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Desalination

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Rich heteroatom doping magnetic carbon electrode for flow-capacitivedeionization with enhanced salt removal abilityDanping Li, Xun-An Ning⁎, Chenghai Yang, Xiaohui Chen, Yi WangGuangzhou Key Laboratory of Environmental Catalysis and Pollution Control, School of Environment Science and Engineering, Institute of Environmental Health andPollution Control, Guangdong University of Technology, Guangzhou 510006, China

G R A P H I C A L A B S T R A C T

A R T I C L E I N F O

Keywords:DeionizationHeteroatomic dopingMagnetic propertiesSelf-assembly

A B S T R A C T

Core-shell nanoparticles were prepared by coating magnetic nanoballs (Fe3O4) with highly cross-linked polymerpoly(-cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) to serve as heteroatomic-rich doping electrodes(MZPS). After formation by carbonization, C-MPZS acts like a flowing electrode used for flow-capacitive deio-nization (FCDI), which has fast ion transfer and diffusion performances because of its rich heteroatom contentand the prominent synergistic effect of N, P, S atoms. Moreover, the FCDI performance of C-MPZS is enhanceddue to its increased synergistic effect from its excellent available specific surface area, unique porous char-acteristics with large micro-/mesopores, and favorable conductivity with high graphitization. The optimalcarbonization temperature was 500 °C with an optimum specific capacitance of 211.54 F g−1, which was testedin a 1 M sodium chloride solution at a scan rate of 1 mV s−1. Then, the C-MPZS flowing electrode was found tohave a high electrosorption capacity (50.07 mg g−1), removal efficiency (68.50%) and removal rate (83.07%) atan applied potential of 1.0 V in a sodium chloride solution with a conductivity of 1200 μS cm−1 at 6 rpm. Anadditional cyclic test demonstrated good stability performance after 10 cycles due to the magnetic nanoparticleseffect. Finally, through the successful application in tail water from printing and dyeing wastewater, the resultsindicated that the as-prepared C-MPZS flowing electrode has promise for application in high-performanceelectrochemical deionization.

https://doi.org/10.1016/j.desal.2020.114374Received 4 November 2019; Received in revised form 27 January 2020; Accepted 4 February 2020

⁎ Corresponding author.E-mail address: ningxunan666@126.com (X.-A. Ning).

Desalination 482 (2020) 114374

Available online 24 February 20200011-9164/ © 2020 Published by Elsevier B.V.

T

1. Introduction

As a consequence of industrialization and urbanization, the re-quirement for fresh and clean water is growing rapidly. However,shortages of water resources, unusable brackish water and difficult-to-regenerate sewage are the main factors that hinder the rapid and sus-tainable development of society [1,2]. Although reverse osmosis,membrane distillation, electrodialysis, and ion-exchange resins are thetraditional methods used to desalt brackish water, [3–6] capacitivedeionization (CDI) is a remarkable alternative technology with an as-cribed higher capacitance efficiency [7,8]. CDI removes and stores saltions from solution by forming electric double layers (EDLs) at the re-sultant electric field [8]. However, there are some drawbacks of CDI,such as its low energy efficiency, large co-ion effect, high interfaceresistance between solution and electrode, and limited ion stores withinelectrode materials [9]. Thus, a flow-electrode capacitive deionization(FCDI) method is investigated to overcome these limitations by repla-cing fixed carbon electrodes with suspended carbon electrodes, whichallows continuous operation to achieve high salt removal efficiency andlarge salt adsorption capacity (SAC) in a concentrated NaCl solution.

Doped carbon materials have great development potential for use aselectrodes with excellent performance and a low price in CDI, but theirefficiency is not yet satisfactory. According to experimental and theo-retical calculations, the doping of heteroatoms into a carbon lattice,such as N (nitrogen), P (phosphorus) and S (sulfur), is a beneficial wayto improve the efficiency of capacitive performance [10–13]. Typically,adding heteroatoms with similar electronegative values (χ), such as S(χ = 2.58), into a carbon lattice (χ = 2.55) can produce more activesites and tune the electron spin density; furthermore, higher electro-negativity values of doped heteroatoms, such as N (χ = 3.04), andlower electronegativity values of doped heteroatoms values, such as P(χ = 2.19), can improve charge sites and regulate the charge density[14–17]. Meanwhile, the transition metal atoms doped with Fe (iron) ina carbon lattice are conducive to providing a valence alternation spacefor chemical absorption of oxygen and enhancing the electroactive site[18–20].

Doping of heteroatom one-dimensional (1D) magnetic nanochainscoated with cross-linked polymers has received greater attention due tocontrolled self-assembly doping. The benefits of this doping method in-clude increasing the special surface area (SSA) and obtaining a high as-pect ratio and large specific capacitance [21–23]. Korth [24] and Kenget al. [25] discussed functional polymers-coated ferromagnetic colloidsused to prepare 1D magnetic chains via a complex and long-term poly-merization at high temperature as well as disintegration that may occurwhen rinsing with a solvent. Cheng et al. [26] reported a three-dimen-sional (3D) coiled structure based on the folding of magnetic-field-in-duced chains. Nevertheless, as-prepared ordered structures are difficultto maintain when the external field is removed because of the infirm or

inappreciable heterogeneous dipolar reciprocity within the magneticblocks. Nevertheless, these synthesis processes are complex, e.g., theyrequire nanostructured templates and doped metal atoms from externalseven when the contents are uncontrollable and low. Therefore, it is ne-cessary to present a simple method for synthesizing homogeneous andessentially doped carbon materials of metal and nonmetal atoms due totheir excellent electrical adsorption properties.

