Preparation of a Novel Polystyrene-Poly(hydroxamic Acid) Copolymer and Its Adsorption Properties for Rare Earth Metal Ions

In this study, a novel polystyrene-poly(hydroxamic acid) copolymer was synthesized as an effective adsorbent for the treatment of rare earth elements. Through the use of elemental analysis as well as FTIR, SEM, XPS, and Brunauer-Emmett-Teller (BET) surface area measurement, the synthesized polymer was found to have a specific surface area of 111.4 m2·g−1. The adsorption performances of rare metal ions were investigated under different pH levels, contact times, initial concentrations of rare earth ions, and temperatures. The adsorption equilibrium for La3+, Ce3+, and Y3+ onto a polystyrene-poly(hydroxamic acid) copolymer is described by the Langmuir model, which confirms the applicability of monolayer coverage of rare earth ions onto a polystyrene-poly(hydroxamic acid) copolymer. The amount of adsorption capacities for La3+, Ce3+, and Y3+ reached 1.27, 1.53, and 1.83 mmol·g−1 within four hours, respectively. The adsorption process was controlled by liquid film diffusion, particle diffusion, and chemical reaction simultaneously. The thermodynamic parameters, including the change of Gibbs free energy (∆G), the change of enthalpy (∆H), and the change of entropy (∆S), were determined. The results indicate that the adsorption of resins for La3+, Ce3+ and Y3+ was spontaneous and endothermic. The polymer was also used as a recyclable adsorbent by the desorption experiment.


Introduction
Rare earth elements, known as the vitamins of modern industry, are important non-recoverable strategic resources. Due to their unique properties, the global demand for rare earth elements and their compounds for use in many high-tech applications has dramatically increased [1][2][3][4]. China is endowed with abundant rare earth mineral resources, but most of them are confined to low-end mixed rare earth products [5,6]. To solve the problem of efficient enrichment and separation of rare earth elements, many studies have been conducted and methods developed, including redox precipitation [7,8], crystallization [9,10], ion exchange adsorption separation [11][12][13][14], and solvent extraction methods [15,16]. Of these methods, ion exchange adsorption separation technology is generally considered to be the most efficient method for separating rare earth elements.
Compared to the other methods, ion exchange is known for its simple operation, high separation efficiency, and reusable adsorbents. Accordingly, the ion exchange method is widely used in the enrichment and separation of rare earth elements. With the development of research, specialists have focused more on adsorption resins with high adsorption capacity, selectivity, and adsorption kinetics.

Purification of Polystyrene-Poly(hydroxamic Acid) Copolymer
Polystyrene-poly(hydroxamic acid) copolymer was extracted using petroleum ether at 85 °C for 3 h then washed with distilled water (or ethyl alcohol) repeatedly until the obtained polymer was free from the residual pore-forming agent. The polymer was then used as an ion exchanger. The particle size of the synthesized polymer was between 0.6 and 0.8 mm.
The synthetic route of the resin is represented in Scheme 1.

Characterization
The specific surface area was measured with N2 adsorption-desorption isotherms using the ASAP2020 (Micromeritics, Norcross, GA, USA) and Brunauer-Emmett-Teller (BET) methods. C, H, N, and O were analyzed using an Elemental Analyzer (Vario EL III, Elementa, Hanau, Germany). Fourier transform infrared spectroscopy (FTIR) (G510FTIR, Nicolet, Madison, WI, USA) provides complete molecular structure information, which facilitates the determination of chemical functional groups. The morphology of the resin was determined by scanning electron microscopy (SEM) (Mira3, Tescan, Brno, Czech Republic). The binding energy of the resin was confirmed by X-ray photoelectron spectrometer (XPS) (Escalab250Xi, Thermo Fisher, Waltham, MA, USA).

Adsorption Experiments
Dry resin (0.2 g) and metal ion solution (25.0 mL, initial concentration was 0.01 mol·L −1 ) were mixed in a 100 mL conical flask. After being shaken at a constant temperature for an appropriate time, the solution was filtered. Then, the absorbance of metal ions in the filtrate was measured using an ultraviolet spectrophotometer. The adsorption capacity of the resin was calculated using the following equation: where Q represents the adsorption capacity, C0 is the initial concentration of rare earth metal ions, Ct is the rare earth metal concentration at t time, V is the volume of experiment solution and m is the mass of dry resin.

