Enhanced removal of vanadium(V) from groundwater by layered double hydroxide-supported nanoscale zerovalent iron

Journal Pre-proof Enhanced removal of vanadium(V) from groundwater by layered double hydroxide-supported nanoscale zerovalent iron Xiangrui Kong (Conceptualization) (Methodology) (Investigation) (Formal analysis) (Writing - original draft) (Writing - review and editing), Jiehao Chen (Software) (Investigation) (Formal analysis) (Visualization), Yunjia Tang (Methodology) (Investigation) (Formal analysis) (Visualization), Yan Lv (Investigation) (Formal analysis) (Validation), Tan Chen (Conceptualization) (Methodology) (Supervision), Hongtao Wang (Conceptualization) (Funding acquisition) (Project administration)

PII:

S0304-3894(20)30380-0

DOI:

https://doi.org/10.1016/j.jhazmat.2020.122392

Reference:

HAZMAT 122392

To appear in:

Journal of Hazardous Materials

Received Date:

3 November 2019

Revised Date:

10 February 2020

Accepted Date:

22 February 2020

Please cite this article as: Kong X, Chen J, Tang Y, Lv Y, Chen T, Wang H, Enhanced removal of vanadium(V) from groundwater by layered double hydroxide-supported nanoscale zerovalent iron, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122392

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Enhanced removal of vanadium(V) from groundwater by layered double hydroxide-supported nanoscale zerovalent iron

Xiangrui Konga, Jiehao Chena#, Yunjia Tanga#, Yan Lvb, Tan Chenb, Hongtao Wanga*

School of environment, Tsinghua University, Beijing, 100084, PR China

b

College of Life and Environmental Sciences, Minzu University of China, Beijing

100081, PR China

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GRAPHICAL ABSTRACT

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Corresponding author. E-mail addresses: [email protected] (H. Wang). # These authors contributed equally to this work.

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a

HIGHLIGHTS 

nZVI dispersed on layered double hydroxide (nZVI@LDH) was prepared.



nZVI@LDH performed best (93.7 mg g-1) with a nZVI/LDH mass ratio of 1:2 at pH 3.



The V(Ⅴ) was reduced and transformed into VO2 and V2O3 to reduce the desorption. nZVI@LDH was an ideal scavenger for V(V)-contaminated groundwater.

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

ABSTRACT

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To reduce the toxicity of vanadium(V) [V(V)] and inhibit the desorption of adsorbed

vanadium in groundwater, we synthesized nanoscale zerovalent iron (nZVI) dispersed

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on layered double hydroxide (LDH) composites (nZVI@LDH) to remove V(V) from

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simulated groundwater. We found that nZVI@LDH could reduce high-valence vanadium to low-valence vanadium, then forming vanadium-containing precipitation to reduce the toxicity and inhibiting vanadium from returning to the groundwater.

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SEM and XRD characterizations exhibited the uniform dispersal of nZVI on the surface of LDH. nZVI@LDH with nZVI/LDH at a mass ratio of 1:2 provided the

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maximum adsorption capacity of 93.7 mg g-1. Coexisting anions and dissolved

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oxygen in groundwater have little effect on V(V) removal. nZVI@LDH performed well across a wide pH range (3.0 to 8.0). The surface characterizations and XPS analysis revealed that LDH as supporting materials inhibited the aggregation and passivation of nZVI. The adsorbed V(V) was reduced to V(IV) and V(III) by nZVI and spontaneously transformed into insoluble VO2 and V2O3. The DFT calculations indicated the strong complexation and better stability of the V(IV) and V(III) species 2

with nZVI@LDH than V(V). This work suggests that nZVI@LDH has the potential to serve as an efficient material for the immobilization of V(V) in groundwater.

Keywords: Nanoscale zerovalent iron; Layered double hydroxide; Vanadium contamination; Groundwater; Removal mechanism

1. Introduction

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Vanadium is widely used in various industrial processes, such as steelmaking,

battery manufacturing, and medicinal processing (H. Liu et al., 2017). At the same

time, vanadium is also a potentially toxic contaminant because it inhibits the growth

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of plants and endangers humans through its strong mutagenic and carcinogenic effects

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(Hidalgo et al., 1988). Vanadium has a variety of valence states, namely, +2, +3, +4, and +5; the V(V) species has the greatest toxicity and the strongest stability (Schiffer

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and Liber, 2017). Extracting vanadium from mines will cause serious pollution to groundwater and soil (Aihemaiti et al., 2018), especially in China, which has the

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largest vanadium ore storage in the world (Yang et al., 2017). A serious issue will arise if vanadium-polluted groundwater is used as a drinking water source. For

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example, the concentration of vanadium from mine tailings was 76–208 μg L-1 in the groundwater of Panzhihua, Sichuan Province, Southeast China; this value is

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0.8–4.7-fold above the limit value (50 μg L-1) set by China’s drinking water standard (GB 5749–2006). Therefore, reducing vanadium pollution in groundwater is increasingly crucial. Various techniques, including ion exchange, surface complexation and electrostatic interaction, have been used to eliminate vanadium pollution. Anion exchanges, such as Amberjet™ 4200 Cl (Keränen et al., 2015) and Amberlite®IRA-400 (Gomes et al., 3

2017) were used for vanadium removal, with the adsorption capacity of 48.9 mg g-1 and 27 mg g-1, respectively. Sirviö et al. (2016) prepared bisphosphonate nanocelluloses to remove vanadium and it obtained high removal efficiency at low solution pH, which was due to the complexation of vanadium with bisphosphonate groups. Surface modification using cationic polymers by electrostatic interaction between anionic vanadium species and cationic groups was investigated for vanadium elimination. For example, glycidyl trimethylammonium chloride (GTMAC) was used

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for the quaternization of pine bark (Zhang and Leiviskä, 2020). According to XPS analysis, the vanadium adsorption ability was closely related to the content of quaternary nitrogen groups. Nonetheless, the binding strength of these techniques

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between vanadium ions and adsorbents may be not strong enough, thus allowing its

easy release back into groundwater during its long-term use (Rončević et al., 2019). In

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addition, further studies are still needed to reduce high toxicity of V(V) and increase

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adsorption capacity. Reducing V(V) to V(IV) and V(III) followed by chemical precipitation, may be an efficient method to reduce V(V) toxicity and decrease the risk of vanadium being released back into the environment.

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Nanoscale zerovalent iron (nZVI), as an efficient reducing material, has been widely used on the reduction of anionic heavy metals, such as selenate, chromate, and

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uranium, from wastewater (Hu et al., 2017; Sheng et al., 2016a; Yu et al., 2019).

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Previous study further found that after Cr(VI) was reduced to Cr(III) by nZVI, the precipitate of FeCr2O4 and Cr2O3, were formed to inhibit chromium desorption (Li et al., 2012). In the previous studies, A preliminary attempt to use nZVI for the V(Ⅴ) removal was conducted (Erdem Yayayürük and Yayayürük, 2017). It proved that nZVI might be potentially used for vanadium removal. The main removal mechanism was surface complexation instead of reduction, although the standard electrode 4

potential of V(H2VO4-)/V(VO2+) (+1.42 V) is higher than Fe2+/Fe0 (－0.44 V). Moreover, the combinations of nZVI with EDTA showed limited promoting effect (Rončević et al., 2019). As explained in that study, the introduction of EDTA may have changed the removal mechanism from reduction to surface adsorption, and the latter was more susceptible to environmental changes, such as changes in the pH and the presence of competing ions. Therefore, it is valuable to investigate the remediation

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mechanism of V(Ⅴ)-contaminated groundwater by nZVI to figure out: (ⅰ) whether V(V) can be reduced by nZVI; (ⅱ) whether the reduced V can form precipitates to prevent vanadium from returning to groundwater; and (ⅲ) whether the property of this adsorbent will be easily affected by groundwater chemistry.

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To eliminate the passivation and aggregation of nZVI, layered double hydroxide

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(LDH) was selected as the supporting material due to its low cost, high elimination efficiency, and environmental friendliness (Yu et al., 2017). Great amount of

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functional groups on the surface of LDH can not only help form LDH-based multifunctional material in pollution control (Liu et al., 2019), but also provide good

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properties of outer-layer adsorption toward contaminants (Pang et al., 2019), which can accelerate electrons transfer from nZVI to contaminants. The function of LDH has

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been verified by research on anionic heavy metals removal from wastewater, such as selenate, chromate, and uranium (Sheng et al., 2016b; Yu et al., 2019; Zhou et al.,

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2015). Therefore, LDH is an ideal support material to eliminate the drawback of nZVI and combine the superiority of LDH. The aim of this paper is to investigate the V(V) removal mechanism by

LDH-supported nZVI (nZVI@LDH) and evaluate the effects of groundwater chemistry, including anoxic condition, dissolved oxygen (DO), pH, ionic strength, and presence of naturally existing ions, on the effectiveness of nZVI@LDH. The results 5

showed that (ⅰ) V(V) could be reduced to V(IV) and V(III) by nZVI@LDH, after which the reduced V ions were spontaneously transformed into insoluble VO2 and V2O3 to decrease the desorption risk; and (ⅱ) nZVI@LDH was an ideal scavenger for the removal of V(V) in polluted groundwater. To the best of our knowledge, this study is the first to explore the vanadium removal mechanism by nZVI@LDH and proposes a new mechanism for V(V) removal. 2. Materials and methods

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2.1 Reagents and chemicals

All chemicals used were of analytical grade without further purification. NaOH,

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HCl, KBH4, FeSO4·7H2O, NaNO3, Na3PO4·12H2O, Mg(NO3)2·6H2O, Al(NO3)3·9H2O, and NaHCO3 were purchased from Sinopharm Chemical Reagent Co., Ltd. NH4VO3,

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Na2SO4·10H2O and (NH2)2CO were obtained from Shanghai Macklin Biochemical Co., Ltd. Anaerobic deionized water was prepared by bubbling nitrogen (N2) into

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deionized (DI) water (18.2 MΩ cm-1) for 30 min. Stock solution (1000 mg L-1) of V(V) was prepared by dissolving NH4VO3 with anaerobic DI water.