Thus, we present a handy one-pot preparation for self-assemblydoping of N, P and S atoms in highly cross-linked polymers to coatmagnetic nanoballs. A nanomagnetic sphere (Fe3O4) combined withparamagnetization and a significant magnetic response is used as thebuilding foundation for 1D nanowires [27]. Poly(-cyclotriphosphazene-co-4,4′-sulfonyldiphenol (PZS)) is utilized to form the highly cross-linked polymer shell that attaches onto the nanomagnetic balls, whichare labeled as MPZS. MPZS inherit the paramagnetization from Fe3O4and retain the good water dispersibility and tailored surface chemistrythat originate in PZS; moreover, MPZS display high stabilization andfunctionalization after self-assembly and coating with PZS. The synth-esis of the electrode materials is shown in Scheme 1. The FCDI per-formance of C-MPZS (after carbonization of the MPZS) electrodes wasinvestigated in a NaCl solution and further compared with that of theAC electrode [28]. According to the results, C-MPZS electrodes have abetter electrochemistry character and a higher desalt property than ACelectrodes. Therefore, the results of this study suggest that the as-pre-pared C-MPZS are a promising type of electrode for application in anFCDI cell. Furthermore, CMPZS as an electrode for FCDI has not beenpreviously reported.

2. Materials and method

2.1. Fabrication

2.1.1. MaterialsHexachlorocyclotriphosphazene (HCCP, 98%), 4,4′-sulfonyldi-

phenol (BPS,> 99.5%), triethylamine (TEA,> 98%), sodium chloride(NaCl, AR), activated carbon (AC, ~1000 m2 g−1) and polyvinyl al-cohol (PVA) were acquired from Aladdin Chemical Reagent Co., Ltd.,Shanghai, CHN. FeCl3∙6H2O (AR), FeSO4∙7H2O (AR), the ammonia so-lution (NH3·H2O, 28%) and methanol (AR) were obtained from JiangsuAlonda High-Tech Industry Co., Ltd., CHN. Carbon black (CB, BP2000)was purchased from CABOT Corp., US. Cation/anion exchange mem-branes (LE-HoCM Grion 0011/1201) were purchased from Guangzhou,CHN, and the part of property index as shown in Table S2. Titaniummesh (GR1, 100 mesh) was purchased from Anping OFL wire meshmanufacturing Co. Ltd., CHN. All solutions were prepared using ultra-pure water (Milli-Q Advantage, Merck Millipore, Germany), and thereagents used in the experiments were analytical grade. Abbreviationsare listed in Table S1.

Scheme 1. The reaction process of C-MPZS.

D. Li, et al. Desalination 482 (2020) 114374

2

2.1.2. Synthesis of PZSSynthesis of PZS was prepared according to a previous report [29].

Typically, 10 mL of methanol was added slowly to 70 mg of HCCP and158 mg of BPS. Then, the solution was stirred for 5 min to dissolvecompletely. Next, 185 μL of TEA was added dropwise and stirred for 6 hcontinuously. After the reaction was complete, the white-products werewashed with methanol and dried in a vacuum at 40 °C for 24 h. Theresulting materials are labeled as PZS.

2.1.3. Synthesis of MPZSSynthesis of Fe3O4 nanoparticles was prepared according to a

previous report [30]. A mixed aqueous solution of FeCl3·6H2O andFeSO4·7H2O (2:1 wt%) was vigorously stirred to dissolve under ni-trogen gas, and NH3·H2O (28%) was quickly added to precipitate Fe2+/Fe3+ ions for the synthesis of magnetite (Fe3O4) nanoparticles at 50 °C.Then, the black products were washed with water and ethanol threetimes and dried under vacuum at 60 °C for 12 h.

500 mg Fe3O4 was mixed in 50 mL methanol using ultrasonic stir-ring for 30 min. Then, the homogeneous solution was added drop-by-drop in 10 mL methanol containing 70 mg of HCCP and 158 mg of BPS,and stirred for 15 min. Subsequently, 185 μL TEA was added dropwiseand stirred for 6 h continuously. The compounds were collected andwashed with methanol and dried in a vacuum at 40 °C for 24 h. Theresulting materials are labeled as MPZS.

2.1.4. Synthesis of C-MPZS-XSyntheses of C-MPZS-X and C-PZS-X were from PZS or MPZS (~1 g)

calcined at a series of temperatures for 2 h in an Ar atmosphere with aphase heating program. The heating program involved increasing thetemperature from room temperature to 200 °C at 5 °C min−1, then itwas enhanced from 200 °C to a target temperature (i.e., 500, 650 or800 °C) at 2 °C min−1, and finally, the temperature was maintained atthe target temperature for 2 h. The obtained products were denoted asC-MPZS-X or C-PZS-X, where X is the carbonated temperature.

