Adsorption behavior of powdered activated carbon to control capacitive deionization fouling of organic matter

Adsorption behavior of powdered activated carbon to control capacitive deionization fouling of organic matter

Journal Pre-proofs Adsorption behavior of powdered activated carbon to control capacitive deionization fouling of organic matter Tianyu Wang, Heng Lia...

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Journal Pre-proofs Adsorption behavior of powdered activated carbon to control capacitive deionization fouling of organic matter Tianyu Wang, Heng Liang, Langming Bai, Xuewu Zhu, Zhendong Gan, Jiajian Xing, Guibai Li, Tejraj M. Aminabhavi PII: DOI: Reference:

S1385-8947(19)32689-0 https://doi.org/10.1016/j.cej.2019.123277 CEJ 123277

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

21 August 2019 4 October 2019 22 October 2019

Please cite this article as: T. Wang, H. Liang, L. Bai, X. Zhu, Z. Gan, J. Xing, G. Li, T.M. Aminabhavi, Adsorption behavior of powdered activated carbon to control capacitive deionization fouling of organic matter, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123277

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© 2019 Published by Elsevier B.V.

Revised Manuscript for Chemical Engineering Journal Date: October-4th-2019

Adsorption behavior of powdered activated carbon to control capacitive deionization fouling of organic matter Tianyu Wanga, Heng Lianga*, Langming Baia, Xuewu Zhua, Zhendong Gana, Jiajian Xinga, Guibai Lia, Tejraj M. Aminabhavib a

State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, PR China b Pharmaceutical Engineering, Soniya College of Pharmacy, Dharwad 580002, India

E-mail: [email protected] (T. Wang); [email protected] (H. Liang); [email protected] (L. Bai); [email protected] (X. Zhu); [email protected] (Z. Gan); [email protected] (J. Xing); [email protected] (G. Li); [email protected] (T.M. Aminabhavi). * Corresponding author. E-mail address: [email protected] (H. Liang).

Abstract: Capacitive deionization (CDI) is a promising desalination technology, but it suffers from serious irreversible fouling caused by the presence of organic matter, which severely reduces the desalination performance. In this research, powdered activated carbon (PAC) was used to reduce the fouling caused by the presence of humic acid (HA). Due to the effective removal of HA by PAC adsorption, HA deposited onto the CDI electrode was reduced by about 40%. Desalination performance of CDI was improved when PAC adsorbed HA entered into the CDI system, resulting in the removal of HA under the action of an electric field in the CDI system, thereby alleviating the irreversible CDI fouling caused by HA. Alternatively, PAC entering the CDI system improved electrical conductivity due to the formation of electrical double layers (EDLs) in the flow path, thereby reducing the internal resistance of CDI system to increase the capacitance of CDI system. The presence of PAC in the CDI system was responsible to reduce the deposition of organic matter onto the electrode as well as the resistance of CDI system, resulting in optimized specific surface area and conductivity to improve the CDI performance. Keywords: Powdered activated carbon; Capacitive deionization; Humic acid; Fouling alleviation

1. Introduction As a new desalination technology, capacitive deionization (CDI) has attracted much attention in recent years[1] due to its high energy utilization and high energy recovery while dealing with low-concentration salt solutions compared to conventional desalination technology[2, 3]. CDI has a high recovery rate[4], low working voltage[5, 6] and low operating pressure[7, 8], which makes it a convenient process for routine studies. Conventional CDI relies primarily on the electrical double layers (EDLs) formed by the Coulombic forces to store ions including charging and discharging. During the charging process, ions of the solution accumulate onto the surface as well as inside the electrode to form the EDLs, thereby facilitating the ion removal. When migration of ions in the EDLs is saturated, CDI would undergo a discharge process and ions are released from the EDLs by a short circuit, which restores the desalination performance of CDI. Thus, desalination performance of CDI mainly depends upon the adsorption capacity during charging as well as velocity of ion desorption and degree of desorption. Actual brackish water is a complex mixture consisting not only toxic ions, but also a large amount of organic matter such as humic acid (HA)[9-12], which represents the major fraction of natural organic matter in ground and surface waters[13-16]; the presence of HA causes severe fouling on CDI, resulting in a severe decline of desalination performance[9, 17]. However, the main component of traditional CDI electrode is carbon material, has good adsorption ability for the organic matter. During discharge process, adsorbed organic matter may not be completely desorbed like those of the ions. Due to Coulombic forces, ions can to combine with the electrodes, so they naturally desorb from the electrode into the solution after a short circuit. However, organic matter depends upon the chemical interactions to tightly bond with the electrodes[18, 19]. When these accumulate onto the electrode surface at longer run time, electrode pores may get blocked, thereby hindering the diffusion of ions into the interior of the electrode and release from the electrode, thus reducing the ion transport. Adsorbed organic matter would minimize the extent of adsorption sites on the electrode surface, thereby reducing the adsorption capacity. Additionally, the deposited organic matter binds to the adsorbed ions, such that ions may not be released from the electrodes during the discharge process. Previous experiments have confirmed that natural organic matter in water causes irreversible CDI fouling, resulting in a severe decline of desalination performance[17, 20, 21]. This has prompted us to explore further to alleviate organic fouling. Majority of efforts have focused on how to increase the adsorption capacity of the electrode, to establish the relationship between pore size distribution of electrodes and to removal of ions[22, 23]. Other studies have modified the carbon-based materials to fabricate electrodes to observe higher adsorption capacity[24-28]. Recently, efforts have also been made to enhance performances of CDI electrodes by controlling the porous structures of porous carbon materials[29]. Metals and organic polymers were also used to fabricate the electrodes[30-33], but only a handful of literature exists on mitigation CDI fouling caused by the presence of organic matter.

