Ind. Eng. Chem. Res. 1994,33, 1765-1770
1766
SEPARATIONS Extraction of Anions with Aliquat 336: Chemical Equilibrium Modeling Berta Galan, Ane M. Urtiaga, Ana I. Alonso, J. Angel Irabien, and M. Inmaculada Ortiz' Departamento de Qulmica, Escuela Politbcnica Superior de Ingenieria, Universidad de Cantabria, Calle Sevilla 6, 39001 Santander, Spain
Quaternary ammonium salts, among them Aliquat 336, are known to be efficient extractants of many solutes, such as anionic species (Cl-, B r , G o d 2 - , ...), mineral acids, organic acids, enzymes, and hormones. However, due to the complex behavior of liquid-liquid systems implying this type of extractants, different approaches have been tried in the literature to find a suitable mathematical model able to predict equilibrium experimental data. In this work, taking into account the difficulty previously reported, the liquid-liquid equilibrium system NaOH-chloride of Aliquat 336 in kerosene has been experimentally studied in the range of concentrations of 0.25 M ICOH-I2.0 M and 1% v/v I Cfiquat336 I 10% v/v. The mathematical model developed under the consideration of a nonideal behavior of both liquid phases and including two fitting parameters satisfactorily predicted the experimental data.
Introduction The development in the early 19608of the high molecular weight amine reagents allowed the application of solvent extraction procedures for the removal of several anions from aqueous solutions, the salts of quaternary ammonium bases being known as efficient extractants of anionic species,i.e., C1-and B r (Ivanovand Zaitsev, 19881,tungsten and vanadium (Marr et al., 1983), gallium and indium (Karve and Khopkar, 1993), molybdenum (Sat0 et al., 1986), iridium (Sano et al., 1993); salts of mineral acids (Bresseland Eyal, 1993);enzymes (Preitschopf et al., 1993); hormones (Li et al., 1993). Treatment of Cr(V1) in spent baths and rinse waters coming from electroplating processes is usually accomplished by reduction to Cr(II1) and subsequent precipitation. In this way the removal of Cr(V1) from the aqueous effluents generates a typically inert residual sludge which is usually disposed in landfills. However, amine extraction applied to the Cr(V1) recovery has been extensively investigated (Hochhauser and Cussler, 1975; Strzelbicki et al., 1984;Weiss and Castaiieda, 1988; Mori et al., 1990; Salazar et al., 1992a,b; Vijayakumar et al., 1993; Alonso et al., 1993a,b, 1994). Aliquat 336 is a quaternary ammonium salt commercialized as a mixture of tri-n-alkylammonium chlorides. The first step in the process of recovery of chromium(V1) with Aliquat 336 is the extraction of the chromate anion from the aqueous phase into the orgapic phase containing Aliquat 336. As a result, the complex Aliquat chromate is formed in the organic phase. Concentration of the chromate anion is achieved in the stripping step when sufficient initial concentrations of the stripping reagent are employed. In this way high chromium concentration factors between the original aqueous effluent and the final stripping phase can be obtained. Moreover, the exchange anion for the chromate in the stripping is selected as a function of the sought composition of the recycled stream which must be suitable for its reintroduction to the electroplating process.
