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Surfactant-enhanced PEG-4000-NZVI for remediating trichloroethylene-contaminated soil
Chemosphere 195 (2018) 585e593
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Surfactant-enhanced PEG-4000-NZVI for remediating trichloroethylene-contaminated soil Huifang Tian, Ying Liang, Tianle Zhu, Xiaolan Zeng, Yifei Sun* Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of Space and Environment, Beihang University, 37 Xueyuan Road, Beijing, China
h i g h l i g h t s A new modified PEG-4000-NZVI was prepared and used in soil remediation. Two surfactants strongly solubilized trichloroethylene. The modified PEG-4000-NZVI worked well within a pH range of 3e11. Almost all trichloroethylene was removed from soil within 3 h.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2017 Received in revised form 7 December 2017 Accepted 11 December 2017 Available online 12 December 2017
In this study a NZVI was prepared by the liquid phase reduction method. The modified NZVI obtained was characterized by BET, TEM and XRD. The results showed that the iron in the PEG-4000 modified material is mainly zero-valent iron with a stable crystal structure. It has a uniform particle size, ranging from 20 to 80 nm, and a larger specific surface area than CTAB modified NZVI, SDS modified NZVI and commercial zero-valent iron. The two surfactants CTAB and SDS are also selected as solubilizers, the results showed that the two selected surfactants obviously solubilize trichloroethylene in soil. Compared with commercial zero-valent iron, PEG-4000 modified NZVI is better removed trichloroethylene from soil; Also, the optimal operational parameters were obtained. When the experimental conditions were: PEG-4000 modified NZVI dosage 1.0 g/L, CTAB/SDS concentration equal to the CMC, SDS concentration was 2.0 CMC, CTAB was concentration 1.0 CMC and the vibration speed 150 r/min, the removal efficiency of trichloroethylene in a soil-water system reached 100% after 4 h. Both NZVI combined with CTAB and NZVI combined with SDS followed fitted first order reaction kinetics during the removal of trichloroethylene and their reaction rate constant k was 0.6869 mg/(L$h) and 0.5659 mg/(L$h), respectively. According to the chloride ion detection test, the trichloroethylene degradation is mainly due to reductive dechlorination. © 2017 Elsevier Ltd. All rights reserved.
Handling Editor: X. Cao Keywords: Modified nano zero-valent iron Trichloroethylene Surfactant Soil remediation Dechlorination
1. Introduction Chlorinated hydrocarbons are widely used as solvents, yet they are also persistent organic pollutants. Their widespread distribution causes serious environmental problems, due to their persistence and toxicity (Gong et al., 2010; Hunt, 2009). TCE is a typical chlorinated hydrocarbon, with low water solubility and strong sorption to soil particles, and it is one of the major challenges for
* Corresponding author. E-mail addresses: [email protected] (H. Tian), [email protected] (Y. Liang), [email protected] (T. Zhu), [email protected] (X. Zeng), sunif@buaa. edu.cn (Y. Sun). https://doi.org/10.1016/j.chemosphere.2017.12.070 0045-6535/© 2017 Elsevier Ltd. All rights reserved.
remediating soil polluted with chlorinated hydrocarbons (Hamby, 1996). Once released into the subsurface, TCE distributes between soil and water (Moran et al., 2007), Because of its low solubility and high adsorbability to soil organic matter, a large fraction of TCE tends to be retained in the solid phase. The traditional methods for remediating such soils include thermal remediation (Acierno et al., 2003; Roland et al., 2007), chemical oxidation (Liang et al., 2004a,b), chemical reduction (Lien and Zhang, 2007), soil vapour extraction (Albergaria et al., 2008, 2012), soil washing (Pavel and Gavrilescu, 2008), solidification/ stabilization and vitrification (US EPA, 1999), However, these traditional techniques also have disadvantages, such as high cost and secondary pollution. Zero-valent iron (Fe0) is strongly reducing and has frequently been used to degrade organic pollutants (Bai
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Abbreviations NZVI PEG-4000 CTAB SDS CMC BET TEM XRD TCE
Nanoscale zero-valent iron Polyethylene glycol Cationic hexadecyl trimethyl ammonium bromide Anionic sodium dodecyl sulfate Critical micelle concentration Brunauer-Emmett-Teller Transmission Electron Microscope X-ray diffraction analysis Trichloroethylene
et al., 2008; Sun et al., 2012), to overcome these disadvantages. However, despite the simplicity and environmental harmlessness of Fe0, its application has been restricted by low reaction speed. In order to solve this problem, continuous efforts have been devoted to explore more efficient reductants. Recently, with the development of nanotechnology, nano zerovalent iron has attracted growing attention as a means for remediating groundwater and soil, due to their high reactivity and easy availability (Arnold and Roberts, 2000; Zhu et al., 2008). Some research reported that was very effective for the transformation and reductive dechlorination of chlorinated organic pollutants and PCBs (Varanasi et al., 2007; Wang and Zhang, 1997; Zhang et al., 2013). Furthermore, various inorganic compounds, such as nitrate, nitrite and Cr6þ(VI) can also be restored in soils by NZVI (Alowitz and Scherer, 2002). However, the easy aggregation of nanoparticles due to their strong van der Waals forces and magnetic attraction limited their rapid delivery in soil and subsequent usage in soil remediation (Saleh et al., 2007; Zhang et al., 2013; Tratnyek and Johnson, 2006). Thus, many researchers tried to overcome this limitation by using various modifiers or dispersants to coat the nanoparticles and increase steric and electrostatic hindrance among the particles, resulting in the enhanced stability of the nanoparticles suspension. Different chemicals were used to modify the particle surface, such as polyacrylic acid, guar gum and carboxymethyl cellulose, but some of these chemicals are not environmentally friendly and also costly (He and Zhao, 2007; Phenrat et al., 2009). Another issue concerning TCE removal in soil is its low water solubility and strong sorption before the reaction begins, which slows down the degradation and prolongs the required reaction time (Shin and Cha, 2008). Alternative strategies have been explored to solve this problem as well and some research reported that surfactant is one of the best options to promote the water solubility and miscibility of TCE in soil (Hill and Ghoshal, 2002; Sailaja et al., 2003). Based on the above analysis, we proposed a modified NZVI preparation method and usage, enhanced with surfactant for remediating TCE contaminated soil (Fan et al., 2013; Kang et al., 2006), assuming that combining a modified NZVI with surfactant might provide both enhanced water solubility and degradation efficiency (Gao et al., 2007; Bao et al., 2010; Bikshapathi et al., 2012). We chose two surfactants (SDS and CTAB). In this study, we investigated the remediating of TCE-contaminated soil by applying a modified NZVI, together with surfactants. The reducibility of using the modified NZVI was tested. The effects of important parameters such as NZVI dosage, surfactant concentration, reaction time, initial pH range and speed of an orbital shaker incubator were all investigated. Besides, NZVI and commercial zero valent iron were compared while remediating TCE contaminated soil. The degradation mechanism, the reduction products and the
possible reductive pathway of TCE were investigated. The objective of this study is to develop a new and effective surfactant-enhanced modified NZVI for remediating TCE contaminated soil. 2. Materials and methods 2.1. Materials Ferrous sulfate (FeSO4$7HO; >99%)), polyethylene glycol (PEG4000), and potassium borohydride (KBH4; >99%) were purchased from Xilong Chemical Co., Ltd, China. Cetyltrimethyl ammonium bromide (CTAB, >99%) and lauryl sodium sulfate (SDS, >99%) were supplied by Lanyi Chemical Products Co., Ltd, China. Sodium hydroxide and methyl alcohol were delivered by Beijing Chemical Works, China. Commercial iron powder was acquired from Tianjin Jinke Chemical Research Institute, China. All chemicals were used as received without further purification. All solutions were prepared with deionized water. 2.2. Soil annalysis The soil was supplied by the China Conservation and Environmental Protection Group (CCEPG), which was collected from a chemical factory, at Henan, China. The soil was air-dried and passed through 60 mesh sieves. The physical and chemical characteristics of the soil are shown in Table 1. 2.3. Preparation of contaminated soil A fixed TCE solution at mass of 100 mg was delivered in a small volume of methyl alcohol, spreading slowly to 100 g of the prepared soil, being mixed with contant temperature oscillator (Taicang Experimental Factory, THZ-C, China) for 48 h. Then the mixture was placed in a fume hood nearly a week for drying. During the drying period, the soil mixture was mixed regularly to ensure uniform drying, and the original value was analyzed before tests. As a result, the TCE concentration in contaminated soils after drying detected by a head-space sampler combined with Gas ChromatographyMass Spectrometer (GC-MS) was 12.56 mg/kg. The contaminated soil was kept out in a closed bottle, at room temperature before experiments. 2.4. Preparation of modified NZVI Modified NZVI particles by different dispersing agents were prepared via a liquid phase reduction method (Wang et al., 2009; Chen et al., 2012). After the rection device was set up, keeping the nitrogen into the three-nected round-bottom flask for 30min, continuously. The aim is to achieve the purpose of eliminating
Table 1 Physical and chemical characteristics of the soil. Soil Parameter
Unit
Value
Sample depth Moisture content pH Soil pore VPCa Hg Pb Benzo [b]&[k] fluoranthene Benzo [b]pyrene Indeno [1,2,3-cd]pyrene Dibenz [a,h]anthracene
cm %
30e50 10.7%e30.7 8.8 35.75e50.89 4.42 107~8.34 104 147 ± 1.8 40 ± 1.3 7.07e13.44 2.59e5.73 1.73e4.79 0.43e1.21
a
% ppm ppm mg/kg mg/kg mg/kg mg/kg
VPC: Vertical permeability coefficient.
