The removal of phosphate ions by capacitive deionization has become one of the most frontier research topics in the water treatment field in recent years. In this work, hybrid electrodes composed of nickel-iron layered double hydroxide (NiFe-LDH) – anchored on activated carbon fiber (ACF)–were synthesized by a hydrothermal method and subsequently applied in capacitive deionization to remove phosphate ions. The adsorption performance of the two hybrid electrodes on phosphate ions was compared by capacitive deionization experiments. The experiment was carried out for 3 hours to reach equilibrium, and the optimum adsorption of 33.48 mg/g was obtained using NiFe-LDH/ACF-2 hybrid electrode at room temperature (25 °C) and pH = 6.0. The results showed that increasing the loading capacity of NiFe-LDH on ACF might enhance the adsorption capacity of phosphate ions. Furthermore, the calculation of adsorption kinetics and adsorption isotherms elucidated that the adsorption capacity increased with the increasing of applied voltage. Meanwhile, the experimental data were fitted well with pseudo-first-order kinetics and Langmuir isotherms. Notably, it was observed that the pH first increased, then decreased during the adsorption due to the electrolysis of water, while the form of phosphate ions was transformed, with low pH favoring the adsorption of phosphate ions.

  • The hybrid electrodes of NiFe-LDH/ACF were prepared through a simple hydrothermal method.

  • The NiFe-LDH loading onto ACF can enhance the adsorption performance of ACF.

  • The NiFe-LDH/ACF-2 adsorption capacity for phosphate ions was about 33.48 mg/g.

As the social economy continues to develop and industrialization increases, the discharge of a great amount of industrial wastewater and domestic sewage has led to water pollution and even eutrophication (Li et al. 2020). This is principally owing to the substandard sewage treatment, causing a glut of nutrients (such as nitrogen, phosphorus, etc.) in the sewage water. Usually, the concentration of phosphate in the water body is below 0.5 mg/L ideally. However, the phosphate content in a batch of fish was once detected to be 5 g/kg, and phosphate concentration was seriously exceeded (Liu et al. 2018). Studies have shown that the presence of phosphate is a critical factor in the eutrophication of water bodies (Qiu et al. 2020). Excessive phosphorus content in water bodies can lead to eutrophication and damage the ecological balance. And excessive phosphorus in the human body can cause osteoporosis, stones and other physical diseases. Therefore, the reasonable protection of water resources and phosphate removal is of great urgency.

The existing phosphate removal technologies include biological and physicochemical dephosphorization (Zuthi et al. 2013), and each has its strengths and weaknesses. Biological phosphate removal technology has low cost, but low removal efficiency and harsh operating conditions. Among physicochemical technologies, chemical precipitation has high phosphate removal efficiency but high cost and the production of large amounts of sludge. And adsorption is the main treatment method with the advantage of low cost, simple operation and without by-products. However, many adsorbents are difficult to reuse, as they are in powder form. Moreover, the adsorbents will sink to the bottom of the water body during the adsorption process, which may cause secondary pollution to the environment. Consequently, a simultaneously economical and effective method for phosphate removal is earnestly required.

Over the past few years, capacitive deionization (CDI), also recognized as an emerging and evolving water treatment technology, has attracted much attention and exploration because of its energy conservation, high efficiency, and simple operation. Nevertheless, since CDI is still in the stage of research and development, CDI also has certain shortcomings, such as higher manufacturing and operating costs of related equipment, etc. (Wang et al. 2017b). CDI has been used to remove various ions, such as nitrogen, fluorine and phosphorus, etc. (Chen et al. 2020). The removal of phosphate ions using CDI is a very desirable and rewarding and interesting approach.

CDI is a technology that builds upon the theory of electric double layer (EDL). With the application of a voltage to two parallel electrodes, an electrode field is generated, and charged ions are absorbed into the surface of the opposite-charged electrodes, thereby decreasing the concentration of the solution and ultimately achieving ion purification (Tan et al. 2018; Tang et al. 2019). By short-circuiting or reversing the potential (Chen et al. 2020), the electrodes with adsorption saturation can then be regenerated. In comparison with other phosphate ion removal methods, CDI is more eco-friendly and will not produce secondary pollution or harmful by-products.

