Temperature effected sorption of europium(III)

Journal of Radioanalytical and Nuclear Chemistry, Vol. 267, No.1 (2006) 147–153

Temperature effected sorption of europium(III) onto 1-(2-pyridylazo)-2-naphthol impregnated polyurethane foam M. M. Saeed,* R. Ahmed Nuclear Chemistry Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan (Received May 10, 2005)

The sorption of Eu(III) ions onto PAN loaded PUF has been optimized and investigated under the influence of various temperatures, i.e., 303, 313 and 323 K. Maximum retention (>96%) of Eu(III) ions (1.79.10–5M) onto PAN loaded PUF (7.75 mg.ml–1) was achieved after 30-minute equilibration time from pH 7 buffer solution. The variation of sorption with temperature yields the thermodynamic parameters ∆H = 79±2 kJ.mol–1, ∆S = 276±7 kJ.mol–1.K–1 and ∆G = –1.4±0.1 kJ.mol–1 at 298 K. The positive value of enthalpy and negative free energy show endothermic and spontaneous nature of sorption, respectively. The sorption data followed Freundlich, Dubinin-Radushkevich (D-R) and Langmuir isotherms at all the three given temperatures. The Freundlich constant 1/n = 0.70, 0.62 and 0.55 and sorption capacities Cm = 10.8 mmol.g–1, 6.1 mmol.g–1 and 4.4 mmol.g–1, respectively, decreased with increasing temperature. Similarly, the sorption capacities of D-R isotherm Xm = 197.6 µmol.g–1, 201.2 µmol.g–1 and 137.4 µmol.g–1, also decreased with temperature. However, the sorption free energy E = 10.2 kJ.mol–1, 11.2 kJ.mol–1 and 12.7 kJ.mol–1, increased with temperature. The monolayer coverage (Q) computed from Langmuir isotherm was 11.1±0.6 µmol.g–1 and remains constant at all the three temperatures investigated. However, the binding energy constant b increased with temperature. The relationship of b with temperature and differential heat of adsorption (∆Hdiff) have been evaluated and discussed.

Introduction With the increasing demand for rare earth metals and their compounds, the purification and separation of these elements have drawn extensive attention in recent years. The various techniques used for the separation and preconcentration of metal ions from aqueous media are the adsorption, filtration, chemical precipitation, reverse osmosis, solvent extraction, and membrane technology. Adsorption is supposed to be superior than all of them due to simple operation, high efficiency, and low cost. Polyurethane foam (PUF) is a cheaper, and well known sorbent used for the separation and preconcentration of metal ions from different aqueous conditions. It was reviewed in the literature.1,2 The sorption properties of PUF can be changed with impregnation of different foreign organic reagents. In previous studies the sorption of Hg(II),3 and Eu(III) and Tm(III) on pyridylazo resorcinol (PAR),4 Tb(III) on thenoyltrifluoroacetone (HTTA)5 indicates that the sorption behavior strictly follows the classical adsorption isotherms, i.e., Freundlich, Langmuir and Dubinin-Radushkevich. The nitrogen atoms of isocyanate and oxygen atom of ether groups of PUF are the strong active sites and play an active role in chemisorption process. PAN is a strong complexing reagent and reacts with many metal ions under different experimental condition.6 It is widely used in solvent extraction for the separation of rare earths,7,8 as loaded resin for heavy metals,9 modified naphthalene for rare earths,10 etc. PAN loaded PUF is also used to preconcentrate Co, Fe, Mn and Cd ions11,12 from aqueous solution, silica gel

loaded for rare earths13 before instrumental analysis. The sorption profile of Eu(III) ions on HTTA loaded PUF has been reported by one of the authors.14 In this study, the sorption behavior of Eu(III) on PAN loaded PUF at different temperature have been investigated. The variation of different adsorption isotherms with temperature and their constants are reported. The relationship of adsorption coefficient of Langmuir isotherm with temperature and thermodynamic parameters are investigated and discussed. Experimental Reagents All reagents used were of Analar grade. Buffer solutions of pH l–10 having ionic strength of 0.1M were prepared by mixing an appropriate volume of 0.2M solutions of HCl and KCl, CH3COOH and CH3COONa and H3BO3 and NaOH for buffers solutions of pH 1–2, 3–6 and 7–10, respectively. The stability of buffer solutions was checked by Metrohm 632 pH meter, periodically. Radiotracer The radiotracer of 152+154Eu used was prepared by irradiating specpure europium oxide (Eu2O3) in a 10 MW swimming pool-type research reactor (PARR-1) of this Institute at a neutron flux of 5.1013 n.cm–2.s–1. The irradiated metal oxide was dissolved in concentrated nitric acid, heated to dryness and diluted up to 25 ml with deionized water. The purity of the tracer was checked on a 4K series of 85 Canberra multichannel analyzer coupled to a 25 cm3 Ge(Li) detector.

