Mechanism of carbon tetrachloride reduction in ferrous ion activated calcium peroxide system in the presence of methanol

Abstract

This study investigated the reductive initiation for the depletion of highly oxidized/perhalogenated pollutants, specifically the degradation of carbon tetrachloride (CT) was induced by adding methanol (MeOH) into a ferrous ion (Fe(II)) activated calcium peroxide (CaO₂) system. The results indicated that CT could be completely degraded within 20 min at CaO₂/Fe(II)/MeOH/CT molar ratio of 30/40/10/1 in this system. Scavenging tests suggested that both superoxide radical anion (O₂^•−) and carbon dioxide radical anion (CO₂^•−) were predominant reactive species responsible for CT destruction. Hydroxymethyl radicals (^•CH₂OH), an intermediate in the transformation of MeOH, could also initiate CT degradation by reducing C-Cl bond. GC/MS analysis identified CHCl₃, C₂Cl₄, and C₂Cl₆ as the intermediates accompanied by CT destruction, and a reduction mechanism for CT degradation was proposed accordingly. In addition, the impact of solution matrix and initial solution pH were evaluated, and the results showed that Cl⁻, NO₃⁻, and HCO₃⁻ had adverse effects on CT degradation. Moreover, the alkaline condition was unfavorable to CT depletion. In conclusion, the results obtained in the actual groundwater tests encouragingly demonstrated that the CaO2/Fe(II)/MeOH process is a highly promising technique for the remediation of CT-contaminated groundwater.

1. Introduction

Halogenated aliphatic hydrocarbon organic compounds, which are frequently detected in the environment, are toxic, mutagenic, and generally resistant to complete microbiological degradation, hence, possess long environmental impacts. Carbon tetrachloride (CT), a typical perchlorinated aliphatic hydrocarbon, was widely utilized in the past as a cleaning solvent, flame retardant, refrigerant, and intermediate for industrial products [1]. The occurrence of CT in groundwater has received much attention because of its high toxicity, environmental persistence, and relatively high solubility (800 mg L⁻¹ at 20°C). CT is highly harmful to liver, lungs and kidneys, and has been classified as a 2B carcinogenic compound by the International Agency for Research on Cancer (IARC) [2] and, hence, the maximum allowable concentration in groundwater is regulated at 0.005 mg L⁻¹ [3].

Advanced oxidation processes (AOPs) with persulfate, ozone, permanganate, and hydrogen peroxide (H₂O₂) have been widely applied to sites remediation contaminated by organic compounds. Reactive oxygen species (ROSs) generated in these AOPs, including S₂O₈²⁻ (E₀ = 2.01 V), O₃ (E₀ = 2.1 V), MnO₄⁻ (E₀ = 1.7 V), OH (E₀ = 2.7 V), are strong oxidants and can degrade multiple unsaturated organics including halogenated aliphatic compounds, halogenated aromatic compounds, and benzene, toluene, ethylbenzene and xylene (BTEX) [3]. However, highly oxidized/perhalogenated organic pollutants, such as hexachlorobenzene, bromotrifluoromethane, and CT, are hardly degraded by these oxidants due to the inefficient reaction between oxidants and C-Cl bonds. Therefore, the use of reduction-based technique will be an alternative approach that is more effective for the degradation of these contaminants.

It is well documented that superoxide radical anion (O₂^•−, E₀ = −2.4 V) [4], carbon dioxide radical anion (CO₂^•−, E₀ = −2.0 V) [5], and excited electron [6] have the potential to transfer an electron to C-Cl bonds, therefore contributing to effective CT destruction. Thus, it is essential to generate these reductants and keep them full activity in the system aiming to destruct these oxidized/perhalogenated organic pollutants. O₂^•−, as a reductive species, can be easily generated in catalytic hydrogen peroxide (CHP) process (reactions 1–3) [4]. Unfortunately, it is found that O₂^•− is unstable and has very short life-time because of rapid disproportionation through reaction 4 [7], and hence it is considered incapable of reducing CT in solution at low concentration. In contrast, Smith et al found that increasing H₂O₂ concentration (>0.1 mol L⁻¹) or adding organic solvents like ethanol, acetone or other aprotic solvents could significantly strengthen the reactivity of O₂^•− due to solvent effects [4, 8]. However, high H₂O₂ concentrations are typically difficult to maintain for subsurface environments, and it also poses issues for handling and implementation. Conversely, the use of small quantities of biodegradable solvents is an effective and practical option for promotion of reductive treatment of recalcitrant pollutants. In addition, adding solvents to the CHP process may also produce other reductants that are favorable for reduction of oxidized/perhalogenated organics.

