Characteristic and model of phosphate adsorption by activated carbon electrodes in capacitive deionization

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Characteristic and model of phosphate adsorption by activated carbon electrodes in capacitive deionization Fang-Fang Chena, Hao-Fei Lia, Xue-Ru Jiaa, Zhao-Yu Wanga, Xuan Lianga, Yu-Ying Qina, ⁎ ⁎ Wen-Qing Chena,b, , Tian-Qi Aob,c, a

College of Architecture and Environment, Sichuan University, Chengdu 610065, China State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu 610065, China c College of Water Resource and Hydropower, Sichuan University, Chengdu 610065, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Capacitive deionization Electric adsorption Phosphate Double layer theory Surface charge

The advanced treatment for phosphate (P) has been focused on in the wastewater treatment field in recent years. Developing an innovative P removal technology with high efficiency, low cost, and low energy consumption has become an imperative requirement. Capacitive deionization (CDI) is an emerging environmentally friendly technology for removing charged ions from aqueous solutions. In this study, we investigated the performances of P removal by CDI with homemade activated carbon electrodes. Furthermore, kinetics, thermodynamics and the equilibrium Gouy-Chapman-Stern (GCS) double-layer model were studied to reveal the mechanism of P adsorption. It was observed that the adsorption capacity was positively correlated with the voltage. The pH first increased and then decreased during the adsorption because of water electrolysis and forms transformation of phosphate. The experimental results were validated by pseudo-first-order adsorption kinetics and Freundlich isotherms. The adsorption rate increased with increasing temperature, voltage, and concentration. The adsorption capacity decreased with increasing temperature, and the maximum adsorption capacity was 8.53 mg/g. Thermodynamic analysis confirmed that the electrosorption of P on the activated carbon electrode was an endothermic process, and the high temperature was conducive to reducing the adsorption potential of P on the activated carbon electrode. The theoretical results of the double layer model were in agreement with the experimental data, which showed that the ion adsorption can be reflected by surface charge. This study can help to understand the adsorption mechanism of P on activated carbon electrodes and provide a theoretical basis for the application of P removal by CDI.

1. Introduction With the rapid development of the economy and increase in human activity, the number of industrial wastewater, living sewage, and agricultural nonpoint pollution sources has increased significantly [1], causing water environmental problems to become increasingly serious. Eutrophication is one of the greatest environmental problems that humanity is faced with [2]. Although phosphorus is essential to the primary productivity of aquatic ecosystems [3,4], excess phosphorus in water bodies such as lakes or reservoirs may lead to eutrophication [5–7]. The abnormal proliferation of algae populations depletes the dissolved oxygen in the water and seriously threatens water quality and aquatic ecological environment security [8]. However, phosphorus removal is still a difficult problem in advanced sewage treatment. Moreover, phosphorus is a nonrenewable resource. Humanity’s mining

⁎

of large amounts of phosphorus rock (mainly marine sedimentary rocks) provides raw materials for phosphate fertilizers. However, the existing phosphate rock reserves on Earth may be depleted in the next 50–100 years [9,10]. Therefore, it is necessary to recycle phosphorus for phosphate fertilizers [11] while also removing phosphorus. Current technologies for phosphorus removal are typically divided into physicochemical techniques and biotechnologies [12,13]. Biological dephosphorization is a common treatment process for urban sewage treatment with the advantage of low cost, but its removal efficiency is low, and polyphosphate-accumulating microorganisms (PAOs) are sensitive to the environment, such as the pH, temperature, and carbon source. Among physicochemical techniques, chemical precipitation is the main treatment method because of its high removal rate of phosphorus. Nevertheless, chemical precipitation has many shortcomings, such as expensive chemicals, high operating costs, and

Corresponding authors at: College of Architecture and Environment, Sichuan University, Chengdu 610065, China. E-mail addresses: [email protected] (W.-Q. Chen), [email protected] (T.-Q. Ao).

https://doi.org/10.1016/j.seppur.2019.116285 Received 27 June 2019; Received in revised form 2 November 2019; Accepted 3 November 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Fang-Fang Chen, et al., Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116285

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described in the supplementary material) was used as the basic material of the electrodes; polyvinylidene fluoride (PVDF, HSV900 of Acomar Company, France), which was used as a binder, and conductive acetylene black (Tianjin Yiborui Chemical Co., Ltd., China), which was used as the conductive material, were mixed and dissolved in N, N-dimethylacetamide (Kelong Chemical Reagent Factory, Chengdu, China) and stirred to ensure homogeneity. Their percentages in the final electrodes were 80, 10 and 10%, respectively. The mixed slurry was coated on a graphite plate with a coating size of 45 mm wide and 50 mm long and then dried in an oven at 60 °C for 4 h and in a vacuum oven a 60 °C for 6 h to remove the organic solvents. The obtained electrode weighed close to 0.2 g, and its thickness was approximately 0.22 mm.

