Combined Electrosorption and Chemisorption of As (V) in Water by

Research Article pubs.acs.org/journal/ascecg

Combined Electrosorption and Chemisorption of As(V) in Water by Using Fe-rGO@AC Electrode Min Dai,†,‡ Man Zhang,§ Ling Xia,† Yanmei Li,∥ Yanyan Liu,§ and Shaoxian Song*,†,§ †

Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China ‡ Doctorado Institucional de Ingeniería y Ciencia de Materiales, Universidad Autonoma de San Luis Potosi, Av. Sierra Leona 530, San Luis Potosi, 78210, Mexico § School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China ∥ Engineering Division, Universidad de Guanajuato, Ex Hacienda de San Matias S/N, Guanajuato, Guanajuato 36000, Mexico S Supporting Information *

ABSTRACT: A novel method of arsenic removal from water, combined electrosorption and chemisorption (CEC), has been presented in this work in order to deeply eliminate arsenic in water. This method was proposed using a mixture of activated carbon (AC) and reduced graphene−iron composite (Fe-rGO) as the electrode (Fe-rGO@AC). The results showed that the AC had a better electrosorption performance, while Fe-rGO was more suitable for chemisorption of As(V). The Fe-rGO@AC electrode combined the electrosorption with chemisorption. It was confirmed that the combination accelerated the adsorption rate of Fe-rGO. The AC on the electrode accelerated the mobility of arsenic ions and concentrated them in the electrical double layer (EDL) by means of electrostatic force underpotential. Meanwhlie, the concentrated As(V) ions reacted with Fe-rGO, contributing to a higher arsenic chemisorption on Fe-rGO. Therefore, the combination of electrosorption with chemisorption was an effective process for arsenic removal. KEYWORDS: Electrosorption, Chemisorption, Combination, As(V), Fe-rGO@AC

■

INTRODUCTION Arsenic pollution in groundwater has gained great attention due to the carcinogenic threat to human beings. In order to avoid the health risk to mankind, the WHO recommends that the maximum concentration of As is 10 μg/L.1,2 Adsorption is among the most common methods to control the distribution of arsenic due to simple operation, low cost, and good regeneration capability, as well as no chemical addition. The basic adsorption mechanism is based on the ligand exchange and the formation of surface complexes.3,4 Owing to the good affinity between arsenic and iron, alumnium, zirconium, lanthanum, and cerium, these metallic elements are involved in most of the high efficient adsorbents for arsenic adsorption. Except for the traditional metal oxides,5−8 the varied modification and synthesis of the novel adsorbents are also based on these metallic elements. It was reported that arsenic removal increased significantly after the modification and synthesis.9−11 The layered double hydroxides (LDH) synthesized with Fe and Al showed a tremendous arsenic adsorption capacity, even above 100 mg/g.12−14 The synthesized CeO2-ZrO2 nanospheres showed a high adsorption capacity of 25.1 and 9.2 mg/g for As(V) and As(III) at the equilibrium concentration of 10 μg/L.15 Electrosorption, or capacitive deionization (CDI), was an eco-friendly technology for desalination of water. The charged © 2017 American Chemical Society

ions in water are attracted to the polarized electrode by electrostatic force and concentrated in the electric double layer (EDL). In the regeneration process, the electro-adsorbed ions were driven by the reversed potential and released to the water.16,17 Because the electrochemical reactions on the electrode consume the electric charge, the electrosoprtion efficiency decreases. The reactions need to be minimized in the CDI process.16 Although the carbon materials are not effective in arsenic adsorption, they are promising in electrosorption as the electrode due to the high surface area, good conductivity, and high chemical stability.18−20 Studies have shown that the arsenic ions were effectively separated by the electrostatic force. Meanwhile, the positive potential promoted the oxidation of As(III) to As(V), serving to reduce the toxicity of the solution.21−24 What’s more, the carbon electrode modified with iron oxide was determined as a potential electrode for CDI.25 Considering the affinity between arsenic and iron, as well as the research above, it was assumed that the electrosorption of arsenic could be combined with chemisorption using the carbon electrode modified by iron. In this study, the reduced Received: February 28, 2017 Revised: May 26, 2017 Published: June 17, 2017 6532

DOI: 10.1021/acssuschemeng.7b00633 ACS Sustainable Chem. Eng. 2017, 5, 6532−6538

Research Article

ACS Sustainable Chemistry & Engineering

circuited in the cell. After the electrosorption for 1 h, a reversed potential was applied to the electrode for another hour. The operation was repeated three times. For the Fe-rGO@AC electrode, the regeneration process was performed by applying a reversed potential for 10 min after the electrosorption of As(V) for 2 h. Then, 200 mL of 0.01 mol/L NaOH was fed and electro-adsorbed in the cell for 1 h, following by desorption with 200 mL of Milli-Q water for 10 min by a reversed potential. After that, another electrosorption experiment was performed. As(V) concentration was 5 mg/L. The applied potential was 1.2 V. The concentration of arsenic was determined by a Thermo Scientific Orion AquaMate 8000 UV−vis spectrophotometer at 840 nm according to the molybdenum blue colorimetric method.27,28 The electrosorption capacity (q), defined as the quantity of arsenic ions adsorbed per unit mass of mixture on the anode, was calculated as follows:

graphene−iron composite (Fe-rGO) was synthesized and chosen as an electrode to explore the feasibility of CEC, as well as the mixture of Fe-rGO and activited carbon (AC). A conventional recycling electrosorption process was conducted in both open circuit and polarized experiments to investigate the combination performance.