$${\text{H}}_2{\text{O}}_2+{\text{Fe}}^{2+}\to {\text{Fe}}^{3+}+{\text{OH}}^{•}+{\text{OH}}^{-}\text{k}=76{\text{M}}^{-1}{\text{s}}^{-1}$$ (1)

$${\text{H}}_2{\text{O}}_2+{\text{OH}}^{•}\to {\text{H}}_2\text{O}+{{\text{HO}}_2}^{•}\text{k}=5.0\times {10}^7{\text{M}}^{-1}{\text{s}}^{-1}$$ (2)

$${{\text{HO}}_2}^{•}\leftrightarrow {\text{H}}^{+}+{{\text{O}}_2}^{•-}\text{k}=1.58\times {10}^5{\text{M}}^{-1}{\text{s}}^{-1}$$ (3)

$$2{{\text{O}}_2}^{•-}+{\text{H}}_2\text{O}\to {\text{O}}_2+{{\text{HO}}_2}^{-}+{\text{OH}}^{-}$$ (4)

α-hydroxyalkyl radical (R^•CHOH), generated from the reaction of aliphatic alcohols with OH^• (reaction 5), is an organic reducing radical that can also transfer an electron to C-Cl bonds [9, 10]. Our previous work has confirmed that hydroxyethyl radical (^•CHCH₃OH) could be generated in the Fe(II)-activated calcium peroxide (CaO₂) system in the presence of ethanol, and concomitantly CT was observed to be completely degraded [11]. According to the chain reactions 1–5, though they cannot directly reflect action with CT, massive H₂O₂ and OH^• are important factors for CT reduction because H₂O₂ and OH^• was produced initially and acted as precursor to generate the reducing species, O₂^•− and R^•UHOH. CaO₂ as a green chemical agent is generally recognized as an effective solid H₂O₂ and O₂ release resource and, hence widely used in in-site chemical oxidation remediation and biological wastewater treatment practices [12]. Compared with liquid H₂O₂, CaO₂ is demonstrated more effective because of regular H₂O₂ release (reaction 6) [13]. Therefore, in this work, CaO₂ was adopted to release H₂O₂ and later initiated to generate OH^• under the aid of Fe(II) activation, and meanwhile, the selected aliphatic alcohol methanol (MeOH) was used for the enhancement of reducing radical’s generation.

$${\text{RCH}}_2\text{OH}+{\text{OH}}^{•}\to {\text{R}}^{•}\text{CHOH}+{\text{H}}_2\text{O}$$ (5)

$${\text{CaO}}_2+2{\text{H}}_2\text{O}\to {\text{H}}_2{\text{O}}_2+\text{Ca}{(\text{OH})}_2$$ (6)

Though the use of alcohol amendment for promoting reduction of chlorinated aliphatic compounds has been confirmed, the reaction mechanism of CT degradation under Fe(II)- activated CaO₂ with MeOH remains unclear. Hence, the purposes of this work are (1) to compare the CT degradation in the CaO₂/Fe(II) system without or with the presence of MeOH, (2) to explore the main radicals responsible for CT destruction, (3) to propose possible CT degradation pathway, and (4) to evaluate the application of the CaO₂/Fe(II)/MeOH process for CT degradation in an actual groundwater remediation.

2. Materials and methods

2.1. Materials and analytical methods

All the materials and analytical methods adopted were provided in Supplementary Material (Text S1 and S2).

2.2. Experimental procedures

The CT stock solutions (1.3 mmol L⁻¹) were prepared by equilibrating the pure non-aqueous phase liquid CT with Milli-Q water or actual groundwater under gentle stirring for 2 h. Then the CT Stock solution was added into a 250 mL cylindrical glass reactor (an inner diameter of 6.0 cm and a height of 9 cm) with two openings at the top: one for dosing and the other for sampling. Subsequently, the predetermined amounts of FeSO₄·7H₂O and MeOH were added into the reactor. The initial solution pH was unadjusted with the exception of experiments for assessing the pH influence, in which the initial solution pH was adjusted with NaOH (0.1 mol L⁻¹) or H₂SO₄ (0.1 mol L⁻¹). The addition of anions and other reagents were then fulfilled. A thermostat circulating water bath (DC, Ningbo, China) was utilized to control the temperature at 20 ± 0.5°C, and a magnetic stirrer was used to keep the uniformity of mixtures in the aqueous solution. The reaction was then initiated by adding the desired CaO₂ dosage. Control tests were conducted in the absence of CaO₂ condition. At the determined time, 1 mL solution was transferred to a brown bottle containing 1 mL n-hexane to stop the reaction before analysis. In order to identify the reactive species for CT degradation, isopropanol (IPA), 1,4-benzoquinone (BQ), and methyl viologen dichloride (MV²⁺) as radical scavengers were applied in the scavenging tests to observe CT degradation changes. Each test was conducted at least in triplicate with the standard deviations below 5%. The average values were used for all presentation and discussion.

