Removal of heavy metal ions by capacitive deionization: Effect of surface modification on ions adsorption

Removal of heavy metal ions by capacitive deionization: Effect of surface modification on ions adsorption

Journal Pre-proof Removal of heavy metal ions by capacitive deionization: Effect of surface modification on ions adsorption Htet Htet Kyaw, Myo Tay Zar...

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Journal Pre-proof Removal of heavy metal ions by capacitive deionization: Effect of surface modification on ions adsorption Htet Htet Kyaw, Myo Tay Zar Myint, Salim Al-Harthi, Mohammed Al-Abri

PII:

S0304-3894(19)31519-5

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121565

Reference:

HAZMAT 121565

To appear in:

Journal of Hazardous Materials

Received Date:

15 April 2019

Revised Date:

9 October 2019

Accepted Date:

28 October 2019

Please cite this article as: Kyaw HH, Myint MTZ, Al-Harthi S, Al-Abri M, Removal of heavy metal ions by capacitive deionization: Effect of surface modification on ions adsorption, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121565

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Removal of heavy metal ions by capacitive deionization: Effect of surface modification on ions adsorption Htet Htet Kyaw1, Myo Tay Zar Myint3, Salim Al-Harthi3 and Mohammed Al-Abri1,2* 1

Nanotechnology Research Center, Sultan Qaboos University, P.O. Box 33, Al-Khoudh,

Muscat 123, Sultanate of Oman 2

Petroleum and Chemical Engineering Department, Sultan Qaboos University, P.O. Box 33,

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Al-Khoudh, Muscat 123, Sultanate of Oman

Physics Department, College of Science, Sultan Qaboos University, P.O. Box 36, Al-

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Khoudh, Muscat 123, Sultanate of Oman

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*Email: [email protected]

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Highlights

Pure and ZnO nanoparticles coated ACC electrodes used in CDI system.



The coating of ZnO NPs on ACC suppresses the surface functional groups.



By adding of ZnO NPs on ACC enhanced electric field at the electrode surface.



Enhanced electric field improved Pb2+ and Cd2+ ions adsorption.

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Abstract

Activated carbon cloth (ACC) coated with zinc oxide (ZnO) nanoparticles (NPs) have been used as electrodes in flow-by capacitive deionization (CDI) system. Aqueous solution of individual Pb2+ and Cd2+ ions and mixed Pb2+ and Cd2+ ions were used as test contaminant in 1

CDI system to study the effect of surface modification upon ions removal efficiency. Due to the aggregated structure of ZnO NPs on ACC surface, the modified ACC electrodes develop the additional surface area as well as dielectric barrier therefore resulting in higher specific capacitance. In addition, coating with ZnO NPs effectively reduced physical adsorption whereby enhanced the ions adsorption rate and capacity during electrosorption process. Upon incorporating with ZnO NPs, the electrosorption efficiency was enhanced from 17% to 33% for Pb2+, from 21% to 29% for Cd2+ and from 21% to 35% for mixed Pb2+ and Cd2+ ions. The

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power consumption of individual ions and mixed ions removal process for ACC and ZnO NPs modified ACC were also discussed. Furthermore, used ACC electrodes surfaces were examined using photoelectron spectroscopy (XPS) and results were also conferred. The CDI

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ACC electrodes with ZnO NPs showed a promising and an effective way for heavy metal

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removal applications.

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oxide nanoparticles

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Keywords: Electrosorption; Desorption; Capacitive deionization; Surface modification; Zinc

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1. Introduction Wastewater desalination is considered as alternative approach for fresh water resources to solve the water scarcity globally [1]. In wastewater, heavy metal pollution has become an

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environmental concern due to the disposal of industrial waste water, modern society in construction sites, in processed food, batteries, mining, pesticides and fertilizers from

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agricultural sector [2-4]. Most metal ions are non-biodegradable and toxic to living organisms

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and have the ability to bind the nucleic acids, proteins and small metabolites. The organic cells get contaminated by heavy metal ions leading to numerous health problems and even

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death in extreme cases [5]. It is, therefore imperative to develop efficient methods to remove heavy metal ions from wastewater. Currently, heavy metal ions removal process includes

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chemical precipitation, ion-exchange, adsorption, membrane filtration, electrochemical processes and so on [6-10]. These technologies have their own disadvantages. For instance

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chemical precipitation technique produce large amount of sludge and secondary pollution, ion-exchange processes produce secondary waste resulting from the regeneration of resins, membrane filtration techniques and electrochemical processes require high energy consumption and hard maintenance [11]. Capacitive deionization (CDI) has received great attention lately due to its environmentally friendly, energy efficient and low cost technology for waste water treatment [12]. CDI is an 3

electrosorption process that operates with low DC voltage (0.6-2.0 V) to remove ions. The basis CDI process contains two consecutive steps: (1) desalination and (2) regeneration. In principle, the negatively charged electrode (cathode) attracts cations and positively charged electrode (anode) attracts anions under an applied electric field during desalination process. In regeneration process, the electric field is removed and short two electrodes or reversing the applied potential to release the adsorbed ions back into the regenerated solution [13]. Porous structure carbon materials similarly activated carbon (AC), activated carbon cloth (ACC),

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carbon aerogel, activated carbon fiber, graphene and carbon nanotubes have been used as electrode material in CDI systems [14-20]. Recently, Chang et al. fabricated threedimensional mesoporous honeycomb graphene cluster based CDI electrodes for abundant

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ions adsorption and a rapid and easy ions transport pathway [21]. Hai et al. developed highperformance porous structure activated carbon electrodes from date seed biomass [22] and