■

MATERIALS AND METHODS

Materials. The flake graphite was obtained from Sanchaya (Yichang), China. The wood AC powder with a BET surface area of 1331 m2/g was from the Sinopharm Chemical Reagent Company, China. Titanium plate was purchased from Yongsheng Company, China. Except for the polyvinylidene fluoride (PVDF) from Sigma-Aldrich, other analytical reagents were purchased from the Sinopharm Chemical Reagent Company, China. The water used in this work was produced by a Millipore Milli-Q Direct 8/16 water purification system. Preparation of Fe-rGO. The graphene oxide (GO) suspension was prepared from graphite oxide (GrO), which was produced by the Hummers method.26 Specific preparation can be found in the Supporting Information. The prepared GO suspension was sonicated on an Elma P30HSE ultrasonic cleaner (320W, 37 kHz) at 300 rev/ min for 30 min before use. And then 0.825 g of FeSO4·7H2O was dissolved in 60 mL of GO suspension. The mixture was stirred and sealed at 90 °C in a water bath for 12 h. After the reaction, the solution was allowed to cool to room temperature. Finally, the production was filtrated though a 0.22 μm filter, rinsed with water, and freezed-dried. Fabrication of AC Electrodes. The electrode was a mixture of AC powder, conductive carbon black (CCB), and Fe-rGO with PVDF as a binder. To achieve a good adhesion between the carbon materials, PVDF was dissolved in the N-dimethylacetamide (DMAc) and then added into the carbon mixture. The mass ratios of AC/CCB/PVDF and Fe-rGO/CCB/PVDF were 8:1:1 and 8:1:0.5, respectively. The mixture was stirred homogeneously before spreading on a titanium plate (60 mm × 40 mm). Afterward, the electrodes were heated at 60 °C for 12 h to remove the residual organic solvent. The prepared electrodes were immersed in the water before use. Electrosorption of As(V). The tests of the electrosorption of As(V) from water were performed in a self-made CDI system, which was schematically represented in Figure 1. Three electrodes with a

q=

(C0 − C) × V m

(1)

where C0 and C were the As(V) concentrations at the beginning and at any time in the electrosorption, respectively; V was the volume of the solution. Measurements. The specific surface area of the AC was calculated using the Brunauer−Emmett−Teller (BET) method by N 2 adsorption−desorption isotherm at 77 K on an ASAP 2020 Physisorption (Micromeritics) device. The X-ray diffraction (XRD) measurements were carried out on a D8 Advance X-ray diffractometer (Bruker-AXS) with Cu Kα, using an operation voltage of 40 kV. The surface chemical composition of the Fe-rGO was analyzed by X-ray photoelectron spectroscopy (XPS; 250XI, Thermo Fisher). The high resolution spectra were obtained with 20 eV pass energy and energy steps of 0.1 V. The peak at 284.6 eV of the C 1s spectrum was used as the binding energy reference. The atomic force microscope (AFM) images of GO were obtained with a MultiMode 8 AFM (Bruker) with the peak force tapping-mode. GO was dropped on a freshly cleaved mica substrate and dried at 60 °C for 2 h. The microstructure of the synthesized Fe-rGO was obtained on the JEOL JSM-6400 scanning electron microscope (SEM), and the distribution of iron on the materials was recorded on an XFlash-4010 energy dispersive X-ray detector (EDX). The electrochemical characterizations of the electrodes were analyzed by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using a VersaStat 4 electrochemical workstation (AMETEK) in a three-electrode cell system. Next, 1.0 mol/L Na2SO4 was used as an electrolyte solution. The electrodes, including AC, Fe-rGO, and Fe-rGO@AC, with a 1 cm2 surface area were used as the working electrodes, while the platinum electrode and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The CV curve was obtained in a potential range of −0.5 to 0.5 V with a scan rate of 5 mV/s. The EIS measurement was performed in a frequency range of 0.01−10000 Hz.

