Enhanced removal of tetrachloroethylene from aqueous solutions by biodegradation coupled with nZVI modified by layered double hydroxide

Chemosphere 243 (2020) 125260

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Enhanced removal of tetrachloroethylene from aqueous solutions by biodegradation coupled with nZVI modified by layered double hydroxide Qing Wang a, Xin Song a, *, Shiyue Tang a, b, Lei Yu c a b c

Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 21008, China University of Chinese Academy of Sciences, Beijing, 100049, China Department of Environmental Engineering, Nanjing Forestry University, Nanjing, 210037, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Coating nZVI with LDH dispersed nZVI and improved the removal efficiency of PCE.
The presence of Cu2þ improved the removal efficiency of PCE by nZVILDH.
Removal of PCE was enhanced by coupling nZVI-LDH and a PCEdegrading consortium.
PCE removal was dominated by nZVILDH, then followed by biodegradation.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 August 2019 Received in revised form 12 October 2019 Accepted 29 October 2019 Available online 6 November 2019

Chlorinated volatile organic compounds, such as tetrachloroethylene (PCE), are the most commonly detected toxic contaminants in groundwater. In this study, the performance of PCE removal by a microbial consortium combined with nZVI modified by layered double hydroxide (nZVI-LDH) was evaluated. The enriched PCE-degrading consortium consisted of 44.49% Clostridium and other potential PCE degraders, and 0.5e2.5 mg/L PCE was completely biodegraded within 4 days. The characterization of nZVI-LDH indicated that LDH was coated on the surfaces of nZVI particles with an increased surface area. The PCE removal kinetics by nZVI-LDH was well described by a second-order model, and the removal rate constant of nZVI-LDH was 0.12 L h/mg, higher than that of native nZVI (0.02 L h/mg). Interestingly, the presence of Cu2þ improved the removal efficiency of PCE by nZVI-LDH, owing to its role as a catalyst or medium for charge transfer during reduction. Removal of PCE was enhanced by coupling the PCEdegrading consortium and nZVI-LDH. The initial removal of PCE was mainly dominated by the abiotic degradation and adsorption of nZVI-LDH, and biodegradation then played a major role in the exhaustion of nZVI-LDH. These results suggest that biodegradation coupled with nZVI-LDH has a great potential for applications in the remediation of chlorinated-solvent contaminated groundwater. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Yongmei Li Keywords: Tetrachloroethylene Zero-valent iron nanoparticles Layered double hydroxides PCE-Degrading consortium

1. Introduction

* Corresponding author. E-mail address: [email protected] (X. Song). https://doi.org/10.1016/j.chemosphere.2019.125260 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

Chlorinated volatile organic compounds (cVOCs), such as tetrachloroethylene (PCE), are the most common organic pollutants in