2.2. Characterization

Thermogravimetric analyses (TGA, TGA5500, USA) were performedat temperatures ranging from room temperature to 1000 °C with a rampof 20 °C min−1 under air atmosphere. The morphology of the productswas identified by a transmission electron microscope (TEM, JEOL, JEM-2100, JP). Fourier transform infrared (FTIR) spectroscopy was con-ducted on a Perkin-Elmer Spectrum GX with a range of 4000–400 cm−1

at room temperature. Powder X-ray diffraction (P-XRD, Bruker, D8Rigaku9000, GER) utilizing a Cu Kα radiation source at a scanning rateof 5°/min at 40 mA and 40 kV with 2θ ranging from 10° to 90° wasmeasured. The element composition and content was tested using X-rayphotoelectron spectrometer (XPS, Thermo ESCALAB 250XI, ThermoFisher Scientific, USA) that contained a twin anode Al Kα (1486.6 eV)X-ray source. The magnetic properties were characterized using vi-brating sample magnetometer (VSM, LakeShore7404, USA) at roomtemperature. N2-adsorption/desorption isotherms (Micromeritics ASAP2460, USA) were used to measure the state of the pores in the samplesafter degassing under vacuum at 200 °C for 10 h. The specific surfacearea (SSA) was calculated according to the multi-point Brunauer-Emmett-Teller (BET) model. Further, the nonlocal density functionaltheory (NLDFT) model was used to analyze the pore size distribution(PSD) based on the desorption branches of the isotherms. C/H/N ele-mental analysis (EA) was performed by combustion analysis (VarioMACRO cube, Elementar, GRE).

2.3. Electrochemical characterization

Electrochemical characterization was carried out by an electro-chemical workstation (CHI 660E, chinstruments CH Instruments, CHN).The configuration of a three-electrode system, which used a reference

electrode of Ag/AgCl (saturated 1 M KCl) and counter electrodes of1 × 1 cm2 Pt foil, was tested in 1 M NaCl. The work electrode slice wasprepared with an 80 wt% active component, 10 wt% CB and 10 wt%PVA to form homogeneous slurries after stirring in the proper amountof DI water. The mixture was coated onto a titanium mesh (1 × 3 cm),and the electrodes were dried at 35 °C for 1 h and then dried at 110 °Cfor 1 h in a vacuum oven.

The cyclic voltammetry (CV) curves were performed at a series ofscan rates (v) from 10 and 50 mV s−1 in the potential range from 0 to1.0 V. The electrochemical impedance spectroscopy (EIS) scan fre-quency ranged from 0.01 Hz to 100 kHz with an oscillation amplitudeof 5 mV at open circuit voltage (OCV). The specific capacitance (Cs) wascalculated directly from using the Eq. (1):

=I

mv VCs

dV2 (1)

where m (g) is the quality of the active substances in the electrode, v(V s−1) is the voltage scan rate, I (A) is the response current and ∆V (V)is the potential difference.

2.4. FCDI experiments

2.4.1. Slurry preparationThe suspension electrodes were prepared by mixing the as-prepared

materials with a dispersing agent (SN-5040, Suzhou Yunfeng ChemicalTechnology Co. Ltd., CHN) in an DI water solution, and the mixture wasstirred for 60 min to disperse completely, and then ultrasonic crushingoccurred for 30 min. The weight ratio of the as-prepared materials tothe SN-5040 was 4.0:1.0 wt%. An active carbon (AC) powder(~1000 m2 g−1) was studied for comparison.

2.4.2. FCDI configurationAs shown in Scheme S1(a), an FCDI cell was composed by a spacer

chamber (thickness of ~1000 μm) made of a silicone gasket(90 × 90 mm) between the anion- and cation- exchange membranes(CEM/AEM, ~600 μm). The carved serpentine flow channels on thetitanium plate were 3 mm wide and 3 mm deep, which were used ascurrent collectors. Each column was a line (length of 60 mm) followinga semicircle (inner/outer diameter of 3 mm and 9 mm) at the end of thecolumn. A total of 11 columns connected from the inlet to the outlet tocreate an effective contact area (ECA) of 19.80 cm2. All the parts arefastened together with two polyvinyl chloride (PVC) end plates.

2.4.3. FCDI operation and performance calculationsAs shown in Scheme S1(b), the operation of the FCDI test was

analyzed in an batch mode with approximately 120 mL of simulatedwastewater continuously cycled by a peristaltic pump at several dif-ferent flow-rate values, and the electrical conductivity (EC) of the feedsolutions were continuously monitored every 1 min by an online con-ductivity gauge (S470 Seven Excellence, METTLER TOLEDO, ZRH).Flow electrode slurries (25 mL, 2 wt% each) were operated in an iso-lated closed-cycle (ICC), which were continuously cycled between theFCDI device and two stirred beakers by a dual-head peristaltic pump(Longer pump, Baoding, CHN) at varying flow rates. In one FCDI op-erating process, electrosorption was carried out at a constant chargingvoltage (0.6–1.2 V) using a DC power supply (MP3094, Powertech,CHN), and then, desorption was performed by mixing two electrodeslurries for electrode regeneration. Furthermore, the experiment of re-moving ions in real wastewater from tail water of a textile printing anddyeing wastewater treatment plant was investigated. The applied po-tential was set at 1.0 V within the FCDI adsorption process unlessotherwise specified. The curve of NaCl concentration vs. conductivitydiagram, as shown in Fig. S1.