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pseudocapacitive RuO2 was electrodeposited onto activated carbon by cyclic voltammetry

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technique to produce superior charge electrosorption capacity electrodes was proposed by Ma et al. [23]. Wu et al. reported non-carbon CDI electrodes made of three-dimensional ordered mesoporous titanium nitride which facilitated rapid ions diffusion with excellent cycling

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stability [24]. The electrode materials are the key component in CDI unit cell and several techniques have been developed to enhance the adsorption capacity for improving the CDI

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performance. The electrode materials modified with metal oxide nanostructures are

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remarkably improve the electrosorption performance [14, 25, 26]. The incorporation of metal alkoxide on the ACC surface reduces the physical adsorption and increases the electrosorption performance [27]. The change in dielectric property of modified nanostructured electrode can be enhanced the effective electric field which in turn to enhance the ions removal efficiency and adsorption capacities [28]. In addition, metal oxide nanorod

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modified ACC surfaces generate uniform electric field along the nanorod surface which make the electrode capacitance increase and improve the desalination efficiency [29]. Extensive studies have been reported on modified electrodes used in salt removal process [26, 27, 29] and some studies have been used CDI technique for removal of heavy metal ions [30, 31]. Chen et al. fabricated amorphous O-doped boron nitride (BNO) nanosheets as electrode material for removal of Cd2+ ions from water by CDI approach. The BNO electrode showed an excellent removal of Cd2+ ions concentration ranging from 600 ppm down to 0.05 ppm

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[30]. Recently, Liu et al. reported 3D graphene surface grafted with ethylenediamine triacetic acid (EDTA) and functionalized with 3-aminopropyltriethoxysilane (APTES) to use as cathode to adsorb Pb2+ and Na+. In their work, the co-ions effect in CDI system was

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minimized for separation and recovery of heavy metal ions and salt ions from wastewater

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[31]. However, no study has reported the modified activated carbon cloth (ACC) electrodes applied to use in CDI system to remove individual and multiple heavy metals ions from

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aqueous solution. In our study described ACC electrodes modified with zinc oxide (ZnO) NPs for the removal of individual Pb2+ and Cd2+ and mixed Pb2+ and Cd2+ from aqueous

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solution by means of capacitive deionization. Our results demonstrate that modified ACC electrodes were more efficient in removing ions than pure ACC (without modified) under

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constant applied potential. The improved in electrosorption of ions can be attributed to the increased specific capacitance with less physical adsorption. Post examination of used pure

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ACC and ZnO-ACC electrodes’ surface properties were examined by XPS technique to understand the surface nature of electrodes after the electrosorption performances. 2. Experimental 2.1. Materials

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Woven activated carbon cloth (Zorflex FM-100, USA) with a thickness of approximately 1 mm and specific surface area of about 1000 m2/g was used as electrodes in CDI system. Activated carbon cloth is chemically inert, naturally hydrophilic because of the presence of hydroxyl groups, less leaching (without using any binder), flexibility and good electrical conductivity. Hydrochloric acid 34% (HCl, analytical grade) was purchased from Merck, Germany. An analytical grade zinc oxide nanoparticle in powder form, cadmium nitrate and lead nitrate were obtained from Sigma-Aldrich. All the chemicals were used as received

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without further purification. 2.2. Activated carbon cloth cleaning

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ACCs were cleaned with concentrated 2 M hydrochloric acid (HCl) and kept boiled for 12 hours to remove any mineral contaminants such as aluminum, zinc, calcium, sodium and

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sulfur etc. that are commonly present in ACC. Subsequently, samples were thoroughly rinsed with tap water and DI water until pH of the rinsing water reached to ca. 6.5 - 7. Then the

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samples were dried in an oven at 90 °C for 48 hours and stored in a desiccator for further use.

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2.3. Surface modification of ZnO nanoparticles on ACC 0.0625 g of commercial zinc oxide nanoparticles with size of 10-30 nm was dispersed in 12

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ml of ethanol. The as prepared solution was sprayed using commercial spray gun on activated carbon cloth (ACC) (with the size of 10 cm  10 cm) while heating at 350 C. After spraying,

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ZnO nanoparticles (NPs) coated ACC substrates were annealed in a furnace (Carbolite, CWF 11/5) at 350 C for 1 hour to enhance the adhesion between ZnO NPs and ACC surface. 2.4. Characterizations XRD patterns of ACC and ZnO-ACC electrodes were obtained using X-ray diffraction (Miniflex 600, Rigaku, Tokyo, Japan) with Cu K radiation in the scanning range from 10 6

to 90 in 0.05/s steps. Surface morphologies of ACC and ZnO-ACC were characterized by field emission scanning electron microscopy (JEOL JSM-7800F, Japan) working at 15 kV and working distance of 10 mm. Energy dispersive X-ray spectroscopy (EDXS) was carried out using Oxford instruments (X-Max, UK) detectors and data were interpreted by AZtec nanoanalysis software. The Fourier-transform infrared spectroscopy (FTIR) spectra were recorded by an infrared spectroscopy (PerkinElmer, SpectraOne, USA) with KBr pellets in the range 400 - 4000 cm-1. The signal resolution of 4 cm-1 with 40 scans was applied in this

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characterization. X-ray photoelectron spectroscopy (XPS) spectra were carried out using multiprobe photoelectron spectroscopy (Omicron Nanotechnology, Germany) system. In this system, a monochromatic Al K radiation (h= 1486.6 eV) was generated with 15 kV and an

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emission current of 20 mA. Constant analyzer transmission energy of 50 eV was used for wide scan and individual element peaks were recorded at 20 eV. As charging effects are