The electrode material is an essential component of CDI, which impacts the adsorption performance of CDI. Over the years, numerous carbon materials have been used in the research of CDI electrodes. Activated carbon fiber (ACF) is a porous material characterized by a large surface area and high porosity, offering wider applications than conventional activated carbon. Compared with activated carbon powder, ACF is more convenient to utilize without extra binders, and ACF has faster adsorption kinetics and holds great promise as a CDI electrode (Wu et al. 2015).

Layered double hydroxides (LDHs) are inorganic materials composing anionic layered compounds formed by two or more metals in combination (Bernardo et al. 2017). By virtue of unique structures and properties, LDHs are the current research hotspot in the adsorption field. LDHs have the advantages of being environmentally friendly, non-polluting, simple to prepare, and low cost. Among the mineral resources, Ni and Fe are relatively abundant. Furthermore, as a typical electrochemical electrode material, NiFe-LDH has been widely used in oxygen evolution reactions and supercapacitors (Sun et al. 2020). However, the phosphorus removal effect of NiFe-LDH/ACF in CDI was rarely examined, and the underlying mechanism remains poorly explored.

LDHs are synthesized mainly by co-precipitation (Rahman et al. 2018) and hydrothermal methods (Luo et al. 2017). In addition, previous studies have demonstrated that hydrothermal methods can successfully synthesize LDH and apply it to supercapacitors with positive performance (Luo et al. 2017; Sun et al. 2020). To our knowledge, LDH-based solid material hybrid electrodes are commonly synthesized by hydrothermal methods in CDI and supercapacitors. In this study, NiFe-LDH/ACF hybrid electrodes were synthesized by hydrothermal method. The study of NiFe-LDH/ACF as a CDI electrode for phosphate ion removal is very promising.

In this work, the physicochemical properties of NiFe-LDH/ACF hybrid electrodes were also characterized, and the influence of different factors was investigated. In addition, the kinetic and isotherm models of electro-sorption were examined to reveal the mechanism of phosphate ion removal. This study aimed to improve the adsorption capacity of phosphate ions through CDI, and NiFe-LDH/ACF hybrid electrodes were applied to CDI as a new electrode option.

Materials

Activated carbon fiber (ACF), polyvinyl alcohol (PVA), conductive acetylene black, nickel nitrate hexahydrate (Ni(NO3)2·6H2O), ferric nitrate nonahydrate (Fe(NO3)3·9H2O), polyvinylpyrrolidone (PVP), urea (CO(NH2)2), and potassium dihydrogen phosphate (KH2PO4) were purchased from Shanghai Macklin Co., Ltd (Shanghai, China). All chemicals were used directly with no further purification in this study.

Preparation of NiFe-LDH/ACFs

NiFe-LDH/ACFs were synthesized by a hydrothermal method. Typically, an aqueous solution (100 mL) was formed by dispersing PVP, Fe(NO3)3·9H2O, Ni(NO3)2·6H2O and urea ([Ni2+] + [Fe3+] = 0.1 mol/L, Ni/Fe = 3, [Urea]/([Ni2+] + [Fe3+]) = 6.6). The mixture was then stirred at a constant speed for 30 minutes while being sonicated to form a homogeneous dispersion. Next, the mixture was placed into a 100 mL polytetrafluoroethylene (PTFE) bottle with immersed ACFs (2 cm × 2 cm) and sealed in a stainless steel autoclave, which was heated to 160 °C for 12 h, and then cooled. Finally, the hybrid composites were rinsed with pure water and ethanol. After drying at 60 °C for 12 h, NiFe-LDH/ACF-1 was obtained. In addition, the NiFe-LDH/ACF-1 was put into the PTFE bottle containing the above mixture again for reaction under the same conditions, and NiFe-LDH/ACF-2 was obtained.

Capacitive deionization experiments

CDI desalination experiment equipment included a direct-current (DC) power supply, a peristaltic pump, a conductivity meter, a temperature electrode, a conductivity electrode, and a single CDI module. The CDI module was composed of a couple of plexiglass plates, two titanium electrode slices, and some of the gasket. The distance between electrodes was adjustable.

In this paper, the CDI experiments were tested at room temperature (25 °C) in circulating mode. The desalting solution was 100 mL KH2PO4 solution. The phosphate solution circulates between the beaker and the CDI module through a peristaltic pump. Meanwhile, the voltage between the two poles of the CDI module was provided by the DC power supply. The electrodes of conductivity and temperature were inserted into the solution to observe the conductivity change in real-time, and record the concentration change. All experiments in the above section were performed three times.