The salt ion removal efficiency (SEC), salt ion adsorption capacity(SAC), charge efficiency (CE) and average salt adsorption rate (ASAR)were characteristic parameters to quantify the performance of the FCDI

D. Li, et al. Desalination 482 (2020) 114374

3

cell and calculated according to Eqs. (2)–(5) [9]

= ×SEC(%) (C C )C

100%0 e

0 (2)

where C0 and Ce (mg/ L) are the initial and final ion concentrations offeed solution, respectively.

= ×SAC(mg g ) (C C ) Vm

1 0 e(3)

where m (g) is the quality of the dry flow electrode; V (L) is the volumeof the feed solution.

= × ××

×Idt

CE (%) F V (C C )M

100%0 e

(4)

where F (96,485C mol−1) is the Faraday constant; I (A) is the current;and M (58.5 g mol−1) is the molar mass.

=×

ASAR (mg min 1 cm 2) (C C )Vt S

0 e(5)

S (19.8 cm2) is the effective contact area; t (min) is the adsorption time.

3. Results and discussion

3.1. Material characterization

In Fig. S2, TGA analysis of the MPZS confirmed that the thermaldecomposition process has two phases that are both below 1000 °C.According to the results of the TGA curve, the two highest weight lossesappeared at point 1 (~500 °C) and 3 (~800 °C), as well as a middleweight loss phase of point 3 (~650 °C). Thus, 500, 650, and 800 °C werechosen as the carbonization temperatures to analyze the as-preparedelectrode materials. Additionally, the increasing peak intensity of 24°suggested that the high carbonization temperature can promote thesuperimposed graphite phase again, which is in agreement with theRaman spectra and TEM results.

Fig. S3(a)–(i) shows the macrostructure of C-MPZS-X by TEMimages. Clearly, the as-prepare materials of C-MPZS-X shows a core–-shell structure and a very mono-disperse regular ball, with an averagediameter of (50.0 ± 15.0) nm. Additionally, these TEM images showthat Fe3O4 has a good adjacent junction to form a short chainlikemorphology instead of a single nanoball within C-MPZS-X; this propertycan be seen in Fig. S3(h) clearly. As the carbonization temperatureincreases, the graphitic phase stacking becomes clearer, the crystallattice of the as-prepared materials becomes more blurred, and thelattice spacing decreases due to the destruction of the uniform core-shell structure that is destabilized at higher temperatures. Meanwhile, itcan be seen that the black Fe3O4 nanoballs are coated by an even, thingray carbon shell to form short 1D nanochains by head-to-tail interac-tions that accumulate with each other. These structures contributesignificantly to increased electrosorption capacitance by adding moreion-accessible space. However, some stacking and interpenetrationwithin the structure of C-MPZS-800 are shown in Fig. S3(i). There aresigns that a nearly graphitic carbon phase is formed after carbonizationby a structural pre-organization as the temperature increases.Meanwhile, the SEM also show the standard ball shape, and C-MPZS-500 has a good adjacent junction to form a short chainlike morphologyinstead of a single nanoball, as shown in Fig. S4.

Furthermore, the FTIR spectra also confirmed the successfulsynthesis of C-MPZS-500/650/800. As shown in Fig. 1(a), the appear-ance of the bands at 3500 cm−1 is attributed to the stretching vibrationof the eOH groups, which has high surface activity to bind other ma-terials. The peak at 907–1067 cm−1 can be assigned to the PeOeC andPeN band, which represents the successful polycondensation reactionbetween BPS and HCCP [31]. The peak at 572 cm−1 corresponds toFeeO stretching vibrations. The peak at 1630 cm−1 was assigned to thestretching vibrations of C]N bonds [9]. The O]S]O group has a peak

at 1382 cm−1 [32]. Meanwhile, the more hydrophilic group containedin C-MPZS is conducive to the colloidal stability in water to develophomogeneous dispersions due to electrostatic repulsion among thegroups. Furthermore, similar results are shown in the analyses of theXRD and Raman spectra.

The element content in C-MPZS-X was analyzed by XPS and C/H/NEA, as summarized in Table 1. Obviously, all doped atoms (N/P/S/Fe)are confirmed to exist in the as-prepared materials of C-MPZS-500/650/800 in the XPS and EA tests. Additionally, the maximum contentsof N, P and S occur at 500 °C and decrease as the temperature increasesto 800 °C due to the formation of more CO2, CO and H2O or others bymining carbon, oxygen and hydrogen during the pyrolysis process asthe temperature increases. Meanwhile, the type content of Fe3O4 is lostat high temperature, which indicates the crystal structure of Fe3O4undergoes some degree of reduction, i.e., the high temperature nega-tively affects the stability of the materials. Clearly, 500 °C is the optimaltemperature for doping more atoms into the carbon skeleton [33].

The XRD pattern was used to investigate the structure of the C-MPZS-X products. As shown in Fig. 1(b), the as-prepared C-PZS signedcharacteristic hexagonal graphitic stacking peaks that correspond to the(002) and (100) plane at 24.1° and 43.5°, respectively, indicate theamorphous nature of the PZS. Meanwhile, the XRD result of C-MPZS-Xis also elucidate a face-centered cubic phase of Fe3O4, and two char-acteristic peaks of the as-synthesized C-PZS can be readily found in C-PZS-X. Thus, the XRD results further confirmed that the surfaces of theFe3O4 core were successfully coated with a PZS shell and the crystalstructures of these substrates were well maintained. However, theFe3O4 was reduced to Fe(0) during the carbonization procedure whentemperature higher than 800 °C, confirmed that the iron content notdecreased much but VSM has reduce.

