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Abstract

For the adsorption and recovery of rare earth elements from aqueous nitric acid, solid-phase extraction resins were prepared by impregnating and immobilizing of bis(2-ethylhexyl)phosphoric acid extractant into the macroporous silica-based polymeric (SiO₂-P) particles. It was found that bis(2-ethylhexyl)phosphoric acid/SiO₂-P had higher adsorption distribution coefficient for heavy rare earth elements than for light rare earth elements. The adsorption capacity of Gd(III) was observed to be 0.315 mmol g⁻¹ by bis(2-ethylhexyl)phosphoric acid/SiO₂-P in 0.1 M HNO₃ at 298 K, which increased slightly when increasing temperature from 298 to 323 K. The adsorption isotherms of Gd(III) matched well with the Langmuir and Freundlich models. The obtained thermodynamic parameters (ΔH^o and ΔG^o) showed that the adsorption of Gd(III) by bis(2-ethylhexyl)phosphoric acid/SiO₂-P was a spontaneous and exothermic process. This study also evaluated the chemical stability of bis(2-ethylhexyl)phosphoric acid/SiO₂-P treated with nitric acid at different temperatures and demonstrated that bis(2-ethylhexyl)phosphoric acid/SiO₂-P had considerable stability against nitric acid and heat.

Keywords

Solid-phase extraction adsorption rare earth elements gadolinium

Introduction

Rare earth elements (REEs) have gained much attention from researchers during the past two decades, not only because of their unique properties, but also due to their potential applications in a wide range of hi-tech industries, such as metallurgy, medicine, lasers, electronic devices, and nuclear fuel control (Gschneidner, 1981; Moldoveanu and Papangelakis, 2012; Rao and Biju, 2000). Approximately 90% of REE minerals are concentrated on Australia, China, and the United States (Clark and Zheng, 1991). With the development of existing industries and new advanced technologies, there is an ongoing increasing requirement for REEs in the global markets; therefore, emphasizing on sustainable utilization of REEs is an urgent issue in order to ensure an abundant supply for present and future generations (Moldoveanu and Papangelakis, 2012).

REEs, particularly light REEs (La, Ce, Pr, Nd, and Pm) and middle–heavy REEs (Sm, Eu, Gd, and Tb) are considered as the main fission products in the waste of nuclear reprocessing plant (Alonso et al., 1995). Furthermore, gadolinium (Gd), as one middle–heavy REE, is mainly used as shielding and fluxing devices in nuclear power reactors (Junk et al., 1974). Therefore, the content of REEs, especially Gd, is significantly considerable in nuclear waste. In recent years, many studies have focused on recovery of REEs and some other fission products from nitric acid solutions of nuclear waste (Anastopoulos et al., 2016; Ansari et al., 2009a, 2009b; Gibilaro et al., 2009; Shimojo et al., 2008), most of which are based on solvent extraction. Although liquid–liquid solvent extraction has been reported as a successful process for industrial recovery of noble metals and hydrometallurgy, generally, it still needs to be applied by a larger volume of organic solvent and equipment (Zhang et al., 2004a).

Solid-phase extraction is one of the effective separation techniques which has some advantages, such as less organic solvent utilization, compact equipment, and less waste accumulation, and it has been widely utilized in separation and preconcentration of elements from environmental samples (Mayyas et al., 2014; Rodríguez et al., 2016; Wei et al., 2000; Zhang et al., 2004a). There are some articles about selective adsorption and recovery of Gd(III) or other elements by using the solid-phase extraction process, in which extractants are immobilized on different solid supports or ion-exchange resins (Aghayan et al., 2013; El-Sofany, 2008; Lück et al., 2010). EI-Sofany (2008) studied removal of La(III) and Gd(III) by using XAD-4 resin impregnated with Aliquat-336. Wei et al. (2000) had reported the synthesis of a novel silica-based extraction resin (SiO₂-P) by inserting the copolymer of styrene–divinylbenzene into macroporous silica materials. When the organic extractants such as octyl(phenyl)-N,N-diisobutylcarbamoyl-methylphosphine oxide (CMPO), N,N,N′,N′-tetraoctyl-3-oxapentane-1,5-diamide (TODGA), and 2,6-bis(5,6-dialkyl-1,2,4-triazine-3-yl)-pyridine are impregnated and immobilized into SiO₂-P resins, it can be used to remove some REEs and other fission products (Pd(II), Fe(III), Zr(IV), and Mo(VI)) from nitric acid medium (Zhang et al., 2006, 2007, 2005a, 2005d).

