Evaluation of D113 cation exchange resin for the removal of Eu(III) from aqueous solution

JOURNAL OF RARE EARTHS, Vol. 28, No. 6, Dec. 2010, p. 862

Evaluation of D113 cation exchange resin for the removal of Eu(III) from aqueous solution XIONG Chunhua (熊春华), ZHENG Zhanwang (郑展望) (Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310012, China) Received 22 October 2010; revised 11 November 2010

Abstract: Batch adsorption experiments were conducted for the adsorption of Eu(III) ions from aqueous solution by D113 resin. The results indicated that D113 resin could adsorb Eu(III) ion effectively from aqueous solution. The adsorption was strongly dependent on pH of the medium with enhanced adsorption as the pH turned from 3.50 to 7.00 and the optimal adsorption condition was in HAc-NaAc medium with pH value of 6.50. The maximum uptake capacity of Eu(III) ions was 290.9 mg/g D113 at 298 K, at an initial pH value of 6.50. The overall adsorption process was best described by Lagergren-first-order kinetics. When Freundlich and Langmuir isotherms were tested, the latter had a better fit with the experimental data. The thermodynamic parameters such as free energy (ΔG) which were all negative, indicated that the adsorption of Eu(III) ions onto D113 resin was spontaneous and the positive value of enthalpy (ΔH) showed that the adsorption was endothermic in nature. Thomas model was applied to experimental column data to determine the characteristic parameters of column useful for process design. Furthermore, Eu(III) could be eluted by using 3.0 mol/L HCl solution and the D113 resin could be regenerated and reused. Keywords: D113 resin; europium(III); adsorption; kinetics; thermodynamic; rare earths

The operation of nuclear power plants, research facilities and the use of radioisotopes in industry and diagnostic medicine produce a wide variety of radioactive wastes. Many of these wastes have to be treated in order to reduce the radionuclide levels to those acceptable for discharge into the environment[1,2]. The safety of nuclear waste repository implies the radionuclides using a methodology that does not allow any release to the environment or biosphere. Therefore, the adsorption of radionuclides, especially the long-life radionuclides, especially the lanthanides and actinides, at the solid-water interface is important for the performance assessment of a nuclear waste repository. Europium is usually taken as a homologue for trivalent actinides because the ionic radii of Eu(III) is almost the same for all the trivalent lanthanides and actinides, which results in a similar physicochemical behavior of Eu(III) with other trivalent lanthanides and actinides. Precipitation, filtration, solvent extraction, ion-exchange and solid phase extraction are conventional processes used for treatment[3,4]. Solvent extraction and ion-exchange are the two most common methodologies for the preconcentration and separation of trace elements from various matrices. Solvent extraction is inefficient due to the requirement of large volume of solvent, which may create health problem. Various adsorbents including TiO2[5], N,N,N’,N’-tetraoctyl diglycolamide[6], zeolites[7], chelating resins[8], hydrous metal-oxides[9], montmorillonite[10] and extraction chromatographic resin[11] were used in extraction of europium ions

from various media. Chelating resins as adsorbent is with good features of easy-functional and chemical stability, as well as shortcomings in the performance of its poor hydrophilicity, slow adsorption rate and bad elution. Cation exchange resins are solid and suitably insolubilized high molecular weight polyelectrolytes which can exchange their mobile ions for ions of equal charge from the surrounding medium. D113 resin is a polymeric material containing a functional group (–COOH). It has not only proton that can exchange with cation, but also oxygen atom that can coordinate directly with metal ions. The extraction studies for traces metal ions on macroporous polyacrylic resins have been performed by many researchers. The present study was performed to evaluated D113 resin as adsorbents for removal of Eu(III) from aqueous solution by systematic evaluation of the parameters involved such as pH of solution, Eu(III) concentration and temperature. Furthermore, the Langmuir and Freundlich adsorption isotherms and Langmuir-first-order and pseudo-second-order reaction were applied to calculate isotherm parameters and to study the kinetics of adsorption. The kinetic and equilibrium data for the adsorption studies were processed to understand the adsorption mechanism. The sample was also characterized with IR spectroscopy.