XPS spectroscopy is utilized to determine the surface compositionsand chemical properties of the doped C-MPZS-X, as shown in Fig. 1(c).The XPS results show N, P, S and Fe exist in C-MPZS-X. Deconvolutionof the Fe 2p signal shows that Fe 2p3/2 (~711.2 eV) and Fe 2p1/2(~724.8 eV) spin orbit peaks exist [34], which suggest that the Fe3O4was incorporated into the composites instead of γ-Fe2O3 (~710.3 and~724.0 eV) [35], as shown in Fig. S5(a). It is worth noting that theFe3O4 nanoball plays an important role for the product of active sitesand broadens the voltage window, which is confirmed in the CV test(Fig. 2(a)). N-deconvolution results, shown in Fig. S5(b), indicate thatfour different types peaks exist at 396.8 eV (pyridinic/FeNx), 398.3 eV(pyrrolic), 399.8 eV (graphitic-N) and 401.4 eV (NeO) [18,36,37].Meanwhile, the P 2p spectra were deconvoluted into a single peak inFig. S5(c). The peaks of PeC and PeO bonding correspond to bindingenergies of ~132.4 eV and ~134.7 eV, respectively. The as-prepared C-MPZS-X also allows the introduction of more active sites that depend onthe oxygen bridge develop by the PeO bond with eCe and Ce(PeOeC) bonds [34]. Furthermore, the S 2p spectra were deconvolutedinto three peaks in Fig. S5(d), including 165.97 eV and 166.93 eV thatcorrespond to S 2p3/2 and S 2p1/2, as well as 168.2 eV that fitted withSeO and eCeSeCe [34].

In Fig. 1(d), the magnetic sensitivity of as-prepared C-MPZS-X wasmeasured by the VSM instrument at room temperature. The highestmagnetization saturation values (Ms) of C-MPZS-500/650/800 were54.99, 49.09 and 39.15 emug−1, respectively. Meanwhile, their coer-civity and remanence were zero, suggesting that they are super-paramagnetic. The Ms value of C-MPZS-X decreased as the carboniza-tion temperature increased due to a small loss of the magneticcomponent in the composite after pyrolysis. However, the magnetism ofall carbonization products already is strong enough for fast and easymagnetic separation after electroadsorption. Therefore, good magneticproperties ensure the simple recovery of electrode particles.

The Raman spectrum curves of C-MPZS-X are shown in Fig. S6. Thecarbon disorder phases (D-band) at ~1350 cm−1 indicate the structuraldefects in the graphitic plane, while the crystallized graphitic sp2

carbon (G-bond) existed at ~1595 cm−1 [9]. The ratio of ID/IG is used

D. Li, et al. Desalination 482 (2020) 114374

4

to estimate the degree of disorder and defect-sates within the graphiticstructure [9]. Obviously, the largest value of ID/IG (3.55), which re-presents a higher level of disorder and defect-states, is carbonization at500 °C. A higher ID/IG means more defects in the carbon structure werecontributed by the defect-sates. Therefore, the ID/IG value decreased at650 °C (2.67) and increased at 800 °C (3.29), which indicates that ahigher temperature is conducive to ordering of the carbon matrix. Theresults also show that heteroatom doping can introduce more defects ofcarbonization products, which is more effective than order/graphiti-zation.

In Fig. S7, the N2 adsorption–desorption isotherms were performed

to investigate the specific surface area (SSA) and pore size distribution(PSD). The results show typical IV isotherms with H4-type hysteresisloops, which represent the micro- and mesoporous properties of C-MPZS-X. Additionally, the data of the PSD curve indicated that thecontrollable pore located at 3.0–4.4 nm was derived from the BJHanalysis, further verified the micro- and mesoporous structure of thesynthesized CDI electrodes. In Fig. S7(b), the pore size is larger withcarbon temperature increased, due to the organic matter lost and gas-forming pore generated. Meanwhile, the SSA of the as-prepared mate-rials ranged between 56.74 and 133.09 m2 g−1, as shown in Table 1.Moreover, SSA decreased as the temperature increased, which is

4000 3500 3000 2500 2000 1500 1000 500

P=NP-O-C

Fe-O

MPZSC-MPZS-500C-MPZS-650C-MPZS-800

Inte

nsit

y/ a

.u.

Wavenumber/ cm-1

(a)C=N

O=S=O

-OH

10 20 30 40 50 60 70

220

311

400422

511

Inte

nsit

y

2

C-MPZS-500 C-MPZS-650 C-MPZS-800 C-PZS

(b)

400

Fe0

0 100 200 300 400 500 600 700

S2p

Fe2sO1s

N1sC1s

P2p

Cou

nts

/ s

Binding Emergy / ev

C-MPZS-500 C-MPZS-650 C-MPZS-800

(c)

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

-50

-25

0

25

50

M /

emu

g-1

Field / G

C-MPZS-500 C-MPZS-650 C-MPZS-800

(d)

Fig. 1. (a) FT-IR survey spectra of C-MPZS-500/650/800; (b) PXRD measurements of C-PZS and C-MPZS-500/650/800; (c) XPS line scans of C-MPZS-500/650/800;(d) VSM curves of C-MPZS-500/650/800.

