Ind. Eng. Chem. Res. 1992,31,1705-1709
1705
SEPARATIONS Anion Exchange and Aggregation of Dicyanocobalamin with Quaternary Ammonium Salts in Apolar Environment Louis Calvarin, Brigitte Roche, and Benri Rsnon* Centre RBacteurs et Processus Cerep, kcole des Mines de Paris, 60 Boulevard Saint-Michel, 75006 Paris, France
Liquid-liquid extraction of dicyanocobalamin was performed, using quaternary ammonium salts (Aliquat 336) as liquid ion exchangers. A chemical model, taking into account aggregation of the cyanide and dicyanmbalamin ammonium salts in the organic phase, results in quantitative agreement with experimental data, covering the whole concentration range of practical interest. Vitamin B12is the moat complex of all vitamin molecules. Although total chemical synthesis was achieved once
in the laboratory, it is made industrially by biosynthesis. Vitamin B12or cyanocobalamin (Cbl) is obtained as a dilute component in a fermentation broth. Its purification to obtain a pharmaceutical grade product can be achieved by a two-stage ion-exchange process using an ion-exchange resin with substituted ammonium salts (Shafer and Holland, 1955). As part of an investigation of other ways to concentrate and purify vitamin B12,(Calvarin, 1989), this article describes a liquid-liquid extraction experiment where Cbl is extracted as an anionic cyanide complex CblCN- by an organic phase containing quaternary ammonium sdta (Aliquat 336) used as anion exchangers. The results are represented by a simple, but credible, empirical ideal associated solution model assuming aggregation of species in both phases. The species involved in this extraction are presented in Figure 1.
Experimental Section Materials. Vitamin B12was obtained from RhonePoulenc as a pharmaceutical grade product of 96% purity. Sodium cyanide and p-xylene were purchased as Prolabo Rsctapur products, dodecane as a Prolabo technical grade product, and l-decanol as a Fluka Purum product. The mixture of trialkylmethylammonium chlorides (trialkyl= C8-Cl0,mainly capryl), known under the name of Aliquat 336, was bought as an organic solution from Fluka. It was converted to cyanide salts before extraction experiments, by repeated contacts with concentrated aqueous sodium cyanide solutions. In the conditions of the experiments, the quaternary ammonium cations form ionpairs only with CN- and CblCN-, denoted respectively as ACN and ACblCN, and various aggregates. It does not diesolve in the aqueous phase as was experimentally verified. Analytical Methods. Water Karl-Fischer titrations were performed on a Tacussel Aquaprocesseur. Interfacial tensions were measured on a Kruas K10 ring tensiometer. Cbl and CblCN- concentrations in the aqueous phase were measured on a Hewlett-Packard 8452 diode-array spectrophotometer, between 480 and 820 nm. They have absorption peaks at 551 nm for Cbl and 580 nm for CblCN-,
and therefore both concentrations can be obtained simultaneously. Cyanide concentrations in aqueous phase were measured on a Metrohm 512 potentiometer by Ag+ titration: this method does not distinguish free cyanide anions and CblCN- complex, and the result is therefore an overall cyanide concentration. The total ammonium concentration - in the organic phase is the sum of the ionpair [ACN] and [ACblCN] concentrations; it is measured by Ag+ potentiometric titration, with ethanol as the common solvent. The free cyanide aqueous concentration [CN-] is not measured directly but calculated from the overall cyanide concentration and cobalamin [Cbl], [CblCN-1, and [ACblCN] concentrations obtained as described above. The calculation takes into account the complexation reaction of cobalamin by cyanide ions with equilibrium constant Kc defined by [CblCN-] Kc (1) [CN-1[Cbll Kc was obtained from spectrophotometric titration of CblCN- and Cbl, because the simultaneous measurement of both is feasible below an overall cobalamin concentration of 2 x 10-4mol/dm3,without any preliminary dilution that would displace the equilibrium. Many measurements were made at overall cyanide concentrations between 5 X lo-" and 2.5 X mol/dm3 and overall cobalamin concentrations between 7 X lo+ and 2 X 10" mol/dm3. The equilibrium constant Kcwas found to be constant, and ita value at 25 OC is 103.esdm3/mol. Extraction Experiments. Extraction experiments were performed in closed glass tubes in a 25 f 2 "C regulated atmosphere. A 10-mL aliquot of the aqueous phase wae mixed with the same volume of the organic phase. The tubes were shaken for 1h on an orbital shaker, and then complete phase separation waa achieved in a temperature-controlled centrifuge (Jouan GT 2oooO SX). Samples were taken from the organic and aqueous phases at equilibrium, using syringes.