1 Experimental

Foundation item: Project supported by the Key (Key grant) Project of Education Department of Zhejiang Province (Z200907459) Corresponding author: XIONG Chunhua (E-mail: [email protected]; Tel.: +86-571-88932083) DOI: 10.1016/S1002-0721(09)60231-3

XIONG Chunhua et al., Evaluation of D113 cation exchange resin for the removal of Eu(III) from aqueous solution

1.1 Materials and instruments D113 resin was provided by Nankai University, activated before use. Its properties are given in Table 1. The stock solutions of Eu(III) was prepared from Eu2O3 (99.99%). HAc-NaAc buffer solution with pH=3.50–7.00 and C6H15O3N-HNO3 buffer solutions with pH 7.20 were prepared from HAc, NaAc, C6H15O3N and HNO3 solutions. The chromophoric reagent of 0.1% arsenazo-I solution was obtained by dissolving 0.1000 g arsenazo-I into 100 ml purified water. All other chemicals were of reagent grade. Eu(III) was determined with a Shimadzu UV-2550 UV-VIS spectrophotometer. A mettler toledo delta 320 pH meter was used for measuring pH of solution. The sample was shaken in the DSHZ-300A temperature constant shaking machine.

name

Functional

Structure

group

D113 Macroporous –COOH resin

Exchange Moisture Wet super- True wet capacity/

content/

ficial den-

density/

(mmol/g) %

sity/(g/ml)

(g/ml)

≥10.8

0.74–0.80

1.15–1.20

45–52

1.2 Analytical method[12] A solution containing lower than 75 μg of Eu(III) was accurately added into a 25 ml colorimetric tube, and then 1 ml 0.1% arsenazo-I solution and 10 ml pH 7.20 C6H15O3NHNO3 buffer solutions were added. After the addition of deionized water to the mark of colorimetric tube, the absorbency was determined in a 1 cm colorimetric vessel at wavelength of 574 nm and compared with blank test. The adsorption capacity (Q) of Eu(III) ions was calculated with the following formula: Q=

Co − Ce W

V

The flask was shaken in a shaker at constant temperature. The upper layer of clear solution was taken for analysis until adsorption equilibrium was reached. Continuous flow adsorption experiments were conducted in a vertical glass column of 0.45 cm inner diameter and 23.5 cm height filled with Eu(III) ion solution. The Eu(III) ion solution was fed from the top at a fixed flow rate. The Eu(III) ion solutions at the outlet of the column were collected periodically and analyzed for the Eu(III) ion concentration using a UV-visible spectrophotometer (Shimadzu UV-2550) at 574 nm. The flow through the column was continued till the outlet and inlet concentrations were equal. All the experiments were carried out at room temperature.

2 Results and discussions

Table 1 Properties of D113 resin Trade

863

(1)

The distribution coefficient (D) of Eu(III) ions between the aqueous phase and the solid phase can be directly obtained using: C − Ce V D= o × (2) Ce W where Co (mg/mL) and Ce (mg/mL) are the initial and equilibrium Eu(III) concentrations, respectively, V/W is the ratio of the volume of metal solution (ml) to the amount of D113 resin (g) in a batch.