3. Results and discussion

3.1. Enhancement of CT degradation in the Fe(II)-activated CaO₂ system by MeOH

The comparison of CT degradation in the Fe(II)-activated CaO₂ system without and with the presence of MeOH were performed, and the results were presented in Fig. 1. Control tests without CaO₂ showed less than 6.5% CT loss due to volatilization and other mass loss reason during the test period. As shown in Fig. 1a, CT degradation was minimal under low chemical dosages when the amounts of CaO₂ and Fe(II) were set equal. However, when the oxidants was increased to much higher than the Fe(II) dosage, greater removal of CT was observed with 25% of CT reduction at the CaO₂/Fe(II)/CT molar ratio of 300/30/1. It was noted that CT removal efficiencies were continuously increased to 31% and 59% when the dosages were further raised to 600/60/1 and 900/90/1, respectively.

Fig. 1.

Comparison of CT degradation in the Fe(II)-activated CaO₂ system: (a) without MeOH and (b) with MeOH ([CT]₀ = 0.13 mmol L⁻¹).

In general, the CaO2/Fe(II) system favored unsaturated organic pollutant degradation owing to its high oxidation of OH^• [3, 14, 15]. But in this work, the reason for CT degradation should differ from those because CT depletion was only reached at remarkably higher CaO₂ dosage condition. Theoretically, when the amount of CaO₂ equals to Fe(II), ROS production, mainly OH^• and minor O₂^•−, would be maximized based on the stoichiometric quantities of reactions 1–3. To the best of our knowledge, the reactions of OH^• with C-Cl bonds are recognized as relatively inefficient, in contrast, the reactions of O₂^•− with C-Cl bonds are more feasible through electron transfer. However, O₂^•− is characterized as minimal reactivity in aqueous solution due to its high solvation by water molecules, which accelerates its rapid disproportionation forming O₂ and HO₂^•−. Teel and Watts [8] reported that high concentrations of H₂O₂ can improve the solvation sphere surrounding O₂^•− and thus reduce its disproportionation. In the experiments where the addition of CaO₂ was much higher than Fe(II), H₂O₂ released from CaO₂ gradually accumulated to a high concentration over the test period. Therefore the effect of high H₂O₂ on O₂^•− caused hyperactive O₂^•− and thus favored CT degradation. However, the drawbacks with such high CaO₂ caused elevation of solution pH, in turn, hampered more H₂O₂ release from CaO2, and at the same time produced a large amount of O₂ [16] which was unconducive to CT degradation. In addition, the operation costs of the use of high CaO₂ concentration would be costly and impractical for field-scale CT remediation.

Compared with the CaO₂/Fe(II) system, the CaO₂/Fe(II)/MeOH process presented significant enhancement in CT depletion (Fig. 1b). CT removal efficiencies of 18%, 35% and 83% were obtained at the CaO₂/Fe(II)/MeOH/CT molar ratios of 10/10/2/1, 20/20/4/1, and 30/30/6/1 within 20 min, respectively, while no CT degradation occurred at the same CaO₂ and Fe(II) dosages without MeOH addition. Based on the literatures [8, 17], the addition of MeOH promoted CT removal in two ways. On one hand, MeOH addition improved the circumstance surrounding O₂^•− through a mixture of MeOH and water molecules, which reduced O₂^•− disproportionation and improved its reactivity in aqueous solution. On the other hand, MeOH generated other reducing species responsible for CT degradation through the reaction between OH^• and MeOH. As aforementioned, a large amount of H₂O₂ released by CaO₂ may play a similar action in increasing O₂^•− reactivity through the solvent effect, but the strength was weaker than MeOH. Overall, the greater O₂^•− reactivity and other generated reactive reducing species were the reasons for remarkable CT degradation in the CaO₂/Fe(II)/MeOH process. Similar tests were repeated by adding equi-molar concentration of acetone (CP) and dimethyl sulfoxide (DMSO) separately. The results showed that CT degradation was also increased to 25% and 36% by CP and DMSO, respectively (Fig. S1). In a word, these results strongly indicated the positive contribution and potential of small- molecule organic compounds in enhancement of CT degradation.