Bis(2-ethylhexyl)phosphoric acid (HDEHP) is a conventional extractant extensively used in industrial separation of REEs by solvent extraction (Gupta and Krishnamurthy, 1992). Recently, it has been demonstrated that HDEHP is suitable to be applied in the solid-phase extraction for separation and recovery of REEs or other ions (Knutson et al., 2014; Mokhodoeva et al., 2011). Cortina et al. (1994) reported extraction of Zn(II), Cu(II), and Cd(II) by HEDHP supported on the commercial resin of Amberlite XAD-2. Saipriya et al. (2016) reported synergic extraction of Am and Eu from a nitric acid medium by impregnating TEHGA+HDEHP into Tulsion ADS 400 in a single-cycle separation.

In Shu et al. (2017), the macroporous SiO₂-P-based HDEHP adsorbent has shown well performance on adsorption and separation characteristics of ²⁴¹Am(III), Ce(III), Sm(III), Eu(III), Gd(III), and Tb(III) from nitric acid solutions. Herein, the main aim of this study was to synthesize the HDEHP/SiO₂-P as a solid-phase adsorbent for adsorption of REEs from nitric acid solutions. In addition, this work puts emphasis on discussing the adsorption thermodynamics and desorption characteristic of Gd(III) by HDEHP/SiO₂-P adsorbent and evaluation on chemical stability of HDEHP/SiO₂-P against nitric acid and temperature.

Experimental

Materials and methods

HDEHP was purchased from J & K Scientific Ltd and purified according to McDowell et al. (1976). RE(NO₃)₃·xH₂O (RE = La(III), Ce(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), and Y(III), x = 3 or 6) were commercial reagents with analytical grade. All other chemicals, such as dichloromethane (CH₂Cl₂) and nitric acid were of reagent grade quality and used without further purification. SiO₂-P resin was synthesized by the previous procedure which given in Wei et al. (2000) and the results of morphological characterization of macroporous SiO₂-P had been reported by Wei et al. (2000) and Wu et al. (2012). The synthesized SiO₂-P was washed with methanol three times prior to use. HDEHP/SiO₂-P adsorbent was synthesized by impregnating and immobilizing HDEHP into the macroporous SiO₂-P particles. The relevant synthesis procedures were the same as Khayambashi et al. (2016) and Shu et al. (2017), and HDEHP supporting proportion about 33.3%. The surface characterization of macroporous HDEHP/SiO₂-P by scanning electron microscopy has been reported in Shu et al. (2017).

Measurements

The concentrations of REEs before and after adsorption were tested using inductively coupled plasma atomic emission spectrometer (ICP-AES, Shimadzu 7510). The concentrations of P and C leaked from HDEHP/SiO₂-P in filtering liquor and washing solution were determined by ICP-AES (Shimadzu 7510) and TOC analyzer (Shimadzu, TOC-500 A), respectively. The FT-IR spectra were collected in the range of 400–4000 cm⁻¹ with FT-IR spectrometers (IR Affinity-1, Shimadzu) as KBr pellets.

Batch adsorption and desorption experiment

Batch experiment was used to investigate the adsorption behavior of REEs by the HDEHP/SiO₂-P resins and the desorption characteristic of Gd(III) from resins, and also evaluate the adsorbability of treated HDEHP/SiO₂-P residues after nitric acid and heat resistance experiment. In the experiment about the effect of HNO₃ concentrations on extraction, 5 ml of 1 mmol l⁻¹ La(III), Ce(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), and Y(III) were mixed with 0.1 g HDEHP/SiO₂-P adsorbent at the concentrations range of HNO₃ from 0.001 to 0.5 M. In the adsorption isotherm experiment, 0.1 g of HDEHP/SiO₂-P was mixed with Gd(III) through altering the initial concentrations from 4 to 13 mmol l⁻¹ in the HNO₃ concentration of pH = 1 at temperature of 298, 303, 313, and 323 K, respectively. Desorption experiment for loaded Gd from HDEHP/SiO₂-P was carried out subsequently after the adsorption experiment (pH = 2). In the chemical stability evaluating experiment, 0.1 g of the treated HDEHP/SiO₂-P residues was combined as solid phase with 5 ml of 10 mmol l⁻¹ Gd(III) solution in the HNO₃ concentration of pH = 1 in a 10 ml glass vial with a screw cap and shaken mechanically at 25℃ in a thermostat water bath for 24 h.