Electrodes used in CDI generally consist of carbon materials and hence, organic matter is mainly bonded onto CDI electrode by the interaction forces between electrode and organic matter. This has prompted us to utilize powdered activated carbon (PAC) to remove organic matter, which would otherwise cause irreversible CDI fouling. In addition, PAC, being environmentally benign, has a strong adsorption capacity for organic substances as well as it has the advantages of using innumerable raw materials having high safety, possessing acid/alkali and heat resistance properties along with its insoluble nature in water and organic solvents. Therefore, in water treatment applications, adsorption of PAC has been employed to remove organic wastes[34, 35]. The study aims to investigate the mitigation of CDI fouling caused by organic pollutant, e.g., HA[36]. Desalination experiments were performed to understand the effect of PAC pretreatment on desalination performance of CDI in the presence of HA. Simultaneously, mitigation effect on CDI fouling was compared in the presence and absence of PAC in a CDI system. The causes of different effects of PAC in the presence and absence of CDI system were analyzed. Additionally, the coupling effect of PAC adsorption on the specific surface area of the electrode and electrochemical characteristics of the CDI system were investigated.

2. Materials and methods 2.1. Materials PAC was purchased from Kuraray Chemical (YP-50F, Japan) and used as the core material of CDI electrode as well as an adsorbent for removing organic matter. Conductive carbon black obtained from Cabot (BP-2000, USA) was used to improve the conductivity of CDI electrode. PVDF provided by Arkema (HSV900, France) acted as a binder to connect the components of the electrode; N,N-dimethylacetamide solvent purchased from Aladdin Chemical (China) was used to dissolve the raw materials for the electrode. HA purchased by Sigma-Aldrich (USA) was originated from the natural organic matter. NaCl was purchased from Aladdin Chemical (China) as a target salt. Ultrapure water (conductivity 18.2 MΩ/cm) was used from Milli-Q purification system (Millipore, U.S.A) [37].

2.2. Experimental protocols 2.2.1. Desalination experiment Batch experiments were performed to investigate the desalination performance of the CDI system. Experiments included two stages of adsorption and desorption. A 500 mg/L NaCl solution was circulated in the CDI system using a peristaltic pump for 20 min at 1.4 V. Until no

further change in NaCl solution, adsorption capacity of CDI reached the equilibrium saturation. The CDI system was then hydraulically cleaned under short circuit conditions until the concentration of NaCl in the effluent did not change. The adsorption capacity of CDI was calculated as: (1) where Q (mg/g) is adsorption capacity, Ci (mg/L) is initial NaCl concentration in the effluent solution, Cs (mg/L) is steady-state NaCl concentration in the effluent solution, V (L) is the volume of NaCl solution and m (g) is mass of the electrode.

2.2.2. Adsorption experiments of PAC HA was used as an organic contaminant to be added to NaCl solution to form a solution of 10 mg/L HA and 500 mg/L NaCl. The solution was then subjected to desalination experiments in the CDI system to investigate the effect of HA. The solution was first adsorbed by 50 mg/L PAC for 30 min, and then filtered through a 0.45 um microfiltration membrane to enter the CDI system for desalination experiments. The effect of PAC pretreatment on desalination performance of CDI was performed. Since the main component of CDI electrode is PAC, it is considered whether PAC entered into the CDI system would improve the desalination performance. For this, two different pretreatments were compared. The first experiment involved the raw solution after pretreatment with PAC, which removed the PAC from the solution through 0.45 um microfiltration membrane, and then entered the CDI system (PACtreated HA). Quite differently, in the second experiment the solution after pretreatment with PAC was not passed through 0.45 um microfiltration membrane, but directly entered into the CDI system (PAC+HA). The main difference between the two experiments was whether PAC entered the CDI system or not. In order to study the effect of electrostatic forces on the removal of HA by PAC, both the application of voltage and in the absence of applied voltage was investigated. The electrode in the CDI system was removed to avoid the effect of the electrode on the removal of HA by PAC, which was coated onto titanium plate.

