Effective degradation of 1,2-dichloroethane in calcium peroxide activated by Fe(III): performance and mechanisms Open Access

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.

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.

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

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.

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 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).

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.

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.

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.

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^−•.

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.

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).

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 HCO₃⁻concentrations 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.

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⁻.

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.

• (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.