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polluted groundwater because of their wide use in industrial production (Kumar et al., 2017; Lin et al., 2018). Chlorinated organic compounds are carcinogenic and cytotoxic, and can cause severe damage to the central nervous system, endocrine system, and immune system in humans, thus posing a severe threat (Danish et al., 2017). Chlorinated organic compounds often have the characteristics of higher density than water and low solubility in water, and generally occur in the form of dense non-aqueous phase liquid (DNAPL) (Dong et al., 2017; Lin et al., 2018). These compounds are thus a potential persistent source of pollution after they enter the underground environment (Dong et al., 2017). Therefore, it is necessary to find economic and effective ways to remediate cVOCs contaminated groundwater. Among current methods for the removal of cVOCs, bioremediation has attracted substantial attention because of its low cost and environmental friendliness (Zemb et al., 2010; Wen et al., 2017). Many bioremediation techniques including reductive dechlorination have been studied using anaerobic bacteria such as Dehalobacter, Sulfurospirillum multivorans, and Desulfuromonas (Agrawal et al., 2002; Wen et al., 2017). Dehalococcoides are the only known organisms that can completely degrade PCE into ethylene under anaerobic conditions (Vogel et al., 2018). However, bioremediation still has many technical challenges, including slow dechlorination rates, long treatment time, insufficient electron donors, and the toxic effects of high concentrations of chlorinated hydrocarbons on dechlorination bacteria (Xiu et al., 2010). Biodegradation, coupled with other remedial reagents, has become more popular in recent years (Sheu et al., 2016; Shanbhogue et al., 2017; Dong et al., 2019). Nanoscale zero-valent iron (nZVI) has been shown to rapidly and effectively dechlorinate cVOCs because of its high specific surface area and high reactivity; hence, it is recognized as a good candidate for in situ groundwater remediation (Kaifas et al., 2014; Zhao et al., 2016). Because of the unique benefits of bioremediation and nZVI applications, previous studies have combined bioremediation with nZVI to exploit the advantages of both methods (Xiu et al., 2010; Dong et al., 2019). However, nZVI particles are severely toxic and limit cell growth (Chaithawiwat et al., 2016). In addition, the agglomeration of nZVI particles poses another challenge to the application of nZVI in the field (Sohn et al., 2006; Zhou et al., 2016). Furthermore, nZVI particles can be oxidized spontaneously when they are exposed to molecular oxygen, significantly decreasing their reactivity (Chen et al., 2014). Coating nZVI with surface modifiers, such as carboxymethyl cellulose (CMC) (Dong et al., 2016), sulfidation reagent (Han and Yan, 2016) and polyethylenimine (Lin et al., 2018), has been found to effectively minimize aggregation and enhance reactivity, and to diminish the toxicity of nZVI (Zhou et al., 2014). In recent years, layered double hydroxides (LDHs), commonly known as hydrotalcite-like anionic clays, have attracted substantial attention because of their layered structure, large surface area, high porosity, high anionic exchange capability, and thermal stability (Gong et al., 2011; Shan et al., 2015). LDHs have been widely used in adsorption and catalyst applications (Parida and Mohapatra, 2012; Hu et al., 2017b). In addition, many hydroxyl groups are found on the surface of LDHs and may provide an ideal carrier for modifications (Hu et al., 2017a). Therefore, a variety of multifunctional materials based on LDHs have been prepared and used for the treatment of pollutants. LDHs can be used as a stabilizer to support Fe3O4 (Lu et al., 2017) and graphene oxide (Zhang et al., 2015) in the preparation of magnetic composite to remove contaminants. However, to the best of our knowledge, no research has been performed using LDHs modified nZVI (nZVI-LDH) coupled with biodegradation for the removal of cVOCs in groundwater. In this study, nZVI-LDH was prepared and then coupled with biodegradation by an acclimated microbial consortium, to address

the challenges of cVOC remediation. The effects of various environmental factors, such as pH, initial concentration of PCE, and coexisting cation and anions commonly found in groundwater, on the removal of PCE by nZVI-LDH were evaluated. The removal efficiency and kinetics of PCE biodegradation coupled with nZVI-LDH were investigated, and the removal mechanisms were explored. 2. Materials and methods 2.1. Chemicals PCE (>99.5%), enthanol (99.0%), Mg(NO3)2,6H2O (99.0%), Al(NO3)3,9H2O (99.0%), Na2SO4 (99.0%), NaCl (99.5%), NaNO3 (99.0%), Na2CO3 (99.8%), NH3,H2O (25e28%), NaBH4 (99.0%), K2HPO4,3H2O (98%), FeSO4,7H2O (99.0%) and vitamin B12 (C63H88CoN14O14P, 99.0%) were purchased from Sinopharm Chemical Reagent Corporation Ltd. (Beijing, China). NaH2PO4,H2O (99.0%), NH4Cl (99.5%), CaCl2 (96.0%), MgCl2 (99.0%), CuSO4,5H2O (99.0%), MgSO4,7H2O (99.0%), ZnSO4,7H2O (99.5%) and MnSO4,H2O (99.0%) were purchased from Xilong Scientific Co., Ltd. (Shanghai, China). N(CH2CO2Na)3,H2O (98%) was obtained from TCI Development Co., Ltd. (Shanghai, China). CoCl2,6H2O (99.0%) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). C3H5NaO3 (60%) was obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China). Methanol was obtained from Merck (Darmstadt, Germany). Deionized water was used to prepare all the solutions. All of the chemicals used were analytical reagent grade or better unless otherwise specified. 2.2. Cultivation of microorganisms The PCE-degrading consortium was acclimated with anaerobic activated sludge through gradient culture in 1000 mL serum bottles with sterile medium. Various concentrations of PCE (1e10 mg/L) were added into the serum bottles weekly. The medium contained (g/L) (Hatzinger et al., 2015): K2HPO4,3H2O, 1.05 g/L; NaH2PO4,H2O, 0.25 g/L; NH4Cl, 0.49 g/L; N(CH2CO2Na)3,H2O, 0.03 g/L; MgSO4,7H2O, 0.05 g/L; FeSO4,7H2O, 0.003 g/L; MnSO4,H2O, 0.74 mg/L; ZnSO4,7H2O, 0.74 mg/L; CoCl2,6H2O, 0.25 mg/L; sodium lactate, 0.112 g/L; Vitamin B12, 0.05 mg/L. The composition of the medium was similar to that of the actual groundwater (Murphy et al., 1997). The medium was purged with N2 to remove oxygen, then autoclaved at 121  C for 20 min before use. The initial pH value was approximately 7.0, and the incubation temperature was 30  C. The PCE concentration was tested at regular time intervals. The microbial composition was identified through 16S rRNA sequence analysis after 3 months of culturing and acclimating. 2.3. Preparation of nZVI, LDH and nZVI-LDH The nZVI was synthesized by using NaBH4 reduction method described in previous studies (Rajajayavel and Ghoshal, 2015). Briefly, an aqueous solution of 0.11 M NaBH4 was added dropwise to a 0.055 M FeSO4,7H2O solution under vigorous stirring, until a 2þ BH molar ratio of 2.0 was reached. The mixed solution was 4 /Fe continuously stirred at room temperature for another ~30 min. The formed nZVI particles were settled and then filtered, after which the solid was washed several times and finally vacuum freezedried. The nZVI-LDH composite was prepared using a co-precipitation method (Lu et al., 2017) in which 1.0 g nZVI was dispersed in 50 mL of solvent (Vmethanol/Vwater ¼ 1/1) by ultrasonication for 20 min to obtain a uniform suspension. Then 50 mL alkaline solution A (6 M ammonia solution, Vmethanol/Vwater ¼ 1/1) and 50 mL salt solution B (3.85 g Mg(NO3)2,6H2O, 1.88 g Al(NO3)3,9H2O, Vmethanol/Vwater ¼ 1/