For most electroplatingprocesses it is necessary to avoid the chloride anion in the composition of the baths since chloride deteriorates cathodes and lead anodes. It attacks lead by selective etching, which causes pitting of anodes, as well as etching of the cathode (Guffie, 1986). The habitual procedure consists of contacting the dilute Aliquat 336 chloride with a concentrated solution of the desired anion up to four contacts or quantitative stripping. Uses of Aliquat 336 in the forms of (i) hydroxide (Salazar et al., 1992a),(ii) cyanide (Calvarin et al., 1992),and (iii) nitrate (Cernl et al., 1993) have been reported. More recently, the extraction of cyanamide and dicyanadiamide with Aliquat 336 in four different ionic forms of the anion exchanger has been reported (Mizelli and Bart, 1994). In the case under study chloride could be eliminated by forcing the exchange of the Aliquat chloride to the Aliquat hydroxide form, usually by contacts with sodium hydroxide. The complexity of the description of the extraction behavior of quaternary ammonium bases has been widely mentioned. Several approacheshave been reported in the literature in order to explain the equilibrium results of different extraction systems. Calvarin et al. (1992) proposed a chemical model for the liquid-liquid extraction of cyanocobalamin cyanide (CblCN-) with Aliquat 336 in cyanide form. The extraction is described by an empirical ideal associated solution model assuming aggregation of species in the organic phase. By applying the strictly quantitative mass action law to the aggregation equilibria, an adequate chemical modeling in agreement with experimental data was obtained. A recent study on the effect of diluents on the amine extraction of citric acid showed that the diluents, especially those with functional groups, can affect the extraction behavior of amine significantly (Blzek et al., 19931, reporting the aggregation of primary acid:amine complexes in the case of non-proton-donating diluents (benzene,toluene, xylene, methyl isobutyl ketone). A different approach to the description of the equilibrium of quaternary ammonium salts is to consider the nonideality in both aqueous and organic phases as well as the influence of the organic diluents. Cernh et al. (1993)
* Author to whom correspondence should be addressed. o s s s - ~ a a ~ i ~ ~ t ~ ~ ~ ~ -0i ~1994 ~ ~American $ o 4 . ~Chemical o t o Society
1766 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994
described in this way the distribution of nitric acid between water or aqueous solutions of ammonium nitrate and organic solutions of Aliquat 336 nitrate. The equilibrium and kinetics of the anionic species of Cr(V1) extraction with Aliquat 336 hydroxide in kerosene has been studied by Salazar et al. (1992a) in the range of initial concentrations of Cr(V1) 1-200 mg/L. The authors applied either the mass action law or an adsorption model to the chromate exchange reaction concluding with a semiempirical model which allowed the evaluation of the equilibrium conditions in the mass transfer kinetic model. Satisfactory application of such equilibrium studies was reported in a subsequent study on the kinetics of the separation-concentration of Cr(V1) with emulsion liquid membranes (Salazar et al., 199213). Arecent work (Alonso et al., 1994) reports the &(VI) extraction with Aliquat 336 by means of nondispersive liquid-liquid extraction in hollow fiber modules; the study includes the mathematical modeling of the system by means of the integration of the mass conservation equation with a nonlinear equilibrium condition, including a variable equilibrium parameter depending on the initial concentration of Cr(V1). In the investigation of the ways to separate and concentrate Cr(V1) by emulsion liquid membranes and nondispersive liquid-liquid extraction, the study of the exchange of the chloride anion of the Aliquat 336 with the hydroxide anion (OH-) from sodium hydroxide aqueous solutions is presented. Taking into account the remarks reported in the literature concerning the complexity of extraction systems implying quaternary ammonium salts, the equilibrium Aliquat chloride *Aliquat hydroxide can be taken as a test system, representing the present work as a systematic study on the modeling of Aliquat 336 extraction equilibria. The accounting of a nonideal behavior of both liquid phases has led to a mathematical model that satisfactorily predicts the results obtained in the experimental range of concentrations 0.25 M I COHI 2.0 M and 1% V/VICuquat338 5 10% V/V.
