Modeling and equilibrium studies on the recovery of praseodymium (III), dysprosium (III) and yttrium (III) using acidic cation exchange resin

In this research, the possibility of using hydrogenated Dowex 50WX8 resin for the recovery and separation of Pr(III), Dy(III) and Y(III) from aqueous nitrate solutions were carried out. Dowex 50WX8 adsorbent was characterized before and after sorption of metal ions using Fourier-transform infrared spectroscopy (FT-IR), Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray Analysis (EDX) techniques. Sorption parameters were studied which included contact time, initial metal ion concentration, nitric acid concentration and adsorbent dose. The equilibrium time has been set at about 15.0 min. The experimental results showed that the sorption efficiency of metal ions under the investigated conditions decreased with increasing nitric acid concentration from 0.50 to 3.0 M. The maximum sorption capacity was found to be 30.0, 50.0 and 60.0 mg/g for Pr(III), DY(III) and Y(III), respectively. The desorption of Pr(III) from the loaded resin was achieved with 1.0 M citric acid at pH = 3 and found to be 58.0%. On the other hand, the maximum desorption of Dy(III) and Y(III) were achieved with 1.0 M nitric acid and 1.0 M ammonium carbonate, respectively. The sorption isotherm results indicated that Pr(III) and Y(II) fitted with nonlinear Langmuir isotherm model with regression factors 0.995 and 0.978, respectively; while, Dy(III) fitted with nonlinear Toth isotherm model with R2 = 0.966. A Flow sheet which summarizes the sorption and desorption processes of Pr(III), DY(III) and Y(III) using Dowex 50WX8 from nitric acid solution under the optimum conditions is also given.


Introduction
Yttrium (Y), Dysprosium (Dy) and Praseodymium (Pr) known as segment of the rare earth elements (REEs) are spirited components in fluorescent lamps, glass polishing and ceramics, computer monitors, lighting, radar, televisions, and X-ray intensifying films. Yttrium is extensively used in the manufacturing of several high-techdevices such as microwave communication for satellite industries, color televisions, computer monitors and temperature sensors [1,2]. Praseodymium is used with neodymium in combination for goggles to shield glassmakers against sodium glare, permanent magnets and cryogenic refrigerant [3]. Dysprosium alloy with neodymium is used for permanent magnets, catalysts, speakers, compact discs and hard discs and medium source rare-earth lamps within the film industry. [4] Ion exchange separation of rare earth elements was used by Spedding and Powell to separate REEs from fission products obtained from nuclear reactors [5][6][7]. Sorption processes for the separation of rare earths have been reviewed in several articles [8][9][10][11]. Strongly acidic cation exchangers were the first artificial functional polymers used for the separation of REE ions, and they are resumed predominating in the fields of chemistry and chemical technology under consideration [11]. Styrene and divinylbenzene copolymers bearing SO 3 H group are utilized. Now, modifications of these exchangers are manufactured under the trade names Amberlite, Dowex, Lewatit, Purolit, which differ by the degree of cross linking sorption capacity, grain size, pore diameter, and other parameter [12]. Dowex 50 W-X8 is one of the ion-exchange resins that have been used for separation of REEs from other ions as well as separation of individual REEs from a mixture of REEs. Al-Thyabat and Zhang, studied the recovery of REEs resulting from phosphoric acid with Dowex 50WX4 and Dowex 50WX8 resins. Their results indicated that the REE-extraction efficiency of Dowex 50WX8 was almost twice that of Dowex 50WX4 resin. This can be explained by the higher exchange capacity, producing more sulfonic groups, of Dowex 50WX8 even though its lower surface area and larger bead size [13]. Felipe et al., studied the recovery of rare earth elements from acid mine drainage by ion exchange [14] and reported that the highest loading capacities were 0.212 mmol g −1 for La and 0.169 mmol g −1 for Ce (Dowex 50WX8) and 0.210 mmol g −1 for La and 0.173 mmol g −1 for Ce (Lewatit MDS 200 H). Recovery of rare earth elements from uranium concentrate by using cation exchange resin (Dowex 50WX8) was studied and the authors reported that the maximum REE sorption capacity was found to 82.74 mg/g which represents about 93.23% of the original capacity of the studied resin [15]. Sorption of rare earth elements from nitric acid solution with macroporous silica-based bis(2-ethylhexyl)phosphoric acid impregnated polymeric adsorbent has been studied by Shu et al. [16] Their results indicated that the adsorption capacity of Gd (III) was found to be 0.315 mmol g −1 by bis(2-ethylhexyl)phosphoric acid/SiO 2 -P in 0.1 M HNO 3 . Adsorption and separation of terbium(III) and gadolinium(III) from aqueous nitrate medium has been investigated using TVEX-PHOR resin by Madbouly et al. [17]. Their work showed that the maximum sorption capacity of this material was 15.49 mg/g and 24.93 mg/g for Gd(III) and Tb(III) from 0.1 M NaNO 3 solution, respectively at pH = 5.2 and V/m = 0.1. El-Dessouky et al. studied the sorption of praseodymium (III), holmium (III) and cobalt (II) from nitrate medium using TVEX-PHOR resin [18] and reported that 85% sorption was achieved for holmium (III), 75% for praseodymium (III), and 12% for cobalt (II) which enables the possibility of separation of cobalt (II) from the investigated lanthanide elements. The extraction and separation of some rare earths from nitric acid solutions by Cyanex 272 impregnated XAD-7 resin has been examined by İnan et al. [19] The obtained results indicated that REEs have a tendency to behave as two different groups that can be separated into two fractions as La, Pr, Nd and Sm, Eu, Gd. Dowex 50wx8 was used previously for the reversible ion exchange of cerium (III) sulfate and Cerium (III) nitrate where the experimental results indicate that the continuous liquid flow reactor studies show a capacity of 0.72 mmol/g sorbent for the Ce nitrate and 0.96 mmol/g sorbent for the Ce sulfate [20].
The main objective of the present work is directed to study the sorption and separation of Praseodymium, Dysprosium and Yttrium from nitric acid solution using strongly acidic cationic exchange resin (Dowex 50WX8) using batch technique. The effects different parameters on the sorption and separation processes will be investigated such as contact time, nitric acid concentration, as well as v/m ratio and temperature. Desorption investigations will be also carried out and evaluated. Separation feasibility between the investigated REEs are also discussed based on the difference between their sorption and desorption behavior.