Distribution coefficient (K_d, cm³ g⁻¹), adsorption amount per unit mass of adsorbent (Q_e, mmol g⁻¹), and desorption percentage were calculated as follows $K_{d} = \frac{C_{o} - C_{e}}{C_{e}} \times \frac{V}{W}$ (1) $Q_{e} = \frac{(C_{o} - C_{e}) \times V}{W}$ (2) $Q_{d} = C_{d} \times 00 A 0 \frac{V}{W}$ (3) $Desorptionpercentage = \frac{Q_{d}}{Q_{e}} \times 100 %$ (4) where C_o and C_e represent the initial and equilibrium concentrations of REEs in the aqueous phase, respectively. Also, V (ml) and W (g) represent the volume of the aqueous phase and weight of HDEHP/SiO₂-P, respectively. C_d (mmol l⁻¹) and Q_d (mmol g⁻¹) denote the concentrations of Gd(III) in eluent and desorption amount, respectively.

Evaluation on chemical stability of HDEHP/SiO₂-P

Two grams of dry HDEHP/SiO₂-P polymeric adsorption materials were mixed with 40 ml of 0.1 and 3 M HNO₃ solution in a 50 ml glass vial with a screw cap and shaken mechanically at 120 r/min for one month under thermostated water bath at 25 and 60℃, respectively. The mixture was filtered by sand core funnel, and the HDEHP/SiO₂-P residues were washed with distilled water to neutral. Then, the HDEHP/SiO₂-P residues were dried in vacuum at 323 K for 48 h and used to evaluate the adsorption properties after the nitric acid and heat resistant experiments. The filtering liquor was collected directly, and the washing solution was diluted to 50 ml, to respectively measure the total organic carbon (TOC) and P concentrations. The corresponding content of HDEHP leaked in solutions was calculated based on the content of P.

Results and discussion

Effect of HNO₃ concentrations on extraction

Effect of HNO₃ on the adsorption of REEs toward HDEHP/SiO₂-P was investigated at different initial concentrations of HNO₃, as is shown in Figure 1. The adsorption results revealed that the K_d value of nine kinds of REEs had slight differences at concentrations of 0.001, 0.01, and 0.1 M HNO₃. HDEHP/SiO₂-P presented stronger adsorption performance for the heavy REEs (Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), and Y(III)) than for the light REEs (La(III), Ce(III), and Nd(III)). In addition, the lowest K_d value of the heavy REE (Sm(III)) was approximately 10 times greater than the highest K_d value of the light REE (Nd(III)). This may be since that lanthanide contraction leads to the decrease of ionic radius, and it makes a better combination of HDEHP adsorbent with the increasing of the atomic number (Xu et al., 2012; Yoshihiro et al., 2006). It can be noted that the distribution coefficient of the light REEs decreased significantly with HNO₃ concentrations increasing from 0.2 to 0.5 M, while the K_d values of the heavy REEs were fluctuated as they first increased in 0.2 M and then decreased while increasing the HNO₃ concentration. According to Nash (1993), Shu et al. (2017), and Wei et al. (2000), the adsorption of REEs (M³⁺) toward HDEHP (HA) can be explained by the following reaction equation $M^{3 +} + 3 (HA)_{2} ⇌ M ({HA}_{2})_{3} + {3 H}^{+}$

Figure 1.

Effect of initial HNO₃ concentration on K_d of typical trivalent REEs in the presence of the HDEHP/SiO₂-P (RE(NO₃)₃·xH₂O (RE = La(III), Ce(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), and Y(III)): 1 mmol l⁻¹, pH = 1, phase ratio: 0.1 g/5 ml, contact time: 24 h, 298 K, and 120 r/min).

Increasing the HNO₃ concentration led the equilibrium to shift to the left and led to decrease of K_d values.