the production of large amounts of sludge [14,15]. Therefore, an increasing number of researchers are concerned with the development of economical and effective dephosphorization methods without secondary pollution. Capacitive deionization (CDI) is a new technology for ion adsorption [16]. CDI employs two parallel electrodes. When a potential difference is applied to the two electrodes, a steady electric field will be generated between them [17–19], and electrified ions will be adsorbed onto the surface of the electrodes, resulting in a reduction in the solution concentration and realization of ion purification. After a period of adsorption, the electrodes will become saturated and then can be regenerated by reversing the potential [20,21] or short-circuiting [22,23]. Research on CDI removal of target pollutants focuses on saline wastewater containing sodium and chloride ions [24–27]. Recently, CDI has been applied to the removal of nitrogen [28], fluorine [29] and phosphate [30], but research on P removal by CDI is rare due to the complex forms of P in aqueous solutions. Compared with other P removal technologies, CDI is an environmentally friendly technology because it does not produce toxic and harmful byproducts or secondary pollution during operation. Moreover, CDI seems promising in terms of energy efficiency [24,28,31]. The adsorption properties of CDI are closely related to the physical and chemical properties of the electrode materials, such as the specific surface area, pore size distribution, hydrophilicity, electrical conductivity and so on [32–34]. Researchers have been actively exploring high specific surface area carbon materials to improve their adsorption properties, such as carbon aerogels(CAs) [32,35], graphene[36], carbon nanotubes(CNTs) [36,37], carbon fibers [38], and activated carbon (AC) [34,37]. Recently, more and more novel carbon materials are used in CDI field, including carbon nanofiber aerogels [39], carbon spheres [40], metal-organic framework(MOFs)-derived carbons [41], covalent organic frameworks (COFs)-derived carbons [42], MOF/CNT hybrid carbon material [43], and MOF/ polypyrrole (PPy) hybrid material [44]. In addition, nitrogen-doped carbon materials were produced to improve the electrical conductivity of CDI electrode, including nitrogen-doped mesostructured carbon (NMCs) [45], nitrogen-doped carbon/graphene (NC/rGO) [46], nitrogen-doped graphene (NG)[47], and CNTs/ nitrogen-doped carbon polyhedral (NCP) hybrid [48]. It provides more choices for CDI electrode materials. Activated carbon (AC) is widely used as a porous electrode material for CDI because it not only has a high surface area, well-developed porosity, and excellent adsorption capacity [49] but also has the advantage of being low cost. Although these materials have a high specific surface area, the adsorption capacity of CDI is not always proportional to the specific surface area of the materials [50]. Pore structure and surface charge of the electrode materials may be the main factors affecting the adsorption [36,51]. However, the mechanism of P removal by CDI and the corresponding adsorption model are not well studied. In this study, the performances of P removal by a CDI unit with homemade activated carbon electrodes at various voltages were investigated. Furthermore, the kinetics, thermodynamics and the equilibrium Gouy-Chapman-Stern (GCS) model of electrosorption at different voltages, temperatures, and concentrations of P solution were studied to reveal the mechanism of the influence of voltage, temperature, and concentration on P adsorption by CDI. The findings not only contribute to understand the adsorption characteristics of CDI for P but also provide a theoretical basis for the application of CDI in P removal.

2.2. Characterization 2.2.1. Characterization of the specific surface area, pore structure, and apparent morphology The specific surface area and pore structure of the activated carbon was characterized by nitrogen adsorption-desorption isotherms at 77 K using a Kubo nitrogen adsorption apparatus (Biaode Elec. and Tech. Co., China). The specific surface area was determined by the BrunauerEmmett-Teller (BET) method [52]. The pore size distribution was derived from the relative pressure range of 0.05–0.35 by the Barrett–Joyner–Halenda (BJH) method [53]. Activated carbon and electrode material were analyzed by scanning electron microscope (SEM, JEOL JSM-7500F) to study their surface morphology and structure properties. 2.2.2. Electrochemical properties of the activated carbon electrodes The cyclic voltammetry (CV) and linear galvanostatic charging and discharging (GCD) experiments were carried out using an electrochemical workstation (CHI660E, CH Instruments, Inc.) with a threeelectrode cell. The counter electrode was a platinum plate, the reference electrode was an Ag/AgCl electrode, and the working electrode was a graphite paper attached with a small piece of carbon electrode material of 1.0 * 1.0 cm in size. The electrolyte was 1.0 M KH2PO4 solution. The CV was performed using a scan rate of 5–50 mV/s in a potential range of −0.4 to +0.8 V, and GCD was performed at the current density of 1.5A/g in a potential range of −1.8 to +1.8 V. The specific capacitances of electrodes Cs were calculated according to the following equation [54]:

Cs = (I × td )/(ΔV × m)

(1)

where I (A) is the current, ΔV (V) is the potential window, td(s) is the discharge time, and m(g) is the mass of the active material of the working electrode. 2.3. Capacitive deionization device The CDI experimental system consisted of a single CDI electrode module that was manually assembled, a DC power supply, an ammeter, a pH meter, and a conductivity meter. The CDI electrode module was composed of a Plexiglas support plate, graphite current collector, activated carbon electrode, and Plexiglas mesh separator. The distance between the electrodes was 2.0 mm (Fig. 1). 2.4. Desalination experiment

2. Materials and methods The P solution was continuously fed into a group of CDI electrode modules by the peristaltic pump until the concentration of the effluent remained basically unchanged. The flow rate was 10 mL/min, the total entry volume was 500 mL. The initial concentration of 0.5 mg/L. To investigate the effect of the applied voltage on the P removal efficiency, the applied voltages varied from 1.2 to 3.6 V in 0.6 V steps. The change in solution concentration with time in the process of CDI adsorption