Desorption Experiments
For desorption studies, 2 g of resin loaded with La(III), Ce(III), or Y(III) was mixed with 50 mL (2 mol·L −1 ) nitric acid. After being shaken at 30 °C for 4 h, the mixture was filtered and the rare earth ions in the aqueous solution were determined by atomic absorption spectrophotometer. The desorption efficiency of the resin was calculated using the following equation:

Characterization
The specific surface area was measured with N 2 adsorption-desorption isotherms using the ASAP2020 (Micromeritics, Norcross, GA, USA) and Brunauer-Emmett-Teller (BET) methods. C, H, N, and O were analyzed using an Elemental Analyzer (Vario EL III, Elementa, Hanau, Germany). Fourier transform infrared spectroscopy (FTIR) (G510FTIR, Nicolet, Madison, WI, USA) provides complete molecular structure information, which facilitates the determination of chemical functional groups. The morphology of the resin was determined by scanning electron microscopy (SEM) (Mira3, Tescan, Brno, Czech Republic). The binding energy of the resin was confirmed by X-ray photoelectron spectrometer (XPS) (Escalab250Xi, Thermo Fisher, Waltham, MA, USA).

Adsorption Experiments
Dry resin (0.2 g) and metal ion solution (25.0 mL, initial concentration was 0.01 mol·L −1 ) were mixed in a 100 mL conical flask. After being shaken at a constant temperature for an appropriate time, the solution was filtered. Then, the absorbance of metal ions in the filtrate was measured using an ultraviolet spectrophotometer. The adsorption capacity of the resin was calculated using the following equation: where Q represents the adsorption capacity, C 0 is the initial concentration of rare earth metal ions, C t is the rare earth metal concentration at t time, V is the volume of experiment solution and m is the mass of dry resin.

Desorption Experiments
For desorption studies, 2 g of resin loaded with La(III), Ce(III), or Y(III) was mixed with 50 mL (2 mol·L −1 ) nitric acid. After being shaken at 30 • C for 4 h, the mixture was filtered and the rare earth ions in the aqueous solution were determined by atomic absorption spectrophotometer. The desorption efficiency of the resin was calculated using the following equation: Polymers 2020, 12, 1905 4 of 14 where R w (%) represents the desorption efficiency, C w (mol·L −1 ) is the concentrations of rare earth metal ions in eluent solutions, V w (L) is the volume of eluent, Q (mmol·g −1 ) is the adsorption capacity, and m (g) is the mass of dry resin.

FTIR Study
The FTIR spectra of the polystyrene-poly(hydroxamic acid) copolymer before and after treatment by La(III), Ce(III), and Y(III) are shown in Figure 1. As shown in Figure 1a, the N-H and O-H stretching bands of the resin appeared at around 3430 cm −1 and were assigned to the superposition peak of stretching vibrations [26]. The bands at around 3024 and 2921 cm −1 belonged to saturated C-H and unsaturated C-H stretching vibrations, respectively. The peaks at around 1719 and 1600 cm −1 were attributed to C=O [11] and benzene ring C=C stretching vibrations, respectively. In addition, the peaks were 1448 cm −1 for saturated C-H bending vibrations, 1269 cm −1 for single bond C-C stretching vibrations, and 697 cm −1 for benzene ring C-H bending vibrations. FTIR spectra showed that C=O and N-H stretching vibration peaks were apparent. Therefore, the desired product was synthesized by combining the results of elemental analysis. After adsorption of La(III), Ce(III), and Y(III), the peaks of N-H and O-H superposition shifted from 3430 cm −1 to a higher frequency nearby of 3445 cm −1 , demonstrating an enhanced electron density of the resin. The difference between the FTIR spectra before and after adsorption suggested that there is coordination between the rare earth ions and the polystyrene-poly(hydroxamic acid) copolymer, and that the resin might react with rare earth ions through its N and O atoms in aqueous solutions. where Rw (%) represents the desorption efficiency, Cw (mol·L −1 ) is the concentrations of rare earth metal ions in eluent solutions, Vw (L) is the volume of eluent, Q (mmol·g −1 ) is the adsorption capacity, and m (g) is the mass of dry resin.

