I n d . Eng. Chem. Res. 1995,34, 4479-4485
4479
Extraction of Anions with Tertiary Amine from Aqueous Solutions of Mixed Acid and Salt Lu-Kwang Ju* and h o o p Verma Department of Chemical Engineering, The University of Akron, Akron, Ohio 44325
Amine extraction was made in aqueous solutions of HC1 (0-0.5 M), HzS04 (0-0.4 M), and HsPO4 (0-0.3 M) with their corresponding salts (0-0.08 M), i.e., NaC1, MgS04, and NazHP04, respectively, covering a wide pH range (Le., 0.4-8.6). The organic phase comprised 15%(w/w) Alamine 336 (tri-Cg to ClO alkylamine) in the diluent, n-hexadecane or oleyl alcohol. The common extraction mechanism involving the protonated tertiary amine was observed in the systems of pure or concentrated acid. The presence of salts was found to enable a nonprotonated extraction mechanism where the anions were postulated to couple with the metallic cations for partition into the organic phase, Among the anions studied, the nonprotonated mechanism was most significant for phosphate extraction and least for chloride extraction. Experimental results have been modeled with simultaneous extraction by protonated and nonprotonated mechanisms, and the best-fit apparent extraction constants are reported.
Introduction There has been increasing interest in fermentative production of organic acids from biomass (Yang et al., 1994). The acid concentrations harvested from these fermentations are usually low ( " 0 3 > HC1> HBr > H2S04 > When these anions are present, the amine extractants may have preference for them over the carboxylates. For example, the addition of HC1 has been found to decrease the yield of lactic acid extraction with Alamine 336 (a straight-chain tertiary amine from Henkel Corp., Kanakakee, IL) (Scholler et al., 1993). Anion competition has also been observed in the extraction of chromium (as chromate and dichromate) by Aliquat 336 (a quaternary amine from Henkel Corp.) (Fuller and Li, 1984). The extraction was found minimal when chloride ions were present in the feed aqueous phase. In our previous study (Ju and Verma, 1994), pure lactic acid solutions were used for determination of the fundamental properties, i.e., the equilibrium extraction constant and the effective diffusion coefficient, of liquid membrane extraction with the tertiary amine Alamine 336. In further pursuit t o understand the effects of the presence of other anions, we have studied the extraction of chloride, sulfate, and phosphate from aqueous solutions of various saluacid combinations, to cover a wide pH range and simulate the condition of acidification. The experimental results and mathematical models, which incorporate both protonated and nonprotonated extraction mechanisms, are reported here.
0888-5885/95/2634-4479$09.0OlO 0 1995 American Chemical Society
4480 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
Chemical Models for Protonated Extraction Mechanisms The "chemical" interactions between the amines and the acids are strong compared to the "physical" interactions in the system. Therefore, the equilibrium behavior for amine extraction of acids is generally described by chemical models, postulating the formation of various stoichiometric complexes of acid and amine (Connors, 1987). The capacity of amines to act as extractants for acids is related to their basicity. The amines bind the proton more strongly than does a water molecule, providing the driving force for the acid extraction through ion pairing (neutralization reaction). The amine salts formed are often also basic and may themselves act as extractants to bind with another acid molecule (addition reaction). Furthermore, the anion of the amine salt can be replaced by other anions via the anion-exchange reaction. Emphasizing the protonated nature of the neutralization reaction, the extraction of acid (H,A) with a tertiary amine (R3N) can be represented by n(Hf>,
+ (A"-), + n(R3N),
-
((R3NH+),A"-),
(1)
where subscripts w and o stand for the aqueous and organic phase, respectively. Quantitatively, it can be described by the law of mass action with an equilibrium extraction constant