Experimental Section Anal& grade NaOH (Eka Nobel) was used to prepare the aqueous solutions. The extractant Aliquat 336 (Fluka), a commercial mixture of trialkylmethylammonium chlorides (trialkyl = C&~O,mainly capryl)was used as received without further purification, the mean molecular weight being 404. Kerosene (Petronor, S.A.) was used as solvent. A 5% v/v solution of a modifier, 3,7-dimethyloctanol (Aldrich-Chemie),was added to all organic mixtures. The addition of a modifier (a high molecular weight alcohol) is necessary in order to avoid the segregation of a third (second organic) phase. Extraction equilibrium experiments were carried out in closed glass tubes in a rotatory SBS stirrer (5-150 rpm). Volumes of initial aqueous and organic phases ranging in proportions of 16:l to 1:16 were equilibrated. Initial hydroxide concentrationsof the aqueous phase were NaOH 0.25,0.5,1, and 2 M. Initial concentrations of chloride of Aliquat 336 in the organic phase were 1,5, and 10% v/v. Successive contacts of partially hydroxide loaded organic phases were carried out in order to increase the number of experiments. The equilibrium concentration of chloride in the aqueous phase was determined by ion chromatography in a Waters 501 HPLC provided with a Waters 430 conductivity detector using an IC-Pack A anionic analytical column. The mobile degasified phase was borate/gluconate (pH = 8.5) at a flow rate of 1.2 mL/min. OH-anions suppose an interference in the chromatographic measurement of
chloride. In order to remove hydroxide anions, aqueous samples were prepared for analysis with a H+ Waters MilliTrap membrane cartridge. Triplicate analyses were performed to determine the experimental error which remained below f15% in the whole range of experimental conditions. Additionally, it was observed that for concentration values of sodium hydroxide lower than 1M the analytical error was below lo%,increasing up to 15% for higher concentrations of NaOH.
Results and Discussion The system under study is the reversible reaction equilibrium of sodium hydroxide with the chloride of a quaternary alkylammonium salt, Aliquat 336:
- -
NaOH + (AL)Cl* (AL)OH + C1- + Na'
(1)
The equilibrium is thermodynamically characterized by the equilibrium constant of the activities of the species
- '(&)OH K,-=--
'C1-
'(AL)Cl
'OH-
- KCq
K yis given as function of the activity coefficients by eq 3: (3)
where y z and y E are the activity coefficients for ?OH- are the the species in the organic phase and ~ c l and activity coefficients for the species in the aqueous phase. K, is the concentrations equilibrium constant defined as follows:
K,=
[(AL)OHl[Cl-I
-
(4)
[(AL)CIl[OH-] Initially it will be assumed that K,= K,,which is a common approximation if dilute solutions are considered leading to Ky= 1. In the experimental conditions under consideration the hydroxide anion concentration is always in large excess in comparison to the chloride concentration so that
An extensive experimental study of the equilibrium was carried out. Starting from initial concentrations of Aliquat 336 in the organic phase of 1,5,and 10%v/v and hydroxide concentrations in the aqueous phase of 0.25,0.5,1, and 2 M, equilibrium experiments were performed. Figures 1-4 plot Y = [(AL)C1l[OH-I/[(AL)OHl in ordinates versus [Cl-I in abscissas at each of the experimental sodium hydroxide molar concentrations. Equation 4 can be expressed in linear form as eq 5. Following eq 5 l/Kc should be obtained from the slope of the linear regression. Table 1collects the regression parameters obtained in the fitting of all the experimental data, i.e., 0.25 M I I 2.0 M and 1% v/v IAliquat 336 I10% v/v to eq 5. The value of the correlation coefficient, r2 = 0.75, is too low to consider that the mathematical model represented by eq
Ind. Eng. Chem. Res., Vol. 33, NO. 7,1994 1767
2-
1-
0 u,OO
0,02
0,Ol
0,03
u,OO
0,04
[CI'] , molldm3 Figure 1. Equilibria of chloride of Aliquat 336 with 0.25 M sodium 1 % v/v; ( 0 )5% v/v; hydroxide. Concentration of Aliquat 336 (0) (A) 10% V/V.
0,04
0,02
0,06
0,08
[CI'] , molldm3
Figure 4. Equilibria of chloride of Aliquat 336 with 2.0 M sodium hydroxide. Concentration of Aliquat 336 ( 0 )1%v/v; ( 0 )5% v/v; (A)10% V/V.