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2.2 Preparation of LDH, nZVI, and nZVI@LDH MgAl-LDH was synthesized using the hydrothermal method at a Mg2+:Al3+ molar

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ratio of 2:1. Briefly, 5 mmol Mg(NO3)2·6H2O and 2.5 mmol Al(NO3)3·9H2O were

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dissolved with 18.8 mmol (NH2)2CO in 50 mL DI water. After stirring for 10 min, the mixed solution was transferred to a 100 mL Teflon autoclave and heated at 150 °C for 6 h. The white precipitates were collected and washed with DI water until the pH was close to neutral. The precipitates were then dried at 80 °C for 24 h. The chemical formula of MgAl-LDH in this paper is Mg0.67Al0.33(OH)2(CO3)0.17·xH2O. nZVI was fabricated via a KBH4 reduction method under nitrogen purging. A total 6

of 9.96 g FeSO4·7H2O was dissolved in 125 mL of anaerobic DI water in three 500 mL flasks. Then, 100 mL of a 3.88 g KBH4 solution was added dropwise to the FeSO4·7H2O solution. After continuous stirring for 30 min, the sample was separated and washed three times by using anaerobic DI water and ethanol. Finally, the sample was freeze-dried in a vacuum freeze dryer. nZVI@LDH was synthesized by using the same approach as that used for nZVI except that a different mass of LDH was immersed in the FeSO4·7H2O solution before

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KBH4 was added (Kong and Wang, 2019). nZVI@LDH at nZVI/LDH mass ratios of 1:4, 1:2, 1:1, and 2:1 was continuously prepared. 2.3 Batch experiments

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2.3.1 Effect of nZVI/LDH mass ratio

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Batch experiments were performed in 50-mL glass vials under anoxic conditions. Basically, 50 mL of a 50 mg L-1 V(V) solution was added to a vial that contained 0.03

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g of nZVI, LDH, nZVI@LDH-1:4, nZVI@LDH-1:2, nZVI@LDH-1:1, or nZVI@LDH-2:1 under stirring (200 rpm) at 303 K. After stirring for 6 h, 6 mL

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supernatant was collected and filtered through a 0.45 μm filter membrane. The entire experiments in this study were performed in triplicate. V(V) and Fe were determined

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by ICP-OES (Prodigy7, Leeman Labs, USA). 2.3.2 Effect of contact time and V(V) concentration

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nZVI@LDH-1:2 was chosen for further experiments, and the particle dosage was

changed to 0.5 g/L. Contact times varying from 0–360 min were applied to 50 mL of a 50 mg L-1 V(V) solution to examine the effect of the contact time on the reaction. A vanadium solution with different concentrations ranging from 10–100 mg L-1 was prepared and reacted with nZVI@LDH-1:2 to investigate the effect of the V(V) concentration. 7

2.3.3 Effect of groundwater chemistry To investigate the pH effect on vanadium removal, the initial pH of 3, 4, 5, 6, 7, 8, and 9 was selected and adjusted by 1.0 or 0.1 mol L-1 HCl and 1.0 or 0.1 mol/L NaOH. The pH of the solution was measured before and after the reaction. To assess the effect of DO concentration, two bottles of a 50 mg L-1 V(V) solution were bubbled by N2 and O2 for 30 min, respectively. The V(V) solution bubbled by N2, the V(V) solution bubbled by O2, and the above two solutions were mixed at a volume ratio of

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1:4, 2:3, 3:2, 4:1 into 50-mL glass vials, respectively. The effects of the ionic strength (0.01–0.1 mol L-1) and co-existing anions (60 mg L-1 PO43-, 600 mg L-1 SO42-, 400 mg L-1 NO3-, and 400 mg L-1 HCO3-) according to the typical concentration of common

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anions in groundwater on V(V) removal were evaluated. 2.3.4 Evaluation of nZVI@LDH for practical application

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To investigate the elimination performance from different water systems, real

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seawater was collected from Qingdao (Shandong, China) and groundwater and river water were collected from Haidian District (Beijing, China). DI water was comparatively analyzed against the collected waters. The composition of these water

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systems is listed in the Table S1. Different water systems were used to prepare a 50-mg L-1 V(V) solution. To assess the long-term effectiveness of nZVI@LDH, the

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antioxidant experiments were conducted by exposing nZVI@LDH in anaerobic DI

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water for 0, 2, 4, 6, 8, and 10 days, after which the suspension was injected into the V(V) solution. 2.4 Characterizations The micromorphology was obtained by scanning electron microscopy (SEM, Zeiss Merlin, Germany) coupled to energy dispersive X-ray spectrometry (EDS, X-MaxN, OXFORD, UK). The samples were suspended in deoxygenated ethanol, and ~20 μL 8

of suspension were placed onto silicon chips before SEM-EDS characterization. The interface structure of nZVI@LDH was characterized by high-resolution transmission electron microscopy (HRTEM, JEM-2010F, JEOL, Japan) and scanning transmission electron microscopy (STEM, Hitachi S-5500, Japan) coupled with energy-dispersive spectrometry (EDS)-elemental mapping analysis. The samples were suspended in deoxygenated ethanol and ~20 μL of suspension were placed onto the micrograte before HRTEM and STEM characterizations. The X-ray diffraction (XRD,

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D/max-2550, Rigaku SmartLab, Japan) was performed with radiation of Cu Kα and scanning rate of 4° min-1 from 5° to 80°. The sample preparation is listed in

Supplementary Material. The magnetic properties were achieved by a vibrating

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sample magnetometer (VSM, PPMS-9, Quantum Design, USA). A magnetic field

varying from −20,000 Oestard to 20,000 Oestard was applied. The removal

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mechanism was achieved from X-ray photoelectron spectroscopy (XPS, ULVAC-PHI,

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Japan) by detecting the surface composition. The ray source was monochromatic AI kα. The parameters for the survey scans were used as: pass energy = 280 eV, step size = 1.0 eV. Similarly, the following parameters were applied to the narrow scans: pass

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energy = 55 eV, step size = 0.1 eV. The sample preparation and fitting parameters of

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XPS were listed in Supplementary Material. 3. Results and discussion

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3.1 Characterizations of nZNI, LDH, and nZVI@LDH The surface morphology of nZVI, LDH, and nZVI@LDH used in this work was

evaluated by SEM. A large SEM magnification was chosen to obtain the microstructure of the nZVI particles. As shown in Fig. 1A, the nZVI particles agglomerated into a chain structure, thereby proving its easy aggregation (Bae et al., 2018). Fig. 1B presents the hexagonal brucite-like layer structure of LDH. On the 9

basis of the SEM-EDS image (Fig. 1C), peaks from Fe were observed in the EDS spectrum of the selected area (Fig. 1C), which confirms the presence of nZVI attached onto LDH. The HRTEM image shows two typical lattice fringes with interplanar spacing of 0.12 nm and 0.14 nm, corresponding to the (110) plane of Fe0 and (606) plane of LDH, respectively (M. Zhang et al., 2019). The STEM morphology of the as-prepared nZVI@LDH (Fig. 1E) demonstrated that the nZVI nanoparticles were well-dispersed on the surface of LDH (Lai et al., 2019). EDS elemental mapping (Fig.

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1F-1H) shows uniform distribution of Fe, Mg, and Al, which indicates the successful synthesis of the nZVI@LDH composites.

The XRD patterns of nZVI, LDH, and nZVI@LDH are shown in Fig. 1I. For nZVI,

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the peak at 44.79° was assigned to the centered cubic plane (110) of Fe0 (Liu et al.,

2015) and the peaks at 29.76° and 30.77° of nZVI were assigned to K2SO4

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(Hashimoto et al., 2005), which were impurities in the process of nZVI synthesis.

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Typical peaks at 2θ = 11.71° and 23.53° were ascribed to the (003) and (006) phases for LDH, respectively (Yang et al., 2014). For nZVI@LDH, typical peaks for both nZVI and LDH as above were observed in the XRD characterization of nZVI@LDH.