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1) were simultaneously added dropwise to the nZVI solution under vigorous stirring, The pH of the mixture was kept at ~11. The solution was stirred for another hour, and then the black powder was collected and washed with ethanol several times and finally vacuum freeze-dried. The preparation method of LDH was similar to that of nZVI-LDH but without the addition of nZVI (Hu et al., 2017b). 2.4. Characterization of nZVI, LDH and nZVI-LDH The morphological analyses of nZVI, LDH, and nZVI-LDH were performed with a transmission electron microscope (TEM; JEM200CX, JEOL, Japan). The X-ray diffraction (XRD) patterns were determined with an X’TRA powder diffractometer (ARL, Ecublens, Switzerland) with CuKa radiation at 40 kV and 40 mA. Samples were scanned from 3 to 100 at a scanning rate of 2 /min. The spectral properties of nZVI and nZVI-LDH were recorded by Fourier transform infrared spectroscopy (FTIR) (Nicolet iS10, Nicolet, USA) with a scanned area of 400e4000 cm1. The specific surface areas of nZVI and nZVI-LDH were analyzed with a surface area analyzer (ASAP2020, Micrometrics, USA) through N2 adsorption-desorption at 77 K. The specific surface area was evaluated using the BrunauerEmmett-Teller (BET) method. The elemental compositions of the nZVI and nZVI-LDH were determined on the basis of X-ray fluorescence (ADVANT’XP, ARL, Switzerland).

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L. PCE biodegradation experiments were conducted without the addition of nZVI-LDH under the same experimental conditions. Control experiments with only PCE in the sterile medium were conducted. All experiments were conducted in triplicate, and the average values with standard deviations are reported. 2.6. 16S rRNA amplification library construction and sequencing The total genomic DNA of the cultivated microbial consortium was extracted with a FastDNA™ SPIN Kit for Soil (MP Bio, USA) according to the manufacturer’s instructions. The universal primers 515F (50 -GTGCCAGCMGCCGCGGTAA-30 ) and 806R (50 -GGACTACHVGGGTWTCTAAT-30 ) were used to amplify 16S rRNA genes. The PCR mixture (25 mL) contained 1  PCR buffer, 1.5 mM MgCl2, 0.4 mM of each deoxynucleoside triphosphate, 1.0 mM of each primer, 0.5 U of Ex Taq, and 10 ng genomic DNA. The PCR conditions consisted of an initial 98  C for 3 min, followed by 15 cycles of 98  C for 45 s, 55  C for 45 s, 72  C for 45 s, and a final extension of 72  C for 7 min. The PCR products were mixed and tested by electrophoresis on a 2% agarose gel, purified with an Axy Prep DNA gel recovery kit (AXYGEN), and eluted with Tris-HCl. The purified samples were then sequenced on the Illumina Mi Seq platform for high-throughput sequencing.

