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Ind. Eng. Chem. Res. 1998, 37, 292-295
RESEARCH NOTES Extraction of Anions from Aqueous Solutions Using Secondary Amines Stefano Brandani, Vincenzo Brandani,* and Francesco Veglio` Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita` de L’Aquila, I-67040 Monteluco di Roio, L’Aquila, Italy
Extraction of strong mineral acids (HCl and H2SO4) from aqueous solutions was carried out using the secondary amine Amberlite LA-2. Experimental measurements were performed with aqueous solutions in the concentration range 0.01-0.5 m. The organic phase comprised 0.5 m Amberlite in diluent toluene. Experimental results were modeled to determine the Nernst distribution coefficients of the two acids and the infinite dilution selectivity. The affinity for H2SO4 is found to be greater than that for HCl. Introduction Aliphatic secondary and tertiary amines dissolved in an organic solvent are powerful extractants for carboxylic acids (Yang et al., 1991; Ju and Verma, 1995; Kirsch and Maurer, 1996). The amine binds the acid in the organic phase through reversible complexation. The protonated amine forms in the organic phase reversed micelles with water pools in which ions are dissolved. Therefore, water has a strong influence on the liquidliquid equilibrium. The anions of strong mineral acids have high affinities to protonated secondary and tertiary amines (Eyal and Baniel, 1982; Cox and Flett, 1983). For the separation of these acids using tertiary amines the selectivity is generally reported in the following order: HClO4 > HNO3 > HCl > HBr > H2SO4. In this study we have used the secondary amine Amberlite LA-2 and toluene as the organic solvent. We have studied the distribution of HCl and H2SO4 between the two phases. The aim of this study is to determine the value of β∞, i.e., the selectivity at infinite dilution of these two inorganic anions, which can be considered the true thermodynamic quantity in the evaluation of affinity and therefore may be used as an indication of the feasibility of a separation process.
inorganic acid solution and an organic solution of Amberlite LA-2 (0.5 m) in a stirred glass flask in a temperature-controlled water bath at 25 ( 0.1 °C. Runs at different total contacting times confirmed that after 1 h complete equilibrium was achieved. After equilibration, the two phases were separated by centrifugation. The concentration of the acid in the aqueous phase was determined by titration with aqueous sodium hydroxide (relative uncertainty 1%). The free amine in the organic phase was determined by titration with perchloric acid in glacial acetic acid (relative uncertainty 1%). The amount of water in the organic phase was determined by Karl-Fischer titration with a relative uncertainty of 3%. Modeling the Liquid-Liquid Equilibrium The experimental results may be modeled assuming that the only species that transfer between the two phases are the acid and water. At equilibrium for solutions of hydrochloric acid we must have (Prausnitz et al., 1986)
f HCl ′ ) f HCl ′′
(1)
a′W ) a′′W
(2)
Materials and Methods HCl, H2SO4, and NaOH Normex (1, 0.1, and 0.01 N, respectively) were purchased from Merck. Toluene (g99.8% GC) and perchloric acid 0.1019 N) in a solution of glacial acetic acid were purchased from Aldrich. Amberlite LA-2 (MW 377) and acetic acid (g99.8%) were purchased from Fluka. All chemicals were used without further purification. Bidistilled deionized water was used. The liquid-liquid equilibrium measurements were carried out by adding equal volumes of an aqueous * Author to whom correspondence should be addressed. Telephone: (+39) 862 434219. Fax: (+39) 862 434203. Email:
[email protected].
where a single prime (′) indicates the aqueous phase and a double prime (′′) is for the organic phase. For the two fugacities (Prausnitz et al., 1986; Pitzer, 1991) we have
f HCl ′ ) γ′(2m′Cl2-H′HCl
(3)
2 f ′′HCl ) γ′′ H+m′′ Cl-H′′ HCl ( m′′
(4)
where H′HCl and H′′HCl are the Henry constants of HCl in the two phases. Substituting eqs 3 and 4 into eq 1 and rearranging, we have
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Ind. Eng. Chem. Res., Vol. 37, No. 1, 1998 293