In general, the activity coefficients (7's) are scarcely available for the organic-phase species (Bullock et al., 1964; Kertes and Markovits, 1968). However, the ratio Y((R~NH+)"A"-)I(~R~N)~ often remains constant for wide concentration ranges of the amine and acid (Shmidt, 1971). When the aqueous-phase acid concentration is low, y* can be further assumed to be 1. Thus, an apparent extraction constant can be used effectively, i.e.,
If the proposed stoichiometry and the law of mass action are valid, the log-log plot of I(R3NH+),An-l0vs ((tR~Nlo~Hf3,>~tA~~l,) must be linear. This is often utilized t o determine the extraction constant and to examine the stoichiometry of acid extraction by neutralization reaction. As mentioned earlier, many amine salts are capable of extracting acids by formation of addition products. The mechanism of this type of extraction can be represented, for a monoprotic acid, by the following reaction: (R3NHA), + (H'),
+ (A-1,
-
((R3NHA)HA)0
(4)
Similarly, the anion-exchange reaction can be described as (R3NHA), + @-Iw
-
(RJWB),
+ (-4-1,
(5)
Materials and Methods The organic phase used in the study contained 15% (w/w) Alamine 336 (tricaprylamine, or tri-Cs to CIO
alkylamine) as the extractant. n-Hexadecane and oleyl alcohol were used as the diluents to represent the nonpolar and polar solvents, respectively. The aqueous solutions studied included various combinations of NaCl (0-0.08 M) with HCl(0-0.5 M), MgS04 (0-0.08 M) with H2S04 (0-0.4 M), and Na2HP04 (0-0.08 M) with H3PO4 (0-0.3 M) to cover a wide range of pH for each system (i.e., chloride, 0.4-7.2; sulfate, 1.2-7.2; phosphate, 1.3-8.6). The two phases, in a fixed water-tooil volume ratio of 5, were mixed at the room temperature (23 f 1 "C) for 15 h in a magnetically stirred, tightly stoppered flask. They were then carefully separated by centrifugation. Aqueous-phase pH measurements were performed with an Orion Model 420A pH meter equipped with an Orion Ross pH electrode. The methods for anion analyses were carefully chosen to provide accurate and reproducible results. Only the aqueous-phase samples were analyzed in the study, and a material balance was used to determine the concentration of the extracted anion species in the organic phase. This approach has been proved adequate in our previous study with lactic acid extraction: the material balance made with the separately measured lactate concentrations in both aqueous and organic phases normally agreed within f 5 % (Ju and Verma, 1994). The approach has also been employed commonly in the literature. The chloride ion concentration in the aqueous phase was determined by the argentometric method (Greenberg et al., 1985). It is suitable for relatively clear samples when 0.15-10 mg of C1- is present in the portion titrated. Extreme care was observed in recognition of the end point of the titration. The results were then corrected for the reagent blank value. The standard deviation of the analysis was normally lower than 3%. The vanadomolybdophosphoric acid colorimetric method was used to determine the phosphorus content of the aqueous phase (Greenberg et al., 1985). In a dilute orthophosphate solution, ammonium molybdate reacts under acid conditions to form molybdophosphoric acid. In the presence of vanadium, yellow vanadomolybdophosphoric acid is formed. The intensity of the yellow color is proportional to the phosphorus concentration. With the concentration of phosphorus in the sample being adjusted to 4-18 mgL, the absorbance reading was taken at 470 nm using a spectrophotometer (Perkin-Elmer Lambda 3B). Analyses were done in triplicates to ensure the standard deviations were within 3%. The sulfate concentrations were determined either by titration with standard NaOH solutions when pure H2SO4solutions were extracted or by turbidometric method (Greenberg et al., 1985) when both MgS04 and HzS04 were present. In the turbidometric method, sulfate ions are precipitated in an acetic acid medium with barium chloride (BaC12)to form barium sulfate (BaS04) crystals of uniform size. Light absorbance of the Bas04 suspension is measured with a spectrophotometer at 420 nm. The sulfate concentration is determined by comparison of the reading with a standard calibration curve. The analysis is sensitive to the stirring speed; hence, identical magnetic stirrers and a fixed stirring speed of 100 rpm were used throughout the analysis.