2-
1 -
P z,OO
0,02
0,Ol
0,03
G,OO
0,04
0,Ol
0,02
0,03
0,04
0,05
[CI'] , molldm3
[CI'] , moVdm3 Figure 2. Equilibria of chloride of Aliquat 336 with 0.5 M sodium hydroxide. Concentration of Aliquat 336: (0) 1%v/v; ( 0 )5% v/v; (A) 10% V/V.
Figure 5. Influence of Aliquat 336 concentration on extraction equilibria; data for a sodium hydroxide concentration = 0.5 M. Concentration of Aliquat 336 (0) 1% v/v; ( 0 )5% v/v; (A)10% v/v.
Table 1. Parameters Obtained from Linear Regression hydroxide slope ordinate at concn (M) (M-9 the origin K, = l/elope 9 0.25-2.0 89.66 0.0181 1.11 X 1b2 0.75 0.25 69.87 -0.053 1.43 X le2 0.93 0.50 82.41 -0.055 1.21 X 0.94 1.0 107.60 0.067 0.93 X 10-2 0.78 2.0 152.57 0.148 0.66 X le2 0.89
o v .
U,OO
'
0,Ol
'
'
0,02
'
' 0,03
'
'
0,04
'
'
0,05
[CI'] , molldm3
Figure 3. Equilibria of chloride of Aliquat 336 with 1.0 M sodium 1% v/v; ( 0 )5% v/v; hydroxide. Concentration of Aliquat 336 (0) (A)10% V/V.
4 satisfactorily describes the equilibrium system under
consideration. Next, the fitting of the data obtained at fixed values of the sodium hydroxide concentration was performed giving the parameters reported in Table 1. Although there is a significant increase in the values of the regression coefficients, specially at low concentrations of NaOH, it is observed from the slopesthat the values of Kcdo not remain constant for the four experimental sodium hydroxide concentrations, as would be expected from ideal behavior.
Thus the dependency of K , on the initial hydroxide concentrations points to an erroneous assumption of ideal behavior in the aqueous phase. Furthermore, a thorough examination of Figures 1-4 reveals that the equilibrium might be influenced by the organic phase since slightly different courses are obtained for each of the total Aliquat concentrations at constant sodium hydroxide concentration. In order to ascertain the equilibrium behavior, the number of experiments was increased, focusing on a 0.5 M hydroxide aqueous concentration, thus minimizing the influence of the analytical error in the interpretation of the data. Figure 5 shows the dependency of [(AL)Cl]. [OH-]/[(AL)OH] with [Cl-I as a function of the total Aliquat concentration. In this way the rigourous analysis of the experimental results made certain that the equilibrium parameter is dependent both on the total concentration of Aliquat chloride in the organic phase and on the hydroxide concentration in the aqueous
(q)
(c)
(G)
1768 Ind.
Eng.Chem. Res., Vol. 33, No. 7, 1994
phase, behavior that can only be explained considering the nonideality of both the aqueous and the organic phases. The nonideality of the organic phase in the amine extraction system is usually expressed in terms of the ideal associated solution complexes. Liquid-liquid extraction of cyanocobalamin was satisfactorily modeled taking into account aggregation of the ammonium salts in the organic phase (Calvarinet al., 1992). Aggregation of primary acidsamine complexes has also been considered for the chemical modeling of the amine extraction of citric acid (Blzek et al., 1993). A different approach to the mathematical modeling of the equilibrium consists of considering the activity coefficients of species in both the aqueous and the organic phases. CernB. et al. (1993) developed empirical expressions for the activity coefficients in the aqueous phase while the activity coefficients in the organic phase were considered as a one-parameter function of the not bound (free) quaternary ammonium salt concentration. Startingfrom this assumption and considering that from the analysis of the previous experimental results the nonideality of the aqueous and the organic phases must be considered, in this work the latter function for the organic phase has been taken as
-3
-4
9
C
-5
-6 u.3
0.7
0.5
0.9
Figure 6. Fitting of experimental data according to eq 8. Concen1% tration of NaOH = 0.25 M. Concentration of Aliquat 336 (0) V/V; ( 0 )5% V/V; (A) 10% V/V.