* E-mail: [email protected] 0236–5731/USD 20.00 © 2006 Akadémiai Kiadó, Budapest

Akadémiai Kiadó, Budapest Springer, Dordrecht

M. M. SAEED, R. AHMED: TEMPERATURE EFFECTED SORPTION OF EUROPIUM(III)

Preparation of the polyurethane foam

%Adsorption =

The foam plugs were prepared and washed as described in Reference 14. The washed foam plugs were soaked in 0.1% PAN solution in acetone for 30 minutes. The excess amount of acetone was drained out and the PUF plugs were pressed between filter paper to remove the excess acetone. After that, the PAN loaded plugs were dried in oven at 50 °C and were kept in airtight plastic bottle for further studies.

100 K d Kd + V /W

(3)

All the experiments were performed at 25±2 °C or at temperature specified otherwise. The results are the average of at least triplicate independent measurements. The precision were in most cases ±2%. The linear regression and slope analyses for the statistical data were carried out. Results and discussion

PAN concentration One plug of PAN loaded foam was soaked in 25 ml acetone to dissolve the loaded PAN. The absorbance was measured by Hitachi 220S spectrophotometer at λmax = 462 nm against acetone. The loaded concentration of PAN on foam was found to be 5.38 mmol.g–1 against standard calibration curve of PAN in acetone. Procedure Five ml buffer solution of known pH was taken in a glass culture tube with a polyethylene cap. A known concentration of Eu(III) ions were added and mixed thoroughly. An aliquot of one ml was taken out for gross gamma-counts (A0). The remaining solution was shaken with PAN loaded PUF for 30 minutes on a Stuart Scientific wrist-action shaker. After shaking, one ml (Ae) of aliquot was withdrawn for assay. For temperature studies, the solution was taken in a culture tube and dipped into water bath at constant temperature (Gallenkamp thermostirrer-100 Model BKL 234) for at least 30 minutes to attain the required temperature. The culture tube was so adjusted in the water bath that about 70% of the total length of the culture tube remained immersed in the water bath for the optimum control of temperature. The sorbed concentration of Eu(III) at equilibrium was calculated by the difference in the amount of europium in aliquot drawn before (A0) and after shaking (Ae). The percentage sorption and distribution coefficient (Kd) were calculated as: %Adsorption =

A0 − Ae × 100 A0

Amount of metal in PUF × Amount of metal in solution Volume of solution (V ) × ml ⋅ g −1 Weight of dry PUF (W )

(1)

Kd =

Effect of the shaking time Maximum sorption can be attained by the effective distribution of sorbate between the sorbent and electrolyte. The influence of agitation time on the sorption of Eu(III) ions (1.97.10−5M) onto PAN loaded PUF from pH 7 buffer solution was studied between 1 and 120 minutes. It is seen in Fig. 2 that the sorption increased up to 30-minute shaking time and then remained constant. Therefore, 30-minute shaking time was used for further experiments. Influence of the amount of sorbent

(2)

The % sorption and the distribution coefficient can be correlated as:

148

The sorption of 1.97.10−5M solution of Eu(III) ions onto 7.75 mg.ml–1 of PAN loaded PUF in 30 minutes was studied in buffer solutions of pH 1–10. The results are presented in Fig. 1. No appreciable sorption was recorded up to pH 4. After that, the sorption of Eu(III) ions increased with increasing pH of buffer solutions up to 6 and there was an abrupt increase in sorption at pH 7 and remained constant up to pH 10. The Kd values were also increased with increasing pH. The abrupt change in sorption at pH 7 is due to the ionization of PAN in complexing with europium and PUF. Depending on the pH of the medium, PAN exists in different monomeric species.6 The yellow cations species (HPAN+) exists at lower pH (>3), while neutral form (PAN), insoluble in aqueous solution and responsible for complex formation with metal ions exists in the pH range of 4–10. The anionic species (PAN–) which is red in color, soluble in aqueous solution exists at pH>12.10 The neutral form of PAN dissociates its hydrogen atom of hydroxyl group with pH and form stable complex with Eu(III) metal ions at pH 7. The pH 7 was selected as an appropriate medium for the sorption of Eu(III) ions onto PAN loaded PUF for further studies.