Materials and chemicals
The chemicals used in this work were of analytical reagent grade (AR) and most of them were used without further purification. Stock solutions of Pr(III), DY(III) and Y(III) (1000 mg/L) were prepared by dissolving a known amount of the metal oxide in minimum concentrated nitric acid and evaporated to near dryness and then made up to the mark in a measuring flask with double distilled water. The desired required concentrations of test solutions were prepared by favorable dilution with a known concentrated nitric acid of the stock solutions. Dowex 50WX8 which is a strong acid cation resin containing 8% divinylbenzene (DVB) [20], Fig. 1 was purchased from sigma Aldrich. The chemical and physical specifications of Dowex 50X8 are given in table given in Table 1.

Sorption experiments
The sorption experiments were carried out under the following conditions, v/m = 0.05 L/g, Pr(III), DY(III) and Y(III) concentrations = 100.0 mg/L in 0.50 M nitric acid solution. In each adsorption experiment, 5.0 ml of the investigated metal ions solution was added to 0.1 g of Dowex 50WX8 resin (100-200 mesh) in stoppered glass bottles which were then shaken at (25 ± 1 °C) in a water thermostatic shaker. The concentrations of Pr(III), Dy(III) and Y(III) ions were measured using UV-visible spectrophotometer (a Shimadzu UV-160, Japan) with Arsenazo (III) method [21,22], and the adsorption capacity (q e ) at equilibrium was given by equation: The sorption efficiency (S %) at equilibrium was calculated from the equation: where C i and C e are the initial and equilibrium metal ions concentrations (mg/L) of metal ions, respectively; v is the volume of the used aqueous solution in liter (L) and w is the weight of the adsorbent (g).

Characterization techniques
Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDX) was used to examine the morphology and determine the elemental composition of the metal ions bonding to Dowex 50WX8 resin under the used experimental conditions. The functional groups included in the used adsorbent were investigated using Fourier transform infrared (FT-IR) spectroscopy (Bruker) in the scanning range of 4000-400 cm −1 and pH was measured using Hannah pH meter.

