Preparation of emulsion hydrogels encapsulating extractant by the Pickering emulsion template method to recover lanthanum ions in aqueous solutions

Lanthanides possess distinctive physical and chemical properties that have extensive applications in fields such as new energy, new materials, energy conservation, environmental protection, aerospace, and electronic information (Zhou et al. 2017). La(III), as a representative of lanthanide elements, is now regarded as a novel contaminant. La(III) is a fission product that may cause harm if released into the environment. La(III) recovery and extraction have consequently attracted a lot of attention.

Now, lanthanides were extracted from an aqueous solution using a variety of techniques, including liquid–liquid extraction (LLE), ion exchange and adsorption, chemical precipitation, membrane extraction separation, and others. Solvent extraction, also known as LLE, was one of the most common methods used in industry to separate and refine lanthanides. This method was mainly through the use of di-(2-ethylhexyl) phosphate (P204), 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (P507), and bis-(2,4,4-trimethylpentyl)-phosphinic acid (Cyanex272) extractant to achieve lanthanide recovery and separation (Panda et al. 2016; Zhang et al. 2021; Xu et al. 2022). Even though LLE was frequently employed in industry, the formation of the third phase and the limited contact area between the two phases make it less efficient in extracting rare earth elements. As a result, a lot of research has been done on solid–liquid extraction (SLE) recently (Kumar et al. 2010; Ngomsik et al. 2012; Radhika et al. 2012; Huang et al. 2017). SLE offers several advantages, including reducing surfactants and organic extractants, improving contact between extractants and lanthanides in the aqueous phase, and preventing the third equivalent formation and the ability to recover solids for multiple cycles. Therefore, SLE is highly efficient for extracting La(III). The solvent-impregnated resin is the SLE technique that is most frequently employed, and it works by impregnating the best extractant onto selected materials like silica and resin to speed up extraction (Xiaoqi et al. 2009; Neagu et al. 2010; Lili & Ji 2011). For instance, Mohammedi et al. found that the extraction order for Cyanex272 impregnated in porous silica was light lanthanides (La), medium lanthanides (Eu), and then heavy lanthanides (Lu) (Mohammedi et al. 2019). El-Sofany prepared impregnated resin that was utilized to extract lanthanum (III) and gadolinium (III) from nitrate media by loading Aliquat-336 onto Amberlite XAD-4 crosslinked polystyrene resin. According to the study, impregnating resin with lanthanum (III) and gadolinium (III) was an efficient way to accomplish so. Additionally, lanthanum (III) and gadolinium (III) had respective adsorption capabilities of 4.73 and 4.44 mg/g (El-Sofany 2008). However, due to the presence of hydrophobic groups, the adsorption efficiency of the impregnated resin prepared by physical impregnation of the extractant into the resin is reduced.

Hydrogel is a three-dimensional network structure polymer material containing a large amount of water (Ahmed 2015), which can provide channels for the diffusion of metal ions (Kobayashi et al. 2010; Puguan et al. 2015). Therefore, the emulsifying hydrogels that extractants were enveloped in hydrogels have received a lot of interest (Zhang et al. 2011; Tokuyama et al. 2018). Since conventional emulsions stabilized by surfactants are thermodynamically unstable, it is necessary to find a way to improve the stability of emulsions. Ramsden and Pickering demonstrated over a century ago that colloidal particles can stabilize emulsions, and such emulsions are called Pickering emulsions (Ramsden 1904). Pickering emulsions have the advantages of cheap cost, minimal emulsifier usage, environmental friendliness, and good emulsion stability over typical surfactant-stabilized emulsions. Therefore, Pickering emulsions can better wrap the extractant. Previous research has shown that Pickering emulsion hydrogels (PEHGs) adsorbed metal cations effectively in aqueous solutions, such as Cu(II), Zn(II), and In(III) (Yuan et al. 2021; Dong et al. 2023; Gao et al. 2023). This technique is generally applicable to other metal ions.