The amount of sorbent effects the sorption. Therefore, sorption of Eu(III) ions (1.97.10−5M) onto PAN loaded PUF was investigated as a function of solid/solution ratio in the range of 2 to 17 mg.ml–1 using


30-minute shaking time. The results presented in Fig. 3 indicates that the percentage sorption increased up to 7.75 mg.ml–1 of PAN loaded PUF and then attained almost constant value. Therefore, 7.75 mg.ml–1 of PAN loaded PUF was the optimum quantity kept constant in further experiments for the removal of europium ions from aqueous solution.

Effect of the temperature The influence of temperature on the sorption of Eu(III) ions (1.97.10−5M) onto PAN loaded PUF was investigated. The increase in sorption with temperature might be due to the acceleration of sorption steps or the creation of some new active sites or due to transport against a concentration gradient, and/or diffusion across the energy barrier.15 The thermodynamic parameters, enthalpy (∆H) and entropy (∆S) were computed from: log K c =

−∆H ∆S + 2.303RT 2.303R

(4)

The plot of log Kc against 1/T is a straight line with a correlation coefficient of r = 0.9975, shown in Fig. 4. The equilibrium constant: Kc = F/(1–F) Fig. 1. Effect of pH on the sorption of 1.97.10−5M solution of Eu(III) ions onto 7.75 mg.ml−1 of PAN loaded PUF in 30 minutes

where F is a fraction metal ions sorbed at equilibrium, K is the temperature in Kelvin and R is the gas constant. The computed values of ∆H and ∆S from the slope and intercept of the plot are 79±2 kJ.mol−1 and 276±7 kJ.mol–1.K−1, respectively. The positive value of enthalpy represents the endothermic behavior of sorption and the large increase in the entropy shows that the stability of the sorbed Eu(III)-PAN complex on the PUF is entropy driven and favors the chemisorption by involving the rupture or creation of new bonds at the surface of the adsorbent. Moreover, the hydration zone formed around the central metal atom in mother liquid is disrupted to a great extent that results in a net endothermic enthalpy and positive entropy effect.16 The Gibb’s free energy (∆G) has been evaluated by: ∆G = −RT ln Kc

Fig. 2. Effect of shaking time for the sorption of 1.97.10−5M solution of Eu(III) ions from pH 7 buffer solution onto 7.75 mg.ml−1 of PAN loaded PUF

(5)

The numerical value of ∆G is found to be –1.4±0.1 kJ.mol−1 at 298 K. The negative value indicates that the sorption process is spontaneous in nature. However, the numerical value of T∆S term in the equation: ∆G = ∆H−T∆S (6) at 298 K is –80.2 kJ.mol−1. The term −T∆S>∆H correspond to a net gain in the degree of freedom of the sorbed complex that favors positive enthalpy.17 Effect of the amount of sorbate

Fig. 3. Effect of weight of PAN loaded PUF (sorbent) for the sorption of 1.97.10−5M solution of Eu(III) ions from pH 7 buffer solution

The sorption of Eu(III) ions at the initial concentration of metal ions onto PAN loaded PUF was investigated in the range of 1.97.10−6−1.18.10−4M using 30-minute shaking time and 7.75 mg.ml−1 of PAN loaded PUF at three different temperatures, at 303, 313 and 323 K. The results are presented in Fig. 5.