■

RESULTS AND DISCUSSIONS Characterization of the Carbon Materials. Figure 2 illustrates the XRD patterns of the flake graphite, the GrO, and the final synthesized product, Fe-rGO. The patterns of the raw material, flake graphite, exhibited a strong and sharp peak at 2θ of 26.5° and a weak peak at 2θ of 54.5°. The peaks corresponded to the (002) and (004) hexagonal lattice planes of the natural graphite, indicating a high purity of the graphite.29,30 After the oxidation, a clear (001) reflection was observed at a 2θ of 10.3° in the patterns of the GrO. The disappearance of (002) and (004) reflections indicated the destruction of the structure of graphite. According to the Bragg equation, the interlayer distance of GrO was 8.58 Å, which was higher than that of graphite (3.36 Å). The increased d-spacing suggested the formation of the hydroxyl, epoxy, and carboxyl groups on the graphite sheets. The (100) reflection implied

Figure 1. Schematic representation of the electrosorption of As(V) in the CDI process. double-sided cathode in the middle were assembled in the poly(methyl methacrylate) cell with a gap distance of 1 mm. A total of 200 mL of As(V) solution was continuously recycled in the system at a flow rate of 50 rpm/min by a peristaltic pump (BT600−2J). The potential applied to the electrode was 1.2 V. The changes of conductivity and pH in the solutions were monitored frequently in the electrosorption. The regeneration of the AC electrode was conducted by a charge/ reverse-charge process in the CDI system. A 20 mg/L As(V) solution 6533


Research Article


Figure S2. The EDX spectra confirmed the existence of C, Fe, and O elements on the material, and Figure S3c shows iron distributed evenly on the surface, suggesting the Fe-rGO was a homogeneous composite. The electrochemical performance of the Fe-rGO and AC electrodes was studied by CV and EIS, in Figure 3. The curve of

Figure 2. XRD patterns of the carbon materials.

that the GO had a turbostratic structure.31 All characteristic peaks of Fe-rGO patterns were consistent with that in the standard card (JCPDS 29-713), which were assigned to goethite.31,32 The peaks were broad and of low intensity due to the poor crystallinity along the stacking direction. The existence of goethite suggested that the Fe2+ was oxidized to Fe3+ after synthesis. Hence, it was deduced that parts of the GO were reduced to rGO. No obvious peaks corresponding to graphite or hydrothermally reduced graphene were observed, such as the (002) reflection, indicating the restacking of graphene sheets was prevented as a result of the deposition of goethite.33 Therefore, the goethite might coat on both sides of the graphene nanosheets. XPS analysis was conducted to obtain further information on the surface chemical composition of the Fe-rGO. Figure S1a in the Supporting Information (SI) shows that the C 1s spectrum of Fe-rGO was deconvoluted into four peaks. The peak located at 284.61 eV was attributed to C−C/CC, which was assigned to graphitic sp2 carbons. The other three peaks at 285.75, 287.66, and 289.32 eV indicated the existence of the oxygenate groups of C−O, CO, and O−CO.34 Therefore, the oxygenate groups were not reduced thoroughly. The Fe 2p spectrum consisted of the peaks of Fe 2p3/2, Fe 2p1/2, and the associated satellites, which centered at 711.58, 724.97, 719.55, and 733.33 eV. The peaks corresponded to the characteristic positions of Fe3+.35,36 The spectrum confirmed the absence of Fe2+, because no peak was located at 709−710 eV.36,37 The results suggested that all Fe2+ was oxidized to Fe3+, in good agreement with XRD results. The AFM image and the height profile of the GO sheets were shown in Figure S2. The thickness of the GO sheets was similar according to the color contrast. And it was 1.5 nm for a given cross-section analysis. As reported, the thickness of a single layer graphene with oxidative functional groups was about 0.78 nm.38 Hence, the GO sheets were of 1−2 layers in this study. And the size of the GO sheets was less than 2 μm. The morphology of the Fe-rGO and the distribution of iron elements on the surface were analyzed by SEM and EDX mapping, respectively, as shown in Figure S3. During the synthesis, the negative charged GO sheets attracted Fe2+ on the surface and edges. The Fe2+ reduced the oxygenate functional groups on GO, resulting in the deposition of nanoparticle crystal goethite on the self-reduced rGO sheets.39 Hence, the rGO sheets were the matrix of the goethite nanoparticles. The rGO sheets covered by goethite overlap each other to form the multilayer structure. The size of Fe-rGO was about 40 μm in Figure S3a, much larger than that of the GO sheets seen in

Figure 3. (a) CV curves of AC and Fe-rGO electrodes at a scan rate of 5 mV/s. (b) Nyquist plots of AC and Fe-rGO electrodes in 1 M NaSO4 solution.

