Synthesis of layered double hydroxide-based hybrid electrode for efficient removal of phosphate ions in capacitive deionization

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.

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.

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.

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.

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.

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).

The electrolysis of water in solution was as follows:

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.

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.

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.

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.