2.5. PCE removal batch experiments

2.7. Analytical methods

PCE removal batch experiments include PCE biodegradation experiments, PCE removal by nanoparticles (nZVI, LDH and nZVILDH), and PCE removal by biodegradation coupled with nZVILDH. PCE biodegradation experiments were conducted in 120 mL serum bottles containing 20 mL inoculation culture and 100 mL sterile medium. The pH of the medium was first adjusted to 7.0, and then the medium was purged with N2 for 30 min before autoclaving. The PCE stock solution was introduced into the 120 mL solution to achieve the concentrations of 0.5e2.5 mg/L as the initial concentration (C0). The same procedures were used for other PCE removal batch experiments. The bottles were incubated in a thermostatic oscillator (MQL-621R) purchased from Minquan Instrument Co., Ltd. (Shanghai, China) at 30  C throughout the experiment. The optimal biodegradation conditions of PCE were performed at 30  C and pH 7 according to our previous study (Tang et al., 2019). Samples were taken from the bottles every 24 h to determine the PCE concentrations. Control experiments with PCE only in the sterile medium were conducted. PCE removal by nZVI, LDH and nZVI-LDH were performed with serum bottles sealed with Teflon-lined rubber septa and aluminum crimp caps. The 50 mL deionized water in the bottles was first purged with N2 for 30 min. Then 100 mg pure nZVI, LDH, or nZVILDH was added, and the stock solution of PCE in methanol was introduced into the system to achieve the C0 of 10 mg/L. The system pH was adjusted to 7.0 by the addition of 10 mM NaOH or HCl as needed. The serum bottles were shaken at 180 rpm at 25  C in the dark throughout the experiment. After a given time interval, serum bottles were removed from the shaker. The effects of the initial pH (5.0, 6.0, 7.0, 8.0 and 9.0) and initial concentrations of PCE (5, 10, 20 and 50 mg/L) on PCE removal by nZVI-LDH were studied. The effects of the co-existing cation (Ca2þ, Mg2þ and Cu2þ) and anioni 2 2 (Cl, NO 3 , SO4 , and CO3 ) commonly found in groundwater on PCE removal were also investigated. Control experiments with only PCE in the deionized water were conducted. To study the coupled removal efficiency of PCE by the acclimated microbial consortium and nZVI-LDH, the sterile nZVI-LDH (1 g/L) were added into 120 mL serum bottles containing 20 mL inoculation culture and 100 mL sterile medium with a C0 of 10 mg/

Gas chromatography (Agilent 7820A, USA) was used for quantitative measurements of components in the aqueous samples in all batch experiments. The equipment included an auto-sampler, a capillary column (DB-624, 60 m  0.25 mm  1.4 mm), and an electron capture detector equipped with gas chromatography. N2 was applied as a carrier gas at a constant flow rate of 30 mL/min 1 mL of sample was injected by the auto-sampler every time, and the injection was performed in split mode (split ratio 20:1). The injector and electron capture detector temperature program were maintained at 200  C and 320  C, respectively. The oven temperature program started at 40  C and was maintained for 5 min, then increased gradually at a rate of 8  C/min up to 100  C and 15  C/min up to 200  C, resulting in the accurate separation of different products. The highest temperature was maintained for 2 min. 3. Results and discussion 3.1. Biodegradation of PCE by the acclimated consortium As shown in Fig. 1a, the composition of the acclimated microbial consortium mainly consisted of Clostridium sp. FCB45 (44.49%), Methylotrophic_bacterium RS X3 (17.34%), Rhodocyclales bacterium TP139 (7.39%), Desulfovibrio (2.61%), and Treponema (0.85%) at the species level. The previously reported dechlorination microorganisms, such as Dehalobacter, Sulfurospirillum multivorans, or Desulfuromonas (Agrawal et al., 2002; Wen et al., 2017; Vogel et al., 2018) were not observed in this study. However, David et al. (2015) reported that Clostridium sp. is a potential PCE-degrading strain. Chaignaud et al. (2017) reported that Methylotrophic bacteria have the dechlorase gene, and can use chlorinated methane as their sole carbon and energy source for growth. Previous research has reported that Rhodocyclales are representative multifunctional bacteria that can grow on a variety of organic compounds (Kittichotirat et al., 2011). Desulfovibrio can participate in the dechlorination and sulfate reduction of several chlorinated pollutants, such as 1, 2dichloroethane (Zemb et al., 2010). Treponema can dechlorinate pentachlorophenol with the addition of acetate and result in the formation of byproducts such as H2, CO2, and formate (Tong et al.,

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Fig. 1. (a) The composition of acclimated microbial consortium at species level, (b) the biodegradation efficiency of PCE by the acclimated microbial consortium. Control experiments with only PCE in the sterile medium were conducted.

