Separation and/or Concentration of Phenolic Materials from Dilute Aqueous Solutions Alan Carpenter, Sidney Siggia, * and Stephen Carter' Department of Chemistv, University of Massachusetts, Amherst, Mass. 0 1002
A cross-linked aqueous insoluble N-vinyl-2-pyrrolidone polymer (PVP) is used in the separation and/or concentration of phenolic compounds from aqueous solution. The percent removal of phenolic compound is studied as a function of solution pH. The pH for maximum bindlng of the phenolic to PVP is dependent on the acldity of the phenolic compound. Binding to PVP is particularly favorable for poiyhydroxyi and extended aromatic compounds. Columns packed with PVP removed >95% of the simple phenolic compounds from aqueous solution and quantitative recovery of the bound phenolic compound was obtained using a 4 M urea eluent.
Phenolic compounds play an important functional part in both natural and synthetic systems. This can be readily appreciated when one considers the presence of phenolics in plant life (Le., lignins and flavonoids), in animal life (i.e., the estrogens and catecholamines), in citrus juices, beers, and wines (Le., the tannins), in disinfectants, in pharmaceuticals (i.e., the hydroxy tetracyclines), and also in our natural waters as industrial pollutants (Le., the cresols and naphthols). Poly(vinylpyrro1idone) (PVP) has been used in a variety of functions. P V P and other polyamides have been shown to bind tannins (1-31, and P V P has long been used to debitter citrus juices ( 4 ) , wines and beers ( 5 ) . Antibiotics which are sparingly soluble in water can be solubilized in aerosols containing a PVP-vinyl acetate copolymer (6). P V P has been demonstrated to be a complexor of phenobarbital and secobarbital and the injection of PVP-10 (mol wt 10000) into the blood of rabbits relieves barbiturate intoxification very rapidly (7). Antimicrobial agents such as chloramphenicol have also been shown to be bindable to P V P (8). Among its other uses, P V P has been used as a transporter of toxins ( 9 ) , as a blood plasma expander (IO), as a protective colloid in cosmetics ( I I ) , and as a means of extracting phenolics from botanical samples (1-3, 12). Several authors have studied the interaction of aromatic compounds containing polar substituents with P V P (13-18). These studies have been largely directed toward the elucidation and measurement of the physical forces involved in the complexation of polar aromatic compounds. None of these studies has investigated the possible application of P V P resins to the quantitative removal of phenolic compounds from aqueous solution. Little work has been performed on the use of selective resin materials suitable for the concentration of phenolic compounds from aqueous solution. Recently an anion exchange resin has been shown to be useful in the removal and recovery of phenolics from basic aqueous solution (19). This system, however, also absorbs some inorganic water soluble components (Le., bicarbonate) and would not be useful for phenolics which oxidize in basic solution such as Present address, Department of Chemistry, University of Arizona, Tucson, Ariz. 85700.
aminophenols and most polyhydroxy1 phenolics (hydroquinone, catechol, phloroglucinol, pyrogallol, etc.). In the present study, the utility of a cross-linked aqueous insoluble, poly(vinylpyrro1idone) (Polyclar AT) in the removal and recovery of phenolics from aqueous solution is investigated. One-plate batch equilibrations between the insoluble P V P resin and phenolic solutions of concentrations from 20-40 ppm are studied a t varying hydrogen ion concentrations. Percent uptake vs. p H profiles have been obtained for some 13 phenolics of interest in biological and environmental systems. Studies have also been performed of the % uptake of a few phenolics vs. changing concentration a t the optimum p H of binding. The stoichiometry of complexation between phenol and aqueous insoluble P V P has been determined, based on the quantity of phenol removed a t the maximum uptake (in mg/g) of the polymer. I t has been suggested in past investigations that the complexation of phenolics by P V P has been due in part to hydrogen bondings (2, 3, 13). An infrared spectroscopic study of the complex has also been performed confirming the presence of hydrogen bonding. The possible application of PVP-based resins to the quantitative removal of phenolics from aqueous solution is of special interest in light of the phenolic pollutants found in our environment. Preliminary studies involving the removal of phenolics from aqueous solution using columns packed with insoluble PVP have been performed.