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unavoidable in the XPS study for non-conducting samples, charge compensation was

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performed by flooding with electrons. The obtained XPS spectra were deconvoluted to their individual components using Gaussian Lorentzian function after background subtraction with

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Shirley function in Casa XPS software (Casa Software Ltd, UK). The binding energies were calibrated with respect to adventitious C 1s feature at 284.6 eV. Cyclic voltammetry (CV) measurement were conducted using Interface-1000 potentiostat (Gamry, USA) with a 0.5 M

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NaCl electrolyte at room temperature. CV was performed at scan rate of 1 mVs-1, 5 mVs-1

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and 10 mVs-1 with scan ranges from -0.4 V DC to +0.4 V DC. The area of the ACC electrode was maintained at 1 cm2 and the weight of the electrodes was approximately 0.037 g for both ACC and ZnO-ACC respectively. The electrochemical impedance spectroscopy (EIS) was conducted by using the three electrode system while pristine ACC and ZnO-ACC were used as working electrodes (electrode area ~ 1 cm2), platinum and Ag/AgCl electrodes were used as counter and reference electrodes. EIS spectra of pristine ACC and ZnO-ACC were

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measured with the frequency range between 110-3 Hz and 1106 Hz at 10 points per decade in the 0.5 M NaCl solution with AC amplitude of 10 mV. 2.5. CDI cell structure, experimental set up and ions removal experiments The CDI unit cell consists of two parallel symmetrical ACC electrodes (anode and cathode) with the dimensions of 4.5 × 4.5 cm (total area of ca. 20.25 cm2) separated by a cellulose spacer (Whatman) having a thickness of approximately 600 μm, a reservoir made up of polymethacrylate

(PMMA)

(Dow

Corning

SYLGARD®

184

(USA) Silicone

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methyl

elastomer kit), graphite electrodes used as current collector were all attached on acrylic sheets as shown in Fig. 1a.

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Batch-mode electrosorption processes (adsorption and regeneration) were conducted in a continuously recirculating system including CDI cell and an online electrical conductivity

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probe with a cell volume of 3.2 ml. The conductivity probe was attached to an online single

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channel conductivity isopod (EPU357) integrated with PodVu software. Prior to the experiment, the conductivity probe was calibrated with 0.1 N (normality) potassium chloride

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solution to obtain the specific conductivity of a solution. Multiple conductivity measurements were carried out and average values were taken during CDI experiment. 80 ml 0.5 mM

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Pb(NO3)2, Cd(NO3)2 and 0.5 mM mixed Pb(NO3)2 and Cd(NO3)2 solutions were continuously pumped using a peristaltic pump drive 5201 (Heidolph Instruments, Germany) into the CDI

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cell with a flow rate of 3 ml/min and the effluent returned to the feed container (See Fig. 1b). Heavy metal ions removal process was carried out by applying the potential of 1.8 V (DC). Prior to the electrosorption process, the test solution was circulating into the CDI system without applying potential in order to saturate the electrodes. The conductivity data was recorded during electrosorption and regeneration processes. The ions removal efficiency and the ions adsorption capacity of ACC and ZnO modified ACC electrodes were estimated by 8

the conductivity change (the solution conductivity was calibrated with the solution concentration before the experiment) during the electrosorption process as [14, 17]: 𝐼𝑜𝑛𝑠 𝑟𝑒𝑚𝑜𝑣𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =

𝐶0 −𝐶𝑓

𝐼𝑜𝑛𝑠 𝑎𝑑𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 =

𝐶0

× 100 %

(𝐶0 −𝐶𝑓 )𝑉

(1)

(2)

𝑚

where C0 and Cf are initial and final concentrations and V (L) is the total volume of the test

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solution and m (g) is the total mass of the electrodes. The current profile of the CDI cell during the electrosorption process was monitored using GW Instek GDM-396 online multimeter (Good Will Instrument Co., Taiwan). Obtained

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current profile was further used to calculate the power consumption for the particular type of test solution. In detail, the total charge deposited on the electrode in one cycle was calculated

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by integrating area under the curve of charging current decay curve in Matlab (Mathworks,

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Natick, Massachusetts, USA). The charge deposited was multiplied by the applied potential (1.8 V) to obtain the work done in joules. Then the work done in joules was divided by the

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duration of one electrosorption cycle to obtain the power consumption in watts (W).

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Fig. 1. Schematic representation of (a) symmetrical CDI cell assembly and (b) batch mode CDI system 3. Results and discussion: 3.1 Electrodes structural morphology and surface characterization Typically, activated carbon cloth is woven structure composed with micron size carbon fibers (image is not shown here). Fig. 2a shows FESEM (Field emission scanning electron

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microscope) image of carbon fibers from a pristine ACC surface. The surface of the carbon fiber is lobular shape with lengthwise striation and lack of contaminations. The average diameter of carbon fiber is approximately 10-14 m and apparently it is a solid structure (See

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Fig. 2a inset). Fig. 2b shows ZnO NPs coated ACC surface and inset in Fig. 2b shows high magnification image of ZnO NPs coated single ACC fiber. It can be clearly seen that ZnO

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NPs are randomly coated on ACC surface with aggregated fashion. Even though the

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morphology of ZnO was revealed as an aggregated structure, ZnO NPs with specific amounts of 0.625 mg/cm2 was deposited on ACC surface. The average particle size of ZnO is between

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10 to 30 nm having hexagonal wurtzite crystal structure (TEM image and XRD data is not shown here). The presence of Zn and O elements were confirmed by EDXS which is depicted

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in Fig. 2c. Fig. 2d shows X-rays diffraction (XRD) pattern of pristine ACC and ZnO NPs coated ACC. The broad XRD peaks at 25 and 43.1 were attributed to (002) and (10) or

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(100) planes of typical graphitic carbon materials originated from ACC [32]. After deposition of ZnO NPs on ACC surface, the characteristic peaks of ACC still exist with an extra peak at 36.5 which may correspond to (101) plane of ZnO NPs (JCPDS No. 361451) [33]. However, any distinct planes from ZnO NPs on ACC surface were not witnessed.