Results and Discussion 1. The Loading Curves. The experimental results obtained in the study for different anions are presented
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4481
4
1.5
A
A
4 j
1.o
I
0
0 0
A
0.5
A
A
0.0
9aM
-10
-8
A
-4
-6
-2
loS(["IJCIlw) Figure 1. Extraction of chloride by 15% (w/w) Alamine 336 in oleyl alcohol (0)or n-hexadecane (A). The filled symbols are for the aqueous solutions of pure acid (HC1).
0.6
0.5 7
-!
0.4
0
0.2
O A
0.1
bf
0.0
A
A
A
-&+qAy+y+-J -18 -16 -14 -12 -10 -8 -6 -4 -2
~os(["!-J~l4 Figure 2. Extraction of sulfate by 15%(w/w) Alamine 336 in oleyl or n-hexadecane (A). The filled symbols are for the alcohol (0) aqueous solutions of pure acid (HzS04).
in Figures 1-3 as the loading curves, i.e., 2 vs Log(([H+l,)fl[AI,). The loading of the extractant, 2, is calculated as [AlJRsNIi. [AI, and [AI, represent the concentrations of the anion species (all forms) present in the equilibrated aqueous and organic phase, respectively, be it iri the undissociated acid, dissociated ions, protonated complex, (R3NH+),An-, or any other forms. [R3N]i is the initial tertiary amine concentration in the organic phase. This form of data presentation has been chosen because it does not rely much on the knowledge of the actual extraction mechanism(s). The characteristics of loading curves, for amine extraction of pure carboxylic acids, have been reported by Tamada et al. (1990). According to the commonly cited protonated mechanism (eq l),with strong enough acids the equilibrium favors complex formation and results in practically total conversion of the initially available amine. With further increase in the acid concentration, no more R3N is available for complexation and a saturation type of behavior may be expected at high ([H+l,)n[Al,. On the other hand, at very low ([H+l,)n[Al,, the formation of amine-acid complex can be expected t o be negligible and Z approaches zero. In this study, the two diluents performed fairly similar for amine extraction of H3P04 (Figure 3). However, before reaching saturation, oleyl alcohol was found more effective than hexadecane for extraction of HC1 and HzSO4: good extraction (i.e., high 2)can be obtained a t lower ([H+l,)n[Al, (Figures 1 and 2). This agrees well with the general finding in the literature (Shmidt, 1971). Several studies on the effects of diluent led to the conclusion that the diluent interacts with both
the amine and the complex through the functional groups (chemical properties) and with the complex through long-range columbic effects (dielectric properties) (Diamond, 1967). Because polar diluents improve the solubility of the complex in the organic phase, they generally yield better extraction (Bucher and Diamond, 1965). The extraction behaviors a t the two extremities, i.e., the saturation level of 2 at high ([H+],)"[A], and the deviation from the protonated mechanism at low ([H+1,~"[A1,, deserve still more description. 1.1. Saturation Level of 2. a. Chloride. For chloride (Figure l),the saturation level of 2 was found t o be nearly 1 in the system with oleyl alcohol as the diluent, indicating the formation of R3NHC1. Overloading (2> l), however, was observed in the system with hexadecane as the diluent, suggesting the occurrence of addition reaction (eq 4). It should be noted that, for the two data points with 2 > 1, the initial acid concentrations were 200 and 400 mM, much higher than the 70 mM required for the complete conversion of R3N to R3NHC1. The formation of addition products during the extraction of acids with amine salts has been confirmed in the literature by spectroscopic methods (e.g., Barrow and Yerger, 1954; Smith and Vitoria, 1968; Duda and Szafran, 1978; Chibizov and Komissarova, 1984; Tamada and King, 1990a). The observation of the current study that the addition products were formed in the system with hexadecane as the diluent but not in that with oleyl alcohol is consistent with the literature reports. When extracting acetic acid with tertiary amines in different diluents, Ricker et al. (1979) found that the 1:l complex, ((R3NH+)Ac-),was favored in polar