where B is a fitting parameter. For aqueous solutions, taking into account that the activity coefficients of the ionic species are determined as a function of the ionic strength of the solution, Ti = ri(l), being
where Zi is the charge and mi the molal concentration of the species i. In the case under study, z = 1 for C1- and OH-; moreover, OH- concentration in the aqueous phase is in a large excess with respect to the C1- concentration, making the ionic strength remain constant during each run. Thus in a first approximation it has been considered that the ratio of the activity coefficients of the species in the aqueous phase YCI-/YOH- takes a constant value. Substitution of eq 6 into eq 2 allows the linear regression of the experimental data as follows:
‘
-6 u,3
0,s
0,7
0,g
[@iiiiCt1
[CTI Figure 7. Fitting of experimental data according to eq 8. Concentration of NaOH = 0.50 M. Concentration of Aliquat 336 ( 0 )1% v/v; ( 0 )5% v/v; (A)10% v/v.
t -5,5’ u,3
The linear regression of the experimentalresults according toeq 8 allows obtaining of the parameters A and B. Figures 6 and 7 represent In K, versus [(AL)ClI/G for 0.25 and 0.5 M initial hydroxide concentrations, respectively, providing similar values of the slopes (B parameter), the small difference being attributed to experimental error. For this reason it has been considered that an average slope can be calculated for the two sets of experiments. In the same way Figures 8 and 9 plot In K, versus [(AL)Cl]/G for 1 and 2 M initial hydroxide concentrations, which are best described by horizontal lines. Next, the origin of ordinates has been obtained for each of the experimental initial hydroxide concentrations consideringthe average slopes previously evaluated. From the origin of ordinates different values of the parameter A are obtained. Table 2 lists the parameters A and B for the initial hydroxide concentrations of 0.25,0.5,1, and 2 M.
I 0.7
0.5
0.9
I
Figure 8. Fitting of experimental data according to eq 8. Concentration of NaOH = 1.0 M. Concentration of Aliquat 336: ( 0 )5% v/v; (A)10% v/v.
The validity of the parameters given in Table 2 will be checked by simulation. The combination of eqs 8 and 5 provides a calculated C1-concentration as follows
XM^A+BX [cl-l = 1 - x 1+
X
cA+Bx
(10)
1-x
where M is the molar initial concentration of sodium hydroxide in the aqueous phasesand x: is the molar fraction -of (ALICl, defined as (AL)Cl/C,. A and B parameters
':I
Ind. Eng. Chem. Res., Vol. 33,No. 7, 1994 1769
~
&
A---A-
-55 u,3
@
0,5
-
-~
~
~
-
!