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oxygen. Using SDS, CTAB and PEG-4000 as dispersing agents, FeSO4$7H2O was made to react with excess KBH4. Firstly, the reaction device with deionized ethanol and water 200 mL (ethanol: water ¼ 2:8, v/v) was deoxygenated by purging it with N2 for 30 min. Under nitrogen protection, 0.2 mol/L FeSO4$7H2O was added to ethanol/water and mechanically agitated for 15 min, then three different kinds of dispersing agents (SDS, CTAB, PEG4000) was added, respectively, and mixed into the FeSO4$7H2O solution. After the solution was mixed, the KBH4 (0.43 mol/L) alkaline solution was added dropwise to the solution. After stirring for 30 min, the mixture turned black. The black particles were separated from the solution by a magnet. They were washed three times with deoxygenated deionized water and deoxygenated ethanol respectively. Then, the black particles were separated by centrifuging, and dried for 24 h under vacuum. Finally we obtained the modified NZVI particles. The reaction can be described by Eq. (1): 0 Fe2þ þ 2BH 4 þ 6H2 O/Fe þ 2BðOHÞ3 þ 7H2 [
(1)
2.5. Characterization of modified NZVI The surface morphology of the modified NZVI particle was characterized by their specific surface area and pore size analysis (BET, NOVA 2200e, Quanta chrome). Transmission electron morphology of the catalyst was investigated by means of a S-3000 scanning electron microscope (TEM, Hitachi Ltd., In Japan). X-ray diffraction patterns of the catalyst were scanned at 2q from 10 to 80 on the Bruker D8 Advance X-ray diffractometer (XRD, Bruker Corporation, In Germany), to verify crystallinity. 2.6. Experimental procedure TCE Removal tested with batch experiments. The reducing activity of modified NZVI was tested while remediating the TCE contaminated soil. All runs were performed under contant temperature oscillator at 150 r/min and room temperature (25 C). 50 mL aqueous solution were added to 10 g of polluted soil. This ratio was selected in order to simulate the removal of TCE entrapped into soil pores in the saturated or vadose zones and treatment can be applied both in situ or off site (Doughty et al., 1999). Runs were carried out by using types of zero-valent iron as reductant, CTAB and SDS was added at its CMC as solubilizer, respectively. The effect of the modified NZVI dosage was investigated in a range from 0 to 2.0 g L1. The effect of surfactant concentration was varied in the range from 0 to 4 CMC. The effect of reaction time was studied between 0 and 4 h. The effect of pH was tested among the range from 3 to 11. Effect of shaker incubator speed was investigated from 0 to 300 r/min. 2.7. Analytical methods Aqueous and soil were separated by centrifugation. pH and chloride ion were analyzed in the aqueous phase. Remaining TCE was tested in both the aqueous phase and soil. The TCE analysis was performed with a head-space sampler (Beijing pury analysis instrument Co. Ltd. RHS-628, China) and Gas ChromatographyMass Spectrometer (GC-MS, Shimadzu Corporation, QC 2010 Plus, Japan). The concentration of dissolved chloride ions in the solution was measured by a chloride ion chromatograph (792, Metrohm China Limited). The extraction method of trichloroethylene in soil-water system was shown at Text S7 in Supplementary Information.
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3. Results and discussion 3.1. Characterization of modified NZVI Table 2 shows the average specific surface area of the modified NZVI, PEG-4000 modified NZVI is larger than SDS modified NZVI and CTAB modified NZVI, whit the average specific surface area of 22.77 m2 g1, 20 times larger than that of commercial iron power and 3 times larger than that of non-modified NZVI (Rare-NZVI). The larger specific surface area also explains the higher activity of the prepared NZVI particles (Wei et al., 2010; Wang et al., 2013). A BET image of ZVI and modified NZVI is shown in Table 2. Fig. 1 shows the XRD patterns of commercial ZVI and modified PEG-4000-NZVI. The peaks at 44.66 and 44.76 were indexed as diffractions of Fe0 and indicate zero-valent iron as the main active component. The PEG-4000, SDS and CTAB added during the preparation of NZVI particles plays a role in the dispersibility of the NZVI particles, by changing the charge distribution of their surface; As shown in the Fig. 1(b),(c),(d) the PEG-4000-NZVI particles are wellcrystallined, which keeps higher intensity than SDS-NZVI and CTAB-NZVI. Also, the PEG-4000-NZVI particles were welldispersed, mutually repulsive and less likely to agglomerate (Gao et al., 2007; Chen et al., 2012). While, since NZVI is easily oxidized, the prepared NZVI contains a certain amount of ferroferric oxide (magnetite), so that a surface oxide layer is formed, hindering further oxidizing contact (Zhao et al., 2011). In order to better understand the characteristics of modified NZVI and ZVI, TEM image of modified NZVI and ZVI were carried out. Pretreatment of samples to be tested: (1) Taking 0.05 ge0.1 g samples into a beaker with 20 mL ethanol, then closed with sealing film, dispersing for 15e20 min in an ultrasonic cleaning machine, with the aim of making sample particle dispersed homogeneously; (2) Putting 200 mesh double-network carbon-supported membrane on the surface of a watch-glass with filter paper. Using the drip tube to make the sample on the double-network carbonsupported membrane. Finally, dried under the nitrogen environment, to be tested. As shown in Fig. 2(b), the PEG-4000-NZVI has a spherical shape, signifying good dispersibility, yet it also features chain-like structures, resulting from the combined effects of surface tension and magnetic properties. The particle size of the PEG-4000-NZVI ranges from 20 to 80 nm, 400 times smaller than that of commercial iron powder. 3.2. TCE degradation in the presence of surfactants Batch experiments were conducted to compare modified NZVI and commercial ZVI with surfactants in their removal of TCE. Compare to SDS modifed NZVI and CTAB modifed NZVI, PEG-4000NZVI was chosed for farther study. Thus, 1.0 g/L zero iron power and
Table 2 BET characterization image of ZVI and modified NZVI. Materials
Specific surface area (m2/g)
Pore volume (cc/g)
Pore diameter (nm)
Commercial ZVIa Rare-NZVIb SDS-NZVIc CTAB-NZVId PEG-4000-NZVIe
1.29 7.56 6.04 11.98 22.77
0.00 0.03 0.03 0.04 0.05
2.47 2.29 2.27 2.25 2.21
a b c d e
ZVI: Zero-valent iron. Rare-NZVI: The non-modified NZVI. SDS-NZVI: The SDS modified NZVI. CTAB-NZVI: The CTAB modified NZVI. PEG-4000-NZVI: The PEG-4000 modified NZVI.
