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Field demonstration of enhanced removal of chlorinated solvents in groundwater using biochar-supported nanoscale zero-valent iron
Science of the Total Environment 698 (2020) 134215
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Field demonstration of enhanced removal of chlorinated solvents in groundwater using biochar-supported nanoscale zero-valent iron Linbo Qian a,b,c, Yun Chen a,b, Da Ouyang a,b, Wenying Zhang a,b, Lu Han a,b, Jingchun Yan a,b,c, Petr Kvapil d, Mengfang Chen a,b,c,⁎ a
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, Jiangsu Province, China University of Chinese Academy of Sciences, Beijing 100049, China National Engineering Laboratory for Site Remediation Technologies, Beijing 100015, China d Technical University of Liberec, Studentska 2, CZ46117 Liberec, Czech Republic b c
H I G H L I G H T S
G R A P H I C A L
• Biochar-nZVI was effectively applied for the first time in a field demonstration. • nZVI significantly reduced concentrations of chlorinated solvents within 24 h. • Biochar-nZVI overcomes the problem of rebound of chlorinated solvents in groundwater. • Biochar-nZVI greatly enhanced removal of chlorinated solvents within 42 days. • Reduction and adsorption are the primary processes to remove chlorinated solvents.
Enhanced removal of chlorinated hydrocarbon by biochar-supported nZVI.
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 24 July 2019 Received in revised form 30 August 2019 Accepted 30 August 2019 Available online 31 August 2019 Editor: Baoliang Chen Keywords: Biochars Nanoscale zero-valent iron Chlorinated solvents In situ groundwater remediation Field demonstration
A B S T R A C T
The application of biochar-supported nanoscale zero-valent iron (biochar-nZVI) was successfully implemented in a field demonstration for the first time. To overcome the significant shortcomings of nZVI agglomeration for in-situ groundwater remediation, biochar-nZVI was injected into groundwater using direct-push and water pressure driven packer techniques for a site impacted by chlorinated solvents in the North China Plain. The field demonstration comprising two-step injections was implemented to demonstrate the effectiveness of nZVI and biochar-nZVI respectively. The outcome of the demonstration revealed a sharp reduction of contaminant concentrations of chlorinated solvents in 24 h following the first injection of nZVI, but the rebound of the concentrations of these contaminants in groundwater has occurred within the next two weeks. However, application of biocharnZVI greatly enhanced the removal of chlorinated solvents in groundwater over the longer period of 42 days. The enhanced removal of chlorinated solvents in groundwater by biochar-nZVI is mainly attributed to the synergistic effects of adsorption and reduction. The adsorption by biochar significantly reduced the level of chlorinated solvents in groundwater. Overall increases in ferrous iron and chloride concentrations after the injections indicated
⁎ Corresponding author at: Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, Jiangsu Province, China. E-mail address: [email protected] (M. Chen).
https://doi.org/10.1016/j.scitotenv.2019.134215 0048-9697/© 2019 Elsevier B.V. All rights reserved.
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that the reduction has occurred during the removal of chlorinated solvents in groundwater. In summary, biocharsupported nZVI could be potentially used for the effective remediation of chlorinated solvents in groundwater. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Chlorinated solvents are organic hydrocarbons that are commonly used in industrial production, and are subsequently found in discontinuous aquifers as pools of residue (Li et al., 2018). Generally, a site that has been contaminated with chlorinated solvents is difficult to remediate, as the extent and characteristics of such contamination are difficult to delineate (Cheng et al., 2016; Ji et al., 2008). Nowadays, nanotechnology has received increasing attention for the remediation of groundwater contaminated with chlorinated solvents (Quinn et al., 2005; Wu et al., 2019). Nanoscale zero valent iron (nZVI) is capable of remediating chlorinated solvents in groundwater via injection into contaminated zones (Comba et al., 2011; Elliott and Zhang, 2001; Lei et al., 2018) and has been used for over 80 sites worldwide since 2000 (Nanorem, 2017). However, many field tests have showed that nZVI is rapidly deposited on the well screen after traveling only a few inches to a few feet before becoming immobilized (Kocur et al., 2014; Krol et al., 2013). This limited mobility of nZVI is caused by its tendency to quickly aggregate and settle primarily because of magnetic attractive forces (Phenrat et al., 2007). The reactivity of nZVI was also found to decrease due to this aggregation (Li et al., 2006). To overcome this bottleneck, nZVI can be coated with various carbon materials to enhance its reactivity and mobility (Vogel et al., 2018; Weil et al., 2019). Several reports have shown that carbon can improve subsurface colloid transport and thus maintain iron reactivity (Chen et al., 2011; Mackenzie et al., 2012; Mackenzie et al., 2016). Natural organic matter and carboxylmethyl cellulose were also found to enhance the mobility of nZVI (Johnson et al., 2009; Johnson et al., 2013). Compared to normal carbon material, biochar includes a large specific surface area and high porosity, which is excellent for nZVI loading (Dong et al., 2017; Qian et al., 2019b; Zhou et al., 2019). The negative surface of carbon materials may also be beneficial for the migration of nZVI (Mackenzie et al., 2012; Qian and Chen, 2013). In laboratory studies, the strengthening effect of carbon-loaded nZVI on the removal of organic contaminants has been confirmed (Yan et al., 2015; Zhang et al., 2018; Zhang et al., 2019), but its role in actual remediation remains to be elucidated. Currently, no published studies have been identified on the removal of chlorinated hydrocarbons in groundwater using biochar-nZVI on a field-scale. Based on the detailed site investigation and risk assessment modelling, a contaminated site located in the North China Plain and impacted by chlorinated solvents was chosen as a pilot test. The objectives of the study were to evaluate the effectiveness of biochar-nZVI in the treatment of chlorinated solvents in groundwater. A potential mechanism for the application of biochar-nZVI was subsequently proposed.