Table 1Summary of characterization results as well as experimental carbon yield of the C-MPZS-X materials.

T/°C 500 650 800

at.% C N P S Fe C N P S Fe C N P S Fe

XPS 64.9 2.9 5.8 3.0 23.4 65.5 2.2 2.5 1.9 27.8 68.8 2.7 4.4 2.9 21.2C/H/N EA 70.9 14.2 – 14.4 – 73.5 13.9 – 12.0 – 76.6 11.7 – 11.1 –SSABET 133.09 m2 g−1 78.06 m2 g−1 56.74 m2 g−1

Contact angle 25.18° 30.25° 41.91°ID/IG 3.55 2.67 3.29VSM 54.99 emug−1 49.09 emug−1 39.15 emug−1

Yield 61.18% 55.11% 41.46%

D. Li, et al. Desalination 482 (2020) 114374

5

ascribed to the destruction of the carbon crystal structure at hightemperatures, as evident by the TEM images (Fig. S3). Additionally,Fe3O4 was reduced to Fe(0) during carbon-reduction procedure at hightemperature. Due to the density of Fe(0) is higher than the Fe3O4, theas-prepared materials would occupy a smaller volume with same massat higher temperature [38]. In a previous study, more micropores andmesoporous brought in more active sites to adsorb ions [39]. Overtly,C-MPZS-500 signs a highly micro- and mesoporous structure, which iscritical for its capacitive and desalt properties.

In addition, the contact angle of the as-fabricated products is shownin Fig. S8. The water droplets method was used to investigate the sur-face hydrophilicity of C-MPZS-X, and the results are summarized inTable 1. Clearly, as the carbonization temperature increases, the hy-drophilicity of C-MPZS-X increases. Hence, C-MPZS-500 has the besthydrophilicity because it contains rich-hydrophilic functional groups,which are caused by the doping of heteroatoms. After all, high hydro-philicity is beneficial to improve the electrosorption performance of theC-MPZS-X electrode.

3.2. Electrochemical performance

The CV survey for all formulated electrode materials is presented inFig. 2(a) to evaluate the electric adsorption performance of the as-synthesized materials. Obviously, all the as-synthesized materials had

no apparent oxidation/reduction peaks under the potential range of−0.6 to 0.4 V, which suggests that the main process of ion adsorption isthe Coulomb interaction instead of the electrochemical reaction to formEDLs on the electrode surface [9]. However, because the intrinsic re-sistance and polarization effect exist in the prepared materials, a slightdifference from the standard rectangular shape exists. Nevertheless, theCV curves show a typical capacitive characteristic and approximaterectangular shapes, indicating that reverse voltage scanning can acquirethe charge platform quickly, which contributes to quick adsorption/desorption of salt ions on the electrodes.

Based on the calculations from the CV curves at 1 mV s−1, thespecific capacitance (Cs) of C-MPZS-500/700/900 and AC is211.54 F g−1, 160.38 F g−1, 151.51 F g−1 and 125.25 F g−1, respec-tively. Obviously, the Cs of the C-MPZS-500 electrode was the highestdue to its low internal resistance and rich micro-/mesoporous. The re-latively integrated structure has more exposed surface and offers moreactive sites, which contributes to the quick transport of ions through theinterface between the solution and material [9].

Moreover, CV analysis of C-MPZS-500 was performed at differentscan rates from 1 to 50 mV s−1, as shown in Fig. 2(b). However, the CVcurve displays a degree of distortion from the typical rectangular shapeat high scan rates due to the effects of inherent resistivity and polar-ization within the electrode and salt solutions. Meanwhile, Fig. 2(c)show that the Cs at scan rates of 1, 5, 10 and 50 mV s−1 are

-0.6 -0.4 -0.2 0.0 0.2 0.4

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8C

urre

nt d

ensi

ty /

A g

-1

Potential / V

CMPZS-800 CMPZS-650 CMPZS-500 AC

(a)

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Fig. 2. (a) CV curves of the C-MPZS-500/650/800 and AC electrode at a potential sweet rate of 1 mV s−1; (b) CV curves of the C-MPZS-500 electrode at different scanrates; (c) the specific capacitance of the C-MPZS-500 electrode at 1–50 mV s−1; (d) Nyquist plots of the C-MPZS-500/650/800 and AC electrode in 1 M NaCl solution.

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211.28 F g−1, 176.37 F g−1, 165.18 F g−1 and 50.41 F g−1, respec-tively. The lower scan rates offer enough time for salt ions to migrateand accumulate from the solution to the electrode materials, whichsuggests that the more pore utilization of SSA occurs within the elec-trode that offers an excellent capacitive behavior.