2.1 Effect of pH The pH of aqueous solution was an important parameter that controlled the adsorption process[13]. The Eu(III) ions were examined within a range of pH 3.50–7.00. Fig. 1 shows the influence of pH in adsorption process of Eu(III) onto D131 resin. It can be seen that the adsorption capacity for Eu(III) was the highest when pH value was 6.50 in the HAcNaAc medium. The adsorption of Eu(III) ions increases quickly at pH<6.50. Then remains level with increasing pH values at pH>6.50. This adsorption trend is likely to be ascribed to the effect of competitive binding between Eu(III) and hydrogen ions for the binding sides on the surface of the resins. At low pH, an excess of hydrogen ions compete effectively with Eu(III) for bonding sites, resulting in a lower recovery. The percentage of ion exchange decreased when the pH was increased above 6.50 owing to the formation of Eu(III) precipitation at higher pH values[14] . 2.2 Adsorption kinetic model The time-dependent adsorption behaviors of Eu(III) were studied with various contact time between resin and Eu(III). Then, the data were used for kinetics study. Several kinetic models are available to examine the controlling mechanism of adsorption from a liquid phase on D113 resin and to interpret the experimental data obtained. The kinetics of adsorption can be described by the Lagergren-first-order rate expression[15], which is given by Eq. (3):

1.3 Adsorption experiments Experiments were run in a certain range of pH, temperature, initial Eu(III) ion concentrations, contact time as well as adsorption isotherms. The operation for the adsorption and desorption of Eu(III) ion is usually carried out in batch vessels and glass columns. Batch experiments were performed under kinetic and equilibrium conditions. A desired amount of treated D113 resin was weighed and added into a conical flask, in which a desired volume of buffer solution was added. After 24 h, a required amount of standard solution of Eu(III) was put in.

Fig. 1 Influence of pH on the distribution coefficient of Eu(III) (Resin 15.0 mg, C0=5.0 mg/30.0 ml, T=298 K, 100 r/min)

864

JOURNAL OF RARE EARTHS, Vol. 28, No. 6, Dec. 2010

lg(Qe − Qt ) = lg Q1 −

k1

(3) t 2.303 The pseudo-second-order kinetic model equation[16] is given as: t Qt

=

1 2

k 2 Q2

+

t

(4)

Q2

where Qe and Qt are the amounts of Eu(III) ions adsorbed on the adsorbent at equilibrium and at various time t (mg/g) respectively; Q1 and Q2 are the calculated adsorption capacity of the Lagergren-first-order model and the pseudo-second-order model (mg/g), respectively; k1 and k2 are the rate constant of the Lagergren-first-order model (h−1) and the pseudo-second-order model (g/(mg·h)). The fitting validity of these models is traditionally checked by the linear plots of lg(Qe−Qt) vs t, and t/Qt vs t, respectively. From the slope and intersection of the straight line obtained, the corresponding constant values for the Lagergren-first-order and pseudo-second-order kinetic models, for each temperature studied, provide the respective ki2 2 netic constants, k1, k2, R1 , R2 parameters (Table 2). The 2 correlation coefficient ( R1 ) for the Lagergren-first-order 2 equation was better than the correlation coefficient ( R2 ) for the pseudo-second-order equation. Thus, good linearity of the Lagergren-first-order plots was revealed that the interactions would follow the first order kinetics. It is shown that the Lagergren-first-order kinetic model can describe the Eu(III)/D113 resin adsorption system studied in our work[17]. 2.3 Adsorption isotherms

The Freundlich and Langmuir isotherm are the two most widely used mathematical description of adsorption, usually fits the experimental data over a wide range of concentration. The Freundlich isotherm gives an empirical expression encompassing the surface heterogeneity and the exponential distribution of active sites and their energies. Its mathematical formula is given as 1 lg Qe = lg K F + lg Ce (5) n Where KF is Freundlich constant and n (dimensionless) is the heterogeneity factor. The Freundlich adsorption isotherm represents the relationship between the corresponding adsorption capacity Qe (mg/g) and the concentration of the metal in solution at equilibrium Ce (mg/ml). The plot of lgQe vs lgCe for various initial concentrations was found to be linear in Fig. 2. The figures facilitate the determination of Freundlich constants 1/n and KF from the slopes and intercepts. The Freundlich constants n is a measure of the devia-