2. Materials and methods 2.1. Site information The field demonstration site, situated on the North China Plain, was contaminated with chlorinated solvents, primarily trichloroethylene (TCE) and trichloromethane (TCM) which were used to clean degreasing tanks and other parts during the maintenance of military vehicles in the past (Fig. 1a, b). These contaminants were released into the groundwater due to inadequate and ignorance of environmental management procedures. Detailed site investigation, groundwater monitoring and risk assessment modelling to establish groundwater remedial
targets were undertaken prior to the design and injection of nZVI and biochar-nZVI in the study area. The geology and hydrogeological conditions and plume distribution of the contaminants in the study area are shown in Fig. 1. The geological sequence of the site is illustrated in Fig. 1c. The top layer is made ground comprising gravel fill or brown reworked silty sand which extends to a depth of approximately 2.5 m bgl (below ground level), followed by silty clay of 0.5 m thick, potentially acting as an aquitard. The third layer consists of inter-bedded sand or sand and silt of approximately 2.5 m thick representing a shallow aquifer. A glacial silt/clay till is beneath the third layer having significantly lower permeability and acting as another aquitard. Detailed site investigation revealed that the groundwater flows from the southwest to the northeast and is in hydraulic continuity with the nearby river (Fig. 1d). The hydraulic conductivity of the aquifer at this site was determined to be 3 m/d, suggesting a reasonably permeable formation. Water table is located approximately 3 m bgl with a saturated thickness of approximately 2.5 m. Groundwater is contaminated by chlorinated solvents including TCE, 1.1Dichloroethene, 1,1-Dichloroethane, 1,2-Dichloroethane, 1,1,2Trichloroethane, Tetrachloroethene, 1,1,2,2-Tetrachloroethane and Chloroform at levels ranging from just above the detection limit (0.5 μg/L) to 21,200 μg/L (Fig. 1e). The distribution of contaminants is illustrated in Fig. 1e and the red, blue, and green areas were delineated based on the risk rankings representing heavy, moderate, and light contaminations, respectively. Based on the direction of groundwater flow and the distribution of contaminants, the test area of approximately 15 m × 15 m was chosen at the upstream of the known heavily contaminated area. Sixteen injection and ten monitoring wells were designed and constructed in the test area (Fig. 1f). Green circles in Fig. 1f represent direct-push injection wells by geoprobe, red circles were designed for the packer injection installed as large diameter wells, and yellow circles represents multilevel monitoring wells (known as micropump wells hereafter). There are totally eleven direct-push injection wells (referred to as DPW hereafter); 5 packer injection wells (referred to as PKW hereafter), 5 micro pump monitoring wells (referred to as PMW hereafter) in the test area and a control well (CK) well outside the test area. PMW and PKW wells have also been used as monitoring points prior to and post the injection. 2.2. The assemblage of nZVI and biochar-supported nZVI The nZVI used in this study is commercially available as NANOFER STAR, which was purchased from Nanoiron (Czech Republic). Biochar was obtained from the Yuanli Huaiyushan activated carbon company. The primary biomass used was wood, and the pyrolysis temperature was 500–600 °C. The specific surface area of biochar was 1793 m2/g, this is due to the activation of phosphate in the preparation of biochar. The nZVI and biochar were dissolved and mechanical blending before injection. Two injections with nZVI and biochar-supported nZVI had been designed in the test area. In case of the test injection with nZVI, the suspension was prepared by dispersing dry iron particles in tap water using a high-speed homogenizer, to form a 200 g/L suspension. The suspension was subsequently injected after the dilution, with a final concentration of 10 g/L of nZVI. The injections were performed at depths of 3.5, 4.5 and 5.5 m bgl at the packer wells and direct-push injection well under a hydraulically adapted pressure with an injection velocity of about 1 m3/h in order to prevent shear failure and tunnel erosion
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Fig. 1. Site location (a) and layout (b), geological structure of the site (c), direction of groundwater flow (d), location of monitoring wells and contaminant plume distribution (e) and injection design (f).
Fig. 2. Setup of the injection procedure.
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within the sediment. In case of injection with biochar-nZVI, it commenced 14 days after the first injection with nZVI, and was performed on the packer injection wells at pre-determined intervals with a suspension concentration of 30 g/L for biochar-nZVI. The concentrations of nZVI and biochar-nZVI were set based on the nZVI concentration. The preparation of nZVI and biochar-nZVI suspension is modified from previous reports (Kim et al., 2018; Soukupova et al., 2015). 2.3. Injection and monitoring of contaminants Firstly, 200 kg of nZVI was injected at a concentration of approximately 10 g/L with a total volume of about 100 m3 of suspension. The injection was undertaken using Geoprobe at eleven directpush wells with an applied fluid pressure of between 1 and 12 bars. The interval of the injection was designed at 3.5 m, 4.5 m and 5.5 m bgl, respectively. The nZVI suspension was continuously stirred during the injection. Second injection was followed fourteen days after the first injection at five packer wells with the biochar-nZVI mixture.
A total of 100 kg of nZVI and 200 kg of biochar were injected with approximately 50 m3 of suspension. The interval of the second injection was the same as the first injection with an applied fluid pressure of between 1 and 5 bars. The setup of the injection procedure is shown in Fig. 2, where the water on the left was used for nZVI reaction and material dilution. The nZVI and biochar were then mixed in a dosing unit and the mixture was injected by either direct push techniques or packer under additional hydraulic pressure. Groundwater was monitored at the micropump and the packer wells and sampled for analyzing chlorinated solvents and some inorganic constituents such as chloride and ferrous iron. The chlorinated hydrocarbons were extracted by liquid-liquid extraction using hexane and quantified using a gas chromatograph (Agilent 7890, USA) equipped with an electron capture detector. The dissolved ferrous ions were measured using the 1,10-phenathroline method (Hach method 8146). The chloride concentration was then analyzed with capillary ion electrophoresis (CE, Waters Quanta 4000E). Groundwater samples were collected prior to the nZVI injection to establish baseline conditions with the
Fig. 3. Trichloroethene concentrations in micropump wells (a–e), packer injection and the background well (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
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frequency of monitoring at 1, 7, 14, 21, 28, and 56 days after the first injection. 3. Results and discussion 3.1. Removal of chlorinated solvents during the field test The concentrations of TCE in the micropump and the packer wells prior to and post the injection are presented in Fig. 3. Depth specific concentrations of TCE in the micropump wells are illustrated in Fig. 3a and increasing concentrations of 2160, 2640 and 3510 μg/L were recorded at depths of 3.5, 4.5 and 5.5 m bgl respectively in MPW1 prior to injection. This may has been caused by the higher density of TCE than groundwater. The TCE concentrations were rapidly decreased to 523, 725 and 982 μg/L, respectively in MPW1–3.5, MPW1–4.5 and MPW1–5.5 following the first injection with nZVI. However, the level of TCE in groundwater had recovered to 1940 μg/L in MPW1–5.5 after 7 days following the first injection. Then the TCE concentrations had been further increased to 2490, 2980 and 3100 μg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively after 14 days following the first injection. The rebound of the TCE concentrations in MPW1 was consistent with previous studies (Elliott and Zhang, 2001). The result exhibited a wavelike pattern that is caused by the advection, dispersion, retardation, and reaction of the nanoparticle plume as it migrated within the test area (Elliott and Zhang, 2001). In order to solve this problem, biochar was chosen as a promising material. Our previous report indicated that biochar can enhance the removal of chlorinated hydrocarbon (Yan et al., 2015). The biochar-nZVI was therefore used for remediation in the pilot site. In order to enhance the removal of the contaminants, the second injection with