Additional experiments were further conducted to explore the effect of various CaO₂, Fe(II) or MeOH dosages on CT degradation, and the results described briefly in Text S3 and Fig. S2 showed that the CaO₂/Fe(II)/MeOH/CT molar ratio of 30/40/10/1 was most suitable for CT removal with over 99% CT depletion. Overall, these significantly encouraging results were obtained by the CaO₂/Fe(II)/MeOH process in this study, which is of great interest to researchers for further indentifying MeOH function in CT degradation mechanism.

3.2. Identification of main reactive species for CT degradation in the CaO₂/Fe(II)/MeOH system

OH^• and O₂^•− are easily generated in the Fe(II)-activated CaO₂ process [14], and their roles in CT degradation need to be confirmed. Taking into account that CO₂^•−, an intermediate during the mineralization of MeOH, can also reduce C-Cl bonds readily [17], the function of CO₂^•− also requires clarification of its contribution to CT removal. Therefore, CT degradation performance was tested both in the absence and presence of appropriate quenchers of these species, individually. The scavengers applied were as below: IPA for OH^• (k _OH• = 3.0 × 10⁹ M⁻¹ s⁻¹) [4, 18], BQ for O₂^•− (k _O2•− = 9.6 × 10⁸ M⁻¹ s⁻¹) [19] and MV²⁺ for CO₂^•− (k _CO2•− = (6.3 ±0.7) × 10⁹ M⁻¹ s⁻¹) [20], and the results of scavenging tests were shown in Fig. 2.

Fig. 2.

The degradation of CT in the CaO₂/Fe(II)/MeOH system with the addition of different scavengers: (a) IPA; (b) BQ and (c) MV²⁺ ([CaO₂]₀ = 3.90 mmol L⁻¹, [Fe(II)]₀ = 5.20 mmol L⁻¹, [MeOH]₀ = 1.30 mmol L⁻¹, [CT]₀ = 0.13 mmol L⁻¹).

Over 99% of CT was degraded with the addition of excess IPA in the initial 3 min, while 84% of CT was removed in the absence of the scavenger (Fig. 2a). A small enhancement caused by IPA not only confirmed that OH^• was not the reactant contributing to direct destruction of CT, but also figured out that IPA played a similar role as MeOH in CT degradation, which consisted with reports in other literatures [8, 19, 21]. Since the concentration of IPA was more than 10 fold higher than that of MeOH, IPA might play a major part in reductants formation and reactivity, thereby causing faster CT removal. The results of O₂^•− scavenging tests with BQ were presented in Fig. 2b. The removal of CT decreased from 93% to 37% and 29%, respectively, in the presence of 15 and 30 mmol L⁻¹ of BQ. Teel and Watts adapted excess chloroform (CF) as O₂^•− scavenger in a modified Fenton’s reaction and observed that CT degradation was decreased to a minimum level, thus indicating that O₂^•− was a main reactive species for CT destruction [4]. Similarly, Xu et al also identified that O₂^•− contributed to CT degradation in the thermally activated persulfate system through a BQ scavenging test [19]. The inhibition by O₂^•− scavenger in this work strongly suggested that O₂^•− played a significant role in CT destruction in the CaO₂/Fe(II)/MeOH system. Since CO₂^•− was probably involved in the oxidation of MeOH by OH^• in the CaO₂/Fe(II)/MeOH system according to the literatures [17, 22], CO₂^•− scavenging test was also carried out to confirm its function. As presented in Fig. 2c, an obvious inhibition on CT removal appeared in the scavenging tests with MV²⁺. 62% and 75% drop in CT removal were observed in the tests with 0.15 and 1.5 mmol L⁻¹ MV²⁺, respectively. The performance that the removal of CT was quenched along with the addition of CO₂^•− scavenger further indirectly demonstrated that CO₂^•−, possibly generated through reaction 7, was one of the active species for the degradation of CT in the CaO₂/Fe(II)/MeOH process.

