Dechlorination of Carbon Tetrachloride by Sulfide-Modified Nanoscale Zerovalent Iron

Abstract

Sulfide-modified nanoscale zerovalent iron (S-nZVI) could potentially be efficient in the degradation of haloalkane contaminants due to its high reactivity. In this study, we investigated the efficiency of carbon tetrachloride (CCl₄, CT) degradation by S-nZVI under various conditions, including initial pH of solution, CT concentration, S-nZVI dosage, and ambient temperature. Results showed that reductive degradation of CT by S-nZVI followed pseudo-first-order kinetics. The rate of CT degradation showed negative correlation to CT concentration, while higher initial pH of solution (in the range of 3–9), higher S-nZVI dosage, and higher ambient temperature would stimulate CT degradation. Comparison among S-nZVI, nZVI, and FeS suggested that S-nZVI had the highest efficiency for CT degradation. This can be result from the unique surface structure and physicochemical property of S-nZVI.

Introduction

Use and discharge of carbon tetrachloride (CCl₄, CT) as solvents or degreasing agents in industry usually results in the soil and groundwater contamination (Tamara and Butler, 2004; Lin et al., 2005; Gao et al., 2011). Although many technologies have been developed to eliminate CT contamination, the improvement of efficiency is still a big concern to researchers.

Iron-based reductants are important components of in-situ reduction technology. Previous research reported that commercial irons could effectively degrade CT (Tamara and Butler, 2004). In recent decade, nanoscale zerovalent iron (nZVI) has received huge attentions due to its large specific surface area and high reactivity (Nurmi et al., 2005; Lien et al., 2007). However, nZVI can also react with water or dissolved oxygen (DO), forming a passivated film which can hinder the further reactions between the reductant and contaminants (Xie and Cwiertny, 2010).

To enhance its reactivity, modifications of nZVI have been extensively studied, including incorporation of metals such as Pd and Ni (Chun et al., 2010; Wei et al., 2014), attachment of stabilizer onto nZVI surface (He and Zhao, 2007), and embedment of nZVI into materials with huge surface area (Xiong et al., 2016). Although these methods can improve the reactivity of nZVI, they also lead to secondary pollution of groundwater at the same time. In addition, the common ions in groundwater such as NO₃⁻, HCO₃⁻ may result in the Pd deactivation of Fe/Pd nanoparticles (Liu et al., 2007; Lim and Zhu, 2008). Besides, several studies have reported that the usage of ZVI in sulfide-containing environment will result in the formation of iron sulfides (He et al., 2008; Kirschling et al., 2010; Fan et al., 2013).

Iron sulfides such as FeS (mackinawite) and FeS₂ (pyrite) have been widely applied in removal of heavy metals (Erdem and Ozverdi, 2006; Jeong and Hayes, 2007) and halogenated organic compounds (Lee and Batchelor, 2002; He et al., 2010; Rajajayavel and Ghoshal, 2015; Lan and Butler, 2016). In comparison with nZVI, iron sulfides exhibit relatively weaker reducibility and lower pHpzc (0.8–3.5) (Dekkers and Schoonen, 1994). Due to low pHpzc, the surface of iron sulfides is prone to deprotonation under neutral conditions, which results in a greater electron density of the deprotonated surface. Thus, combined iron sulfides with nZVI will not only alter the surface structure and physicochemical property of nZVI but also retain its high reducibility. This may have a positive impact on contaminant removal.

Recent studies have demonstrated that sulfide-modified nanoscale zerovalent iron (S-nZVI) has a better performance on removing heavy metals and chlorinate solvents than nZVI (Kim et al., 2013; Su et al., 2015). Currently, the studies of chlorinated solvents degradation mainly focused on chloro-alkene contaminants such as TCE (Kim et al., 2013; Rajajayavel and Ghoshal, 2015). There is a significant need to study the degradation of CT, which is a common chloro-alkane pollutant in groundwater.

The objective of this research was to study the CT degradation by S-nZVI. The impacts of several factors, including initial pH of solution, CT concentration, S-nZVI dosage, and ambient temperature, were also investigated in the study. In addition, the efficiency of CT degradation by S-nZVI was compared with other reductants such as nZVI and FeS under the same conditions.