biochar-nZVI was carried out. The TCE concentration at 7 d after the second injection has been decreased sharply to 386 and 15.4 μg/L in MPW1–4.5 and MPW1–3.5, respectively. By 14 d after the second injection (28 d after the first injection), the levels of TCE concentration in all three micropump wells were lower than 40 μg/L, and the concentration in all wells were lower than remediation target within 56 d. The TCE concentrations in MPW2, MPW3, MPW4 and MPW5 before the injection were lower than that of MPW1, except for MPW1–4.5 m. After the injection, the TCE concentrations in MPW2, MPW3, MPW4 and MPW5 were following the same trend as that of MPW1. The results of the monitoring from the micropump wells therefore indicated that the injection of nZVI is effective for the removal of TCE, especially in the form of biochar-nZVI. TCE concentrations in micropump wells being rapidly decreased implied that the nZVI was transported more than 1.5 m to remove the TCE. The TCE concentrations were higher in micropump wells than those in the packer wells (PKW2) before the injection, at 21200 μg/L. After the first injection with nZVI, the TCE concentration was decreased (PKW1PKW5). The concentration of TCE was subsequently decreased to lower than remediation target after biochar-nZVI. All the packer well also appears to have suffered from less rebound, which can mainly be attributed to the large amount of nZVI in the packer well in the short term. By comparison, the TCE concentration in the well (CK) without material injection demonstrated no significant change within 56 d. Tetrachloroethene (PCE) concentration before and after the injection is presented in Fig. 4. Before the injection, the PCE concentrations were 98.4, 263 and 238 μg/L, respectively in MPW1–3.5, MPW1–4.5 and MPW1–5.5. Similar to TCE, this trend of concentrations increasing with depths is confirmed in the remaining micropump wells. After the first injection with nZVI, PCE concentrations were rapidly decreased to 25.2, 78.9 and 96.1 μg/L, respectively in MPW1–3.5, MPW1–4.5 and MPW1–5.5. However, PCE concentration was increased by 7 d after the first injection in MPW1–3.5 and MPW1–4.5. By 14 d after the first injection, PCE concentrations in these two wells were further increased to 97.0 and 110 μg/L, respectively. Previous reports have indicated that the injection of nZVI is prone to rebound, leading to an increase in the
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PCE concentration after ten weeks (Mackenzie et al., 2016). PCE concentrations at 7 d after the second injection was observed to have been sharply decreased to 0.8, 4.7, and 6.4 μg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively. By 14 d after the second injection, PCE concentrations at three depths in the micropump well (MPW1) were lower than 2 μg/L. The highest PCE concentrations before the injection in MPW2, MPW3, MPW4, and MPW5 was 168, 212, 45.4 and 493 μg/L, respectively. After the first injection, the rebound of the PCE concentration in MPW2–4.5 had occurred and then sharply decreased following the second injection. The PCE concentrations in MPW3, MPW4 and MPW5 were decreased after the first injection. These results indicate that nZVI is more effective in the removal of PCE than that of TCE. This can mainly be attributed to a lower initial concentration of PCE than that of TCE. It is therefore indicated that the removal of TCE with biocharnZVI is more effective than that of PCE. Contaminant concentrations that are reduced via adsorption by biochar during the first step are thought to be one of the key factors enhancing removal. Previous reports had indicated that there is an optimum concentration of nZVI for the removal of pollutants (Qian et al., 2019a). This can mainly be attributed to the fact that high concentrations of pollutant hinder removal with nZVI. PCE concentrations were higher in the micropump wells than those in the packer wells before the injection. After the first injection with nZVI, the PCE concentrations were decreased, and continued to decrease to be lower than the limit of detection after the second injection. Due to the large amount of nZVI in the packer wells, the rebound did not happen immediately. As expected, the PCE concentration in the well (CK) without material injection showed no significant change. The changes in trichloromethane (TCM) concentration are presented in Fig. 5. TCM concentrations in the micropump wells are presented in Fig. 5a–e. The TCM concentrations in MPW1–3.5, MPW1–4.5 and MPW1–5.5 were 445 μg/L, 370 μg/L and 69.6 μg/L before the injection. However, the concentrations of TCM and other chlorinated solvents were lower than that of TCE in the corresponding wells, indicating that TCE is the main contaminant in the test area. After the first injection with nZVI, the TCM concentration was rapidly decreased. The TCM concentrations in MPW1–3.5, MPW1–4.5 and MPW1–5.5 was 29.2, 128 and 62.6 μg/L, respectively. The TCM concentrations were slightly increased at both 7 d and 14 d after the first injection in MPW1–5.5 m. The TCM concentration in MPW1 was then decreased by 7 d after the second injection with biochar-nZVI. By 28 d after the second injection the TCM concentrations at three depths in the micropump wells were lower than the limit of detection. The TCM concentrations in MPW2, MPW3, MPW4, and MPW5 before the injection were lower than that of MPW1. After the injection, the TCM concentrations in MPW2, MPW3, MPW4, and MPW5 followed the same trend as that of MPW1. The results from the micropump well indicate that the nZVI injection was effective for removing TCM. However, the TCM concentrations in MPW2 and MPW3 were sharply increased by 14 d after the first injection, confirming the concentrationdependent rebound. This rebound is apparent when the concentrations of contaminants exceed the removal capacity of nZVI, although the concentrations of contaminants tend to decrease again over a short time. After the injection of biochar-nZVI, the TCM concentrations were decreased. At the same period after the second injection, the TCM concentration had decreased to lower than remediation target. Compared to the micropump well, the TCM concentration was higher in the packer well (PKW1 and PKW2) before the injection, at 1860 μg/L. After the first injection with nZVI, the TCM concentration decreased, and had decreased to lower than the limits for detection within 28 d. In order to better illustrate the contamination of the pilot site, the main chlorinated hydrocarbons found in the area other than TCE, PCE and TCM are presented in Fig. 6. The concentrations of chlorinated hydrocarbons in the micropump wells are presented in Fig. 6a. Before the injection, the concentrations of chlorinated hydrocarbons were 1492, 1293 and 832 μg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5,
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Fig. 4. Tetrachloroethene concentrations in micropump wells (a–e), packer injection and the background well (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
respectively. After the first injection with nZVI, the concentrations of chlorinated hydrocarbon were rapidly decreased within 1 d to 251, 535 and 581 μg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5 respectively. However, the chlorinated hydrocarbon concentrations were increased by 7 d after the first injection. By contrast, the chlorinated hydrocarbon concentrations were sharply decreased to 225, 169 and 216 μg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively by 7 d after the second injection with biochar-nZVI. Then the concentrations of chlorinated hydrocarbon at three depths in the micropump wells were continue to decrease. By 32 d after the second injection with biochar-nZVI the concentrations of chlorinated hydrocarbons remained stable at approximately 11.7, 146 and 220 μg/L, respectively. The chlorinated hydrocarbon concentrations in MPW2, MPW3, MPW4 and MPW5 before injection were lower than that of MPW1. After the injection, the chlorinated hydrocarbon concentration in MPW2, MPW3, MPW4 and MPW5 followed the same trend as in MPW1. The results from the micropump well indicated that injection with nZVI was effective for
the removal of chlorinated hydrocarbon, especially the biochar-nZVI. Compared to the micropump well, the chlorinated hydrocarbon concentration was higher in the packer well before injection, at 109156 μg/L. After the first injection with nZVI, the concentration of chlorinated hydrocarbon decreased. The chlorinated hydrocarbon concentration then decreased to the limits for detection after the second injection. The rebound after nZVI injection was therefore acting as a bottleneck for the use of nZVI in general application for the removal of contaminants. 3.2. Ferrous iron and chloride transformation The ferrous iron concentrations in the micropump and the packer wells are presented in Fig. 7. Fig. 7a shows the concentrations of ferrous iron to be 0.05, 0.05 and 0.11 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively, before the injection. After the first injection with nZVI, the concentrations of ferrous iron were increased to 0.31,