2.3. Characterization of the CDI system 2.3.1. Pore structure of the electrode To investigate the effect of PAC entering the CDI system on the pore structure of CDI electrode, N2 sorption-desorption isotherms were generated and the surface area as well as the pore size distribution of the electrodes were assessed using the Micromeritics BrunauerEmmett-Teller (BET) analyzer (ASAP 2020). Experiments were run on samples that were dried

under vacuum and BET method as well as quenched-solid density functional theory (QSDFT) was used to analyze the surface area and pore size distribution[38].

2.3.2. Electrochemical analysis of CDI system The resistance and capacitance of the CDI system were measured, respectively by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), using Metrohm Autolab potentiostat (PARSTAT302N). The CDI system consisting of a two-electrode system (anode and cathode) was used to evaluate the electrochemical properties of the CDI system in 500 mg/L NaCl solution at room temperature. The frequency range was maintained between 100 kHz and 0.01 Hz at the fixed open-circuit voltage (OCV) at 5 mV in the EIS measurement. The sweep potential ranging from -1.4 to 1.4 V at the scan rate of 10 mV/s was performed using CV and the specific capacitance (Cs) was calculated using[39-41]: (2) where

(A) is current, v (m/s) is scan rate, m (g) is mass of the two electrodes, and ∆U (V) is the

window voltage.

2.4. Organic matter analysis The UV254 absorbance was measured by a UV-Vis spectrophotometer (T6, Puxi, China) to assess the concentration of HA, which showed the linear relationship (R2=0.999) between UV254 and HA concentrations[9, 42, 43].

3. Results and discussion 3.1. Kinetics and thermodynamics of adsorption of PAC for HA and NaCl Mixtures Isothermal and kinetics of adsorption of HA onto PAC in the presence of NaCl was investigated and the results are presented in Figs. 1 and 2, respectively. The adsorption of HA was highly dependent on the initial HA concentration (see Fig. 1). By increasing the initial concentration of HA from 2.5 to 50 mg/L, a substantial increase in HA adsorption was observed from 38.34 to 119.61 mg/g. The equilibrium isotherm adsorption indicates the distribution of adsorbate between the liquid phase and the solid phase. From the analysis of equilibrium

isotherms using Langmuir and Freundlich models (Fig. 1), the results of HA are found to be in good agreement with the Freundlich model (R2=0.983) given by[44, 45]: (3)

where qe is the amount of adsorbate/mass of the adsorbent, Kf is Freundlich constant, Ce is equilibrium concentration of the adsorbate, and n is heterogeneity parameter. Lower value of 1/n indicates that adsorption followed non-linear trend, while the value of 1/n down to 0.1 suggests the adsorption to be irreversible. From the parameter values of Kf and 1/n summarized in Table. 1, it was found that the value of 1/n, which indicates the adsorption to follow the non-linear and reversible trends[46]. 130 Experimental data Freundlich Langmuir

120 110 100

qe (mg/g)

90 80 70 60 50 40 30 20 0

10

20

30

40

50

Ce (mg/L)

Fig. 1. Isothermal adsorption of HA on PAC

Table 1. Langmuir and Freundlich parameters of HA on PAC Langmuir Freundlich qm KL Absorbent R2 KF 1/n R2 (mg/g) (L/mg) PAC 113.2 0.2245 0.828 33.86 0.3174 0.983 The results of adsorption kinetics of HA on PAC are presented in Fig. 2. The removal of HA showed similar trends that gradually increased with time for all the studied concentrations and the removal of HA was > 80% of the total removal within 30 min. However, the removal of HA was only increased by < 2% with the adsorption time extended by one more hour. Therefore, the majority of HA was removed during the first 30 min and it was found that the removal of HA increased with increasing dosing amount of PAC. The removal of HA was almost doubled from 15% to 31% in 30 min, when the dosing amount of PAC was increased from 10 mg/L to 100 mg/L.