To research the recyclability of the electrode material, the desorption of the electrode material was carried out by short-connecting, and the desalting experiment was repeated ten times. To determine the relationship between conductivity and phosphate solution concentration, the standard curve was measured.

The performance of the CDI module is evaluated based on its phosphate removal efficiency and adsorption capability (Equations (1) and (2)):
formula
(1)
formula
(2)
where and are the concentrations of phosphate solution at times 0 and t (mg/L), separately. is the volume of solution (L), and is the mass of the active substance (g).

Characterization

X-ray diffraction (XRD) patterns of hybrid electrode were characterized by an Ultima IV Focus X-ray diffractometer in the range of 2θ from 5° to 80°. A nitrogen adsorption system (ASAP 2020, Micromeritics) was employed to record the nitrogen adsorption-desorption isotherms at 77 K, and then gained the specific surface area of hybrid electrodes. The morphology and structure observation of electrode material were observed under the scanning electronic microscope (SEM, S4800, Hitachi Corporation, Germany). The cyclic voltammetry (CV), as well as electrochemical impedance spectroscopy (EIS) experiment, were performed in 1 mol·L−1 KH2PO4 solution utilizing an electrochemical workstation containing a three-electrode cell.

Characterizations

Figure 1 shows the XRD pattern of the as-prepared NiFe-LDH sample. The typical characteristic peaks on the NiFe-LDH were found to be 11.3°, 22.6°, 34.2°, 38.3°, 46.3°, 59.7° and 61.1° with the corresponding diffraction peaks of (003), (006), (012), (015), (018), (110), and (113), respectively, which matched well with the standard diffraction patterns of the NiFe-LDH structures (JCPDS card No. 40-0215) (Munonde & Zheng 2021). The sharp and undifferentiated peak shape could be observed in Figure 1, indicating that the prepared NiFe-LDH had good crystallization (Wang et al. 2017a). In addition, the crystal size of NiFe-LDH could be calculated to be about 15.5 nm according to the Scherrer equation.
Figure 1

XRD pattern of NiFe-LDH.

Figure 1

XRD pattern of NiFe-LDH.

Close modal
According to the SEM images (Figure 2), the samples were NiFe-LDH nanosheets with a rhombic structure and an average diameter of approximately 0.5–1 μm. What's more, the ACFs displayed excellent pore size and specific surface area, and NiFe-LDH could be well wrapped on its surface. As shown in Figure 2, the morphology and microstructure of NiFe-LDH/ACF-1 were mainly composed of two NiFe-LDH nanosheets pressed tightly together, so that the contact area between the hybrid electrodes and the phosphate solution was reduced. Therefore, the adsorption effect was not satisfactory. Conversely, the morphology and microstructure of NiFe-LDH/ACF-2 were formed by the arrangement of individually layered nanosheets, thereby increasing the contact area and exposing more active sites. Thus, the adsorption effect of NiFe-LDH/ACF-2 was better. the difference between NiFe-LDH/ACF-1 and NiFe-LDH/ACF-2 may be due to the synthesis time. Finally, it was concluded that NiFe-LDH was successfully loaded to ACF, and increasing the number of loads can effectively increase the load volume. Therefore, results from both SEM and XRD indicated that NiFe-LDH/ACFs were successfully prepared.
Figure 2

SEM images of NiFe-LDH/ACF-1 (a, b) and NiFe-LDH/ACF-2 (c, d).

Figure 2

SEM images of NiFe-LDH/ACF-1 (a, b) and NiFe-LDH/ACF-2 (c, d).

Close modal
The Brunauer–Emmett–Teller (BET) measurements were performed to further evaluate the specific surface area of the NiFe-LDH/ACFs. Figure 3 shows the isotherms of nitrogen adsorption-desorption for NiFe-LDH/ACF-1 and NiFe-LDH/ACF-2. Based on the IUPAC standard classification, it was distinct that all isotherms are of type IV with hysteresis loops of type H3 (Xiao et al. 2013), which indicated the coexistence of mesopores and micropores (Luo et al. 2021). NiFe-LDH/ACF-1 had a specific surface area of 684 m2/g, while that of NiFe-LDH/ACF-2 was 935 m2/g, an increase of 37%. It has been documented that the increase in specific surface area was beneficial for the enhanced adsorption capacity (Li et al. 2011). This was attributed to the fact that the large specific surface area led to the exposure of more active sites, thus improving the adsorption performance. And this also explained the higher adsorption capacity of NiFe-LDH/ACF-2. Furthermore, the presence of micropores and mesopores has been demonstrated to promote the contact and wettability between the hybrid electrodes and the phosphate solution, thereby enhancing the adsorption performance (Luo et al. 2021).
Figure 3

Nitrogen adsorption-desorption isotherms of NiFe-LDH/ACFs.