0,9
0,7
@Zijiii1 r-61 Figure 9. Fitting of experimental data according to eq 8.Concentration of NaOH = 2.0 M.Concentrationof Aliquat 336 ( 0 )5% v/v; (A)10% v/v. Table 2. Values of A and B Parameters A parameter
-
CT (% V/V) 1 5 10
0.25 M -3.66 -3.08 -2.98 E
0.5 M -3.85 -3.46 -3.14 -1.40
1M
2M -5.44 -5.09 -4.83
-4.76 -4.55 -4.46
B=O
0,oa m
E
'0
1
0,06
E6 -#
z m
0,04
[CI'] Experimental, moVdm3 Figure 10. Parity graph. Simulated chloride concentrations vs experimental chloride concentrations. Table 3. Weighted Standard Deviations Corresponding to Each Sodium Hydroxide Concentration hydroxide concn (M) Uw (%) no. of exptl points 0.25 6.2 14 9.7 55 0.5 1.0 12.2 16 2.0 14.1 30
used in the calculation of the chloride concentrations are taken from Table 2. Comparison between the experimental and simulated chloride concentrations was performed on the basis of the minimum weighted standard deviation, a,. The weighted standard deviation is defined in eq 11:
Figure 10 shows the parity graph for 0.25,0.5,1,and 2 M initial hydroxide concentration. One hundred percent of the results of Cairn fall within the interval Cexp f 7 7% Cexp. The good agreement between experimental data and simulated values is presented in Table 3,showing that the standard deviation is kept below 10% for hydroxide concentrations of 0.25and 0.5 M and under 15% for 1 and
2 M concentrations. The increase in a, is related to the lower certainty of the analytical method of chloride concentration measurement since the difficulty in removing the totality of OH- rises when high hydroxide concentrations are present in the aqueous sample, lowering the accuracy of the analysis. For this reason data obtained at high hydroxide concentrations (1 and 2 M) and low total Aliquat concentration (1% v/v) show a higher dispersion. As soon as the weighted standard deviations range from 6.2% to 14.1%, values that fall in the range of the analytical error, the validity of the equilibrium model can be taken as satisfactory. If any comparison of the models in the literature accounting for the extraction equilibria of anions with Aliquat 336 and the model proposed in this work is established, several differential features can be encountered. The chemical model given by Calvarin et al. (1992) accounts for the aggregation of species in the organic phase, leading to six fitting parameters. The credibility of the model is undoubted since the experimental study in a wide range of concentrations is exhaustive. A simplified model from the point of view of the chemistry of the species and accounting for the nonideal behavior of the solute with respect to both aqueous and organic phases is given by Cernd et al. (19931,but up to eight fitting parameters are necessary in order to predict the extraction of nitric acid with Aliquat 336. The model proposed in the present study requires only two fitting parameters, and its validity has been checked through an exhaustive planning of experiments in a wide range of experimental conditions of practical importance. From a technical point of view, the fewer number of parameters facilitates the use of the model when the equilibrium description is needed for the design of a mass transport process system, e.g., the analysis and design of the nondispersive liquid-liquid extraction process in hollow fiber modules, the scope where the present work is focused.
Conclusions In this work the equilibrium of the exchange of the chloride anion of the Aliquat 336 with the hydroxide anion (OH-) from sodium hydroxide aqueous solutions has been studied. When ideal behavior of the liquid phases was considered, unsatisfactory results were obtained in the fitting of the experimental data. The accounting of nonideal behavior of the aqueous and organic phases together with an exhaustive experimental planning has led to an equilibrium model with two fitting parameters, which satisfactorily correlates the experimental data. The validity of the model has been checked through the comparison between experimental data and simulated values, in which reasonable fitting parameters were obtained using the minimum weighted standard deviation as the criterion. Furthermore this work develops a systematic methodology for the analysis and modeling of liquid-liquid equilibria implying the use of quaternary ammonium salts such as Aliquat 336. However the empirical nature of the fitting parameters makes it necessary to perform a broader study in order to analyze their physicochemical meaning as well as to validate the applicability of the reported model to other experimental systems containing different ammonium salts. Acknowledgment Financial support from the Spanish CICYT (MEC) under Project AMB93-0316 is gratefully acknowledged.
1770 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994
Nomenclature A = parameter in eq 9 a = chemical activity of solutes B = parameter in eq 6 C, = total concentration of Aliquat 336 in the organic phase C, = experimental values of concentration Ch = simulated values of concentration I = ionic strength K, = concentration equilibrium constant, defined by eq 4 Ke= equilibrium constant, defined by eq 2 K.,= equilibrium parameter, defined by eq 3 m = molal concentration r = molar fraction zj charge of the species i Y = [(AL)ClI[OH-I/[(AL)OHl, mol/dm3 Greek Letters y = activity coefficient a, = weighted standard deviation
.
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Received for reuiew December 17, 1993 Accepted April 5, 1994. 0
Abstractpubliihed in Aduance ACSAbstracts, May 15,1994.