Desorption investigations
Various reagents such as mineral acids, sodium carbonate, ammonium carbonate and citric acid (at different pH) were used for the desorption investigations of the metal ions under study. In this context, 0.1 g of Dowex 50WX8 loaded with about 100.0 mg/L of each individual Dy(III) or Pr(III) or Y(III) was shaken with 5.0 mL of the stripping solution for 20.0 min under the same sorption experimental conditions.    [20]. The morphology investigations and particle surface variations of the sorbent were given by EDX. Map and SEM analyses were performed both before and after the adsorption of metal ions, Fig. 3a-d. The obtained results show the presence of a variety of pores with a wide range of pore size on the surface of the Dowex 50WX8 resin; the pore space could be attributed to the adsorption process.

Characterization of Dowex 50WX8
The results of the EDX and Map analyses indicate the presence of different elements on the surface of the Dowex 50WX8, including Praseodymium, Dysprosium and yttrium, with the elements distributed uniformly across the resin surface Fig. 4(a-d). Consequently, the results of the EDX and Map analyses affirm the successful bonding of the metal ions on the surface of Dowex 50WX8.

Sorption batch experiments Effect of shaking time
The impact of contact time on the sorption efficiency of Pr(III), Dy(III) and Y(III) ions using Dowex 50WX8 Fig. 3 SEM images of, a Dowex 50WX8 before sorption process, b after sorption of Pr(III), c after sorption of Dy(III), d after sorption of Y(III) from nitrate medium sorbent from nitrate medium was carried out in the range 1.0-90.0 min. The results indicated that the rare earth ions sorption process took 1-15 min to occur and was based on the availability of vacant active sites The rate of adsorption on the surface of the adsorbents was significantly decreased at contact times beyond 15.0 min, Fig. 5a possibly because of the saturation of the available active sites on the sorbent surface; accordingly the equilibrium contact time for the sorption of the investigated metal ions using Dowex adsorbent was fixed at 15.0 min.

Adsorbent dosage
The sorbent dosage is another key factor imposing large contribution to the sorption process, as it can determine the adsorption efficiency of the sorbent for a given initial concentration of the investigated metal ions [24]. In this respect, the impact of the sorbent dosage (v/m) on the adsorption efficiency of Pr(III), Dy(III) and Y(III) ions was investigated in the range of 0.02-0.25 L/g, Fig. 5b. The obtained results show that the adsorption efficiency yield takes the order DY(III) > Pr(III) > Y(III) enhanced abruptly by increasing v/m from 0.1 L/g up to 0.25 L/g. Based on the obtained results, the optimal adsorbent dosage on Dowex was fixed at 0.05 L/g in all experiments carried out in this work.

Effect of nitric acid concentration
The adsorption of Pr(III), Dy(III) and Y(III) from different nitric acid concentrations is given in Fig. 5c and the experimental results revealed that the adsorption efficiency of Dowex ion exchanger decreased rapidly upon increasing nitric acid concentration from 0.50 to 3.0 M. This may be attributed to the compensation of H + with higher increasing of acid concentration which leads to a decrease in the exchange rate between hydrogen ion and metal ions; with further increase in the acid molarity higher than 3.0 M the adsorption capacity became stable, Fig. 5c. However, the experiments were performed at 0.5 M HNO 3 .

Effect of metal ions concentration
The effect of the initial concentration of Pr(III), Dy(III) and Y(III) on the adsorption capacity (q e ) was studied in the range of 50-500 mg L −1 through their sorption by Dowex from nitrate medium. The experiments were carried out by shaking 5.0 mL of the investigated metal ions solution individually with 0.05 g of the adsorbent for 15.0 min at 25 °C. The obtained data are represented in Fig. 5d, The adsorption efficiency increased as the concentration of REEs +3 increased and the highest adsorption capacities of 29.0 mg/g, 25.0 mg/g and 24.0 mg/g were achieved at 500 mg/L for Dy, Pr and Y, respectively. This saturation can be ascribed to the interactions between the adsorbent active sites and these metal ions. [25].

