Cobalt oxide, sulfide and phosphide-decorated carbon felt for the capacitive deionization of lead ions

Journal Pre-proofs Cobalt oxide, sulfide and phosphide-decorated carbon felt for the capacitive deionization of lead ions Wei Jin, Meiqing Hu PII: DOI: Reference:

S1383-5866(19)34558-7 https://doi.org/10.1016/j.seppur.2019.116343 SEPPUR 116343

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Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

5 October 2019 9 November 2019 22 November 2019

Please cite this article as: W. Jin, M. Hu, Cobalt oxide, sulfide and phosphide-decorated carbon felt for the capacitive deionization of lead ions, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/ j.seppur.2019.116343

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Cobalt oxide, sulfide and phosphide-decorated carbon felt for the capacitive deionization of lead ions

Wei Jin *a,b, Meiqing Hu a

a School

of Chemical and Material Engineering, Jiangnan University, No. 1800 Lihu Avenue, Wuxi 214122, China

b

Institute of Process Engineering, Chinese Academy of Sciences, 1th Ber-er-tiao Zhongguancun, Beijing 100190, People’s Republic of China

*Corresponding author: E-mail: [email protected] (Wei Jin)

Abstract Remediation and recovery of highly toxic lead ions are highly important for water purification. In this study, a simple two-step method is developed to obtain the cobalt phosphide-decorated active carbon felt (ACF-CoP) materials for capacitive deionization of dilute Pb2+ (50 mg/L). The as-prepared ACF-CoP materials exhibit higher capacitance of 182.4 F/g than the corresponding ones of pristine ACF, ACF-CoS and ACF-Co3O4, which is possibly due to the surface Co3+ groups and abundant defect sites. Therefore, the Pb2+ ions were successfully recovered with an effective capacity of 90 mg/g at low voltage of -0.6 V. Furthermore, great charge/discharge stability and large voltage application range were also achieved in

the as-fabricated ACF-CoP materials.

Key words: Capacitive deionization (CDI); cobalt phosphide; lead ions; specific capacitance; carbon felt

1. Introduction According to the World Health Organization (WHO), lead ion (Pb2+) is one of the 10 most hazardous chemical pollutants towards public health [1-3]. Due to the considerable discharge and high toxicity/mobility, the widespread Pb2+ can cause potential nausea, convulsions, cancer, damages on metabolism and intelligence [4-8]. Strict permissible Pb2+ limit is set at 15-50 mg/L in drinking water systems of different countries [9-13], thus the remediation of Pb2+-based wastewaters is highly desirable to solve the water pollution and heavy metal recovery [14-17]. Many techniques have been employed for the removal of Pb2+, such as flocculation, solvent extraction, adsorption and chemical precipitation [18-21]. Nevertheless, these technologies usually suffer from the mass transfer bottleneck due to the dilute Pb2+ concentration in the industrial effluents (200-500 mg/L or even lower), and the complicate process flow or secondary pollution generation[22-25]. Recently, the emerging energy-efficient and environmental friendly technique of capacitive deionization technology (CDI) attracts increasing attentions to remove various salt and heavy metal ions of Pb, Cr, Cu, Ni and Cd [17, 26-32]. In CDI, target ions can be extracted from water via electrosorption between a pair of highly porous

electrodes, while this process is reversible to regenerate the electrodes. Generally, capacitive deionization can be carried out with high removal efficiency (>85%) and low energy consumption (1.35 Wh/m3) [33, 34]. It has been demonstrated that the exploration of electrode materials with excellent adsorption/desorption capacity is of great importance for the CDI performance. As a result, carbon materials including activated carbon, carbon aerogel and graphene have been widely used as the CDI electrodes, originating from their good chemical/mechanical durability, high conductivity and specific surface area [13, 28, 35-39]. Huang et al. [33] identified that the 81% and 43% Pb2+ removal can be achieved using 1.2 V in the individual or multi-component metal-containing solutions, respectively. Furthermore, considerable investigations have been carried out to improve the performance by doping or functionalization of other nanomaterials with carbon, such as transition metal oxides. It has been reported that the incorporated metal oxides can provide abundant surface hydroxyl/carboxylic groups for CDI[40, 41]. However, there is potential pore structure blocking and electrical conductivity decreasing issues of this oxide doping strategy. Recently, transition metal sulfides (TMS) and phosphide (TMP) mixed carbon nanomaterials have been widely developed as the alternative electrodes. In our previous study, nanoporous ZnS-decorated carbon felt has been fabricated for the high-performance dilute 100 mg/L-1 Cu2+ capacitive deionization, in which ultra-high remove capacity of 27.4 mg/g-1 is obtained at low voltages of -0.2 V [26, 42]. The performance is 2-5 times better than the available carbon nanofiber, graphene and