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Fig. 2. (a) FE-SEM images of pristine ACC and (b) ZnO NPs coated ACC (Inset shows high magnification image of ZnO NPs coated single fiber). (c) XRD pattern of pristine ACC and

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ZnO NPs coated ACC (d) XRD diffraction patterns of ACC and ZnO NPs coated ACC surface

XPS was conducted to investigate the surface composition and the chemical state of the elements presented on ACC surfaces. Fig. 3a shows XPS survey spectra of ACC and ZnO NPs coated ACC surfaces. From the survey spectra, C1s peak at 284.6 eV was from graphitic

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nature of ACC surface as well as the adventitious carbon. Insignificant amount of Zn (Zn 2p peak) was observed on pristine sample surface which might come from the presence of impurities on the surface. High intensity Zn peaks (Zn 2p, Zn 3p, Zn 3d and Zn 3s) were detected on ZnO-ACC survey spectrum. No other traces of impurities were observed on both of the sample surfaces. After depositing with ZnO NPs on ACC surface, the enhancement in O 1s peak intensity was observed which emerged from the deposited ZnO nanomaterial. From high resolution XPS spectrum of O 1s peak for ZnO-ACC sample (Fig. 3b), two

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Gaussian components can be observed which centred at 531.3 eV and 532.9 eV respectively. The peak at lower binding energy (BE), 531.3 eV, can be assigned as chemisorbed oxygen (O2) while higher BE, 532.9 eV, attributed to OH group absorbed on the ZnO NPs surface

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[34]. Fig. 3c shows core level Zn 2p peaks for ZnO-ACC sample where Zn 2p3/2 and Zn 2p1/2 were observed at binding energies of 1022.2 eV and 1045.6 eV with a spin orbit splitting of ~

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23 eV which affirmed the zinc in ZnO form [34].

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(c)

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Fig. 3. (a) XPS survey spectra of pristine ACC and ZnO NPs coated ACC (b) high resolution XPS spectra of O 1s and (c) Zn 2p FTIR spectroscopy was employed to study the functional groups of pristine ACC and ZnOACC (see Fig. 4) surface. A broad peak at 3450 cm-1 corresponds to the O-H stretching vibration. The bands at 2920 cm-1 and 2850 cm-1 are assigned to the stretching modes of C-H bands. The transmission bands at 1554 cm-1 and 1635 cm-1 are related to the stretching vibrations of C=O in quinone and carboxylate groups. The band around 1165 cm-1 is

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attributed to the stretching modes of C-O species which can be observed in acids, esters or ethers [35, 36]. From the results, it can be realized that after coating ACC surface with ZnO shows a reduction in functional groups such as C-H, C=O and C-O respectively (See Fig. 4).

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The functional groups obtained from FTIR results on ACC and ZnO-ACC is in a good

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agreement with XPS spectra of C 1s peaks in Fig. S1 (supporting information).

Fig. 4. FTIR spectra of pristine ACC and ZnO-ACC 3.2 Electrochemical characterization of electrodes Cyclic voltammetry (CV) measurements for pristine ACC and ZnO-ACC with a scan rate of 1 mVs-1, 5 mVs-1 and 10 mVs-1 and the corresponding specific capacitance are shown in Fig. 5. At lower scan rate (Fig. 5a), both pristine ACC and ZnO NPs coated ACC displayed the 13

quasi-rectangle shape. The shapes of voltammogram were changed to leaf structure when the scan rates were increased to 5 and 10 mVs-1 (Fig. 5b and c). The changes of the shape can be explained by the following pattern. At higher scan rates, the ions have less time to be adsorbed into the pores of ACC from the electrolyte solution. Additionally, ionic diffusion was limited in order to access to the small pore (for instance, micropore with diameter of less than 2 nm) at higher scan rate. This limitation of ions diffusion into the pores creates overlapping double layer at the entrance of the pores. Therefore, there was no obvious

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electrochemical responses shown in Fig. 5 under 5mV/s and 10mV/s scan rates as a results of the overlapping double layer formation at the entrance of the micropores and that can decrease the capacitance [15]. On the other hand, at lower scan rate, the ions can migrate

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easily into the pores which simply interpreted to the higher capacitance. The CV curves for pristine ACC and ZnO-ACC for all scan rates were symmetrical structure with smooth trend.

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These can be attributed to the absence of electrochemical reaction (non-faradaic reaction)

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occurred on the pristine ACC and ZnO-ACC surfaces.