γ′(2 H′HCl γ′(2 ) KN,HCl γ′′2 H′′HCl γ′′2
m′′H+m′′Cl)
2 m′′ Cl-
(
(5)
(
where KN,HCl is the Nernst distribution coefficient of HCl between the two phases. Taking into account the equilibrium of protonation of the amine
A + H+ ) AH+
(6)
with
m′′AH+ γ′′AH+ m′′Am′′H+ γ′′Aγ′′H+
(7)
m′′AH+ γ′′Aγ′′H+ )K m′′Am′′H+ γ′′AH+
(8)
K)
From electroneutrality we have
m′′H+ + m′′AH+ ) m′′Cl-
(9)
Combining eqs 8 and 9, we have
m′′H+(1 + Qγm′′A) ) m′′Cl-
m′′Cl(mol/kg)
1.71 × 10-4 3.89 × 10-4 5.34 × 10-4 1.23 × 10-3 1.26 × 10-3 1.82 × 10-3 4.18 × 10-3 6.98 × 10-3
3.15 × 10-2 4.45 × 10-2 5.97 × 10-2 7.08 × 10-2 8.85 × 10-2 1.03 × 10-1 1.12 × 10-1 1.36 × 10-1
(10)
m′′H+ (mol/kg)
7.38 × 10-5 1.03 × 10-4 4.29 × 10-4 7.21 × 10-3 4.18 × 10-3 6.10 × 10-3 8.79 × 10-3
9.41 × 10-2 1.39 × 10-1 2.01 × 10-1 2.62 × 10-1 2.82 × 10-2 3.33 × 10-1 3.56 × 10-1
5.48 × 10-2 1.01 × 10-1 1.62 × 10-1 2.45 × 10-1 2.65 × 10-1 3.39 × 10-1 3.62 × 10-1
m′′SO42-
m′SO42-f0
(11)
(
In the limit to infinite dilution we may derive two useful relationships
lim
m′′H+m′′Clm′Cl2-
) KN,HCl
(12)
3 4m′SO 24
)
γ′(3 K 3 N,H2SO4 γ′′ (
γ′(3 ) 4 K (1 + Qγm′′A)2 3 3 N,H2SO4 m′SO γ′′ 2( 4
lim
m′SO42-f0
3 4m′SO 24
) KN,H2SO4
m′′A m′′AH+ WH2O (mol/kg) (mol/kg) (g/kg) 0.367 0.324 0.260 0.221 0.202 0.172 0.150
0.133 0.176 0.240 0.278 0.298 0.328 0.350
0.78 1.52 2.46 3.04 3.18 3.33 3.53
2/3 ) (4KN,H2SO4)1/3(1 + Ka′′* (17) A )
bm′Anion 1 + bcm′Anion
(18)
where
(14)
b ) lim
m′Anionf0
m′′Anion m′Anion
(19)
From appropriate plots of the experimental values it is therefore possible to determine the Nernst constant and the infinite dilution selectivity, defined as
β∞ ) lim
m′Anionf0
m′′SO42-/m′SO42m′′Cl-/m′Cl-
bSO42)
bCl-
)
(4KN,H2SO4)1/3 (1 + Ka′′A*)1/6 (20) (KN,HCl)1/2 (15)
and taking the limits to infinite dilution 2 m′′ SO42H+ m′′
0.83 0.85 0.71 0.61 0.75 0.84 0.83 0.66
(13)
and 3 m′′ SO42-
m′SO42-
m′′Anion )
where a′′A* is the activity of free amine in the initial organic phase. For H2SO4 the same procedure may be applied, and considering that at low concentrations complete dissociation may be assumed, the following relationships may be derived 2 m′′ SO42H+ m′′
0.026 0.037 0.053 0.056 0.078 0.086 0.094 0.097
An alternative way to describe the equilibrium may be obtained considering a simplified physical picture of the contacted mixtures. The water pools in the reverse micelles may be seen as “adsorption” sites. This leads to a langmuirian type of equilibrium isotherm as shown by Brandani et al. (1994). This approach yields a simple expression which may be used to correlate the experimental results
and
m′′Cl) xKN,HCl(1 + Ka′′* lim A ) m′Cl f0 m′Cl-
0.474 0.462 0.447 0.444 0.422 0.414 0.405 0.403
organic phase m′′SO42(mol/kg)
lim
m′Cl-f0
5.64 × 10-3 7.46 × 10-3 6.30 × 10-3 1.47 × 10-2 1.02 × 10-2 1.70 × 10-2 1.78 × 10-2 3.90 × 10-2
aqueous phase m′SO42(mol/kg)
and therefore 2 m′′ γ′(2 Cl) K (1 + Qγm′′A) m′Cl- γ′′2 N,HCl
organic phase m′′H+ m′′A m′′AH+ WH2O (mol/kg) (mol/kg) (mol/kg) (g/kg)
aqueous phase m′Cl(mol/kg)
Table 2. Experimental Results for the Liquid-Liquid Equilibrium in the System Sulfuric Acid/Water/Toluene + Amberlite LA-2
and
Qγ )
Table 1. Experimental Results for the Liquid-Liquid Equilibrium in the System Hydrochloridric Acid/Water/ Toluene + Amberlite LA-2
(16)
Results and Discussion The experimental results obtained for the systems containing HCl and H2SO4 are reported in Tables 1 and 2, respectively. The experimentally measured quantities were all the initial compositions, the free amine in the organic phase, the amount of water in the organic phase, and the final acid composition in the aqueous
294 Ind. Eng. Chem. Res., Vol. 37, No. 1, 1998
Figure 1. Determination of the Nernst distribution coefficient for HCl.
Figure 2. Comparison between calculated and experimental Clconcentrations in organic and aqueous phases.
Figure 3. Determination of the Nernst distribution coefficient for H2SO4.
Figure 4. Comparison between calculated and experimental SO42- concentrations in organic and aqueous phases.
Table 3. Parameter Estimation of the Models (12) and (18) for the HCl Systema regressed param KN,HCl ( s.d. (×10-4)
b ( s.d.
c ( s.d.