2017). These species could play important roles in the biodegradation process of PCE. The biodegradation efficiency of PCE by the acclimated microbial consortium is shown in Fig. 1b. The PCE was completely removed within 4 days at initial PCE concentrations of 0.5e2.5 mg/ L, and the half-life of PCE biodegradation was 0.55e0.75 d. The biodegradation rate of PCE by the acclimated microbial consortium was higher than that of some reported dechlorinating bacteria. Ge et al. (2019) reported that the biodegradation efficiency of TCE and PCE by Acidimicrobiaceae sp. A6 were 32% and 55% within 12 days with the initial concentration of 1 mg/L, respectively. The halfdegradation time of 1 mg/L TCE was 7 days for Burkholderia sp. (Chee, 2011). The biodegradation rate of PCE increased with the increasing initial PCE concentration. This lower PCE biodegradation rate associated with lower initial concentrations was probably due to the decreased activity of the microorganisms as a result of the lower induction of PCE degradation enzymes by PCE (Xie et al., 2017). The reductive dehalogenases, such as TceA, PceA, VcrA and BvcA, and the genes of reductive dehalogenases in anaerobic bacteria such as tceA, bvcA and vcrA played a major role in anaerobic dehalogenation (Krajmalnik-Brown et al., 2004; Chow et al., 2010; Yoshikawa et al., 2017). The reduced dehalogenase gene mbrA is involved in the generation of trans-DCE in the dechlorination pathway (Chow et al., 2010).

3.2. Characterizations of nZVI, LDH, and nZVI-LDH The surface morphologies of nZVI, LDH, and nZVI-LDH were characterized by TEM (Fig. 2). As shown in Fig. 2a, the surface of LDH was smooth and lamellar, in agreement with observations by Gong et al. (2011), who have reported that LDH consists of a dispersed platelet-like structure. nZVI particles were spherical with particle sizes in the range of 10e100 nm (Fig. 2b). Because of the high surface energies and magnetic interactions of nZVI particles,

they were aggregated into a chain-like conformation (Rajajayavel and Ghoshal, 2015; Hu et al., 2017a). The TEM image (Fig. 2c) of the nZVI-LDH showed that nZVI was well dispersed onto the surface of LDH without clear aggregation. We proposed that coating nZVI with LDH could enhance the dispersibility of nZVI, owing to the decreased aggregation and increased mechanical strength (Sheng et al., 2016; Zhou et al., 2016). Fig. 3a shows the XRD patterns of nZVI, LDH, and nZVI-LDH. There were three remarkable diffractions at 2q to (110), (200), and (211) planes (Kim et al., 2010) in nZVI, which were indexed as body-centered cubic phase iron. There was no significant difference in the intensities of the characteristic reflections in nZVI-LDH, as compared with those in pure nZVI and LDH, demonstrating that the synthesized nZVI-LDH was a composite of LDH and nZVI nanoparticles. The FTIR spectra of nZVI and nZVI-LDH are shown in Fig. 3b. A broad band near 3426 cm1 was assigned to the stretching of OeH on the surface of nZVI (Lu et al., 2017). The peaks around 1385 cm1 and 421 cm1 were attributed to NO 3 and Mg/AleO, respectively, indicating the successful coating of LDH (Gong et al., 2011; Zhang et al., 2015). The peaks at 653 cm1 were assigned to FeeO, indicating the oxidation of nZVI particles during the process of LDH loading. Moreover, the peaks around 1385 cm1 and 421 cm1 were stronger in the spectra of nZVI-LDH than nZVI, indicating the presence of LDH on the nZVI surface. The BET method was used to obtain nitrogen adsorptiondesorption isotherms to analyze the morphology of the samples (Fig. 3c and d). According to the IUPAC classification, the adsorption-desorption isotherms of samples showed a type IV curve with a H3-like hysteresis loop, suggesting the existence of mesopores and the occurrence of capillary condensation (Lin et al., 2018). The type IV hysteretic isotherm was considered to reflect capillary condensation, because it leveled off before the saturation pressure. The BET surface area of the nZVI particles was 32.5 m2/g,

Fig. 2. TEM images of (a) LDH, (b) nZVI and (c) nZVI-LDH.

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Fig. 3. (a) XRD patterns of nZVI, LDH and nZVI-LDH, (b) FTIR spectra of nZVI and nZVI-LDH; (c) (d) N2 adsorption/desorption isotherms of nZVI and nZVI-LDH, respectively.