$${\text{CH}}_3\text{OH}+{\text{H}}_2{\text{O}}_2/{\text{OH}}^{•}\to {{\text{CO}}_2}^{•-}+{\text{H}}_2\text{O}$$ (7)

Besides O₂^•− and CO₂^•−, hydroxymethyl radical ^•CH₂OH) generated between the reaction of MeOH and OH^• (reaction 8) can also transfer an electron to C-Cl bonds, which was the intrinsic cause for CT destruction [23]. We have observed hydroxyethyl radical by electron paramagnetic resonance (EPR) technique in the Fe(II)-activated CaO₂ system in the presence of ethanol [11]. Therefore, the EPR technique was similarly applied in this work. However, the signal of ^•CH₂OH was not detected due to the difficulty in distinguishing its weak response values in this work. Nevertheless, ^•CH₂OH should still be deduced as a reactive species for CT degradation in the CaO₂/Fe(II)/MeOH system. Overall, the reactive reducing radicals including O₂^•−, CO₂^•−, and ^•CH₂OH were directly involved in the reductive destruction of CT, while OH^• might played an indirect role by controlling the yield of these reductants.

$${\text{CH}}_3\text{OH}+{\text{OH}}^{•}\to {}^{•}{\text{CH}}_2\text{OH}+{\text{H}}_2\text{O}\text{k}=9.7\times {10}^8{\text{M}}^{-1}{\text{s}}^{-1}$$ (8)

3.3. Mechanism of CT reduction in the CaO₂/Fe(II)/MeOH system

Based on the outstanding CT depletion performance, the intermediates, including the volatile chlorinated organics and chloride ions in solution were further investigated to reveal the pathway of CT destruction through GC/MS and ion chromatography analyses. Chloroform (CF), hexachloroethane (HCA), and tetrachloroethene (PCE) were identified by GC/MS analyses and regarded as the chlorinated organic intermediates as presented in Fig. S3. Further experiments showed that all three of the chlorinated intermediates were produced within the initial 5 min reaction and subsequently degraded within later 15 min (Fig. 3). The generation and later elimination of CF, HCA, and PCE demonstrated that CT destruction might follow a multi-step process in system. Since CF, HCA and PCE were the main intermediates of CT, it was worthy to confirm the role of MeOH in the removal of these chlorinated products, individually. The results of CF, HCA, and PCE depletion in the Fe(II)- activated CaO₂ system without or with the presence of MeOH were compared in Fig. S4. It suggested that the CaO2/Fe(II)/MeOH process is also suitable for HCA degradation, but not favorable for PCE which should be more amenable to an oxidation process instead of a reductive one [24, 25]. Meanwhile, CF is minimally degraded by the two systems.

Fig. 3.

Evolution of the chlorinated organic intermediates in the CaO2/Fe(II)/MeOH system ([CaO₂]₀ = 3.90 mmol L⁻¹, [Fe(II)]₀ = 5.20 mmol L⁻¹, [MeOH]₀ = 1.30 mmol L⁻¹, [CT]₀ = 0.13 mmol L⁻¹).

The amounts of the chlorinated organic intermediates produced along with CT degradation were detected by GC/MS, and it was found that their contents were far less than the amount of CT depleted. Besides chlorinated organics, the Cl⁻ concentration in solution was also monitored in the experiment along with the CT depletion by ion chromatography in order to check the extent of CT mineralization. Cl⁻ released in solution at maximum represented 85% of total theoretical Cl⁻ formation, suggesting that CT dechlorination performed well in the CaO₂/Fe(II)/MeOH process. The Cl balance was quantified by combining the amount of the chlorinated organic intermediates produced and the Cl⁻ released from CT compound (Fig. 4). The theoretical quantity of Cl in parent CT was regarded as the total amount of Cl in system (100%), which was shared by four parts: (1) Cl in CT compound, (2) Cl in chlorinated organic intermediates, (3) Cl released in solution and, (4) unknown Cl. As it can be seen, Cl content in chlorinated organic intermediates was much less, while Cl released in solution appeared larger and increased gradually versus CT degradation. The unknown Cl was caused by CT volatilization and other possible chlorinated-acid formation [17]. Since the addition of MeOH would increase the TOC level, which is a discouraging point in practical treatment, TOC analysis was conducted to evaluate the mineralization of MeOH. The final TOC was below 7.2 mg L⁻¹ while the initial TOC value was 17.2 mg L⁻¹.HCHO, identified by GC/MS (showed in Fig. S3), was regarded as an oxidation product of MeOH, and it could be continually mineralized when considering that the theoretical organic carbon of CT only contributed to 1.6 mg L⁻¹ of TOC in this process. Based on the literature [26], after a series of complex reactions with O₂, H₂O₂ and OH^•, HCHO would be transformed into HCOOH, which has been confirmed to produce CO₂^•− under the aid of OH^• [23, 27]. Unfortunately, the presence of HCOOH was not detected in the experiment, probably due to its low concentration and short life-time. Although adding MeOH is indeed a risk for actual groundwater, the fact that MeOH and its by-products showed less toxicity and favorable biodegradability compared with CT make it reasonable and acceptable. Nevertheless, more efforts are needed to improve the process and investigate the risk in actual remediation, and more environmentally friendly chemicals instead of MeOH will be focused on in our future work.