Sorption mechanism of REE + with Dowex-H +
Based on the experimental results and considering that M (NO 3 ) 2 + is the predominant species in 1.0 M nitric acid solution [21,26], where M represents Pr(III), Dy(III) and Y(III),The ion exchange extraction mechanism of REEs metal ion (M) with Dowex-H was suggested to proceed via different reaction pathways from Eqs. (3) and (4), [20] Equation (3) suggests that the extraction mechanism occurs via partial ion exchange reactions during the REE diffusion and interaction at the Dowex active sites SO 3 -H + where the extracted metal ions species according to Eq. (3) were found to be Dowex-SO 3 -Pr(NO 3 ), Dowex-SO 3 -Dy(NO 3 ) and Dowex-SO 3 -Y(NO 3 ) for Pr, Dy and Y respectively.

Adsorption isotherm of Pr, Dy and Y on the Dowex 50WX8 cation exchanger
Adsorption isotherms were used to describe the distribution of metal ions between the sample solution (liquid phase) and the resin (solid phase) when the ion exchange process reaches equilibrium [27,28]. The Langmuir isotherm model describes a homogeneous monolayer chemical adsorption process, while the Freundlich isotherm model describes a heterogeneous physical adsorption process [29]. Non-liner models achieved the most flexible curve fitting functionality. In this context, Langmuir, Freundlich, Temkin, D-R isotherm and Toth isotherm were employed for studying the nonlinear adsorption isotherm of Pr, Dy and Y on the cation exchanger resin (Dowex 50WX8).
Nonlinear Langmuir isotherm equation is given as: where q e is the equilibrium adsorption capacity of ions on the adsorbent (mg g −1 ), C e is the equilibrium ions Toth isotherm model is another empirical equation developed to improve Langmuir isotherm fittings and take into consideration both low and high-end boundary of the concentration and is given as, [30,31] The relationship between q e and C e for each nonlinear isotherm model is plotted in (Fig. 6 a-e) and the values of the obtained parameters are tabulated in Table 2. The results indicate that Pr and Y fitted with nonlinear Langmuir isotherm model with regression factors 0.995 and 0.978 respectively, while, Dy was fitted with nonlinear Toth isotherm model with R = 0.966.

Desorption, reusability and separation between Dy, Pr and Y from nitrate medium using Dowex 50WX8
The most effective separation obtained between the investigated metal ions was obtained from the stripping process. This process was carried out by contacting the loaded Dowex 50WX8 with different stripping agents at experimental conditions (contact time = 60.0 min, v/m = 0.05 at 25 ± 1 °C). The results illustrated in Table 3 show that the maximum stripping of Pr(III) is 58% and was achieved with 1.0 M citric acid at pH = 3. In the case of Dy(III) and Y(III) the maximum desorption is 55%  Fig. 7.
The desorption results indicate that Pr(III) can be separated from Dy(III) and Y(III) as follows:  Table 3.
Finally, the reported results show that Dowex 50WX8 resin is relatively selective, high efficient and cost effective for Pr(III), Dy(III) and Y(III) adsorption and is also easily regenerated rather than other reported adsorbent/ ion exchangers which were used in the adsorption from acidic nitrate medium. The reusability was carried out for 4.0 adsorption stages with sorption capacity of 15.0, 30.0, 35.0 mg/g for Pr, DY and Y, respectively, under the used experimental conditions.

Conclusions
Dowex 50WX8 was successfully used for the recovery of DY(III), Pr(III) and Y(III) from acidic nitrate medium. The calculated maximum capacity of Dowex 50WX8 is 30, 50, 60 mg/g for Pr, DY and Y respectively at the optimum batch conditions; the maximum stripping of Pr(III) is 58.0% and was achieved with 1.0 M citric acid at pH = 3. The results indicate that Pr(III) and Y(III) fitted with nonlinear Langmuir isotherm model with regression factors 0.995 and 0.978 respectively. The regenerated Dowex 50WX8 gave sorption capacities of 15.0, 30.0, 35.0 mg/g for Pr, DY and Y, respectively under the used experimental conditions.   Diglycolamic-acid modified chitosan sponges 40.7 0.5-7 12.0 h [17]