Materials and Methods

Chemicals and synthesis of nZVI, FeS, and S-nZVI

CT, chloroform (CF), ferric chloride (FeCl₃·6H₂O), ferrous sulfate (FeSO₄·7H₂O), sodium sulfide (Na₂S · 9H₂O), sodium borohydride (NaBH₄), sodium hyposulfite (Na₂S₂O₄), methanol (>99.7%), and 1, 10-phenanthroline were purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). All chemicals were of analytical grade without further purification. The water used for all experimental procedures was the deionized water after bubbling nitrogen gas for 2 h to expel DO.

S-nZVI was synthesized according to a method called one-pot method proposed by Kim et al. (2011). In brief, 0.6 g Na₂S₂O₄ was dissolved in 300 mL of 30.24 g/L NaBH₄ aqueous solution. Then, the mixed solution was dropwise added into the reactor containing 100 mL of 135.1 g/L FeCl₃·6H₂O aqueous solution to acquire the S-nZVI nanoparticles. The solution in the reactor was kept stirring at 300 rpm and sparking with pure nitrogen (99.99%) throughout the synthesis process. In addition, nZVI was synthesized by reducing FeSO₄·7H₂O (0.5 M, 0.1 L) with NaBH₄ (0.5 M, 0.3 L); FeS was prepared by mixing FeSO₄·7H₂O (0.5 M, 0.1 L) with Na₂S · 9H₂O (1.1 M, 0.5 L).

The resulting black suspensions were allowed to settle for an hour. Then, the supernatant was discarded, and the solid was washed by deoxygenated deionized water and ethanol several times before centrifugation at 6,000 rpm for 8 min. Finally, these reductants were dried by vacuum freeze-drying (SiHuan, Beijing, China) and stored in desiccators under a nitrogen atmosphere for further experiments.

Experimental procedure and analysis

Static experiments of CT degradation by S-nZVI were conducted in serum bottles (100 mL) sealed with polytetrafluoroethylene (PTFE)-lined rubber septa and aluminum cap (CNW, Germany). First, a given amount of S-nZVI was added into the bottle contained 99 mL deoxygenated deionized water and then the bottle was sonicated with a few seconds to disperse the nanoparticles. Afterward, the degradation reaction began with rapidly injecting a certain volume of CT stock solution into the serum bottle. During the reaction period, the bottle was placed on a water bath shaker (SH-A, Changzhou, China) at 150 rpm in the dark.

At sampling time, 1.2 mL sample was collected from the solution by syringe and then it was immediately filtered through a filter film (0.22 μm). The acquired filtrate (1 mL) was quickly transferred to the headspace vial (CNW) containing 9 mL of deionized water (preserved with 1 drop of concentrated H₂SO₄). The headspace vial was preheated in the water of 60°C for 30 min before 1 mL gas of the headspace vial was extracted to analyze. Blank experiment without adding S-nZVI was carried out to observe the loss of CT due to volatilization. Control experiment of CT degradation by different reductants (S-nZVI, nZVI, and FeS) was also conducted.

To examine the effect of initial solution pH (3–9) on the CT degradation by S-nZVI, the pH of deionized water was adjusted by H₂SO₄ (0.1 M) and NaOH (0.1 M) before bubbling nitrogen. The pH value was measured by pH meter (Thermo Scientific). Moreover, a series of batch tests were carried out to evaluate the effect of CT concentration (1–5 mg/L), S-nZVI dosage (0.1–0.5 g/L), and environmental temperature (10–40°C) on CT degradation. All experiments were conducted in triplicate. The stability of S-nZVI under different pH conditions was determined by measuring the Fe²⁺ concentration in solution. In brief, 0.05 g of S-nZVI was separately added into the serum bottles containing 100 mL of deoxygenated deionized water with different pH values. Then, these bottles were sonicated and placed on a water bath shaker (30°C). In sampling intervals, 2 mL sample was withdrawn and filtered to analyze.

Concentrations of CT and its degradation products were analyzed by a gas chromatograph (ECD; Shimadzu 2014) with manual injection according to a procedure established in HJ 620-2011. The lower limits of quantification for CT and CF were 0.12 μg/L and 0.08 μg/L. The injection temperature and the detector temperature were 220°C and 320°C, respectively. The original column temperature kept 40°C for 5 min followed by increasing to 100°C with a rate of 8°C/min, then increased to 200°C at a rate of 6°C/min and held for 10 min. The carrier gas was high-purity nitrogen (99.99%) at a flow rate of 1 mL/min and the split ratio was 20:1.