diluents capable of solvating the complex, while higher order complexes and aggregates were more likely to be formed in hydrocarbonlike diluents. In a similar study, Hogfeldt and Fredlund (1967) proposed the formation of ((R3N),(HAc),) with the n:m ratio of 2 and 4 for the nonpolar diluents n-heptane and o-xylene, respectively. b. Sulfate. For sulfate (Figure 21, the saturation level of 2 was close to 0.5 in oleyl alcohol, suggesting the formation of the complex ( R ~ N H + ) Z S O ~Accord~-. ingly, 2 has been plotted against log((~H+l,)2[Sl,) in Figure 2. Again, in the system with nonpolar nhexadecane as the diluent, a higher saturation loading (ca. 0.65) was found, indicating that (R3NH+)HS04-was also present. Depending on the pH of the aqueous solution, sulfate is present as either monobasic bisulfate ions (HS04-) or dibasic sulfate ions (s04'-). Thus, [SI, = [HS04-lw [S042-lw. The pKa value of HS04- is 1.9 at 25 "C (Kask and Rawn, 1993). In this study, the saturation level of 2 of 0.5 in oleyl alcohol was observed when the aqueous phase contained HS04- predominantly at equilibrium (pH = 0.5-1.5). It is postulated that once the complex (R3NH+)HS04-has been formed in the organic phase, it may combine with the free amine t o form the complex ( R ~ N H + ) Z S O ~ ~ - . c. Phosphate. For phosphate (Figure 31, the saturation levels of 2 cannot be determined clearly from the data obtained in the study. Nevertheless, the value of 2 as high as 1.2 has been found in both oleyl alcohol and hexadecane systems. Therefore, the complex (R3NH+)HzP04-is likely the dominant form, with some addition products being also present. 1.2. Deviation from Protonated Mechanism. As mentioned earlier, 2 should approach zero a t low
+
4482 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
1.2 1.0
i
3
or ([Na+lw[Pl,) is clearly shown in both figures.
+
1
2
3
4
5
6
7
8
PH
Figure 4. Dependency of Kp on equilibrium pH of the aqueous solutions of mixed NaCl and HCl, assuming that the extraction of chloride is only by the protonated mechanism; ( 0 )oleyl alcohol and ( A ) n-hexadecane.
([H+l,)n[A1, if the protonated mechanism is solely responsible. This is different from the experimental results obtained in the study. In particular, a second peak of 2 appears at log([H+lw2[S1,)= -14 in the loading curve for the sulfate extraction with hexadecane as the diluent (Figure 21, and for phosphate extraction 2 rises to about 1.0 at very low ([H+l,[Pl,) (Figure 3). Even for the chloride extraction from aqueous solutions of mixed acid and salt, the deviation becomes more obvious in Figure 4,where the values of Kp calculated from experimental data by assuming that the extraction is enabled by the protonated mechanism only are plotted against the equilibrium [H+l, logarithmically. Kpwas found strongly dependent on [H+l, while no dependency should be expected ideally. The existence of residual extraction higher than what can be accounted for by the common protonated mechanism, at low ([H+l,)n[Alw,has been seen but unexplained in the literature (Ricker et al., 1979; Wennersten, 1983; Yang et al., 1991). On the other hand, another type of deviation from the law of mass action in amine-facilitated extraction has been reported previously. In the extraction of uranyl sulfate from acidic aqueous solutions by benzene solutions of tri-n-octylamine sulfate and didecylamine sulfate, an anomalous dependency of the uranium extraction constant on the extractant concentration in the diluent (benzene) was observed (Allen, 1958a,b; McDowell and Baes, 1958). Hypothesizing that this dependency was due t o the metastable conditions induced by the vigorous agitation customarily used in liquid-liquid extraction, Allen and
Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 4483 For phosphate: protonated
(R,N),
+ (H’), + (H,PO,-),
KP
((R3NH+)H2P04-),(12) nonprotonated
2(Naf),
+ (HPO:-),
-
Kw
((Naf),HP0,2-), (13)
Figure 6. Three-dimensional plot ofZ as a function of log([H+Iw2[SI,) and log([Mg2+l,[Sl,) for sulfate extraction from aqueous solutions of mixed HzS04 and MgS04 with n-hexadecane as the diluent.