Fig. 4.

Cl balance during CT degradation in the CaO₂/Fe(II)/MeOH process ([CaO₂]₀ = 3.90 mmol L⁻¹, [Fe(II)]₀ = 5.20 mmol L⁻¹, [MeOH]₀ = 1.30 mmol L⁻¹, [CT]₀ = 0.13 mmol L⁻¹)

According to the above discussion, a CT degradation pathway was proposed for understanding the mechanisms involved (Fig. 5). In the first step, a series of OH^•-based chain reactions generated the reductants including O₂^•−, CO₂^•−, and ^•CH₂OH. Once the reductants were produced and the associated reactivity was maximized, all of the reductants attacked CT by transferring electron to C-Cl bonds yield ^•CCl₃ (reactions 9–11), after that ^•CCl₃ was continually dechlorinated to :CCl₂ by these reductants. Finally, :CCl₂ rapidly hydrolyzed to generate chlorinated-acid, Cl⁻, and CO₂ under the influence of water molecule. It has to be mentioned that, accompanied with the formation of ^•CCl₃ and :CCl₂, recombination between them would yield HCA and PCE, and H-abstraction from MeOH by ^•CCl₃ would form CF. But once the chlorinated organic intermediates yielded, all of them would be degraded subsequently to completion. The CT dechlorination pathway in this system was similar to that in other system [28–33] where Fe⁰, ascorbic acid, CO₂^•−, Zn⁰ and other reactive species acted as the electron donor transferring an electron to C-Cl bonds, but the quantity and evolution of the intermediates were different.

Fig. 5.

Proposed CT degradation pathway in the CaO₂/Fe(II)/MeOH process.

$${\text{CCl}}_4+{{\text{O}}_2}^{•-}\to {}^{•}{\text{CCl}}_3+{\text{Cl}}^{-}+{\text{O}}_2$$ (9)

$${\text{CCl}}_4+{}^{•}{\text{CH}}_2\text{OH}\to {}^{•}{\text{CCl}}_3+\text{HCHO}+{\text{Cl}}^{-}+{\text{H}}^{+}$$ (10)

$${\text{CCl}}_4+{{\text{CO}}_2}^{•-}\to {}^{•}{\text{CCl}}_3+{\text{CO}}_2+{\text{Cl}}^{-}+{\text{H}}^{+}$$ (11)

As aforementioned, CT could be destructed by high CaO₂ concentration, and therefore we also carried out the experiments to evaluate the intermediate of CT in Fe(II)-activated CaO₂ system in the absence of MeOH. The results presented in Fig. 6 showed that CT depletion was accompanied with significant CF formation even at complete CT destruction in 260 mmol L⁻¹ CaO₂ dosage. Since CF appears more persistent and toxic than CT in the environment [34], and considering the difference of CT degradation effectiveness as well as the intermediates evolution in the CaO₂/Fe(II) and CaO₂/Fe(II)/MeOH processes, our study strongly suggested that the CaO₂/Fe(II)/MeOH process possesses much more advantageous than the CaO₂/Fe(II) process. In addition, MeOH addition would also increase the CT solubility at actual CT DNAPLs contaminated sites, which will be benefit for CT reaction. Since ethanol has the similar ability to enhance CT destruction [11], the comparison of CT degradation in the CaO₂/Fe(II) system with the presence ethanol and MeOH were performed and the results were presented in Fig. S5. Although the enhancement of ethanol on CT degradation seemed to be better than MeOH, ethanol used in the CaO₂/Fe(II)/CT process would cause more complex intermediates like 1,1,1-trichloroethane and 1,1-dichloroethylene. In addition, TOC caused by ethanol would be double when compared to MeOH. Based upon the above two aspects, MeOH has obviously comparative advantage compared to ethanol in practical remediation.