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Fig. 5. Trichloromethane concentrations in micropump wells (a–e), packer injection and the background well (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
0.12 and 0.14 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5 respectively by 1 d after the first injection. By 14 d after the first injection, the ferrous iron concentrations were increased to 0.53, 0.45 and 0.53 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively. As the concentration of chlorinated hydrocarbons were high in the corresponding period. This indicates that nZVI was inadequate for the removal of ferrous iron. By 7 d after the injection with biochar-nZVI, the ferrous iron concentrations were decreased to 0.12 and 0.22 mg/L in MPW1–3.5 and MPW1–4.5 respectively. The concentration of ferrous iron in MPW1–5.5 continued to increase to 1.18 mg/L. By 56 d after the second injection the ferrous iron concentrations were decreased to 0.05, 0.12 and 0.18 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively. The ferrous iron concentrations in MPW2, MPW3 and MPW4 were higher than that in MPW1 at the corresponding period. The ferrous iron concentrations were significantly increased after the injection, indicating that the ferrous iron can be transported some distance. This result also suggested that the ferrous iron was not fully removed. The concentrations of ferrous iron were significantly decreased after the
biochar-nZVI injection. There are two means by which the ferrous concentrations could be decreased: participating in the reduction of chlorinated hydrocarbon and the adsorption by biochar. Previous study demonstrated that activated carbon could act as an adsorbent for ferrous iron, and then take part in the reduction process (Huang et al., 2019). The concentrations of ferrous iron were higher in the packer wells (PKW2) than those in the micropump wells, and were found to be at 9.27 mg/L after injection. This result indicated that much of ferrous iron was not fully used during the removal. After the injection with biochar-nZVI, the concentrations of ferrous iron were decreased. Then the ferrous concentrations in the packer well had decreased to the same concentrations as before injection within 56 d. By comparison, the ferrous concentration in the well without material injection remained at a low level from the beginning to the end of the test period. The chloride concentrations in the micropump and the packer wells are presented in Fig. 8. Fig. 8a shows the chloride concentrations of 1130, 490 and 490 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5 respectively before the injection. The chloride concentrations were
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Fig. 6. Chlorinated solvents in micropump wells (a–e), packer injection and the background well (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
increased with depths in the other micropump wells (MPW2-MPW5). After the first injection with nZVI, the chloride concentrations were rapidly decreased. This result indicates that chloride can be reduced by iron oxide. By 1 d after the injection, the chloride concentrations in MPW1–3.5, MPW1–4.5 and MPW1–5.5 was 348, 348 and 564 mg/L, respectively. However, the chloride concentrations were increased by 7 d after the first injection. The highest chloride concentration was reached at 897 mg/L in MPW1–3.5. By 14 d after the first injection, the chloride concentrations were further increased to 471, 518 and 948 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively, indicating that a reductive dechlorination process occurred during the removal of the contaminants. By 7 d after the injection with biochar-nZVI, the chloride concentrations were decreased in MPW1–5.5, and then increased again by 14 d and 42 d. The highest chloride concentration was reached at 1080 mg/L in MPW1–5.5. This is mainly due to the initial chloride being adsorbed by iron oxides in the first step, and then the released chloride by dichlorination increased the chloride concentration in the second step. The result indicates that biochar-nZVI enhanced the
dechlorination of the contaminants. The chloride concentrations in MPW2, MPW3, MPW4 and MPW5 followed the same trend as that of MPW1. The results from the micropump well indicate that the nZVI injection was effective for the dechlorination of the contaminants, especially in the form biochar-nZVI. The chloride concentrations were increased significantly after the injection with nZVI in the packer wells. This increase in chloride likely resulted from the dechlorination of chlorinated organics and provided strong evidence that dechlorination did occur (Su et al., 2012). The results above have highlighted the dechlorination efficiency of biochar-nZVI, which is of fundamental importance if the full potential of using biochar-nZVI for the groundwater remediation is to be achieved. Biochar, with a large specific surface area and a rich pore structure, was used as support for nZVI, which solved the bottleneck of reduced degradation efficiency of nZVI due to the agglomeration effect. The results from the test strongly indicated that biocharsupported nZVI significantly enhanced the removal of chlorinated hydrocarbons. The enhanced dechlorination of chlorinated hydrocarbon
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Fig. 7. Concentrations of ferrous iron in micropump wells (a–e), packer injection well and the background (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
by nZVI is mainly due to the following potential mechanisms. Firstly, during the dechlorination of chlorinated hydrocarbon by nZVI, intermediate products are more likely to be transferred to the surface of biochar because of the larger surface area. Secondly, biochar is served as a good adsorbent, and the concentrations of pollutants were decreased via adsorption by biochar. Thirdly, biochar is acted as an electron-transfer mediator and an adsorption-support material for the coprecipitation of reduction products with iron (Qian et al., 2017; Yan et al., 2015). Therefore, biochar-supported nZVI is an effective functional material that can be used as a readily accessible alternative for the remediation of groundwater contaminated with chlorinated hydrocarbons. 4. Conclusions In this study, the field demonstration of in-situ groundwater remediation using nZVI and biochar-nZVI mixture was undertaken at a site
contaminated by chlorinated solvents in the north-east China. Comprehensive groundwater monitoring prior to and post the injection was undertaken to evaluate the removal efficiency of the field test. The results suggested that monometallic nZVI was able to degrade the target contaminants over a short time, however the biochar-nZVI mixture promoted enhanced removal of chlorinated hydrocarbons over the longer period. The contaminant concentrations in the pilot area were decreased by more than 90%. The field test also demonstrated that the injected fluid could be delivered to adjacent micropump wells. This study also suggests that the injection of biochar-nZVI suspension would not result in the pore clogging that tends to limit subsequent remediation efforts. Further work is needed to assess the extent of the reaction with target contaminants, water, or natural reductants with the ultimate goal of designing nanometal formulation that optimizes the reaction with the target contaminants while maintaining good mobility. Additional work is needed to develop a greater understanding of the
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Fig. 8. Chloride concentrations in micropump wells (a–e), packer injection well and the background (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
transport of nZVI and the processes governing the degradation of contaminants at a range of field sites.
We are grateful to Prof. Miroslav Černík of Technical University of Libere and Dr. Jan Slunský of NANO IRON, s.r.o., for assisting in the design and implementation of the pilot test.
Declaration of competing interest References The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This article is financially supported by the National Key Research and Development Program of China (Grant No. 2017YFA0207002 and 2018YFC1803002); the Frontier Fields of the Thirteenth FiveYear Plan Period of the Institute of Soil Science, Chinese Academy of Sciences (Grant No. ISSASIP1656); National Engineering Laboratory for Site Remediation Technologies (Grant No. NELSRT201710).