In Fig. 2(d), the electrochemical impedance spectra (EIS) weremeasured in the 1.0-M NaCl solution. The EIS curves consist of twoparts. One part is the intersection with the coordinate axis that

corresponds to the solution inner-resistance and contact resistance be-tween the salt ion solution and electrode and the semi-circle suggestingthe charge transfer resistance (Rct) [40]. The other part in the lowfrequency region is a straight line showing the ionic diffusion im-pedance. Clearly, all four samples have small semicircles at high fre-quencies, suggesting a low Rct. Meanwhile, as the carbonization tem-perature increases, the C-MPZS electrode conductivity increases andionic diffusion occurs more quickly at the low frequency region as

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Fig. 3. (a) The conductivity variation with deionization time; (b) the SAC, SEC and ASAR curves of C-MPZS-500/650/800 and AC electrodes in in an initialconductivity of NaCl solution at a cell voltage of 1.0 V with a flow rate of 6 rpm; FCDI performance of the C-MPZS-500 electrode: (c) the variation of conductivityalong with the desalination time and (d) the SAC, SEC and ASAR curves at different applied voltages; (e) the SAC, SEC and ASAR curves at different salt concentrationand (f) CE with different initial NaCl concentrations at different voltage.

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confirmed by the straight lines sharply sloping toward the imaginaryaxis [40]. However, overall, the interface contact resistance values ofall the as-fabricated materials are low, and it can be ensure the elec-trosorption process.

3.3. FCDI performance

The performance of the FCDI cell is investigated under differentoperating conditions in the NaCl aqueous solution. The FCDI cell isassembled by a symmetrical flowing-electrodes solution that is fabri-cated by the as-synthetized electrodes. Then, the effect of deionizationwas compared with commercial AC electrodes.

In Fig. 3(a), the influence of the as-fabricated electrodes at differentcarbonization temperatures on the FCDI effect was investigated with aninitial conductivity of 1200 μS cm−1 at a potential of 1.0 V with aneffluent speed of 6 rpm and a flowing-electrode solution. Clearly, all theFCDI cells that have the as-synthetized electrodes have a drastic de-crease within 60 min, which indicates that those electrodes have a rapidand easy adsorption of the salt ions. Meanwhile, a better deionizationperformance was observed by C-MPZS-X than the commercial ACelectrode. As the absorption progresses, the conductivity graduallydecreases until approaching electrosorption equilibrium within100 min due to the electrode reaching its electrosorption saturationwith an increased electrostatic repulsion that takes place at the ad-sorbed ions on the electrode surface [41]. Fig. 3(b) depicts the SACvalues of C-MPZS-500, C-MPZS-650 and C-MPZS-800 electrodes are50.07 mg g−1, 44.35 mg g−1 and 32.89 mg g−1, respectively, as well asSEC are 68.50%, 60.67% and 45.00%, respectively, which are bothhigher than AC electrode. Furthermore, good FCDI performance is de-fined by a high value of ASAR, that is, the C-MPZS-500 shown a bettersalt ion removal rate. The high desalination performance of preparedelectrodes at low carbonization can be attributed to (i) rich-functionalgroups of hydrophilicity on the surfaces of synthetized electrode, (ii)the excellent core-shell morphological structure that allows salt ions to

rapidly and simply accumulate on the surface of the electrode throughintra-channels, and (iii) heteroatoms doping into the carbon lattice thatleads to more active sites, tuning of the charge/electron spin density,offering a variable-valence space for chemisorption of oxygen and in-crease of active sites that can be taken for a successful path towardachieving enhanced special capacitance and increased ion concentra-tion to enlarge the ion store space [18–20].

In Fig. 3(c), the effects of applied potentials (0.6–1.2 V) on the FCDIcell performance of the C-MPZS-500 electrode were investigated. Ob-viously, the adsorption curves show a sharp dropped within 60 min andreached adsorption equilibrium near 120 min, suggesting that C-MPZS-500 electrode absorbs salt ions quickly. In Fig. 3(d), as the appliedvoltage increased, the SAC improved from 32.89 to 55.43 mg g−1, theSEC enhance from 45.00% to 75.83% and the ASAR increased. Thoseexhibiting higher potential could generate stronger electrostatic inter-action, thus providing greater removal of salt ions.

To further investigate the stability of the FCDI performance under ahigh concentration NaCl solution at an applied voltage of 1.0 V. TheFCDI cell was tested in a concentration of NaCl aqueous that rangedfrom 0.4 to 1 g L−1. The results in Fig. 3(e) show that as the NaClconcentration increased, the SAC of C-MPZS-500 electrode enhancedfrom 43.25 mg g−1 to 53.30 mg g−1 but SEC decreased from 90.10% to44.42%. Meanwhile, the ASAR was increased with salt ion concentra-tion enhance, due to the high concentration has high conductivity that

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Fig. 4. FCDI regeneration performance of the C-MPZS-500 electrode in an in-itial conductivity of NaCl solution at 1.0 V with a flow rate of 6 rpm.

Table 2Comparison with other methods.

Raw materials Applied voltage/V Salt solution/NaCl CE/% SAC/mg g−1 ASAR Ref.

Two-Dimensional Molybdenum Carbide (CNTs/MXene) 0.8 5–600 mM ~95 15 2.1–5.9 μg g−1 s−1 [42]CNT-MoS2 0.8 5–500 mM 76–95 10–25 2.5–7 μg g−1 s−1 [43]ZIE-8/ZIF-67 1.4 500 mg/L – 11.4–12.3 [44]Activated carbon-amination/sulfonation 1.2 55 mS cm−1 68.7–85.6 – 1.5–5.5 mM s−1 m−2 [45]C-MPZS-X 0.6–1.0 ~1 g L−1 64.0–93.0 43.3–53.3 2.5–3.1 mg s−1 m−2 Our work

Table 3Index and treatment effect of tail water come from three textile printing anddyeing wastewater treatment plant.