2.1. Preparation of the electrode material The electrodes for capacitive deionization were prepared by the coating method. Homemade activated carbon(prepared by coupling physical and chemical activation methods of sodium hydroxide; the raw material consisted of apricot shells, the preparation process as 2

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Fig. 1. Schematic of the CDI experimental system.

was measured. Meanwhile, the changes in pH and conductivity in the solution were measured by a pH meter (Leici pHS-3C, China) and conductivity meter (Leici DDB-303, China), respectively. To study the isotherms and kinetics of electrosorption at various solution temperatures (298, 303, or 308 K) and concentrations (1, 2, 5, 7, 10, or 13 mg/L), a series of adsorption experiments was carried out at an applied voltage of 3.6 V. The mechanism of P adsorption on the CDI electrode was determined by thermodynamic calculations. During the above experiments, samples were collected every hour (from 0 to 6 h), and 1 mL of the phosphorus solution was collected each time. The samples were analyzed by ion chromatography(IC, HIC-20A, SHIMADZU). All experiments were repeated three times, and the results were averaged.

W=φ

(C − Ct ) × V Γt = 0 m

F × V × (C0 − Ct ) × 100% Mi × ∫ Idt

W 1000 × V

(6)

Wm =

W m × Γt

(7)

3. Results and discussion 3.1. Characterization of the activated carbon and electrode materials

(2)

The performance of CDI was closely related to the specific surface area and pore size distribution of the activated carbon electrode (Fig. S1). The homemade activated carbon had a mixed structure of micropores and mesopores with a specific surface area of 961.35 m2/g. The total pore volume and average pore diameter were 0.595 cm3/g and 2.47 nm, respectively, and the volume distribution ratio of the mesopores to the total volume was 35%. The microstructure of homemade activated carbon and electrode material is shown in Fig. S2. For the activated carbon (Fig.S2a), the activated carbon is the amorphous structure with a large number of pores. Comparison of the activated carbon, the surface of the electrode material is rough, and the carbon particles, carbon black and binder aggregate in an irregular network (Fig.S2b). Cyclic voltammetry is an important method to evaluate the characteristics of carbon electrodes because the working principle of CDI is similar to that of supercapacitors. The cyclic voltammetry (CV) of

(3)

where C0 and Ct (mg/L) are the initial concentration and concentration of phosphorus solution at time t, respectively; V is the solution volume (L), m is the active substance mass on the electrode (g). The current efficiency is calculated based on Eq. (4) [55]:

ηcharge =

Wv =

where V is the volume of wastewater treated by the adsorption process (m3), m is the mass of the active substance on the electrode (g) and Γt is the amount of P adsorbed by the electrode at time t (mg/g).

The P removal performance of the CDI unit was mainly evaluated by the P removal efficiency (Eq. (2)) and adsorption capacity (Eq. (3)):

(C0 − Ct ) × 100% C0

(5)

where W is the energy consumption (kWh), φ is the voltage (V). The energy consumption for treating a unit water volume (kWh/m3) and removing a unit mass P (kWh/g) are calculated according to Eqs. (6) and (7), respectively:

2.5. Removal effect and calculation of the power consumption

η=

∫ Idt

(4)

where F is the Faraday constant (96485 C/mol), Mi is the relative molecular weight of P, and I is the current supplied during the adsorption process (A). The energy consumption calculation during the adsorption process is based on Eq. (5): 3

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Fig. 2. Changes in the total phosphorus concentration, removal efficiency with time (a) and adsorption capacity (b) at various applied voltages.

radius of phosphate ion (H2PO4−/HPO42−) is between 0.302 and 0.327 nm, which is larger than that of chloride ion. Therefore, a higher voltage in P removal is preferred.

activated carbon electrode at four different scanning rates (5, 10, 20, 50 mv/s) is shown in Fig. S3. The specific capacitance is 94.99 F/g at the rate of 5 mv/s in 1.0 M KH2PO4 solution. There is no obvious redox peak on the CV curve, which indicates that the specific capacitance mainly comes from the double layer capacitance at the interface of the carbon electrode and electrolyte. The ions adsorption on the electrode is mainly electrostatic, not Faraday reaction [56].

3.2.2. Changes in the pH during CDI adsorption at various applied voltages Fig. 3 shows the changes in the pH with time during CDI adsorption at various applied voltages. During the adsorption process, the pH changed significantly, first increasing, then decreasing and finally stabilizing at 7.5–8. The change in the pH was determined by the transformation of different forms of phosphate [59] (Eqs. (8) - (10)) and the electrolysis of water (Eqs. (11)–(15)) in the aqueous solution. The forms of phosphate in water were H2PO4−, HPO42−, and PO43−, and their transformations at different pH values were as follows:

3.2. Effect of the applied voltage on the phosphate removal performance of CDI 3.2.1. Effect of the applied voltage on the phosphate removal efficiency Fig. 2 show the variation in the P concentration, removal efficiency and adsorption capacity with time in capacitive deionization devices at various applied voltages (0, 1.2, 1.8, 2.4, 3.0, and 3.6 V). There was no electrostatic field between the two electrodes without an applied voltage, and the removal of P in solution was only due to the adsorption onto the activated carbon. The removal efficiency and adsorption capacity were 6% and 0.08 mg/g, respectively, which indicated that the adsorption onto activated carbon contributed little to the removal of P in solution. Once a voltage was applied, orthophosphate ions were adsorbed onto the anode surface due to the electric field, and the P concentration gradually decreased until the electrosorption process reached equilibrium at 6 h (Fig. 2a). The results showed that electrosorption capacity and removal efficiency increased with increasing applied voltage (Fig. 2) and reached their maximum values at 3.6 V, which were 1.611 mg/g and 86.6%, respectively. These results indicated that the higher the applied voltage, the greater the P removal efficiency of CDI. According to the GCS model [57], the double layer on the electrode surface consisted of a compact layer and a diffuse layer. With the application of voltage, the charge on the surface of the anode was compensated by the anions enriched by the double layer in the solution. The charge of the compact layer mainly depended on the applied voltage. The surface charges of the anode compact layer increased with increasing voltage. Meanwhile, the number of ions adsorbed on the surface of the electrode increased, and the adsorption capacity increased accordingly. These results were consistent with the conclusions of Ge et al.[55]. Generally, the applied voltage for desalination by CDI is limited to less than 2.0 V to avoid energy loss caused by Faraday current. The 3.6 V voltage used for P removal in this study can lead to potential water decomposition [58]. However, this may not be a major issue; it provides additional functions of disinfection as well as P removal. In addition, according to the research of Ge et al, a greater driving force is necessary for effectively removing P from water, because the hydration

H3 PO4 ⇌ H2 PO4− + H+pK a = 2.12

(8)

H2 PO4− ⇌ HPO42 − + H+pK a = 7.20

(9)

HPO42 − ⇌ PO43 − + H+pK a = 12.36

(10)

According to the literature, the adsorption of ions by CDI is related to the ion concentration, ion charge, and radius of the hydrated ion [31,32]. The phosphate solution in this study was prepared with potassium dihydrogen phosphate (KH2PO4). The initial solution was dominated by H2PO4− with a pH of 5–6, and the radius of hydrated H2PO4− ions (0.302 nm) was smaller than that of HPO42− (0.327 nm)

Fig. 3. Changes in the pH with time during CDI adsorption at various applied voltages. 4

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Fig. 4. Changes of P concentration (a) and adsorption capacity (b) at different pH.

[60]. Therefore, H2PO4− was preferentially adsorbed onto the anode. As H2PO4− was adsorbed, Eq. (8) shifted to the left, and the H+ in the solution was consumed. Meanwhile, the hydrogen evolution reaction on the cathode also consumed H+ because the applied voltage exceeded the theoretical hydrogen evolution voltage of water (1.23 V) [61]. The above comprehensive effect led to the rise in the pH of the solution. When the pH increased to 8–9, the solution was dominated by HPO42−, and the latter was preferentially adsorbed. As Eq. (8) shifted to the right, H+ increased in the solution, and the pH of the solution decreased. The pH remained stable at 7.5–8 until phosphate was no longer adsorbed onto the anode (Fig. 3).

removal but also provided the possibility of disinfection by the oxidation-reduction reaction on the electrode surface. Current efficiency refers to the ratio of the number of ions adsorbed to the charge transferred between the electrodes. Fig. S4 shows the current efficiency at various voltages. The current efficiency was always below unity at each voltage, indicating that the charges transferred between electrodes were used not only for adsorbing ions but also for other energy-consuming processes, such as the Faraday reaction and resistance loss [63]. The current efficiency decreased with the increase of voltage, which indicated that the proportion of charge used in redox reaction increases with the increase in voltage. The result was consistent with the conclusions of Ge et al. [55]. The oxidation of water (Eqs. (11) and (12)) may occur on the anode [64,65] as follows:

3.3. Effect of initial solution pH on phosphorus adsorption by CDI Fig. 4 shows the variation in the P concentration with time and adsorption capacity in capacitive deionization devices at different initial solution pH (5–10). The adsorption capacity decreased with the increase of initial pH. The adsorption capacity of P by CDI decreased from 3.8 mg/g to 3.0 mg/g with an initial pH from 6 to 10. This may be due to the change in the proportion of different forms of phosphorus (H2PO4−, HPO42−). Huang et al. [30] believed that the decrease of phosphorus removal rate was mainly due to the competition between phosphate and hydroxyl ions with the increase of pH. However, in this study, the removal efficiency of P began to decrease when the initial solution pH was 7, and the concentration of OH– ion was 10−4 mM, which could not compete with the concentration of phosphorus (~10−2 mM) in the solution. The initial solution was dominated by H2PO4− with a pH of 6, the proportion of H2PO4−decreased and that of HPO42−increased gradually with the increase of pH. When the pH increased to 10, the solution was dominated by HPO42−. From the aspect of charge balance, the number of charges required to adsorb 1 mol of HPO42− is twice that of adsorbed 1 mol of H2PO4−. Therefore, the decrease of P adsorption capacity seems reasonable due to the increase in the proportion of HPO42−. From the perspective of hydration radius, the radius of hydrated H2PO4− ions (0.302 nm) is smaller than that of HPO42− (0.327 nm) [60], and the ions with smaller hydration radius are more easily adsorbed by CDI [31]. In conclusion, the more effective adsorption of P by CDI at low pH is due to the radius of hydrated ions and the number of charges of H2PO4 −,which is consistent with Huang et al. [62].