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Qian, L., Liu, S., Zhang, W., Chen, Y., Ouyang, D., Han, L., Yan, J., Chen, M., 2019a. Enhanced reduction and adsorption of hexavalent chromium by palladium and silicon rich biochar supported nanoscale zero-valent iron. J. Colloid Interf. Sci 533 (428–436), 153–160. Qian, L., Shang, X., Zhang, B., Zhang, W., Su, A., Chen, Y., Ouyang, D., Han, L., Yan, J., Chen, M., 2019b. Enhanced removal of Cr(VI) by silicon rich biochar-supported nanoscale zero-valent iron. Chemosphere 215, 739–745. Quinn, J., Geiger, C., Clausen, C., Brooks, K., Coon, C., O'Hara, S., Krug, T., Major, D., Yoon, W.S., Gavaskar, A., Holdsworth, T., 2005. Field demonstration of DNAPL dehalogenation using emulsified zero-valent iron. Environ.l Sci. Technol. 39 (5), 1309–1318. Soukupova, J., Zboril, R., Medrik, I., Filip, J., Safarova, K., Ledl, R., Miroslav, M., Jaroslav, N., Miroslav, C., 2015. Highly concentrated, reactive and stable dispersion of zero-valent iron nanoparticles: direct surface modification and site application. 262, 813–822. Su, C., Puls, R.W., Krug, T.A., Watling, M.T., O'Hara, S.K., Quinn, J.W., Ruiz, N.E., 2012. A two and half-year-performance evaluation of a field test on treatment of source zone tetrachloroethene and its chlorinated daughter products using emulsified zero valent iron nanoparticles. Water Res. 46 (16), 5071–5084. Vogel, M., Nijenhuis, I., Lloyd, J., Boothman, C., Poeritz, M., Mackenzie, K., 2018. Combined chemical and microbiological degradation of tetrachloroethene during the application of carbo-iron at a contaminated field site. Sci. Total Environ. 628-629, 1027–1036. Weil, M., Mackenzie, K., Foit, K., Kuehnel, D., Busch, W., Bundschuh, M., Schulz, R., Duis, K., 2019. Environmental risk or benefit? Comprehensive risk assessment of groundwater treated with nano Fe-0-based carbo-iron (R). Sci. Total Environ. 677, 156–166. Wu, Y., Pang, H., Liu, Y., Wang, X., Yu, S., Fu, D., Chen, J., Wang, X., 2019. Environmental remediation of heavy metal ions by novel-nanomaterials: a review. Environ. Pollut. 246, 608–620. Yan, J., Han, L., Gao, W., Xue, S., Chen, M., 2015. Biochar supported nanoscale zerovalent iron composite used as persulfate activator for removing trichloroethylene. Bioresour. Technol. 175, 269–274. Zhang, D., Li, Y., Sun, A., Tong, S., Jiang, X., Wang, L., Sun, X., Li, J., Liu, X., Shen, J., 2018. Biochar supported sulfide-modified nanoscale zero-valent iron for the reduction of nitrobenzene. RSC Adv. 8, 22161–22168. Zhang, D., Li, Y., Sun, A., Tong, S., Su, G., Jiang, X., Li, J., Han, W., Sun, X., Wang, L., Shen, J., 2019. Enhanced nitrobenzene reduction by modified biochar supported sulfidated nano zerovalent iron: comparison of surface modification methods. Sci. Total Environ. 694, 133701. Zhou, Y., He, Y., He, Y., Liu, X., Xu, B., Yu, J., Dai, C., Huang, A., Pang, Y., Luo, L., 2019. Analyses of tetracycline adsorption on alkali-acid modified magnetic biochar: site energy distribution consideration. Sci. Total Environ. 650, 2260–2266.
Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Field demonstration of enhanced removal of chlorinated solvents in groundwater using biochar-supported nanoscale zero-valent iron Linbo Qian a,b,c, Yun Chen a,b, Da Ouyang a,b, Wenying Zhang a,b, Lu Han a,b, Jingchun Yan a,b,c, Petr Kvapil d, Mengfang Chen a,b,c,⁎ a
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, Jiangsu Province, China University of Chinese Academy of Sciences, Beijing 100049, China National Engineering Laboratory for Site Remediation Technologies, Beijing 100015, China d Technical University of Liberec, Studentska 2, CZ46117 Liberec, Czech Republic b c
H I G H L I G H T S
G R A P H I C A L
• Biochar-nZVI was effectively applied for the first time in a field demonstration. • nZVI significantly reduced concentrations of chlorinated solvents within 24 h. • Biochar-nZVI overcomes the problem of rebound of chlorinated solvents in groundwater. • Biochar-nZVI greatly enhanced removal of chlorinated solvents within 42 days. • Reduction and adsorption are the primary processes to remove chlorinated solvents.
Enhanced removal of chlorinated hydrocarbon by biochar-supported nZVI.
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 24 July 2019 Received in revised form 30 August 2019 Accepted 30 August 2019 Available online 31 August 2019 Editor: Baoliang Chen Keywords: Biochars Nanoscale zero-valent iron Chlorinated solvents In situ groundwater remediation Field demonstration
A B S T R A C T
The application of biochar-supported nanoscale zero-valent iron (biochar-nZVI) was successfully implemented in a field demonstration for the first time. To overcome the significant shortcomings of nZVI agglomeration for in-situ groundwater remediation, biochar-nZVI was injected into groundwater using direct-push and water pressure driven packer techniques for a site impacted by chlorinated solvents in the North China Plain. The field demonstration comprising two-step injections was implemented to demonstrate the effectiveness of nZVI and biochar-nZVI respectively. The outcome of the demonstration revealed a sharp reduction of contaminant concentrations of chlorinated solvents in 24 h following the first injection of nZVI, but the rebound of the concentrations of these contaminants in groundwater has occurred within the next two weeks. However, application of biocharnZVI greatly enhanced the removal of chlorinated solvents in groundwater over the longer period of 42 days. The enhanced removal of chlorinated solvents in groundwater by biochar-nZVI is mainly attributed to the synergistic effects of adsorption and reduction. The adsorption by biochar significantly reduced the level of chlorinated solvents in groundwater. Overall increases in ferrous iron and chloride concentrations after the injections indicated
⁎ Corresponding author at: Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, Jiangsu Province, China. E-mail address: [email protected] (M. Chen).
https://doi.org/10.1016/j.scitotenv.2019.134215 0048-9697/© 2019 Elsevier B.V. All rights reserved.
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that the reduction has occurred during the removal of chlorinated solvents in groundwater. In summary, biocharsupported nZVI could be potentially used for the effective remediation of chlorinated solvents in groundwater. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Chlorinated solvents are organic hydrocarbons that are commonly used in industrial production, and are subsequently found in discontinuous aquifers as pools of residue (Li et al., 2018). Generally, a site that has been contaminated with chlorinated solvents is difficult to remediate, as the extent and characteristics of such contamination are difficult to delineate (Cheng et al., 2016; Ji et al., 2008). Nowadays, nanotechnology has received increasing attention for the remediation of groundwater contaminated with chlorinated solvents (Quinn et al., 2005; Wu et al., 2019). Nanoscale zero valent iron (nZVI) is capable of remediating chlorinated solvents in groundwater via injection into contaminated zones (Comba et al., 2011; Elliott and Zhang, 2001; Lei et al., 2018) and has been used for over 80 sites worldwide since 2000 (Nanorem, 2017). However, many field tests have showed that nZVI is rapidly deposited on the well screen after traveling only a few inches to a few feet before becoming immobilized (Kocur et al., 2014; Krol et al., 2013). This limited mobility of nZVI is caused by its tendency to quickly aggregate and settle primarily because of magnetic attractive forces (Phenrat et al., 2007). The reactivity of nZVI was also found to decrease due to this aggregation (Li et al., 2006). To overcome this bottleneck, nZVI can be coated with various carbon materials to enhance its reactivity and mobility (Vogel et al., 2018; Weil et al., 2019). Several reports have shown that carbon can improve subsurface colloid transport and thus maintain iron reactivity (Chen et al., 2011; Mackenzie et al., 2012; Mackenzie et al., 2016). Natural organic matter and carboxylmethyl cellulose were also found to enhance the mobility of nZVI (Johnson et al., 2009; Johnson et al., 2013). Compared to normal carbon material, biochar includes a large specific surface area and high porosity, which is excellent for nZVI loading (Dong et al., 2017; Qian et al., 2019b; Zhou et al., 2019). The negative surface of carbon materials may also be beneficial for the migration of nZVI (Mackenzie et al., 2012; Qian and Chen, 2013). In laboratory studies, the strengthening effect of carbon-loaded nZVI on the removal of organic contaminants has been confirmed (Yan et al., 2015; Zhang et al., 2018; Zhang et al., 2019), but its role in actual remediation remains to be elucidated. Currently, no published studies have been identified on the removal of chlorinated hydrocarbons in groundwater using biochar-nZVI on a field-scale. Based on the detailed site investigation and risk assessment modelling, a contaminated site located in the North China Plain and impacted by chlorinated solvents was chosen as a pilot test. The objectives of the study were to evaluate the effectiveness of biochar-nZVI in the treatment of chlorinated solvents in groundwater. A potential mechanism for the application of biochar-nZVI was subsequently proposed.