carbon aerogels electrodes [43-47]. Furthermore, it has be demonstrated that there is significant activity and durability enhancement of supercapacitor and electrochemistry performance at the CoP-based nanomaterials, suggesting its great potential in electrochemical water purifications [48-51]. In order to further evaluate the improved CDI performance, the aim of this article is to fabricate the systematic transition metal (cobalt) oxide, sulfide and phosphide-decorated carbon felt materials for the CDI removal of Pb2+ [52-54]. The effect of incorporated transition metal compounds towards the surface area, pore structure, electrical conductivity and Pb2+ adsorption-desorption kinetics were systematically illustrated.

2. Materials and methods 2.1.

Synthesis of transition metal cobalt-decorated active carbon felt electrodes. Cobaltous nitrate hexahydrate (99.999 wt%), thioacetamide (>99.0 wt%), urea

(99 wt%), sodium hypophosphite monohydrate (>99.0 wt%) and lead nitrate (>99.0 wt%) were analytical purity from Sigma Aldrich. Transition metal cobalt-decorated active carbon felt electrodes were obtained by the hydrothermal, calcination and phosphating methods (Fig. 1). Active carbon felt (3*2*0.5, 0.50 g) from Wuxi Teda Biotechnology Co., Ltd., China was used as the substrate. It should be noted that ACF-CoS electrode was synthesized in one-step by hydrothermal method at 160 oC for 4h, and the mass in the ACF substrate of the CoS particles was 77.6-93 mg/g. ACF-Co3O4 electrode was prepared by hydrothermal at 120 oC for 4h and calcination at 300 oC for 2h, and the mass in the ACF substrate of the Co3O4 was 48.2-82.0 mg/g.

Then the ACF-Co3O4 electrode was phosphorized at 300 oC for 2h to obtain the ACF-CoP electrode and the final mass in the ACF substrate of the CoP particles on the electrode was 38.3-46.0 mg/g.

2.2.

Materials characterization. The surface morphology of electrodes was determined by the field emission

scanning

electron

microscopy

(FE-SEM,

QUANTA250-FEI,

USA)

with

energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction patterns were carried out using an X-ray diffractometer (XRD, BUKER AXS, Germany) with a Cu Kα source at a constant scan rate (2° min-1). The chemical compositions of the electrodes were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, USA). The specific surface area and pore size distribution were measured by a surface analyzed (Autosorb IQ, Quantachrome, USA) with nitrogen adsorption/desorption isotherm according to the Brunauer-Emmetl-Teller (BET) method. The Raman spectra of the electrodes were carried out by a Microscopic confocal Raman spectrometer (inVia, RENISHAW, USA). The Pb2+ and other metal ions concentration were determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES) (ICAP7400, Thermo Scientific, USA).

2.3.

Electrochemical measurements. The electrochemical performance of the transition metal cobalt electrodes was

analyzed with an electrochemical workstation (CHI760E, CH instruments, China).