Figure 3

Nitrogen adsorption-desorption isotherms of NiFe-LDH/ACFs.

Close modal
Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted to investigate the electrochemical properties of NiFe-LDH/ACFs. CV is an important method for evaluating the electrode properties, since CDI working principle is quite similar to supercapacitors. Figure 4 shows the CV curves of NiFe-LDH/ACFs. It can be noticed that the CV curves had no obvious redox peaks, thus indicating that no Faraday reaction occurred on the electrode surface. In addition, it was confirmed that the hybrid electrode material had a better electrical double layer capacitance behavior generated by the Coulomb reaction (Zhang et al. 2012), and the phosphate ions were adsorbed to the electrodes through the electrostatic reaction. The shape of the CV curve deviates from a rectangle because of the decrease in the resistivity and charging speed of the phosphate solution (Yan et al. 2018). It was possible to calculate the specific capacitance from the CV curves, area. The larger the area of the curve, the larger the specific capacitance and the better the adsorption efficiency was, obviously. According to Figure 4, the area of NiFe-LDH/ACF-2 was larger, and NiFe-LDH/ACF-2 had a better adsorption efficiency on phosphate ions. According to the literature, the specific capacitance of the NiFe-LDH/ACF-2 hybrid electrode was calculated to be 0.39 F/g (Chen 2013).
Figure 4

CV curves of NiFe-LDH/ACFs.

Figure 4

CV curves of NiFe-LDH/ACFs.

Close modal
The EIS test is an effective tool to discuss electrode resistance in electrochemical processes. Figure 5 shows the measured Nyquist plots of NiFe-LDH/ACFs. The diameter of the semicircular arc of the EIS Nyquist plot could represent the electron transfer resistance of the CDI electrode material. It can be observed that the radius of NiFe-LDH/ACF-2 was smaller than that of NiFe-LDH/ACF-1, which indicated that NiFe-LDH/ACF-2 had a relatively lower charge transfer resistance and better conductivity. CV and EIS analyses were consistent with the experimental results.
Figure 5

Nyquist plots of NiFe-LDH/ACFs.

Figure 5

Nyquist plots of NiFe-LDH/ACFs.

Close modal

Factors affecting the phosphate removal

Effect of applied voltage

The effect of applying different voltages (0, 0.6, 1.2, 1.8, 2.4 V) on the removal of phosphate in CDI was investigated (Figure 6). The applied voltage was a single variable, NiFe-LDH/ACF-1 was used as electrode, the plate spacing was 1 mm, and the flow rate was 15 mL/min. When the applied voltage was 0 V, it was apparent that no electric field existed between the two electrodes of the CDI module, and the CDI at this time was only the physical adsorption of phosphate ions by NiFe-LDH/ACF-1. Neither the adsorption capacity nor the removal efficiency was significant, which were 2.63 mg/g and 2.12%, respectively. Phosphate ions were absorbed to the anode surface when there was an electric field applied, the measured value of conductivity dropped, the concentration of phosphorus gradually decreased, and the electro-sorption equilibrium was reached after 3 hours. At a voltage of 2.4 V, both adsorption capacity and phosphate removal efficiency reached the maximum value of 20.91 mg/g and 15.71%, separately. The results showed that a rise in voltage assisted in removing the phosphate ions.
Figure 6

Changes in adsorption capacity when different voltages were applied.

Figure 6

Changes in adsorption capacity when different voltages were applied.

Close modal

In addition, the applied voltage of CDI was ordinarily set within 2.0 V to reduce energy loss, while a greater applied voltage may cause water splitting (He et al. 2016). It was not a major concern, and it also provided the added functionality of disinfection and phosphate removal (Chen et al. 2020).