FTIR Study
The FTIR spectra of the polystyrene-poly(hydroxamic acid) copolymer before and after treatment by La(III), Ce(III), and Y(III) are shown in Figure 1. As shown in Figure 1a, the N-H and O-H stretching bands of the resin appeared at around 3430 cm −1 and were assigned to the superposition peak of stretching vibrations [26]. The bands at around 3024 and 2921 cm −1 belonged to saturated C-H and unsaturated C-H stretching vibrations, respectively. The peaks at around 1719 and 1600 cm −1 were attributed to C=O [11] and benzene ring C=C stretching vibrations, respectively. In addition, the peaks were 1448 cm −1 for saturated C-H bending vibrations, 1269 cm −1 for single bond C-C stretching vibrations, and 697 cm −1 for benzene ring C-H bending vibrations. FTIR spectra showed that C=O and N-H stretching vibration peaks were apparent. Therefore, the desired product was synthesized by combining the results of elemental analysis. After adsorption of La(III), Ce(III), and Y(III), the peaks of N-H and O-H superposition shifted from 3430 cm −1 to a higher frequency nearby of 3445 cm −1 , demonstrating an enhanced electron density of the resin. The difference between the FTIR spectra before and after adsorption suggested that there is coordination between the rare earth ions and the polystyrene-poly(hydroxamic acid) copolymer, and that the resin might react with rare earth ions through its N and O atoms in aqueous solutions.

SEM Analysis
SEM analysis is an efficient method for observing morphology. The morphologies of the polystyrene-poly(hydroxamic acid) copolymer before and after adsorption of rare earth ions are presented in Figure 2. The surface of the resin was not flat and presented coarse pore structures, which may have contributed to the adsorption of metal ions. After being adsorbed, the morphology surfaces changed significantly. The structure and porosity of the resin changed, and a metallic luster appeared at the resin surface. We concluded that rare metal ions adsorbed on the surface of the resin. The BET surface area, average pore size, and pore volume for the resin were 111.4 m 2 ·g −1 , 5.93 nm, and 0.4869 cm 3 ·g −1 , respectively. The results demonstrated that the porous structure of the resin is

SEM Analysis
SEM analysis is an efficient method for observing morphology. The morphologies of the polystyrene-poly(hydroxamic acid) copolymer before and after adsorption of rare earth ions are presented in Figure 2. The surface of the resin was not flat and presented coarse pore structures, which may have contributed to the adsorption of metal ions. After being adsorbed, the morphology surfaces changed significantly. The structure and porosity of the resin changed, and a metallic luster appeared at the resin surface. We concluded that rare metal ions adsorbed on the surface of the resin. The BET surface area, average pore size, and pore volume for the resin were 111.4 m 2 ·g −1 , 5.93 nm, and 0.4869 cm 3 ·g −1 , respectively. The results demonstrated that the porous structure of the resin is mainly mesoporous. Elemental analysis showed that the nitrogen content of polystyrene-poly(hydroxamic acid) copolymer is 1.759%. mainly mesoporous. Elemental analysis showed that the nitrogen content of polystyrene-poly(hydroxamic acid) copolymer is 1.759%.

XPS Analysis
The surface composition and chemical state of the element were investigated by XPS. Figure 3 shows the full XPS spectrum of polystyrene-poly(hydroxamic acid) copolymer before and after treatment by La(III), Ce(III), and Y(III) over a binding energy range of 0~1200 eV. Compared with the spectrum before adsorption, new signals of La, Ce, and Y demonstrate the presence of La(III), Ce(III), and Y(III) cations on the resin, respectively. The binding energy of element nitrogen decreased and that of oxygen increased, which also indicated that there was coordination between the rare earth ions with polystyrene-poly(hydroxamic acid) copolymer and that rare metal ions were adsorbed on the surface of the resin.