Adsorption isotherms

Gd is a heavy REE which is largely used in nuclear reactors (Junk et al., 1974) and thus, adsorption of Gd(III) was emphasized in this research. Equilibrium adsorption isotherms for Gd(III) were studied at four different temperatures (298, 303, 313, and 323 K). As is shown in Figure 2, the Gd uptake capacities increased quickly at low concentration, indicating that HDEHP/SiO₂-P had a strong affinity for Gd(III) (Park and Tavlarides, 2010). It was found that the maximum adsorption capacity of Gd(III) was 0.315 mmol g⁻¹ at 298 K, whereas at 303, 313, and 323 K the maximum capacities were 0.319, 0.322, and 0.324 mmol g⁻¹, respectively. This phenomenon was also shown in Alghouti et al. (2005) and Chiou and Li (2002, 2003), with the increasing of the temperature led to an increase of adsorption capacity, indicating the Gd(III) adsorption toward HDEHP/SiO₂-P could be a kinetically controlled process and the higher temperature increased the adsorption rate.

Figure 2.

Adsorption isotherms of Gd(III) onto the HDEHP/SiO₂-P resin in 0.1 M HNO₃ at different temperatures (phase ratio: 0.1 g/5 ml, contact time: 48 h, 120 r/min, the initial concentration of Gd(III): 4, 5, 6, 7, 8, 9, 10, 11, 12, and 13 mmol l⁻¹, pH = 1).

In order to well understand the adsorption properties of Gd(III) toward HDEHP/SiO₂-P, Langmuir and Freundlich models were established to fit the experimental data, respectively. The primitive equation of the Langmuir model (3) and Freundlich model (4) can be described, respectively, as follows (Freundlich, 1937; Langmuir, 1917) $Q_{e} = \frac{Q_{o} K_{L} C_{e}}{1 + K_{L} C_{e}}$ (5) $Q_{e} = K_{F} C_{e}^{(1 / n)}$ (6)

These equations have also been transformed to the linearity relation as follows (Qureshi et al., 2009; Yuan et al., 2010)

Linearity relation of the Langmuir model $\frac{C_{e}}{Q_{e}} = \frac{1}{Q_{o} K_{L}} + \frac{C_{e}}{Q_{o}}$ (7)

Linearity relation of the Freundlich model $\lg Q_{e} = \frac{1}{n} \lg C_{e} + \lg K_{F}$ (8) where C_e is the equilibrium concentration of Gd(III), Q_e is the adsorption amount in an equilibrium state, Q_o is the monolayer adsorption capacity in the Langmuir model, K_L is the Langmuir constant, K_F is the Freundlich constant, R² is the coefficient of determination, and n is the heterogeneity factor. The R2 was coefficient of determination which calculated by fitting equation. Figures 3 and 4 show the linear plots of C_e/Q_e versus C_e and lgQ_e versus lgC_e for Gd(III) toward HDEHP/SiO₂-P in the Langmuir and Freundlich isotherm model, respectively. The corresponding constants and coefficient of determination (R²) calculated by the two isotherm models were listed in Table 1. It was found that both the Langmuir and Freundlich model had a good linear relation under experimental conditions, while the adsorption capacity obtained from the Langmuir isotherm was more consistent with the experimental data and plotting of C_e/Q_e versus C_e given a higher value of coefficient of determination. It was indicated that both the Langmuir and Freundlich isotherm models were suitable to study the equilibrium adsorption of Gd(III) by HDEHP/SiO₂-P on the experimental conditions.

Figure 3.

The Langmuir plots for the adsorption isotherms of Gd(III) by HDEHP/SiO₂-P adsorbent.

Figure 4.

Freundlich plots for the adsorption isotherms of Gd(III) by HDEHP/SiO₂-P adsorbent.

Table 1.

Isotherm parameters for adsorption of Gd(III) onto HDEHP/SiO₂-P adsorbent at different temperatures.

Temperature (K)	Langmuir isotherm			Freundlich isotherm
Temperature (K)	Q_o (mmol g⁻¹)	K_L (l mmol⁻¹)	R²	K_F (mmol^1−1/ ⁿ g⁻¹ l^1/ ⁿ )	n	R²
298	0.315	118.04	1.00	0.310	100.20	0.986
303	0.319	117.32	1.00	0.313	96.06	0.969
313	0.322	105.58	1.00	0.315	70.87	0.982
323	0.324	96.69	0.99	0.316	65.06	0.981

HDEHP: bis(2-ethylhexyl)phosphoric acid.