The tested electrolyte was prepared with different concentration (50 mg/L-500 mg/L) of lead nitrate and nitric acid (pH adjustment). It should be noted that the lead ions tend to precipitate in other acidic solutions (H2SO4 and HCl), thus the nitric acid was selected. Besides, copper nitrate (Cu(NO3)2·3H2O, Aldrich, 50 mg/L), nickel nitrate (Ni(NO3)2·6H2O, Aldrich, 50 mg/L) and ferrous nitrate (Fe(NO3)2·9H2O, Aldrich, 50 mg/L) were used for the selectivity tests. Electrochemical measurements were conducted at room temperature using the fabricated ACF-based materials and Ag/AgCl reference electrode, respectively. Cyclic voltammograms (CV) and amperometric i-t curves were performed at various scan rates. CV was performed using a three-electrode system with 1.0 M NaCl solution as the electrolyte, while the EIS was obtained under the frequency range of 0.01 to 20 kHz in 1.0 M NaCl solution. Furthermore, the detailed operation model and system information are presented in Table 1. The specific capacitance was calculated by the following equation: V2

SC   IdV / mv(V2  V1 ) V1

(1)

in which I is the current range (  IdV is the integrated area of CV curves), m is the mass of the cathodic electrode, v is the potential scan rate, V1 and V2 are the low and high voltage limits, respectively [55, 56]. Galvanostatic charge/discharge (GCD) curves were utilized to determine the electrochemical behavior of the ACF-CoP electrode with a three-electrode system in 1.0 M NaCl solution with an ACF film counter electrode and an Ag/AgCl reference electrode. Multiple charge/discharge curves were conducted from -0.6 to 0.6V to verify the stability of ACF-CoP electrode. The regeneration tests were conducted at a direct voltage of -0.6V to release the

adsorbed ions, and the initial concentration of Pb2+ solution was 50 mg/L.

3. Results and discussions 3.1.

Cobalt oxide, sulfide, phosphide-decorated active carbon felt. The surface morphology of various ACF electrodes were characterized by SEM.

As shown in Fig. 2a-c, a layer of nanoparticles is uniformly wrapped on the surface of the ACF. As compared to the planar structure of ACF-Co3O4 electrode and ACF-CoS electrode, the ACF-CoP electrode has a three dimension (3D) complex appearance which consists of C, O, P and Co (Fig.2d-g). Fig. 3 display the XRD and EDS patterns of ACF-Co3O4, ACF-CoS and ACF-CoP, further confirming the existence of cobalt compounds. In the XRD characterization (Fig. 3a), the peaks at 25.9° is corresponding to the C of ACF substrates, while the nanoparticles on the different ACF materials surface are identified as the Co3O4 (PDF# 74-2120), CoS (PDF# 75-0605) and CoP (PDF# 29-0497), respectively. As shown in Fig. 2b, the EDS patterns of ACF-CoP electrode show that the atomic ratio of P to Co in the catalysis is approximate 1:1, suggesting the formation of CoP. Moreover, the EDS spectra of ACF-CoS and ACF-Co3O4 materials show that the atomic ratio of Co to S and Co to O are approximately 1:1 and 3:4, respectively. The surface element species and binding information were investigated by XPS analysis. In Fig. 4a, there are clear Co, O and C peaks. The peak at 533.0 eV of O 1s suggests the presence of the Co-O or oxidized sulfate and oxidized phosphate species, which is possibly due to the air exposure and incomplete phosphating. Co 2p spectra

of various electrode materials are illustrated in Fig. 4b. The characteristic peaks at 779.6 eV and 795.5 eV in Co 2p spectrum are associated with the Co 2p3/2 and Co 2p1/2 of Co3+-O and Co2+-O, indicating the existence of CoO/Co2O3 (Co3O4). And other apparent satellite (Sat.) peaks at 784.6 eV and 802.1 eV are related to the Co2+ arising from surface oxidation under air exposure. Notably, the peaks at 777.1 eV and 791.8 eV of ACF-CoP electrode can be ascribed to binding energies for Co 2p3/2 and Co 2p1/2. As shown in Fig. 4c, the P 2p spectra for ACF-CoP depicts two peaks of the BEs of 2p 3/2 and 2p 1/2 at 129.8 eV and 131.0 eV, respectively. The peak at 162.5 eV in S 2p for ACF-CoS is due to the formation of metal sulfides, while another peak at 168.8 eV suggests the bonding of S-O possibly owing to the partly oxidized sulfur atoms. Raman spectrum is applied to analyze the degree of structure disorder or defects in graphitic-like materials. As shown in Fig. 4d, two apparent peaks of various electrode materials at 1358 and 1591 cm-1 can be observed, which are corresponding to the D band of disordered carbon and G band of graphitic carbon, respectively. Generally, the ID/IG value can be used to determine the graphitization degree and the amounts of active site at carbon materials. The ID/IG values of ACF, ACF-Co3O4, ACF-CoS and ACF-CoP are 1.20, 1.21, 1.23 and 1.25, respectively, indicating the presence of highly active defect sites. The peaks at 475, 514 and 674 cm-1 are associated with the Raman spectrum of CoO/Co2O3, in consistent with the XPS results. N2 adsorption-desorption isotherm were analyzed to study surface area and