Fig. 2 Freundlich isotherms for Eu(III) on D113-III resin at different temperatures (Resin 15.0 mg, C0=4.0 mg/30 ml, 5.0 mg/30 ml, 6.0 mg/30 ml, 7.0 mg/30 ml, 8.0 mg/30 ml, 100 r/min)

tion from linearity of the adsorption. The numerical values of n at equilibrium lay between 3.9 and 4.5, indicating that Eu(III) ion was favorably adsorbed by D113 resin at all the studied temperatures. The Langmuir isotherm assumes that solid surface has a finite number of identical sites which are energetically uniform. According to this model, there is no interaction between adsorbed species, which means that the amount adsorbed has no influence on the rate of adsorption. A monolayer was formed when the equilibrium was attained. This model can be expressed as Ce C 1 = + e (6) Qe Qm K L Qm where Qe is the equilibrium Eu(III) ions concentration on the adsorbent (mg/g), Ce is the equilibrium Eu(III) ions concentration in solution (mg/ml), Qm is the monolayer capacity of the adsorbent (mg/g) and KL is the Langmuir constant and related to the free energy of adsorption. A graft of Ce/Qe versus Ce that gives a straight line with correlation coefficient (R2) of 0.9934 and 0.9996 at all experimental temperatures was plotted (Fig. 3). The slopes and intercepts of the graft were used to calculate the Qm and KL. The Langmuir and Freundlich parameters for the adsorption of Eu(III) ion onto D113 resin are listed in Table 3. It is evident that the adsorption of Eu(III) ion onto D113 resin is fitted better to the Langmuir isotherm model than that of the Freundlich isotherm models. The adsorption capacity increased with an increase in temperature.

Table 2 Comparison of Lagergren-first-order and pseudo-second-order kinetics models T/

Qe/

Lagergren-order-kinetic model

Second-order-kinetic model

K–1 (mg/g) Q1/(mg/g)

k1/min–1

R2

Q2/(mg/g)

k2/min–1 R2

288 226

224.1

0.0465

0.9846

322.6

0.0001

0.7503

298 267

260.9

0.0500

0.9780

416.7

0.0001

0.7110

308 282

270.7

0.0571

0.9812

344.8

0.0002

0.9301

Fig.3 Langmuir isotherms for Eu(III) on D113-III resin at different temperatures (Resin 15.0 mg, C0=4.0 mg/30 ml, 5.0 mg/30 ml, 6.0 mg/30 ml, 7.0 mg/30 ml, 8.0 mg/30 ml, 100 r/min)

XIONG Chunhua et al., Evaluation of D113 cation exchange resin for the removal of Eu(III) from aqueous solution Table 3 Isotherms parameters for the adsorption of Eu(III) ions by D113 resin Langmuir isotherm

T/K–1

Freundlich isotherm 2

Qmax/(mg/g)

KL/(ml/mg)

R

n

KF/(mg/g)

R2

288

303.0

41.3

0.9934

3.9

439.9

0.9268

298

333.3

60.0

0.9993

4.1

497.9

0.9446

308

357.1

70.0

0.9996

4.5

526.4

0.9391

2.4 Thermodynamic parameters

In any adsorption procedure, both energy and entropy considerations should be taken into account in order to determine which process will take place spontaneously. Values of thermodynamic parameters are the actual indicators for practical application of a process. The amounts of Eu(III) ions adsorbed at equilibrium at different temperatures, which are 288, 298 and 308 K, have been examined to obtain thermodynamic parameters for the adsorption system (Fig. 4). The free energy change ΔG, enthalpy change ΔH, and entropy change ΔS for the adsorption process can be calculated by the following equations: lgD=–ΔH/2.303RT+ΔS/2.303R (7) ΔG=ΔH–TΔS (8) where D is the distribution coefficient, R refers to gas constant, and T presents the absolute temperature. The plot of lgD versus 1/T gives the straight line from which ΔH and ΔS was calculated from the slope and intercept of the linearised form. Table 4 shows the values of thermodynamic parameters of Eu(III) ion adsorption on D113 resin. The negative value of ΔG confirms the spontaneity of the adsorption process with increasing temperature and the positive value of ΔH suggests that the adsorption is endothermic in nature. Although there are no certain criteria related to the ΔH values that define the adsorption type, the heat of adsorption values between 20.9–418.4 kJ/mol, which are heats of chemical reactions, are frequently assumed as the comparable values for the chemical adsorption process. In addition, the values of ΔS were found to be positive due to the exchange of the metal ions with more mobile ions present on the exchanger, which