2H2 O → O2 + 4H+ + 4e−E 0 = 1.23V / SHE

(11)

H2 O → HO·+H+ + e−E 0 = 2.80V / SHE

(12)

The possible reduction reactions of the cathode [65] are shown in Eqs. (13)- (15):

O2 + 2H+ + 2e− → H2 O2 E 0 = 0.69V / SHE

(13)

H2 O2 + 2H+ + 2e− → 2H2 OE 0 = 1.78V / SHE

(14)

2H2 O +

2e−

→ H2 +

2OH−E 0

= 0V / SHE

(15)

The HO∙ and H2O2 produced on the CDI electrode at a high voltage can be used for sewage disinfection [66]. In addition, the hydrogen peroxide produced by cathodic reduction combined with ultraviolet irradiation may also have led to bacterial inactivation. It has been reported that a low concentration of H2O2 (20–150 μM) combined with ultraviolet irradiation or O3 resulted in an increase in the number of inactivated bacteria by 0.5–2.0 log units [67,68]. Wang et al. [69] has done research on CDI disinfection using activated carbon electrodes and found that the electric field between the two electrodes increased with the increase of voltage, and more bacteria were adsorbed onto the electrode surface and killed. Next, CDI will be used for upgrading after anaerobic treatment in sewage treatment plants. It can not only be used for advanced treatment of P but also be used to replace the disinfection process. Therefore, we will further study the disinfection characteristics and efficacy of CDI under various voltages. 3.5. Kinetics model of electrosorption

3.4. Disinfection with high voltage

The main driving forces of ion adsorption onto the CDI electrode

The high voltages applied to CDI not only ensured the efficiency of P 5

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Fig. 5. The electrosorption kinetics of phosphate onto the CDI electrode at various voltages (a), concentrations and temperatures (b)–(d).

were the applied voltage, temperature and solution concentration [70]. Lagergren's pseudo-first-order and pseudo-second-order kinetic models were used to fit the electrosorption behavior of activated carbon electrodes in the phosphate solution at various voltages, temperatures, and concentrations. The forms of these kinetic equations are expressed in Eqs. (16) and (17). The adsorption rate constant could be estimated by the slope of the plot of the adsorption amount and time.

log (qe − qt ) = logqe − t 1 t = + qt qe k2 qe2

k1 t 2.303

Table 1 Pseudofirst-order and pseudosecond-order dynamic parameters at various voltages. Kinetics

where qe is the equilibrium adsorption capacity of TP (mg/g), qt is the adsorption capacity of TP at time t (mg/g), and k1 (min−1) and k2 (g/ mg min) are the pseudo-first-order and pseudo-second-order adsorption rate constants, respectively.

Value 1.2 V

1.8 V

2.4 V

3.0 V

3.6 V

First-order

qe K1 R2

0.6985 0.30503 0.9897

1.1802 0.2295 0.9967

1.2529 0.4265 0.9871

1.3777 0.4777 0.9986

1.6568 0.6882 0.9949

Second-order

qe K2 R2

1.0342 0.2156 0.9896

1.8542 0.0826 0.9964

1.7646 0.1902 0.9809

1.8463 0.2192 0.9982

2.1106 0.3109 0.9847

(16)

(17)

Parameter

Because the voltage was a key factor in controlling the formation of a double layer [57], and a higher voltage could accelerate the movement of charged ions to the opposite-charged electrode to form a thicker double layer for ion adsorption.

3.5.1. Kinetics model of electrosorption at various voltages Fig. 5a shows the fitting of experimental data and kinetic model equations at the applied voltages. Table 1 summarizes the fitting parameters and regression coefficient (R2) values. The results showed that both the pseudo-first-order and pseudo-second-order model could verify the experimental data, but the pseudo-first-order correlation was better. The results showed that the higher the voltage, the greater the electrosorption capacity at equilibrium and adsorption rate of phosphorus on the active carbon electrode, indicating that the adsorption capacity and rate were positively correlated with the applied voltage.

3.5.2. Kinetics model of electrosorption at various temperatures and concentrations Fig. 5(b, c, and d) show the fitting of experimental data and kinetic model equations at various temperatures and initial concentrations while maintaining the applied voltage constant at 3.6 V. Tables 2 and 3 summarize the fitting parameters and regression coefficients R2. The results indicated that the kinetic experimental data at various temperatures and concentrations correlated well with the pseudo-firstorder kinetics model fitting results. This was consistent with the result 6

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Table 2 The pseudo-first-order kinetic parameters at various concentrations and temperatures. Initial concentration(mg/L)