Experimental Results and Discussion Ammonium cyanide ion pairs carry lipophilic as well as hydrophilic groups and therefore have tensioactive properties with well-documented aggregation tendencies
0888-588519212631-1705$03.OO/0 0 1992 American Chemical Society
1706 Ind. Eng.Chem. Res., Vol. 31, No. 7, 1992 0,05
ICN.I= 012moWdm3
ICN 1
0,001
0,002
i
ICN-I = 036 molldm3
018 moWdmS
0,003 0,004 [CblCN-I ,mol /dm3
0,005
0,Ol
Figure 4. Illustration of the influence of the cyanide concentration on extraction efficiency, dodecane-decanol/ACN equilibrium data.
[ACNI , moV dm3
Figure 2. Interfacial tension curve. The emooth, higher concentration slope change is typical of an aggregation phenomenon. 10 para-xylene diluent
0,Ol
-
5*
--0 '
~
3 :
Tbird phase formatio-
0,M 0,08 ratio X = [CblCN-I / E N - I
0.04
0,02
0,lO
Figure 5. Dodecane-decanol/ACN CblCN- extraction equilibria: experimental data represented vs the value of ratio X.
perimental results of the concentration of water in a pxylene organic phase. The curve is described by the following equation: straight line in loganthmic coordinates
[H,O] = 1.86 X
lo-, + 5.4([-]
or
[m])(2)
Part of water is solubilized by the solvent (1.86 X mol/dm3), and the rest represents hydration of ion pairs. There is a constant number of water molecules (5or 6) per ion pair, independent from the total concentration of ion pairs. Therefore, exchange between cyanide and cyanocobalamin anions in the ammonium ion pair involves aggregated and hydrated ion pairs: the aggregation number, not the hydration level, is the sensitive variable in the proposed extraction model. Anion extraction cannot be called an exchange reaction unless the cation, ammonium in the present case, is insoluble in water. In the present extraction, only two anions are present at significant concentration levels in the aqueous phase and therefore, the extraction is a function of the sole ratio of the concentrations of the two anions involved, becawe the ratio of activity coefficient is assumed constant:
X=
[CblCN-]
[CN-I
(3)
The relevance of the X variable appears clearly by comparison of Figure 4 with Figure 5: the effect of the aqueous cyanide concentration, illustrated on Figure 4 by the multiplicity of extraction curves, results in a unique curve in Figure 5, through the use of variable X in the abscissa. The use of ratio X enables one to ignore all but the relevant relative variations of concentrations [CblCN-] and [CN-1. All curves remain superposed right up to the
Ind. Eng. Chem. Res., Vol. 31,No. 7, 1992 1707 0,08
total [ACNI = 10%
f 1
*
0,OO
0.02
0.04 0,08 0,08 Ratio XI [CbICN.] / [CN-I
0,lO
0,12
0,14
Figure 6. Equilibrium data obtained - with the p-xylene/ACN organic phase: illustration of the ACN total initial concentration influence on extraction yield.
0,0016J
.I
0
-. J g
Table I. Fitted Parameter Values of the Model and the Corresbonding Standard Deviations diluent diluent p-xylene dodecanedecanol parameters 11.2 0.3 10.2 1.1 KI 4.14 x 1013 1.3 x 104 5.00 x 1014 f 5.7 x 103 KII 10.6 0.2 a 7.63 0.41 1.65 6 1.65 3.4 . lo-' 1.52 7 1.52 f 5.6 . lo-' 9.95 0.03 9.35 f 0.21 B 10.7% 13.0% root mean square av error 78 104 no. of exptl points
0,0012
+ + +
0,0000 , A + 0.00
+ *
+ +++
++
I
+
j3CblCN-
.
0,02 0,M Ratio X = [CblCN-I / [CN-I
approach, representative aggregation numbers are mean values, whereas in the stepwise approach, only integer values are considered. The experimental extraction curves of CblCN- show two separate slopes joined by a sharply pronounced bend: this corresponds to two populations with different ACblCN aggregation numbers. Therefore, the second approach has been adopted in this study. It is assumed that the ammonium cyanide exists in the organic phase only in aggregated form (A,CN,) with aggregation number a. The cyanocobalamin anion is incorporated in organic aggregates either alone or in number (j3 + 1) according to the following reactions: CblCN- +),-(6
*+++ + + + *
0,06
Figure 7. Illustration of the main features of the CblCN- anion extraction by ACN in p-xylene.
