectively, only 32.7% of 1,2-DCA was removed within 180 min in the CaO2/Fe(III) system due to the scavenging effect of high Cl and HCO3concentrations on HO and O2−•. However, 1,2-DCA removal increased to 73.4% with the concentrations of CaO2 and Fe(III) increasing to 20 and 80 mM, respectively. This result revealed that increasing the chemical amounts could enhance 1,2-DCA removal in the CaO2/Fe(III) system in actual groundwater.

Table 4

The main characteristics of the actual groundwater

ParameterValueParameterValue
pH 7.91 SO42− 32.7 mg L−1 
TOC 7.89 mg L−1 Total Fe <0.010 mg L−1 
Cl 117 mg L−1 Ca2+ 21 mg L−1 
HCO3 332 mg L−1 Mg2+ 12 mg L−1 
NO3 2.44 mg L−1 Mn2+ <0.010 mg L−1 
ParameterValueParameterValue
pH 7.91 SO42− 32.7 mg L−1 
TOC 7.89 mg L−1 Total Fe <0.010 mg L−1 
Cl 117 mg L−1 Ca2+ 21 mg L−1 
HCO3 332 mg L−1 Mg2+ 12 mg L−1 
NO3 2.44 mg L−1 Mn2+ <0.010 mg L−1 
Figure 10

1,2-DCA degradation performance in actual groundwater in the CaO2/Fe(III) system ([1,2-DCA]0 = 0.2 mM).

Figure 10

1,2-DCA degradation performance in actual groundwater in the CaO2/Fe(III) system ([1,2-DCA]0 = 0.2 mM).

Close modal

Possible 1,2-DCA degradation pathway

The dechlorination performance of 1,2-DCA in the CaO2/Fe(III) system was investigated. Theoretically, 2 M Cl would be released from 1 M 1,2-DCA when being dechlorinated completely. As shown in Figure 11, the released amount of Cl reached 70% in the CaO2/Fe(III) system, demonstrating that most of the chlorine in the 1,2-DCA could be dechlorinated to Cl.

Figure 11

The dechlorination performance of 1,2-DCA in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM).

Figure 11

The dechlorination performance of 1,2-DCA in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM).

Close modal

Furthermore, the degradation intermediates of 1,2-DCA in the CaO2/Fe(III) system were analyzed by GC-MS. The GC-MS spectrum of the intermediates is supplied in Figure 12. Based on the above analytical results, a degradation pathway of 1,2-DCA was proposed in the CaO2/Fe(III) system (Figure 13). Firstly, O2−• attacked 1,2-DCA and generated vinyl chloride (VC). Then VC was transformed to 2-chloroethanol or 1-chloroethanol through an HO addition reaction, resulting in the generation of ethylene glycol and 1,1-ethanediol. Moreover, the ethylene glycol and 1,1-ethanediol were unstable and were converted to acetaldehyde. Finally, the acetaldehyde was oxidized by HO and transformed to H2O and CO2.

Figure 12

GC-MS chromatogram of 1,2-DCA degradation intermediates at the reaction time of 60 min in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM).

Figure 12

GC-MS chromatogram of 1,2-DCA degradation intermediates at the reaction time of 60 min in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM).

Close modal
Figure 13

Proposed 1,2-DCA degradation pathway in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM).

Figure 13

Proposed 1,2-DCA degradation pathway in the CaO2/Fe(III) system ([CaO2]0 = 6.0 mM, [Fe(III)]0 = 24 mM, [1,2-DCA]0 = 0.2 mM).

Close modal
  • (1)

    A total of 83.3% of 1,2-DCA could be effectively degraded in the CaO2/Fe(III) system when the molar ratio of CaO2/Fe(III)/1,2-DCA was 30/120/1, and both HO and O2−• contributed to the 1,2-DCA removal.

  • (2)

    The 1,2-DCA could be efficiently removed in the CaO2/Fe(III) system in the initial pH range of 3–11, and the negative effect of anions followed the order of HCO3 > Cl > SO42− > NO3.

  • (3)

    The released concentration of Cl (84.1%) revealed that most of the chlorine in 1,2-DCA could be dechlorinated. Based on the detection results of GC-MS, 1,2-DCA was firstly reduced to VC and later oxidized to CO2 and H2O.

  • (4)

    The degradation performance of 1,2-DCA in actual groundwater was not as efficient as that in ultrapure water due to the buffering and coexistence of ions in the actual groundwater, but increasing chemical dosages could overcome this shortcoming, demonstrating that the CaO2/Fe(III) system has a remarkable prospect in 1,2-DCA-contaminated groundwater remediation.

This work was funded by the National Key R&D Program of China (No. 2018YFC1802505).

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

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