XPS Analysis
The surface composition and chemical state of the element were investigated by XPS. Figure 3 shows the full XPS spectrum of polystyrene-poly(hydroxamic acid) copolymer before and after treatment by La(III), Ce(III), and Y(III) over a binding energy range of 0~1200 eV. Compared with the spectrum before adsorption, new signals of La, Ce, and Y demonstrate the presence of La(III), Ce(III), and Y(III) cations on the resin, respectively. The binding energy of element nitrogen decreased and that of oxygen increased, which also indicated that there was coordination between the rare earth ions with polystyrene-poly(hydroxamic acid) copolymer and that rare metal ions were adsorbed on the surface of the resin.

Adsorption Study
The effects of contact time on the adsorption of the resin for La 3+ , Ce 3+ , and Y 3+ were investigated

Effect of Contact Time
The effects of contact time on the adsorption of the resin for La 3+ , Ce 3+ , and Y 3+ were investigated and the results are shown in Figure 4. It is clear that the adsorption capacities of rare earth ions increased with increases in contact time, reached equilibrium after around 3.5 h, 4 h and 3.5 h respectively, and remained steady the rest of the time. Remarkably, the amount of adsorption was obviously faster at the initial stage, potentially because, initially, the adsorbent site was vacant and the solute concentration gradient was high. Thus, it appeared that the adsorption capacity of rare earth ions on the resin was a rapid process [27], with equilibrium reached in four hours. Therefore, the contact time of four hours was considered appropriate in the following experiments.

Effect of Contact Time
The effects of contact time on the adsorption of the resin for La 3+ , Ce 3+ , and Y 3+ were investigated and the results are shown in Figure 4. It is clear that the adsorption capacities of rare earth ions increased with increases in contact time, reached equilibrium after around 3.5 h, 4 h and 3.5 h respectively, and remained steady the rest of the time. Remarkably, the amount of adsorption was obviously faster at the initial stage, potentially because, initially, the adsorbent site was vacant and the solute concentration gradient was high. Thus, it appeared that the adsorption capacity of rare earth ions on the resin was a rapid process [27], with equilibrium reached in four hours. Therefore, the contact time of four hours was considered appropriate in the following experiments.

Effect of pH
The effect of pH solution is an important parameter that influences the adsorption capacity of the adsorbent for metal ions [28,29]. Figure 5 shows the effect of pH on the adsorption of resin for La 3+ , Ce 3+ , and Y 3+ . Study at higher pH range was avoided since lanthanides tend to hydrolyze above pH 6 [11]. Therefore, the adsorption experiment was conducted at pH values from 1.0 to 6.0. As shown in Figure 5, the adsorption capacity for Y(III) increased with increasing pH, then decreased and tended to stabilize. The adsorption capacity for Ce(III) decreased with increasing pH and the

Effect of pH
The effect of pH solution is an important parameter that influences the adsorption capacity of the adsorbent for metal ions [28,29]. Figure 5 shows the effect of pH on the adsorption of resin for La 3+ , Ce 3+ , and Y 3+ . Study at higher pH range was avoided since lanthanides tend to hydrolyze above pH 6 [11]. Therefore, the adsorption experiment was conducted at pH values from 1.0 to 6.0. As shown in Figure 5, the adsorption capacity for Y(III) increased with increasing pH, then decreased and tended to stabilize. The adsorption capacity for Ce(III) decreased with increasing pH and the adsorption trend of La(III) was the same as that of Ce(III). We concluded that the optimal pH levels for La 3+ , Ce 3+ , and Y 3+ are 2.0, 1.0 and 3.0, respectively.
Polymers 2020, 12, x 7 of 14 adsorption trend of La(III) was the same as that of Ce(III). We concluded that the optimal pH levels for La 3+ , Ce 3+ , and Y 3+ are 2.0, 1.0 and 3.0, respectively.

