Enhanced degradation of trichloroethene by sodium percarbonate activated with Fe(II) in the presence of citric acid

Trichloroethene (TCE), a typical chlorinated solvent which was widely used in metal degreasing, dry cleaning, etc., is a ubiquitous contaminant. Due to its harmful characteristics such as cytotoxicity, carcinogenicity and persistence in groundwater environments, TCE has been identified as one of the priority existing chemicals under the toxic substances control act by the United States Environmental Protection Agency (USEPA 2014).

During the last few decades, various techniques have been developed for chlorinated solvent contaminated groundwater remediation, such as permeable reactive barriers, soil vapor extraction, air sparging and in situ chemical oxidation (ISCO). ISCO accomplished by injecting chemical oxidants (including ozone, potassium permanganate, persulfate, and Fenton's reagent) directly into contaminated groundwater to degrade contaminants is an increasingly popular technology (Krembs et al. 2010). Among the above-mentioned chemical oxidants, Fenton's reagent has attracted much attention because of its strong capacity for oxidizing organic contaminants via the hydroxyl radical (HO•) (2.76 V) generated from the decomposition of H₂O₂. Though HO• has a higher redox potential than ozone (2.07 V), permanganate (1.68 V), and persulfate (2.01 V) (Gu et al. 2011), other radicals may also play roles in the process, such as the superoxide radical (O₂⁻•) and perhydroxyl radical (HO₂•), which promote chain free-radical reactions in the presence of Fe(II) or Fe(III).

However, Fenton's reagent has some stubborn drawbacks, e.g. the rapid accumulation of Fe(III), easy precipitation of Fe(OH)₃, and the narrow applicable pH ranges (2–4), therefore limiting its widespread application (Ahn et al. 2013). It is reported that the addition of chelating agents can effectively stabilize and minimize the loss of soluble iron (Luca et al. 2014). For example, chelating agents such as citric acid (CA) (Jho et al. 2010), ethylenediaminetetraacetic (EDTA) (Chang et al. 2013) and (S,S)-ethylenediamine-N,N-disuccinic acid (EDDS) (Ahmad et al. 2012), have been documented as efficient agents in modified Fenton processes in the degradation of contaminants.

Sodium percarbonate (SPC) (2Na₂CO₃·3H₂O₂) is a cheap, stable and environmentally friendly oxidant (Kabalka et al. 1989). In recent years, SPC has been used as an alternative oxidant in ISCO practice (de la Calle et al. 2012). SPC naturally decomposes generating a theoretical available active oxygen as shown in Equation (1) (Wu & Zhou 2000):

To the best of our knowledge, the ability of CA in enhancing the Fe(II)-catalyzed oxidative capacity of SPC for TCE degradation has not been investigated. Therefore, the main purpose in this study is to figure out the effect of CA on TCE degradation. We will specifically focus on: (1) investigating the effect of CA dosage on TCE degradation performance in the SPC/Fe(II)/CA system; (2) assessing the effects of inorganic anions (Cl⁻, HCO₃⁻ ions) and natural organic matter (NOM) on TCE degradation performance; and (3) identifying the reactive oxygen radicals generated in the SPC/Fe(II)/CA system through probe tests and their contribution to TCE degradation through scavenger tests.

Trichloroethene (TCE, >99.0%), SPC (2Na₂CO₃·3H₂O₂, >98%), hydroxylamine hydrochloride (>98.5%) and 1,4-benzoquinone (>97%) were obtained from Aladdin (Shanghai, China). Ferrous sulfate heptahydrate (>99%), sodium bicarbonate (>99.5%), EDTA acid (>99%), CA (>99%), humic acid (HA, fulvic acid >90%), nitrobenzene (>97%), carbon tetrachloride (>97%) and tert-butyl alcohol (>99%) were purchased from Shanghai Jingchun Reagent Ltd Co. (Shanghai, China). (S,S)-ethylenediamine-N,N-disuccinic acid (EDDS, 35% in water) was obtained from Sigma-Aldrich (Shanghai, China). All of these reagents were used as received without further purification. Ultrapure water from a Milli-Q water process (Classic DI, ELGA) was used for the preparation of aqueous solutions.

TCE stock solution was prepared by allowing the pure non-aqueous-phase liquid TCE to equilibrate with Milli-Q water overnight under gentle stirring in darkness. The TCE stock solution was then diluted to the desired concentration (initial TCE concentration = 0.15 mM). Batch tests were conducted in a 250 mL cylindrical glass reactor to evaluate the effectiveness of activated SPC oxidation for the remediation of TCE-contaminated groundwater. A magnetic stirrer was used to ensure uniform mixing of contaminants in the aqueous solution. At the start of a run, the reactor was completely filled by the TCE-containing solution to remove head space and avoid loss of TCE from volatilization. The initial solution pH in all experiments was unadjusted and the temperature was controlled at 20 °C. Control tests without Fe(II) and SPC were also conducted in parallel to examine the behavior of the contaminants in an aqueous system. Aqueous samples were taken at the desired time intervals and analyzed immediately, and the tests were conducted at least in duplicate and the mean values were reported.