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However, three differences were found:

(ⅰ) Basal spacing (d) refers to the sum of the thickness of a brucite-like layer and

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the interlayer spacing between two brucite-like layers (Fig. S1). In general, the

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thickness of a brucite-like layer for Mg/Al-LDH is 0.48 nm (Chen and Qu, 2003) whereas the thickness of interlayer spacing is related to the size of charge balancing interlayer substances (Olfs et al., 2009). Compared with 0.76 nm basal spacing (d003) of the pristine CO32- intercalated LDH, the nZVI@LDH exhibited an enlarged basal spacing (d003) of 0.88 nm, demonstrating the larger introduction of nZVI in the interlayer space of LDH; 10

(ⅱ) Bragg’s equation is defined as nλ = 2d’sinθ (Mallakpour et al., 2015), where n is an integer, λ is the wavelength of radiation, θ is the glancing angle of incidence, and d’ is the interplanar spacing of the crystal, d’=3d (Duan and Zhang, 2009). After intercalation of nZVI, the increasing basal spacing (d) resulted in the decrease of θ. Therefore, the (003) and (006) phases shifted to the low 2θ; (ⅲ) The peak of nZVI@LDH at 44.79° was broader than that of nZVI, indicating the poor crystallinity of nZVI on LDH. The peaks at 33.81° referred to Fe2O3 (Suresh

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et al., 2016) due to the partial oxidation of nZVI. The magnetization hysteresis loops were measured by a VSM instrument (Fig. 1J). The values of saturation magnetization were 43.59, 0, and 33.83 emu g-1 for nZVI,

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LDH, and nZVI@LDH, respectively. The addition of LDH resulted in lower magnetic

saturation as compared with nZVI, but contributed to the lowered aggregation of

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nZVI via the steric stabilization effect (Elbasuney, 2015), as confirmed by the SEM

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for practical applications.

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characterization results. The magnetic property of the samples increases its potential

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image of nZVI@LDH (D). STEM image of nZVI@LDH (E) and EDS elemental mapping images (F-H). The XRD pattern of nZVI, LDH, and nZVI@LDH (I). The VSM characterization of nZVI, LDH, and nZVI@LDH (J).

3.2 Effect of nZVI/LDH mass ratio The effect of the nZVI/LDH mass ratio on V(V) removal is illustrated in Fig. 2. Notably, nZVI exhibited an adsorption capacity of 46.6 mg g-1, which was higher than

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that of LDH (16.8 mg g-1). The different performances between nZVI and LDH imply that the reduction mechanism of nZVI was more efficient than the adsorption process

of LDH for V(V) elimination, and nZVI of nZVI@LDH may play an important role

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in the V(V) removal process.

The efficacy of V(V) removal was strongly affected by the nZVI/LDH mass ratio.

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At a low nZVI proportion of 1:4, poor removal capacity (38.2 mg g-1) was observed

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due to the relatively low reactivity. As the mass ratio of nZVI/LDH increased to 1:2, the adsorption capacity correspondingly increased to 70.0 mg g. With increasing nZVI ratio, more reactive sites enhanced the removal performance through the good

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dispersion of nZVI on the LDH surface (Dong et al., 2017). However, lowered V(V) removals to 59.6 mg g-1 and 35.5 mg/g were observed at higher mass ratios of 1:1 and

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2:1, respectively. These results may be due to excessive nZVI loading on LDH,

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leading to the aggregation and agglomeration, and resulting in a loss of reducing ability. A similar phenomenon was verified by other studies (Han et al., 2015; Qian et al., 2017).

As a result, the optimal mass ratio of nZVI/LDH was 1:2, and the V(V) removal capacity was 1.5- and 4.2-fold higher than that of nZVI and LDH, respectively. Furthermore, the removal capacity of nZVI@LDH-1:2 exceeded the total capacity for 13

nZVI and LDH, which indicated a 1 + 1 > 2 synergistic effect between LDH and nZVI. These results further confirm the good potential of nZVI@LDH for V(V)

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elimination.

Fig. 2. Effect of nZVI/LDH mass ratio on V(V) removal by nZVI@LDH. Conditions: V(V) initial

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temperature = 298 K, contact time = 6 h.

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concentration = 50 mg L-1, adsorbent dosage = 0.6 g L-1, solution volume = 50 mL, experimental

3.3 Effect of contact time and V(V) concentration

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To examine the practical application, the effect of contact time was investigated in a 50 mg L-1 V(V) solution (Fig. 3A). The elimination rate was fast in the first 10 min,

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and above 50% V(V) was binding to nZVI@LDH. The removal reached equilibrium

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after 360 min because of the saturation of the active sites and the falling off of the reducing agents. The maximum adsorption capacity was determined to be 69.6 mg g-1, and the corresponding removal capacity was measured at 77.1%. Two typical kinetic models (pseudo-first-order and pseudo-second-order) were applied to examine the V(V) immobilization rate. The corresponding parameters are shown in Table 1. The removal of V(V) was better fitted to the pseudo-second-order 14

(R2 = 0.9999) than by the pseudo-first-order (R2 = 0.9182), thereby indicating that the rate-limiting step was adsorption (Ho and McKay, 2000). To identify the interaction between the adsorbate and adsorbent at equilibrium, the removal performance at different V(V) concentrations (10–100 mg L-1) was investigated (Fig. 3B). The adsorption capacity increased from 19.9 mg g-1 to 86.2 mg g-1 with increasing V(V) concentrations, while the removal percentage gradually decreased from 99.5% to 43.3%. Fig. 3C showed the fitting curve for Langmuir and

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Freundlich isotherms, and the isotherm parameters are listed in Table 1. The Freundlich model fitted better toward the V(V) removal process (R2 = 0.9974). The n value of 4.73 was in the range of 1–10, indicating favorable adsorption (Treybal,

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1980). The better fitting of the Freundlich model demonstrated the multilayer

adsorption over the heterogeneous surface of nZVI@LDH, which was consistent with

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chemical adsorption process.

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the result from the kinetic model and indicated that the V(V) removal was due to the

Table 2 presents the V(V) adsorption capacities of different adsorbents with nZVI@LDH. A direct comparison of the capacity was difficult due to complex

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experiment conditions, but the V(V) adsorption value of nZVI@LDH in this study

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was highly competitive as compared to those reported in previous work.

Fig. 3. Effect of contact time (A) and V(V) concentration (B) on V(V) removal by nZVI@LDH.

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Fitting curve for Langmuir and Freundlich isotherms (C). Conditions: V(V) initial concentration = 50 mg L-1, adsorbent dosage = 0.5 g L-1, solution volume = 50 mL, experimental temperature = 298 K, contact time = 6 h.

Table 1 The parameters of kinetic and thermodynamics models for the adsorption of V(V) on nZVI@LDH.

Parameters Qe (mg g-1)

R2

0.0181

20.48

0.9182

k2 (mg g-1 min-1)

Qe (mg g-1)

R2

0.0025

70.92

0.9999

KL (L mg-1)

R2

86.96

0.50

0.9951

n

KF (mg1-n Ln g-1)

R2

4.73

37.81

0.9974

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Table 2

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Qmax (mg g-1) Langmuir model

Freundlich model

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k1 (min-1)

Pseudo-first-order model

Pseudo-second-order model

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Modals

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Comparison of V(V) adsorption capacities of different adsorbents with nZVI@LDH.

Adsorbent

C0 (mg L-1)

Particle dosage (g L-1)

pH

qm (mg g-1)

Ref.

ZVI synthesized by iron-rich sludge combined with kaolin clay Ferric groundwater treatment residual modified peat Octylamine functionalized magnetite nanoparticles

80

2.0

5.0

15.0

(Bello et al., 2019)

166

2.0

4.0

16.3

(R. Zhang et al., 2019)

244

1.9

3.2

25.7

(Parijaee et al., 2014)

PGTFS-NH3+Cl−

300

2.0

6.0

45.9

(Anirudhan and Radhakrishnan, 2010)

16

Polypyrrole coated magnetized natural zeolite

250

3.0

4.5

65.1

(Mthombeni et al., 2016)

PdO-MWCNTs nanocomposites

60

1.0

3.0

85.1

(Gupta et al., 2017)

nZVI functionalized by EDTA

10

1.0

-

9.5

(Rončević et al., 2019)

nZVI functionalized by PDCA

10

1.0

-

7.3

(Rončević et al., 2019)

nZVI@LDH

50

0.5

3.0

93.7

Current study

3.4 Effects of groundwater chemistry on V(V) removal For the removal media, many previous studies focused on the removal performance

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of functional material-supported nZVI in wastewater, but few studies reported on the

applicability in simulated groundwater (Li and Zhu, 2014). However, wastewater and polluted groundwater have major differences: (ⅰ) the anoxic condition in deep

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groundwater and the existence of DO in shallow groundwater may have different

effects on the corrosion products of nZVI and the pollutant removal process (A. Liu et

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al., 2017); and (ⅱ) the concentration of contaminants is much lower than that of

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naturally existing ions in groundwater. Thus, a high concentration of naturally existing ions may significantly affect the capability and reactivity of nZVI@LDH. Therefore,

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the feasibility of nZVI@LDH applications in groundwater remediation is needed to acquire further insights.