Effect of Initial Concentration of Metal Ions
The effect of initial concentration of metal ions is shown in Figure 6. The concentration of metal ions was evaluated in the range of 0.005~0.020 mol·L −1 . The adsorption capacity of metal ions was sharply enhanced by increasing the rare earth ion concentration from 0.005 to 0.015 mol·L −1 and

Effect of Initial Concentration of Metal Ions
The effect of initial concentration of metal ions is shown in Figure 6. The concentration of metal ions was evaluated in the range of 0.005~0.020 mol·L −1 . The adsorption capacity of metal ions was sharply enhanced by increasing the rare earth ion concentration from 0.005 to 0.015 mol·L −1 and remained in a state of slow growth thereafter. This phenomenon could be interpreted as showing that the increase in the initial concentration can lead to an increase in the driving force of the adsorption process and can improve contact opportunities between metal ions and effective active sites in the solution [30]. With the increase of initial metal ion concentration, most of the adsorption sites became occupied and the adsorption efficiency began to be constant [26]. When the adsorption process reached the saturated adsorption capacity, the increase in ion concentration had less effect on adsorption capacity.

Effect of Initial Concentration of Metal Ions
The effect of initial concentration of metal ions is shown in Figure 6. The concentration of metal ions was evaluated in the range of 0.005~0.020 mol·L −1 . The adsorption capacity of metal ions was sharply enhanced by increasing the rare earth ion concentration from 0.005 to 0.015 mol·L −1 and remained in a state of slow growth thereafter. This phenomenon could be interpreted as showing that the increase in the initial concentration can lead to an increase in the driving force of the adsorption process and can improve contact opportunities between metal ions and effective active sites in the solution [30]. With the increase of initial metal ion concentration, most of the adsorption sites became occupied and the adsorption efficiency began to be constant [26]. When the adsorption process reached the saturated adsorption capacity, the increase in ion concentration had less effect on adsorption capacity.

Effect of Temperature
The effect of temperature on the adsorption of resin for La 3+ , Ce 3+ , and Y 3+ was investigated and the results are shown in Figure 7. It can be seen that the adsorption capacity of metal ions increased with the temperature. However, the increase happened very slowly. This result showed that temperature has a lower effect on the adsorption capacity of metal ions.

Effect of Temperature
The effect of temperature on the adsorption of resin for La 3+ , Ce 3+ , and Y 3+ was investigated and the results are shown in Figure 7. It can be seen that the adsorption capacity of metal ions increased with the temperature. However, the increase happened very slowly. This result showed that temperature has a lower effect on the adsorption capacity of metal ions.

Adsorption Kinetics
The kinetic study was carried out to describe the reaction rate of the adsorption process and explore the adsorption mechanism. Adsorption kinetics data were fitted using liquid film diffusion, particle diffusion, and chemical reaction, which are expressed as Equations (3), (4), and (5) [31,32], respectively:

Adsorption Kinetics
The kinetic study was carried out to describe the reaction rate of the adsorption process and explore the adsorption mechanism. Adsorption kinetics data were fitted using liquid film diffusion, particle diffusion, and chemical reaction, which are expressed as Equations (3), (4), and (5) [31,32], respectively: where F = Q t /Q e , where Q e , and Q t are adsorption capacities (mmol·g −1 ) at equilibrium and at time t (min); and k 1 , k 2 , k 3 denote the rate constants of the liquid film diffusion, particle diffusion, and chemical reaction (min −1 ), respectively. Figure 8. The constants calculated from the slope and intercept of those plots are given in Table 1. The values of the correlation coefficient (R 2 ) of the three kinetics models were very high (close to 0.99). Based on the correlation coefficients, the three models match better with experimental data, suggesting that the adsorption process might be simultaneously controlled by liquid membrane diffusion, particle diffusion, and chemical reaction [33].

Adsorption Isotherm
The adsorption isotherm reflects the variation of adsorption capacity compared to initial ion concentration at the same temperature. In this study, the experimental results obtained were tested by the Langmuir and Freundlich isotherms [34][35][36]. Their linear equations are expressed as follows:

Adsorption Isotherm
The adsorption isotherm reflects the variation of adsorption capacity compared to initial ion concentration at the same temperature. In this study, the experimental results obtained were tested by the Langmuir and Freundlich isotherms [34][35][36]. Their linear equations are expressed as follows: ln Q e = 1 n ln C e + ln K F where Q e (mmol·g −1 ) is the adsorption capacity at equilibrium, Q m (mmol·g −1 ) is the maximum Langmuir adsorption capacity, C e (mol·L −1 ) is the equilibrium concentration, K L (L·mol −1 ) is the Langmuir constant, and n and K F are Freundlich adsorption coefficients. The fitted results are shown in Figure 9 and the parameters are listed in Table 2. Figure 9a shows that the equilibrium data were well fitted by the Langmuir isotherm. Moreover, the correlation coefficient (R 2 ) of the Langmuir isotherm was much higher than that of the Freundlich isotherm, suggesting that the adsorption process of the resin to rare metal ions might be monolayer adsorption. Similar observations were reported for the adsorption of Ce(III) onto D151 resin [37].