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Fig. 1. X-ray diffractogram of commercial ZVI and modified NZVI by different dispersants.
Fig. 2. TEM image of commercial ZVI and PEG-4000-NZVI.
1.0 CMC of surfactant was added. After 3 h reaction time, trichloroethylene showed no loss or reduction in a blank experiment involving a soil-water system. Fig. 3(A) shows the TCE removal efficiency of commercial ZVI, reaching 51% and 78% in commercial ZVI combined with SDS and commercial ZVI combined with CTAB, respectively. Fig. 3(B) indicates that only 1% TCE remains in the supernatant when PEG-4000-NZVI was alone added, suggesting that PEG-4000-NZVI removes almost all TCE from the aqueous phase. For PEG-4000-NZVI combined with CTAB, 83% of TCE can be removed from soil. Compared to a commercial ZVI and PEG-4000-NZVI, our PEG-4000-NZVI has a higher removal efficiency than commercial ZVI in soil-water systems. For similar reaction conditions, the smaller particle size of zero-valent iron offers
good advantages for removing TCE: the smaller the particle size, the larger the specific surface area, and the more active sites supplied. All increase the contacting probability with pollutants and as a result, the removal efficiency can of the target pollutants is improved (Liu et al., 2005). TCE removal efficiency was defined as follows: TCE removal efficiency: hTCE (%) ¼ C0C C0 100% C0 correspond to the TCE concentration in the original soil solution part or supernatant solution part; and C correspond to the remaining concentration of TCE in the soil solution orsupernatant solution after the reaction. The presence of anionic surfactant SDS and cationic surfactant CTAB leads to obvious solubilization of TCE; because the alkyl chain
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Fig. 3. Removal of TCE by surfactants with commercial ZVI and NZVI. (A) Commercial ZVI, (B) PEG-4000-NZVI. (a) Blank; (b) (Control); (c) (Commercial ZVI); (d) (SDS); (e) (CTAB); (f) (Commercial ZVI combined with CTAB); (f-1) (PEG-4000-NZVI combined with CTAB); (g) (Commercial ZVI combined with SDS); (g-1) (PEG-4000-NZVI combined with SDS).
of CTAB is longer than for SDS, CTAB shows superior solubilization ability than SDS, in agreement with Gao et al. (1999) and Pennell K D et al. (Pennell et al., 1997) reported that the longer the length of the alkyl chain, the larger the volume of micellar nucleus, and the more hydrophobic organic compounds can be absorbed; as a result, the solubilization of surfactants improves more. Compared to commercial ZVI combined with CTAB/SDS process, the PEG-4000NZVI combined with CTAB/SDS process, after 3 h rection, the remains of TCE in supernatant solution are higher, it can be result from the higher adsorbability of PEG-4000-NZVI (Zhang et al., 2013; Azzam et al., 2016). So the TCE adsorpted by surfactants are desorption in supernatant solution. Also, it was a short time for PEG-4000-NZVI to remove TCE in 3 h, so a longer reaction time is needed for farther study. 3.3. pH range The pH of initial samples was adjusted by 0.05 mol/L H2SO4 and 0.05 mol/L NaOH with a pH meter. Fig. 4 shows the TCE removal obtained for initial pH values of 3.0, 5.0, 7.0, 9.0 and 11.0. In the original sample, with initial pH values of the soil-water system of 8.25, TCE were almost completely removed, for both NZV combined with CTAB and PEG-4000-NZVI combined with SDS. In PEG-4000NZVI combined with CTAB, both neutral and acidic conditions were
appropriate for TCE degradation; in alkaline conditions there was a slight decrease of TCE dechlorination. For PEG-4000-NZVI combined with SDS, both strongly acidic and neutral conditions were the most appropriate for TCE degradation, compared with the other conditions tested. In an acidic aqueous solution, the oxide layer at the surface of PEG-4000-NZVI can react with Hþ, which is beneficial for fresh PEG-4000-NZVI. However, in alkaline condition, the reaction mechanism becomes more complex. On the one hand, it induces two ways to accelerate TCE degradation: (1) PEG-4000-NZVI directly degrades TCE; (2) iron hydroxides generated from PEG4000-NZVI undergo coordinated reactions with TCE, improving the removal efficiency of TCE (Chatterjee et al., 2010). On the other hand, iron hydroxides also prevent PEG-4000-NZVI to contact TCE; as a result, the TCE removal efficiency slightly decreases. As a whole, PEG-4000-NZVI can adapt to a wide pH range in TCE degradation. In the experiment, the solution pH value was determined after 4 h; as shown in Table 3., the pH values changed with time: the acidic pH values all present slightly alkaline values (7.5e8.5) after the reaction. As shown in Eq. (3) and (4), during the degradation of chlorinated organic compounds by Fe0, Hþ was consumed, leading to pH value increases. According to the degradation mechanism of chlorinated organic compounds by Fe0 (as shown in Eq. (2)), it can indirectly reflect that TCE is degraded by reductive dechlorination. Besides, the alkaline pH values all present slightly alkaline values (8.0e9.5) after the reaction. In alkaline system, the iron hydroxides are produced by Fe0 (as shown in Eq. (2)), leading to a slightly pH value decreases.
3.4. Effects of operating parameters The effect of three operating parameters (PEG-4000-NZVI dosage, surfactant concentration and oscillation strength) is
Table 3 The pH-value before and after the reaction.
Fig. 4. Effect of initial pH on TCE removal efficiency. (cPEG-4000-NZVI ¼ 1.0 g/L, cSDS ¼ 2.0 CMC, cCTAB ¼ 1.0 CMC, T ¼ 4 h).
Before reaction
After reaction
3 5 7 9 11 Initial 8.25
SDS 7.81 8.5 8.74 8.81 9.35 8.89
CTAB 7.79 8.26 8.28 8.3 8.8 8.37
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Fig. 5. Effects of operating parameters on the degradation of TCE.
illustrated in Fig. 5. Fig. 5 a, b, and c suggest that each of these parameters has an important influence, however, possibly with mutual interactions. PEG-4000-NZVI dosage. The amount of PEG-4000-NZVI dosage is one of the main parameters influencing degradation and its effects are illustrated in Fig. 5(a). When the PEG-4000-NZVI dosage amplifies from 0.1 to 1.0 g/L, the dechlorination efficiency of TCE augments stepwise: such results can tentatively be explained by a rising amount of active sites becoming available from PEG-4000NZVI (Gao et al., 2007; Prak, 2008). When the number of active sites increases, the probability of contact with target pollutants grows, and as a result, the reaction rate surges higher. When the PEG-4000-NZVI dose further swells from 1.0 to 2.0 g/L, the removal efficiency remains steady. For PEG-4000-NZVI combine with SDS and PEG-4000-NZVI combine with CTAB, when the PEG-4000-NZVI dosage reaches 1.0 g/L, the removal efficiency of TCE amounts to 80 and 91%, respectively, illustrating the better synergistic effect on TCE degradation of CTAB with PEG-4000-NZVI than for SDS. Surfactant concentration. The effect of surfactants on the degradation of TCE was tested using PEG-4000-NZVI. The experimental data displayed a two-stage rate profile. Fig. 5(b) shows an almost complete TCE removal in soil-water systems above 1 CMC. When the concentration of SDS is between nil and 2.0 CMC, an initial rapid rate is followed by a subsequent slower dechlorination rate. When SDS is added at 2.0 CMC, 87.32% of TCE was degraded, When further increasing the concentration of SDS to 4.0 CMC, the removal efficiency no longer improves. However, for CTAB, about 90% of TCE was destroyed at 1.0 CMC after 4 h. In the presence of CTAB, more TCE can be removed, at lower concentration. When surfactant is added below 1.0 CMC, it still has some solubilization capabilities, because the single molecule of SDS and CTAB both have the ability to increase the solubility of hydrophobic organics (Qiu and Fang, 2010). In addition, the dimer or trimer formed from surfactant molecules has this same ability (Urum et al., 2006).