Most colloidal nanoparticles can stabilize Pickering emulsions, and silica nanoparticles are frequently utilized in the stabilization of Pickering emulsions because they are simple to manufacture and manipulate. However, Binks & Lumsdon found that emulsions prepared stabilized by either very hydrophilic or very hydrophobic particles are large and unstable to coalescence (Binks & Lumsdon 2000), because particles are too lipophilic or too hydrophilic and tend to disperse in the oil or aqueous phases, respectively. Binks et al. then studied in detail the behavior of a stable dodecane–water emulsion consisting of a mixture of silica nanoparticles and cationic surfactants. Studies have shown that cationic surfactants and nanoparticles can be able to maintain the emulsion, while hydrophilic nanoparticles cannot (Binks et al. 2007). Therefore, PEHGs prepared by cationic surfactant-modified silica-stabilized emulsions have high encapsulation efficiency for extractants and have broad application prospects in the recovery of lanthanides in water. However, there are no related reports on this method's recovery of La(III).

In this study, nano-silica (SiO₂) treated with hexadecyl trimethyl ammonium bromide (CTAB) was employed to improve the stability of SiO₂ at the oil–water interface. The oil-in-water (O/W) Pickering emulsions were then stabilized using the modified silica to produce the PEHGs that contain the P204. The encapsulation ability of PEHGs was tested, and they were then used to extract lanthanum ions from the solution. PEHGs were assessed using thermogravimetric (TG) analysis, scanning electron microscopy (SEM), and Fourier transform infrared (FTIR). The impact of temperature, pH level, extractant concentration, and oscillation length on La(III) extraction was then studied. PEHG's ability to adsorb La(III) was studied in terms of thermodynamics and kinetics. The objective of this study was to develop a functional material that efficiently extracts La(III) from industrial wastewater while being economical and environmentally benign.

All used reagents in the present study are analytical grade and used without any additional purification. Sodium alginate (SA) and calcium chloride (CaCl₂) were purchased from Aladdin. Sulfonated kerosene and di-(2-ethylhexyl) phosphate (D2EHPA, P204 C₁₆H₃₅O₄P) were purchased from Zhengzhou Hecheng New Materials. Hydrophilic nano-SiO₂ was purchased from Wacker Company. Arsenazo III (C₂₂H₁₈O₁₄N₄S₂As₂), glacial acetic acid (C₂H₄O₂), sodium acetate (C₂H₃O₂Na), nitric acid (HNO₃), sodium hydroxide (NaOH), and hexadecyl trimethyl ammonium bromide (CTAB, C₁₉H₄₂BrN) were purchased from Tianjin Kemio Chemical Reagent Co., Ltd. Lanthanum nitrate (La(NO₃)₃) was purchased from Sinopharm Chemical Reagent Co., Ltd.

Ultrasonic cleaner (KQ5200E, China Co. Ltd), FTIR spectrometer (Nicolet IS5, Thermo Fisher Scientific, USA), optical microscope (OPTEC-BK5000, China Co. Ltd), TG analysis (TG, Q600SDT, TA, USA), scanning electron microscope (SEM, JSM-6460LV, JEOL, Japan), and Toup View (Toup View Technology Co., Ltd., China Ltd) were used in this study.

The modification of SiO₂ was carried out according to previous reports with slight modifications (Khan et al. 2019). SiO₂ (3 wt%) was added to the CTAB aqueous solution (10⁻² mol/L) and stirred for 4 h at 25 °C to ensure the adsorption of CTAB on the surface of SiO₂. The precipitate was then collected by centrifugation at a speed of 9,000 rpm and washed three times with deionized water to remove excess CTAB. Finally, the precipitate was dried at 80 °C for 10 h.

SA was completely dissolved in deionized water at 80 °C using mechanical stirring to prepare a 0.8% aqueous solution of SA. Then, modified SiO₂ of different quality was added to the 0.8 wt% SA aqueous solution, and the mixture was ultrasonic oscillated at 2,450 Hz for 5 min to obtain a continuous phase. Kerosene and P204 were mixed in 3:7, 4:6, 5:5, 6:4, and 7:3 ratios as the dispersed phase of Pickering emulsions. The dispersed phase was mixed with the continuous phase in a ratio of 4:6 and then shaken for 30 s to form O/W Pickering emulsions. The type of emulsion was determined by performing a ‘dropping’ experiment (Binks & Lumsdon 2000). The prepared emulsions were dropped into distilled water, and if they could be dispersed in water, it was determined to be O/W type. If the droplet remained intact, it was determined to be W/O type.

To compare the adsorption effect, SA hydrogels (HGs) without extraction were prepared as the control group.