Table 1 BET surface area, pore volume and pore diameter of nZVI and nZVI-LDH. Element

BET surface area (m2/g)

Total pore volume (cm3/g)

Average pore diameter (nm)

nZVI nZVI-LDH

32.5 43.4

0.12 0.18

14.18 16.59

while the nZVI-LDH surface area increased to 43.4 m2/g (Table 1), confirming that coating nZVI with LDH increased the surface area. The average pore diameters of nZVI and nZVI-LDH were 14.18 nm and 16.59 nm. These results suggest that the processing of loading LDH increased the number of pores in all pore width ranges, and the pores were predominantly mesopores, defined by diameters in the range of 2 nme50 nm (Dong et al., 2017). The elements of nZVI samples before and after LDH loading are summarized in Table 2. The Fe content on the nZVI-LDH surface decreased from 96.33% to 68.74%, whereas the Mg and Al content increased from 0.0048% to 0.0061%e4.83% and 5.05%, respectively, suggesting the successful loading of LDH on nZVI. The nitrate used in the preparation of LDH resulted in the appearance of the N element after LDH coating. The decline in the Fe content could be explained by the addition of LDH, resulting in the proportional

Table 2 XRF analyses of nZVI and nZVI-LDH. Elementa

O

Fe

Mg

Al

N

nZVI nZVI-LDH

2.27 18.82

96.33 68.74

0.0048 4.83

0.0061 5.05

Not detected 1.44

a

Ratios of each element are expressed in mass percent (mass%).

decrease. The mass fraction of the O element substantially increased from 2.27% in nZVI to 18.82% in nZVI-LDH, mainly because of the corrosion of Fe as a reductant forming iron oxides/ hydroxides during the preparation of nZVI-LDH. 3.3. PCE removal by nZVI, LDH and nZVI-LDH The removal of PCE by nZVI and nZVI-LDH is shown in Fig. 4a. The PCE removal by nanoparticles was much faster than that by the acclimated microbial consortium (hours vs. days), indicating a two stage PCE removal if both technologies are applied in a coupled manner. To shed light on the removal mechanisms, the PCE removal by LDH was explored. As shown in Fig. 4a, PCE removal by nZVI and nZVI-LDH particles showed similar trends, although the removal efficiency of PCE by nZVI-LDH was higher than that of nZVI particles: 81.5% and 96.3% PCE was removed with 24 h by nZVI and nZVILDH, respectively. The enhancement induced by the LDH coating in nZVI-LDH was mainly due to the functions of LDH as a dispersant and stabilizer of nZVI (Shi et al., 2011). Moreover, LDH removed PCE rapidly, and the removal efficiency was 54.9% at the end of the reaction, suggesting that LDH contributed to PCE removal by the nZVI-LDH composite nanoparticles. The PCE degradation kinetic data by nZVI and nZVI-LDH were

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Fig. 4. (a) The removal of PCE by nZVI, LDH and nZVI-LDH, (b) the kinetics analysis of PCE using second-order kinetic model. Control experiments with only PCE in the deionized water were conducted.

fitted with a second-order kinetic model (Fig. 4b) expressed as Eq. (1)

1=C ¼ k t þ 1=C0

(1)

where C (mg/L) and C0 (mg/L) are the concentrations of PCE at time t (h) and at initial concentrations of PCE, k (L h/mg) is the removal rate constant. The correlation coefficients (R2) of 0.95 and 0.97, respectively, suggested that the second-order kinetic model fit both sets of data well, and PCE degradation follows second-order kinetic behavior. The degradation rate constant of nZVI-LDH was 0.12 L h/ mg, which is much higher than the 0.02 L h/mg for nZVI. These results indicated that the enhanced removal of PCE was mainly attributed to the synergistic effect between LDH adsorption and nZVI reduction. In addition, coating with LDH improved the dispersion of nZVI, which also contributed to the enhance removal of PCE by nZVI-LDH. Removal of pollutants by nZVI is an interfacial process including mass transfer on the solid liquid interface, and then pollutants was reduced at the surface, where electron transfer occurred; the enrichment of PCE on the solid surface is an important initial stage (Hu et al., 2017a). Therefore, we propose the removal process of PCE by nZVI-LDH as follows: at the first step, more PCE can be enriched at the interface, facilitating easier approach of PCE to the reactive sites of nZVI-LDH and resulting in the acceleration of surficial mass transfer. After PCE molecules arrive at the surface, both sorption of PCE by LDH and reductive reaction of nZVI can occur. The redox reaction occurred on the surfaces of nZVI-LDH particles; consequently, iron oxide mainly deposited on the nanoparticle surfaces, resulting in gradual slowing of the PCE removal rate. The effects of initial concentration, pH, coexisting anions, and cations on the removal of PCE by nZVI-LDH are shown in Fig. 5. As shown in Fig. 5a, as the initial concentrations of PCE increased, both the efficiency and the removal rate decreased. PCE was almost completely removed in 8 h when the initial concentration of PCE was less than 10 mg/L, whereas incomplete removal of PCE was observed when the PCE concentration was increased; the removal efficiency of PCE was 88.3% and 79.6% at 20 and 50 mg/L, respectively. This result was mainly because when the amount of iron is fixed, the number of available reactive sites is limited; after the initial concentration of PCE increased, a large number of active sites were occupied, and more iron was oxidized and formed an oxidation layer (Yoon et al., 2011), decreasing the electron transfer from Fe0 to PCE and the removal rate of PCE. The initial pH effect on PCE removal by nZVI-LDH is presented in Fig. 5b. The removal efficiency of PCE decreased from 96.7% at pH 5.0e81.4% at pH 9.0, whereas the initial pH values of 6.0e8.0 had no