In addition, ZnO NPs coated ACC (namely “electrode”) has higher specific capacitance than

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pristine ACC for all scan rates (Fig. 5d). An increase in specific capacitance attributed to the enhancement in electric field distribution at the electrode surface [15]. The specific

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capacitances (Cs) of ACC and ZnO-ACC electrodes were calculated by the following

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equation [14];

𝐸

𝑖(𝐸)𝑑𝐸

𝐶𝑠 = ∫𝐸 2 2(𝐸

2 −𝐸1 )𝑚𝑣

1

(3)

where E1 and E2 are the initial and final voltage, m is the mass of the electrode, v is the scan rate and i(E) dE is the area under the curve of CV graph. By applying equation 3, the specific capacitance of ACC electrode was found to be 55 F/g at 1 mVs-1, 14 F/g at 5 mVs-1 and 4 F/g at 10 mVs-1 respectively. In the case of ZnO NPs coated ACC electrode, the specific 14

capacitance was 63 F/g at 1 mVs-1, whereas decreased to 19 F/g and 7 F/g at a scan rate of 5 mVs-1 and 10 mVs-1. After coating with low dielectric material like ZnO, capacitance value was enhanced by 15 %, 40 % and 65 % for 1, 5 and 10 mVs-1. Previously, we reported that the enhancement of specific capacitance by coating with one dimensional nanostructure metal oxide semiconductor which built up localized electric field under applied potential [14, 15]. Recently, Laxman et al. reported the effect of dielectric materials (ZnO and TiO2) coating on ACC electrodes by a finite model approach. Their simulation results showed that NPs

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dielectric coating improved the localized electric field due to the induced dipoles formation within the NPs. Furthermore, cyclic voltammetry and impedance measurements showed stronger double-layer capacitance behaviour of the ACC electrodes coated with ZnO or TiO2

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[28]. (b)

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Fig. 5. CV curves of 1cm2 areas of pristine ACC and ZnO NPs coated ACC electrodes with scan rates of (a) 1 mVs-1, (b) 5 mVs-1, (c) 10 mVs-1 and (d) the specific capacitance of pristine ACC and ZnO NPs coated ACC Electrochemical impedance spectroscopy was carried out to study the electrical properties of pristine ACC and ZnO-ACC. Fig. 6 displays real and imaginary impedance in Nyquist plot of pristine ACC and ZnO-ACC. Three main components were observed as a semi-circle in high frequency region, a non-vertical line in intermediate frequency region (almost semi-circle in

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this study) and a straight line in low frequency region. The semi-circle portion resembles to the parallel combination of the charge transfer resistance and double layer capacitance between the electrode and electrolyte [37]. In ZnO-ACC, the decreased in semi-circle

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indicated the smaller charge transfer resistance mainly from the coating of ZnO nanoparticles

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on ACC surface. In addition, the change in series resistance was also observed in ZnO-ACC when compared with the pristine ACC. The series resistance can be estimated by the real-axis

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(Zreal) intersection of the impedance spectrum. The coating of ZnO on ACC provides ZnOACC electrode has more contact area (increased in surface area which can be seen in BET

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results in Table S1) to expose with the ionic species from the electrolyte solution which resulted in the reduction of series resistance and charge transfer resistance. In the

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intermediate frequency region, the unusual second semi-circle appears (non-vertical straight line was observed by some reported studies in this frequency region which related to the ions

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transport limitation and non-uniform electrodes pore sizes and roughness [38, 39]) in both ACC and ZnO-ACC probably due to the large amount of micropores (<2nm) present in ACC which hinder the diffusion of ions into the micropores (BET results in Table S1 show high content of micropores). This phenomenon can also be observed in CV (Fig. 5b and c) as the micropores cannot easily access the ions at higher scan rates and limits the ions diffusion. The straight line in low frequency region resembles to the ions diffusion process of Warburg 16

diffusion and the capacitive behaviour obtained from the electrical double layer formation at

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the electrode/electrolyte interface [40].

Fig. 6. The Nyquist impedance plot of pristine ACC and ZnO-ACC

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3.3 Heavy metal ions removal

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3.3.1. Pb2+ ions removal

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Fig. 7a shows the electrosorption performance of 0.5 mM Pb(NO3)2 on ACC and ZnO-ACC electrodes with an applied potential of 1.8 V DC. Prior to the electrosorption process,

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electrodes were saturated with test solution for overnight to avoid the disruption of physisorption in the process. Once both of the electrodes were reached to the saturation, the

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electrosorption process was started under constant voltage mode. When DC voltage (1.8 V) was applied to the CDI system with pristine ACC electrodes, the solution conductivity was

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slightly increased (shown by dotted ellipse, Fig. 7a). The increased electrical conductivity was due to the releasing of some physisorbed ions on the electrodes surface while counter ions were repelled by charged electrodes (cations were driven off by anode electrode and anions were driven off by cathode electrode). The physisorbed ions so called physical absorption was obtained from the accumulation of ions on ACC surface without any applying electrical potential. This is due to the presence of copious polar functional groups on pristine 17

ACC surface which serve as ions adsorption sites for physical adsorption [27]. The existence of numerous functional groups on ACC surface can be observed in FTIR (Fig. 4) and C1s in XPS analysis (Fig. S1). Conversely, no physical adsorption was occurred for ZnO NPs modified ACC electrodes in which the solution conductivity was barely increased at the initial state of applying potential. In the case of incorporating with ZnO NPs on ACC surface reduced the functional groups of ACC which resulted in a substantial decrease in the physical adsorption (see Fig. 7a.). Although ZnO NPs were not covered all over ACC surface (see Fig.