1.00

10 mg/L PAC 20 mg/L PAC 50 mg/L PAC 100 mg/L PAC

0.95

HA (Ct/C0 )

0.90 0.85 0.80 0.75 0.70 0.65 0.60 0

30

60

90

120

150

180

210

Time (min)

Fig. 2. Adsorption kinetics of HA on PAC

Nitrogen sorption isotherms were used to evaluate the pore distribution and surface area (see Fig. 3). The nitrogen adsorption-desorption curve with Hysteresis loops of Type H4 showed a rapid nitrogen adsorption at a relatively low relative pressure, demonstrating that PAC is microporous[47]. The values of specific surface area and micropore area of the PAC as calculated from the nitrogen adsorption-desorption curves are found to be 1664 and 1448 m2/g, respectively. The pore size distribution of PAC is displayed in Fig. 3, while half-pore-width of PAC is mainly concentrated around 0.3~0.7 nm, indicating PAC as microporous material. The molecular weight distribution of HA is around 4 to 23 kDa[48] and the hydrodynamic size of high molecular weight HA is larger than the size of micropores (< 2nm), suggesting that high molecular weight HA was not easily removed because of the size exclusion effect[49-51]. 550

450 400

2.0 1.8 1.6

350

1.4

Dv(r) (cm3/g/nm)

Volumn absorbed @ STP (cm3/g)

500

300

1.2 1.0 0.8 0.6 0.4

250

0.2 0.0

200

-0.2 0

2

4

6

8

10

12

Half pore width (nm)

150 0.0

0.2

0.4

0.6

Relative pressure (P/P0)

0.8

1.0

Fig. 3. Nitrogen sorption isotherms of PAC

3.2. Effect of PAC pretreatment on CDI desalination performance The effect of PAC adsorption on CDI fouling caused by HA was investigated, and the results are given in Fig. 4. The reduction of desalination performance on CDI system caused by HA was particularly effective for a decline of approximately 24% at the end of the fifth cycle. In the meanwhile, specific adsorption capacity decreased as the number of cycles increased, and specific adsorption capacity was reduced from 0.906 in the first cycle to 0.664 in the fifth cycle. These results indicate that the desalination performance recovery by the hydraulic cleaning was limited and irreversible fouling might have played a major role in HA fouling of the CDI system. In order to evaluate the effect of PAC pretreatment on CDI fouling caused by the presence of HA, a 50 mg/L of PAC was added to NaCl solution containing 10 mg/L of HA. After 30 min of adsorption, the mixture was filtered through 0.45 um microfiltration membrane, and the filtrate was introduced into the CDI system. It was found that after pretreatment with PAC, desalination performance of CDI was improved compared to that of the raw HA solution. The desalination performance was decreased by 27%, indicating that PAC pretreatment might have alleviated the CDI fouling caused by HA. With increasing run time, fouling was more problematic. Therefore, HA solution after the adsorption of PAC directly entered the CDI system without filtration, and the desalination performance of CDI system was further improved. Compared to the results of removing PAC, specific adsorption capacity was increased from 0.755 to 0.824, which might have delayed the organic fouling of CDI caused by HA. From the analysis of these two situations, it can be observed that the interaction between PAC and HA or CDI systems may be the possible reason to further improve the desalination performance of CDI system. This might be attributed to two reasons. For HA, PAC entering the CDI system might have further adsorbed the HA, which would reduce the deposition of HA onto the CDI electrode surface and thereby, alleviating the organic fouling of CDI. However, the overall HA removal was not substantially improved with an increase of adsorption time, suggesting that adsorption of HA by PAC might have been promoted by the applied voltage. For the CDI system, the entry of PAC into CDI might have altered the characteristics of the overall system, especially the electrochemical properties associated with the ion transport, resulting in a greater desalination performance of CDI.

Specific absorption capacity (Q/Q0)

1.00

PAC+HA PAC-treated HA HA

0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 1

2

3

4

5

Cycle

Fig. 4. Desalination performance of the CDI system in the presence or absence of PAC (HA---Raw

HA solution; PAC-treat HA---Raw HA solution after PAC pretreatment with removing activated carbon; PAC+HA---Raw HA solution after PAC pretreatment without removing activated carbon).

