Article pubs.acs.org/IECR
Solvent Extraction Equilibrium of Zinc from Nitrate Solutions with 2‑Ethylhexylphosphonic Acid Mono-2-ethylhexyl Ester Chaitanya R. Adhikari, Hirokazu Narita, and Mikiya Tanaka* Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan S Supporting Information *
ABSTRACT: The solvent extraction equilibrium of zinc with 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA) in an alkane diluent, Shellsol D70, was studied at a constant ionic strength of 1 mol/L from nitrate solutions considering the correction of the extractant nonideality in the organic phase by Alstad’s method. On the basis of the slope analysis of the obtained data, a molar ratio of 1:2 for the zinc to the EHPNA dimer in the extracted complex was indicated. Apparent extraction equilibrium constants obtained at each temperature from 283 to 323 K showed the endothermic nature of the extraction reaction and gave the apparent standard enthalpy and entropy changes. These two constants enable the calculation of the zinc distribution ratio at each temperature which correlates the experimental data with good accuracy.
1. INTRODUCTION Solvent extraction has become one of the most important and well-established separation methods in hydrometallurgy. Nowadays, a very large number of extractants are available for solvent extraction, of which about a dozen reagents are in everyday use.1 Among the various extractants, acidic organophosphorus compounds such as bis(2-ethylhexyl)phosphoric acid (DEHPA), 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA), and bis(2,4,4-trimethylpentyl)phosphinic acid (BTMPPA) have attracted considerable interest from researchers due to their special ability to selectively extract some metal ions such as zinc,2−9 cobalt,7,10−12 and rare earth elements.13,14 Therefore, a large number of papers dealing with metal solvent extraction employing acidic organophosphorus compounds are available in the literature.2−28 We have developed a process for separating zinc from nickel using solvent extraction with EHPNA or BTMPPA with a view to solving the zinc accumulation/contamination problem faced by the electroless nickel plating industry.6 In order to control and improve the process efficiently, it is necessary to have fundamental data on the equilibria of zinc solvent extraction with EHPNA. A literature survey has revealed that there is a discrepancy in the compositions of the formed complexes in the low loading region: the reported complexes depend on the authors and are ZnL2·HL,5,16,18,19,23,24,27 ZnL2·2HL,3,4,15,20,21,26 or both,2,17,22 where HL represents the extractant monomer. One of the reasons for this discrepancy seems to lie in the nonideal behavior of acidic organophosphorus compounds in alkane diluents due to the weak solute−solute interactions.31,32 Danesi and Vandegrift32 pointed out that a deviation from ideal behavior pronounced with the DEHPA dimer concentration was the most probable reason for the different interpretations during the slope analysis. Alstad et al.31 presented an empirical method in order to correct this nonideality, and this method was used for the equilibrium analyses of some metal extraction systems with DEHPA.15,32 Recently, we have also reported that this method © 2012 American Chemical Society
is effective in order to analyze the equilibrium extraction of dysprosium with EHPNA.28 Thus, in the present paper, we study the zinc extraction from a nitrate medium of constant ionic strength with EHPNA dissolved in an alkane diluent by applying this method. The purpose of this is to determine the relevant apparent thermodynamic constants and thus to enable the quantitative calculation of the equilibrium zinc extraction behavior in the low loading region. Nitrate medium is selected as the initial step because of the relatively lower degree of complexation of zinc(II) ion with nitrate ion. The extraction was done at different pHs, EHPNA concentrations, and temperatures.