149


It is clear from the figure that the sorption in all the cases decreased with the increase in the europium concentration. This decrease is gradual till 7.9.10−5M and beyond that an abrupt decrease occurred that might be due to the decrease of the available sites for bonding. However, the distribution coefficient (Kd) values are decreased accordingly. The variation of sorption with the concentration of sorbate is subjected to different adsorption isotherms namely Freundlich, Dubinin-Radushkevich (D-R) and Langmuir. All these isotherms are obeyed the data very well for all three temperatures in the entire concentration range investigated. The Freundlich isotherm is empirical and gives some information on the surface heterogeneity and exponential distribution of the energy sites of the adsorbent.18 The Freundlich sorption isotherm was tested by the linear form: log Cads = log Cm +

1 log Ce n

1 − 2β

(10)

The corresponding values of β, Xm and E computed for Eu(III) ions sorbed onto PAN loaded PUF are listed in Table 1. The D-R isotherm is powerful and simple in concept and applications and can be employed from trace to high metal ion concentrations. Polanyi potential, ε is the work required to remove a molecule or ion to infinity from its location in the “sorption space”. It postulates a fixed volume or “sorption space” close to the sorbent surface where sorption takes place. It envisages heterogeneity of sorption energies within the sorbent surface that is independent of temperature.

(7)

where Cads is the concentration of Eu(III) on PAN loaded PUF in mol.g−1 and Ce is the concentration of Eu(III) in solution in mol.l−1 at equilibrium, Cm and 1/n are Freundlich constants indicating maximum sorption capacity and intensity of the sorption. The plot of log Cads versus log Ce for all three temperatures gave straight lines as shown in Fig. 6. The numerical values of 1/n and Cm were calculated from the slope and intercept of the line and are summarized in Table 1. The characteristic Freundlich constant 1/n, less than unity indicates the surface heterogeneity of the sorbent. The higher fractional value of 1/n describes higher heterogeneous nature of the sorbent surface and vice versa.19 The sorption data of Eu(III) ions sorbed onto PAN loaded PUF at different temperatures was also subjected to Dubinin-Radushkevich (D-R) isotherm,20 in the following form: ln Cads = ln Xm–βε2

E=

Fig. 4. Variation of sorption equilibrium of 1.97.10−5M solution of Eu(III) ions from pH 7 buffer solution on 7.75 mg.ml−1 of PAN loaded PUF with temperature

(8)

where Xm is the maximum sorption capacity at sorbent surface, β is a constant related to energy, and ε is the Polanyi potential which is equal to:

ε = RT ln 1 +

1 Ce

(9)

where R is the universal gas constant in kJ.mol−1, T is the temperature in Kelvin, Cads and Ce are described previously. The plot of ln Cads against ε2 are straight lines as shown in Fig. 7. The slope of the linear plot gives the value of β and intercept yields the value of maximum sorption capacity, Xm, at different temperatures. The value of β can be correlated to sorption energy (E) by:21 150

Fig. 5. Variation of percentage sorption and distribution coefficient (Kd) with initial Eu(III) metal ion concentration onto PAN loaded PUF


Table 1. Freundlich, D-R, and Langmuir isotherms constants of Eu(III) at different temperatures Constant Freundlich isotherm 1/n Cm, mmol.g−1 r D-R isotherm β, kJ2.mol−2 Xm, µmol.g−1 E, kJ.mol−1 r Langmuir isotherm Q, µmol.g−1 b ×104, l.mol−1 r

303 K

Values 313 K

323 K

0.70 ± 0.03 10.8 ± 0.7 0.9924

0.62 ± 0.04 6.1 ± 0.6 0.9804

0.55 ± 0.03 4.4 ± 0.2 0.9921

−0.0048 ± 0.0005 197.6 ± 34.7 10.2 ± 0.5 0.9911

−0.0040 ± 0.0002 201.2 ± 32.9 11.2 ± 0.2 0.9908

−0.0031 ± 0.0002 137.4 ± 17.7 12.7 ± 0.2 0.9939

11.1 ± 0.6 5.7 ± 0.1 0.9887

11.3 ± 0.8 10.7 ± 0.6 0.977

11.1 ± 0.4 20.5 ± 1.9 0.9918

The Langmuir22 isotherm envisages that ions are sorbed on definite sites that are monoenergetic and each site can accommodate only to one molecule or ion. The sorbed ions cannot migrate across the surface or interact with neighboring molecules. The Langmuir sorption isotherm is tested in the following linearized form: Ce C 1 = e+ Cads Q Qb