porous structure of ACF, ACF-Co3O4, ACF-CoS and ACF-CoP materials. As shown in Fig. 5a, four electrode materials exhibit type IV isotherm characteristics with hysteresis loop in the relative pressure (P/P0) with steep upward slope, indicating the existence of both coarse mesoporous and Macropore. All pore size distribution (PSD) calculations were performed using the numerical algorithm SAIEUS, which solves the adsorption integral equation utilizing splines and regularization procedures. Therefore, the results of calculated PSDs with NLDFT model are presented in Fig. 5b, where ACF-CoP material showed distinct peaks at 3-15 nm and other samples exhibited mesoporous structure at 25-40 nm. The BET specific area and pore size results are illustrated in Table 2, in which the surface area of ACF-CoP, ACF-CoS and ACF-Co3O4 materials are higher (35.7, 24.6 and 24.3 m2/g) than ACF materials. It should be noted that the surface area of ACF-CoP materials is approximate 3 times that of the ACF materials.

3.2.

Capacitive deionization performance towards Pb2+ The Nyquist plots can determine the potential electron transfer kinetics and

capacitance behavior. As shown in Fig. 6a, the semicircle of ACF-CoP electrode suggests lower charge transfer resistance than the ACF-Co3O4 and ACF-CoS electrodes. And the slope in the low frequency region is similar, indicating those cobalt-decorated ACF electrodes have similar diffusion resistance. As shown in Fig. 6b, the amperometric i-t curves display the comparison of reaction currents at -0.6 V. It can be seen that the average current of the ACF-CO3O4, ACF-CoS and ACF-CoP

electrode are 0.4, 1.25 and 1.65 mA, respectively. The larger average current of ACF-CoP electrode is in consistent with its lower resistance of EIS curves. In the CV curves of Fig. 6c, the ACF-CoP electrode presents much bigger CV area and current density, indicating that the much higher electrochemical activity is obtained by the phosphating process. And the integrated area of the CV curves for ACF-CoP is larger than that for ACF-Co3O4 and ACF-CoS electrode, suggesting the larger specific capacitance. As shown in Fig. 6d, the required charge/discharge time for the ACF-CoP electrode is longer than for the ACF-CoS and ACF-Co3O4 electrodes. It can be seen that ACF-CoP electrode exhibits quasi rectangular CV curves in Fig. 6e, indicating typical electrochemical double layer character and excellent rate property. The specific capacitance calculated from CV curves[57, 58] are 182.4 and 46.3 F/g at scan rate of 5 and 100 mV/s, respectively, which are higher than that of ACF-CoS and ACF-Co3O4 electrodes. As compared to the ACF-CoS and ACF-Co3O4, the better capacitance performance of the ACF-CoP electrode originates from the higher specific surface area, surface Co3+ groups and abundant defect sites [59, 60]. Capacitive deionization system was applied to remove 50 mg/L Pb2+ ions at various electrodes and applied voltage, in which the final solution pH was 2.5. As shown in Fig. 7a, the curve pattern shows a continuous increasing trend followed by a stable plateau, and the electrosorption capacity (EC) for Pb2+ gradually approaches maximum value of 25, 35, 67 and 90 mg/g at ACF, ACF-Co3O4, ACF-CoS and ACF-CoP electrodes (Fig. 7b), respectively. The electrosorption tests were conducted