Fig. 6.

Evolution of CF during CT degradation in the CaO₂/Fe(II) process (CaO₂/Fe(II) molar ratio = 10/1, [CT]₀ = 0.13 mmol L⁻¹)

3.4. CT degradation performance in actual groundwater application

Although CT degradation was effective in ultrapure water condition, it was critical to carry out further test to assess CT degradation in actual groundwater because the solution matrix including inorganic anions (such as NO₃⁻, Cl⁻, SO₄²⁻, and HCO₃⁻), solution pH, and natural organic matter (NOM) can affect the category and intensity of reactive species, and ultimately influence overall remediation effectiveness [35, 36]. Hence, the experiments were further conducted to evaluate the effect of solution matrix individually, and finally using the actual groundwater (Table S1) to assess the applicability of the CaO₂/Fe(II)/MeOH process for actual CT contaminated groundwater remediation.

3.4.1. Effect of inorganic anions, initial solution pH and NOM

As shown in Fig. 7a and b, NO₃⁻ and Cl⁻ had a slight suppression on CT removal, whereas the effect of SO₄²⁻ was found to be negligible (Fig. 7c), meanwhile HCO₃⁻ had an obvious inhibitive effect (Fig. 7d). Hasan et al. confirmed that solvated electrons could be produced by decomposition of H₂O₂ [37], and similarly here, it was speculated that solvated electrons were also generated and were responsible for CT destruction. Since high concentration of NO₃⁻ was a stronger scavenger of solvated electrons [18], the presence of NO₃⁻ would reduce CT depletion in terms of hampering solvated electrons-based reduction. Moreover, NO₃⁻ might also compete with CT for other reductants. The presence of Cl⁻ also suppressed CT degradation, and the inhibitory effect was slightly higher than NO₃⁻. An approximate 7% drop in CT removal was observed when Cl⁻ concentration was 100 mmol L⁻¹ The reasons for this inhibitive effect were that Cl⁻ could scavenge the precursor OH^• for the reductants through reaction 12 [38], and consume O₂^•− through competing reaction (reaction 13) [39]. Although NO₃⁻ and Cl⁻ only appeared slight inhibition in this work, a higher concentration of their presence would obviously suppress CT degradation. Compared with NO₃⁻ and Cl⁻, a much more significantly inhibitive influence occurred in the presence of HCO₃⁻. CT degradation efficiency was reduced to 20% and 15% with the addition of 10 and 100 mmol L⁻¹ HCO₃⁻, respectively. The main reasons were as follow: (1) the presence of HCO₃⁻ would increase the solution pH and buffering capacity which were unfavorable to CT destruction (discussion in later section). (2) HCO₃⁻ could sweep OH^• through reactions 14 and 15 [40], which was emphasized to be vital for reductants formation in the system. (3) HCO₃⁻ could directly consume O₂^•− and CO₂^•− via reactions 16 and 17 [41, 42].

Fig. 7.

Effect of solution matrix on CT degradation in the CaO₂/Fe(II)/MeOH system (a) NO₃⁻; (b) Cl⁻; (c) SO₄²⁻; (d) HCO₃⁻; (e) pH and (f) NOM ([CaO₂]₀ = 3.90 mmol L⁻¹, [Fe(II)]₀ = 5.20 mmol L⁻¹, [MeOH]₀ = 1.30 mmol L⁻¹, [CT]₀ = 0.13 mmol L⁻¹).

$${\text{Cl}}^{-}+{\text{OH}}^{•}\to {\text{ClOH}}^{•-}$$ (12)

$${\text{Cl}}^{-}+{{\text{O}}_2}^{•-}\to \text{product}$$ (13)

$${{\text{HCO}}_3}^{-}+{\text{OH}}^{•}\to {{\text{CO}}_3}^{•-}+{\text{H}}_2\text{O}$$ (14)

$${{\text{HCO}}_3}^{-}+{\text{OH}}^{•}\to {{\text{HCO}}_3}^{-}+{\text{OH}}^{-}$$ (15)

$${{\text{HCO}}_3}^{-}+{{\text{O}}_2}^{•-}\to {{\text{HO}}_2}^{-}+{{\text{CO}}_3}^{•-}$$ (16)

$${{\text{HCO}}_3}^{-}+{{\text{CO}}_2}^{•-}\to {{\text{HCO}}_2}^{-}+{{\text{CO}}_3}^{•-}$$ (17)