Oscillation can be proposed to stop or break the recombination of nanoparticles, which is beneficial for strengthening pollutants' degradation. Fig. 5(c) shows the results of optimizing oscillation strength: TCE decomposition was boosted by using a constant speed with an oscillation intensity from 0 to 200 r/min for PEG-4000-NZVI combined with SDS and 0e150 r/min for PEG-4000-NZVI combined with CTAB. This phenomenon can be explained as follows: as the oscillation intensity strengthens, the reunion of PEG-4000-NZVI is better prevented and the dispersion of PEG-4000-NZVI particles is superior. Hence, the possibility for PEG-4000-NZVI to contact with TCE increases, as a result, the reaction conditions improve. Thus, 200 r/min in PEG-4000-NZVI combined with SDS process and 150 r/min in PEG-4000-NZVI combined with CTAB process were selected as optimal oscillation intensity. In summary, Fig. 5. (a),(b),(c) summarize the empirical results obtained during testing. During the experimental work, for each of the systems studied, a domain of successful activity was identified. 3.5. Reaction and kinetics The effect of reaction time on TCE removal was also investigated. As shown in Fig. 6., the removal efficiency of TCE increased gradually and almost completely removed after 4 h. Nevertheless, also the conversion kinetics show a complex behavior. There is an initial latency period, during which destruction is still slow, whereas the final data point seems unexpectedly high. In between the two, a fitted first order law applies reasonably well. According to the first order kinetics equation: dC/dt ¼ kobsC0, after 3.0 h, PEG-4000-NZVI combine with CTAB leads to fitted first order reaction kinetics for the removal of trichloroethylene, with the rate contants (kobs) of the two systems were 0.6869 h1 and a related correlation coefficient (R2) was 0.9945; when the reaction time was 0e3.5 h, PEG-4000-NZVI combined with SDS leads to
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Fig. 6. Reaction kinetics of TCE removed by surfactant with PEG-4000-NZVI. (a) PEG-4000-NZVI combined with SDS; (b) PEG-4000-NZVI combined with CTAB.
fitted first order reaction kinetics for the removal of trichloroethylene, with the rate contants (kobs) of the two systems were 0.5659 h1 and a related coefficient (R2) was 0.9934.
3.6. Dechlorination mechanisms There is no unified view on the mechanism of action of PEG4000-NZVI: both reduction (Manning et al., 2007; Shin and Cha, 2008; Zhang, 2003) and oxidation (Noradoun et al., 2003; Joo et al., 2005; Chang et al., 2009; Ghauch and Tuqan, 2009) reactions have been put forward to explain earlier findings. During reduction chlorinated organic compounds can be dechlorinated by Fe0, as shown in Eqs. (2)e(3). Besides, in the aqueous phase, Hþ is generated by the reaction of chlorinated organic compounds removed by Fe0, chlorinated organic compounds, solution pH to be acid can co-dechlorine the target pollutant, as shown in Eq. (4).
Fe0 þRCl þ H2 O/FeðOHÞ2 þRH þ Cl1 þHþ
(2)
Fe0 þ2Hþ þ RCl/Fe2þ þCl1 þ H2
(3)
RCl þ Hþ þ2e /þRH þ Cl1
(4)
According to the law of conservation of mass, the concentration of TCE can also be reflected indirectly, by monitoring some of its decomposition products. A mass balance for chloride ions during the TCE reductive degradation by PEG-4000-NZVI combine with surfactants in soil-water system was performed, the results are shown in Fig. 7. The operating conditions are as follow: cNZVI ¼ 1.0 g/L, cSDS ¼ 2.0 CMC, cCTAB ¼ 1.0 CMC, T ¼ 4 h. The total concentration of TCE is 12.56 mg/kg, the concentration of chlorine ions is (C0) 0.057 mol/L. In the research, the dechlorination amount of TCE was detacted by measuring the concentration of chloride ions released in soil-water system and chlorine in chlorinated hydrocarbons. The dechlorination efficiency of TCE was measured by the ratio of dechlorination amount to total chlorine atoms amount in TCE. The higher the amount of chlorine ions, the more TCE has been removed. The results show that SDS and CTAB surfactants do no reduce TCE, but exert a synergistic effect on the TCE degradation by PEG-4000-NZVI. The dechlorination efficiency of TCE were 74%, 77% and 92% in PEG-4000-NZVI, PEG-4000-NZVI combined with CTAB and PEG-4000-NZVI combined with SDS, respectively. The results indicated that there were desorption and reductive dechlorination in the removal of TCE, and reductive dechlorination played a leading role.
Fig. 7. A mass balance for chloride ions during the TCE reductive degradation in soilwater system.
3.7. Possible pathways Chlorinated TCE degradation necessitates a series of reaction steps and also creates toxic chlorinated intermediates. In this study, the TCE dechlorination efficiency were evaluated as shown in Fig. 7. Dichloroethylenes (DCEs) and vinyl chloride (VC) were found during the experiment. With reaction time increasing, vinyl chloride were removed completely, This is agree with the result of that VC were not detected after 3 h reaction time (Shanbhogue et al., 2017). It indicated that the vinyl chloride was degraded to non-chlorine micromolecules. After 4 h, 5.33% of DCEs can still be detected, Kaifas D. et al. (Kaifas et al., 2014) found that small amount of DCEs can be detected by during the experiment, and were generally not detected any more before the end of the experiment in 500 h. So it can be explained that DCEs detected in our study after 4 h, may result from the much shorter time than 500 h. Further experiments, a longer time would be necessary to determine the degradation of DCEs. None of TCE was detected after 4 h by PEG-4000-NZVI combine with surfactants in soil-water system, The end products were mostly nonchlorinated hydrocarbons. The possible removal pathways of TCE by PEG-4000-NZVI combine with surfactants comprise two processes: a reduction and an adsorption step (Arnold and Roberts, 1998, 2000). According to the chlorination mechanisms and experimental results, Fe0 and
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Fig. 8. Possible degradation pathway.
Fe2þ was the electron donor to organic pollutants in the reduction process. As shown in Fig. 8., firstly, TCE was broken by electron attack originating from a Fe0 surface, and dichloroethylenes are formed as intermediates. Reportedly, cis-dichloroethylene is dechlorinated to vinyl chloride (VC), then all dichloroethylene formed was completely dechlorinated to ethylene and ethane (Hara et al., 2005). Finally, these intermediates may be oxidized to nonchlorine products (Kaifas et al., 2014). In the adsorption process, Fe0 and the surfactant play as adsorbing materials. Small amount of trichloroethylene may be adsorbed in the surface by Fe0 and the surfactants.
forthe Central Universities (Project No. KG12031101) and National Natural Science Foundation of China international communication and cooperation programs (NSFCdJSPS, Project No. 2141101075) for providing financial assistances. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.chemosphere.2017.12.070.