Fe-rGO was similar to that of AC, indicating a typical EDL capacitor formed on the electrode surface. There were no evident oxidation/reduction peaks in the potential range for the Fe-rGO electrode. The specific capacitance of the electrode was 57.19 and 47.76 F/g for AC and Fe-rGO, respectively. The results were calculated on the basis of the following expression:40 Cs =

∫V

Vc

a

I dV /2(Vc − Va)mv

(7)

where Cs is the specific capacitance of the individual sample, Va and Vc are the cutoff potentials in cyclic voltammetry, I is the instantaneous current, m is the mass of the sample, and v is the potential scan rate. Figure 3b illustrated the Nyquist plots of the electrodes. An equivalent circuit was used to calculate the parameters. The x intercept of the plot at high frequency indicated the electrolyte resistance (R1). The semicircle corresponded to the polarization resistance, implying the charge transfer limit on the electrode (R2). The nearly vertical lines in the low frequency zones represented the Warburg impedance (W1), corresponding to the ion-diffusion process. CPE was used to overcome the nonideal behavior of the electrode. The value of R2 was calculated as 64.79 and 8.62Ω for the AC and Fe-rGO electrodes, respectively, suggesting that the charge transfer 6534


Research Article


electrode still worked even at 240 min, in line with its chemisorption. It was concluded that the electrosorption of AC electrode in the EDL was much higher than that of the Fe-rGO electrode, resulting from the relatively high specific surface area. Relatively, the material Fe-rGO was better in chemisorption due to the reaction of arsenate and iron and the formation of a surface complex on the goethite. The pseudo-first-order kinetics model and the pseudosecond-order kinetics model were used to fit the kinetics of As(V) on the two electrodes.

resistance in the Fe-rGO was much smaller than that of AC (Table S1). The surface characterization of the Fe-rGO was analyzed by the nitrogen adsorption isotherm, and the results were compared with AC, listed in Table 1. The specific surface Table 1. Surface Characterization of the Carbon Materials surface area (m2/g)

pore volume (cm3/g)

sample

SBET

VMicro

VMeso

VTotal

AC Fe-rGO

1396.11 247.34

0.6022 0.0081

0.2674 0.5577

0.8696 0.5658

area of AC was 5 times higher than that of Fe-rGO. In addition, the two materials differed in the pore distribution. Micropores were predominant in the AC. However, most pores in the FerGO were mesoporous, contributing to the connection of goethite and the reduced GO sheets. Electrosorption Kinetics. The electrosorption behavior of the Fe-rGO and AC electrodes was investigated in As(V) solution with an initial concentration of 10 ppm. The applied voltage was 1.2 V. When the potential was applied on the electrode, the electrosorption capacity was higher than the adsorption capacity of the electrode in the open circuit, suggesting that except for the physisorption (AC)41 and chemisorption (Fe-rGO) of As(V), the electrodes worked like a capacitor, concentrating the arsenic ions to the EDL of the anode, shown in Figure 4a,b. Although the Fe-rGO electrode showed a higher electrosorption capacity, the increase after applying potential was almost half that of the AC electrode, implying a better electrosorption performance of the AC electrode. In addition, the AC electrode reached equilibrium before 120 min, while the electrosorption of the Fe-rGO

ln(qe − qt) = ln qe − k1t

(2)

t 1 t = + 2 qt qe k 2qe

(3)

where qt (mg/g) is the adsorption capacity at time t (min), k1 (min−1) is the pseudo-first-order adsorption rate constant, k2 (g·(mg·min)−1) is the pseudo-second-order adsorption rate constant, and qe (mg/g) is the equilibrium adsorption capacity. The calculated parameters were summarized in Table 2. In contrast to that of the AC electrode, the adsorption of the FeTable 2. Adsorption Kinetic Parameter for As(V) Adsorption on the Electrodes pseudo-first-order kinetic model AC no potential AC potential Fe-rGO no potential Fe-rGO potential

pseudo-first-order kinetic model qe

k2

R2

0.9566

0.4898

0.0220

0.9533

0.0517

0.9841

7.67

0.0088

0.9422

9.78

0.0187

0.9936

12.31

0.0015

0.9939

13.00

0.0184

0.9748

16.16

0.0012

0.9855

qe

k1

R

0.32

0.0154

6.57

2

rGO electrode fitted well with the pseudo-second-order kinetics model. The adsorption rates k1 and k2 showed the electrosorption of AC was faster than that of Fe-rGO when applied the potential. Since the ion storage properties of the electrode for CDI were to adsorb the ions in the EDL of the electrode, the concentration of ions in the EDL was much higher than that in the bulk solution. Considering the chemisorption capacity increased with the equilibrium concentration before reaching the equilibrium, it was assumed that the electrosorption would promote the chemisorption of the electrode if they were combined together. Meanwhile, the fact that the charged ions were attracted by the electrostatic force would contribute to a faster adsorption. Although the Fe-rGO electrode could realize the combination, the improvement in arsenic removal was not satisfied, shown in Figure 4a. On the basis of the results above, the combination could also be realized by the mixture of AC and Fe-rGO as the electrode, since AC showed a better performance in electrosorption, and Fe-rGO was better in chemisorption. Combination of Electrosorption and Chemisorption. The AC and Fe-rGO were mixed at different ratios, and the results are illustrated in Figure 5a. The elctrosorption time was fixed at 120 min. The electrosorption capacity of the mixture electrode was higher than that of the AC electrode, suggesting the chemisorption worked during the electrosorption. Assuming that the adsorbing capacity (q) of the mixture was equal to

Figure 4. Electrosorption kinetics of As(V) on the electrode with and without potential (As(V) 10 ppm, 1.2 V). 6535


Research Article


suggested that the adsorption rate of the electrode underpotential followed as AC > Fe-rGO@AC > Fe-rGO. The adsorption isotherms of As(V) on the electrodes were modeled using both the Langmuir and the Freundlich equations.