The Langmuir equation assumed that the adsorption sites were identical and energetically equivalent. Also, the equation was based on the assumption of homogeneous adsorbent by one-on-one monolayer adsorption manner (Xiong and Yao, 2009). Therefore, once a Gd(III) ion occupied a site, no further adsorption occurred at the same site. According to Barkat et al. (2009), Hall et al. (1966), and Langmuir (1918), the dimensionless constant separation factor, R_L, was an essential characteristic of the Langmuir isotherm, defined by $R_{L} = \frac{1}{1 + K_{L} C_{o}}$ (9) where C_o is the initial concentration of Gd(III). The value of R_L denotes the type of the Langmuir isotherm to be irreversible (R_L = 0), favorable (0 < R_L < 1), linear (R_L = 1), or unfavorable (R_L > 1), respectively. In this research, the calculated values of R_L were all in the range of 0 < R_L < 1, indicating that adsorption of Gd(III) toward HDEHP/SiO₂-P was favorable in all cases on the experimental conditions.

Thermodynamics studies

The investigations on adsorption thermodynamics were used to get an insight into the adsorption behavior of Gd(III) toward HDEHP/SiO₂-P. The thermodynamic parameters including standard enthalpy change (ΔH^o, kJ mol⁻¹), standard entropy change (ΔS^o, kJ mol⁻¹ K⁻¹), and standard Gibbs free energy (ΔG^o, kJ mol⁻¹) were calculated using the following thermodynamic equations (Ajmal et al., 1998; Anastopoulos and Kyzas, 2016; Oguz, 2005) $lnK = \frac{{Δ S}^{o}}{R} - \frac{{Δ H}^{o}}{RT}$ (10) ${Δ G}^{o} = {Δ H}^{o} - {T Δ S}^{o}$ (11) where K is the equilibrium constant obtained from the Langmuir isotherms, R is the gas constant (8.314 J mol⁻¹ K⁻¹), and T is the absolute temperature (K). The values of ΔH^o and ΔS^o were calculated from slope and intercept of the linear plotting of lnK versus 1/T (Figure 5), respectively, and the related parameters were listed in Table 2. The negative value of enthalpy change ΔH^o indicated that the adsorption process of Gd(III) by HDEHP/SiO₂-P was an exothermic reaction (Liu et al., 2010). The positive value of entropy change ΔS^o suggested the increased randomness at the solid/solution interface during the adsorption of Gd(III) onto the HDEHP/SiO₂-P (Liu et al., 2010). Negative Gibbs free energy ΔG^o values in all cases demonstrated the feasibility of the processes and the spontaneous property of these adsorptions (El-Kamash et al., 2007; Gupta and Ali, 2004; Li et al., 2005; Namasivayam and Sangeetha, 2005).

Figure 5.

Plot of the Langmuir constant (lnK) versus temperature (1/T).

Table 2.

Thermodynamic parameters for Gd(III) adsorption on HDEHP/SiO₂-P adsorbent.

Temperature (K)	ΔH^o (kJ mol⁻¹)	ΔS^o (kJ mol⁻¹ K⁻¹)	ΔG^o (kJ mol⁻¹)
298	−6.77	0.017	−11.86
303			−11.95
313			−12.12
323			−12.29

HDEHP: bis(2-ethylhexyl)phosphoric acid.

FT-IR analysis

The IR spectroscopy of a neat HDEHP sample and the Gd(III) complex of HDEHP were examined by Shimadzu IR Affinity-1 instrument and the results are shown in Figure 6. According to Thomas and Chittenden’s assignment (1964a, 1964b) about the characteristic frequencies for the type of (RO)₂POOH, HDEHP displays a broad band at ∼1680 cm⁻¹ which is ascribed to the P–O–H group. The characteristic bands at 1230 and 1030 cm⁻¹ are assigned to P=O and P–O–C, respectively. The IR spectrum of complex Gd(III)-HDEHP has substantial changes compared with that of neat HDEHP. The broad band at 1680 cm⁻¹ (P–O–H group) and sharp band at 1230 cm⁻¹ (P=O group) in HDEHP do not display in the IR spectrum of complex Gd(III)-HDEHP. However, the spectrum in complex of Gd(III)-HDEHP appears a sharp symmetric (O–P–O) stretching band which is seen at 1096 cm⁻¹ and asymmetric (O–P–O) stretching band centered at 1185 cm⁻¹, which are consistent with the ν_s(PO₂) and ν_as(PO₂) bands in that of H₂PO₄⁻ (Steger and Herzog, 1964). It indicates the deprotonation of HDEHP and formation of the (RO)₂POO⁻ moiety (Lumetta et al., 2016). In addition, the P–O–C band in the spectrum of complex Gd(III)-HDEHP is not significantly changed comparing with that in neat HDEHP, both centering at ∼1030 cm⁻¹.