Influence of NiFe-LDH/ACFs

The adsorption efficiencies of the different hybrid electrodes were studied. As shown in the Figure 7, the adsorption properties of NiFe-LDH/ACF-2 for phosphate ions were greater than those of NiFe-LDH/ACF-1. In addition, the adsorption capacity and removal efficiency of NiFe-LDH/ACF-1 were 20.92 mg/g and 15.71%, and those of NiFe-LDH/ACF-2 were 33.48 mg/g and 24.61%, respectively. This was due to the more complete interlayer structure of NiFe-LDH/ACF-2, which could adsorb more anions. Different reaction times would lead to different loadings of NiFe-LDH on the hybrid electrodes, which were about 14 and 32 mg, respectively. It was concluded that increasing the loading could improve the adsorption capacity. The phosphate ion adsorption capacity of activated carbon mentioned in previous research was 8.53 mg/g (Chen et al. 2020). The results showed that the hybrid electrode could significantly improve the adsorption capacity of phosphate ions.
Figure 7

The adsorption capacity and removal efficiency of phosphate ions applying the different hybrid electrodes.

Figure 7

The adsorption capacity and removal efficiency of phosphate ions applying the different hybrid electrodes.

Close modal

Effect of electrode spacing

The electrode spacing was set as a single variable to study the removal efficiency of phosphate ions. According to Figure 8, as electrode spacing increased, the capacity of CDI in removing phosphate ions decreased. This was mainly because the smaller the electrode spacing, the thicker the electric double layer formed at the same voltage, and the larger the electro-sorption capacity that could be achieved. Meanwhile, as the ions, diffusion distance between hybrid electrodes was also closer, the time for ions to reach the electric double layer was shorter, and the adsorption rate also increased accordingly. When the electrode spacing was widened to 3 mm, the resistance in the solution became larger, and the repulsion of ions by electrostatic force was affected by the resistance; as a result, phosphate ion removal performance decreased. In case the electrode contact was short circuit as a result of the too-small electrode spacing, in this experiment, a non-woven separator would be clamped in the device to avoid this phenomenon. In summary, the electrode spacing of 1 mm was taken for all subsequent tests.
Figure 8

Effect of different electrode spacing on the adsorption of phosphate at NiFe-LDH/ACF-2.

Figure 8

Effect of different electrode spacing on the adsorption of phosphate at NiFe-LDH/ACF-2.

Close modal

Effect of flow rates

Flow rate is also a factor affecting the capacity of CDI for phosphate ion removal. The flow rate was set as the only variable to explore the adsorption capacity of the NiFe-LDH/ACF-2 electrode at different flow rates (5, 10, and 15 mL/min) (Figure 9). It could be inferred that the effect of flow rate on adsorption was very weak. With the flow rate improving, the adsorption amount slightly increased from 31.61 to 33.48 mg/g. Moreover, the faster flow rate could make the time to reach adsorption equilibrium shorter. It was concluded that the residence time of the phosphate solution in the CDI module decreased as the flow rate increased, and the difference in concentration of the phosphate solution circulating in and out of the CDI module was reduced. The solution treated by the CDI module was relatively large, which enhanced the absorption rate and thus reduced the time to reach equilibrium. In light of the above conclusions, 15 mL/min was selected as the optimal inlet flow rate for the CDI.
Figure 9

The adsorption capacity at different flow rates.

Figure 9

The adsorption capacity at different flow rates.

Close modal

Effect of pH

Figure 10(a) shows the changes of pH in CDI process at 2.4 V. The value of pH first increased, then decreased, and finally stabilized during adsorption process. The change in pH was related to the conversion of phosphate forms (Equations (3)–(5)) and the electrolysis of water (Equations (6)–(10)) in solution. The transformations of phosphate at different pH were as follows:
formula
(3)
formula
(4)
formula
(5)
Figure 10

Changes in the pH with time at 2.4 V (a), and adsorption capacity (b) at various pH.

Figure 10

Changes in the pH with time at 2.4 V (a), and adsorption capacity (b) at various pH.

Close modal

The electrolysis of water in solution was as follows:

The anode:
formula
(6)
formula
(7)
The cathode:
formula
(8)
formula
(9)
formula
(10)

It has been shown that the removal of ions by CDI is related to the hydration radius of the ions. The initial pH of KH2PO4 solution was about 5.5. The ions were dominated by H2PO4 in the solution and its hydration radius was smaller than HPO42− (Seo et al. 2010). Thus, the H2PO4 was preferentially absorbed to the anode. The H2PO4 was absorbed, as in Equation (3), and H+ was consumed. Furthermore, the hydrogen evolution reaction occurred at the cathode, consuming H+. Therefore, the value of pH was increased. When the pH = 8.5–9, the priority to be absorbed was HPO42−. The H+ increased, as in Equation (3), and the pH decreased in the solution. Eventually, the pH tended to be stable at about 7.5, and the phosphate ions were no longer absorbed.