third phase formation (organic-phase demixtion), that is in the whole range of practical interest: it means that CblCN- extraction is a genuine exchange. Dodecane-decanol/ACN extraction data are shown on Figures 4 and 5; similar curves in Figure 6 represent the experimental data measured with the p-xylene organic phase. Each curve in Figure 6 corresponds to one ammonium overall concentration. Figure 7 is an enlargement of the area near the origin in Figure 5: it illustrates the characteristic features of CblCN- liquid-liquid extraction by anion exchange. Extraction efficiency increases sharply for values of X higher than 0.05 it means that when CblCN- anions are already present inside the polar cores of liquid exchanger aggregates, extraction is enhanced further; ACblCN ion pairs themselves tend to aggregate. The chemical model developed below takes ACblCN aggregation into account in order to interpret the experimental data illustrated in Figures 4-7. A true aggregation model can take into account stepwise aggregation starting from the monomer and going up to the highest possible aggregation number, with a specific equilibrium constant for each aggregation step. When no direct physical evidence about the size of the aggregates themselves is available and equilibrium constants have to be fitted, full aggregation models become impractical because the parameters to be fitted (equilibrium constants) are too numerous and excessively correlated one has to resort to simpler models. A frequent assumption is that most constants (usually starting from the second or third) have identical values. Another is that the whole aggregates population may be divided into classes, each class being characterized by an average aggregation number; in this
*
CN- (I)
= (A,,CN,,,CblCN)+
+ ~(m,) + (As,CN6,,Cb1CN) (A,,,+T)CN(,+T),(,+l)CblCN,+,)
=
+ pCN- (1'
6 is the average number of A,CN, in organic aggregates containing one CblCN-, and 6 7,the average number of A,CN, in aggregates containing (0 1)CblCN-. The six parameters of the model are a,@,a, 7,and the equilibrium constants KI and KII. The average numbers a,j3,6, and 7 are average in a class of aggregates where the real number of anions and cations are around this value. The models assume only three types of aggregates. Models with more than three types of aggregates have been tested without significant improvement of the agreement between calculated and experimental points. Mass balances of extracted cyanmbnlnmin anions and Aliquat cations, combined with equilibrium equations for reactions I and 11, yield the total (stoichiometric) concentration in the organic phase.
+
+
The total quaternary ammonium salts concentration [FItod is given, and [Cb1CN-ltod is thus a function of two variables, X and [(A,CN,)], the latter being an implicit function of [A+],d as the only positive root of eq 5. The parameters of the chemical model were obtained by minimization of the root mean square average relative deviation between experimental and calculated [CblCN],d values. The resulting parameter values and the corresponding standard deviations are given in Table I. Each set of parameters corresponds to one organic solvent. In the case of the dodecane-decanol/ACN organic concentration was used, the 6 phase, as only one [PIbd and 7 parameters cannot be fitted meaningfully the values fitted on the p-xylene/ACN data were used. Table I1 gives the correlation coefficient matrices corresponding to
1708 Ind. Eng. Chem. Res., Vol. 31, No. 7,1992 100
Table 11. Correlation Coefficient Matrices D-Xylene
KI Ku a b T
6
KI
KII
a
1 -09639 0.2411 0.5506 0.4613 -0.5639
-0.03661 -0.697 -0.5972 0.7094
1 -0.6225 0.03233 0.607
KI KII a
B
0,04
0.00
[ACN]23[ACblCN] 11
b
T
B
1
Dodecane-Decanol(6 and K, K,, 1 1 -0.7275 0.5686 -0.3129 -0.1532 0.7332
1 0.37 1 -0,9975 -0.3595 T
O W 0,06 Ratio X = [CblCN-I / [CN-I
0,oo
1
-0.2336
)
0,08
0,lO
0,08 total [ACNI = 10%
0,02
0.04
0,06 0.08 0,lO R a b o X = [CblCN-I / [CN-I
0,12
0,02
0,04 0,M 0,08 h t i o XI [CblCN-I I [CN-I
0,lO
Figure 10. Dodecane-decanol/ACN extraction equilibria: relative amount of the different aggregate populations.
1
Figure 8. Dodecane-decanol/ACN CblCN- extraction equilibria: comparison between calculated and experimental data.