The leaching Fe²⁺ concentration was analyzed with a UV–vis spectrophotometer (JK-753, Shanghai, China) using the 1,10-phenanthroline method (Son et al., 2010). The pHpzc (point of zero charge) of S-nZVI was measured by the method described by Sun et al. (Sun et al., 2012). The recovery value of CT and its degradation products was calculated by dividing total molarity of CT and its products at the last sampling time by initial CT molarity.

Particles characterization

S-nZVI and nZVI used for characterization were prepared as dried nanoparticles according to the procedure mentioned above. The used S-nZVI was the recycling of the S-nZVI after reacting with CT under certain pH condition. The recycling procedure included the following: separation of the S-nZVI suspension by strong Nd-Fe-B magnet, rinse the solid with deoxygenated deionized water and ethanol several times, and then vacuum freeze-drying.

Scanning electron microscope (SEM, JSM-6301F; JEOL) was performed to observe the morphology of S-nZVI and nZVI. X-ray photoelectron spectroscopy (XPS, PHI-5300) with Al Kα radiation (1486.6 eV) was measured to show S-nZVI element composition and corresponding chemical forms. To explore the change of S-nZVI crystalline phase before and after reaction, the fresh and used S-nZVI were characterized by X-ray diffraction (XRD, D8-advance; Bruker) at room temperature with Cu Kα radiation at 40 kV and 40 mA.

Results and Discussion

SEM, XPS, and XRD study

Figure 1 shows the SEM images of nZVI and S-nZVI, indicating that the nanoparticles of S-nZVI and nZVI have similar size with their diameters in the range of 100–200 nm and have obviously different surface morphology. The surface of nZVI was smooth, while the surface of the S-nZVI was wrinkled. According to reported studies, the wrinkled surface of S-nZVI was more similar to FeS morphology (Mariëtte et al., 2003; Chen et al., 2015). Therefore, we speculate that the components of the wrinkled surface could be iron sulfides, formed by the addition of Na₂S₂O₄.

FIG. 1.

Scanning electron microscope images of nZVI (a) and S-nZVI (b). S-nZVI, sulfide-modified nanoscale zerovalent iron.

Measurements of XPS not only determined the chemical elements but also further revealed their chemical forms. In the XPS survey scan, elements of Fe, O, C, and S were detected, and corresponding atomic concentrations were 30.68%, 41.83%, 25.55%, and 1.94% (Fig. 2a). Obviously, the relative content of S element was very low. Figure 2b–d present the narrow region of Fe (2p), O (1s), and S (2p), respectively.

FIG. 2.

X-ray photoelectron spectroscopy survey scan of S-nZVI and the narrow region scans of (a, b) Fe (2p_3/2), (c) O (1s), and (d) S (2p).

The Fe (2p) spectrum showed that iron element was presented in two forms, corresponding to Fe (0) at 705.68 eV and Fe(III) at 711.68 and 724.83 eV (Fig. 2b) (Kim et al., 2011; Gong et al., 2016). Consistent with previous studies, Fe spectrum indicates that iron-core was covered by a thin layer of iron oxides (Kim et al., 2011; Yan et al., 2013; Zhang et al., 2013). The O spectrum shown in Fig. 2c had three peaks, the prominent peak of which at 531.63 eV was fitted to OH⁻, and the other peaks were attributed to O²⁻ (530.30 eV) and adsorbed H₂O (532.75 eV), respectively. Therefore, the iron oxides coated on S-nZVI surface might be FeOOH and Fe₃O₄ (Zhang et al., 2013). Figure 2d illustrates the chemical forms of S element. The peaks at 162.16, 163.08, and 167.95 eV were associated with Fe₂³⁺Fe²⁺S₄ (greigite), polysulfide (S_n²⁻), and SO₃²⁻, respectively (Neal et al., 2001; Su et al., 2015). This indicates that iron sulfides were successfully formed when Na₂S₂O₄ was added.

XRD was conducted to analyze the change of S-nZVI crystalline phase before and after reaction (Fig. 3). It was obvious that both fresh and used S-nZVI had a remarkable Fe⁰ peak at 2θ = 44.6° (Fig. 3a). However, a relatively weak peak at 2θ = 35.5° only appeared at the used S-nZVI pattern (pH of 9). Figure 3b specifically shows the pattern of used S-nZVI (pH of 9). The peak at 2θ = 35.5° was fitted to Fe₃O₄, which was the typical product of Fe oxidation. The existence of Fe₃O₄ in the used S-nZVI (pH 9) pattern suggested that S-nZVI suffered severe oxidation after reacting with CT under initial pH of 9. The pattern of used S-nZVI (pH 7) was similar to that of used S-nZVI (pH 9) (data not shown). In addition, no obvious characteristic peaks of iron sulfides were detected both before and after the reaction, presumably due to its low content or poor crystalline phase (Kim et al., 2011; Su et al., 2015).