Subsequently, the desorption experiment of La(III) was carried out. Around 0.3 g of PEHGs adsorbed with La(III) was soaked in 30 ml of 0.1 mol/L HNO₃ solution for 10 h. Afterwards, the PEHGs were washed with deionized water until pH > 5. The washed samples were used for the next step of adsorption.

The size and shape of the emulsified Pickering emulsion droplets were observed with an optical microscope, and the encapsulation of the extractant was analyzed by FTIR spectroscopy and TG. To investigate the microstructure of PEHGs, a SEM was employed to observe and record the results after vacuum freeze drying. The FTIR spectra of PEHGs before and after adsorption were recorded in a transmission mode.

The effects of particle concentrations of 0.4–1.4 wt% on the stability of Pickering emulsions were studied by using CTAB-SiO₂ to stabilize the emulsions. All emulsions were O/W emulsions, and the emulsions’ particle size observed by the optical microscope is shown in Figure 2(c). The figure shows that as particle concentration increases, the particle size of the emulsions reduces progressively and eventually tends to remain stable. The particle size distribution is shown in Figure 2(b). The concentration of CTAB-SiO₂ increased from 0.4 to 1.4 wt%, and the average particle size of the emulsions decreased from 52 to 24 μm. Specifically, the increase in particle concentration allows more nanoparticles to reach the oil–water interface, which in turn expands the area of the interface and reduces the size of the emulsions. The average size of the emulsions is also shown to stabilize at 1.0 wt% of particle concentration, so CTAB-SiO₂ with a particle concentration of 1.0 wt% is selected to stabilize the O/W emulsions.

To verify that the extractant was encapsulated in SA hydrogels, and curcumin was dissolved in kerosene to prepare the oil phase. The prepared PEHGs are shown in Figure 5(b), and it can be seen that the prepared PEHGs are yellow and have no obvious leakage.

Moreover, Rhodamine-B was dissolved in the SA aqueous solution to prepare hydrogel, and the results were compared with PEHGs. Figure 5(c) shows that the hydrogels without the extractant were submerged at the bottom, while the PEHGs with the extractant were floated on the surface. Because the density of the oil phase is lower than that of water, the PEHGs are less dense and therefore float on the surface. This phenomenon indicated the successful encapsulation of the extractant.

Figure 3(b) shows the extraction process of lanthanum ions by P204 extractant in PEHGs. This process was achieved by placing an Erlenmeyer flask containing lanthanum ions in an oscillator. SA is a hydrophilic hydrogel that provides a channel for the diffusion of La(III) from the aqueous phase to the oil–water interface, then La(III) complexes with P204 dimer to generate a 6-coordinated La(HA₂)₃ complex (Zheng et al. 2022). The resultant lanthanum ion complex diffused into the interior of the oil phase, and the unreacted extractant diffused to the oil–water interface to continue the reaction. The FTIR spectrum also proved its complexation mechanism, and Figure 4(b) shows the infrared spectrum of PEHGs before and after adsorption. It can be seen that the P–OH characteristic peak at 810 cm⁻¹ after adsorption is significantly weakened, indicating that H⁺ in P–OH of P204 is replaced by La(III), and the P = O characteristic absorption peak after adsorption moved from 1,235 to 1,183 cm⁻¹. This displacement change indicated that P204 formed a P = O → La coordination bond with the lanthanum ion. This phenomenon was confirmed by the adsorption mechanism (Figure 3(b)).

• (1)

  Effect of extraction time on adsorption content

To study the effect of adsorption time on lanthanum adsorption, 0.5 g of PEHGs were added into the lanthanum solution at an initial concentration of 1 mg/mL (50 mL). As shown in Figure 7(a), the adsorption content increased as the adsorption time increased and reached equilibrium after 14 h, and the maximum adsorption amount was 38.05 mg/g. Therefore, in the subsequent experiments, the adsorption time was set as 14 h. As a control, the HGs without encapsulating P204 reached adsorption equilibrium within 3 h, and the adsorption amount was only 5.42 mg/g. This indicated that the adsorption of PEHGs mainly depends on the extraction of P204. The equilibrium time was longer when compared to the adsorption capacity of La(III) by PEHGs and the impregnated resin (İnan et al. 2018; Yarahmadi et al. 2023) adsorption capacity of 7.324 and 11.1 mg/g, but the extraction capacity was greatly enhanced.

• (2)

  Effect of pH on adsorption content

HA is P204, (HA)₂ is a dimer of P204, and La (HA₂)₃ is a complex of La(III) and P204.