3.4.1 Effect of initial groundwater pH

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The pH of groundwater is the most important influence factor in heavy metal

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removal because the species distribution of aqueous heavy metal ions and the surface charge property of adsorbent are influenced by solution pH (Han et al., 2015). In general, the pH value of natural groundwater ranges from 5.0 to 8.0. As such, we chose to test the impact of V(V) removal by nZVI@LDH at a pH ranging from 3.0 to 9.0. Fig. 4A shows the predominant role of pH on V(V) removal. nZVI@LDH 17

achieved >98.5% removal with a pH 3.0 solution, and exhibited a corresponding adsorption capacity of 93.7 mg g-1. When the pH increased to 6.0, the removal percentage gradually decreased to 71.5%. In acidic solution, the improved V(V) removal could be attributed to two aspects: (ⅰ) the enhanced corrosion of nZVI promoted V(V) reduction, which was proved by increasing the dissolved Fe concentration (Fig. S2) at pH 3.0; and (ⅱ) the increasing H+ concentration resulted in the protonation of the surface hydroxyl groups (-OH2+), hence improving the

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electrostatic attraction between V(V) anions and LDH (Shi et al., 2011). As a result, the opportunity for electron transfer from nZVI to V(Ⅴ) was enhanced.

In cases where the pH value varied from 7.0 to 8.0, the removal percentage

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decreased from 66.4% to 62.9% but was still over 60% in the 50 mg L-1 V(V) solution. With a solution pH of 9.0, the immobilization efficiency sharply declined to 44.6%.

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This trend has a strong consistency with previous studies (Bello et al., 2019). In

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neutral and alkaline solutions, on the basis of the results of Visual MINTEQ simulation (Fig. 4B), the anionic form of V(V) (H2VO4- and HVO42-) was dominant at a solution pH that ranges from 5.0 to 9.0. With the increase in pH, OH- competed with

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V(V) anions for the opportunity to react with nZVI. The reaction of OH- and nZVI would form iron oxides and iron hydroxides coating on nZVI surface, which occupied

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the reactive sites on the nZVI surface and reduce the reactivity (Hu et al., 2017). Iron

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(hydr)oxide formation was indirectly confirmed by lowered dissolved Fe concentration in the solution as the pH increased (Fig. S2). The above results show a strong correlation between the solution pH and the V(V)

sorption behavior, wherein an acidic environment favored the immobilization of V(V) onto nZVI@LDH. Given that natural groundwater has a pH value range of 5.0 to 8.0, nZVI@LDH has the potential to perform well in groundwater environments. 18

Fig. 4. Effect of pH on V(V) removal by nZVI@LDH (A). Species distribution of V(V) as a

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function of pH (B). Conditions: V(V) initial concentration = 50 mg L-1, adsorbent dosage = 0.5 g L-1, solution volume = 50 mL, experimental temperature = 298 K, contact time = 6 h.

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3.4.2 Effect of DO in groundwater

DO exists in shallow groundwater because of fluctuating water tables. It is a typical

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oxidant that can accept electrons from nZVI according to Eqs. (5) and (6). The

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influence of DO on heavy metal removal is complicated. On the one hand, the interaction between DO and nZVI@LDH can accelerate Fe0 corrosion. The additional

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Fe2+ generated can serve as a reductant for V(V) reduction (Qin et al., 2016). On the other hand, DO competes with V(V) for electrons donated by Fe0 and adsorption sites

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to impede the removal of contaminants (Mu et al., 2015).

2Fe0 + 2H2O + O2  2Fe2 + 4OH

(5)

4Fe2 +10H2O + O2  4Fe(OH)3 (s) +8H

(6)

Fig. 5 shows the dependence of V(V) removal by nZVI@LDH on DO. The

existence of DO slightly decreased the removal percentage. Specifically, the removal percentage was 69.0% under N2 purging then dropped to 57.7% under O2 purging. According to the result in this study, the side effect that DO competed for active sites

19

and electrons played a bigger role in the reaction process. The decreasing trend of dissolved Fe concentration shown in Fig. 5 implies that dissolved Fe2+ may react with DO to form insoluble iron oxides or hydroxides. The similar negative effect of DO on Cr(VI) and U(VI) removal by nZVI was also reported in other studies (Du et al.,

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2016).

Fig. 5. Effect of DO volume ratio on V(V) removal by nZVI@LDH. Conditions: V(V) initial

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concentration = 50 mg L-1, adsorbent dosage = 0.5 g L-1, solution volume = 50 mL, experimental

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temperature = 298 K, contact time = 6 h.

3.4.3 Effects of ionic strength and co-existing anions present in groundwater

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Various anions are present in the groundwater system, and the concentration of natural existing anions is much higher than that of contaminants in groundwater. The

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reactivity of nZVI@LDH in V(V)-contaminated groundwater may be affected by the presence of background anions. Therefore, the effects of ionic strength (0.01, 0.05, and 0.1 M) and co-existing anions, including HCO3-, PO43-, NO3-, and SO42-, were investigated to acquire further information for in-situ remediation. To simulate the real situation closely, the concentration of co-existing anions was consistent with the typical concentration in groundwater. 20

The effects of ionic strength are shown in Fig. 6. The removal behavior was strengthened from 67.6% to 79.0% as the ionic strength increased from 0 M to 0.1 M. Two possible reasons may explain the increasing removal percentage: (ⅰ) increasing electrolyte concentration might strengthen the corrosion of Fe0, as tested by Grieger et al. (2010); and (ⅱ) increasing ionic strength accelerated the formation of new iron oxide through old iron oxide diffusing away from the iron surfaces (Farrell et al., 2000). In addition, the ionic strength representing the concentration of background

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electrolyte would have an impact on the binding species of adsorption (Lv et al., 2013). The removal process of V(V) was sensitive to the ionic strength variations, thereby indicating the typical outer-sphere surface complexation.

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The effect of co-existing anions (Fig. 6) indicates that HCO3- and SO42- promoted

V(V) removal to 79.9% and 75.7%, respectively, while PO43- and NO3- restrained the

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removal process to 53.0% and 60.2%, respectively. The influence mechanisms of

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co-existing anions on V(V) removal could be summarized as follows: (ⅰ) the existence of co-existing anions could improve the ionic strength, thus promoting the removal process; and (ⅱ) co-existing anions would compete for active sites with V(V),

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leading to inhibition for V(V) removal (Mu et al., 2015). (ⅲ) LDH could adsorb co-existing anions through the ion exchange process, thus occupying the available

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sites of nZVI@LDH.

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The favorable effect of SO42- and HCO3- is consistent with Dan’s research (Lv et al., 2019). Previous studies have shown that SO42- strengthened corrosion and increased the reactive sites of nZVI by forming outer-sphere complexes with iron (oxy)hydroxides (Kim et al., 2014). The negative influence of HCO3- was negligible in this study, indicating that the activity of nZVI@LDH was insensitive to HCO3(Setshedi et al., 2013). The mechanism of PO43- was contrasted with that of SO42-. 21

PO43- formed inner-sphere complexes on the reacted nanoparticles (Su and Puls, 2001), leading to the decrease in the adsorptive sites on the nZVI surface. NO3- is a competitive redox anion that could be reduced to ammonia by nZVI (Li et al., 2010). Therefore, NO3- might compete for electrons with V(V) in redox reactions, thereby

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decreasing V(V) removal.

Fig. 6. Effect of ionic strength (IS) and coexisting anions on V(V) removal by nZVI@LDH.

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Conditions: V(V) initial concentration = 50 mg L-1, adsorbent dosage = 0.5 g L-1, solution volume = 50 mL, experimental temperature = 298 K, contact time = 6 h, HCO3- concentration = 400

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mg L-1, PO43- concentration = 60 mg L-1, NO3- concentration = 400 mg L-1, SO42-

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concentration = 600 mg L-1.

3.5 Evaluation of nZVI@LDH for practical application

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3.5.1 V(V) removal performance in different water matrices To investigate the feasibility of practical application of nZVI@LDH in natural

waters, batch experiments were performed in different water matrices, including groundwater, seawater, and river water. The important properties and compositions of these water systems are shown in Table S1. As shown in Fig. 7A, nZVI@LDH showed better removal percentage in river water (74.7%) than in groundwater and sea 22

water (54.9% and 53.1%, respectively). According to the study of groundwater chemistry in this paper, the solution pH had the most important effect on the V(V) removal. The higher pH of groundwater sample than other water matrices may be responsible for the passivation of nZVI and decrease in reactivity. According to the higher conductivity and ions concentration of seawater, the ionic strength was estimated to be much higher than others, leading to the aggregation of nZVI (Liu et al., 2018). The >50% removal percentage suggested that nZVI@LDH can be capable of

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V(V) removal in different water matrices. 3.5.2 Long-term effectiveness of nZVI@LDH

Aging is generally related to the aggregation and passivation of nZVI. The

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formation of lepidocrocite, magnetite, goethite, and maghemite may mask the redox

active sites and result in diminished nZVI reactivity (Luciani et al., 2011). The

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negative effect of aging has been reported in the previous studies. Li et al. (2016)

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revealed that the nZVI synthesized in their study found little or no reactivity with TBBPA after two weeks of aging.