Ka′′A*
1.2 ( 0.4
170 ( 7
7.0 ( 0.5
1.4
a
s.d. ) standard deviation.
Table 4. Parameter Estimation of the Models (12) and (18) for the H2SO4 Systema regressed param KN,H2SO4 ( s.d. (×10-8)
b ( s.d.
c ( s.d.
Ka′′A*
6(3
1830 ( 57
3.2 ( 0.2
0.7
a
s.d. ) standard deviation.
phase. All other quantities are obtained from mass balances and electroneutrality relationships. The values of the Nernst constant for HCl is obtained from an extrapolation to zero concentration in the plot shown in Figure 1. The experimental data are also correlated using eq 18, and Figure 2 shows the comparison between calculated and experimental concentrations. The values of the parameters obtained are reported in Table 3. The same procedure is applied to the system containing H2SO4, and the results are shown in Figures 3 and 4, with the corresponding parameters
Figure 5. 95% confidence ellipse for the parameters obtained for HCl.
reported in Table 4. The simple langmuirian type isotherm is able to represent the equilibrium behavior in the concentration range which we investigated. It is possible to evaluate the activity of the amine in
Ind. Eng. Chem. Res., Vol. 37, No. 1, 1998 295
toluene using the Flory-Huggins model for activity coefficients (Prausnitz et al., 1986). The value we obtain is a′′A* ) 0.84 m. This yields an average value for the amine protonation equilibrium constant of 1.2, which can be considered a rough estimate. The most important quantity that we want to evaluate is the infinite dilution selectivity. Since this is necessarily obtained indirectly, we also report in Tables 3 and 4 the standard deviations on the regressed parameters. The estimated value for β∞ is 11 ( 4. This uncertainty is in part due to the fact that a langmuirian type of isotherm results in parameters which are strongly correlated as can be seen from the 95% confidence ellipse for HCl shown in Figure 5. The result obtained clearly shows that secondary amines have a greater affinity for H2SO4. The opposite behavior is observed for tertiary amines (Ju and Verma, 1995). Conclusions The liquid-liquid equilibrium of aqueous acid solutions containing HCl or H2SO4 in contact with toluene and the secondary commercial amine Amberlite LA-2 has been investigated at 25 °C. The experimental equilibrium concentrations were correlated using a langmuirian type isotherm based on a simplified physical picture of the contacted solutions. Nernst distribution coefficients for both solutes were obtained. An estimate for the amine protonation equilibrium constant was also derived. Finally the infinite dilution selectivity clearly shows that Amberlite LA-2 has a greater affinity for H2SO4 than for HCl. Acknowledgment The authors thank G. Spagnoli for his help in the experimental measurements. Financial support from MURST is gratefully acknowledged. List of Symbols a ) activity b ) distribution ratio at infinite dilution c ) constant in eq 18 f ) fugacity
H ) Henry’s constant K ) equilibrium constant of amine protonation KN ) Nernst distribution coefficient m ) molality Qγ ) defined in eq 8 Greek Letters β∞ ) selectivity at infinite dilution γ( ) mean activity coefficient Superscripts and Subscripts A ) amine W ) water ′ ) aqueous phase ′′ ) organic phase
Literature Cited Brandani, S.; Brandani, V.; Di Giacomo, G.; Spera, L. A Thermodynamic Model for Protein Partitioning in Reversed Micellar Systems. Chem. Eng. Sci. 1994, 49, 3681-3686. Cox, M.; Flett, D. S. Metal Extractant Chemistry. In Handbook of Solvent Extraction; Wiley: New York, 1983. Eyal, A.; Baniel, A. Extraction of Strong Mineral Acids by Organic Acid-Base Couples. Ind. Eng. Chem. Res. 1982, 21, 334-337. Ju, L. K.; Varma, A. Extraction of Anions with Tertiary Amine from Aqueous Solutions of Mixed Acid and Salt. Ind. Eng. Chem. Res. 1995, 34, 4479-4485. Kirsch, T.; Maurer, G. Distribution of Oxalic Acid between Water and Organic Solutions of Tri-n-octylamine. Ind. Eng. Chem. Res. 1996, 35, 1722-1735. Pitzer, K. S. Activity Coefficients in Electrolyte Solutions, 2nd ed.; CRC Press: Boca Raton, FL, 1991. Prausnitz, J. M.; Lichtenthaler, R. N.; de Azevedo, E. G. Molecular Thermodynamics of Fluid Phase Equilibria, 2nd ed.; PrenticeHall: Englewood Cliffs, NJ, 1986. Yang, S. T.; White, S. A.; Hsu, S. T. Extraction of Carboxylic Acids with Tertiary and Quaternary Amines: Effect of pH. Ind. Eng. Chem. Res. 1991, 30, 1335-1342.
Received for review June 23, 1997 Revised manuscript received October 22, 1997 Accepted October 27, 1997X IE970447P
X Abstract published in Advance ACS Abstracts, December 1, 1997.