3.3. Migration of HA in the presence or absence of PAC in CDI systems In order to verify whether applied voltage has promoted the removal of HA by PAC, an experiment was conducted wherein HA was first pretreated by PAC for 30 min, and then directly entered into the CDI system for desalination experiments. The only difference is that the electrode was not coated onto titanium plate to eliminate the interference of CDI electrode on the removal of HA. The HA removed by PAC under the applied voltage condition was investigated. After pretreatment for 30 min, voltage was applied to the solution containing PAC and HA. Here, the removal of HA by PAC pretreatment was about 25%, but the total removal of HA was 34% as shown in Fig. 5(a). The removal of HA was further increased by 1/3 and at the same time, kinetic adsorption experiments of HA showed that adsorption of HA onto PAC almost reached equilibrium after 30 min of pretreatment, which did not change with the increase in adsorption time. Such an improvement in the removal of HA may be attributed to the effect of voltage, which might have weakened the electrostatic repulsion between PAC and HA, thereby promoting the removal of HA onto PAC[52, 53]. Thus, the total removal of HA in the two situations of PAC entering the CDI system and not entering was compared. From the data presented in Fig. 5b, it can be observed that relative to the raw HA solution, the overall removal of HA in the presence of PAC pretreatment was increased by 6%, 8%, 10%,

13%, and 15% at the end of the first, second, third, fourth and fifth cycles, respectively. The removal of HA by the PAC pretreatment was around 25% and hence, the HA adsorbed by CDI system was 22~15%, which decreased slightly with the increase of running cycle. Compared to raw HA, the HA adsorbed by the CDI system was reduced by 13~21%, and the extent of adsorption of HA onto CDI system was reduced by 45%~50% per cycle. These results confirm that PAC pretreatment has reduced the adsorption of HA onto the electrode, thereby eliminating the organic fouling of CDI caused by HA and prolonging the operating life of CDI. However, the adsorption of HA onto CDI system decreased with the increase of running cycle, since the adsorption site of organic matter onto the electrode was limited. With the accumulation of organic matter, adsorption site decreased, such that adsorption of organic matter onto CDI system decreased with increasing operating cycle. In addition, the downward trend became slower than that observed in the absence of PAC pretreatment. This may be mainly due to the fact that HA adsorbed by CDI system has decreased, which might have impaired the interference of the existing HA onto the electrode to the HA in solution and more organic matter might have deposited onto the electrode. Furthermore, when PAC was not removed, but when it entered the CDI system together with HA after treatment, the total removal of HA did not change significantly, which was about 3% higher than that in the absence of PAC. On the other hand, PAC entered into the CDI system was able to further remove about 10% of HA under the application of voltage. Therefore, HA which might be attached to the electrode was removed when PAC entered the CDI system, thus preventing the adsorption of HA by the CDI system. a 0.45

b

0.35

0.5

HA removal

0.30

HA removal

PAC-treated HA PAC+HA HA

0.6

0.40

0.25 0.20 0.15

0.4 0.3 0.2

0.10 0.1

0.05 0.00

0.0

PAC pretreatment

Further adsorption under voltage

1

2

3

4

5

Cycle

Fig. 5. HA removal: (a) PAC pretreatment and under voltage; (b) CDI system in the presence and absence of PAC (HA removal represented the ratio of the concentration difference between the influent HA and the effluent HA and the influent concentration of HA). Surface area and pore size distribution of CDI electrode in both the cases were investigated by the N2 sorption-desorption isotherms. The deposition of organic matter onto CDI electrode was further analyzed and the results are displayed in Fig. 6. Specific surface area and micropore area of the raw electrode were 1424 and 1135 m2/g, respectively. The HA pretreated by PAC might be responsible to decrease the specific surface area and micropore area of the CDI electrode by 143 and 164 m2/g, respectively. When PAC was not removed, but directly entered the CDI system, specific surface areas as well as micropore area of CDI electrode were decreased by 111 and 114 m2/g, respectively. Compared to the removal of PAC, the decrease in specific surface area and micropore area were reduced by around 20% and

30%, respectively. This confirms that entry of PAC into the CDI system further reduced the amount of HA attached to the CDI electrode. By reducing the deposition of HA, PAC pretreatment reduced the irreversible fouling, and improved the desalination performance of CDI to extend the running life of CDI. At the same time, pore size distribution of the electrode in both the cases, where PAC entered the CDI system and did not enter the CDI system are compared (see results in Fig. 7). It is observed that pore size distribution did not show any significant difference. The PAC as the main component of the electrode material is a microporous material, and most of the micropores could be blocked by HA due to the pore size exclusion, such that PAC retained its original pore size. 650 PAC-treated HA PAC+HA Raw electrode

Volume adsorbed @ STP (cm3/g)

600 550 500 450 400 350 300 250 200 150 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Fig. 6. Nitrogen sorption isotherms of CDI electrodes Table 2. Surface area of the electrodes Specific surface Micropore surface External surface Material 2 2 area (m /g) area (m /g) area (m2/g) PAC-treated 1281 971 310 HA PAC+HA 1313 1021 292 Raw 1424 1135 288 electrode