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By contrast, our results showed that nZVI@LDH maintained high reactivity with V(V) after aging for 10 days. The removal percentage of 10-day aging nZVI@LDH

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was 65.3% and maintained 91.1% of freshly-prepared nZVI@LDH capacity (Fig. 7B). The excellent long-term effectiveness implies that the aggregation and passivation of

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nZVI may be reduced due to the introduction of LDH. Well-dispersed nZVI on the LDH decreased the tendency for the particles to aggregate. Moreover, the transfer of V(V) to the surface of nZVI was enhanced by V(V) adsorption process of LDH to sustain the reduction reaction of nZVI.

23

Fig. 7. V(V) removal percentage in different water matrices by nZVI@LDH (A). Effect of aging on the V(V) removal by nZVI@LDH (B). Conditions: V(V) initial concentration = 50 mg L-1,

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adsorbent dosage = 0.5 g L-1, solution volume = 50 mL, experimental temperature = 298 K,

3.6 Mechanism for V(V) removal by nZVI@LDH

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3.6.1 Solid phase characterizations

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contact time = 6 h.

The composition evolution of nZVI@LDH after the reaction with V(V) was

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investigated by XRD analysis (Fig. 8A). Compared with nZVI@LDH before reaction with V(V) (Fig. 1I), the peak of Fe0 from nZVI@LDH dramatically weakened and

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Fe2O3 (35.04°) and Fe3O4 (35.15°, 62.18°) appeared after the reaction (Dong et al., 2017). The peaks at 39.35° and 60.89° were assigned to VO2 and the peaks at 32.50°

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and 63.93° were ascribed to V2O3 after careful analysis, which was consistent with previous studies (Wu et al., 2005; Zhang et al., 2014). In the vanadium adsorption

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process, the content of vanadium was much lower than that of nZVI@LDH. Therefore, the peaks of vanadium were not strong to some extent, but it can still be identified to prove the existence of vanadium oxide. The appearance of Fe(II), Fe(III), V(III), and V(VI) implied a redox reaction between the nZVI and V(V). SEM-EDS, HRTEM, and STEM-EDS characterizations were conducted to further understand the removal mechanism. The SEM image of nZVI@LDH after the 24

reaction with V(V) is presented in Fig. 8B. By visual inspection, the amount of nZVI particles was reduced and the surface of LDH was covered tightly with cluster-like particles. According to the EDS analysis, the high level of V and O suggested the presence of vanadium oxides in the cluster-like particles. The HRTEM image (Fig. 8C) demonstrated two different lattice fringes with interplanar spacings of 0.25 nm and 0.26 nm, corresponding to the (200) plane of VO2 and the (104) plane of V2O3, respectively (Jin et al., 2011; Kong et al., 2011). The STEM with EDS elemental

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mapping results showed uniform Fe and V covering on the LDH (Fig. 8E-8H). Vanadium oxides and iron oxides were well-dispersed on the surface of LDH, which slowed down the passivation of nZVI. Therefore, LDH as the supporting material

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promoting the persistent reactivity of nZVI.

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Fig. 8. The XRD pattern of nZVI@LDH after reaction with V(V) under different conditions.

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Peaks are referred to magnetite/maghemite (Fe3O4/γ-Fe2O3) (A). SEM-EDX image of nZVI@LDH after reaction with V(V) (B). HRTEM image of nZVI@LDH (C). STEM image of

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nZVI@LDH (D) and EDS elemental mapping images (E-H). Conditions: V(V) initial concentration = 50 mg L-1, adsorbent dosage = 0.5 g L-1, solution volume = 50 mL, experimental

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temperature = 298 K, contact time = 6 h.

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3.6.2 XPS analysis

XPS measurements were conducted to study the mechanism for V(V) removal by

nZVI-LDH. The XPS survey and high-resolution spectra of the primary elements are shown in Fig. 9. Fig. 9A depicts the photoelectron lines of LDH and nZVI@LDH before and after adsorption. The presence of Mg 2p, Al 2p, and Fe 2p indicated the good synthesis of nZVI@LDH. After V(V) capture, the new peak of 514.2 eV was 26

attributed to V 2p3/2, suggesting that V was strongly adsorbed on nZVI@LDH by chemical bonding. Fig. 9B shows the XPS spectra of V 2p3/2. The three peaks at binding energies of 518.0, 517.0, and 515.8 eV were the characteristic to the presence of V(V), V(IV) oxide (VO2), and V(III) oxide (V2O3), respectively (Fan et al., 2016). The quantitative result revealed that 85.8% of V(V) was reduced to V(IV) oxide and V(III) oxide on the nZVI@LDH surface. The high-resolution XPS shows that the Fe 2p of nZVI@LDH before and after V(V)

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removal was divided into two major peaks (Fe 2p1/2 and Fe 2p3/2) and several small peaks (Fig. 9C). To be specific, the peaks area of 731.9, 724.1, and 710.2 eV were correlated to FeOOH, Fe2O3, and Fe3O4, respectively (Lv et al., 2019; Tran et al.,

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2019). For pristine nZVI@LDH, the peak at 707 eV was related to Fe0. The coexistence of Fe0 and iron oxides (Fe2O3, and Fe3O4) indicate the typical core-shell

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structure of nZVI (Sun et al., 2014). Similar results were also found in previous

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studies (Bae and Lee, 2010; Huang et al., 2013). The peak of the Fe0 intensity vanished sharply after the reaction with V(V), suggesting that Fe0 was consumed in the V(V) reduction process. Notably, the area of FeOOH, Fe2O3, and Fe3O4 increased

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after the reaction, implying the formation of new iron (hydr)oxides (Pang et al., 2019). The O 1s XPS spectra shown in Fig. 9D confirms the presence of abundant

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functional groups, such as C=O, Mg–O, Al–O, and –OH (Yu et al., 2019). After the

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reaction with V(V), all the peaks of the O 1s binding energy changed and the Al 2p peaks at 74.1 eV shifted to 74.4 eV (Fig. S3). The distinct variation revealed that a new chemical interaction formed between V and O and changed the metal-oxide bonds that took part in the reaction (Hu et al., 2016). Furthermore, the sharply decreased -OH peak intensity after reaction with V(V) revealed the formation of a hydrogen bond between the V oxyanions and hydroxyl groups on the LDH surface 27

(Chao et al., 2018). A great amount of functional groups on the surface of LDH can not only help form LDH-based nZVI material in pollution control but can also provide good outer-layer adsorption properties toward V(V) (Goh et al., 2008), which can

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accelerate electron transfer from nZVI to the contaminants.

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Fig. 9. The XPS survey spectra of LDH and nZVI@LDH before and after adsorption (A). The XPS high-resolution of V 2p3/2 (B), Fe 2p (C), and O 1s (D). Conditions: V(V) initial

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concentration = 50 mg L-1, adsorbent dosage = 0.5 g L-1, solution volume = 50 mL, experimental temperature = 298 K, contact time = 6 h.

3.6.3 DFT calculation According to the abovementioned results, after the V(V) reduction, the core-shell structure of nZVI was transformed and was mainly composed of an Fe3O4 shell layer. 28

H2VO4-, VO2 and V2O3 were considered in the density functional theory (DFT) method to further characterize the V(V), V(IV), and V(III) properties on nZVI. The DFT calculations were performed using the Vienna Ab-initio Simulation Package (VASP) code (Hohenberg and Kohn, 1964; Kresse and Furthmüller, 1996). The plane wave cutoff energy was set as 400 eV for the total energy calculation. Ultrasoft pseudopotentials within the framework of the projector-augmented wave (PAW) method were used to represent the ion-electron interactions (Blöchl, 1994).

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The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was adopted as the exchange-correlation functional calculations (Blöchl, 1994; Perdew et al., 1996). The Brillouin zone integration was approximated

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by a sum over special selected k-points using the 2×2×1 MonkhorstPack method

(Monkhorst and Pack, 1976). The geometries were optimized until the energy was

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converged to 1×10-5 eV atom-1 and the forces to 0.02 eV Å-1 to ensure calculation

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accuracy.

For Fe3O4, HRTEM image showed that (110) was the dominantly exposed surface (Fig. 8C). Thus, the (110) surface was modeled using a three-layer p (1×1) super cell.