Ce C 1 = + e qe θb θ log qe = log KF +

(5)

1 log Ce n

(6)

where Ce (mg/L) is the equilibrium adsorbate concentration, θ (mg/g) is the maximum adsorption capacity, and b (L/mg) is the equilibrium adsorption constant. KF (mg1−1/nL1/ng−1) and n are the Freundlich constants. Figure 6 depicts the experimental data and the fitting line of the models above. The results showed that the adsorption of

Figure 5. (a) Electrosorption capacity on the electrode as a function of the ratio of AC and Fe-rGO (As(V) 10 ppm, 1.2 V). (b) Electrosorption kinetics of As(V) on the Fe-rGO@ AC electrode.

the combination of electrosorption (q 1 ) of AC and chemisorption (q2) of Fe-rGO: q = q1 + q2 (4)

Figure 6. Adsorption isotherm of As(V) on the electrode with and without potential.

Then, q2 was roughly calculated as 24.96, 23.21, 29.64, and 11.44 mg/g with an increasing ratio of AC and Fe-rGO if we fixed the value of q1. The result was higher than the independent chemisorption of the Fe-rGO, shown in Figure 4a. Therefore, the combination was feasible with the mixture electrode. The concentration of As(V) on the AC promoted the chemisorption of Fe-rGO on the electrode. However, the massive Fe-rGO would hinder the electrosorption of AC, resulting in less arsenic retention on the electrode. Considering the removal efficiency, the optimal AC/Fe-rGO ratio in this study was 5:1, and it was denoted as Fe-rGO@AC. The electrochemical performance of the Fe-rGO@AC electrode was analyzed by CV and EIS, shown in Figure S4. The CV curve exhibited a typical capacitive behavior of the electrode. No obvious redox reaction was found in the potential range. The equivalent circuit was used to calculate R2 from the Nyquist plots. The results were compared with those of the AC and Fe-rGO electrodes, listed in Table S1. The specific capacitance of Fe-rGO@AC was the highest as compared with the other two electrodes. Also, the value of R2 was smaller than that of the AC electrode, indicating that the addition of Fe-rGO decreased the charge transfer limit of the electrode. Thus, the combination increased the electrochemical performance of the electrode as compared to the AC electrode. The kinetics of As(V) on the Fe-rGO@AC electrode are shown in Figure 5b, and the data were fitted with the pseudofirst and second-order kinetics models. The regression coefficient was 0.9647 and 0.9810, respectively. Compared with the AC, Fe-rGO’s k1 value of 0.0517 and k2 value of 0.0059

the Fe-rGO@AC with and without potential fitted well with the Langmuir model, whereas the Freundlich model was more suitable for the AC electrode. The Langmuir model indicated a monolayer coverage of arsenic due to the chemisorption of FerGO. And the concentration of As(V) in the EDL was better evaluated by the Freundlich model. The CDI performance of the Fe-rGO@AC was investigated and compared with that of AC. The electrosorption capacity of the AC-Fe-rGO was higher than that of AC, and the advantage was more obvious at a low arsenic concentration. The results agreed well with the hypothesis. On account of the storage of As(V) in the EDL by AC underpotential, the As(V) concentration around the electrode was much higher than that in the bulk solution. The local concentrated ions contributed to a higher chemisorption of Fe-rGO on the electrode surface. Compared with the adsorption capacity of the Fe-rGO@AC electrode in the open circuit, it was concluded that the CEC dramatically increased the removal of As(V), especially at a low arsenic concentration. The morphology of the Fe-rGO@AC electrode is shown in Figure 7. The carbon materials connected to each other with some macropores on the surface, facilitating the entrance of ions. Despite the different size, the EDX-mapping image of Fe suggested the Fe-rGO distributed evenly on the electrode. The C element was observed in the location of Fe, suggesting a thin covering of goethite on the Fe-rGO. Both AC and Fe-rGO contained As after the electrosorption. Considering the weak connection between AC and arsenic, the detected arsenic on 6536


Research Article


regeneration. Considering the reversibility of the AC electrode, the electrosorption might contribute to AC on the electrode after regeneration. The 20% decrease was due to the failure of desorption of arsenic on Fe-rGO. Further study on desorption needs to be carried out.