When the pH < 4, the concentration of H⁺ decreases with the increase of pH, leading to the positive direction of the reversible reaction, which is beneficial for the complexation of La(III) with the extractant. When pH > 4, La(III) in the solution may form a complex with OH⁻ in the solution, thus hindering the complexation reaction between the extractant and La(III) (El-Sofany 2008).

• (3)

  Effect of P204 concentration on adsorption content

Kerosene was selected as a diluent, and extractants with volume fractions of 30, 40, 50, 60, and 70% were configured as organic phases. The effect of extractant concentration on the extraction of La(III) was studied under the condition of pH 4 and an extraction time of 14 h.

The results are shown in Figure 7(c), and the amount of La(III) extracted increases dramatically as the concentration of the extractant increases. However, it seems that raising the content of P204 above 40% did not lead to a significant improvement in adsorption capacity for La(III). This could be owing to P204's high viscosity, which reduces the diffusion rate of La(III) in the organic phase, resulting in a decrease in La(III) adsorption. Therefore, more focus was on the concentration of P204 at 40% to maximize adsorption capacity.

• (4)

  Effect of initial solution concentration on adsorption capacity

PEHGs adsorbed La(III) solutions with different initial concentrations, as shown in Figure 7(d). As the initial concentration of La(III) increases, the adsorption capacity of the PEHGs also increases. Intuitively, larger concentrations result in faster transfer rates and more frequent collisions, which are necessary for chemical reactions to occur. However, it is interesting to note that as the starting concentration increased, the extraction rate declined. This phenomenon is consistent with previous reports (El-Sofany 2008).

The R_L indicates the nature of the isotherm accordingly: 0 < R_L < 1 shows favorable adsorption, R_L = 1 indicates linear adsorption, and R_L = 0 indicates irreversible adsorption. At the temperature of 298–318 K, when c₀ changes from 200 to 1,000 mg/L, the range of R_L value is 0.007–0.229. This result indicated that the removal of La(III) using PEHGs is a favorable adsorption process.

As shown in Figure 8(c), it reflects the linear plot of ln K_L and 1/T. Thus, ΔH and ΔS could be calculated from the slope and intercept of the linear plot of ln K_L and 1/T, and the relevant parameters are listed in Table 2. The positive value ΔH indicated that the reaction is endothermic. With increasing temperature, the reaction progresses with greater efficiency, thereby facilitating the extraction process. The positive value of ΔS indicated that the randomness of the solid/solution interface increases during the adsorption of La(III) by PEHGs. A negative value of ΔG indicated in all cases that the reaction of P204 to extract La(III) is spontaneous.

Figure 8(d)–8(f) presents the fitting lines of three different models: the pseudo-first-order kinetic model, the pseudo-second-order kinetic model, and the internal diffusion model. The kinetic parameters are summarized in Table 3.

Upon examining the fitting line and relevant data, it was evident that the pseudo-second-order kinetic model aligns well with the experimental data, and the calculated theoretical saturation quantities q_e,cal (50.25 mg/g) were in good agreement with the experimental values. Hence, it could be concluded that the pseudo-second-order kinetic model is more suitable for explaining the adsorption process of La(III) on PEHGs, indicating a chemisorption mechanism.

The correlation coefficient R² = 0.9886 of the straight line fitted by the internal diffusion model fits the experimental data, demonstrating that internal diffusion was involved in adsorption. This phenomenon and the increase in the concentration of P204 mentioned above would increase the viscosity of the oil phase, resulting in the diffusion rate of La(III) reducing the adsorption efficiency of La(III).

In summary, the modified silica could stabilize the emulsions to prepare PEHGs that contain the P204, and no obvious leakage of the extraction agent was seen. The adsorption capacity of La(III) by PEHGs was up to 48 mg/g. The adsorption isotherm of PEHGs on La(III) is in good agreement with the Langmuir model, and the adsorption process was a spontaneous endothermic process. The pseudo-second-order kinetic model could explain the adsorption process of La(III) on PEHGs. Through the internal diffusion model, it has been found that the adsorption process was related to internal diffusion. The results of adsorption/desorption experiments showed that PEHGs could be recycled, which indicated that this work provided an efficient and clean method for recovering rare earth metals in an aqueous solution.

This work was supported by the Xi'an University of Science and Technology, Grant/Award Number: 19023.

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

The authors declare there is no conflict.