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2b), most of the functional groups were suppressed by binding with ZnO NPs. Furthermore, decrease in surface functional group on ACC surface was also confirmed by FTIR (see Fig. 4) and C1s peaks in XPS spectra (Fig. S1). The quantitative analysis from C 1s peak revealed

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that the amount of intrinsic carbon was reduced in ZnO-ACC compared with pristine ACC

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which proved the suppression of surface functional groups after coating with ZnO NPs. Ions adsorption process was carried out for 50 minutes and desorption process was performed

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for 54 minutes in which 4 minutes for reversing the polarity and 50 minutes for shorting the electrodes to return the initial conductivity value for both pristine ACC and ZnO-ACC

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electrodes. The electrical conductivity profile shows reproducible results for multiple cycles of ions adsorption and desorption processes for both cases (Here, we present only 3 cycles

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and actual process is carried out for more than 20 cycles). Fig. 7b shows the current profile for CDI system equipped with pristine ACC and ZnO-ACC electrodes during electrosorption

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and regeneration process. Slightly higher current reading (during charging) was observed for ZnO-ACC CDI system than pristine ACC and ZnO-ACC CDI system showed faster ion adsorption. It was noted that the electrical conductivity decreased from 240 S/cm to 200 S/cm for pristine ACC and 240 S/cm to 160 S/cm for ZnO-ACC after 50 minutes of electrosorption process. Two fold increment of electrosorption was observed for ZnO-ACC surface in which total ions removal of ~ 17% for pristine ACC and ~ 33% for ZnO-ACC 18

were achieved. Slight improvement of specific capacitance could help to escalate the electrosorption process. Detail of the electrosorption enhancement will be discussed in the later section. Furthermore, ZnO-ACC electrodes were saturated slower than pristine ACC which could lead to higher ion adsorption capacity. The specific ions adsorption capacity for pristine ACC was ca. 6.41 mg/g and 15.67 mg/g for ZnO-ACC, respectively. The current profile during adsorption process (Fig. 7b; inset) was used to estimate total power consumed by CDI cell. The total power consumption for pristine ACC was 0.047 kWh and ZnO-ACC

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was 0.044 kWh (power consumption and ion adsorption capacity are depicted in table 1) and a slight reduction in power consumption was obtained for ZnO-ACC system.

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Fig. 7. (a) The conductivity profile of Pb2+ ions adsorption and desorption onto pristine ACC and ZnO-ACC electrodes over three repeated cycles and (b) the current profile of Pb2+ ions

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adsorption and desorption process for one cycle; Inset shows the area under the Pb2+ ions adsorption curve for one cycle to calculate the total charge deposited on the electrodes during the adsorption process 3.3.2. Cd2+ ions removal

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Electrosorption and desorption process of 0.5mM Cd(NO3)2 on pristine ACC and ZnO-ACC electrodes with the applied potential of 1.8 V is shown in Fig. 8a. Ions adsorption and desorption process time and preconditioning of the CDI electrodes were same as mentioned above. When the potential was introduced to CDI system with ZnO-ACC electrodes, the conductivity reading was less compare with pristine ACC electrodes (see Fig. 8a indicated with black arrow) due to less physical adsorption of Cd2+. During electrosorption process, the conductivity decreased from 240 μS/cm to 190 S/cm whereby 21 % of Cd2+ ions was

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removed by CDI system equipped with pristine ACC electrodes. Slight enhancement of electrosorption on Cd2+ ions over Pb2+ was observed in which low physisorption of Cd2+ ions at the initial state made this enhancement. In contrast, Haung et al. reported that Cd2+ has less

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tendency to remove due to large hydrated radii (4.26 Å) and small charge valence (+2) [41]. A small physical adsorption on ZnO-ACC electrodes surface was due to more affinity of Cd2+

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ions than Pb2+ ions under zero electrical potential (so called physisorption process). The

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conductivity decreased from 240 μS/cm to 170 S/cm by which 29 % of Cd2+ ions were removed during electrosorption process. Notably, high conductivity reading was observed on

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ZnO-ACC electrodes during regeneration process (Fig. 8a indicated with dotted ellipse) due to fast desorption of ions from the electrodes surface by reversing the polarity of applied

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potential. This phenomenon was only experienced on ZnO-ACC electrodes and pristine ACC electrodes were responded as typical conductivity profile.

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Cd2+ ions adsorption capacity for pristine ACC and ZnO-ACC were found to be 7.69 mg/g and 12.33 mg/g respectively. A small physical adsorption occurred in ZnO-ACC system could lower Cd2+ ions electrosorption performance when compared to Pb2+ ions. The smaller physisorption could impact on electrosorption by following process. Under an applied potential, the physisorbed ions released back to the feed solution repelling by charged electrode (positively charged electrode repels cations). In the meantime, the electrodes attract 20

opposite charged ions (positively charged electrode attracts anions) and there was a competition between co-ions and counter-ions resulted in lower electrosorption efficiency for Cd2+ ions. Fig. 8b shows current-time transient of Cd2+ ions adsorption and desorption on pristine ACC and ZnO-ACC electrodes. Decreasing of charging current for ZnO-ACC CDI system led to lower ions removal while pristine ACC CDI system maintained the same current value. Thus, insignificant changes of ions removal were noted for pristine ACC electrodes. The power consumption was estimated and found to be 0.061 kWh for pristine

consumption was observed for ZnO-ACC system (table 1).

(b)

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(a)

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ACC and for ZnO-ACC was 0.034 kWh. Due to lower charging current, lower power

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Fig. 8. (a) The conductivity profile of Cd2+ ions adsorption and desorption onto pristine ACC and ZnO-ACC electrodes and (b) the current profile of Cd2+ ions adsorption and desorption

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process for one cycle

3.3.3. Mixed Pb2+ and Cd2+ ions removal Fig. 9a shows the electrosorption results under an applied voltage of 1.8 V DC and 0.5 mM of mixed Pb(NO3)2 and Cd(NO3)2 solution used as test contaminant. CDI system was equipped with pristine ACC and ZnO-ACC electrodes. In order to compare with the individual ions 21

adsorption processes, all the experimental parameters such as physisorption and electrosorption were maintained the same as mentioned before. While the DC potential was introduced to the pristine ACC electrodes, physisorbed ions from the electrodes surface released back to the solution by observing the increased conductivity of the solution. However similar response was not observed in ZnO-ACC electrodes CDI system. During electrosorption process, the conductivity reduced from 240 μS/cm to 190 S/cm which displayed the ions removal of 21 % for the CDI system equipped with pristine ACC

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electrodes. As expected, ions removal performance of ZnO-ACC electrodes was higher than pristine ACC electrodes and ions removal efficiency about 35 % was attained.