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During the calculations, the bottom one layer was fixed, while the top two layers and the adsorbed species were relaxed. Moreover, in order to eliminate the interactions in

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vertical direction, the vacuum region was set to 15 Å in the z-direction to separate the

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slabs. The adsorption energy (Eads) was defined as follows:

Eads = EAB  EA  EB

(7)

where EAB is the total energy of the optimized adsorption structure of vanadium species over Fe3O4; EA is the energy of nZVI@LDH; and EB is the energy of the vanadium species. With this definition, more negative values reflect the stronger interaction and better stability of vanadium species with Fe3O4 (Duan et al., 2016; 29

Kattel et al., 2016). The optimized geometric structure of Fe3O4-H2VO4-, Fe3O4-VO2 and Fe3O4-V2O3 were shown in Fig. 10. During the adsorption process, two kinds of covalent bonds (V-O and Fe-O) were formed. Specifically, V-O bonds were 3.690 Å, 1.794 Å and 1.787 Å for Fe3O4-H2VO4-, Fe3O4-VO2 and Fe3O4-V2O3, respectively. Fe-O bonds were 1.850 Å, 1.864 Å and 1.970 Å for Fe3O4-H2VO4-, Fe3O4-VO2 and Fe3O4-V2O3, respectively. The shorter V-O bonds and longer Fe-O bonds were beneficial for the

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stability of vanadium on the nZVI@LDH (Chen et al., 2018). The calculation results of H2VO4-, VO2, and V2O3 towards Fe3O4 were-1.28 eV, -3.46 eV, and -5.59 eV, respectively. The larger adsorption energies of the VO2 and V2O3 as compared to the

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V(V) species indicated the strong complexation and better stability of the VO2 and V2O3 with nZVI@LDH than V(V), and the adsorption capacity will be enhanced after

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V(Ⅴ) reduction to V(Ⅳ) and V(Ⅲ) by nZVI@LDH (Wang et al., 2019).

Fig. 10. The optimized structures for H2VO4- adsorbed on Fe3O4 (A). The optimized structures for VO2 adsorbed on Fe3O4 (B). The optimized structures for V2O3 adsorbed on Fe3O4 (C). Bond

30

lengths are in angstroms.

In summary, the removal of V(V) by nZVI@LDH can be deduced as follows: (ⅰ) the V(V) oxyanions were adsorbed by electrostatic attraction and the H-bonding of LDH; (ⅱ) V(V) was reduced to V(IV) or V(III) by nZVI due to the high redox potential value; and (ⅲ) soluble V(IV) or V(III) ions adsorbed on the surface of LDH were spontaneously transformed into insoluble VO2 and V2O3. Previous researches (Wilson and Weber, 1979; Wan and Ning, 2010; Sturini et al., 2013) showed that

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VO2+ as the main form of V(IV) ions, would be formed during the reduction of V(Ⅴ) to V(IV). Therefore, VO2+ was an reasonable intermediate in the reduction of V(V) by nZVI@LDH. Further research will be carried out to investigate the intermediate

and nZVI@LDH can be summarized as follows:

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process of vanadium removal by nZVI@LDH. The reaction equations between V(V)

(8)

VO2 + Fe3 + 5OH  VO2 (s) + Fe(OH)3 (s) + H2O

(9)

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3H2 VO4 + Fe0 +12H+  3VO2 + Fe3 + 9H2O

(10)

2V3 + Fe3 + 9OH  V2O3 (s) + Fe(OH)3 (s) + 3H2O

(11)

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4. Conclusion

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3VO2 + Fe0 + 6H+  3V3+ + Fe3 + 3H2O

The present study prepared nZVI@LDH as the sorbent for V(V) removal from

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assembled groundwater. The characterizations demonstrated the uniform distribution of nZVI on the LDH surface. nZVI@LDH showed remarkable adsorption capacity, with a maximum value of 93.7 mg g-1. The groundwater chemistry showed little impact on the nZVI@LDH removal performance. The improved Fe0 corrosion and electrostatic attraction enhanced the V(V) removal in an acidic solution. DO slightly impeded the removal process by competing for electrons and available sites with 31

V(V). The removal behavior was improved with increasing ionic strength. The good removal performance in different natural water matrices and long-term effectiveness even after 10-day aging suggested its feasibility for practical applications. The XRD, SEM, STEM-EDS, and XPS characterizations results demonstrated that nZVI played a dominant role in the removal process for the reduction of V(V) to V(IV) and V(III) by nZVI, followed by its spontaneous transformation into insoluble VO2 and V2O3, which can reduce the vanadium toxicity and desorption risks. The DFT calculation

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revealed the strong complexation and better stability of VO2 and V2O3 on the nZVI@LDH as well as the enhanced adsorption capacity after the V(V) reduction

process. These results provide new perspectives on nZVI@LDH as an effective

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sorbent for V(V) removal in groundwater.

For successful application of long-term in-situ groundwater remediation by

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nZVI@LDH, future investigation can be focused on the improvement about electron

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selectivity of nZVI@LDH towards contaminants, as the reduction in the fraction nZVI with H2O and O2 will consume many electrons to reduce the removal efficiency for contaminants and investigation about simultaneous removal performance of

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various pollutants by nZVI@LDH.

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Credit Author Statement

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Xiangrui Kong: Conceptualization, Methodology, Investigation, Formal analysis, Writing - Original Draft, Writing - Review & Editing Jiehao Chen: Software, Investigation, Formal analysis, Visualization Yunjia Tang: Methodology, Investigation, Formal analysis, Visualization Yan Lv: Investigation, Formal analysis, Validation Tan Chen: Conceptualization, Methodology, Supervision 32

Hongtao Wang: Conceptualization, Funding acquisition, Project administration

Declaration of interests 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 work was supported by the National Natural Science Foundation of China

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(Grant Nos. 41877188 and 51578312). References

Aihemaiti, A., Jiang, J., Li, D., Liu, N., Yang, M., Meng, Y., Zou, Q., 2018. The

-p

interactions of metal concentrations and soil properties on toxic metal

re

accumulation of native plants in vanadium mining area. J. Environ. Manage. 222, 216–226.

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Anirudhan, T.S., Radhakrishnan, P.G., 2010. Adsorptive performance of an amine-functionalized poly(hydroxyethylmethacrylate)-grafted tamarind fruit

142–150.

na

shell for vanadium(V) removal from aqueous solutions. Chem. Eng. J. 165,

ur

Bae, S., Collins, R.N., Waite, T.D., Hanna, K., 2018. Advances in Surface Passivation

Jo

of Nanoscale Zerovalent Iron: A Critical Review. Environ. Sci. Technol. 52, 12010–12025.

Bae, S., Lee, W., 2010. Inhibition of nZVI reactivity by magnetite during the reductive degradation of 1, 1, 1-TCA in nZVI/magnetite suspension. Appl. Catal. B Environ. 96, 10–17. Bello, A., Leiviskä, T., Zhang, R., Tanskanen, J., Maziarz, P., Matusik, J., Bhatnagar, 33

A., 2019. Synthesis of zerovalent iron from water treatment residue as a conjugate with kaolin and its application for vanadium removal. J. Hazard. Mater. 374, 372–381. Blöchl, P.E., 1994. Projector augmented-wave method. Phys. Rev. B 50, 17953. Chao, H.-P., Wang, Y.-C., Tran, H.N., 2018. Removal of hexavalent chromium from groundwater by Mg/Al-layered double hydroxides using characteristics of in-situ

ro of

synthesis. Environ. Pollut. 243, 620–629. Chen, W., Qu, B., 2003. Structural characteristics and thermal properties of

PE-g-MA/MgAl-LDH exfoliation nanocomposites synthesized by solution

-p

intercalation. Chem. Mater. 15, 3208–3213.

Chen, Z., Xing, J., Pu, Z., Wang, Xiangxue, Yang, S., Wei, B., Ai, Y., Li, X., Chen,

re

D., Wang, Xiangke, 2018. Preparation of nano-Fe0 modified coal fly-ash composite and its application for U (VI) sequestration. J. Mol. Liq. 266,

lP

824–833.

na

Dong, H., Deng, J., Xie, Y., Zhang, C., Jiang, Z., Cheng, Y., Hou, K., Zeng, G., 2017. Stabilization of nanoscale zero-valent iron (nZVI) with modified biochar for

ur

Cr(VI) removal from aqueous solution. J. Hazard. Mater. 332, 79–86. Du, J., Bao, J., Lu, C., Werner, D., 2016. Reductive sequestration of chromate by

Jo

hierarchical FeS@Fe0 particles. Water Res. 102, 73–81. Duan, T., Zhang, R., Ling, L., Wang, B., 2016. Insights into the effect of Pt atomic ensemble on HCOOH oxidation over Pt-decorated Au bimetallic catalyst to maximize Pt utilization. J. Phys. Chem. C 120, 2234–2246. Duan, X., Zhang, F.Z., 2009. Intercalative Assembly of Inorganic Supramolecular

34

Materials. Elbasuney, S., 2015. Surface engineering of layered double hydroxide (LDH) nanoparticles for polymer flame retardancy. Powder Technol. 277, 63–73. Erdem Yayayürük, A., Yayayürük, O., 2017. Adsorptive performance of nanosized zero-valent iron for V (V) removal from aqueous solutions. J. Chem. Technol. Biotechnol. 92, 1891–1898.