Pseudo-first-order kinetic parameters 298 K

1.0 2.0 5.0 7.0 10.0 13.0

303 K 2

308 K

qe

K1

R

qe

K1

R

qe

K1

R2

1.921 3.176 6.105 6.495 7.291 7.079

0.596 1.007 1.049 1.463 1.206 2.348

0.9955 0.9964 0.9939 0.9948 0.9100 0.9634

2.301 3.698 5.389 5.997 6.732 6.876

0.719 1.324 1.599 1.078 1.456 2.046

0.9959 0.9729 0.9945 0.9925 0.9668 0.9157

2.396 3.461 5.789 6.257 6.778 6.805

0.954 1.714 1.910 2.374 2.541 3.011

0.9999 0.9945 0.9866 0.9852 0.9951 0.9827

of the electroabsorption kinetics of NaCl on CNT-CNFs film [71]. As the concentration increased, the electrosorption capacity and adsorption rate of the activated carbon electrode for phosphate increased, which may be related to the increased driving force of the high concentration gradient. The electrosorption rate increased with increasing temperature. This may have been due to the increase in ionic thermal motion caused by the temperature rise, which increased the chaos degree of the system; the latter resulted in a more effective contact between the charged ions in the aqueous solution and the electrode surface, and as a result, the adsorption rate of phosphate onto the activated carbon electrode increased. However, the adsorption capacity at equilibrium increased with increasing temperature at low concentrations (1–2 mg/ L) and decreased with increasing temperature at high concentrations (5–13 mg/L). The adsorption process was endothermic when the initial concentration was lower and exothermic when the initial concentration was higher. It could be inferred that chemical adsorption between the adsorbent and adsorbate may have occurred at lower concentrations. With increasing concentration, the adsorptive sites for chemical adsorption were gradually occupied by phosphate, followed by physical adsorption. As the temperature increased, the total energy of adsorbate molecules in the aqueous solution increased, which increased the trend of phosphate desorption from the activated carbon electrode and led to a decrease in the phosphate adsorption capacity. Although the pseudo-first-order model could describe the experimental data, the model did not consider the effect of the voltage on the adsorption. Therefore, it was necessary to consider the voltage effect to improve the model. Biesheuvel et al. [72] established a dynamic adsorption process model of CDI based on the GCS model. The model results were in good agreement with the experimental data of the electron current and salt ion concentration and could be used to analyze the effects of the voltage, ion strength and electrode area on the CDI performance. However, the model did not consider the effect of the temperature on the CDI performance. Therefore, the model could be improved by considering the temperature effect.

2

3.6. Isothermal model of electrosorption and Thermodynamic analysis 3.6.1. Isothermal model of electrosorption Fig. 6 shows the electrosorption isotherms at various solution temperatures (298, 303, and 308 K) while maintaining the applied voltage constant at 3.6 V. The initial concentrations were 1.0, 2.0, 5.0, 7.0, 10.0 and 13.0 mg/L. As expected, the electrosorption capacity increased with increasing initial concentration. The high driving force at high concentrations led to an increase in the electrosorption capacity [73]. The Langmuir (Eq. (18)) and Freundlich (Eq. (19)) isothermal models were used to fitting the experimental data of the phosphorus adsorption onto activated carbon electrodes.

qe =

qm KL Ce 1 + KL Ce

(18)

qe = KF Ce1/ n

(19)

where qe is the equilibrium adsorption capacity of TP (mg/g), qm is the maximum adsorption capacity (mg/g), KL is the Langmuir constant related to the adsorption binding energy (the higher the value, the stronger the adsorbent's adsorbability), Ce is the equilibrium concentration of the solution (mg/L), KF is the Freundlich constant related to the adsorption capacity, and n is a measure of the adsorption intensity or surface heterogeneity. If 1/n is less than 1, the adsorbate is easily adsorbed by adsorbents, and new adsorption sites appear with the progress of adsorption. If 1/n is greater than 1, it is difficult for adsorbates to be adsorbed by adsorbents [74,75]. Table 4 shows the fitting parameters and R2. The results showed that the Freundlich isotherm described the experimental data better than the Langmuir isotherm, indicating that the adsorption of phosphorus by the activated carbon electrode was a multilayer adsorption process. In addition, the maximum adsorption capacity fitted by the Langmuir isotherm could be used as the theoretical adsorption capacity at this temperature. The results showed that the maximum adsorption capacity of the activated carbon electrode for phosphate decreased with increasing temperature, and the maximum adsorption capacity was 8.53 mg/g at 298 K. The Freundlich constant 1/n was less than 1, indicating that new adsorption sites appeared as the adsorption process

Table 3 The pseudo-second-order kinetic parameters at various concentrations and temperatures. Initial concentration(mg/L)

Pseudo-second-order kinetic parameters 298 K

1.0 2.0 5.0 7.0 10.0 13.0

303 K 2

qe

K2

R

2.4530 3.6780 7.0101 7.4688 8.0765 7.4370

0.2250 0.3206 0.1810 0.1680 0.2091 0.6136

0.9838 0.9819 0.9860 0.9808 0.8862 0.9509

308 K 2

qe

K2

R

2.835 4.131 5.870 6.855 7.360 7.294

0.254 0.418 0.406 0.193 0.291 0.484

0.9917 0.9397 0.9889 0.9838 0.9527 0.8662

7

qe

K2

R2

2.809 3.751 6.187 6.583 7.098 7.052

0.383 0.693 0.517 0.686 0.725 0.996

0.9944 0.9747 0.9749 0.9636 0.9856 0.9675

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Table 4 The parameters determined by fitting the Langmuir and Freundlich isotherms at various temperatures. Isotherm