EXPERIMENTAL Infrared Spectroscopic Study. IR spectra were obtained for Polyclar AT, phenol, and for Polyclar AT after equilibration with a concentrated (-1000 ppm) phenol solution. The PVP resin before and after equilibration were dried in a desiccator -24-48 hr before they were prepared as the KBr pellets. The KBr pellets were made under a N2 atmosphere in order to limit adsorption of H20 by the resin which is very hygroscopic. Study of Uptake vs. pH. At least 0.1 g of reagent grade phenolic compound was dissolved in l l. of distilled deionized water to make a stock solution of about 100 ppm. Care was taken to minimize sublimation with those phenols which were volatile (Le., phenol and the cresols). Catechol, resorcinol, hydroquinone, pyrogallol, phloroglucinol, and the catecholamines were dissolved in pH 2-3 water to prevent oxidation. It was also necessary to refrigerate the catecholamines at -10 "C to prevent oxidation which occurs over long periods of time even a t acidic pH's. Aliquots taken from the stock solution of each phenolic were diluted to a concentration suitable for uv analysis and the pH's were adjusted using HC1 or NaOH with a Corning pH meter and Ag/ AgCl and reference calomel electrodes. Washed and dried Polyclar AT, 0.25 g, was placed in a 125-ml Erlenmeyer flask with 25 ml of the phenolic solution of concentration 20-40 ppm and the mixture was shaken on a mechanical shaker for at least 1 hr. Distilled water blanks were prepared similarly at each pH studied. After equilibration, the Polyclar was allowed to settle for at least 4 hr since premature analysis could lead to severe difficulties caused by light scattering as a result of the suspended polymer. The clear supernatent over the Polyclar AT was then analyzed for its absorbance in the uv at the wavelength of maximum absorbance for the phenol studied vs. the blank described earlier. The solution concentrations were read off a standard curve which had ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
225
Table I. Column Conditions "IO
u 70P a60-
b5040-
Phenolic
N, pressure, lb/in2
Phenol p-Cresol Catechol Norepinephrine Epinephrine
30 30 30 40 40
PH 4.5-5.0 4.5-5.0 3.0 3.2 3.2
Flow rate, ml/min
-1.0 -1.0 -1.0 -0.5 0.5
-
30-
20-
'
20
30
40
5.0
60
70
80
Table 11. pH Studies-Batch Equilibration
t
90
00
PH
Figure 1. % uptake vs. pH.
= catechol, 0-0-0
O-O-O
=
1-naphthol
307
L
t
;
20
30
40
50
70 80
60
90
100
PH Figure 2. % uptake vs. pH. O-O-O
= phenol
been prepared earlier. The % uptake was calculated from the concentrations of phenolic present in solution before and after equilibration with the insoluble PVP. Study of the Uptake vs. Concentration at Constant pH. The procedure employed here was much the same as that employed in the pH studies, only requiring dilution of the supernatent over the insoluble PVP before uv measurenients were made. Preparation of Polyclar AT. PVP polymers were obtained from General Aniline and Film Corp. The Polyclar A T was obtained in a 10-lb package and initial studies using this resin indicated the presence of lower molecular weight water soluble fractions of PVP which resulted in a strong uv absorbance at -215 nm. To remove or limit this difficulty -25 g of Polyclar was equilibrated first with pH -12 deionized water, then with -pH 2 deionized water, and finally was washed with neutral pH water. The transmittance of the final wash solution was no less than 90% at 270 nm. The PVP resin was then dried in an oven at -110 "C for >24 hr before use in uptake studies. Instrumental. The IR spectra were obtained using the PerkinElmer 137 Infrared Spectrophotometer with a slit width of 30 fi and a slow scan rate was employed. The uv analyses were performed using the Heath modular double beam spectrophotometer consisting of the EU-701-50 light source, the EU-707-11 double beam sample chamber, the EU-7930 PM module, and the EU-805 DVM as the readout. A deuterium lamp was used as the light source, the monochromator slit width was set at 250 p , and the PM tube voltage was -900 V. Column Studies. The insoluble PVP was prepared as described earlier. Before column packing, the PVP was washed with distilled water adjusted to the same pH at which the phenolic was being studied. The column was then packed with 3-5 g of Polyclar and 50 ml of -40-ppm phenolic solution adjusted to the pH of maximum complexation was then passed through under pressure exerted by nitrogen gas. With the 1-cm diameter column used, a height of -10 cm of Polyclar was obtained and the rate of the phenolic solution passed through the column was extremely slow. The conditions used in these column studies are given in Table I. The solution collected after passage through the column was analyzed as before (via uv absorbance measurements) for the uncomplexed phenolic. The phenolic bound on the Polyclar column was released with 50 ml of 4 M urea ( 2 ) .Investigation beforehand indi226
* ANALYTICAL CHEMISTRY, VOL.