significant effect on the removal of PCE. Similar results have been reported in other studies that the removal of PCE by nZVI-LDH at acidic pH was better than that at alkaline pH (Yoon et al., 2011). When nZVI in the composite nanoparticles react with PCE, Fe0 is oxidized to Fe3þ/Fe2þ, and then PCE was reduced by dechlorination (Chen et al., 2014). At a low pH value, the presence of Hþ led to the accelerated corrosion of nZVI, which resulting in less precipitation of iron oxide on the surface of iron, thus increasing the removal of PCE (Dong et al., 2017). When the pH was high, the iron oxide coating on the surface of nZVI occupied the reaction site, thus decreasing the reactivity of nZVI (Fu et al., 2015; Zhou et al., 2016). The results of cations effects are shown in Fig. 5c. It was found the removal rate of PCE by nZVI-LDH without cation was higher than that in the presence of Ca2þ and Mg2þ ions, suggesting the inhibition of Ca2þ and Mg2þ on the removal of PCE. This finding probably due to the formation of precipitates on the surface of iron, such as CaCl2 and Mg(OH)2, which block electron transfer on the iron surface (Hu et al., 2017a), thus decreasing the removal rate of PCE by nZVI-LDH. It should be noticed that the existence of Cu2þ did slightly improve the removal of PCE, which may be associated with the role of Cu2þ as a catalyst or charge transfer medium in the reaction (Bransfield et al., 2006; Hou et al., 2008). The effects of the anions on PCE removal by nZVI-LDH are shown in Fig. 5d. It can be seen that the removal trends of PCE by nZVI-LDH were similar under different anions conditions, while the removal rate of PCE was slightly inhibited in the presence of any anions. The 2 inhibition effect of SO2 4 was relatively obvious, followed by CO3 ,   Cl and NO3 . This phenomenon was mainly due to the formation of iron-anion complexes on the surface of nZVI, which resulting the passivation of iron reactive sites, thereby decreasing the removal rate of PCE by ZVI-LDH (Liu et al., 2007). It was reported NO 3 can be reduced to NO 2 and NH3 by taking electrons from the surface of iron (Suzuki et al., 2012), resulting a competitive process between  PCE and NO 3 on iron surface. For Cl , the effect may have been caused by the passivation of the reaction site due to the formation of FeeCl complexes on nZVI surface (Chen et al., 2014). The slight inhibition of CO2 3 may have been due to the formation of a passive film mainly composed of FeCO3 (Agrawal et al., 2002; Fu et al., 2015). Cheng et al. (2019) have observed that sulfate has a slight inhibition on the degradation efficiency of TCE by acrylic copolymer stabilized NZVI (acNZVI), owing to competition with TCE for reactive sites of acNZVI surface. 3.4. PCE biodegradation coupled with nZVI-LDH removal As shown in Fig. 6 and 65.9% of PCE was biodegraded within 6 days by the PCE-degrading consortium at the initial concentration

Q. Wang et al. / Chemosphere 243 (2020) 125260

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Fig. 5. The effect of (a) initial concentration of PCE, (b) initial pH, (c) co-existing cations and (d) anions on the removal of PCE by nZVI-LDH. Control experiments with only PCE in the deionized water were conducted.

of 10 mg/L PCE, and the removal rate was lower than that at initial concentrations of 0.5e2.5 mg/L. These results indicated that the degradation efficiency of PCE was inhibited under high PCE concentrations. For the combined system of the acclimated microbial consortium coupled with the composite of nZVI-LDH, the removal rate of PCE was higher than that of only the PCE-degrading consortium. The PCE concentration decreased rapidly, and the removal efficiency was almost 69.0% after 1 day of reaction; subsequently, the concentration of PCE was reduced at a much slower rate. PCE was completely removed within 5 days. Both biodegradation and removal of nZVI-LDH contributed to the fast removal of PCE at the earlier stage, as evidenced by a 24.1% biodegradation efficiency and a total removal of 69.4% by the coupled biodegradation with nZVI-