The specific ions adsorption capacity for pristine ACC and ZnO-ACC on mixed Pb2+ and

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Cd2+ ionic solution system were noted as 8.40 mg/g and 15.67 mg/g respectively. Fig. 9b

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shows current-time transient of pristine ACC and ZnO-ACC electrodes during adsorption and desorption process using mixed Pb2+ and Cd2+ ionic solution. The power consumption was

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estimated to be 0.035 kWh for pristine ACC and 0.036 kWh for ZnO-ACC.

(b)

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(a)

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Fig. 9. (a) The conductivity profile of mixed Pb2+ and Cd2+ ions adsorption and desorption onto pristine ACC and ZnO-ACC electrodes and (b) the current profile of mixed Pb2+ and Cd2+ ions adsorption and desorption process for one cycle In order to compare the performance of ions adsorption rate, electrosorption kinetics of pristine ACC and ZnO-ACC electrodes, 1st order pseudokinetics reactions and normalized relative to the mass of the electrodes were used in equation 1 as: 𝐶

ln ( 0 ) = 𝑘𝑡

(1)

𝐶𝑡

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1 𝑚

where Ct is the ions concentration at time t (min) and k (1/g-min) is the rate constant of the 1st order pseudokinetics reaction [42, 43]. Fig. 10a, b and c show Pb2+, Cd2+ and mixed Pb2+

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and Cd2+ ions adsorption rates of pristine ACC and ZnO-ACC electrodes. ZnO-ACC

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electrodes show faster electrosorption than pristine ACC for Pb2+ (0.057 and 0.011 1/g-min), Cd2+ (0.045 and 0.026 1/g-min) and mixed Pb2+ and Cd2+ ions (0.040 and 0.026 1/g-min)

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respectively. For pristine ACC, the lower ions adsorption rate was observed due to the releasing of some physisorbed ions on the polar functional groups of ACC surface prior to the

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electrosorption process. To support the electrosorption kinetic of pristine ACC and ZnO-ACC electrodes, ions adsorption capacity versus ions adsorption rate (Ragone plot) was derived

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from the conductivity profiles of Pb2+, Cd2+ and mixed Pb2+ and Cd2+ ions. CDI Ragone plot in Fig. 10d, e and f, ZnO-ACC electrodes displayed faster ions adsorption rates and better

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ions adsorption capacity than pristine ACC on all cases. This is attributed to the incorporation of ZnO nanoparticles on ACC surface which suppressed the surface fuctional groups to reduce the physical adsorption and enhanced electric field at ZnO surface for higher ions adsorption capacity. Based on electrosorption kinetic and adsorption capacity, Cd2+ ion was more affinity than Pb2+ on pristine ACC electrode and Pb2+ ions was faster and higher ions adsorption than Cd2+ on ZnO-ACC electrode surface for individual ions removal process. For 23

mixed Pb2+ and Cd2+ ions, slowest ions adsorption rate and reduction of adsorption capacity was observed on both pristine ACC and ZnO ACC electrodes. (b)

(c)

(d)

(e)

(f)

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Fig. 10. Electrosorption kinetics of (a) Pb2+, (b) Cd2+, (c) mixed Pb2+ and Cd2+ and CDI Ragone (Kim-Yoon) plots of (d) Pb2+, (e) Cd2+, (f) mixed Pb2+ and Cd2+ for ACC and ZnO-

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ACC

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Enhanced ions adsorption was achieved for ZnO-ACC electrodes for individual (Pb2+ or Cd2+) as well as mixed (Pb2+ and Cd2+) ions compared with pristine ACC electrodes. In

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principle, semiconductors can enhance the charge storage under applied potential by generating dipoles within the material core and on its surface. These store charges produce an

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associated electric field around them and a combination of each charges magnify the effective electric field to attract more ions in their surrounding area. Laxman et al. reported a theoretical model specifically the effect of semiconductor dielectric materials upon the generation of electric field by using different oxide materials. In their model, materials with low dielectric constant (SiO2 and ZnO) could generate a stronger surface electric field than high dielectric materials (ZrO2 and TiO2). In order to compare the performance of low 24

dielectric materials with high value materials, we also used TiO2 as high dielectric material (results not shown here) and deposited on ACC electrodes used in CDI system. It was observed that ZnO NPs coating on ACC showed higher CDI performance than TiO2 coating on ACC. The performance enhancement was due to lower dielectric constant of ZnO than TiO2 where low dielectric constant develops stronger electric field. The enhanced electric field at the surface could favour higher ions adsorption within a CDI system [44]. Conversely, the coating of ZnO NPs on ACC (ZnO-ACC) could reduce the number of micro

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and mesopores on ACC surfaces (confirmed by BET results) which could trigger the reduction of the active area for ion adsorption. Therefore, coating with ZnO NPs at specific concentration develops the additional surface area (confirmed by BET results) which

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moderately compensate the blocking of micro and mesopores on ACC surfaces. However,

efficiency (the results are not shown here).