at different pH values by using ACF-CoP electrode (inset of Fig. 7b), and the results show that the ACF-CoP electrode exhibits excellent adsorption properties both in acid and neutral solution (leads ions precipitate in alkaline solution). When the pH is much lower, the Pb2+ uptake decreased due to the presence of considerable competitive H+ ions [61, 62]. Besides, CoP material is unstable in an concentrated acidic solution (pH<2), the loss of CoP materials in the ACF-CoP electrode is estimated to be ~5%. Fig. 7c presents the adsorption removal capacity of Pb2+ ions at various applied voltages. As the voltage is applied, Pb2+ ions are rapidly adsorbed via the strong electrostatic interactions. It should be noted that with the gradual occupation of the adsorption sites, the subsequent ions would be electrostatically repulsive towards the previously adsorbed ions, leading to the stabilization of the adsorption rate. Moreover, as the voltage increases, the adsorption capacity achieves the maximum adsorption amount of 102.5 mg/g at the voltage of -1.0 V. However, the excessive voltage can also cause side reaction of electrodeposition. The Langmuir isotherm is applied to correlate the CDI behavior of the ACF-CoP electrode. Fig. 7d and Table 3 present the simulation and experimental data, respectively. The maximum removal capacity obtained from the isotherm are 30.2, 50.4, 128.4 and 155 mg/g at the voltage of -0.2, -0.4, -0.8 and -1.0 V, respectively. It can be seen that the Langmuir isotherm agrees with the experimental data, since the coefficient r2 are close to 1.0. In order to determine the electrosorption rate (ER), the Ragone plots (Fig. 7e and Fig. 7f) are given, in which the ACF-CoP electrode has a higher Pb2+ adsorption capacity and faster electrosorption rate. In Fig. 7f, the plots

shift upwards to the right with increasing voltage and time, indicating that the electrosorption capacity and rate simultaneously increase due to the stronger voltage-dependent Coulombic interaction [63-65]. However, it should be noted that excessive voltage can cause side reaction and decrease electrode stability [26]. As shown in Fig. 8a, the Pb2+ removal capacity of ACF-CoP electrodes is higher than other electrodes. Besides, the recently reported data on the electrosorption capacity and removal efficiencies for the different ions are summarized in Table 4. In order to investigate the selectivity of Pb2+ on ACF-CoP materials, the mixture solution containing 50 mg/L Pb2+, 50 mg/L Cu2+, 50 mg/L Ni2+ and 50 mg/L Fe3+ were used. After operation at -0.6 V for 120 min, the removal capacity of ACF-CoP toward Pb2+ ions can still reach 74.8 mg/g in presence of Pb2+, Cu2+, Ni2+ and Fe3+ (Fig.8b), indicating good selectivity for the potential practical applications. As shown in Fig. 8c, the removal efficiency of Pb2+ ions rapidly increases in the first 60 min and gradually reached maximum (85%) in 60-120 min. ACF-CoP electrode also exhibits removal capacity for other ions (Cu2+ 41%, Ni2+ 39% and Fe3+ 27%), which is much lower than than that of the Pb2+ ions due to the favorable selectivity.

3.3.

Stability and mechanism of Pb2+ capacitive deionization The GCD process can illustrate the maximum capacitance and stability of the

electrode at certain applied voltage. In Fig. 9a, it can be seen that the ACF-CoP electrode displays the good charging-discharging curves, and the GCD curves well maintain their initial shape without obvious change after 10000 cycles at 5 A/g (Fig.

9b). Furthermore, the XPS analysis was carried out to investigate detailed information about lead ions at the electrode surface after capacitive deionization. It should be noted that the electrodes were rinsed with distilled water and dried to remove the aqueous components on the surface, then the remained absorbed species were detected for XPS. In Fig. 9c, a peak appears at approximate 138.5 eV for Pb(II) 4f, which can be attribute to the PbO [66, 67]. In Fig. 9d, two peaks occur at 436.1 and 413.6 eV for 4d3/2 and 4d5/2, which possibly due to the contribution of Pb(0). However, XRD (Fig. 9e) and EDS (Fig. 9f) spectra were further used to determine the atomic composition on the surface of electrodes after adsorption process [68, 69]. It can be concluded that Pb(0) and PbO coexist on the surface of electrodes. There is good regeneration behavior of ACF-CoP after 50 CDI cycles in 50 mg/L Pb2+ aqueous solution at -0.6V (Fig. 9g). By analyzing the EDS spectrum before and after 50 times cycles (inset in Fig. 9g), the content of CoP on the surface is well maintained, indicating that ACF-CoP electrode has excellent stability. As shown in Fig. 9h, the removal capacity of ACF-CoP electrode rapidly increases in the first 30 min and reaches equilibrium within 60 min. The adsorption capacity of 90 mg/g is observed for an adsorbate concentration of 50 mg/L, while higher feed concentration (100 mg/L) has less effect towards the adsorption capacity. Capacitive deionization is a promising heavy metal ions extraction method (Fig. 10a). The material covering the ACF substrate will provide the additional active site for the Pb2+ ions, in which the high removal efficiency, selectivity and durability are desirable. As shown in Fig. 10b, the lead ions were adsorbed on the surface of the