0,02
[ACN]16 ACblCN
1
8
a
401
1
Not Fitted)
Calculated curve ( chemical aggregation model
0,02
1
c
0,14
Figure 9. p-XylenelACN CblCN- extraction equilibria: comparison between calculated and experimental data.
the minimization run for each diluent. The calculated curves are shown in Figures 8 and 9,together with the experimental points: agreement is good. Data found in the literature are consistent with the fitted values: thus the LY parameter values are inside the range reported for the aggregation numbers of tetraheptylammonium chloride in benzene or trioctylmethylammonium nitrate in o-xylene, which were found to vary from 1 to 13 or 2 to 14,respectively, as a function of overall ammonium concentration (Kertes and Gutmann, 1976; Dyrkacz et al., 1979). The calculateddistribution of ammonium cations among the various aggregates is shown in Figure 10 whereas the ACblCN monomers predominate at low X values (which corresponds to [CblCNIua values), the high order ACblCN aggregates become predominant at high X values, when the extraction yield is high.
Conclusion The proposed chemical model gives a good representation of CblCN extraction by anion exchange with Aliquat 336 in cyanide form. It takes into account monomers and higher order ACblCN aggregates with an aggregation of ACN molecules prior to any CblCN- extraction. The average high order aggregates in dodecane-decanol/ACN organic phase are [(ACN)23(ACblCN)ll]. The estimate of the area of one aggregate of stoichioS = 6070 A2, metric composition ((ACN),(ACblCN),,) is assuming that the hydration numbers of (ACN) is 5 and of (ACblCN) is 42 and that the aggregate behaves as a spherical ‘reverse micelle”. Several authors have shown that the interfacial area widely depends on the nature of the counterion paired with the ammonium cation (Scibona et al., 1971;Vandegrift - et al., 1980). The interfacial area occupied by one ACN molecule in p-xylene is estimated from the interfacial tension variation with concentration (Figure 2). Ita value is 141 A2,which can be compared with the AC1 interfacial area value (48A2)and the AN03value (223A2)given in the above-cited -references. The total interfacial area occupied by ACN molecules is S1= 3240 A2. If one assumes that the area occupied by the 11 ACblCN molecules is S2 = S - S1, one finds an area of 257 A2for one ACblCN. Registry No. Dicyanocobalamin, 15041-09-5;Aliquat 336, 63393-96-4.
Literature Cited Bourrel, M.; Schechter, R. S. In Microemulsions and Related Systems: Formulation, Solvency and Physical Properties; Surfactant Science Series, Vol. 30,Bourrel, M., Schechter, R. S., Eds.; Dekker: New York, 1988; pp 31-126. Calvarin, L. Echange #Anions et Dicyanocobalamine. Ph.D. The&, ENSMP, Paris, 1989. Dyrkacz, G. R.;Vandegriff, G. F.; Thomsen, M. W.;Horwitz, E. P. Extraction of Pertechnetate with Tri(alky1)methylammonium Nitrates. Kinetics and Mechanism in the System o-XyleneNitric Acid. J. Phys. Chem. 1979,83,670-674. Eicke, H.F. Surfactants in Nonpolar Solvents. Aggregation and Micellization; Topics in Current Chemistry, Vol. 87;SpringerVerlag: Berlin, 1980,pp 85-145. Kertes, A. S.; Gutmann, H.Surfactants in Organic Solvente: The Physical Chemistry of Aggregation and Micellization. Surf. Colloid Sci. 1986,8,193-195. Levy, 0.; Markovita, G.; Kertee, A. S. Molecular Aesociation and the Dielectric Constant of Long-chain Alkylammonium Salts in Benzene. J . Phys. Chem. 1971,75,542-547. Marcou, L.Method- d’etudes des agents de surface. In Galenica 5, agents de surface et emulsions; Puisieux, F., Seiller, M., Ede.; Technique et Documentation: Paris, 1983;pp 95-152.
Ind. Eng. Chem. Res. 1992,31, 1709-1717 Marcus, Y.; Kertes, A. S. Extraction by Ion-Pairs Formation. In Zon Exchange and Solvent Extraction of Metal Complexes; Marcus, Y., Kertes, A. S., Eds.; Wiley Interscience: New York, 1969;pp 737-814. Scibona, G.; Danesi, P. R.; Conte, A.; Shuppa, B. Interfacial Equilibria with Quaternary Alkylammonium Salts. J. Colloid Interface Sci. 1971,35,631-635.
1709
Shafer, H. M.; Holland, A. J. US.Patent 2 709 669,Merck and Co., 1955. Vandegrift, G. F.; McCarty Lewey, S.; Dyrkacz, G. R.; Horwitz, E. P.J . Znorg. Nucl. Chem. 1980,42,127-130.