References 4. Conclusions This study showed that both PEG-4000-NZVI combined with CTAB and PEG-4000-NZVI combined with SDS process can remove TCE efficiently, using PEG-4000-NZVI particles, in combination with surfactants, and ensuring the complete removal of TCE from soil-water systems. Moreover, both PEG-4000-NZVI combined with CTAB and PEG-4000-NZVI combined with SDS can adapt to a wide range of pH values. The TCE removal efficiency was influenced by various factors, such as PEG-4000-NZVI dose, surfactants concentration and oscillation intensity. For a dosage of PEG-4000-NZVI of 0.8 g L1, the CTAB concentration was 1.0 CMC, the SDS concentration was 2.0 CMC, and thus TCE can be completely removed in 4 h. The removal of TCE by PEG-4000-NZVI combine with surfactant follows fitted first order kinetics, after an initial period. The removal efficiency of TCE by PEG-4000-NZVI was much higher than for commercial ZVI, due to its smaller particle size, its larger specific surface area and higher surface reactivity, also, the nanoparticles of PEG-4000-NZVI can easily transferred by surfactant from soil into water and simultaneously contacted with TCE. The possible removal of TCE by PEG-4000-NZVI may include two pathways: reduction and adsorption, yet the reductive dechlorination of TCE by PEG-4000-NZVI plays a main role. Acknowledgement The authors are grateful to National Natural Science Foundation of China (Project No. 21677007), the Fundamental Research Funds
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Surfactant-enhanced PEG-4000-NZVI for remediating trichloroethylene-contaminated soil Huifang Tian, Ying Liang, Tianle Zhu, Xiaolan Zeng, Yifei Sun* Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of Space and Environment, Beihang University, 37 Xueyuan Road, Beijing, China
h i g h l i g h t s A new modified PEG-4000-NZVI was prepared and used in soil remediation. Two surfactants strongly solubilized trichloroethylene. The modified PEG-4000-NZVI worked well within a pH range of 3e11. Almost all trichloroethylene was removed from soil within 3 h.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2017 Received in revised form 7 December 2017 Accepted 11 December 2017 Available online 12 December 2017
In this study a NZVI was prepared by the liquid phase reduction method. The modified NZVI obtained was characterized by BET, TEM and XRD. The results showed that the iron in the PEG-4000 modified material is mainly zero-valent iron with a stable crystal structure. It has a uniform particle size, ranging from 20 to 80 nm, and a larger specific surface area than CTAB modified NZVI, SDS modified NZVI and commercial zero-valent iron. The two surfactants CTAB and SDS are also selected as solubilizers, the results showed that the two selected surfactants obviously solubilize trichloroethylene in soil. Compared with commercial zero-valent iron, PEG-4000 modified NZVI is better removed trichloroethylene from soil; Also, the optimal operational parameters were obtained. When the experimental conditions were: PEG-4000 modified NZVI dosage 1.0 g/L, CTAB/SDS concentration equal to the CMC, SDS concentration was 2.0 CMC, CTAB was concentration 1.0 CMC and the vibration speed 150 r/min, the removal efficiency of trichloroethylene in a soil-water system reached 100% after 4 h. Both NZVI combined with CTAB and NZVI combined with SDS followed fitted first order reaction kinetics during the removal of trichloroethylene and their reaction rate constant k was 0.6869 mg/(L$h) and 0.5659 mg/(L$h), respectively. According to the chloride ion detection test, the trichloroethylene degradation is mainly due to reductive dechlorination. © 2017 Elsevier Ltd. All rights reserved.
Handling Editor: X. Cao Keywords: Modified nano zero-valent iron Trichloroethylene Surfactant Soil remediation Dechlorination
1. Introduction Chlorinated hydrocarbons are widely used as solvents, yet they are also persistent organic pollutants. Their widespread distribution causes serious environmental problems, due to their persistence and toxicity (Gong et al., 2010; Hunt, 2009). TCE is a typical chlorinated hydrocarbon, with low water solubility and strong sorption to soil particles, and it is one of the major challenges for
* Corresponding author. E-mail addresses: [email protected] (H. Tian), [email protected] (Y. Liang), [email protected] (T. Zhu), [email protected] (X. Zeng), sunif@buaa. edu.cn (Y. Sun). https://doi.org/10.1016/j.chemosphere.2017.12.070 0045-6535/© 2017 Elsevier Ltd. All rights reserved.
remediating soil polluted with chlorinated hydrocarbons (Hamby, 1996). Once released into the subsurface, TCE distributes between soil and water (Moran et al., 2007), Because of its low solubility and high adsorbability to soil organic matter, a large fraction of TCE tends to be retained in the solid phase. The traditional methods for remediating such soils include thermal remediation (Acierno et al., 2003; Roland et al., 2007), chemical oxidation (Liang et al., 2004a,b), chemical reduction (Lien and Zhang, 2007), soil vapour extraction (Albergaria et al., 2008, 2012), soil washing (Pavel and Gavrilescu, 2008), solidification/ stabilization and vitrification (US EPA, 1999), However, these traditional techniques also have disadvantages, such as high cost and secondary pollution. Zero-valent iron (Fe0) is strongly reducing and has frequently been used to degrade organic pollutants (Bai
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Abbreviations NZVI PEG-4000 CTAB SDS CMC BET TEM XRD TCE
Nanoscale zero-valent iron Polyethylene glycol Cationic hexadecyl trimethyl ammonium bromide Anionic sodium dodecyl sulfate Critical micelle concentration Brunauer-Emmett-Teller Transmission Electron Microscope X-ray diffraction analysis Trichloroethylene
et al., 2008; Sun et al., 2012), to overcome these disadvantages. However, despite the simplicity and environmental harmlessness of Fe0, its application has been restricted by low reaction speed. In order to solve this problem, continuous efforts have been devoted to explore more efficient reductants. Recently, with the development of nanotechnology, nano zerovalent iron has attracted growing attention as a means for remediating groundwater and soil, due to their high reactivity and easy availability (Arnold and Roberts, 2000; Zhu et al., 2008). Some research reported that was very effective for the transformation and reductive dechlorination of chlorinated organic pollutants and PCBs (Varanasi et al., 2007; Wang and Zhang, 1997; Zhang et al., 2013). Furthermore, various inorganic compounds, such as nitrate, nitrite and Cr6þ(VI) can also be restored in soils by NZVI (Alowitz and Scherer, 2002). However, the easy aggregation of nanoparticles due to their strong van der Waals forces and magnetic attraction limited their rapid delivery in soil and subsequent usage in soil remediation (Saleh et al., 2007; Zhang et al., 2013; Tratnyek and Johnson, 2006). Thus, many researchers tried to overcome this limitation by using various modifiers or dispersants to coat the nanoparticles and increase steric and electrostatic hindrance among the particles, resulting in the enhanced stability of the nanoparticles suspension. Different chemicals were used to modify the particle surface, such as polyacrylic acid, guar gum and carboxymethyl cellulose, but some of these chemicals are not environmentally friendly and also costly (He and Zhao, 2007; Phenrat et al., 2009). Another issue concerning TCE removal in soil is its low water solubility and strong sorption before the reaction begins, which slows down the degradation and prolongs the required reaction time (Shin and Cha, 2008). Alternative strategies have been explored to solve this problem as well and some research reported that surfactant is one of the best options to promote the water solubility and miscibility of TCE in soil (Hill and Ghoshal, 2002; Sailaja et al., 2003). Based on the above analysis, we proposed a modified NZVI preparation method and usage, enhanced with surfactant for remediating TCE contaminated soil (Fan et al., 2013; Kang et al., 2006), assuming that combining a modified NZVI with surfactant might provide both enhanced water solubility and degradation efficiency (Gao et al., 2007; Bao et al., 2010; Bikshapathi et al., 2012). We chose two surfactants (SDS and CTAB). In this study, we investigated the remediating of TCE-contaminated soil by applying a modified NZVI, together with surfactants. The reducibility of using the modified NZVI was tested. The effects of important parameters such as NZVI dosage, surfactant concentration, reaction time, initial pH range and speed of an orbital shaker incubator were all investigated. Besides, NZVI and commercial zero valent iron were compared while remediating TCE contaminated soil. The degradation mechanism, the reduction products and the
possible reductive pathway of TCE were investigated. The objective of this study is to develop a new and effective surfactant-enhanced modified NZVI for remediating TCE contaminated soil. 2. Materials and methods 2.1. Materials Ferrous sulfate (FeSO4$7HO; >99%)), polyethylene glycol (PEG4000), and potassium borohydride (KBH4; >99%) were purchased from Xilong Chemical Co., Ltd, China. Cetyltrimethyl ammonium bromide (CTAB, >99%) and lauryl sodium sulfate (SDS, >99%) were supplied by Lanyi Chemical Products Co., Ltd, China. Sodium hydroxide and methyl alcohol were delivered by Beijing Chemical Works, China. Commercial iron powder was acquired from Tianjin Jinke Chemical Research Institute, China. All chemicals were used as received without further purification. All solutions were prepared with deionized water. 2.2. Soil annalysis The soil was supplied by the China Conservation and Environmental Protection Group (CCEPG), which was collected from a chemical factory, at Henan, China. The soil was air-dried and passed through 60 mesh sieves. The physical and chemical characteristics of the soil are shown in Table 1. 2.3. Preparation of contaminated soil A fixed TCE solution at mass of 100 mg was delivered in a small volume of methyl alcohol, spreading slowly to 100 g of the prepared soil, being mixed with contant temperature oscillator (Taicang Experimental Factory, THZ-C, China) for 48 h. Then the mixture was placed in a fume hood nearly a week for drying. During the drying period, the soil mixture was mixed regularly to ensure uniform drying, and the original value was analyzed before tests. As a result, the TCE concentration in contaminated soils after drying detected by a head-space sampler combined with Gas ChromatographyMass Spectrometer (GC-MS) was 12.56 mg/kg. The contaminated soil was kept out in a closed bottle, at room temperature before experiments. 2.4. Preparation of modified NZVI Modified NZVI particles by different dispersing agents were prepared via a liquid phase reduction method (Wang et al., 2009; Chen et al., 2012). After the rection device was set up, keeping the nitrogen into the three-nected round-bottom flask for 30min, continuously. The aim is to achieve the purpose of eliminating
Table 1 Physical and chemical characteristics of the soil. Soil Parameter
Unit
Value
Sample depth Moisture content pH Soil pore VPCa Hg Pb Benzo [b]&[k] fluoranthene Benzo [b]pyrene Indeno [1,2,3-cd]pyrene Dibenz [a,h]anthracene
cm %
30e50 10.7%e30.7 8.8 35.75e50.89 4.42 107~8.34 104 147 ± 1.8 40 ± 1.3 7.07e13.44 2.59e5.73 1.73e4.79 0.43e1.21
a
% ppm ppm mg/kg mg/kg mg/kg mg/kg
VPC: Vertical permeability coefficient.