48, NO. 1, JANUARY 1976
%re-
Phenolic
Initial concn, ppm
pH range for max uptake
moval
Phenol p-Cresol o-Cresol p-Nitrophenol Resorcinol Hydroquinone Catechol Epinephrine Norepinephrine Pyrogallol Phloroglucinol Tannic acid 1-Naphthol
47.1 23.0 28.0 40.0 19.0 19.0 29.0 28.0 22.0 40.0 40.0 40.0 40.0
4.0-5.0 4.0-5.0 3.0-4.0 2.0-3.5 3.0-6.0 4.0 3.0 3.2 3.2 2.5-3.5 2.0-2.5 2.0-3.0 2.5-6.0
26. 43. 42. 29. 54. 77. 73. 5. 10. 79. 91. 93. 84.
cated no discernible absorbtivity change for the phenolics in the urea solutions and A,, for each phenolic remained the same in the urea matrix.
RESULTS IR Study. Hydrogen bonding is a typical interaction of hydroxyl compounds and amides in general, and it has been suggested by some authors that this type of bond plays a n integral part in the binding of phenols by P V P (2, 3, 13). T h e ir spectra obtained for Polyclar AT, for phenol, and for Polyclar A T after equilibration show differences indicative of hydrogen bonding. T h e 0-H stretching mode absorption of phenol is shifted from 3600 cm-' to 3200 cm-' when bound t o Polyclar. Also as a result of H-bond interactions, the carbonyl absorption band of Polyclar is shifted slightly from 1690 cm-' t o 1670 cm-'. Uptake vs. pH. T h e % uptake of phenolics was dependent on the p H of the phenol solution. In some cases, the maximum % uptake was observed over a narrow range of p H as typified by catechol, while in other cases the maximum % uptake was observed over a wide range of p H as typified by 1-naphthol (see Figure 1). As shown by the plot of uptake vs. p H for phenol in Figure 2 there is no binding of phenol to P V P above p H 10. This basic p H range corresponds t o that in which only the phenolate anion exists in solution and the lack of complexation is due to the absence of hydrogen bond interactions. This uptake vs. p H behavior of phenol is typical of all the phenolics studied. The pH's of maximum uptake and the % uptake at these pH's are listed in Table I1 for the respective phenols. T h e relative order of % uptake for various phenolic compounds shows that % uptake increases with a n increasing number of hydroxyl groups. For example, the order of uptake for some simple phenol derivatives is phloroglucinol > resorcinol > phenol; and pyrogallol > catechol > phenol. T h e extent of aromaticity of the phenolic is also seen t o greatly effect the max % uptake value as 1-naphthol is removed from solution to a much greater extent than phenol. This is consistent with the fact that charge transfer bond-
Table 111. Column Study
i
% recovInitial concn
Phenol p-Cresol Catechol Epinephrine Norepinephrine
38.8 40.6 40.7 37.1 40.0
% removal
ery of % overall Complexed efficiency
98. i 2. 100 i 95. i 2. 100 f 97. i 1. 99 i 92. i 3. 101 f 67. ( 2 trials) 97.
3. 96. 3. 94. 2. 95.