Adsorption Thermodynamics
Thermodynamic parameters are necessary to determine the spontaneity and heat exchange of adsorption. The thermodynamic parameters included change in Gibbs free energy (∆G), enthalpy (∆H), and entropy (∆S), which were determined using Equations (8)-(10) [38][39][40]. The fitted plots are shown in Figure 10 and the results are presented in Table 3.
where D is the distribution coefficient (L·g −1 ), V is the volume of solution (L), m is the mass of the resin (g), R is the gas constant (8.314 J·mol −1 ·K −1 ), and T stands for temperature (K). C 0 (mol·L −1 ) and C e (mol·L −1 ) are the initial concentration and equilibrium concentration of La(III), Ce(III), and Y(III) ions, respectively. As shown in Table 3, the enthalpy change (∆H) of the adsorption process was greater than zero, suggesting that the process was an endothermic reaction and the increase of temperature was beneficial to the adsorption, which is consistent with the previous conclusion. The negative ∆G values indicated that the adsorption was spontaneous. The positive ∆S indicated there was a high affinity between rare earth ions and the resin [41]. The adsorption was an endothermic chemical adsorption.

Desorption Experiments
The regeneration of the adsorbent is an important factor required for practical application. In this study, desorption experiments were carried out and the results are listed in Table 4. After the resin desorption happened three times, desorption efficiency of the resin could reach 98.7%. We found that 1.6 mol·L −1 nitric acid can be effectively desorbed from rare earth ion complexes, suggesting the obtained resin has good regeneration properties.

Desorption Experiments
The regeneration of the adsorbent is an important factor required for practical application. In this study, desorption experiments were carried out and the results are listed in Table 4. After the resin desorption happened three times, desorption efficiency of the resin could reach 98.7%. We found that 1.6 mol ·L −1 nitric acid can be effectively desorbed from rare earth ion complexes, suggesting the obtained resin has good regeneration properties.

Comparison between Our Results and the References
In this study, the adsorption amount of polystyrene-poly(hydroxamic acid) resin was 1.27 mmol·g −1 for La(III), 1.53 mmol·g −1 for Ce(III), and 1.83 mmol·g −1 for Y(III). To evaluate the adsorption capacity of the synthesized polymer, some other adsorbent materials reported in previous studies for Ce (III) adsorption are listed in Table 5. Polystyrene-poly(hydroxamic acid) resin showed a significantly larger adsorption capacity for Ce(III) than most of the reported adsorbents. The synthesized resin exhibits considerable potential as an absorbent for rare earth ions.

Conclusions
In this paper, we reported the synthesis of polystyrene-poly(hydroxamic acid) copolymer by suspension polymerization. Based on our systematic characterizations and batch adsorption experimental results, the obtained polymer shows a large adsorption capacity for rare earth ions and the maximum adsorption capacities are 1.27 mmol·g −1 for La 3+ , 1.53 mmol·g −1 for Ce 3+ , and 1.83 mmol·g −1 for Y 3+ . The adsorption kinetics study indicated that the adsorption process of the resin for rare earth ions involves three stages: liquid membrane diffusion, particle diffusion, and chemical reaction. The Langmuir isotherm was more appropriate for fitting the equilibrium data than the Freundlich isotherm. The thermodynamics studies indicated that the adsorption process is spontaneous and endothermic. Spectral results implied that there is coordination between the rare earth ions with the polystyrene-poly(hydroxamic acid) copolymer and that chemical reactions might occur between rare earth ions and the N and O atoms of the polymer. In addition, the desorption tests proved that the obtained polymer can be used as a recyclable adsorbent.