Aqueous samples (1.0 mL) were analyzed following extraction with hexane (1.0 mL) for 3 min using a vortex stirrer, and allowed to separate for 5 min. Then the organic phase (TCE in hexane) was transferred to a 2 mL GC vial. TCE, nitrobenzene and carbon tetrachloride in hexane were analyzed using a gas chromatograph (Agilent 7890A, Palo Alto, CA), and specific conditions can be found in our previous research (Zhang et al. 2015). The recovery of TCE through the above procedure was in the range of 87%–95%. The chloride anion was analyzed by ion chromatography (Dionex ICS-I000, Sunnyvale, CA). The concentrations of ferrous ion (Fe(II)) and total iron ions (Fe(II) and Fe(III)) were measured by using the 1,10-phenanthroline method (Tamura et al. 1974). Specifically, for ferrous ion measurement, a 1.0 mL sample was mixed with 5.0 mL potassium biphthalate (0.2 M) and 10 mL 1,10-phenanthroline monohydrate (0.3%) in a 25 mL colorimetric tube. After complete mixing the absorbance at 512 nm was measured. The total iron ions were measured with the same procedure, but with the addition of 1.0 mL hydroxylamine hydrochloride (10%) in the samples. The solution pH was measured with a pH meter (Mettler-Toledo DELTA 320).

It should be noted that the addition of EDTA and EDDS showed no positive effect on TCE degradation (Figure 1). The reasons for this result could be due to: (1) the different chelating ability of different chelators, leading to the different activated capacity of chelated Fe(II), and finally affecting free-radical production and TCE degradation. Specifically EDTA and EDDS have strong chelating ability with Fe(II), hence limited available Fe(II) involved in HO• generation chain reactions; (2) the high reaction rate of EDTA with HO• (k_EDTA/HO• = 4.0 × 10⁸ L M⁻¹ s⁻¹ at pH 4.0) (Pignatello 1992). In addition, the solution pH decreased below 4.0 in the SPC/Fe(II)/EDTA system (Table 2) because EDDS was a weak acid compared with CA and EDTA. As a biodegradable chelating agent, EDDS could also react with HO• (Vandevivere et al. 2001). So EDTA and EDDS could not elevate TCE degradation by scavenging the HO• radical.

Although the traditional Fenton reagent has a strong capacity for oxidizing organic contaminants, when applying Fenton's reagent, the pH had to be adjusted to 2–4, and the hydrogen peroxide (H₂O₂) was unstable and had a short life-time after injection into the subsurface (Duesterberg & Waite 2006). However, based on the following experiment, the SPC/Fe(II)/CA system did not need adjustment of the solution pH and the concentration of inorganic carbon from SPC was too low to inhibit TCE degradation. Because of the positive effect of CA on TCE degradation, the CA chelating system was investigated in detail in the following experiments.

As the traditional Fenton process could generate lots of reactive oxygen species (HO•, O₂⁻• and so on) (Watts et al. 2005), and SPC has the similar function with a Fenton reagent, it is expected that these reactive oxygen species may also exist similarly in the SPC/Fe(II)/CA system. Based on our previous research, nitrobenzene and carbon tetrachloride were using as a chemical probe for assessing HO• and O₂⁻•, respectively (Miao et al. 2015), and the initial concentrations of both nitrobenzene and carbon tetrachloride were kept at 0.15 mM.

Based on the results of the probe tests, free-radical scavenging tests were conducted to elucidate the role of HO• and O₂⁻• in TCE degradation. Experiments were conducted independently with the addition of different free-radical scavengers. Tert-butyl alcohol was an effective scavenger for HO• due to the rate constant between tert-butyl alcohol and HO• (k = 5.2 × 10⁸ M⁻¹ s⁻¹) (Sun et al. 2013). And 1,4-benzoquinone could scavenge O₂⁻• effectively (k = 9.6 × 10⁸ M⁻¹ s⁻¹) through rapid transference of electrons and generation of benzoquinone radicals (Haag & Yao 1992).

TCE degradation in the SPC/Fe(II)/CA system in the presence of 1,4-benzoquinone is shown in Figure 6(b). As can be seen, the degradation of TCE decreased from 90.1% to 76.9% and 74.7% respectively with the addition of 10 and 20 mM 1,4-benzoquinone. The scavenging effect of 1,4-benzoquinone demonstrated the presence of O₂⁻• in the SPC/Fe(II)/CA system and its contribution to the degradation of TCE. Thus it can be concluded that HO• was mainly responsible for TCE degradation in the SPC/Fe(II)/CA system. In the traditional Fenton process, the main principle in Fenton's reaction owes to the Fe(II)-catalyzed decomposition of H₂O₂ in producing HO• (Walling 1975).

This study investigated TCE degradation performance using chelated Fe(II)-activated SPC in the presence of CA in aqueous solution. The experimental results showed that the presence of CA promoted H₂O₂ generation in solution, resulting in significant improvement in TCE degradation. The presence of Cl⁻ and HCO₃⁻ has an inhibitive effect on TCE oxidation, and HA has a negative effect on TCE degradation only at relatively high concentration. The initial solution pH appeared to be not negligible for TCE degradation. The generation of HO• and O₂⁻• in the SPC/Fe(II)/CA system was confirmed with chemical probes. Meanwhile, free-radical scavenging tests verified that HO• was the dominant radical that contributed to TCE degradation and O₂⁻• promoted TCE degradation by participating in HO• generation. In summary, the findings in this study strongly support the prospect of using CA to enhance Fe(II)-activated SPC in situ for remediation of TCE-contaminated groundwater.

This study was financially supported by grants from the National Natural Science Foundation of China (No. 41373094 and No. 51208199), China Postdoctoral Science Foundation (2015M570341) and the Fundamental Research Funds for the Central Universities (22A201514057).