2. 92. 65.
f
i f
i
3. 2. 1. 3.
ing interactions are present as well as hydrogen bond interactions in the complexation of phenolics by PVP. The pK, value of the phenolic does not have significant effect on the % maximum uptake as exemplified by the similar % uptake values for p-nitrophenol (pK, = 7.3) and phenol (pK, = 9.8). The p H of maximum uptake does not follow a regular trend with p K values of the phenols; however, the p H of maximum uptake decreases with an increasing number of hydroxyl groups. Uptake vs. Concentration at Constant pH. The uptakes of o-cresol, p-cresol, and phenol over the concentration range 20-600 ppm, at the optimum pH’s of complexation for each, are shown in Figure 3. There is no regular trend noticeable except it is significant that the plots are not linear, which is an indication t h a t the polymer structure may be changing with changing concentration. This change in configuration was particularly evident when a 56.4 ppt solution of phenol caused the Polyclar to agglomerate in large clumps in the flask in contrast to the finely divided resin present a t low phenolic concentration. The maximum uptake of phenol by insoluble P V P was reached a t 56.4 ppt and was found t o be 0.866 g phenollg PVP. Since the cross-linkages of the insoluble polymer account for only about 1-2% of the polymer weight, it can be estimated that the stoichiometry of complexation is approximately 1to 1.
1 e Polvclar AT 112 g/mol monomer
10.0
= 1.02
0.866 g uptake 94.1 g phenol/mol This evidence supports the conclusions of other workers who have also determined that the phenol-pyrrolidone binding is 1 to 1 (17,20). Column Studies. The results of initial column studies are shown in Table I11 for phenol, catechol, o-cresol, epinephrine, and norepinephrine. For the simple phenols > 95% removal from aqueous solution was realized. Norepinephrine was removed -90% from solution while only 67.3% of epinephrine in solution was tied up on the Polyclar AT column. Because of the slow flow rates used and because this work was performed in normal laboratory lighting, a t least part of the catechol amine not bound up on the column were oxidation products. Study in the future will be conducted with “dark columns” when catechol amines are being worked with in order to limit this photoinduced oxidation. Plant phenolics, such as tannins, bound t o insoluble P V P have been reported to be recoverable by equilibrating the phenolic bound to P V P with 8 M urea. At least part of the success obtained with this eluent is due to the ability of urea to hydrogen-bond to PVP. Using a 4 M urea solution in this study, it was found that this was adequate to recover the complexed phenolics a t about 100%efficiency.
100 100
a0 a0
300 300
400 400
500 500
600 + 600
conc (PW) Figure 3. uptake (mg/g) vs. concn. 0-0-0 O--O-O
= pcresol,
A-A-A
=
phenol,
= 0-cresol
Overall efficiencies for removal and recovery of the simple phenols fell in the range of 95-100%.
DISCUSSION The binding of phenolics to aqueous insoluble P V P has been shown to be dependent on the hydrogen-ion concentration of the solution. Infrared and % uptake vs. pH results indicate that hydrogen bonding plays a part in the complexation. In terms of the hydrogen-bonding capabilities of phenolic compounds, the equilibrium +OH e 4-0H + explains the fact that the pH’s of maximum binding of the phenolics studied were in all cases observed in the acid pH range. A regular trend has been observed for the p H of maximum complexation vs. the number of hydroxyl substituents. Molyneux and Frank (13) have postulated that interaction of the amide group of the polymer with the aromatic A electrons explains, in part, the observed bonding of polar aromatic compounds with PVP. This theory is supported by the results reported here which show that a substantial increase in % uptake is realized with an extended aromatic system (1-naphthol; maximum % uptake = 84%) relative to a simple phenolic (phenol; maximum uptake = 26%). In agreement with the conclusions of other authors, the phenol-pyrrolidone interaction has been seen to be 1 to 1 (17). The degree of binding to insoluble P V P is dependent on the substituents present in the phenol. Particularly noticeable is the increase in % uptake with an increasing number of hydroxyl groups present on the aromatic ring. The maximum % uptake is not, however, a linear function of the number of hydroxyl groups as the maximum uptakes of phenol, catechol, and pyrogallol are 26, 77, and 79%, respectively. The binding of the cresols t o Polyclar is more favorable than the complexation of phenol which indicates that inductive effects of substituent groups can be important in the degree of uptake. In this respect, the similar uptake of phenol and p-nitrophenol is a t first glance somewhat surprising. Inductive effects of an electron withdrawing substituent such as a nitro group would be expected to lower the % uptake of nitrophenol vs. phenol through a decrease in the hydrogen bonding ability of the hydroxyl group. This effect, however, appears to be negated by the additional strength of a charge transfer bonding interaction between p-nitrophenol and the pyrrolidone amide linkage. This conclusion is sup-
+
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
* 227
ported by the fact t h a t nitrobenzene is also bindable to P V P (13). The low efficiency of removal of the catecholamines from solution in one plate equilibria with Polyclar is most probably due to the fact t h a t steric hindrance caused by the bulky side chains of epinephrine and norepinephrine prevents the proper alignment of these phenolics with the pyrrolidine ring. This steric hinderance lowers the energies of both hydrogen and charge transfer bonds. Recovery of the complexed phenolics from the Polyclar column using 4 M urea has been observed to proceed with near 100% efficiency. Urea is a weaker hydrogen bonder than phenols and, because of this, high concentrations were needed to force the equilibrium to favor the release of the phenolic from the polymer.