Fig. 4 Plot of lgD vs 1/T (Resin 15.0 mg, C0=4 mg/30.0 ml, pH=6.50, 100 r/min) Table 4 Thermodynamic parameters for Eu(III) on D113 resin ΔH/ (kJ/mol) 25.5

ΔS/(J/(K·mol)) 149

ΔG/(kJ/mol) T=288 K

T=298 K

T=308 K

– 17.5

– 19.0

– 20.5

865

would cause increase in the entropy, during the adsorption process[18]. 2.5 Elution

15.0 mg D113 resin was added into a mixed solution composed of pH 6.50 buffer solution and desired amount of Eu(III) solution. After equilibrium was reached, the concentration of Eu(III) in the aqueous phase was determined, and the adsorption capacity of D113 resin for Eu(III) was obtained. Then, the D113 resin separated from aqueous phase was washed three times with pH 6.50 buffer solution. The D113 resin adsorbed Eu(III) was shaken with 30.0 ml HCl eluant. After equilibrium was reached, the concentration of Eu(III) in aqueous phase was determined and then the percentage of elution for Eu(III) was obtained. The results showed that the percentage of elution for Eu(III) was different when the concentration of HCl is changed (Table 5). It was evident from data that the maximum percentages of elution for Eu(III) was obtained by using the 3.0 mol/L HCl solution as an eluant. Table 5 Elution test of Eu(III) ions Concentration of HCl/(mol/L)

0.5

1.0

2.0

3.0

Elution percentage/%

88.4

89.3

98.9

100

2.6 Dynamic adsorption and desorption curves

2.6.1 Dynamic adsorption curve The performance of packed beds is described through the concept of the breakthrough curve. The breakthrough curve shows the loading behavior of Eu(III) ion to be adsorbed from solution in a fixed bed and is usually expressed in terms of adsorbed Eu(III) ion concentration (Cad=inlet Eu(III) ion concentration, C0=outlet Eu(III) ion concentration) or normalized concentration defined as the ratio of effluent Eu(III) ion concentration to inlet Eu(III) ion concentration (Ce/C0) as a function of time or volume of effluent for a given bed height. The area under the breakthrough curve obtained by integrating the adsorbed concentration (Cad; mg/ml) vs the throughput volume (V; ml) plot can be used to find the total adsorbed Er(III) ion quantity (the maximum column capacity). Total adsorbed Eu(III) ion quantity (Q; mg/g) in the column for a given feed concentration and flow rate is calculated from Eq. (9): v (C − C ) o e Q=∫ dV (9) 0 m where m(g) is the mass of the adsorbent. The capacity value Q was obtained by graphical integration as 255.4 mg/g. Successful design of a column adsorption process requires prediction of the concentration versus time profile or breakthrough curve for the effluent. The maximum sorption capacity of D113 resin is also in design. Traditionally, the Thomas model is used to fulfill the purpose. The model has the following form[19]: Ce 1 (10) = Co 1 + exp [ K T (Qm − CoV ) / θ ]