Micropore volume (cm3/g) 0.327 0.341 0.373

1.4 PAC-treat HA PAC+HA

1.2 1.4

PAC-treat HA PAC+HA

1.2

Dv(r) (cm3/g/nm)

Dv(r) (cm3/g/nm)

1.0 0.8 0.6

1.0 0.8 0.6 0.4 0.2

0.4

0.0 0.0

0.2

0.5

1.0

2.0

1.5

2.5

3.0

Half pore width (nm)

0.0 0

2

4

6

8

10

12

Half pore width (nm)

Fig. 7. Pore size distribution of the CDI electrodes

3.4. Electrochemical properties of CDI in the presence or absence of PAC The effect of PAC on the electrochemical properties of CDI after entering the CDI system was investigated. The ions in solution move towards the electrode under the action of an electrostatic force and are finally removed due to electrode adsorption. The migration and accumulation of ions onto the electrodes are the two main processes. EIS measures the resistance of CDI system to reflect how quickly the ions migrate through the device, while the CV measures the ability of ions to accumulate onto the electrode surface by measuring the capacitance of the CDI system.

3.4.1. Electrochemical impedance spectroscopy As can be observed from Fig. 8, high frequency region of the measured frequency in the impedance spectrum showed an approximate semicircle but starting point of the semicircle was not at the origin with an intercept on the Z axis. As the test frequency gradually decreased, a diagonal line close to 45° was observed in the low frequency region, which is typical of EIS technique[54]. The intercept on the Z axis represents series resistance (Rs) of the CDI system, mainly including electrical resistance of the current collector, the intrinsic resistance of the electrodes, the ionic resistance of NaCl solution and resistance of the wires[55]. As can be seen, the series resistance of CDI system was 2.38 Ω without the PAC, but when PAC entered the CDI system, the series resistance of the system was dropped to 1.72 Ω. Compared to the raw CDI

system, series resistance decreased by 27.8%, while the semicircle in the high frequency region represents charge transfer resistance (Rct). From Figure 8, it can be observed that when PAC entered the CDI system, charge transfer resistance of the CDI system dropped by about 20%. As an adsorbent with a large specific surface area, PAC adsorbed ions in the CDI flow channel formed the EDLs on the surface, thereby increasing the conductivity of the flow channel[56]. Thus, overall, PAC might have decreased the internal resistance in the flow channel of the CDI system to improve the adsorption rate as well as desalination performance of the CDI system by increasing the conductivity of the CDI system[56, 57].

a

3.0 500 mg/L NaCl 50 mg/L PAC+500 mg/L NaCl 2.5 0.10

-Zim (Ω)

0.08

-Zim (Ω)

2.0

50 mg/L PAC+500 mg/L NaCl

0.06 0.04 0.02

1.5

0.00 1.70

1.75

1.80

1.85

1.90

Zreal (Ω) 0.10

1.0

500 mg/L NaCl

-Zim (Ω)

0.08 0.06 0.04

0.5

0.02 0.00

2.40

0.0 1.5

2.0

2.5

3.0

3.5

4.0

2.45

2.50

Zreal (Ω)

2.55

4.5

5.0

Zreal (Ω)

b

3.5 Rct Rs

3.0

Resistance (Ω)

2.5 2.0 1.5 1.0 0.5 0.0 500 mg/L NaCl

50 mg/L PAC+500 mg/L NaCl

Fig. 8. Electrochemical impedance spectroscopy of CDI: (a) Nyquist plot; (b) Resistance.

3.4.2. Cyclic voltammetry Cyclic voltammetry was used to measure the capacitance of the CDI system, which indicates the ability of the ions to accumulate at the EDLs formed between the porous electrodes and the electrolyte[40]. Larger the capacitance, more ions will accumulate, and desalination performance of CDI would be superior. The results shown in Fig. 9(a) suggest that all the samples exhibited similar CV curves. However, there was no obvious redox peak on the curve, which confirmed that removal of ions by the CDI system mainly depends on the EDLs formed by the Coulombic forces, rather than the redox reactions[58, 59]. Symmetry of the curve indicates that adsorption of ions might be highly reversible. The capacitance of CDI system was calculated from the area of the curve and these results are shown in Fig. 9(b). As can be seen, CDI system showed a specific capacitance of 46 F/g without PAC, but when PAC entered the CDI system, specific capacitance of the system was increased to 64 F/g. Thus, in comparison to raw CDI system, the capacitance was increased by about 40%. Ionic conductivity is another important parameter affecting the capacitance characteristics of the CDI systems. When PAC entered the CDI system, the conductivity was increased, resulting in improving ion transport of CDI system. Therefore, the entry of PAC into the CDI system has increased the capacitance of the CDI system to provide enhanced desalination performance of the CDI system. a

b

0.25 500 mg/L NaCl 50 mg/L PAC+500 mg/L NaCl

0.15 0.10

Current (A)

70 65

Specific capacitiance (F/g)

0.20

0.05 0.00 -0.05 -0.10

60 55 50 45 40 35

-0.15

30

-0.20 -1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

500 mg/L NaCl

50 mg/L PAC+500 mg/L NaCl

Potential applied (V)

Fig. 9. Capacitance characteristics of CDI system: (a) cyclic voltammetry; (b) specific capacitance.