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Fan, K., Chen, H., Ji, Y., Huang, H., Claesson, P.M., Daniel, Q., Philippe, B., Rensmo, H., Li, F., Luo, Y., 2016. Nickel–vanadium monolayer double hydroxide for efficient electrochemical water oxidation. Nat. Commun. 7, 11981.

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Farrell, J., Kason, M., Melitas, N., Li, T., 2000. Investigation of the long-term performance of zero-valent iron for reductive dechlorination of trichloroethylene.

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Environ. Sci. Technol. 34, 514–521.

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Goh, K.-H., Lim, T.-T., Dong, Z., 2008. Application of layered double hydroxides for removal of oxyanions: a review. Water Res. 42, 1343–1368.

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Gomes, H.I., Jones, A., Rogerson, M., Greenway, G.M., Lisbona, D.F., Burke, I.T., Mayes, W.M., 2017. Removal and recovery of vanadium from alkaline steel slag

ur

leachates with anion exchange resins. J. Environ. Manage. 187, 384–392. Grieger, K.D., Fjordbøge, A., Hartmann, N.B., Eriksson, E., Bjerg, P.L., Baun, A.,

Jo

2010. Environmental benefits and risks of zero-valent iron nanoparticles (nZVI) for in situ remediation: risk mitigation or trade-off? J. Contam. Hydrol. 118, 165–183.

Gupta, V.K., Fakhri, A., Bharti, A.K., Agarwal, S., Naji, M., 2017. Optimization by response surface methodology for vanadium (V) removal from aqueous solutions

35

using PdO-MWCNTs nanocomposites. J. Mol. Liq. 234, 117–123. Han, L., Xue, S., Zhao, S., Yan, J., Qian, L., Chen, M., 2015. Biochar supported nanoscale iron particles for the efficient removal of methyl orange dye in aqueous solutions. PLoS One 10, e0132067. Hashimoto, S., Yamaguchi, A., Fukuda, K., Zhang, S., 2005. Low-temperature synthesis of leucite crystals using kaolin. Mater. Res. Bull. 40, 1577–1583.

ro of

Hidalgo, A., Navas, P., Garcia-Herdugo, G., 1988. Growth inhibition induced by vanadate in onion roots. Environ. Exp. Bot. 28, 131–136.

Ho, Y.-S., McKay, G., 2000. The kinetics of sorption of divalent metal ions onto

-p

sphagnum moss peat. Water Res. 34, 735–742.

Hohenberg, P., Kohn, W., 1964. Inhomogeneous electron gas. Phys. Rev. B 136,

re

864–871.

lP

Hu, B., Ye, F., Jin, C., Ma, X., Huang, C., Sheng, G., Ma, J., Wang, X., Huang, Y., 2017. The enhancement roles of layered double hydroxide on the reductive

na

immobilization of selenate by nanoscale zero valent iron: Macroscopic and microscopic approaches. Chemosphere 184, 408–416.

ur

Hu, B., Ye, F., Ren, X., Zhao, D., Sheng, G., Li, H., Ma, J., Wang, X., Huang, Y., 2016. X-ray absorption fine structure study of enhanced sequestration of U (VI)

Jo

and Se (IV) by montmorillonite decorated with zero-valent iron nanoparticles. Environ. Sci. Nano 3, 1460–1472.

Huang, P., Ye, Z., Xie, W., Chen, Q., Li, J., Xu, Z., Yao, M., 2013. Rapid magnetic removal of aqueous heavy metals and their relevant mechanisms using nanoscale zero valent iron (nZVI) particles. Water Res. 47, 4050–4058.

36

Jin, Y., Yang, C.P., Rui, X.H., Cheng, T., Chen, C.H., 2011. V2O3 modified LiFePO4/C composite with improved electrochemical performance. J. Power Sources 196, 5623–5630. Kattel, S., Yan, B., Chen, J.G., Liu, P., 2016. CO2 hydrogenation on Pt, Pt/SiO2 and Pt/TiO2: Importance of synergy between Pt and oxide support. J. Catal. 343, 115–126. Keränen, A., Leiviskä, T., Salakka, A., Tanskanen, J., 2015. Removal of nickel and

ro of

vanadium from ammoniacal industrial wastewater by ion exchange and adsorption on activated carbon. Desalin. Water Treat. 53, 2645–2654.

-p

Kim, H.-S., Ahn, J.-Y., Kim, C., Lee, S., Hwang, I., 2014. Effect of anions and humic

acid on the performance of nanoscale zero-valent iron particles coated with

re

polyacrylic acid. Chemosphere 113, 93–100.

Kong, F.Y., Li, M., Pan, S.S., Zhang, Y.X., Li, G.H., 2011. Synthesis and thermal

lP

stability of W-doped VO2 nanocrystals. Mater. Res. Bull. 46, 2100–2104.

na

Kong, X., Wang, H., n.d. Removal of vanadium from groundwater by layered double hydroxide supported nanoscale zerovalent iron, in: 17th International Waste

ur

Management and Landfill Symposium 2019. Kresse, G., Furthmüller, J., 1996. Efficiency of ab-initio total energy calculations for

Jo

metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6, 15–50.

Lai, C., Zhang, M., Li, B., Huang, D., Zeng, G., Qin, L., Liu, X., Yi, H., Cheng, M., Li, L., 2019. Fabrication of CuS/BiVO4 (0 4 0) binary heterojunction photocatalysts

with

enhanced

photocatalytic

activity for Ciprofloxacin

degradation and mechanism insight. Chem. Eng. J. 358, 891–902. 37

Li, B., Zhu, J., 2014. Removal of p-chloronitrobenzene from groundwater: effectiveness and degradation mechanism of a heterogeneous nanoparticulate zero-valent iron (NZVI)-induced Fenton process. Chem. Eng. J. 255, 225–232. Li, D., Mao, Z., Zhong, Y., Huang, W., Wu, Y., Peng, P., 2016. Reductive transformation of tetrabromobisphenol A by sulfidated nano zerovalent iron. Water Res. 103, 1–9. Li, J., Li, Y., Meng, Q., 2010. Removal of nitrate by zero-valent iron and pillared

ro of

bentonite. J. Hazard. Mater. 174, 188–193.

Li, Y., Li, J., Zhang, Y., 2012. Mechanism insights into enhanced Cr (VI) removal

-p

using nanoscale zerovalent iron supported on the pillared bentonite by macroscopic and spectroscopic studies. J. Hazard. Mater. 227, 211–218.

re

Liu, A., Liu, J., Han, J., Zhang, W., 2017. Evolution of nanoscale zero-valent iron (nZVI) in water: microscopic and spectroscopic evidence on the formation of

lP

nano-and micro-structured iron oxides. J. Hazard. Mater. 322, 129–135.

na

Liu, A., Liu, J., Zhang, W., 2015. Transformation and composition evolution of nanoscale zero valent iron (nZVI) synthesized by borohydride reduction in static

ur

water. Chemosphere 119, 1068–1074. Liu, H., Zhang, B., Yuan, H., Cheng, Y., Wang, S., He, Z., 2017. Microbial reduction

Jo

of vanadium (V) in groundwater: Interactions with coexisting common electron acceptors and analysis of microbial community. Environ. Pollut. 231, 1362–1369.

Liu, X., Cao, Z., Yuan, Z., Zhang, J., Guo, X., Yang, Y., He, F., Zhao, Y., Xu, J., 2018. Insight into the kinetics and mechanism of removal of aqueous chlorinated nitroaromatic antibiotic chloramphenicol by nanoscale zero-valent iron. Chem. 38

Eng. J. 334, 508–518. Liu, X., Ma, R., Wang, Xiangxue, Ma, Y., Yang, Y., Zhuang, L., Zhang, S., Jehan, R., Chen, J., Wang, Xiangke, 2019. Graphene oxide-based materials for efficient removal of heavy metal ions from aqueous solution: A review. Environ. Pollut. Luciani, S., Cavani, F., Dal Santo, V., Dimitratos, N., Rossi, M., Bianchi, C.L., 2011. The mechanism of surface doping in vanadyl pyrophosphate, catalyst for n-butane oxidation to maleic anhydride: The role of Au promoter. Catal. Today

ro of

169, 200–206.

Lv, D., Zhou, J., Cao, Z., Xu, J., Liu, Y., Li, Y., Yang, K., Lou, Z., Lou, L., Xu, X.,

-p

2019. Mechanism and influence factors of chromium (VI) removal by sulfide-modified nanoscale zerovalent iron. Chemosphere 224, 306–315.

re

Lv, X., Hu, Y., Tang, J., Sheng, T., Jiang, G., Xu, X., 2013. Effects of co-existing ions and natural organic matter on removal of chromium (VI) from aqueous

na

Eng. J. 218, 55–64.

lP

solution by nanoscale zero valent iron (nZVI)-Fe3O4 nanocomposites. Chem.

Mallakpour, S., Dinari, M., Nabiyan, A., 2015. Production of NiAl-layered double hydroxide intercalated with bio-safe amino acid containing organic dianion and

ur

its utilization in formation of LDH/poly (amide-imide) nanocomposites. J. Polym.