2. Materials and methods 2.1. Site information The field demonstration site, situated on the North China Plain, was contaminated with chlorinated solvents, primarily trichloroethylene (TCE) and trichloromethane (TCM) which were used to clean degreasing tanks and other parts during the maintenance of military vehicles in the past (Fig. 1a, b). These contaminants were released into the groundwater due to inadequate and ignorance of environmental management procedures. Detailed site investigation, groundwater monitoring and risk assessment modelling to establish groundwater remedial
targets were undertaken prior to the design and injection of nZVI and biochar-nZVI in the study area. The geology and hydrogeological conditions and plume distribution of the contaminants in the study area are shown in Fig. 1. The geological sequence of the site is illustrated in Fig. 1c. The top layer is made ground comprising gravel fill or brown reworked silty sand which extends to a depth of approximately 2.5 m bgl (below ground level), followed by silty clay of 0.5 m thick, potentially acting as an aquitard. The third layer consists of inter-bedded sand or sand and silt of approximately 2.5 m thick representing a shallow aquifer. A glacial silt/clay till is beneath the third layer having significantly lower permeability and acting as another aquitard. Detailed site investigation revealed that the groundwater flows from the southwest to the northeast and is in hydraulic continuity with the nearby river (Fig. 1d). The hydraulic conductivity of the aquifer at this site was determined to be 3 m/d, suggesting a reasonably permeable formation. Water table is located approximately 3 m bgl with a saturated thickness of approximately 2.5 m. Groundwater is contaminated by chlorinated solvents including TCE, 1.1Dichloroethene, 1,1-Dichloroethane, 1,2-Dichloroethane, 1,1,2Trichloroethane, Tetrachloroethene, 1,1,2,2-Tetrachloroethane and Chloroform at levels ranging from just above the detection limit (0.5 μg/L) to 21,200 μg/L (Fig. 1e). The distribution of contaminants is illustrated in Fig. 1e and the red, blue, and green areas were delineated based on the risk rankings representing heavy, moderate, and light contaminations, respectively. Based on the direction of groundwater flow and the distribution of contaminants, the test area of approximately 15 m × 15 m was chosen at the upstream of the known heavily contaminated area. Sixteen injection and ten monitoring wells were designed and constructed in the test area (Fig. 1f). Green circles in Fig. 1f represent direct-push injection wells by geoprobe, red circles were designed for the packer injection installed as large diameter wells, and yellow circles represents multilevel monitoring wells (known as micropump wells hereafter). There are totally eleven direct-push injection wells (referred to as DPW hereafter); 5 packer injection wells (referred to as PKW hereafter), 5 micro pump monitoring wells (referred to as PMW hereafter) in the test area and a control well (CK) well outside the test area. PMW and PKW wells have also been used as monitoring points prior to and post the injection. 2.2. The assemblage of nZVI and biochar-supported nZVI The nZVI used in this study is commercially available as NANOFER STAR, which was purchased from Nanoiron (Czech Republic). Biochar was obtained from the Yuanli Huaiyushan activated carbon company. The primary biomass used was wood, and the pyrolysis temperature was 500–600 °C. The specific surface area of biochar was 1793 m2/g, this is due to the activation of phosphate in the preparation of biochar. The nZVI and biochar were dissolved and mechanical blending before injection. Two injections with nZVI and biochar-supported nZVI had been designed in the test area. In case of the test injection with nZVI, the suspension was prepared by dispersing dry iron particles in tap water using a high-speed homogenizer, to form a 200 g/L suspension. The suspension was subsequently injected after the dilution, with a final concentration of 10 g/L of nZVI. The injections were performed at depths of 3.5, 4.5 and 5.5 m bgl at the packer wells and direct-push injection well under a hydraulically adapted pressure with an injection velocity of about 1 m3/h in order to prevent shear failure and tunnel erosion
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Fig. 1. Site location (a) and layout (b), geological structure of the site (c), direction of groundwater flow (d), location of monitoring wells and contaminant plume distribution (e) and injection design (f).
Fig. 2. Setup of the injection procedure.
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within the sediment. In case of injection with biochar-nZVI, it commenced 14 days after the first injection with nZVI, and was performed on the packer injection wells at pre-determined intervals with a suspension concentration of 30 g/L for biochar-nZVI. The concentrations of nZVI and biochar-nZVI were set based on the nZVI concentration. The preparation of nZVI and biochar-nZVI suspension is modified from previous reports (Kim et al., 2018; Soukupova et al., 2015). 2.3. Injection and monitoring of contaminants Firstly, 200 kg of nZVI was injected at a concentration of approximately 10 g/L with a total volume of about 100 m3 of suspension. The injection was undertaken using Geoprobe at eleven directpush wells with an applied fluid pressure of between 1 and 12 bars. The interval of the injection was designed at 3.5 m, 4.5 m and 5.5 m bgl, respectively. The nZVI suspension was continuously stirred during the injection. Second injection was followed fourteen days after the first injection at five packer wells with the biochar-nZVI mixture.
A total of 100 kg of nZVI and 200 kg of biochar were injected with approximately 50 m3 of suspension. The interval of the second injection was the same as the first injection with an applied fluid pressure of between 1 and 5 bars. The setup of the injection procedure is shown in Fig. 2, where the water on the left was used for nZVI reaction and material dilution. The nZVI and biochar were then mixed in a dosing unit and the mixture was injected by either direct push techniques or packer under additional hydraulic pressure. Groundwater was monitored at the micropump and the packer wells and sampled for analyzing chlorinated solvents and some inorganic constituents such as chloride and ferrous iron. The chlorinated hydrocarbons were extracted by liquid-liquid extraction using hexane and quantified using a gas chromatograph (Agilent 7890, USA) equipped with an electron capture detector. The dissolved ferrous ions were measured using the 1,10-phenathroline method (Hach method 8146). The chloride concentration was then analyzed with capillary ion electrophoresis (CE, Waters Quanta 4000E). Groundwater samples were collected prior to the nZVI injection to establish baseline conditions with the
Fig. 3. Trichloroethene concentrations in micropump wells (a–e), packer injection and the background well (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
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frequency of monitoring at 1, 7, 14, 21, 28, and 56 days after the first injection. 3. Results and discussion 3.1. Removal of chlorinated solvents during the field test The concentrations of TCE in the micropump and the packer wells prior to and post the injection are presented in Fig. 3. Depth specific concentrations of TCE in the micropump wells are illustrated in Fig. 3a and increasing concentrations of 2160, 2640 and 3510 μg/L were recorded at depths of 3.5, 4.5 and 5.5 m bgl respectively in MPW1 prior to injection. This may has been caused by the higher density of TCE than groundwater. The TCE concentrations were rapidly decreased to 523, 725 and 982 μg/L, respectively in MPW1–3.5, MPW1–4.5 and MPW1–5.5 following the first injection with nZVI. However, the level of TCE in groundwater had recovered to 1940 μg/L in MPW1–5.5 after 7 days following the first injection. Then the TCE concentrations had been further increased to 2490, 2980 and 3100 μg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively after 14 days following the first injection. The rebound of the TCE concentrations in MPW1 was consistent with previous studies (Elliott and Zhang, 2001). The result exhibited a wavelike pattern that is caused by the advection, dispersion, retardation, and reaction of the nanoparticle plume as it migrated within the test area (Elliott and Zhang, 2001). In order to solve this problem, biochar was chosen as a promising material. Our previous report indicated that biochar can enhance the removal of chlorinated hydrocarbon (Yan et al., 2015). The biochar-nZVI was therefore used for remediation in the pilot site. In order to enhance the removal of the contaminants, the second injection with biochar-nZVI was carried out. The