3.5. Effect of PAC on CDI fouling caused by HA Four types of PAC were selected in this study as adsorbents for the pretreatment to investigate the effect of PAC type on HA fouling of the CDI. The results shown in Fig. 10(a) indicate that the removal of HA by PAC-1 is highest, for which the removal rate reached to 30%, while the removal of HA by PAC-2, PAC-3 and PAC-4 have successively decreased to 25%, 20%

and 10%, respectively. This could be due to the differences in the pore structures of four PACs selected. The nitrogen adsorption-desorption curves and pore size distribution of various PACs are shown in Fig. 10(b) and (c). The PAC-2 has a specific surface area of 1,660, which is the largest specific surface area among the four PACs investigated. Compared to other three types of PACs, the PAC-2 is highly microporous, and its micropore area accounted for 85% of the specific surface area. The distribution of micropores, mesopores and macropores of PAC-1, PAC-3 and PAC-4 is relatively balanced, while the specific surface area has successively decreased. Considering the results of removal of organic matter, high specific surface area suggests that more of the organic matter is adsorbed except for PAC-2. Although PAC-2 shows the highest specific surface area, of which majority of the surface area may be contributed by the micropores. HA is a polymer with a broad molecular weight distribution. Due to the pore blocking effect, only a part of HA with a small molecular weight could enter into the micropores, while most of the HA might be adsorbed by the mesopores of the PAC. The external area of PAC is the main determining factor for the extent of adsorption of organic matter. a

b

0.40

PAC-1 PAC-2 PAC-3 PAC-4

550

Volumn adsorbed @ STP (cm3/g)

0.35 0.30 0.25 0.20 0.15 0.10 0.05

500 450 400 350 300 250 200 150 100 50

0.00 PAC-1

PAC-2

PAC-3

0

PAC-4

0.0

0.2

0.4

0.6

0.8

Relative pressure (P/P0)

c 1.8

PAC-1 PAC-2 PAC-3 PAC-4

1.6 1.4 1.8

1.2

PAC-1 PAC-2 PAC-3 PAC-4

1.5

1.0

Dv(r) (cm3/g/nm)

Dv(r) (cm3/g/nm)

HA removal

600

0.8 0.6

1.2 0.9 0.6 0.3

0.4

0.0 0.0

0.2

0.5

1.0 1.5 2.0 2.5 Half pore width (nm)

3.0

0.0 0

2

4

6

8

Half pore width (nm)

10

12

14

1.0

Fig. 10. (a) HA removal by PAC pretreatment; (b) nitrogen adsorption and desorption curve of PAC; (c) pore size distribution of PAC. Table 3. Surface area data of different PACs Specific surface Micropore surface External surface Micropore volume Material area (m2/g) area (m2/g) area (m2/g) (cm3/g) PAC-1 598 241 357 0.0883 PAC-2 1665 1419 246 0.4756 PAC-3 219 64 155 0.0216 PAC-4 125 30 95 0.0091 When HA solution pretreated by PAC entered the CDI system, and the adsorption of organic matter onto the CDI electrode was investigated, it was observed from Fig. 11 that four PAC pretreatments showed similar trends of organic removal rate and extent of adsorption of HA onto the electrode at every cycle was reduced with increasing run time. On the other hand, HA after the pretreatment with PAC-2 is the least adsorbed by the electrode, and the extent of adsorption is reduced by almost half of its original value compared to raw HA. The deposited organic matter onto the electrode surface is thus responsible for the fouling of CDI. Thus, only part of the organic matter is possible to be removed by PAC-1, PAC-3 and PAC-4 adsorbent, but the organic matter adsorbed by the electrode remained in solution. Compared to all three PACs, PAC-2 alleviated the CDI fouling caused by HA more effectively than the other PACs due to the difference in surface chemistry of PACs. 0.45 PAC-1 PAC-2 PAC-3 PAC-4

HA adsorption amount (mg/L)