Jo

Res. 22, 11.

Monkhorst, H.J., Pack, J.D., 1976. Special points for Brillouin-zone integrations. Phys. Rev. B 13, 5188.

Mthombeni, N.H., Mbakop, S., Ochieng, A., Onyango, M.S., 2016. Vanadium (V) adsorption isotherms and kinetics using polypyrrole coated magnetized natural zeolite. J. Taiwan Inst. Chem. Eng. 66, 172–180. 39

Mu, Y., Wu, H., Ai, Z., 2015. Negative impact of oxygen molecular activation on Cr (VI) removal with core–shell Fe@ Fe2O3 nanowires. J. Hazard. Mater. 298, 1–10. Olfs, H.-W., Torres-Dorante, L.O., Eckelt, R., Kosslick, H., 2009. Comparison of different synthesis routes for Mg–Al layered double hydroxides (LDH): Characterization of the structural phases and anion exchange properties. Appl. Clay Sci. 43, 459–464.

ro of

Pang, H., Wu, Y., Wang, Xiangxue, Hu, B., Wang, Xiangke, 2019. Recent Advances

in Composites of Graphene and Layered Double Hydroxides for Water

-p

Remediation: A Review. Chem. Asian J. 14, 2542–2552.

Parijaee, M., Noaparast, M., Saberyan, K., Shafaie-Tonkaboni, S.Z., 2014. Adsorption

re

of vanadium(V) from acidic solutions by using octylamine functionalized

2237–2244.

lP

magnetite nanoparticles as a novel adsorbent. Korean J. Chem. Eng. 31,

Perdew, J.P., Burke, K., Ernzerhof, M., 1996. Generalized gradient approximation

na

made simple. Phys. Rev. Lett. 77, 3865.

Qian, L., Zhang, W., Yan, J., Han, L., Chen, Y., Ouyang, D., Chen, M., 2017.

ur

Nanoscale zero-valent iron supported by biochars produced at different

Jo

temperatures: Synthesis mechanism and effect on Cr (VI) removal. Environ. Pollut. 223, 153–160.

Qin, H., Li, J., Bao, Q., Li, L., Guan, X., 2016. Role of dissolved oxygen in metal (loid) removal by zerovalent iron at different pH: its dependence on the removal mechanisms. RSC Adv. 6, 50144–50152. Rončević, S., Nemet, I., Ferri, T.Z., Matković-Čalogović, D., 2019. Characterization 40

of nZVI nanoparticles functionalized by EDTA and dipicolinic acid: a comparative study of metal ion removal from aqueous solutions. RSC Adv. 9, 31043–31051. Schiffer, S., Liber, K., 2017. Toxicity of aqueous vanadium to zooplankton and phytoplankton species of relevance to the athabasca oil sands region. Ecotoxicol. Environ. Saf. 137, 1–11. Setshedi, K.Z., Bhaumik, M., Songwane, S., Onyango, M.S., Maity, A., 2013. polypyrrole-organically

modified

montmorillonite

clay

ro of

Exfoliated

nanocomposite as a potential adsorbent for Cr (VI) removal. Chem. Eng. J. 222,

-p

186–197.

Sheng, G., Hu, J., Li, H., Li, J., Huang, Y., 2016a. Enhanced sequestration of Cr(VI)

re

by nanoscale zero-valent iron supported on layered double hydroxide by batch and XAFS study. Chemosphere 148, 227–232.

lP

Sheng, G., Tang, Y., Linghu, W., Wang, L., Li, J., Li, H., Wang, X., Huang, Y., 2016b. Enhanced immobilization of ReO4− by nanoscale zerovalent iron

na

supported on layered double hydroxide via an advanced XAFS approach: Implications for TcO4− sequestration. Appl. Catal. B Environ. 192, 268–276.

ur

Shi, L., Zhang, X., Chen, Z., 2011. Removal of chromium (VI) from wastewater using

Jo

bentonite-supported nanoscale zero-valent iron. Water Res. 45, 886–892. Sirviö, J.A., Hasa, T., Leiviskä, T., Liimatainen, H., Hormi, O., 2016. Bisphosphonate nanocellulose in the removal of vanadium (V) from water. Cellulose 23, 689–697. Sturini, M., Rivagli, E., Maraschi, F., Speltini, A., Profumo, A., Albini, A., 2013. Photocatalytic reduction of vanadium (V) in TiO2 suspension: Chemometric 41

optimization and application to wastewaters. J. Hazard. Mater. 254, 179–184. Su, C., Puls, R.W., 2001. Arsenate and arsenite removal by zerovalent iron: effects of phosphate, silicate, carbonate, borate, sulfate, chromate, molybdate, and nitrate, relative to chloride. Environ. Sci. Technol. 35, 4562–4568. Sun, Y., Ding, C., Cheng, W., Wang, X., 2014. Simultaneous adsorption and reduction of U (VI) on reduced graphene oxide-supported nanoscale zerovalent

ro of

iron. J. Hazard. Mater. 280, 399–408. Suresh, S., Karthikeyan, S., Jayamoorthy, K., 2016. Effect of bulk and nano-Fe2O3 particles on peanut plant leaves studied by Fourier transform infrared spectral

-p

studies. J. Adv. Res. 7, 739–747.

Tran, H.N., Nguyen, D.T., Le, G.T., Tomul, F., Lima, E.C., Woo, S.H., Sarmah, A.K.,

re

Nguyen, H.Q., Nguyen, P.T., Nguyen, D.D., 2019. Adsorption mechanism of hexavalent chromium onto layered double hydroxides-based adsorbents: A

lP

systematic in-depth review. J. Hazard. Mater. 373, 258–270.

na

Treybal, R.E., 1980. Mass transfer operations. New York 466. Wan, H.-Q., Ning, S.-M., 2010. Kinetics study on adsorption of vanadium by

ur

ion-exchange resin. Min. Metall. Eng. 30, 73–76. Wang, Y., Peng, M., Ye, C., Gan, C., Zhang, J., Guo, C., 2019. Enhanced catalytic

Jo

performance

of

Pd-Ga

bimetallic

catalysts

for

2-ethylanthraquinone

hydrogenation. Appl. Organomet. Chem. 33, e5076.

Wilson, S.A., Weber, J.H., 1979. An EPR study of the reduction of vanadium (V) to vanadium (IV) by fulvic acid. Chem. Geol. 26, 345–354. Wu, X., Tao, Y., Dong, L., Wang, Z., Hu, Z., 2005. Preparation of VO2 nanowires and 42

their electric characterization. Mater. Res. Bull. 40, 315–321. Yang, J., Teng, Y., Wu, J., Chen, H., Wang, G., Song, L., Yue, W., Zuo, R., Zhai, Y., 2017. Current status and associated human health risk of vanadium in soil in China. Chemosphere 171, 635–643. Yang, K., Yan, L., Yang, Y., Yu, S., Shan, R., Yu, H., Zhu, B., Du, B., 2014. Adsorptive removal of phosphate by Mg-Al and Zn-Al layered double hydroxides: Kinetics, isotherms and mechanisms. Sep. Purif. Technol. 124,

ro of

36–42.

Yu, S., Wang, Xiangxue, Liu, Yafeng, Chen, Z., Wu, Y., Liu, Yue, Pang, H., Song, G.,

-p

Chen, J., Wang, Xiangke, 2019. Efficient removal of uranium(VI) by layered

double hydroxides supported nanoscale zero-valent iron: A combined

re

experimental and spectroscopic studies. Chem. Eng. J. 365, 51–59. Zhang, M., Lai, C., Li, B., Huang, D., Zeng, G., Xu, P., Qin, L., Liu, S., Liu, X., Yi,

lP

H., 2019. Rational design 2D/2D BiOBr/CDs/g-C3N4 Z-scheme heterojunction photocatalyst with carbon dots as solid-state electron mediators for enhanced

na

visible and NIR photocatalytic activity: Kinetics, intermediates, and mechanism insight. J. Catal. 369, 469–481.

ur

Zhang, R., Leiviskä, T., 2020. Surface modification of pine bark with quaternary

Jo

ammonium groups and its use for vanadium removal. Chem. Eng. J. 385, 123967.

Zhang, R., Leiviskä, T., Tanskanen, J., Gao, B., Yue, Q., 2019. Utilization of ferric groundwater treatment residuals for inorganic-organic hybrid biosorbent preparation and its use for vanadium removal. Chem. Eng. J. 361, 680–689. Zhang, Y., Pan, A., Liang, S., Chen, T., Tang, Y., Tan, X., 2014. Reduced graphene 43

oxide modified V2O3 with enhanced performance for lithium-ion battery. Mater. Lett. 137, 174–177. Zhou, J., Su, Y., Zhang, J., Xu, X., Zhao, J., Qian, G., Xu, Y., 2015. Distribution of OH bond to metal-oxide in Mg3-xCaxFe-layered double hydroxide (x=0-1.5): Its

Jo

ur

na

lP

re

-p

ro of

role in adsorption of selenate and chromate. Chem. Eng. J. 262, 383–389.

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