H. Tian et al. / Chemosphere 195 (2018) 585e593
oxygen. Using SDS, CTAB and PEG-4000 as dispersing agents, FeSO4$7H2O was made to react with excess KBH4. Firstly, the reaction device with deionized ethanol and water 200 mL (ethanol: water ¼ 2:8, v/v) was deoxygenated by purging it with N2 for 30 min. Under nitrogen protection, 0.2 mol/L FeSO4$7H2O was added to ethanol/water and mechanically agitated for 15 min, then three different kinds of dispersing agents (SDS, CTAB, PEG4000) was added, respectively, and mixed into the FeSO4$7H2O solution. After the solution was mixed, the KBH4 (0.43 mol/L) alkaline solution was added dropwise to the solution. After stirring for 30 min, the mixture turned black. The black particles were separated from the solution by a magnet. They were washed three times with deoxygenated deionized water and deoxygenated ethanol respectively. Then, the black particles were separated by centrifuging, and dried for 24 h under vacuum. Finally we obtained the modified NZVI particles. The reaction can be described by Eq. (1): 0 Fe2þ þ 2BH 4 þ 6H2 O/Fe þ 2BðOHÞ3 þ 7H2 [
(1)
2.5. Characterization of modified NZVI The surface morphology of the modified NZVI particle was characterized by their specific surface area and pore size analysis (BET, NOVA 2200e, Quanta chrome). Transmission electron morphology of the catalyst was investigated by means of a S-3000 scanning electron microscope (TEM, Hitachi Ltd., In Japan). X-ray diffraction patterns of the catalyst were scanned at 2q from 10 to 80 on the Bruker D8 Advance X-ray diffractometer (XRD, Bruker Corporation, In Germany), to verify crystallinity. 2.6. Experimental procedure TCE Removal tested with batch experiments. The reducing activity of modified NZVI was tested while remediating the TCE contaminated soil. All runs were performed under contant temperature oscillator at 150 r/min and room temperature (25 C). 50 mL aqueous solution were added to 10 g of polluted soil. This ratio was selected in order to simulate the removal of TCE entrapped into soil pores in the saturated or vadose zones and treatment can be applied both in situ or off site (Doughty et al., 1999). Runs were carried out by using types of zero-valent iron as reductant, CTAB and SDS was added at its CMC as solubilizer, respectively. The effect of the modified NZVI dosage was investigated in a range from 0 to 2.0 g L1. The effect of surfactant concentration was varied in the range from 0 to 4 CMC. The effect of reaction time was studied between 0 and 4 h. The effect of pH was tested among the range from 3 to 11. Effect of shaker incubator speed was investigated from 0 to 300 r/min. 2.7. Analytical methods Aqueous and soil were separated by centrifugation. pH and chloride ion were analyzed in the aqueous phase. Remaining TCE was tested in both the aqueous phase and soil. The TCE analysis was performed with a head-space sampler (Beijing pury analysis instrument Co. Ltd. RHS-628, China) and Gas ChromatographyMass Spectrometer (GC-MS, Shimadzu Corporation, QC 2010 Plus, Japan). The concentration of dissolved chloride ions in the solution was measured by a chloride ion chromatograph (792, Metrohm China Limited). The extraction method of trichloroethylene in soil-water system was shown at Text S7 in Supplementary Information.
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3. Results and discussion 3.1. Characterization of modified NZVI Table 2 shows the average specific surface area of the modified NZVI, PEG-4000 modified NZVI is larger than SDS modified NZVI and CTAB modified NZVI, whit the average specific surface area of 22.77 m2 g1, 20 times larger than that of commercial iron power and 3 times larger than that of non-modified NZVI (Rare-NZVI). The larger specific surface area also explains the higher activity of the prepared NZVI particles (Wei et al., 2010; Wang et al., 2013). A BET image of ZVI and modified NZVI is shown in Table 2. Fig. 1 shows the XRD patterns of commercial ZVI and modified PEG-4000-NZVI. The peaks at 44.66 and 44.76 were indexed as diffractions of Fe0 and indicate zero-valent iron as the main active component. The PEG-4000, SDS and CTAB added during the preparation of NZVI particles plays a role in the dispersibility of the NZVI particles, by changing the charge distribution of their surface; As shown in the Fig. 1(b),(c),(d) the PEG-4000-NZVI particles are wellcrystallined, which keeps higher intensity than SDS-NZVI and CTAB-NZVI. Also, the PEG-4000-NZVI particles were welldispersed, mutually repulsive and less likely to agglomerate (Gao et al., 2007; Chen et al., 2012). While, since NZVI is easily oxidized, the prepared NZVI contains a certain amount of ferroferric oxide (magnetite), so that a surface oxide layer is formed, hindering further oxidizing contact (Zhao et al., 2011). In order to better understand the characteristics of modified NZVI and ZVI, TEM image of modified NZVI and ZVI were carried out. Pretreatment of samples to be tested: (1) Taking 0.05 ge0.1 g samples into a beaker with 20 mL ethanol, then closed with sealing film, dispersing for 15e20 min in an ultrasonic cleaning machine, with the aim of making sample particle dispersed homogeneously; (2) Putting 200 mesh double-network carbon-supported membrane on the surface of a watch-glass with filter paper. Using the drip tube to make the sample on the double-network carbonsupported membrane. Finally, dried under the nitrogen environment, to be tested. As shown in Fig. 2(b), the PEG-4000-NZVI has a spherical shape, signifying good dispersibility, yet it also features chain-like structures, resulting from the combined effects of surface tension and magnetic properties. The particle size of the PEG-4000-NZVI ranges from 20 to 80 nm, 400 times smaller than that of commercial iron powder. 3.2. TCE degradation in the presence of surfactants Batch experiments were conducted to compare modified NZVI and commercial ZVI with surfactants in their removal of TCE. Compare to SDS modifed NZVI and CTAB modifed NZVI, PEG-4000NZVI was chosed for farther study. Thus, 1.0 g/L zero iron power and
Table 2 BET characterization image of ZVI and modified NZVI. Materials
Specific surface area (m2/g)
Pore volume (cc/g)
Pore diameter (nm)
Commercial ZVIa Rare-NZVIb SDS-NZVIc CTAB-NZVId PEG-4000-NZVIe
1.29 7.56 6.04 11.98 22.77
0.00 0.03 0.03 0.04 0.05
2.47 2.29 2.27 2.25 2.21
a b c d e
ZVI: Zero-valent iron. Rare-NZVI: The non-modified NZVI. SDS-NZVI: The SDS modified NZVI. CTAB-NZVI: The CTAB modified NZVI. PEG-4000-NZVI: The PEG-4000 modified NZVI.