■

CONCLUSIONS The CEC of arsenic could be realized with the Fe-rGO@AC electrode. The charged ions (As(V)) were attracted to the anode and concentrated in the EDL mainly by the electrosorption of AC. Meanwhile, the Fe-rGO on the electrode chemisorbed the concentrated As(V) ions, contributing to a higher arsenic chemisorption. The combination accelerated the adsorption rate of Fe-rGO by applying the electrostatic force on the charged ions. The storage of arsenic ions in EDL and promotion of the chemisorption on Fe-rGO makes CEC a promising process in arsenic removal at low concentrations.

Figure 7. EDX-mapping of AC-Fe-rGO electrode after arsenic electrosorption.

■

ASSOCIATED CONTENT

S Supporting Information *

AC was attributed to the arsenic retention in the EDL. It was intuitive that the arsenic density on the Fe-rGO was higher than that on the AC, implying the Fe-rGO had a higher arsenic adsorption capacity. In this study, the electrosorption capacity of the Fe-rGO@ AC electrode was a little poorer as compared with that of the Fe-rGO electrode. But Fe-rGO@AC was more suitable in the process. In the case of Fe-rGO as the electrode, the electrosorption capacity was only 1.32 times as high as that in the open circuit, while the increase was 9.67 times higher on the Fe-rGO@AC electrode. By introducing AC to Fe-rGO, the chemisorption of Fe-rGO was promoted, and the adsorption rate was accelerated. In addition, the AC was much cheaper than graphene. Therefore, it was reasonable to mix Fe-rGO with AC to form the new material, Fe-rGO@AC, as the CEC electrode. Considering that the Fe-rGO@AC electrode only contained 13.68% Fe-rGO, which was responsible for arsenic adsorption, it was easy to understand that the Fe-rGO@AC had a lower adsorption capacity than Fe-rGO. However, it was possible that the combination showed a higher arsenic adsorption than Fe-rGO if the content of AC decreased a little bit properly, or the electrochemical performance of AC was improved. Further investigation can be focused on these aspects. Recycling of the Electrodes. The regeneration process of the AC electrode was conducted by charge/reverse-charge process in the CDI system. As illustrated in Figure S5a, the arsenic concentration decreased during the electrosorption process. Then, it increased sharply in the first 10 min after applying a reversed potential. Later, the concentration decreased again in the following 50 min. The results indicated that the AC electrode was recovered in 10 min, followed by readsorption of As(V) in another 50 min on the opposite electrode. Except for the first electrosorption, no obvious variation of As(V) concentration was observed in the next three recycles. Therefore, the electrosorption/desorption of As(V) on the AC electrode was a reversible process. The regeneration of the Fe-rGO@AC electrode was not as successful as that of the AC electrode, shown in Figure S5b. The use of the concentrated OH− by AC to desorb arsenic on the goethite was not satisfied. After the regeneration, the electrosorption capacity decreased 20% as compared to the first one. The decrease was not significant for the second

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00633. Description of the reparation of GO suspension, XPS analysis of Fe-rGO, AFM image of the GO, SEM image and EDX spectrum of Fe-rGO, figure of electrochemistry performance of Fe-rGO@AC electrode, figure of regeneration of the electrodes, table of electrochemical results of the electrodes (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shaoxian Song: 0000-0001-7278-7875 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial support for this work from the National Natural Science Foundation of China under the projects no. 51474167 and no. 51674183 is gratefully acknowledged. Also, M.D. would like to thank the Consejo Nacional de Ciencia y Tecnologia of Mexico for offering her the scholarship no. 393918.

■

REFERENCES

(1) Singh, R.; Singh, S.; Parihar, P.; Singh, V. P.; Prasad, S. M. Arsenic contamination, consequences and remediation techniques: A review. Ecotoxicol. Environ. Saf. 2015, 112, 247−270. (2) Guidelines for Drinking Water Quality, Fourth ed.; World Health Organisation: Geneva, Switzerland, 2011. (3) Müller, K.; Ciminelli, V. S. T.; Dantas, M. S. S.; Willscher, S. A comparative study of As(III) and As(V) in aqueous solutions and adsorbed on iron oxy-hydroxides by Raman spectroscopy. Water Res. 2010, 44 (19), 5660−5672. (4) Fendorf, S.; Eick, M. J.; Grossl, P.; Sparks, D. L. Arsenate and chromate retention mechanisms on goethite. 1. Surface structure. Environ. Sci. Technol. 1997, 31 (2), 315−320. (5) Giménez, J.; Martínez, M.; de Pablo, J.; Rovira, M.; Duro, L. Arsenic sorption onto natural hematite, magnetite, and goethite. J. Hazard. Mater. 2007, 141 (3), 575−580. 6537