2. EXPERIMENTAL SECTION 2.1. Reagents and Chemicals. All chemicals used in this experiment were of reagent grade except for the extractant and the diluent. The extractant, EHPNA, was a Daihachi product, and the diluent, Shellsol D70, was a mixture of 60% alkanes and 40% cycloalkanes and was obtained from Shell Chemicals. Both were used without further purification. Zinc aqueous phases (1 mmol/L) were prepared by dissolving the required amount of zinc nitrate in 1 mol/L (H, Na)NO3 solutions. 2.2. Extraction Procedure. Equal volumes (15 mL) of the organic phase (0.04, 0.08, 0.16, 0.24, and 0.32 mol/L as EHPNA dimer) and the aqueous phase were mixed in a glassstoppered conical flask and horizontally shaken at 140 rpm for over 4 h in a water bath maintained at specified temperatures of 283, 293, 298, 303, 313, and 323 ± 0.1 K. On the basis of a preliminary experiment, it was confirmed that this shaking time was sufficient to ensure equilibration. The mixture was then centrifuged, and the organic phase was removed. The zinc concentration in the aqueous raffinate was measured by ICPReceived: Revised: Accepted: Published: 16433
June 17, 2012 November 10, 2012 November 24, 2012 November 25, 2012 dx.doi.org/10.1021/ie301589s | Ind. Eng. Chem. Res. 2012, 51, 16433−16437
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the Alstad constant, respectively. The A value for EHPNA in Shellsol D70 at 298 K was determined to be 0.473 (L/mol)1/2 by a previous study.28 The free EHPNA concentration, [H2L2], was regarded to be equal to the total EHPNA concentration, [H2L2]T, because the zinc concentration in the present experiment was much lower than that of the total EHPNA concentration. Although the previous researchers31,32 used the terms “activity” and “activity coefficient”, respectively, for [H2L2]* and y*, we avoid the term “activity” in this paper considering the awkward definition of the reference state (infinite dilution) due to the use of the industrial diluent and the presence of a small and saturated amount of water in the organic phase. The apparent extraction equilibrium constant, Kex, is therefore expressed as
AES (Horiba ULTIMA 2) after proper dilution. The pH of the aqueous phase was measured by a DKK-TOA pH meter (HM 60G model). The zinc concentration in the organic raffinate was calculated from the difference in the zinc concentrations in the aqueous phases before and after extraction. The distribution ratio of zinc, D, was calculated as D = [Zn]T /[Zn]T
(1)
where the overbar and the subscript “T” denote the organic phase and the total concentration, respectively. The quadruplicate runs under three different conditions at 298 K and EHPNA dimer concentration of 0.08 mol/L revealed that the standard deviations in log D were 0.078, 0.012, and 0.047 at log D of ca. −0.9, 0, and 1.4, respectively.
3. RESULTS AND DISCUSSION 3.1. Determination of the Stoichiometry of the Extraction. Figure 1 shows the log D values at 298 K as a
Kex =
[ZnL 2·2(n − 1)HL][H+]2 [Zn 2 +][H 2L 2]*n
(5)
By considering the complex formation of zinc with nitrate ion as Zn 2 + + NO3− ⇄ ZnNO3+
with the stability constant, β, of 10 is arranged to
(6) −0.76
33
L/mol at 298 K, eq 5
log D = 2pH + n log([H 2L 2]*/mol L−1) − log(1 + β1[NO3−]) + log(Kex /mol2 − n L−2 + n) (7)
on the assumption that ZnL2·2(n − 1)HL is the only extracted species in the organic phase. A plot of log D versus the logarithm of the effective EHPNA dimer concentration at pH 1.5 is shown in Figure 2. Because Figure 1. Log D of zinc at 298 K as a function of the equilibrium pH at various EHPNA concentrations.
function of the equilibrium pH at different concentrations of the EHPNA dimer. Figure 1 shows that, at the same pH, the values of log D increase with the EHPNA concentration due to the availability of more reactive sites for zinc. Figure 1 further shows that the log D values at each EHPNA concentration are on a straight line with a slope of 2, indicating that zinc extraction is a cation exchange process in which two protons in the extractant molecule are replaced by one zinc ion; thus, the stoichiometric relation of the extraction can be written as Zn 2 + + nH 2L 2 ⇄ ZnL 2· 2(n − 1)HL + 2H+
(2)
where H2L2 represents the extractant dimer. Here EHPNA is known to dimerize in nonpolar diluents.29,30 Alstad et al.31 introduced a correction term depending on the concentration of the DEHPA dimer for expressing the nonideality in the organic phase, which was later employed by Danesi and Vandegrift32 in their study to evaluate the activity coefficients of DEHPA in n-dodecane during extraction of some metals. Alstad’s empirical equation is expressed as
[H 2L 2]* = y*[H 2L 2]
(3)
log y* = −A[H 2L 2]1/2
(4)
Figure 2. Variation in log D of zinc at 298 K with the logarithm of the effective concentration of the EHPNA dimer at pH 1.5.
the slope of the fitted straight line is 2.0, this indicates that two EHPNA dimers combine with a zinc ion (i.e., n = 2). Without the correction by Alstad’s method, the slope of the corresponding line is 1.5. In order to further verify the value of n, the average Kex values were determined using the experimental D values, Dexpt, in Figure 1 and eq 7 by assuming various n values, and the calculated values, Dcalc, at the same pHs as those in the experiment were then obtained using eq 7 for each Kex value. In Figure 3, the mean square errors in log D, (1/N)∑(log Dcalc −
where [H2L2]*, y*, and A are the effective concentration of the EHPNA dimer, a coefficient for the effective concentration, and 16434
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Figure 3. Variation in (1/N)∑(log Dcalc − log Dexpt)2 with n for the data in Figure 1.