TCE concentration at 7 d after the second injection has been decreased sharply to 386 and 15.4 μg/L in MPW1–4.5 and MPW1–3.5, respectively. By 14 d after the second injection (28 d after the first injection), the levels of TCE concentration in all three micropump wells were lower than 40 μg/L, and the concentration in all wells were lower than remediation target within 56 d. The TCE concentrations in MPW2, MPW3, MPW4 and MPW5 before the injection were lower than that of MPW1, except for MPW1–4.5 m. After the injection, the TCE concentrations in MPW2, MPW3, MPW4 and MPW5 were following the same trend as that of MPW1. The results of the monitoring from the micropump wells therefore indicated that the injection of nZVI is effective for the removal of TCE, especially in the form of biochar-nZVI. TCE concentrations in micropump wells being rapidly decreased implied that the nZVI was transported more than 1.5 m to remove the TCE. The TCE concentrations were higher in micropump wells than those in the packer wells (PKW2) before the injection, at 21200 μg/L. After the first injection with nZVI, the TCE concentration was decreased (PKW1PKW5). The concentration of TCE was subsequently decreased to lower than remediation target after biochar-nZVI. All the packer well also appears to have suffered from less rebound, which can mainly be attributed to the large amount of nZVI in the packer well in the short term. By comparison, the TCE concentration in the well (CK) without material injection demonstrated no significant change within 56 d. Tetrachloroethene (PCE) concentration before and after the injection is presented in Fig. 4. Before the injection, the PCE concentrations were 98.4, 263 and 238 μg/L, respectively in MPW1–3.5, MPW1–4.5 and MPW1–5.5. Similar to TCE, this trend of concentrations increasing with depths is confirmed in the remaining micropump wells. After the first injection with nZVI, PCE concentrations were rapidly decreased to 25.2, 78.9 and 96.1 μg/L, respectively in MPW1–3.5, MPW1–4.5 and MPW1–5.5. However, PCE concentration was increased by 7 d after the first injection in MPW1–3.5 and MPW1–4.5. By 14 d after the first injection, PCE concentrations in these two wells were further increased to 97.0 and 110 μg/L, respectively. Previous reports have indicated that the injection of nZVI is prone to rebound, leading to an increase in the
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PCE concentration after ten weeks (Mackenzie et al., 2016). PCE concentrations at 7 d after the second injection was observed to have been sharply decreased to 0.8, 4.7, and 6.4 μg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively. By 14 d after the second injection, PCE concentrations at three depths in the micropump well (MPW1) were lower than 2 μg/L. The highest PCE concentrations before the injection in MPW2, MPW3, MPW4, and MPW5 was 168, 212, 45.4 and 493 μg/L, respectively. After the first injection, the rebound of the PCE concentration in MPW2–4.5 had occurred and then sharply decreased following the second injection. The PCE concentrations in MPW3, MPW4 and MPW5 were decreased after the first injection. These results indicate that nZVI is more effective in the removal of PCE than that of TCE. This can mainly be attributed to a lower initial concentration of PCE than that of TCE. It is therefore indicated that the removal of TCE with biocharnZVI is more effective than that of PCE. Contaminant concentrations that are reduced via adsorption by biochar during the first step are thought to be one of the key factors enhancing removal. Previous reports had indicated that there is an optimum concentration of nZVI for the removal of pollutants (Qian et al., 2019a). This can mainly be attributed to the fact that high concentrations of pollutant hinder removal with nZVI. PCE concentrations were higher in the micropump wells than those in the packer wells before the injection. After the first injection with nZVI, the PCE concentrations were decreased, and continued to decrease to be lower than the limit of detection after the second injection. Due to the large amount of nZVI in the packer wells, the rebound did not happen immediately. As expected, the PCE concentration in the well (CK) without material injection showed no significant change. The changes in trichloromethane (TCM) concentration are presented in Fig. 5. TCM concentrations in the micropump wells are presented in Fig. 5a–e. The TCM concentrations in MPW1–3.5, MPW1–4.5 and MPW1–5.5 were 445 μg/L, 370 μg/L and 69.6 μg/L before the injection. However, the concentrations of TCM and other chlorinated solvents were lower than that of TCE in the corresponding wells, indicating that TCE is the main contaminant in the test area. After the first injection with nZVI, the TCM concentration was rapidly decreased. The TCM concentrations in MPW1–3.5, MPW1–4.5 and MPW1–5.5 was 29.2, 128 and 62.6 μg/L, respectively. The TCM concentrations were slightly increased at both 7 d and 14 d after the first injection in MPW1–5.5 m. The TCM concentration in MPW1 was then decreased by 7 d after the second injection with biochar-nZVI. By 28 d after the second injection the TCM concentrations at three depths in the micropump wells were lower than the limit of detection. The TCM concentrations in MPW2, MPW3, MPW4, and MPW5 before the injection were lower than that of MPW1. After the injection, the TCM concentrations in MPW2, MPW3, MPW4, and MPW5 followed the same trend as that of MPW1. The results from the micropump well indicate that the nZVI injection was effective for removing TCM. However, the TCM concentrations in MPW2 and MPW3 were sharply increased by 14 d after the first injection, confirming the concentrationdependent rebound. This rebound is apparent when the concentrations of contaminants exceed the removal capacity of nZVI, although the concentrations of contaminants tend to decrease again over a short time. After the injection of biochar-nZVI, the TCM concentrations were decreased. At the same period after the second injection, the TCM concentration had decreased to lower than remediation target. Compared to the micropump well, the TCM concentration was higher in the packer well (PKW1 and PKW2) before the injection, at 1860 μg/L. After the first injection with nZVI, the TCM concentration decreased, and had decreased to lower than the limits for detection within 28 d. In order to better illustrate the contamination of the pilot site, the main chlorinated hydrocarbons found in the area other than TCE, PCE and TCM are presented in Fig. 6. The concentrations of chlorinated hydrocarbons in the micropump wells are presented in Fig. 6a. Before the injection, the concentrations of chlorinated hydrocarbons were 1492, 1293 and 832 μg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5,
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Fig. 4. Tetrachloroethene concentrations in micropump wells (a–e), packer injection and the background well (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
respectively. After the first injection with nZVI, the concentrations of chlorinated hydrocarbon were rapidly decreased within 1 d to 251, 535 and 581 μg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5 respectively. However, the chlorinated hydrocarbon concentrations were increased by 7 d after the first injection. By contrast, the chlorinated hydrocarbon concentrations were sharply decreased to 225, 169 and 216 μg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively by 7 d after the second injection with biochar-nZVI. Then the concentrations of chlorinated hydrocarbon at three depths in the micropump wells were continue to decrease. By 32 d after the second injection with biochar-nZVI the concentrations of chlorinated hydrocarbons remained stable at approximately 11.7, 146 and 220 μg/L, respectively. The chlorinated hydrocarbon concentrations in MPW2, MPW3, MPW4 and MPW5 before injection were lower than that of MPW1. After the injection, the chlorinated hydrocarbon concentration in MPW2, MPW3, MPW4 and MPW5 followed the same trend as in MPW1. The results from the micropump well indicated that injection with nZVI was effective for
the removal of chlorinated hydrocarbon, especially the biochar-nZVI. Compared to the micropump well, the chlorinated hydrocarbon concentration was higher in the packer well before injection, at 109156 μg/L. After the first injection with nZVI, the concentration of chlorinated hydrocarbon decreased. The chlorinated hydrocarbon concentration then decreased to the limits for detection after the second injection. The rebound after nZVI injection was therefore acting as a bottleneck for the use of nZVI in general application for the removal of contaminants. 3.2. Ferrous iron and chloride transformation The ferrous iron concentrations in the micropump and the packer wells are presented in Fig. 7. Fig. 7a shows the concentrations of ferrous iron to be 0.05, 0.05 and 0.11 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively, before the injection. After the first injection with nZVI, the concentrations of ferrous iron were increased to 0.31,