Conductivity/μS cm−1 pH/25 °C TOC/ppm η/%

Plant 1 883 8.2 6.48 98.86Plant 2 1441 7.9 5.86 71.66Plant 3 2356 8.3 4.15 50.34

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Fig. 5. The treatment effect of tail water come from three textile printing anddyeing wastewater treatment plant.

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majority of the charged ions were adsorbed in the flow electrodes ea-sily. Obviously, the results show that a high adsorption mass and anexcellent removal ability of NaCl allow EDLs to easily and fleetly formin a high conductivity salt solution due to a quick transportation rate ofsalt ions toward the electrode spacing channel that depends on a de-crease in ionic resistance. Whereas, the acquired quantity of ions in thesalt solution is much larger than the adsorption capacity of the as-prepared material until storage saturation of the ions in the pores re-sults in reduced removal rates. In addition, the CE of C-MPZS-500 en-hances with salt ion concentration at same applied voltage, as welldecreases with applied voltage at same concentration, as shown inFig. 3(f). The results show that the lower potential can limit to avoidparasitic reactions and rapid performance loss, as well the low ioncontent obtain in low ion concentration also can acquire low chargeefficiency.

Furthermore, the FCDI performance stability of the C-MPZS-500electrode during continuous cycling was investigated with an initialconductivity of 1200 μS cm−1, applied voltage of 1.0 V, and flowelectrode weight content of 5.0 wt%. Over 10 cycles, the results inFig. 4 show an excellent stable desalt performance with SAC that rangesfrom 50.07 to 41.42 mg g−1. Therefore, the good reuse process ex-hibited good regeneration performance. In Table 2, compared withother published carbon electrode materials, the desalt property of ourwork is superior to that of previous work. Due to the rich heteroatomiccontent and excellent water wettability, which can bring a pseudoca-pacitance increased specific capacitance to promote capacitive deioni-zation at low applied voltage. As well, it can obtain a larger specificcapacitance and faster ions adsorption rate [9,46]. Meanwhile, our as-prepared materials as flow-electrode can realized recycling easily bymagnetism, which show excellent recycle performance compared withother reported materials. In summary, the electrode of C-MPZS-500 hasbroad application prospects in deionization.

Finally, to evaluate the applicability of the FCDI method in apractical matrix, it was applied to removing salt ions from tail water ofan actual textile printing and dyeing wastewater treatment plant thatwas treated by a process that included pre-sedimentation, A2O, ozoneand biological aerated filter (BAF) technology in sequence. The realwastewater samples were acquired from plants in Zhongshan,Guangdong province; then, they were filtered through a 0.25 μm hy-drophobic membrane and stored at 4 °C in the dark until analysis. Thetail water index and treatment effect on the printing and dyeing was-tewater are shown in Table 3. Obviously, the pH values of all sampleswere< 8.3 at 297 K, which indicates that these water samples werenearly neutral, which is beneficial to the stable use of electrode mate-rials. Meanwhile, the appropriate values of TOC and initial conductivityshow that the FCDI process is good for treating the tail water of textileprinting and dyeing wastewater. Removal studies were carried out by aFCDI device comprising the C-MPZS-500 electrode, and the results areshown in Fig. 5. The conductivity removal efficiency of Plant 1, Plant 2and Plant 3 are 98.86, 71.66 and 50.34%, respectively, after 240 min.However, the different ions removal results are mainly caused by thedifference of initial concentration of wastewater. The better removaleffect is attributed to the low initial salt ion concentration which isconfirmed in Fig. 3(e). Meanwhile, it can be observed that the lowerTOC value has a weak influence on the deionization process. Overall,the results show that the FCDI device matched with the C-MPZS elec-trode has good desalinization performance and has potential applica-tion prospects.

4. Conclusion

In conclusion, a simple route to synthesize an intrinsic and homo-geneous transition metal (Fe) and non-metal atoms (N, P, S) dopedcore-shell structure electrode material is presented. The C-MPZS elec-trodes inherited all the advantages of PZS, i.e., good wettability, lowimpedance, high specific capacitance, long-term colloid stability and

favorable water dispersibility due to the non-metallic atom doping.Meanwhile, the perfect uniform distribution of transition metal atomsin the carbon lattice lead to a favorable recycle performance based onexcellent superparamagnetism, as well as rapid charge transport due tothe plentiful reaction active sites. Finally, the successful application totail water of printing and dyeing wastewater treatment proved that theC-MPZS electrode has broad application prospects in deionization.

CRediT authorship contribution statement

Danping Li: Methodology, Validation, Formal analysis,Investigation, Data curation, Writing - original draft, Writing - review &editing, Visualization. Xun-An Ning: Conceptualization, Supervision,Funding acquisition, Writing - review & editing. Chenghai Yang:Software. Xiaohui Chen: Resources. Yi Wang: Methodology, Software.

Declaration of competing interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgments

The authors thank the support of Local Innovative and ResearchTeams Project of Guangdong Pearl River Talents Program(2017BT01Z032), the 2017 Central Special Fund for Soil, PreliminaryStudy on Harmless Treatment and Comprehensive Utilization ofTailings in Dabao Mountain (18HK0108), and Natural ScienceFoundation of China (Grant No. 21577027).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.desal.2020.114374.

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