Parameter

Value 298 K

303 K

308 K

Langmuir

qm KL R2

8.5309 1.0219 0.9853

7.3799 1.3029 0.9161

7.1158 2.1378 0.9293

Freundlich

KF 1/n R2

3.8504 0.3209 0.9164

3.7193 0.2804 0.9753

4.0204 0.2474 0.9533

et al. (2009) [72]developed a more comprehensive model that considered the voltage. 3.6.2. Thermodynamic analysis Thermodynamic parameters related to adsorption, such as standard Gibbs free energy change (ΔG ), standard enthalpy change (ΔH ) and standard entropy change (ΔS ) were determined at 298, 302 and 308 K. The changes in the standard enthalpy (ΔH ) and standard entropy (ΔS ) could be obtained from the slope and intercept, respectively, of the Van’t Hoff equation (Eq. (20)) of lnKd versus 1/ T (Fig. S6a):

1 1 ⎞ ΔH + ΔS lnK d = −⎛ R ⎝ RT ⎠

(20)

where Kd is the adsorption equilibrium constant, which can be estimated from the intercept of a linear plot of Ln(qe/Ce) versus qe (Fig. S6b); R is the universal gas constant (8.314 × 10−3 kJ/mol * K); T is the temperature of the solution (K). The standard Gibbs free energy ΔG could be obtained from ΔG = −RTlnK d andΔG = ΔH − TS , as shown in Table S1. The ΔG at all temperatures was negative and became more negative with increasing temperature, indicating that the electrosorption of P onto the activated carbon was a spontaneous process and was favored at a higher temperature, this result was consistent with the conclusions of Li et al. [71,76]. The enthalpy change (ΔH ) was greater than zero, indicating that the electrosorption process was endothermic. It may be because the radius of hydrated ions of phosphate is larger, and the most probable pore size of the homemade activated carbon was 2–4 nm, belonging to the small range of mesopores. The adsorbates needed to overcome high barriers to enter the pores, which hindered the diffusion of adsorbates in the pores. Therefore, the adsorbates became unstable after adsorption, resulting in an increase in the system energy [77]. 3.7. Double-layer adsorption model based on Gouy-Chapman-Stern (GCS) The double-layer model is used to study the relationship between the surface charge on the pore of carbon electrode particles and the counter ions amount in solution at adsorption equilibrium. GCS model was used to fit the experimental data and theoretical results for the P electrosorption. This study was based on the following assumptions: (1) The electric double layer in the pore was assumed to be non-overlapping, i.e. the size of the pore with effective adsorption of P was much larger than the length of Debye; (2) The charge distribution on the electrode surface was assumed to be uniform; (3) Ions in solution were assumed to be point charges and conformed to Boltzman distribution law; (4) The permittivity at each position of the diffusion layer in solution was assumed to be the same.

Fig. 6. Electrosorption isotherms under various temperatures.

progressed, forming multilayer adsorption. Moreover, 1/n decreased with increasing temperature, indicating that the increase in temperature was conducive to the adsorption of phosphate by CDI, which was consistent with the pseudo-first-order kinetic finding. Although the experimental data of phosphate adsorption by CDI conformed to the Freundlich model, one drawback of the isotherms was that the effect of the applied voltage was not considered. The calculations in this study were based on a constant voltage of 3.6 V. Biesheuvel

Based on the above assumptions, the GCS model was used to 8

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data and theoretical results are presented in Fig. 7 using as parameters setting as Cst = 0.2 F/m2, m = 0.2 g (proportion of active substances was 85%), Vcell ranged from 1.2 V to 3.6 V, and c0 ranged from 0.5 mg/L to 10 mg/L. The theoretical results were in good agreement with the experimental results. The model showed that the adsorption capacity depended strongly on voltage and concentration. It indicated that the model can be used to describe the relationship between the adsorption capacity of solution and surface charge. However, not all the pores in activated carbon electrodes have an adsorption effect on ions, and it was found that the concentration had an effect on the effective inner surface area. Therefore, it is necessary to find the pore size range of the carbon electrode for P adsorption, and to develop more accurate models including pore size and effective inner surface area, so as to make the double layer model more applicable.

analyze the experimental data. The total voltage Vcell is divided into two double layers consisting of a stern layer voltage (Vst) and the diffusion layer voltage (Vd). The voltage of the two electrodes is assumed to be the same. Thus,

Vcell = Vst + Vd 2

(21)

The distribution of ion concentration in solution accords with Boltzman distribution law, and the ion concentration × distance from the electrode surface is

ci (x ) = c0·exp (−z i ·φ (x ))

(22)

where zi is the valence charge of the ions, c0 is the ions concentration in solution far from the electrode surface (φ = 0), φ is dimensionless potential (which is the voltage Vcell divided by thermal voltage VT, VT = RT/F, VT is 25.6 mV at room temperature). The surface charge density (σ, mol/m2) per unit electrode area is given by