Figure 6.

FT-IR spectrum of a neat HDEHP sample and complex of Gd(III)-HDEHP in KBr pellets.

Desorption characteristic

To evaluate the desorption characteristic of Gd from HDEHP/SiO₂-P adsorbent, HNO₃ solution was used as eluent. We also investigated the effect of HNO₃ concentrations on the desorption percentage and the result was shown in Figure 7. The desorption percentage increased significantly with the increasing of HNO₃ concentration in the range from 0.1 to 2 M and reached about 95% at the concentration of 2 M HNO₃ solution. It indicated that Gd(III) desorption from HDEHP/SiO₂-P could be carried out under moderate and simple conditions.

Figure 7.

Effect of HNO₃ concentrations on Gd(III) desorption from HDEHP/SiO₂-P. Adsorption conditions: initial Gd(III) concentration, 10 mmol l⁻¹; initial pH = 2.0; solution volume, 25 ml; HDEHP/SiO₂-P, 0.5 g. Desorption conditions: solution volume, 5 ml; HDEHP/SiO₂-P loaded with Gd(III), 0.1 g.

Chemical stability of HDEHP/SiO₂-P

Since HNO₃ solutions were commonly used to dissolve the metals and materials during nuclear spent fuel reprocessing process, and the previous research demonstrated that temperature had an influence on separation performance about the silica-based solid-phase extraction (Zhang et al., 2003, 2004b), the resistant properties against HNO₃ solutions and heat were significant parameters for adsorbent materials (Zhang et al., 2005b). This research mainly discusses the adsorption behavior of Gd(III) toward the treated HDEHP/SiO₂-P adsorbent and the leakage from the adsorbent after being treated with different concentrations of HNO₃ solutions at different temperatures.

The uptake of Gd(III) was studied in 0.1 M HNO₃ solutions to evaluate the adsorption performance of HDEHP/SiO₂-P adsorbent after acid and heat resistant experiment. The adsorption amounts of Gd(III) as a function of contact time toward fresh HDEHP/SiO₂-P and treated HDEHP/SiO₂-P in 0.1 M HNO₃ at 25℃ and in 3 M HNO₃ at 25 and 60℃ were compared in Figure 8. Obviously, all the adsorption kinetics of Gd(III) toward treated HDEHP/SiO₂-P with different conditions were similar to the fresh HDEHP/SiO₂-P resin. The adsorption amounts increased very quickly by increasing the contact time in the initial stage of adsorption and then reached equilibrium gradually within about 6 h. The corresponding equilibrium adsorption amounts of Gd(III) by treated HDEHP/SiO₂-P with 0.1 M HNO₃ at 25℃, 3 M HNO₃ at 25℃, and 3 M HNO₃ at 60℃ were 0.295, 0.292, and 0.283 mmol g⁻¹, respectively, which were reduced by 4.53, 5.50, and 8.41%, respectively, compared with 0.309 mmol g⁻¹ in the fresh HDEHP/SiO₂-P. Calculated from the experimental data of Zhang et al. (2005b, 2005c), the equilibrium adsorption amounts of Nd(III) by treated CMPO/SiO₂-P were reduced by 3.36% with 0.01 M HNO₃ at 25℃, 1.92% with 3 M HNO₃ at 25℃, and 5.65% with 3 M HNO₃ at 80℃, compared with the fresh CMPO/SiO₂-P, which was regarded as the most potential solid-phase extractant for utilizing in the field of nuclear fuel reprocessing. The corresponding percentages reduced by 3.27, 6.54, and 66.93% for treated TODGA/SiO₂-P with 0.01 M HNO₃ at 25℃, 3 M HNO₃ at 25℃, and 3 M HNO₃ at 80℃, respectively, compared with fresh TODGA/SiO₂-P. Therefore, the resistance properties of HDEHP/SiO₂-P against nitric acid and temperature were similar with that of CMPO/SiO₂-P and were more superior to that of TODGA/SiO₂-P. The effect of temperature on the stability of the HDEHP/SiO₂-P resin was slightly greater than that of the HNO₃ concentration.