Figure 10(b) shows the adsorption capacity using NiFe-LDH/ACF-2 in CDI at various initial pH. The adsorption capacity decreased from 33.48 to 26.83 mg/g with the increase of initial pH. The reason for this may be the change in proportion of the forms of phosphorus. With pH increasing, the H2PO4 decreased and the HPO42− increased. The ions with smaller hydration radius were more favorable for CDI (Chen et al. 2020). So, there was better adsorption of phosphate ions at low pH.

Cycling stability of NiFe-LDH/ACF-2 hybrid electrode

Based on the principle of saving raw material costs and protecting the environment, the recycling of hybrid electrodes is essential to the application. Under identical conditions, the NiFe-LDH/ACF-2 hybrid electrode decreased in adsorption capacity from 33.48 to 30.27 mg/g after ten recycling experiments, a decrease of 9.6% (Figure 11). It was proved that the hybrid electrode possesses fine reusability. The decrease in desalination performance can be explained by the consumption of the edge arrays of NiFe-LDH nanosheets and the decrease in specific capacitance during the desorption process (Wang et al. 2021). Moreover, the absence of residual nickel or iron ions in the desalting solution after the adsorption process suggested that the NiFe-LDH/ACF hybrid electrode has good stability. This was probably thanks to the presence of nickel and iron in solid form, as well as the repeated rinsing and sonication of the product during the preparation stage to remove excess unconsolidated precipitates.

Adsorption kinetic models

The adsorption kinetic models were considered to fit the adsorption behavior of the NiFe-LDH/ACF-2 hybrid electrode. The pseudo-first-order and pseudo-second-order kinetic equations were shown in Equations (11) and (12):
formula
(11)
formula
(12)
where is equilibrium adsorption capacity (mg/g), is adsorption capacity at time t (mg/g), (min−1) and (g/mg·min−1) are the pseudo-first-order and pseudo-second-order adsorption equilibrium rate constants, respectively.
Figure 11

Adsorption capacity of the recycled NiFe-LDH/ACF-2.

Figure 11

Adsorption capacity of the recycled NiFe-LDH/ACF-2.

Close modal
Table 1 shows the fitting parameters and regression coefficient values. Figure 12 shows the pseudo-first-order and pseudo-second-order kinetic equations fitting curves at different applied voltage. It could be seen that the pseudo-first-order kinetics model correlated better than pseudo-second-order. It can be drawn from Figure 12 that the higher the applied voltage, the better the adsorption capacity of phosphate ions on the NiFe-LDH/ACF-2 hybrid electrode. They were positively correlated. This was because a double layer was formed by applying voltage. And the higher the voltage, the thicker the double layer, which could promote the movement of charged ions.
Table 1

The pseudo-first-order and pseudo-second-order kinetic parameters at various voltages

KineticsParametersValue
0V0.6V1.2V1.8V2.4V
Pseudo-first-order  4.750 14.879 27.149 32.232 35.579 
 0.498 0.753 0.654 0.866 1.040 
 0.990 0.997 0.991 0.994 0.998 
Pseudo-second-order  7.447 21.146 39.966 44.277 46.867 
 0.045 0.028 0.012 0.016 0.020 
 0.987 0.994 0.987 0.990 0.994 
KineticsParametersValue
0V0.6V1.2V1.8V2.4V
Pseudo-first-order  4.750 14.879 27.149 32.232 35.579 
 0.498 0.753 0.654 0.866 1.040 
 0.990 0.997 0.991 0.994 0.998 
Pseudo-second-order  7.447 21.146 39.966 44.277 46.867 
 0.045 0.028 0.012 0.016 0.020 
 0.987 0.994 0.987 0.990 0.994 
Figure 12

The adsorption kinetic models of phosphate ions on the NiFe-LDH/ACF-2 hybrid electrode.

Figure 12

The adsorption kinetic models of phosphate ions on the NiFe-LDH/ACF-2 hybrid electrode.