Received f o r review February 18, 1992 Accepted March 4,1992
Membrane Solvent Extraction Removal of Priority Organic Pollutants from Aqueous Waste Streams Chang H. Yun, €&vi Prasad,?and Kamalesh K. Sirkar* Department of Chemistry and Chemical Engineering, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, New Jersey 07030
A recently developed nondispersive microporous membrane-based solvent extraction technique has been used to remove a number of priority organic pollutants simultaneously from a synthetic high strength aqueous waste stream. A microporous hydrophobic hollow fiber based membrane extractor having an order of magnitude higher contact area than conventional extraction devices has been used. The pollutants were phenol, 2-chlorophenol, nitrobenzene, toluene, and acrylonitrile. The extracting solvents were methyl isobutyl ketone (MIBK), isopropyl acetate (PAC),and hexane. The distribution coefficient of each pollutant has been measured for each solvent over a wide concentration range. In the once-through extracting mode, the concentrations of all pollutants in the aqueous phase flowing through the hollow fiber lumen were reduced to less than 20 mg/L using either MIBK or IPAc as solvent flowing countercurrently on the hollow fiber module shell-side. A lumped masstransfer analysis has been made to characterize the observed mass transfer for all pollutants. This technique was shown to be efficient in cleaning high strength wastewaters containing pollutants, which may be polar, nonpolar, high boiling, low boiling, aromatic, aliphatic, etc.
Introduction A common commercial method of recovering phenol from concentrated waste streams is extraction with an immiscible organic solvent (Cusack et al., 1991; Patterson, 1985). The phenol concentration in such treated effluents is low enough for biological treatment to take over without upsets from fluctuations in toxic species loading. A question of interest is could we remove priority organic pollutants, in general, successfully by solvent extraction? Equilibrium distribution studies of priority polar organic pollutants, e.g., 2-chlorophenol, nitrobenzene, isophorone, acrylonitrile, acrolein, and N-nitrosodimethylaniline,between water and various organic solventa indicate that, indeed, there exists considerable potential for solvent extraction (Joshi et al., 1984). Recovery of pollutanta thus extracted and solvent recycle can be achieved in a number of ways including distillation. Conceptual designs and economic analyses of such solvent extraction processes for removal and recovery of priority polar organic pollutants, e.g., acrylonitrile, etc., indicate a cost in the range of $4-10/10oO gal of water (Joshi et al., 1984). Compare this with the orders of magnitude higher cost of incineration: high BTU waste costa $1-2/gal whereas low BTU waste costs $7-8/gal (Magee, 1988). Conventional solvent extraction employs dispersion of one phase as drops in another phase and subsequent coalescence of dispersed phase and phase separation. This mode of operation frequently leads to solvent loss by *To whom correspondence should be addressed. Current address: Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, N J 07102. Current address: Separations Products Division, Hoechst Celanese, Charlotte, NC 28273.
emulsion formation. This is detrimental to the process economics. For example, the loss of costly extractants or chelating agents in heavy metal removal poses a signifcant economic minus. Such solvent loss may also unduly increase the organic loading of the treated stream requiring additional cleanup. Emulsion formation in conventional dispersion-based solvent extraction is thus a major shortcoming. In conventional solvent extraction, a given solvent extraction column can operate efficiently within a small flow range; larger flow variations lead to flooding (Treybal, 1963). Thus, fluctuations in waste stream rate can load or flood the column. Further, there has to be a density difference between the aqueous and the organic stream. Moreover, particulates in an aqueous waste stream are a significant problem in such columns. We have recently developed a dispersion-free solvent extraction technique which eliminates all such problems (Kiani et al., 1984; Frank and Sirkar, 1985; Prasad et al., 1986; Frank and Sirkar,1986, Prasad and Sirkar,1987a,b). In this technique (Figure la) the aqueoumxganic interface is immobilized in the pores of highly open microporous polymeric membranes as the two phases flow on two sides of the membrane. The membrane pores are filled with the phase, preferentially wetting it, while the other immiscible phase is completely excluded. Solvent extraction is easily achieved by transfer of solutes through the aqueous-organic interfaces immobilized at the pore mouths due to a pressure difference between the two phases. There is no coalescence problem since there is no dispersion. Each phase can have any flow rate or any density. Fermentation broths having suspended cells have been easily handled (Frank and Sirkar, 1985,19861, suggesting little problem with fouling streams. However, certain precautions or pretreatments may be necessary with particulates in
0888-588519212631-1709$03.00/00 1992 American Chemical Society