(11)

where Ce and Cads are described previously, Q and b are characteristic constants of Langmuir isotherm related to the monolayer sorption coverage and binding energy of sorption, respectively. When Ce/Cads is plotted against Ce for Eu(III) ions at different temperatures, straight lines are obtained as presented in Fig. 8. The numerical values of Q and b are evaluated from the slope and intercept and are given in Table 1. Table 1 clearly indicates that the sorption capacities Cm and Xm derived from Freundlich and D-R isotherms decreases with temperature while the sorption energy, E, from D-R isotherm increases with temperature. The plausible explanation of this event is based on the multilayer sorption phenomena at lower temperature that have a low sorption energy which is converted to submonolayer coverage of the sorbent at high temperature. The Freundlich constant 1/n is temperature dependent and its decrease with temperature indicates more specific surface heterogeneity of the sorbent at high temperature and supports the endothermic chemisorption. The Langmuir isotherm based on mono-layer

coverage of the sorbent indicates that the sorption capacity Q is almost constant at all temperatures and has much lower values than the sorption capacities Cm and Xm evaluated from Freundlich and D-R isotherms, respectively. The binding energy related to the heat of sorption, b, of Langmuir isotherm increases with temperature similarly to the sorption free energy, E, of D-R isotherm indicates the stability of the sorbed complex at high temperature and endothermic chemisorption.

Fig. 6. Freundlich isotherm plot of Eu(III) ions on 7.75 mg.ml−1 of PAN loaded PUF at different temperatures from pH 7 buffer solution

151


The linear relationship of adsorption coefficient, b, with temperature is related to the differential enthalpy of adsorption (∆Hdiff) in the form of:19,23 ln b = ln b'+

Fig. 7. Dubinin-Radushkevich isotherm plot of Eu(III) ions on 7.75 mg.ml–1 of PAN loaded PUF at different temperatures from pH 7 buffer solution

−∆H diff RT

(12)

where R is the gas constant, K is the temperature in Kelvin and b' is the constant related to the entropy. The plot of lnb' vs. 1/T is a straight line as shown in Fig. 9. The numerical value of enthalpy (∆Hdiff) was calculated from the slope of graph and found to be 54 kJ.mol–1. The positive value indicates the endothermic chemisorption. However, the equilibrium enthalpy (∆H) is much higher than the differential enthalpy (∆Hdiff).24 The magnitude and sign of the enthalpy change (∆H) associated with the adsorption process in aqueous solution comprise:16,25 ∆H = ∆Hde + ∆Hc + ∆Hads The stepwise enthalpy changes are defined as: (1) ∆Hde is the enthalpy change of dehydration which will cause ∆H to be endothermic because energy is required to break the ion–water and water–water bonding of hydrated metal ions; (2) ∆Hc is the enthalpy change for complexation that will make ∆H more endothermic due to formation of metal complex; and (3) ∆Hads is the enthalpy change due to sorption of metal complexes and new bond formation at the sorbent surface in chemisorption and thus makes ∆H more positive. The differential enthalpy (∆Hdiff) change occurs at the surface of the sorbent and comprises: ∆Hdiff = ∆Hc + ∆Hads

Fig. 8. Langmuir isotherm plot of Eu(III) ions on 7.75 mg.ml−1 of PAN loaded PUF at different temperatures from pH 7 buffer solution

The difference in the enthalpy (25 kJ.mol–1) can be attributed to the dehydration sub-reaction of the metal ions in the sorption process, which is the slowest step and is more significant in endothermic chemisorption.

Conclusions

Fig. 9. Plot of adsorption coefficient, b, of Langmuir isotherm with temperature

152

The PAN loaded PUF can remove/preconcentrate Eu(III) ions from aqueous solutions present at trace or subtrace levels quantitatively, in a single stage operation from a solution of pH 7. The thermodynamic parameters ∆H, ∆S and ∆G worked out for Eu(III) ions indicate endothermic and spontaneous nature of sorption. The sorption data obey Langmuir, Freundlich, and DubininRadushkevich (D-R) isotherms. The sorption capacity and energy have been evaluated from the characteristic constants of Eu(III) metal ions at different temperature. The monolayer coverage of the sorbent evaluated from Langmuir isotherm is constant but the adsorption


coefficient b increases with temperature envisaging the endothermic chemisorption. The sorption capacities of Freundlich, and D-R isotherms are converted from multi-layer to sub-monolayer. The increase of surface heterogeneity of the sorbent and sorption free energy of the metal ions with temperature reflects the stability Eu(III)-PAN complex on PUF.

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