Research Article


carbon in the presence of other heavy metals. Energy Fuels 2010, 24 (6), 3415−3421. (25) Zafra, M. C.; Lavela, P.; Rasines, G.; Macías, C.; Tirado, J. L.; Ania, C. O. A novel method for metal oxide deposition on carbon aerogels with potential application in capacitive deionization of saline water. Electrochim. Acta 2014, 135, 208−216. (26) Peng, W.; Li, H.; Liu, Y.; Song, S. Comparison of Pb(II) adsorption onto graphene oxide prepared from natural graphites: Diagramming the Pb(II) adsorption sites. Appl. Surf. Sci. 2016, 364, 620−627. (27) Arsenic by the molybdenum blue method. Annual Book of ASTM Standards; ASTM: Philadelphia, 1982. (28) Dhar, R. K.; Zheng, Y.; Rubenstone, J.; Van Geen, A. A rapid colorimetric method for measuring arsenic concentrations in groundwater. Anal. Chim. Acta 2004, 526 (2), 203−209. (29) Sun, G.; Li, X.; Qu, Y.; Wang, X.; Yan, H.; Zhang, Y. Preparation and characterization of graphite nanosheets from detonation technique. Mater. Lett. 2008, 62 (4−5), 703−706. (30) Benjwal, P.; Kumar, M.; Chamoli, P.; Kar, K. K. Enhanced photocatalytic degradation of methylene blue and adsorption of arsenic(iii) by reduced graphene oxide (rGO)−metal oxide (TiO 2 /Fe 3 O 4) based nanocomposites. RSC Adv. 2015, 5 (89), 73249− 73260. (31) Xu, H.; Hu, Z.; Lu, A.; Hu, Y.; Li, L.; Yang, Y.; Zhang, Z.; Wu, H. Synthesis and super capacitance of goethite/reduced graphene oxide for supercapacitors. Mater. Chem. Phys. 2013, 141 (1), 310−317. (32) Shou, Q.; Cheng, J.; Zhang, L.; Nelson, B. J.; Zhang, X. Synthesis and characterization of a nanocomposite of goethite nanorods and reduced graphene oxide for electrochemical capacitors. J. Solid State Chem. 2012, 185, 191−197. (33) Yuan, X.; Wang, Y.; Wang, J.; Zhou, C.; Tang, Q.; Rao, X. Calcined graphene/MgAl-layered double hydroxides for enhanced Cr(VI) removal. Chem. Eng. J. 2013, 221, 204−213. (34) Shin, H. J.; Kim, K. K.; Benayad, A.; Yoon, S. M.; Park, H. K.; Jung, I. S.; Jin, M. H.; Jeong, H. K.; Kim, J. M.; Choi, J. Y.; et al. Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance. Adv. Funct. Mater. 2009, 19 (12), 1987−1992. (35) Mäkie, P.; Persson, P.; österlund, L. Adsorption of trimethyl phosphate and triethyl phosphate on dry and water pre-covered hematite, maghemite, and goethite nanoparticles. J. Colloid Interface Sci. 2013, 392 (1), 349−358. (36) Mesquita, A. M.; Guimarães, I. R.; Castro, G. M. M.; de Gonçalves, M. A.; Ramalho, T. C.; Guerreiro, M. C. Boron as a promoter in the goethite (α-FeOOH) phase: Organic compound degradation by Fenton reaction. Appl. Catal., B 2016, 192, 286−295. (37) Yamashita, T.; Hayes, P. Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci. 2008, 254 (8), 2441− 2449. (38) Fan, X.; Peng, W.; Li, Y.; Li, X.; Wang, S.; Zhang, G.; Zhang, F. Deoxygenation of exfoliated graphite oxide under alkaline conditions: a green route to graphene preparation. Adv. Mater. 2008, 20 (23), 4490−4493. (39) Wang, Z.; Liu, C. J. Preparation and application of iron oxide/ graphene based composites for electrochemical energy storage and energy conversion devices: Current status and perspective. Nano Energy 2015, 11, 277−293. (40) Fan, Z.; Yan, J.; Zhi, L.; Zhang, Q.; Wei, T.; Feng, J.; Zhang, M.; Qian, W.; Wei, F. A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors. Adv. Mater. 2010, 22 (33), 3723−3728. (41) Eguez, H. E.; Cho, E. H. Adsorption of Arsenic on activated charcoal. JOM 1987, 39 (7), 38−41.