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Fig. 1. X-ray diffractogram of commercial ZVI and modified NZVI by different dispersants.
Fig. 2. TEM image of commercial ZVI and PEG-4000-NZVI.
1.0 CMC of surfactant was added. After 3 h reaction time, trichloroethylene showed no loss or reduction in a blank experiment involving a soil-water system. Fig. 3(A) shows the TCE removal efficiency of commercial ZVI, reaching 51% and 78% in commercial ZVI combined with SDS and commercial ZVI combined with CTAB, respectively. Fig. 3(B) indicates that only 1% TCE remains in the supernatant when PEG-4000-NZVI was alone added, suggesting that PEG-4000-NZVI removes almost all TCE from the aqueous phase. For PEG-4000-NZVI combined with CTAB, 83% of TCE can be removed from soil. Compared to a commercial ZVI and PEG-4000-NZVI, our PEG-4000-NZVI has a higher removal efficiency than commercial ZVI in soil-water systems. For similar reaction conditions, the smaller particle size of zero-valent iron offers
good advantages for removing TCE: the smaller the particle size, the larger the specific surface area, and the more active sites supplied. All increase the contacting probability with pollutants and as a result, the removal efficiency can of the target pollutants is improved (Liu et al., 2005). TCE removal efficiency was defined as follows: TCE removal efficiency: hTCE (%) ¼ C0C C0 100% C0 correspond to the TCE concentration in the original soil solution part or supernatant solution part; and C correspond to the remaining concentration of TCE in the soil solution orsupernatant solution after the reaction. The presence of anionic surfactant SDS and cationic surfactant CTAB leads to obvious solubilization of TCE; because the alkyl chain
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Fig. 3. Removal of TCE by surfactants with commercial ZVI and NZVI. (A) Commercial ZVI, (B) PEG-4000-NZVI. (a) Blank; (b) (Control); (c) (Commercial ZVI); (d) (SDS); (e) (CTAB); (f) (Commercial ZVI combined with CTAB); (f-1) (PEG-4000-NZVI combined with CTAB); (g) (Commercial ZVI combined with SDS); (g-1) (PEG-4000-NZVI combined with SDS).
of CTAB is longer than for SDS, CTAB shows superior solubilization ability than SDS, in agreement with Gao et al. (1999) and Pennell K D et al. (Pennell et al., 1997) reported that the longer the length of the alkyl chain, the larger the volume of micellar nucleus, and the more hydrophobic organic compounds can be absorbed; as a result, the solubilization of surfactants improves more. Compared to commercial ZVI combined with CTAB/SDS process, the PEG-4000NZVI combined with CTAB/SDS process, after 3 h rection, the remains of TCE in supernatant solution are higher, it can be result from the higher adsorbability of PEG-4000-NZVI (Zhang et al., 2013; Azzam et al., 2016). So the TCE adsorpted by surfactants are desorption in supernatant solution. Also, it was a short time for PEG-4000-NZVI to remove TCE in 3 h, so a longer reaction time is needed for farther study. 3.3. pH range The pH of initial samples was adjusted by 0.05 mol/L H2SO4 and 0.05 mol/L NaOH with a pH meter. Fig. 4 shows the TCE removal obtained for initial pH values of 3.0, 5.0, 7.0, 9.0 and 11.0. In the original sample, with initial pH values of the soil-water system of 8.25, TCE were almost completely removed, for both NZV combined with CTAB and PEG-4000-NZVI combined with SDS. In PEG-4000NZVI combined with CTAB, both neutral and acidic conditions were
appropriate for TCE degradation; in alkaline conditions there was a slight decrease of TCE dechlorination. For PEG-4000-NZVI combined with SDS, both strongly acidic and neutral conditions were the most appropriate for TCE degradation, compared with the other conditions tested. In an acidic aqueous solution, the oxide layer at the surface of PEG-4000-NZVI can react with Hþ, which is beneficial for fresh PEG-4000-NZVI. However, in alkaline condition, the reaction mechanism becomes more complex. On the one hand, it induces two ways to accelerate TCE degradation: (1) PEG-4000-NZVI directly degrades TCE; (2) iron hydroxides generated from PEG4000-NZVI undergo coordinated reactions with TCE, improving the removal efficiency of TCE (Chatterjee et al., 2010). On the other hand, iron hydroxides also prevent PEG-4000-NZVI to contact TCE; as a result, the TCE removal efficiency slightly decreases. As a whole, PEG-4000-NZVI can adapt to a wide pH range in TCE degradation. In the experiment, the solution pH value was determined after 4 h; as shown in Table 3., the pH values changed with time: the acidic pH values all present slightly alkaline values (7.5e8.5) after the reaction. As shown in Eq. (3) and (4), during the degradation of chlorinated organic compounds by Fe0, Hþ was consumed, leading to pH value increases. According to the degradation mechanism of chlorinated organic compounds by Fe0 (as shown in Eq. (2)), it can indirectly reflect that TCE is degraded by reductive dechlorination. Besides, the alkaline pH values all present slightly alkaline values (8.0e9.5) after the reaction. In alkaline system, the iron hydroxides are produced by Fe0 (as shown in Eq. (2)), leading to a slightly pH value decreases.
3.4. Effects of operating parameters The effect of three operating parameters (PEG-4000-NZVI dosage, surfactant concentration and oscillation strength) is
Table 3 The pH-value before and after the reaction.
Fig. 4. Effect of initial pH on TCE removal efficiency. (cPEG-4000-NZVI ¼ 1.0 g/L, cSDS ¼ 2.0 CMC, cCTAB ¼ 1.0 CMC, T ¼ 4 h).
Before reaction
After reaction
3 5 7 9 11 Initial 8.25
SDS 7.81 8.5 8.74 8.81 9.35 8.89
CTAB 7.79 8.26 8.28 8.3 8.8 8.37
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Fig. 5. Effects of operating parameters on the degradation of TCE.
illustrated in Fig. 5. Fig. 5 a, b, and c suggest that each of these parameters has an important influence, however, possibly with mutual interactions. PEG-4000-NZVI dosage. The amount of PEG-4000-NZVI dosage is one of the main parameters influencing degradation and its effects are illustrated in Fig. 5(a). When the PEG-4000-NZVI dosage amplifies from 0.1 to 1.0 g/L, the dechlorination efficiency of TCE augments stepwise: such results can tentatively be explained by a rising amount of active sites becoming available from PEG-4000NZVI (Gao et al., 2007; Prak, 2008). When the number of active sites increases, the probability of contact with target pollutants grows, and as a result, the reaction rate surges higher. When the PEG-4000-NZVI dose further swells from 1.0 to 2.0 g/L, the removal efficiency remains steady. For PEG-4000-NZVI combine with SDS and PEG-4000-NZVI combine with CTAB, when the PEG-4000-NZVI dosage reaches 1.0 g/L, the removal efficiency of TCE amounts to 80 and 91%, respectively, illustrating the better synergistic effect on TCE degradation of CTAB with PEG-4000-NZVI than for SDS. Surfactant concentration. The effect of surfactants on the degradation of TCE was tested using PEG-4000-NZVI. The experimental data displayed a two-stage rate profile. Fig. 5(b) shows an almost complete TCE removal in soil-water systems above 1 CMC. When the concentration of SDS is between nil and 2.0 CMC, an initial rapid rate is followed by a subsequent slower dechlorination rate. When SDS is added at 2.0 CMC, 87.32% of TCE was degraded, When further increasing the concentration of SDS to 4.0 CMC, the removal efficiency no longer improves. However, for CTAB, about 90% of TCE was destroyed at 1.0 CMC after 4 h. In the presence of CTAB, more TCE can be removed, at lower concentration. When surfactant is added below 1.0 CMC, it still has some solubilization capabilities, because the single molecule of SDS and CTAB both have the ability to increase the solubility of hydrophobic organics (Qiu and Fang, 2010). In addition, the dimer or trimer formed from surfactant molecules has this same ability (Urum et al., 2006).
