Highlights

*The maximum adsorption capacities of Dy(III), Pr(III) and Y(III) were 30, 50 and 60 mg/g respectively using 500 mg/L of Dowex 50WX8 resin.

*Maximum stripping (58%) of Pr(III) was achieved using 1 M Citric acid at pH 3.

*The maximum separation ratios obtained were 56, 35 and 4.6 for Y(III)/Pr(III), Y(III)/Dy(III) and Dy(III)/Pr(III), respectively.

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-tech-devices 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 SO3H 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/SiO2-P in 0.1 M HNO3. 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 NaNO3 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.

Experimental

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.

Fig.1
figure 1

Chemical structure of hydrogenated Dowex 50WX8 resin

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Table 1 Specification data sheet of Dowex 50WX8
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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 (qe) at equilibrium was given by equation:

$$ q_{e} = \,(C_{i} - C_{e} ) \times \left( {\tfrac{V}{w}} \right)\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, $$
(1)

The sorption efficiency (S %) at equilibrium was calculated from the equation:

$$ S\% \, = \,\tfrac{{C_{i} - C_{e} }}{{C_{{_{e} }} }}\,\,\, \times 100\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, $$
(2)

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.

Results and discussion

Characterization of Dowex 50WX8

Figure 2a–d shows the FT-IR spectra of Dowex 50WX8 before and after loading of rare earth metal ions (Pr(III), DY(III) and Y(III)), The spectrum of (Pure Dowex 50WX8) showed adsorption peaks in the ranges of 3330–3450 cm−1 and 1632–1645 cm−1, which may be attributed to stretching vibrations of the O–H functional group in the structure of the adsorbents [20]. Moreover, bands at 1658–1648 cm−1 correspond to the alkene group (C=C) in the Dowex skeleton and bands at1350–1340 cm−1 are assigned to the stretching S=O of sulfonic acid. Dowex 50WX8 gives characteristic IR bands for the SO3 vibration (1169 cm−1), (S–C) vibration (1121 cm−1), and (S–O) vibrations (1093 and 1037 cm−1) [23]. Previous studies have shown that hydration of the sulfonate site with H2O leads to the formation of hydronium ion (H3O+‏) species which is the interaction moiety with metal ions.

Fig. 2
figure 2

FT-IR spectra of, a Dowex 50WX8, b Dowex -Dy, c Dowex -Pr, d Dowex -Y for sorption of rare earth metal ions from nitrate medium

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The FT-IR spectrum of Dowex after interaction with REE metal ions, indicate that adsorbing species (Dowex/Pr/Dy/Y) were formed at the counter ion of the Dowex 50WX8 (H+) which protonates with H2O to form the Dowex 50WX8(H3O+) species [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.

Fig. 3
figure 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

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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.

Fig. 4
figure 4

EDX-analysis 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

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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 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.

Fig. 5
figure 5

a Effect of contact time on the adsorption efficiency of Pr(III), Dy(III) and Y(III) using Dowex 50WX8 from nitrate medium (temperature: 25 °C, initial ion concentration: 100 mg/L, v/m = 0.05 L/g, b Effect of the sorbent dosage on the adsorption efficiency (at optimal pH value, contact time = 15.0 min, initial metal ion concentration 100 mg/L, temperature: 25 °C, c Effect of nitric acid concentration, d Effect of metal ion concentrations on the sorption of Dy, Pr and Y using Dowex 50WX8

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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 HNO3.

Effect of metal ions concentration

The effect of the initial concentration of Pr(III), Dy(III) and Y(III) on the adsorption capacity (qe) 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 (NO3)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]

$$ 3DOWEX - SO_{3} - 3H^{ + } + M\left( {NO_{3} } \right)_{2}^{ + } \rightleftharpoons 2{\text{DOWEX}} - SO_{3} - {\text{M}}NO_{3} {\text{ + HNO3 + }}DOWEX - SO_{3} - H^{ + } \, $$
(3)
$$ 3DOWEX - SO_{3} - 3H^{ + } + M\left( {NO_{3} } \right)_{2}^{ + } \rightleftharpoons {\text{DOWEX}} - SO_{3} - {\text{M}}NO_{3} { + 2}DOWEX - SO_{3} - 2H^{ + } \, $$
(4)