electrodes when the applied voltage ranging from 0 to -0.4V (Area I). When the electrodes were stopped or reversed, the Pb2+ ions were desorbed into the solution. However, a part of Pb2+ ions on the surface of ACF-CoS and ACF-Co3O4 were reduced and electrolyzed to Pb (0) in the voltage ranging from -0.4 to -0.6 V (Area II). When the applied voltage exceeded -0.8 V, the reduction and electrolysis reactions occurred on the surface of all three electrodes (Area III), indicating excessive voltage may cause undesired reaction. Obviously, the ACF-CoP electrode can process a high-speed capacitive deionization process by applying higher voltage.

4. Conclusions In summary, we developed a simple two-step method to obtain the ACF-CoP materials for capacitive deionization of Pb2+ ions. The as-prepared ACF-CoP exhibits a larger surface area with 35.7 m2/g and a higher capacitance of 182.4 F/g, as compared to the corresponding ones of pristine ACF, ACF-CoS and ACF-Co3O4. Therefore, the Pb2+ ions were successfully extracted via electrostatic at low voltage of -0.6V, in which the effective removal capacity of 90 mg/g is much better than the corresponding ones of pristine ACF (20 mg/g), ACF-CoS (67 mg/g) and ACF-Co3O4 (35 mg/g) electrodes. The XPS and EDS were carried out to character the elemental composition of three electrodes, and ACF-CoP electrode has larger voltage application range. Furthermore, great charge/discharge stability and regeneration performance were also observed in the as-fabricated ACF-CoP materials.

Acknowledgements We acknowledge the funding support from National Natural Science Foundation of China (Grant No. 51604253) and Fundamental Research Funds for the Central Universities (JUSRP21936).

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Figures and tables captions Fig. 1 Schematic illustration of the fabrication of the cobalt oxide, sulfide and phosphide-decorated active carbon felt electrodes. Fig.2 SEM images of (a) ACF-Co3O4, (b) ACF-CoS, and (c) ACF-CoP electrodes. Elemental mapping of ACF-CoP electrode (d-g) correspond to C, Co, O and P maps, respectively. Fig. 3 The XRD and EDS spectra of ACF-Co3O4, ACF-CoS and ACF-CoP electrode materials. Fig. 4 XPS survey (a) and Co 2p (b) spectra of ACF-Co3O4, ACF-CoS and ACF-CoP electrodes; (c) XPS spectrum of P 2p (ACF-CoP) and S 2p (ACF-CoS); (d) The Raman spectra of various electrode materials. Fig. 5 (a) N2 adsorption and desorption isotherms of ACF, ACF-CO3O4. ACF-CoS and ACF-CoP electrode materials；(b) Pore size distribution (PSD) plots calculated by NLDFT silt model. Fig. 6 (a) EIS spectra and (b) amperometric i-t curves of various electrodes in 100 ppm Pb2+ aqueous solution; (c) CV and (d) GCD curves of as-prepared electrodes at the scan rate of 5 mV s-1 and at the current density of 0.2 A/g, respectively; (e) CV curves of ACF-CoP electrode at different scan rates ranging from 10-50 mV s-1; (f) Specific capacitance of various electrodes at different scan rates. Fig. 7 (a) The removal capacity for 100 mg/L Pb2+ of ACF, ACF-CoP, ACF-CoS and ACF-Co3O4 electrodes at -0.6 V; (b) The maximum removal capacity for Pb2+ ions on different electrodes (the inset is the adsorption capacity in different pH); (c)