Figure 7. Three-dimensional plot of 2 as a function of log([H+l,[PI,) and log([Nafl,[Pl,) for phosphate extraction from aqueous solutions of mixed H3P04 and NazHP04 with n-hexadecane ( 0 ) or oleyl alcohol (0)as the diluent.
The experimental results are therefore modelled with simultaneous extraction by protonated and nonprotonated mechanisms. For chloride: protonated
(R,N),
+ (H’), + (Cl-),
KP
((R,NH+)Cl->, (6) (addition)
((R,NH+)Cl-),
+ (H?, + (Cl-),
-
Kad
(((R~NHc~)H+)c~-), nonprotonated
(Na’),
-
+ (Cl-IwKw (Na’Cl-),
For sulfate:
Several points should be noted about the above mechanisms. First, the above form of nonprotonated extraction mechanism includes implicitly the possible effects of the diluents, such as chemical complexation or physical interactions. As the diluent is present in great excess, its concentration remains practically unchanged and can be lumped into the apparent equilibrium constant Knp. Second, the addition reactions for extraction of chloride and sulfate are considered only for the systems with hexadecane as the diluent. Third, although rather arbitrary, the choice of the nonprotonated mechanism for phosphate extraction is based on the observation of better extraction at higher pH (891, which favors the dominance of HP04’- in the aqueous phase as the values of pKa for phosphoric acids at 25 “C are 2.10, 7.21, and 12.66, subsequently (Kask and Rawn, 1993). The best-fit values for the equilibrium constants Kp, and Knpare summarized in Table 1. While any physical effects of the tertiary amine, in modifylng the properties of the organic phase, on the nonprotonated extraction are incorporated in the above Knp,the role of its possible “chemical”effects is uncertain. Literature abounds with the information on extraction of metal salts with amines, although mainly for valuable elements (with uranium being the most important), noble metals, and rare metals (Shmidt, 1971). Two types of extraction reactions were identified: incorporation and double salt formation. The former relies on the formation of stable metal-nitrogen coordinate bonds; the latter involves the extraction of the metal salts by the amine-acid complexes already present in the organic phase, leading to the formation of the double salts (amineHA),Me&. “he incorporation reaction is not common because most metals do not display a marked tendency to metal-nitrogen bond formation in the presence of water (Gindin et al., 1967). Most reported extractions of metal salts, therefore, were based on the mechanism of double salt formation (Shmidt, 1971). In this study, however, the nonprotonated extraction occurred in the systems of relatively high pH. The presence of high concentrations of amine-acid complex for the formation of double salts is inconceivable. Therefore, the possible chemical effects of the tertiary amine have to be considered, e.g., in the following reaction for chloride extraction:
+
+
(addition) ((R3NH+)2S0,2-), (H’), + (HSO,-), 2((R3NH+)HS04-), (10)
+
-
Knpl
n(R,N), (Na+), ((217, ((R,N),Na+Cl-), (14) The best-fit parameters are n = (8.7 f 1.8) and Kn; = (1.1f 4.0) x lo7 for the system with oleyl alcohol as the diluent, and n = (6.9 f 0.9) and Knpl = (3.3 & 5.0) x lo5 for n-hexadecane. While the standard deviations for Kn< are very large, the best-fit values of n suggest that, on the average, seven to nine molecules of amine may “combine”with each ion-paired NaCl to enable its partition into the oil phase. As the physical meaning
4484 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 Table 1. Best-Fit Values of the Equilibrium Constants anion diluent
chloride oleyl alcohol
chloride n-hexadecane
sulfate oleyl alcohol
sulfate n-hexadecane
phosphate oleyl alcohol
phosphate n-hexadecane
1.5 x lo6 9.1 x 102 4.4 107 1.0 x 106 6.9 104 7.0 103 KPa -d 1.4 x lo2 7.4 x 102 Kadb 1.9 14 7.6 8.9 x 103 2.6 x 104 KIlpC a The units of Kp are M-2 for extraction of chloride and phosphate and M-4 for extraction of sulfate. * The units of Kad are M-2 for extraction of chloride and M-I for extraction of sulfate. The units of Knpare M-' for extraction of chloride and M-2 for extraction of phosphate. -, values assumed to be negligible and excluded from the fitting.