0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 1

2

3

4

5

Cycle

Fig. 11. Extent of HA adsorption onto the electrode From the FTIR analysis, it is observed that functional groups on the surface of four PACs are different (see Fig. 12). Since the electrode material is to be tested repeatedly, PAC-2 deliberately reduced the functional groups onto the surface of PAC in order to maintain the durability of the material. These findings are consistent with the FTIR data. For instance, PAC-2

shows a weak C-O stretching at 1148 cm-1 with no appearance of any functional groups, but other PACs have shown several strong functional group peaks. HA is a mixture containing amino N-H (3340 cm-1), carboxyl C=O (2919 cm-1), carbonyl C=O (1702 cm-1), aromatic C-C vibration (1607 cm-1), alcoholic/phenolic O-H (1114 cm-1) and aliphatic C−O (1033 cm-1) moieties (see Fig. S4). Carboxyl group and hydroxyl group of the negatively charged group on HA were combined with the Lewis basic sites of the activated carbon by electrostatic forces, while amino group was bonded to carboxyl group and hydroxyl group on the activated carbon by H-bonding. In addition, aromatic portion of HA could be combined with electron rich sites at the graphene surfaces of activated carbon by π-π interactions. Previous studies[18, 19, 60-62] indicated several mechanisms including electrostatic attraction, H-bonding and π-π interactions might be responsible for the adsorption of HA onto PAC. Because of different surface functional groups of various PACs, organic substances adsorbed by each type of PAC are quite different. However, PAC-2 is the same as the core material of CDI electrode, but HA adsorbed by PAC-2 and the electrode are basicallyathe same. Thus, PAC-2 is a better adsorbent than other PACs. 110 PAC-1 PAC-2 PAC-3 PAC-4

100

PAC-3

Absorbance

90 1038 1036 1062

80

2361

70 60

PAC-4

2513

PAC-1

50 40 30

PAC-2

778 795 874 880

1426 1448 1457

20 500

1000

1500

2000

2500

3000

3500

4000

Wavelength (cm-1)

Absorbance

b Fig. 12. FTIR spectra of PACs Table 4. FTIR peak assignments of PAC (based on published reports)[63-70]. Band Assignments 2361, 2513 C≡C 1426, 1448, 1457 CH2 1148, 1062, 1038, 1036 C-O 880, 874, 795, 778 C-H 3399

2919 1702

1033 1607

4000

3500

3000

2500

2000

1500

Wavelength (cm-1)

1114

1000

500

4. Conclusions The present investigation concludes that the extent of adsorption of HA by PAC increased as the amount of PAC increased, following the Freundlich adsorption model. The removal rate of HA increased with increase in adsorption time, and most of the removal occurred during the first 30 min. The pretreatment of PAC was responsible to reduce the HA adsorption onto CDI by 27%, which significantly alleviated the fouling of CDI system caused by the presence of HA, thereby improving the desalination performance of CDI. Upon entry of PAC into the CDI system, specific adsorption capacity of CDI increased from 0.755 to 0.824. Compared to the removal of PAC, the PAC entering the CDI system has resulted in further removal of HA, thus alleviating the irreversible fouling of CDI caused by the presence of HA. When the PAC entered the CDI system, electrical conductivity of the flow channel also increased due to the formation of EDLs on the PAC surface[56, 57], which is responsible to improve the desalination performance. Meanwhile, it also reduced the internal resistance of CDI, especially the series resistance from 2.38 to 1.72, which further improved the conductivity of the flow channel. On the other hand, the capacitance of CDI system was increased by about 40%, thereby increasing the adsorption of ions by the CDI system. Additionally, comparing the four different PACs, the PAC with the same material as that of CDI electrode was found to be quite efficient to alleviate CDI fouling caused by the presence of HA, which was mainly because most of the organic matter removed by PAC2 was an organic substance that was responsible for causing the CDI fouling. Overall, PAC hindered the blockage of organic matter into the pore size of CDI electrode by removing the organic matter, resulting in optimized surface area of the electrode. On the other hand, PAC reduced the internal resistance of the CDI system, thereby improving the conductivity of CDI system. Hence, the presence of PAC absorbent in the CDI system improved the overall CDI performance.

Acknowledgements This research was jointly supported by the National Key R&D Program of China (2018YFC0408001), the National Natural Science Foundation of China (51778170), State Key Laboratory of Urban Water Resource and Environment (2017DX06) and Fundamental Research Funds for Central Universities.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Graphical Abstract

Highlights 

Pretreatment of activated carbon alleviated organic fouling of CDI system.



Electric field promoted the adsorption of organic matter by activated carbon.



Activated carbon improved the conductivity of CDI system.



Functional groups on activated carbon determined the adsorption of organic matter.