Close modal

Adsorption isotherm models

The fitting parameters and are shown in Table 2. The adsorption isotherms at the voltage of 2.4 V are shown in Figure 13. As the initial concentration of the solution increased, the adsorption capacity also increased because the high concentrations gave ions a large driving force (Chen et al. 2020). The Langmuir and Freundlich equations (Equations (13) and (14)) were applied to fit the experimental data of phosphorus adsorption on the NiFe-LDH/ACF-2 hybrid electrode.
formula
(13)
formula
(14)
where is the equilibrium concentration of KH2PO4 solutions (mg/L), is adsorption capacity (mg/g), (L/mg) and (mg/g) are Langmuir constants, ((mg/g)/(mg/L)n) and n are Freundlich constants, respectively.
Table 2

The parameters of the Langmuir and Freundlich isotherms

Langmuir
Freundlich
(mg/g)
38.294 0.016 0.992 3.531 6.115 0.969 
Langmuir
Freundlich
(mg/g)
38.294 0.016 0.992 3.531 6.115 0.969 
Figure 13

The adsorption isotherms of phosphate ions.

Figure 13

The adsorption isotherms of phosphate ions.

Close modal

According to the fitting curves and regression coefficients, the experimental data matched the Langmuir isotherm model ( = 0.992) better than the Freundlich isotherm ( = 0.969). Monolayer adsorption occurred at the NiFe-LDH/ACF-2 hybrid electrode surface. And the maximum adsorption capacity (38.29 mg/g) obtained by fitting the Langmuir isotherm could be used as the theoretical adsorption capacity under this condition.

Removal mechanism

As shown in Figure 14, the NiFe-LDH/ACF hybrid electrodes were used as the cathode and anode of the CDI device. And a DC voltage was applied to the cathode and anode to form an electrostatic field. Therefore, the negatively charged phosphate ions in the electrostatic field moved toward the positively charged anode under the action of electrostatic force and thus formed an electric double layer on the surface of the electrode plate. Phosphate ions were adsorbed by hybrid electrode and temporarily stored in the electric double layer until the electrode reached saturation, so as to achieve the purpose of removing phosphate ions in the solution. By reversing the power supply, the DC electric field disappeared, and the phosphate ions stored in the bilayer were returned to the solution. Desorption was achieved and regenerated NiFe-LDH/ACF hybrid electrodes were obtained.
Figure 14

Schematic diagram of the adsorption mechanism of phosphate ions with NiFe-LDH/ACF hybrid electrodes in CDI system.

Figure 14

Schematic diagram of the adsorption mechanism of phosphate ions with NiFe-LDH/ACF hybrid electrodes in CDI system.

Close modal

In this study, the NiFe-LDH-based hybrid electrodes were synthesized by a hydrothermal method. The effects of various factors on the removal of phosphate ions using NiFe-LDH/ACF hybrid electrodes via CDI were systematically investigated. It was observed that the adsorption capacity was positively correlated with the applied voltage. Meanwhile, with the increase of NiFe-LDH loading on ACFs, the adsorption capacity of phosphate ions was significant. And the NiFe-LDH/ACF-2 electrode had a high adsorption capacity for phosphate ions (33.48 mg/g). Furthermore, the experimental data fitted well with pseudo-first-order kinetics and Langmuir isotherms. The adsorption rate increased with the increase of applied voltage. And low pH was more conducive to the adsorption of phosphate ions. Following ten cycles of experiments, the NiFe-LDH/ACF-2 adsorption capacity for phosphate ions could still be maintained at about 30 mg/g, which expressed superior recyclability. This study provided a new option for LDH-based hybrid electrodes and the removal of phosphate ions in CDI.

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 41877132 and 51908242), Shandong Provincial Natural Science Foundation of China (Grant No. ZR2020KB009).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