Figure 8.

The adsorption properties of Gd(III) onto the HDEHP/SiO₂-P resin treated with different concentration of HNO₃ solutions and different temperatures ([Gd(III)] = 10 mmol l⁻¹, [HNO₃] = 0.1 M, phase ratio: 0.1 g/5 ml, and 120 r/min). HDEHP: bis(2-ethylhexyl)phosphoric acid.

On the other hand, the leakage from HDEHP/SiO₂-P in HNO₃ solutions was analyzed by testing the TOC and P content in the filtering liquor and washing solution, and the corresponding results were listed in Table 3. The total leakage results had no visible changes under treatment with different conditions. While the leakage percentage of 4.10% under treatment with 60℃ in 3 M HNO₃ solutions was inconsistent with the descending percentage of equilibrium adsorption amount of Gd(III), which was reduced by 8.41%. This deviation might be caused by decomposition of HDEHP molecules at the high temperature acidic solution while the fragments were still immobilized into SiO₂-P materials; however, those fragments had lost the adsorption capacity for Gd(III).

Table 3.

The contents of P, C, and HDEHP leaked from the HDEHP/SiO₂-P adsorbent after being shaken for one month on different experiment conditions.

Solution	P (ppm)	C (ppm)	HDEHP (ppm)	Leakage ratio (%) (based on P)
40 ml of 0.1 M HNO₃ + 2.0 g HDEHP/SiO₂-P, 25℃
Filtering liquor	36.82	96.80	395.21 (based on P)	4.25
Washing solution	23.47	79.71	251.92 (based on P)	4.25
40 ml of 3 M HNO₃ + 2.0 g HDEHP/SiO₂-P, 25℃
Filtering liquor	39.88	102.10	428.06 (based on P)	3.86
Washing solution	16.01	59.22	171.85 (based on P)	3.86
40 ml of 3 M HNO₃ + 2.0 g HDEHP/SiO₂-P, 60℃
Filtering liquor	45.00	131.90	483.01 (based on P)	4.10
Washing solution	14.69	55.04	157.68 (based on P)	4.10

HDEHP: bis(2-ethylhexyl)phosphoric acid.

Conclusion

The present study investigated the application of HDEHP loaded on macroporous silica-based polymeric adsorbent for adsorption and removal of REEs from nitric acid solutions by the solid-phase extraction. The obtained results demonstrated that HDEHP/SiO₂-P had better adsorption performance for the heavy REEs than the light REEs. The adsorption isotherms of Gd(III) showed that the uptake capacities had no evident difference within the temperature range from 298 to 323 K. The Langmuir and Freundlich isotherm models were both suitable for fitting the experimental data of Gd(III) adsorption. The obtained thermodynamic parameters indicated that the adsorption process of Gd(III) by HDEHP/SiO₂-P was a spontaneous and exothermic reaction. Gd(III) desorption from loaded HDEHP/SiO₂-P was easy with moderate conditions. The equilibrium adsorption amount of Gd(III) was reduced by 8.41% after resin treatment at 60℃, being at such high temperature in high acidic solution for a relatively long period, indicating that the HDEHP/SiO₂-P adsorbent had satisfactory resistance against nitric acid at high temperature. Finally, it was concluded that HDEHP/SiO₂-P can be potentially used for recovery of REEs from nitric acid medium.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant No. 91126006 and Grant No. 11305102) and Major Science and Technology Program for Water Pollution Control and Treatment (Grant No. 2015ZX07406006-06).

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Studies on adsorption of rare earth elements from nitric acid solution with macroporous silica-based bis(2-ethylhexyl)phosphoric acid impregnated polymeric adsorbent

Abstract

Keywords

Introduction

Experimental

Materials and methods

Measurements

Batch adsorption and desorption experiment

Evaluation on chemical stability of HDEHP/SiO2-P

Results and discussion

Effect of HNO3 concentrations on extraction

Adsorption isotherms

Thermodynamics studies

FT-IR analysis

Desorption characteristic

Chemical stability of HDEHP/SiO2-P

Conclusion

Footnotes

Declaration of Conflicting Interests

Funding

References

Evaluation on chemical stability of HDEHP/SiO₂-P

Effect of HNO₃ concentrations on extraction

Chemical stability of HDEHP/SiO₂-P