1 σ = 4λD c0·sinh ⎛ ·Δφd⎞ ⎠ ⎝2

3.8. Energy consumption and recycling test of CDI electrode for phosphorus adsorption

(23) The effect of the applied voltages on the energy consumption is shown in Fig. 8. The energy consumption increased with increasing voltage, as expected. When the voltage was increased from 1.2 to 3.6 V, the energy consumption (kWh/m3 treated water and kWh/g TP) increased from 0.0057 to 0.075 kWh/m3 treated water and from 0.029 to 0.157 kWh/g TP. The energy consumption (kWh/m3 treated water and kWh/g TP) increased sharply with the operating voltage over 2.0 V. Although the effect of phosphorus removal by the CDI electrode increased obviously with the increase of voltage, it is more economical to control the voltage below 2.0 V without considering the added benefits such as disinfection. According to the previous study [79], when the initial concentration of TP is 5–7 mg/L, the operating cost of the high-efficiency biological dephosphorization process is 0.274 RMB/m3, and the operating cost of chemical dephosphorization is 0.469 RMB/m3, while the operating cost of using CDI device to remove phosphorus is 0.161 RMB/m3 (Fig.S5). Thus, according to the above comparison, CDI dephosphorization has lower energy consumption, which has the potential for industrial application. Fig. 9 shows the cyclic experiment results of adsorption and desorption of phosphate by CDI with an applied voltage of 1.8 V and an initial phosphate concentration of 1 mg/L. The experimental process was intended to adsorb phosphate for 2 h and desorb phosphate for 2 h to regenerate the adsorbent, and then the cycle was repeated five times. The results showed that the first adsorption capacity of phosphate by

Δφd is the diffuse layer dimensionless voltage difference, λD is the length of Debye, given by

λD =

1 κ

(24)

κ2 =

2F 2c0 = 8πλB c0 NA εr ε0 RT

(25)

where λB is the Bjerrum length related to temperature and permittivity of the solution, λB is 0.72 nm at room temperature; NA is Avogadro’s number, 6.02 × 1023 mol−1. According to Biesheuvel et al. [72] and Eq. (22), the amount of total ions adsorbed from solution per unit electrode area, Γ is given by

Γ = 8λD c0·sinh2 (Δφd /4)

(26)

The voltage of the stern layer (Vst = Δφst ·VT ) can be directly related to the surface charge density σ according to Gauss law:

Cst ·Δφst ·VT = σ·F

(27)

According to Frumkin et al. [78] and Yang et al. [51], the stern layer capacitance Cst is a constant at low surface charge density. In this study, Cst is assumed to be 0.2 F/m2. Based on the GCS model, the surface charge density σ can be obtained. According to the data analysis, the inner surface area a, i.e. the effective adsorption area, can be estimated. Results of the experimental

Fig.7. Total ion capacity as a function of different voltages (a) and P concentrations (b). 9

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Fig. 8. Energy consumption of CDI at various voltages.

homemade activated carbon electrodes at various applied voltages were investigated. Furthermore, the kinetics, thermodynamics and the equilibrium GCS model were studied to reveal the mechanism of P adsorption by CDI. It was observed that the adsorption capacity and removal efficiency were positively correlated with the voltage. The change in the pH was determined by the electrolysis of water and transformation of different forms of phosphate in the aqueous solution, the latter resulted in an increase in the pH at first, then a decrease, and finally, the pH remained stable at a value of 7.5–8. The HO∙ and H2O2 produced in a high voltage gave CDI its potential for sewage disinfection. The experimental results were validated by pseudo-first-order adsorption kinetics and Freundlich isotherms. The adsorption rate increased with increasing concentration, temperature, and voltage. The adsorption capacity decreased with increasing temperature, and the maximum adsorption capacity was 8.53 mg/g. Thermodynamic analysis confirmed that the electrosorption of P onto the activated carbon electrode was a spontaneous and endothermic process and was favored at a higher temperature. The GCS theoretical results were in good agreement with the experimental results, indicating that the adsorption capacity depended strongly on voltage and concentration. It indicated that the surface charge can reflect the number of ions adsorbed. It was found that the concentration had an effect on the effective inner surface area. In the future, it is necessary to develop more accurate models including pore size and effective inner surface area. This study not only helps to understand the mechanism of P adsorption on activated carbon electrodes but also provides a theoretical basis for the application of P removal by CDI.

Fig. 9. Adsorption and desorption regeneration cycles of phosphate by CDI.

CDI was 1.290 mg/g, and the first desorption rate was 99.8%. After regeneration, some of the ions remained in the pores of the electrode. After five adsorption-desorption experiments, the adsorption capacity was 1.173 mg/g, which was 91% of the adsorption capacity at the first adsorption experiment. And the fifth desorption rate was 99.2%. It could be inferred that the homemade activated carbon electrode with a high specific surface area exhibited strong regeneration ability after P adsorption, and the removal and recovery of P from the aqueous solution by CDI was feasible. Fig. 10 shows the results of a 100-time charge-discharge cycle of an activated carbon electrode in a P solution at a current density of 1.5 A/ g. The specific capacitance is calculated every 5 cycles according to Eq. (1), and the specific capacitance changed with the recycle times is shown in Fig. 10b. The results showed that the specific capacitance of the activated carbon electrode during the first charge and discharge cycle was 92.93F/g. After 100 cycles of charge and discharge, the specific capacitance of the 100th time was 84.48F/g, which was 9.09% lower than the specific capacitance of the first cycle. The specific capacitance of the activated carbon electrode did not decrease significantly during the whole cycle, indicating that the activated carbon electrode has a better regeneration ability.

Declaration of Competing Interest 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.

Acknowledgment We thank American Journal Experts (AJE) for English language editing.

Appendix A. Supplementary material 4. Conclusion Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.116285.

In this study, the performances of P removal by a CDI unit with 10

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