of this phenomenon is not clear, the modeling with this more complicated approach is not further pursued here. The answer to the question about the involvement of the tertiary amine in the nonprotonated mechanism is, however, very important. It dictates whether the two mechanisms, protonated and nonprotonated, are competitive for the available extractants or not. Furthermore, the observed nonprotonated extraction may be significant for the design of amine extraction of carboxylic acids. The extractive fermentations that use amines as the extractants have been commonly designed on the basis of the protonated mechanism. Their effectiveness was shown to be hampered by the dilemma in the choice of pH for fermentation. The optimal pH for cell growth and product formation is higher than the pKa of the acid product and yields very poor extraction in situ; the lower pH which favors the extraction is inhibitory or even fatal for the microorganisms. A thorough understanding of the nonprotonated mechanism may lead t o the development of new extraction design for more effective removal of the inhibitory products at the optimal fermentation pH. More study is certainly warranted.
Conclusion When aqueous solutions of pure or strong acids were used, the protonated mechanism prevailed and practically all tertiary amine molecules would be converted to the amine-acid complexes with high enough acid concentrations. In the system with polar oleyl alcohol as the diluent, this led to a typical saturation-like behavior of the loading curve. According t o the saturated loading level, the complexes formed at saturation were (R3NH+)Cl-,(R3NH+)2S0d2-,and (R3NH+)H2P04-, respectively. Loading a t higher than the saturation levels was, however, observed in the systems with nonpolar n-hexadecane as the diluent under this condition of high acid concentration. Consistent with the literature reports, this has been attributed to the addition reaction due to the basicity of the complexes themselves. Nevertheless, before reaching the saturation, oleyl alcohol was much more effective than hexadecane in offering good extraction at lower acid concentrations. The presence of salts was shown to enable a proposed nonprotonated extraction where the anions are coupled with the cations of the salts for partition into the organic phase. Its significance depends on the aqueous-phase pH and salt concentration as well as the type of anion involved. Among the anions studied, the nonprotonated mechanism appeared to be most significant for phosphate extraction and least for chloride extraction. Experimental results have been modeled with simultaneous extraction by protonated and nonprotonated mechanisms, and the best-fit apparent equilibrium extraction constants are reported.
Nomenclature K , = dissociation constant of acid, mol& Kad = equilibrium constant for the addition reaction, i.e., eqs 7 and 10 K,, = equilibrium constant for the extraction by nonprotonated mechanism, Le., eqs 8, 11, and 13 Kn< = equilibrium constant for the extraction by nonprotonated mechanism considering the possible chemical effects of the tertiary amine, i.e., eq 14 Kp= apparent extraction constant based on the protonated mechanism, as defined in eq 3 K,' = equilibrium extraction constant, as defined in eq 2 2 = loading of extractant, mol of the anion species of the acid (in all forms)/molof amine y = activity coefficient Subscripts i = initial concentration o = organic phase w = aqueous phase
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Received for review April 11, 1995 Accepted July 25, 1995@ IE950238+
Abstract published in Advance A C S Abstracts, November 1, 1995. @