log Dexpt)2, are plotted as a function of n, where N denotes the number of data points. Figure 3 shows that when n is increased from 1 to 3, the mean square error gradually decreases, reaches a minimum at n = 2.1, and gradually increases thereafter, indicating that the most probable n value is 2. The n values for EHPNA and DEHPA in the low loading region available in the literature are summarized in the Supporting Information together with the diluents and the methods for correcting nonidealities of the extractants and determining the n values. The results are divided into three groups depending on the n values: (i) 1.5, (ii) 2, and (iii) 1.5 and 2 (two complexes are coexisting). Although it is difficult to give a consistent interpretation for this discrepancy, one important key would be whether they properly corrected the nonideality of the extractant particularly in an alkane diluent. In some studies, such a nonideality was not considered or only monomer−dimer equilibrium was considered, while there is an indication of the formation of the higher-order aggregates than the dimer.21,32 If the higher-order aggregates are really formed, the slope of log D versus the logarithm of total extractant concentration (as dimer) diagram obtained without any correction or by the monomer−dimer model would be lower than the real n value.28,32 Alstad’s method seems to be useful to overcome this problem because it implicitly considers the formation of the higher-order extractatnt aggregates. 3.2. Effect of Temperature. The effect of temperature on zinc extraction was investigated using various concentrations of EHPNA at different temperatures. Figure 4 shows a typical result (with 0.24 mol/L EHPNA as dimer) indicating that log D gradually increases with temperature. Similar observations are reported for DEHPA in kerosene by Sato et al.4 and DEHPA in toluene by Ajawin et al.,16 whereas Kunzmann and Kolarik21 noted that the distribution of zinc was suppressed by a factor of 1.4 when the temperature was elevated from 298 to 318 K. The log D versus pH diagram at each temperature with different EHPNA concentrations is available in the Supporting Information. The observed data were further analyzed by the least-squares method in order to simultaneously determine the values of A and log Kex at each temperature, where the β value at each temperature was calculated from the β value at 298 K as previously described and the standard enthalpy change for eq 6 (−9.00 kJ/mol).33 Based on the result, the A value was found to be little affected by temperature and thus can be set constant at our previously obtained value of 0.473 (L/mol)1/2. The van’t Hoff plot in Figure 5 shows the determined values of log Kex with standard deviations, σ, at various temperatures. From the
Figure 4. Log D of zinc at various temperatures with 0.24 mol/L EHPNA dimer as a function of the equilibrium pH.
Figure 5. van’t Hoff plot for the extraction of zinc by EHPNA.
plot, the apparent standard changes in enthalpy, ΔH°, and entropy, ΔS°, were found to be 12.2 ± 3.0 kJ/mol and 25 ± 10 J/mol·K, respectively. Nash and Choppin34 reported a ΔH° of 10.8 kJ/mol for zinc extraction with DEHPA in benzene based on the distribution experiments at various temperatures. Similarly, Sato et al.4 reported 11.8 kJ/mol for DEHPA in kerosene. These values are in good agreement with our result, which suggests that the nature of the zinc complexation with EHPNA is the same as that with DEHPA. Using these values of ΔH° and ΔS°, the log D values were calculated from eq 7, where log Kex at each temperature was obtained using the following thermodynamic relationship: 2.303R log Kex = −ΔH °/T + ΔS°
(8)
The calculated log D values are shown as lines in Figure 1, Figure 4, and the Supporting Information. A good correlation was observed between the calculated and experimental values of log D, with 0.984 as the coefficient of determination, in which the root-mean-square error in log D was 0.088.
4. CONCLUSIONS The solvent extraction equilibrium of zinc with EHPNA in Shellsol D70 was studied at a constant ionic strength 16435
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considering the nonideality of the extractant in the organic phase. The most probable molar ratio of zinc to the EHPNA dimer in the extracted complex was found to be 1:2. The temperature dependence of the zinc extraction revealed that the reaction is endothermic with a ΔH° of 12.2 kJ/mol. Good thermodynamic correlation was obtained for all the data from 283 to 323 K.
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ASSOCIATED CONTENT
S Supporting Information *
Comparison of the n values for the zinc solvent extraction with EHPNA and DEHPA. Log D of zinc versus equilibrium pH diagrams under various EHPNA concentrations at different temperatures except 298 K. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors express their sincere thanks to the New Energy and Industrial Technology Development Organization (NEDO) for its financial support.
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REFERENCES
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