LDH at day 1 (Fig. 6a). However, the difference of 45.3% in the removal efficiency between the coupled removal and biodegradation only indicates that nZVI-LDH played the major role at the early stage. This is consistent with the fast removal kinetics observed in the experiment with the nZVI-LDH only (Fig. 4b). Later, with the consumption of nZVI-LDH, the removal contribution shifted to the biodegradation of PCE by the acclimated microbial consortium in a later phase. A possible explanation for the lag in biodegradation by the acclimated microbial consortium was that when nZVI-LDH was first added to cell suspensions, it was sufficiently reactive to inhibit the dechlorinating bacteria (Xiu et al., 2010). nZVI-LDH was partially oxidized and passivated over time, and later allowed the dechlorinating culture to recover to biodegrade PCE. This finding is

Fig. 6. (a) Removal efficiency of PCE by biodegradation only and biodegradation coupled with nZVI-LDH, (b) the kinetics analysis of PCE using pseudo first-order kinetic model. Control experiments with only PCE in the sterile medium were conducted.

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very important, because it indicates that a two-stage in situ remediation is feasible for PCE removal in engineering field applications. The PCE removal by the consortium alone and the combined system of microbial consortium with nZVI-LDH followed pseudofirst order reaction kinetics, and the removal rate constants were 0.16 and 0.55 d1, respectively. The synergistic effects in enhancing the removal rate occurred because PCE at higher initial concentrations was degraded by nZVI-LDH to lower concentrations in the initial stage and was conducive to the growth of microorganisms. Moreover, the addition of nZVI-LDH can create reducing conditions and produce hydrogen as an electron donor, thus enhancing the PCE biodegradation efficiency (Xie et al., 2017). Similar results have been reported in which the growth of native organohaliderespiring microorganisms was stimulated by the injection of nZVI/CMC (Kocur et al., 2015). Rosenthal et al. (2004) have reported that Fe0 can provide electrons for complete dechlorination of PCE by Dehalococcoides strains. These results suggest that the combination of abiotic and biotic degradation can enhance the removal of chlorinated organics in field sites. In addition, the effectiveness of removing PCE at higher concentrations with the combined system suggests the potential for remedial applications to remove cVOCs. 4. Conclusions The PCE removal performance by a PCE-degrading consortium coupled with nZVI-LDH was extensively evaluated. The observed increase in PCE removal efficiency by the composite nZVI-LDH in the presence of Cu2þ was attributed to the role of this ion as a catalyst or medium of charge transfer during the reaction. This discovery was interesting and may have important implications for field applications. Generally, the combined PCE-degrading microbial consortium with nZVI-LDH increases the application range for cVOCs removal, because: (1) it has a potential to be applied at high PCE concentrations (hence, further evaluation of the applicability of DNAPL is warranted); and (2) it shortens the remediation duration compared with that of bioremediation alone. In the coupled system, the initial removal of PCE was mainly dominated by the abiotic degradation and adsorption of nZVI-LDH, supplemented by the PCE biodegradation process; subsequently, biodegradation played a major role after the exhaustion of nZVI-LDH. The observed twostage remediation may potentially enhance the in situ biodegradation of cVOCs, and contribute to the success of cVOCs remediation in the field, especially for those contaminated sites with high cVOCs concentrations. Acknowledgments This research was supported by the Youth Fund of the Natural Science Foundation of Jiangsu, China (No. BK20161094); the Youth Foundation of the National Natural Science Foundation of China, China (No. 41701365); the “135” Plan and Frontiers Program of the Institute of Soil Science, Chinese Academy of Sciences, China (No. ISSASIP1657); and the Jiangsu Province Social Development Project, China (No. BE2017779). References Agrawal, A., Ferguson, W.J., Gardner, B.O., Christ, J.A., Bandstra, J.Z., Tratnyek, P.G., 2002. Effects of carbonate species on the kinetics of dechlorination of 1,1,1trichloroethane by zero-valent iron. Environ. Sci. Technol. 36, 4326e4333. Bransfield, S.J., Cwiertny, D.M., Roberts, A.L., Fairbrother, D.H., 2006. Influence of copper loading and surface coverage on the reactivity of granular iron toward 1,1,1-trichloroethane. Environ. Sci. Technol. 40, 1485e1490. Chaignaud, P., Maucourt, B., Weiman, M., Alberti, A., Kolb, S., Cruveiller, S., Vuilleumier, S., Bringel, F., 2017. Genomic and transcriptomic analysis of growth-supporting dehalogenation of chlorinated methanes in methylobacterium. Front. Microbiol. 8.

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