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Fig. 5. Trichloromethane concentrations in micropump wells (a–e), packer injection and the background well (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
0.12 and 0.14 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5 respectively by 1 d after the first injection. By 14 d after the first injection, the ferrous iron concentrations were increased to 0.53, 0.45 and 0.53 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively. As the concentration of chlorinated hydrocarbons were high in the corresponding period. This indicates that nZVI was inadequate for the removal of ferrous iron. By 7 d after the injection with biochar-nZVI, the ferrous iron concentrations were decreased to 0.12 and 0.22 mg/L in MPW1–3.5 and MPW1–4.5 respectively. The concentration of ferrous iron in MPW1–5.5 continued to increase to 1.18 mg/L. By 56 d after the second injection the ferrous iron concentrations were decreased to 0.05, 0.12 and 0.18 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively. The ferrous iron concentrations in MPW2, MPW3 and MPW4 were higher than that in MPW1 at the corresponding period. The ferrous iron concentrations were significantly increased after the injection, indicating that the ferrous iron can be transported some distance. This result also suggested that the ferrous iron was not fully removed. The concentrations of ferrous iron were significantly decreased after the
biochar-nZVI injection. There are two means by which the ferrous concentrations could be decreased: participating in the reduction of chlorinated hydrocarbon and the adsorption by biochar. Previous study demonstrated that activated carbon could act as an adsorbent for ferrous iron, and then take part in the reduction process (Huang et al., 2019). The concentrations of ferrous iron were higher in the packer wells (PKW2) than those in the micropump wells, and were found to be at 9.27 mg/L after injection. This result indicated that much of ferrous iron was not fully used during the removal. After the injection with biochar-nZVI, the concentrations of ferrous iron were decreased. Then the ferrous concentrations in the packer well had decreased to the same concentrations as before injection within 56 d. By comparison, the ferrous concentration in the well without material injection remained at a low level from the beginning to the end of the test period. The chloride concentrations in the micropump and the packer wells are presented in Fig. 8. Fig. 8a shows the chloride concentrations of 1130, 490 and 490 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5 respectively before the injection. The chloride concentrations were
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Fig. 6. Chlorinated solvents in micropump wells (a–e), packer injection and the background well (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
increased with depths in the other micropump wells (MPW2-MPW5). After the first injection with nZVI, the chloride concentrations were rapidly decreased. This result indicates that chloride can be reduced by iron oxide. By 1 d after the injection, the chloride concentrations in MPW1–3.5, MPW1–4.5 and MPW1–5.5 was 348, 348 and 564 mg/L, respectively. However, the chloride concentrations were increased by 7 d after the first injection. The highest chloride concentration was reached at 897 mg/L in MPW1–3.5. By 14 d after the first injection, the chloride concentrations were further increased to 471, 518 and 948 mg/L in MPW1–3.5, MPW1–4.5 and MPW1–5.5, respectively, indicating that a reductive dechlorination process occurred during the removal of the contaminants. By 7 d after the injection with biochar-nZVI, the chloride concentrations were decreased in MPW1–5.5, and then increased again by 14 d and 42 d. The highest chloride concentration was reached at 1080 mg/L in MPW1–5.5. This is mainly due to the initial chloride being adsorbed by iron oxides in the first step, and then the released chloride by dichlorination increased the chloride concentration in the second step. The result indicates that biochar-nZVI enhanced the
dechlorination of the contaminants. The chloride concentrations in MPW2, MPW3, MPW4 and MPW5 followed the same trend as that of MPW1. The results from the micropump well indicate that the nZVI injection was effective for the dechlorination of the contaminants, especially in the form biochar-nZVI. The chloride concentrations were increased significantly after the injection with nZVI in the packer wells. This increase in chloride likely resulted from the dechlorination of chlorinated organics and provided strong evidence that dechlorination did occur (Su et al., 2012). The results above have highlighted the dechlorination efficiency of biochar-nZVI, which is of fundamental importance if the full potential of using biochar-nZVI for the groundwater remediation is to be achieved. Biochar, with a large specific surface area and a rich pore structure, was used as support for nZVI, which solved the bottleneck of reduced degradation efficiency of nZVI due to the agglomeration effect. The results from the test strongly indicated that biocharsupported nZVI significantly enhanced the removal of chlorinated hydrocarbons. The enhanced dechlorination of chlorinated hydrocarbon
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Fig. 7. Concentrations of ferrous iron in micropump wells (a–e), packer injection well and the background (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
by nZVI is mainly due to the following potential mechanisms. Firstly, during the dechlorination of chlorinated hydrocarbon by nZVI, intermediate products are more likely to be transferred to the surface of biochar because of the larger surface area. Secondly, biochar is served as a good adsorbent, and the concentrations of pollutants were decreased via adsorption by biochar. Thirdly, biochar is acted as an electron-transfer mediator and an adsorption-support material for the coprecipitation of reduction products with iron (Qian et al., 2017; Yan et al., 2015). Therefore, biochar-supported nZVI is an effective functional material that can be used as a readily accessible alternative for the remediation of groundwater contaminated with chlorinated hydrocarbons. 4. Conclusions In this study, the field demonstration of in-situ groundwater remediation using nZVI and biochar-nZVI mixture was undertaken at a site
contaminated by chlorinated solvents in the north-east China. Comprehensive groundwater monitoring prior to and post the injection was undertaken to evaluate the removal efficiency of the field test. The results suggested that monometallic nZVI was able to degrade the target contaminants over a short time, however the biochar-nZVI mixture promoted enhanced removal of chlorinated hydrocarbons over the longer period. The contaminant concentrations in the pilot area were decreased by more than 90%. The field test also demonstrated that the injected fluid could be delivered to adjacent micropump wells. This study also suggests that the injection of biochar-nZVI suspension would not result in the pore clogging that tends to limit subsequent remediation efforts. Further work is needed to assess the extent of the reaction with target contaminants, water, or natural reductants with the ultimate goal of designing nanometal formulation that optimizes the reaction with the target contaminants while maintaining good mobility. Additional work is needed to develop a greater understanding of the
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Fig. 8. Chloride concentrations in micropump wells (a–e), packer injection well and the background (f). Note: nZVI and biochar-nZVI were used in the first and second injection respectively.
transport of nZVI and the processes governing the degradation of contaminants at a range of field sites.
We are grateful to Prof. Miroslav Černík of Technical University of Libere and Dr. Jan Slunský of NANO IRON, s.r.o., for assisting in the design and implementation of the pilot test.
Declaration of competing interest References The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This article is financially supported by the National Key Research and Development Program of China (Grant No. 2017YFA0207002 and 2018YFC1803002); the Frontier Fields of the Thirteenth FiveYear Plan Period of the Institute of Soil Science, Chinese Academy of Sciences (Grant No. ISSASIP1656); National Engineering Laboratory for Site Remediation Technologies (Grant No. NELSRT201710).
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