Electrosorption removal capacity on ACF-CoP electrode at various applied voltage; (d) Langmuir isotherms of Pb2+ ions on ACF-CoP electrode at different initial concentration; (e) Ragone plots of ER vs. EC of different electrodes in a 50 mg/L Pb2+ solution at -0.6 V; (f) Ragone plots of ER vs. EC of ACF-CoP electrode in a 50 mg/L Pb2+ solution at different voltages. Fig. 8 (a) Comparison of different adsorbents for the removal of Pb2+ ions; (b) The selective removal capacity of Pb2+ ions in heavy metal ions solution; (c) Removal efficiency of different ions with time. Fig. 9 (a) GCD curves of ACF-CoP electrode at the current density of 0.1 to 1.0 A/g; (b) GCD cycle curves of first 10 cycles and the last cycles (from the 9990th cycles to 10 000th cycles) at the current density of 5 A/g; Pb 4f (c) and Pb 4d (d) XPS spectra of surface on ACF-CoP after capacitive deionization process at -0.6 V; The XRD spectrum (e) and EDS spectrum (f) of lead adsorption species at different electrode materials after electrosorption; (g) Regeneration plots of ACF-CoP electrodes (the inset is the EDS spectrum of ACF-CoP electrode before adsorption and after 50 cycles). (h) Effect of different adsorbate concentration (5 to 100 mg/L) with ACF-CoP electrodes at pH of 2.5. Fig. 10 Schematic diagram of (a) the capacitive deionization process and (b) reaction process of three electrodes at different voltage ranges. Table 1 CDI separation reports of lead ions removal used ACF-CoP materials. Table 2 Specific surface area and pore size on various electrode materials. Table 3 Coefficient of Langmuir fitting.

Table 4 Comparison of the electrosorption capacities and operate modes of various electrodes for the different ions.

Fig. 1

Fig. 2

b

ACF-Co3O4

C

Intensity (a.u.)

Intensity (a.u.)

a

PDF#74-2120 Co3O4

ACF-CoS PDF#75-0605 CoS

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ACF-Co3O4

Co O C

ACF-CoS ACF-CoP

P S

Co

P 12.3 At% Co 15.2 At% S 12.1 At% Co 12.3 At% O 45.1 At% Co 38.2 At%

PDF#29-0497 CoP

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ER (mg/g/min)

1

ACF-CoP ACF-CoS ACF-Co3O4

0.1

1

-0.2 V -0.4 V -0.6 V -0.8 V -1.0 V

0.1

0.01

ACF 0.01

1

10

100

1

EC (mg/g)

Voltage increase 10

EC (mg/g)

Fig.7

100

90

a

140

This work

Removal capacity (mg/g)

Removal capacity (mg/g)

180 160

Surface-modified Biosorbent [70] hematic nanopiticles [71]

120 100

CNT [72]

80

Modified soga lignin [73]

60

A Mannich base biosorbent [74]

40

Fe-modified sporopollenin [75]

20 0 600

500

400

300

200

100

Removal efficiency (%)

c

80

Pb2+ Cu2+ Ni2+ Fe3+

60

40

20

0

0

20

40

60

80

100

120

Time (min)

Fig. 8

b

70 60 50 40 30 20 10 0

0

Time (min)

100

80

Pb2+

Cu2+

Ni2+

Fe3+

0.0 -0.2 -0.4 -0.6 -0.8 -1.0

b

0.8

0.1 A/g 0.2 A/g 0.5 A/g 1.0 A/g

Potential (V)

Current (V vs. Ag/AgCl)

a

0.2

5

1

10 9990

0.4 

0.0 -0.4 -0.8

0

50

100

150

200

250

300

350

400

0

20

40

60

c

d

ACF-Co3O4 ACF-CoS ACF-CoP

140

139

138

137

4d3/2

440

f

 Pb  



ACF-Co3O4





ACF-CoS

 

 

20

60

80

0

Removal capacity (mg/g)

80 Initial

Intensity (a.u.)