866

where KT (ml/(min·mg)) is the Thomas rate constant and θ (ml/min) is the volumetric flow rate. The linearized form of the Thomas model is as follows: C K Qm K T Co ln( o − 1) = T − V (11) Ce θ θ The kinetic coefficient KT and the adsorption capacity of the bed Q can be determined from a plot of ln[(C0/Ce)–1] vs t at a certain flow rate as shown in Fig. 5. The Thomas equation coefficients for Eu(III) ion adsorption were KT = 0.0168 ml/(min·mg) and Q= 255.4 mg/g. The theoretical predictions based on the model parameters were compared with the experimental data as shown in Fig. 5. The Thomas model was found in a relatively good fitness with breakthrough curves for adsorption of Eu(III) ion on D113 resin with the high R2 value (R2=0.9908), and the theoretical Q value was very close to the experimental one. Therefore, it can be concluded that the experimental data fitted well to the Thomas model, which indicated that the external and internal diffusion were not the limiting step. 2.6.2 Dynamic desorption curve Efficient elution of adsorbed solute from D113 resin in column is essential to ensure the reuse of D113 resin for repeated adsorption/desorption cycles. With respect to the stripping of Eu(III) ion from D113 resin, the 3.0 mol/L HCl eluant was employed. Desorption curve was plotted with the effluent concentration (Ce) vs elution volume (V) from the column at a certain flow rate. Fig. 7 revealed that the adsorption flow rate was less than previous research result so that the volume of elution was less, which was beneficial to the easy handling and high concentration for economical recovery of Eu(III) ion. It was observed that the total volume of eluent was 25 ml, after which further desorption was negligible. Therefore, the 3.0 mol/L HCl eluant was helpful in easy handling and remov-

Fig.5 Plot of ln(C0/Ce–1) vs t (Resin 150 mg, pH=6.50, C0=0.1 mg/ml, flow rate=0.109 ml/min)

JOURNAL OF RARE EARTHS, Vol. 28, No. 6, Dec. 2010

ing of Eu(III) ion. 2.7 Infrared spectra analysis[20]

Fig. 8 shows the FT-IR spectrum of D113 resin before and after the adsorption of Eu(III) ions. FTIR spectra of the adsorbents were recorded in the range of 400–4000 cm−1 to identify the possibility of Er(III) ion bonding to resin. According to the FT-IR spectrum, there are significant changes in the IR spectra of D113 resin before and after Eu(III) ion adsorption. It should be noticed that the bands at 3437 and 342 1 cm−1 are stretching vibrations of the hydroxyl groups, 171 2 and 154 9 cm−1 is an indication of C=O. It was found that the characteristic absorption peak of the bond C=O (1712 cm−1) disappeared after Eu(III) ion adsorption. The characteristic peak of the hydroxyl groups shifts from 343 7 cm−1 to 342 1 cm−1. These results show that there are coordination bonds between oxygen atoms and Eu(III) ion. The relevance explanation of a shift in the spectra, reduction and disappearance of the peaks were that there is an effect of metal adsorption on the functional groups.

Fig. 7 Dynamic desorption curve (Resin 150 mg, flow rate=0.125 ml/min)

Fig. 8 Infrared spectra of modified bamboo charcoal (1) After adsorption; (2) Before adsorption

3 Conclusions

Fig.6 Breakthrough curve for adsorption of Eu(III) (Resin 150 mg, pH=6.50, C0=0.1 mg/ml, flow rate=0.109 ml/min)

(1) The D113 resin containing carboxylic group had very good potential for utilization as an adsorbent for Eu(III) from aqueous medium. (2) Variables, such as pH, contact time, and temperature could affect the sorption behavior. The maximum Eu(III) recovery was obtained at pH 6.50, 298 K. (3) The kinetics of adsorption of Eu(III) on D113 resin was complexed, while the results were tested with models

XIONG Chunhua et al., Evaluation of D113 cation exchange resin for the removal of Eu(III) from aqueous solution

based on the Lagergren-first-order, and pseudo-second- order, close conformity could be obtained with Lagergrenfirst-order mechanism. (4) It is evident from the experimental data that the adsorption of Eu(III) ions fitted well to the Langmuir isotherm model than that of the Freundlich isotherm models and the adsorption coefficients agreed well with the conditions supporting favourable adsorption. (5) The adsorption process was endothermal and spontaneous at higher temperatures.

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