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further increment of ZnO concentration could lead to decrease in heavy metal ions removal

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Table 1. Power consumption and ions adsorption capacity obtained from ACC and ZnOACC electrodes

Pb2+

Cd2+

Pb2+ + Cd2+

Pb2+

Cd2+

Pb2+ + Cd2+

0.047

0.061

0.035

6.41

7.69

8.40

0.044

0.034

0.036

15.67

12.33

15.67

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ACC

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ZnO-ACC

ions adsorption capacity (mg/g)

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Power consumption (kWh)

Electrodes

3.3.4. Post characterization of surface chemistry of ACC electrodes To study the surface nature of pristine ACC and ZnO-ACC electrodes toward individual Pb2+, Cd2+ and mixed Pb2+ and Cd2+ ions, the ACC electrodes after electrosorption experiments were qualitatively analysed by XPS. Fig. 11 shows the survey spectra and high resolution 25

scan of used ACC and ZnO-ACC electrodes. Significant amount of individual Pb2+, Cd2+ and mixed Pb2+ and Cd2+ ions were detected on used ACC and ZnO-ACC electrodes which affirmed the existence of these metals after electrosorption processes. Insets in Fig. 11a and d show high resolution spectra of Pb 4f7/2 and Pb 4f5/2 at binding energies of 138.8 eV and 143.6 eV on used ACC (ACC-Pb) and 141.7 eV and 146.6 eV on ZnO-ACC (ZnO-ACC-Pb) surface with a spin splitting of 4.7 eV. Fig. 11b and e show Cd 3d5/2 and Cd 3d3/2 at binding energies of 405.5 eV and 412.4 eV for used ACC (ACC-Cd) and 405.6 eV and 412.3 eV for

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ZnO-ACC (ZnO-ACC-Cd) regions with a spin splitting of ~ 6.7 eV [45]. Qualitative analysis of Pb2+, Cd2+ and mixed Pb2+ and Cd2+ ions was carried out by comparing area under the graph of the main peak (Pb 4f and Cd 3d). From the results, it was observed that ZnO-ACC

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electrodes has 31.1% and 11% reduction of Pb and Cd element for individual Pb2+ and Cd2+ ions electrosorption process. For mixed ions electrosorption (Fig. 11c and f), the amount of

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both Pb and Cd ions were decreased to 7.4% and 41.3% respectively. The decreased in Pb

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and Cd in CDI system equipped with ZnO NPs coated ACC electrodes were due to the reduction of surface functional group which could favour chemically bonded to ions. These are in good agreement with the results of individual ions (Pb or Cd) and mixed ions (Pb +

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Cd) electrosorption performances where the electrosorption efficiencies were improved by

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coating with ZnO NPs.

ZnO is known to be amphoteric material which could dissolve under slight changes in pH of

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the solution. This type of dissociation might have happened in our system where some of the coated ZnO NPs were released during electrosorption and desorption processes. The attached Pb2+ or Cd2+ and both Pb2+ and Cd2+ ions were also released along with ZnO NPs resulted in reduction of Pb and Cd on ZnO-ACC electrodes after used. Notably, N 1s peak was also detected in all spectra together with cations peaks (Pb and Cd). Nitrogen peak was appeared from the nitrate compound (NO3)2 with BE value of ~ 400 eV. The presence of nitrate 26

compound (probably as nitrate ions (anions)) could be due to electrosorption when the electrodes polarities were reversed in regeneration process. Please note that in our CDI system the electrodes were experienced both positive and negative polarity during desalination and regeneration processes.

(b)

(d)

(e)

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(f)

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Fig.11. XPS survey spectra of individual Pb2+, Cd2+ and mixed Pb2+ and Cd2+ ions attachment

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on pristine ACC (a, b and c) and ZnO-ACC (d,e and f) after electrosorption process; insets in (a) and (d) show high resolution spectra of Pb 4f and in (b), (c), (e) and (f) show high resolution spectra of Cd 3d

Declaration of interests

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☒ 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.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Conclusions

Pristine ACC and ZnO NPs coated ACC have been used as electrodes in CDI system for heavy metal ions removal process. It was observed that the coating of ZnO NPs on ACC

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surface significantly improved the specific capacitance of the electrodes due to the enhanced electric field at the electrode surfaces. The enhanced electric field favoured higher ions

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adsorption and thereby improved the single Pb2+, Cd2+ ions and mixed Pb2+ and Cd2+ ions

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electrosorption performance. In addition, ZnO NPs coating on ACC surface suppresses the surface functional groups with significantly decreased the physical adsorption. As a result, the electrosorption efficiency of ZnO-ACC electrode increased from 17% to 33% for Pb2+, from 21% to 29% for Cd2+ and from 21% to 35% for mixed Pb2+ and Cd2+ compared to pristine ACC electrode. Post examination of used ACC and ZnO-ACC electrodes’ surface properties revealed the coating of ZnO NPs reduced the adsorption or binding of Pb and Cd.

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Although XPS results show detection of nitrate compound (nitrate anion), more comprehensive research of different anions on the performance of CDI will be conducted in the future study. This work suggested that semiconductor modified ACC electrode was a promising candidate to improve the CDI performance in removing heavy metal ions. For further and future studies, ZnO can decorate with noble metal NPs or SiO2 NPs to prevent the dissociation in harsh environment as well as to enhance the deionization performance in CDI system.

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Acknowledgement

The authors acknowledge the partial support of Nanotechnology Research Centre and Surface

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Science Lab, Department of Physics, College of Science. We would also like to thank Mr. Nasser Mohammed Al-Mandhari, Petroleum and Chemical Engineering lab, College of

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Engineering, Sultan Qaboos University for BET measurement.

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