Removal capacity (mg/g)

100

100

40 20

P 13.4 Wt% Co 18.7 Wt%

50 times

0

2

4

P 12.8 Wt% Co 18.1 Wt% 6

8

10

Energy (eV)

0

10

20

30

40

Cycle number

Pb 1.8 At% O 29.8 At%

1

2

Pb 8.7 At% O 27.2 At%

Pb 3

4

5

6

Energy (eV)

8

9

5 mg/L 10 mg/L 30 mg/L 50 mg/L 100 mg/L

60 40 20

0

20

40

60

80

Time (min)

Fig. 9

7

h

80

0

50

Pb 1.1 At% O 46.7 At%

Pb P

g

60

400

S

2 Theta (degree)

120

410

Pb

 ACF-CoP

40

420

ACF-CoP ACF-CoS ACF-Co3O4

O

Intensity (a.u.)

Intensity (a.u.)

 PbO 

430

4d5/2 413.6 eV

Binding energy (eV)

Binding energy (eV)

e

ACF-CoS ACF-CoP

436.1 eV

450

136

Pb 4d

ACF-Co3O4

Intensity (a.u.)

Intensity (a.u.)

Pb 4f

141

45140 45160 45180

Time (s)

Time (s)

0

104

9995

100

120

10

Fig. 10

Table 1 Parameter

Value

Notes

Cfeed (mg L-1)

50-500

Feed water salt concentration.

∆c (mg L-1)

5±0.2

Average concentration of each cycle.

Separation

Salt

Pb(NO3)2

Feed water composition.

Conditions

κ (mS/cm)

3.3

Measured feed water electrical conductivity.

Operate mode

CV

Constant voltage, 0.8-1.5 mA current window.

n

1

Number of the cells (electrode pairs) used in the separation.

nm

0

Number membranes used per cell.

Cell

A (cm2)

19.6

Projected face area per CDI cell.

Characteristics

le (μm)

3±0.2

Electrode thickness.

lsp (μm)

1.5

Electrode distance.

m (g)

1.0

Total mass of electrodes.

ρ (g/cm3)

0.33

Electrode mass density.

C (F/g)

182.4

Calculated from cell capacitance and electrodes mass.

M (cm3/g)

35.7

Specific surface area.

V (cm3/nm/g)

3.05

Pore size calculated by BET method.

Performance indicators

Table 2 Materials

Specific surface area (m2/g)

Pore size (nm)

ACF

12.1

3.05

ACF-CO3O4

24.3

3.42

ACF-CoS

24.6

3.82

ACF-CoP

35.7

3.15

Table 3 Model equation

qe 

qKC 1  KC

Parameter

Value -0.2V

-0.4V

-0.8V

-1.0V

q

30.2

54.0

128.4

155.0

K

0.0012

0.0014

0.0035

0.0025

r2

0.984

0.994

0.97

0.887

Table 4 Electrode

Ions

Method

Applied voltage

Time

pH

Initial conc. (mg/L)

Removal efficiency

Qe (mg/g)

Ref.

BBL/PVA α-Fe2O3 NPs Clays Acacia gum AC CNT PCS HAT-CN ACF-CoP

Pb2+ Pb2+ Pb2+ Pb2+ Pb2+ Na+ Cr3+ Na+ Pb2+

Biosorbent Adsorption Adsorption Adsorption Electrosorption Constant voltage Electrosorption Electrosorption Electrosorption

/ / / / -0.13V 1.2V 1.3V 1.2V -0.6V

12h 6h 3h 24h 10h 1h 2.6h 2h 2h

6 7 4.5 7 5 7 / 7 2.5

200 10 30 100 5 3500 50 500 50

98% 98% 90% 98.3% 97.5% 19% 72.2% / 98%

113.84 111 / 12.2 17.17 / / 24.66 155

[70] [71] [76] [77] [78] [79] [80] [81] This work

Graphical abstract

Highlights 1. Cobalt oxide, sulfide and phosphide-decorated carbon felt were prepared; 2. Modified electrodes exhibit great capacitive deionization towards Pb2+; 3. The detailed electro-sorption kinetics and mechanism are illustrated.

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.

Wei Jin Professor, Jiangnan University, China