FIG. 3.

(a) XRD patterns of fresh and used S-nZVI. (b) XRD pattern of used S-nZVI at pH 9. XRD, X-ray diffraction.

Effect of initial solution pH

Figure 4a presents the reductive degradation of CT by S-nZVI under different initial pH (3, 6, 7, 8, and 9). The rate of CT degradation increased with increasing solution pH from the acid to alkaline condition. Figure 4b shows a linear relationship between initial pH (3–9) and lnK_obs (observed rate constants), with a slope of 0.081 ± 0.007 (R² = 0.968). When the pH value was between 6 and 9, initial pH and lnK_obs showed a better linear correlation, with a slope of 0.058 ± 0.003 (R² = 0.9915). This pH-dependence of CT degradation is similar to the results reported by Kim et al. They found that the rate of TCE degradation by Fe/FeS nanoparticles increased with increasing solution, pH in the range of 6 to 9 (Kim et al., 2013).

FIG. 4.

(a) Effect of initial solution pH on CT degradation by S-nZVI (S-nZVI = 0.5 g/L, CT = 3 mg/L and temperature = 30°C). (b) Linear relationship between lnK_obs and initial solution pH. CT, carbon tetrachloride.

Moreover, the stability of S-nZVI was further studied to deeply understand the effect of solution pH on CT degradation. The stability of S-nZVI was determined by measuring the Fe²⁺ concentration in solution. As shown in Fig. 5, the Fe²⁺ concentration at pH 7 was far more less than that at pH 3, which indicates that S-nZVI is unstable at the acid condition. The abundant Fe²⁺ at pH 3 should be derived from dissolution of iron oxides-, iron sulfides-shell, and Fe-core. At pH 3, the decrease in S-nZVI reactivity due to iron loss could be the reason of the lowest rate of CT degradation. Despite this, according to the reported study, iron loss of S-nZVI was still less than that of nZVI at the same acid conditions (pH <5) (Su et al., 2015).

FIG. 5.

Concentration of Fe²⁺ dissolved from S-nZVI under different pH values.

On the contrary, when solution pH values increased from 6 to 9, the rate of CT degradation also increased progressively. This phenomenon might have a relationship with the surface charge of S-nZVI. As we all know, the surface charge of S-nZVI is determined by the pHpzc of S-nZVI and the solution pH. In this study, the pHpzc of S-nZVI was measured as 4.22 (Fig. 6). Therefore, it was reasonable that S-nZVI began to deprotonation when solution pH was higher than its pHpzc. Deprotonated S-nZVI carried a large number of negative charges on its surface, which will enhance the driving force of electron transfer from S-nZVI to CT and further accelerate CT degradation (Kim et al., 2013).

FIG. 6.

The pHpzc of S-nZVI measured by titration of NaOH.

Effect of CT concentration

Figure 7a shows the effect of initial CT concentration (1 to 5 mg/L) on CT degradation. Obviously, the efficiency of CT degradation had a negative linear correlation to the initial CT concentration. Figure 7b describes the processes of CT degradation and CF formation with initial CT concentration of 3 mg/L. CF formation was apparently behind CT degradation at the beginning of the reaction, which indicates that the CT adsorbed on the reductant surface is not completely degraded to CF. This phenomenon is consistent with prior research showing that the rate of electron transfer was slower than that of adsorption.

FIG. 7.

(a) Effect of CT initial concentration on the reductive degradation of CT (S-nZVI = 0.5 g/L, temperature = 30°C and pH = 7). (b) The processes of CT degradation and CF formation with CT initial concentration of 3 mg/L. CF, chloroform.

During the adsorption process, iron sulfides coated on the reductant surface played a key role in CT adsorption due to its wrinkled surface and hydrophobicity (Park et al., 2006; Chen et al., 2015). Except for CF, no other chlorine-containing daughter products were detected. Figure 8 presents the accumulation of CF and the recovery of CT and CF under different initial CT concentrations. The five values of recovery were all higher than 95% and the incompleteness might be due to the formation of nondetectable products or volatilization loss during sampling (Lee and Batchelor, 2002).

FIG. 8.

Recovery values at different CT initial concentrations.