Oscillation can be proposed to stop or break the recombination of nanoparticles, which is beneficial for strengthening pollutants' degradation. Fig. 5(c) shows the results of optimizing oscillation strength: TCE decomposition was boosted by using a constant speed with an oscillation intensity from 0 to 200 r/min for PEG-4000-NZVI combined with SDS and 0e150 r/min for PEG-4000-NZVI combined with CTAB. This phenomenon can be explained as follows: as the oscillation intensity strengthens, the reunion of PEG-4000-NZVI is better prevented and the dispersion of PEG-4000-NZVI particles is superior. Hence, the possibility for PEG-4000-NZVI to contact with TCE increases, as a result, the reaction conditions improve. Thus, 200 r/min in PEG-4000-NZVI combined with SDS process and 150 r/min in PEG-4000-NZVI combined with CTAB process were selected as optimal oscillation intensity. In summary, Fig. 5. (a),(b),(c) summarize the empirical results obtained during testing. During the experimental work, for each of the systems studied, a domain of successful activity was identified. 3.5. Reaction and kinetics The effect of reaction time on TCE removal was also investigated. As shown in Fig. 6., the removal efficiency of TCE increased gradually and almost completely removed after 4 h. Nevertheless, also the conversion kinetics show a complex behavior. There is an initial latency period, during which destruction is still slow, whereas the final data point seems unexpectedly high. In between the two, a fitted first order law applies reasonably well. According to the first order kinetics equation: dC/dt ¼ kobsC0, after 3.0 h, PEG-4000-NZVI combine with CTAB leads to fitted first order reaction kinetics for the removal of trichloroethylene, with the rate contants (kobs) of the two systems were 0.6869 h1 and a related correlation coefficient (R2) was 0.9945; when the reaction time was 0e3.5 h, PEG-4000-NZVI combined with SDS leads to
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Fig. 6. Reaction kinetics of TCE removed by surfactant with PEG-4000-NZVI. (a) PEG-4000-NZVI combined with SDS; (b) PEG-4000-NZVI combined with CTAB.
fitted first order reaction kinetics for the removal of trichloroethylene, with the rate contants (kobs) of the two systems were 0.5659 h1 and a related coefficient (R2) was 0.9934.
3.6. Dechlorination mechanisms There is no unified view on the mechanism of action of PEG4000-NZVI: both reduction (Manning et al., 2007; Shin and Cha, 2008; Zhang, 2003) and oxidation (Noradoun et al., 2003; Joo et al., 2005; Chang et al., 2009; Ghauch and Tuqan, 2009) reactions have been put forward to explain earlier findings. During reduction chlorinated organic compounds can be dechlorinated by Fe0, as shown in Eqs. (2)e(3). Besides, in the aqueous phase, Hþ is generated by the reaction of chlorinated organic compounds removed by Fe0, chlorinated organic compounds, solution pH to be acid can co-dechlorine the target pollutant, as shown in Eq. (4).
Fe0 þRCl þ H2 O/FeðOHÞ2 þRH þ Cl1 þHþ
(2)
Fe0 þ2Hþ þ RCl/Fe2þ þCl1 þ H2
(3)
RCl þ Hþ þ2e /þRH þ Cl1
(4)
According to the law of conservation of mass, the concentration of TCE can also be reflected indirectly, by monitoring some of its decomposition products. A mass balance for chloride ions during the TCE reductive degradation by PEG-4000-NZVI combine with surfactants in soil-water system was performed, the results are shown in Fig. 7. The operating conditions are as follow: cNZVI ¼ 1.0 g/L, cSDS ¼ 2.0 CMC, cCTAB ¼ 1.0 CMC, T ¼ 4 h. The total concentration of TCE is 12.56 mg/kg, the concentration of chlorine ions is (C0) 0.057 mol/L. In the research, the dechlorination amount of TCE was detacted by measuring the concentration of chloride ions released in soil-water system and chlorine in chlorinated hydrocarbons. The dechlorination efficiency of TCE was measured by the ratio of dechlorination amount to total chlorine atoms amount in TCE. The higher the amount of chlorine ions, the more TCE has been removed. The results show that SDS and CTAB surfactants do no reduce TCE, but exert a synergistic effect on the TCE degradation by PEG-4000-NZVI. The dechlorination efficiency of TCE were 74%, 77% and 92% in PEG-4000-NZVI, PEG-4000-NZVI combined with CTAB and PEG-4000-NZVI combined with SDS, respectively. The results indicated that there were desorption and reductive dechlorination in the removal of TCE, and reductive dechlorination played a leading role.
Fig. 7. A mass balance for chloride ions during the TCE reductive degradation in soilwater system.
3.7. Possible pathways Chlorinated TCE degradation necessitates a series of reaction steps and also creates toxic chlorinated intermediates. In this study, the TCE dechlorination efficiency were evaluated as shown in Fig. 7. Dichloroethylenes (DCEs) and vinyl chloride (VC) were found during the experiment. With reaction time increasing, vinyl chloride were removed completely, This is agree with the result of that VC were not detected after 3 h reaction time (Shanbhogue et al., 2017). It indicated that the vinyl chloride was degraded to non-chlorine micromolecules. After 4 h, 5.33% of DCEs can still be detected, Kaifas D. et al. (Kaifas et al., 2014) found that small amount of DCEs can be detected by during the experiment, and were generally not detected any more before the end of the experiment in 500 h. So it can be explained that DCEs detected in our study after 4 h, may result from the much shorter time than 500 h. Further experiments, a longer time would be necessary to determine the degradation of DCEs. None of TCE was detected after 4 h by PEG-4000-NZVI combine with surfactants in soil-water system, The end products were mostly nonchlorinated hydrocarbons. The possible removal pathways of TCE by PEG-4000-NZVI combine with surfactants comprise two processes: a reduction and an adsorption step (Arnold and Roberts, 1998, 2000). According to the chlorination mechanisms and experimental results, Fe0 and
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Fig. 8. Possible degradation pathway.
Fe2þ was the electron donor to organic pollutants in the reduction process. As shown in Fig. 8., firstly, TCE was broken by electron attack originating from a Fe0 surface, and dichloroethylenes are formed as intermediates. Reportedly, cis-dichloroethylene is dechlorinated to vinyl chloride (VC), then all dichloroethylene formed was completely dechlorinated to ethylene and ethane (Hara et al., 2005). Finally, these intermediates may be oxidized to nonchlorine products (Kaifas et al., 2014). In the adsorption process, Fe0 and the surfactant play as adsorbing materials. Small amount of trichloroethylene may be adsorbed in the surface by Fe0 and the surfactants.
forthe Central Universities (Project No. KG12031101) and National Natural Science Foundation of China international communication and cooperation programs (NSFCdJSPS, Project No. 2141101075) for providing financial assistances. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.chemosphere.2017.12.070.
References 4. Conclusions This study showed that both PEG-4000-NZVI combined with CTAB and PEG-4000-NZVI combined with SDS process can remove TCE efficiently, using PEG-4000-NZVI particles, in combination with surfactants, and ensuring the complete removal of TCE from soil-water systems. Moreover, both PEG-4000-NZVI combined with CTAB and PEG-4000-NZVI combined with SDS can adapt to a wide range of pH values. The TCE removal efficiency was influenced by various factors, such as PEG-4000-NZVI dose, surfactants concentration and oscillation intensity. For a dosage of PEG-4000-NZVI of 0.8 g L1, the CTAB concentration was 1.0 CMC, the SDS concentration was 2.0 CMC, and thus TCE can be completely removed in 4 h. The removal of TCE by PEG-4000-NZVI combine with surfactant follows fitted first order kinetics, after an initial period. The removal efficiency of TCE by PEG-4000-NZVI was much higher than for commercial ZVI, due to its smaller particle size, its larger specific surface area and higher surface reactivity, also, the nanoparticles of PEG-4000-NZVI can easily transferred by surfactant from soil into water and simultaneously contacted with TCE. The possible removal of TCE by PEG-4000-NZVI may include two pathways: reduction and adsorption, yet the reductive dechlorination of TCE by PEG-4000-NZVI plays a main role. Acknowledgement The authors are grateful to National Natural Science Foundation of China (Project No. 21677007), the Fundamental Research Funds
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