Equation (3) suggests that the extraction mechanism occurs via partial ion exchange reactions during the REE diffusion and interaction at the Dowex active sites SO3-H+ where the extracted metal ions species according to Eq. (3) were found to be Dowex-SO3-Pr(NO3), Dowex-SO3-Dy(NO3) and Dowex-SO3-Y(NO3) 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:

$$ {\text{q}}_{{\text{e}}} \,{ = }\,{\text{Q}}\frac{{{\text{bC}}_{{\text{e}}} }}{{{\text{1 + bC}}_{{\text{e}}} }} \, $$
(5)

where qe is the equilibrium adsorption capacity of ions on the adsorbent (mg g−1), Ce is the equilibrium ions concentration in solution (mg L−1), Q the maximum capacity of the adsorbent. (mg g−1), and b the Langmuir adsorption constant (L mg−1). Nonlinear Freundlich isotherm equation is given as:

$$ q_{e\,} \, = \,K_{f} \,C_{e}^{1/n} \, $$
(6)

where Kf is the Freundlich constant (mg/g).

Nonlinear Temkin isotherm model which takes into account the interactions of ions of the aqueous solution and the adsorbent and is given as:

$$ q_{e} = \frac{RT}{b}\ln (A_{T} C_{e} ) \, $$
(7)

where R is the universal gas constant (8.314 J/mol K), T the absolute temperature, b a constant related to the heat of sorption (J/mol), AT the equilibrium binding constant (L/g) and b the adsorption constant (J/mol K).

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]

$$ q_{e} = \, q_{m} \exp ( - \beta \varepsilon^{2} ) \, $$
(8)
$$ \varepsilon = RT\ln \left( {1 + \frac{1}{{C_{e} }}} \right) \, $$
(9)
$$ E = \frac{1}{{(2\beta )^{0.5} }} \, $$
(10)

The relationship between qe and Ce 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.

Fig. 6
figure 6

a Nonlinear isotherm plot of Langmuir model, b Nonlinear isotherm plot of Freundlich model, c Nonlinear isotherm plot of D-R, d Nonlinear isotherm plot of Temkin model, d Nonlinear isotherm plot Toth model for adsorption of Pr(III), DY(III) and Y(III) onto Dowex 50WX8

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Table 2 Nonlinear Freundlich, Langmuir, Dubinin–Radushkevich, Temkin and Toth isotherm parameters for adsorption of metal ions onto Dowex 50WX8
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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% and 56% and was achieved with [HNO3] = 1.0 M and [(NH4)2CO3] = 1.0 M, respectively. A flow sheet which illustrates the sorption and desorption processes of the investigated rare earth using D-50WX8 from 0.5 M HNO3 solution at v/m = 0.05 at 25 ± 1 °C is given in Fig. 7.

Table 3 Desorption of metal ions (III) with different reagents after their adsorption with the Dowex 50WX8 resin at v/m = 0.05 at 25 ± 1 °C
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Fig. 7
figure 7

Flow sheet for the sorption and desorption processes of Pr, DY and Y using Dowex 50WX8

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The desorption results indicate that Pr(III) can be separated from Dy(III) and Y(III) as follows:

  1. i.

    Stripping of Dy(III) and Y(III) using [(NH4)2CO3] = 1.0 M from Loaded Dowex 50WX8 after 2 cycles

  2. ii.

    Dowex 50WX8 containing Pr(III) was then stripped with 1.0 M citric acid at pH = 3 after two stripping cycles

Furthermore, the separation ratio (S-ratio) between the investigated metal ions were calculated by dividing their desorption percentages. The results indicate that the maximum S-ratios are 56.0, 35.0 and 4.6 for Y/Pr, Dy/Pr and were achieved with [(NH4)2CO3] = 1.0 and [HNO3] = 5.0 M respectively, 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.

Comparison study of REEs/Dowex 50WX8 with other reported materials

Comparison of REEs/Dowex 50WX8 system under the used optimum conditions of batch technique with other commercially reported materials [18, 32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51] and given in Table 4, shows the advantages and efficiency of Dowex 50WX8 adsorbent. The results of comparison in the term of maximum capacity (Q0) (30, 50, 60 mg/g for Pr, DY and Y), pH = 1, and contact time (15 min) and which were achieved in the current study indicate that Dowex 50WX8 is more efficient and affordable than other reported materials.

Table 4 Comparison study of REEs/Dowex 50WX8 with other reported materials
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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.

Declaration