As depicted in Fig. 7e, it was apparent that CT depletion was greater at acidic pH range from 3.0–6.0, whereas CT depletion was markedly suppressed at pH over 7.0. This phenomenon was caused by the following three reasons. First, a high solution pH encouraged Fe(II) precipitation, and thus blocked the chain reaction of reducing species generation. Second, the elevated solution pH constrained H₂O₂ release from CaO₂ dissolution [16]. Third, the reactivity of ROSs involved in this process was also influenced by high pH condition. It is noted that a slightly inhibitory effect could also appear at pH<3, and this result was due to the relatively low reaction rate between Fe(II) and H₂O₂ [43]. Although CT degradation performed well in acidic pH range from 3.0–6.0 in this work, pH should still need to pay attention in practical application because the actual groundwater, in general, has a high pH buffering capacity.

Fig. 7f showed a significant inhibitive effect on CT degradation by humic acid (HA), a main fraction of NOM. CT degradation decreased by 28%, 22% and 16% in the initial 3 min when the addition of HA was 10, 50 and 100 mg L⁻¹, respectively. On one hand, it has been documented that the presence of HA could consume OH^• [44], which was vital for reductants formation in this work, therefore the diminution of OH^• would hinder CT degradation ultimately. On the other hand, a slight alleviation of CT degradation inhibition with increasing the concentration of HA from 1.0 to 100 mg L⁻¹ was speculated to be related to chelation of Fe(II) with HA [45].

3.4.2. The performance of CT degradation in actual groundwater

To validly evaluate the applicability of the CaO2/Fe(II)/MeOH process for actual groundwater remediation, tests were conducted using the actual groundwater. Fig. 8 shows the removal of CT in the actual groundwater when the CaO₂/Fe(II)/MeOH/CT molar ratios were set at 30/40/10/1, 60/80/20/1, 120/160/40/1, 180/240/60/1 and 240/320/80/1, respectively. It was observed that CT degradation was greatly reduced in the actual groundwater compared to the complete CT depletion in ultrapure water system at the same chemical addition (30/40/10/1). This may be caused by the presence of various constituents in actual groundwater, such as the high anion contents of NO₃⁻ (27.37 mg L⁻¹), Cl⁻ (35.73 mg L⁻¹) and HCO₃⁻ (251.31 mg L⁻¹), the pH buffer capacity (pH = 6.86), as well as high total organic carbon (20.59 mg L⁻¹). Nevertheless, complete CT removal could still be achieved after a relatively longer reaction time by increasing dosages of reagents to 180/240/60/1, encouragingly suggesting that the CaO₂/Fe(II)/MeOH process can be applied for remediation of CT contaminated groundwater.

Fig. 8.

CT degradation performance in the actual groundwater ([CT]₀ = 0.13 mmol L⁻¹).

4. Conclusions

In this work, the degradation of CT in the CaO₂/Fe(II) system was greatly promoted by addition of MeOH. The optimal CT removal was achieved at the CaO₂/Fe(II)/MeOH/CT molar ratio of 30/40/10/1. The enhancement of CT degradation by three other solvents, namely CP, DMSO, and IPA, indicated that small-molecule solvent amendment is a viable approach for CT remediation. Radical scavenger tests demonstrated that O₂^•− and CO₂^•− were the dominant species, and meanwhile ^•CH₂OH, based on literature survey, was also regarded as a reductant responsible for CT degradation. The detected intermediates, CF, HCA and PCE, were quantitatively analyzed and a chlorine balance was calculated along with CT degradation, indicating that CT could be well mineralized. Moreover, a multi-step process for CT mineralization was proposed. The solution matrix tests showed that NO₃⁻ and Cl⁻ had a slight scavenging effect on CT depletion, whereas HCO₃⁻ and HA had a significant inhibitive function. The CT removal performed well in weak acidic to nearly neutral pH condition. Finally, CT degradation in actual groundwater demonstrated that the CaO₂/Fe(II)/MeOH process has a great potential for application in CT contaminated groundwater remediation.

Highlights:

• CT degradation in CaO₂/Fe(II) system is promoted by MeOH addition.

• The leading species for CT degradation are O₂^•−, CO₂^•−.

• CHCl₃, C₂Cl₄, and C₂Cl₆ were the intermediates of CT.

• A reduction mechanism for CT degradation was proposed.

• Efficient removal of CT in groundwater was attained.