(6) Arai, Y.; Elzinga, E. J.; Sparks, D. L. X-ray Absorption Spectroscopic Investigation of Arsenite and Arsenate Adsorption at the Aluminum Oxide-Water Interface. J. Colloid Interface Sci. 2001, 235 (1), 80−88. (7) Banerjee, K.; Amy, G. L.; Prevost, M.; Nour, S.; Jekel, M.; Gallagher, P. M.; Blumenschein, C. D. Kinetic and thermodynamic aspects of adsorption of arsenic onto granular ferric hydroxide (GFH). Water Res. 2008, 42 (13), 3371−3378. (8) Tuček, J.; Prucek, R.; Kolarik, J.; Zoppellaro, G.; Petr, M.; Filip, J.; Sharma, V. K.; Zboril, R. Zero-Valent Iron Nanoparticles Reduce Arsenites and Arsenates to As(0) Firmly Embedded in Core-Shell Superstructure: Challenging Strategy of Arsenic Treatment under Anoxic Conditions. ACS Sustainable Chem. Eng. 2017, 5, 3027. (9) Yürüm, A.; Kocabaş-Atakli, Z. Ö .; Sezen, M.; Semiat, R.; Yürüm, Y. Fast deposition of porous iron oxide on activated carbon by microwave heating and arsenic (V) removal from water. Chem. Eng. J. 2014, 242, 321−332. (10) Huang, Y.; Yang, J.-K.; Keller, A. A. Removal of Arsenic and Phosphate from Aqueous Solution by Metal (Hydr-)oxide Coated Sand. ACS Sustainable Chem. Eng. 2014, 2 (5), 1128−1138. (11) Dwivedi, A. D.; Sanandiya, N. D.; Singh, J. P.; Husnain, S. M.; Chae, K. H.; Hwang, D. S.; Chang, Y.-S. Tuning and characterizing nanocellulose interface for enhanced removal of dual-sorbate (As V and Cr VI) from water matrices. ACS Sustainable Chem. Eng. 2017, 5, 518−528. (12) Lu, H.; Zhu, Z.; Zhang, H.; Zhu, J.; Qiu, Y. Simultaneous removal of arsenate and antimonate in simulated and practical water samples by adsorption onto Zn/Fe layered double hydroxide. Chem. Eng. J. 2015, 276, 365−375. (13) Wu, X.; Tan, X.; Yang, S.; Wen, T.; Guo, H.; Wang, X.; Xu, A. Coexistence of adsorption and coagulation processes of both arsenate and NOM from contaminated groundwater by nanocrystallined Mg/ Al layered double hydroxides. Water Res. 2013, 47 (12), 4159−4168. (14) Yu, X.-Y.; Luo, T.; Jia, Y.; Xu, R.-X.; Gao, C.; Zhang, Y.-X.; Liu, J.-H.; Huang, X.-J. Three-dimensional hierarchical flower-like Mg−Allayered double hydroxides: highly efficient adsorbents for As(v) and Cr(vi) removal. Nanoscale 2012, 4 (11), 3466−3474. (15) Xu, W.; Wang, J.; Wang, L.; Sheng, G.; Liu, J.; Yu, H.; Huang, X. J. Enhanced arsenic removal from water by hierarchically porous CeO2-ZrO2 nanospheres: Role of surface- and structure-dependent properties. J. Hazard. Mater. 2013, 260, 498−507. (16) Porada, S.; Zhao, R.; Van Der Wal, A.; Presser, V.; Biesheuvel, P. M. Review on the science and technology of water desalination by capacitive deionization. Prog. Mater. Sci. 2013, 58 (8), 1388−1442. (17) Liu, Y. H.; Hsi, H. C.; Li, K. C.; Hou, C. H. Electrodeposited manganese dioxide/activated carbon composite as a high-performance electrode material for capacitive deionization. ACS Sustainable Chem. Eng. 2016, 4 (9), 4762−4770. (18) Oren, Y. Capacitive deionization (CDI) for desalination and water treatment - past, present and future (a review). Desalination 2008, 228 (1−3), 10−29. (19) Li, H.; Zou, L.; Pan, L.; Sun, Z. Novel graphene-like electrodes for capacitive deionization. Environ. Sci. Technol. 2010, 44 (22), 8692− 8697. (20) Liang, P.; Yuan, L.; Yang, X.; Zhou, S.; Huang, X. Coupling ionexchangers with inexpensive activated carbon fiber electrodes to enhance the performance of capacitive deionization cells for domestic wastewater desalination. Water Res. 2013, 47 (7), 2523−2530. (21) Fan, C.-S.; Tseng, S.-C.; Li, K.-C.; Hou, C.-H. Electro-removal of arsenic(III) and arsenic(V) from aqueous solutions by capacitive deionization. J. Hazard. Mater. 2016, 312, 208−215. (22) Beralus, J.-M.; Ruiz-Rosas, R.; Cazorla-Amorós, D.; Morallón, E. Electroadsorption of arsenic from natural water in granular activated carbon. Front. Mater. 2014, 1, 28. (23) Beralus, J.-M.; Ruiz-Rosas, R.; Cazorla-Amorós, D.; Morallón, E. Electroadsorption of Arsenic from Natural Water in Granular Activated Carbon. Front. Mater. 2014, 1, 1−9. (24) Bain, E. J.; Calo, J. M.; Spitz-Steinberg, R.; Kirchner, J.; Axen, J. Electrosorption/electrodesorption of arsenic on a granular activated 6538


Combined Electrosorption and Chemisorption of As (V) in Water by

Recommend Documents