Chen
G. Z.
2013
Understanding supercapacitors based on nano-hybrid materials with interfacial conjugation
.
Progress in Natural Science: Materials International
23
(
3
),
245
255
.
Chen
F.-F.
,
Li
H.-F.
,
Jia
X.-R.
,
Wang
Z.-Y.
,
Liang
X.
,
Qin
Y.-Y.
,
Chen
W.-Q.
&
Ao
T.-Q.
2020
Characteristic and model of phosphate adsorption by activated carbon electrodes in capacitive deionization
.
Separation and Purification Technology
236
, 116285.
He
D.
,
Wong
C. E.
,
Tang
W.
,
Kovalsky
P.
&
Waite
T. D.
2016
Faradaic reactions in water desalination by batch-mode capacitive deionization
.
Environmental Science & Technology Letters
3
(
5
),
222
226
.
Li
H.
,
Pan
L.
,
Lu
T.
,
Zhan
Y.
,
Nie
C.
&
Sun
Z.
2011
A comparative study on electrosorptive behavior of carbon nanotubes and graphene for capacitive deionization
.
Journal of Electroanalytical Chemistry
653
(
1–2
),
40
44
.
Li
J.
,
Li
B.
,
Huang
H.
,
Zhao
N.
,
Zhang
M.
&
Cao
L.
2020
Investigation into lanthanum-coated biochar obtained from urban dewatered sewage sludge for enhanced phosphate adsorption
.
Science of the Total Environment
714
,
136839
.
Luo
L.
,
Zhou
Y.
,
Yan
W.
,
Luo
L.
,
Deng
J.
,
Du
G.
,
Fan
M.
&
Zhao
W.
2021
Design and construction of hierarchical sea urchin-like NiCo-LDH@ACF composites for high-performance supercapacitors
.
Industrial Crops and Products
171
, 113900.
Rahman
M. T.
,
Kameda
T.
,
Miura
T.
,
Kumagai
S.
&
Yoshioka
T.
2018
Application of Mg–Al layered double hydroxide for treating acidic mine wastewater: a novel approach to sludge reduction
.
Chemistry and Ecology
35
(
2
),
128
142
.
Seo
S. J.
,
Jeon
H.
,
Lee
J. K.
,
Kim
G. Y.
,
Park
D.
,
Nojima
H.
,
Lee
J.
&
Moon
S. H.
2010
Investigation on removal of hardness ions by capacitive deionization (CDI) for water softening applications
.
Water Research
44
(
7
),
2267
2275
.
Tang
W.
,
Liang
J.
,
He
D.
,
Gong
J.
,
Tang
L.
,
Liu
Z.
,
Wang
D.
&
Zeng
G.
2019
Various cell architectures of capacitive deionization: recent advances and future trends
.
Water Research
150
,
225
251
.
Wang
M.
,
Lian
J.
,
Zhou
X.
&
Lian
Y.
2017a
Supercapacitors based on the composite of graphene and NiFe-layered double hydroxides
.
Science of Advanced Materials
9
(
2
),
220
226
.
Wang
Z.
,
Yan
T.
,
Chen
G.
,
Shi
L.
&
Zhang
D.
2017b
High salt removal capacity of metal–organic gel derived porous carbon for capacitive deionization
.
ACS Sustainable Chemistry & Engineering
5
(
12
),
11637
11644
.
Wang
J.
,
Li
M.
,
Zhai
Y.
,
Wang
F.
,
Zhang
X.
,
Lv
H.
,
Yu
T.
&
Zhang
W.
2021
Construction of three-dimensional nanocube-on-sheet arrays electrode derived from Prussian blue analogue with high electrochemical performance
.
Applied Surface Science
556
, 149789.
Wu
T.
,
Wang
G.
,
Dong
Q.
,
Qian
B.
,
Meng
Y.
&
Qiu
J.
2015
Asymmetric capacitive deionization utilizing nitric acid treated activated carbon fiber as the cathode
.
Electrochimica Acta
176
,
426
433
.
Xiao
N.
,
Tan
H.
,
Zhu
J.
,
Tan
L.
,
Rui
X.
,
Dong
X.
&
Yan
Q.
2013
High-performance supercapacitor electrodes based on graphene achieved by thermal treatment with the aid of nitric acid
.
ACS Appl Materials & Interfaces
5
(
19
),
9656
9662
.
Yan
T.
,
Liu
J.
,
Lei
H.
,
Shi
L.
,
An
Z.
,
Park
H. S.
&
Zhang
D.
2018
Capacitive deionization of saline water using sandwich-like nitrogen-doped graphene composites via a self-assembling strategy
.
Environmental Science: Nano
5
(
11
),
2722
2730
.
Zhang
H.
,
Bhat
V. V.
,
Gallego
N. C.
&
Contescu
C. I.
2012
Thermal treatment effects on charge storage performance of graphene-based materials for supercapacitors
.
ACS Applied Materials & Interfaces
4
(
6
),
3239
3246
.
Zuthi
M. F.
,
Guo
W. S.
,
Ngo
H. H.
,
Nghiem
L. D.
&
Hai
F. I.
2013
Enhanced biological phosphorus removal and its modeling for the activated sludge and membrane bioreactor processes
.
Bioresource Technology
139
,
363
374
.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY-NC-ND 4.0), which permits copying and redistribution for non-commercial purposes with no derivatives, provided the original work is properly cited (http://creativecommons.org/licenses/by-nc-nd/4.0/).