Effect of S-nZVI dosage

Effect of S-nZVI dosage (0.1 to 0.5 g/L) on CT degradation is shown in Fig. 9. The efficiency of CT degradation obviously increased with the increment of S-nZVI dosage from 0.1 to 0.5 g/L. It is generally accepted that the reactive sites of reductants or catalysts play an important role in contaminants degradation (Kadam et al., 2014; Chu et al., 2016a; Kulkarni et al., 2016). When the dosage of S-nZVI was 0.1 g/L, the limited reactive sites could not effectively degrade CT with initial concentration of 3 mg/L. When added more S-nZVI, the reactive sites would increase accordingly, and more CT could be degraded. Moreover, excessive S-nZVI could also react with water to produce a trace amount of hydrogen, which might be the additional electron donor [Eq. (1)] (Zhu and Lim, 2007). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{F}}{{ \rm{e}}^0} + {{ \rm{H}}_2}{ \rm{O}} \to { \rm{F}}{{ \rm{e}}^{2 + }} + {{ \rm{H}}_2} + 2{ \rm{O}}{{ \rm{H}}^ - } \tag{1} \end{align*} \end{document}

FIG. 9.

Effect of S-nZVI dosage on the CT degradation (temperature = 30°C, CT = 3 mg/L, and pH = 7).

Effect of temperature

Figure 10 illustrates the effect of temperature on CT reductive degradation by S-nZVI. The rate of CT degradation had a positive correlation with the ambient temperature. This temperature dependence of CT degradation is similar to the previous studies and can be explained by Frontier Orbital Theory (Chu et al., 2016b). Luo et al. (2015) reported that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were the two places used for electron exchange. HOMO is occupied by electrons and can donate electrons while LUMO with an empty orbit has a strong electron affinity. During the degradation reaction, the higher environment temperature would provide more energy to electrons and stimulate them quickly escaping from the HOMO of S-nZVI to the LUMO of CT, which leads to the higher temperature, the more rapid rate of CT degradation.

FIG. 10.

Effect of temperature on the CT degradation (S-nZVI = 0.5 g/L, CT = 3 mg/L, and pH = 7).

Effect of different reductants on CT degradation

To explore the effect of different reductants (nZVI, FeS, and S-nZVI) on CT degradation, control experiment was conducted at 3 mg/L CT, 0.5 g/L reductant, initial solution pH 7, and temperature of 30°C (Fig. 11). It was obvious that S-nZVI had the highest efficiency of CT degradation followed by nZVI and FeS. The efficiency of CT degradation by S-nZVI was more than 90%, whereas the efficiency of CT degradation by nZVI was less than 50%. In addition, FeS seemed to have no obvious degradation effect on CT within 2 h. Thus, under the same conditions, S-nZVI achieves the highest efficiency in the degradation of CT compared with nZVI and FeS.

FIG. 11.

CT degradation by FeS, nZVI, and S-nZVI (CT = 3 mg/L, reductant = 0.5 g/L, and temperature = 30°C).

Conclusion

We studied the reductive degradation of CT by S-nZVI under various conditions, including initial solution pH, CT concentration, S-nZVI dosage, and temperature. Moreover, the efficiency of CT degradation by S-nZVI was compared with the efficiency by other reductants (such as S-nZVI and FeS). At the acid condition (pH 3), the rate of CT degradation by S-nZVI was the lowest. However, the rate increased progressively with increasing pH value from 6 to 9, which should be attributed to S-nZVI deprotonation. Both S-nZVI dosage and ambient environment had positive effects on the rate of CT degradation. The higher dosage of S-nZVI could supply more reaction sites for CT degradation and the higher ambient temperature could stimulate electrons quickly transferring from S-nZVI to CT. CF as the primary product of CT degradation indicated that the mechanism of CT degradation was mainly reductive dehalogenation. Compared with nZVI and FeS, S-nZVI degraded CT with the highest efficiency, which might have a close relationship with the surface structure and physicochemical property of S-nZVI.

Footnotes

Acknowledgments

This work was supported by the National Natural Science Foundation of China (50578151), the National Science and Technology Major Project of China (2012ZX07201-005-06-01; 2015ZX07406-005), and the Fundamental Research Funds for the Central Universities (2652016025, 2652016023, 2652016024, 2652016022). The authors thank Haijiao Xie from shiyanjia laboratory for the support of XPS analysis.

Author Disclosure Statement

No competing financial interests exist.

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