LITERATURE CITED (1) (2) (3) (4)
R. A. Anderson and J. A. Sowers, Phytochemistry, 7, 293, (1968). K. H. von Gustavson. Leder, 14(2), 27 (1963). W. D. Loomis and J. Batailla. Phytochemistry, 5, 423 (1966). R. V. Dahlstrom and M. R. Sfat, Brew. Dig., 47(5), 75 (1972). (5) R. A. Ciemens and A. J. Martineiii, Wines Vines, 39(4), April 1958.
(6) (7) (8) (9) (10) (1 1) (12) (13) (14) (15) (16) (17) (18) (19) (20)
Chem. Ind. Ltd., Britain, 1,099,722(Cl. A61K). Jan. 17, 1968. J. Burdy and W. Chernecki, Can. J. Physiol. Pharmacol., (6), 46 (1968). M. A. Kassem and A. E. M. EiNimr, Sci. Pharm., 3, 38 (1970). J. W. V. Cordice, J. E. Suess. and J. Scudder, J. Surg. Gyn. Obstet.. 97, 39 (1953). H. A. Ravin. A. M. Seligman. and J. Fine, N. Engl. J. Med., 247(24). 921 (1952). M. Freifeld, J. R. Lyons, and A. J. Martinelli, Am. Perfumer, 77(2), 25-27 (1962). R. A. Anderson and J. R. Todd, T06. Sci,, 12, 1078 (1966). P. Molyneux and H. P. Frank, J. Am. Chem. Soc., 83, 3169 (1969). T. Higuchi and R . Kuramoto, J. Am. Pharm. Assoc, Sci. Ed., 398, July 1954. D. i. Randall, E. M. Smolin, and J. P. Copes, Nature, 244, 369 (1973). B. N. Kabodi and E. R . Hammarlund, J. Pharm. Sci., 55, 1069 (1966). D. Guttmann and T. Higuchi, J. Am. Pharm. Assoc., 45, 659 (1956). H. P. Frank, S. Barkin, and F. R . Eirich, J. Phys. Chem., 61, 1375 (1957). C. D. Chriswell, R. C. Chang, and J. S. Fritz, Anal. Chem., 47, 1325 (1975). L. Chien, "An NMR Study of the Complexation of Phenol by Poiyvinyipyrrolidone"; Research Project, University of Massachusetts, unpublished, 1974.
RECEIVEDfor review July 7, 1975. Accepted October 6, 1975. This work was supported by Grant No. G P 37493X from the National Science Foundation.
1 CORRESPONDENCE Analysis of Errors in the Capillary Method for Determining Diffusion Coefficients Sir: The method of Anderson and Saddington ( I ) for determining diffusion coefficients (D) by observing the rate a t which substances diffuse from capillaries has been used to obtain D values applicable to electroanalytical chemistry. Adams and his group, for example, have clearly demonstrated the importance of determining D values independent of electrochemical techniques but under conditions used in electrochemistry, and they have used the capillary method in their work (2-5). Although the method has been described in numerous papers and has been discussed in a well-known monograph (6),to our knowledge a differential error analysis has not appeared. Such an analysis clearly shows the relative importance of the various errors that may be encountered and should prove useful to those who plan to use the method. Briefly, the experimental procedure consists of filling a capillary of length 1 with a solution of diffusant a t concentration COand then suspending the capillary vertically in a large excess of pure solvent. The capillary is closed a t the lower end but open a t the top so that material can diffuse out. Also, it is customary to stir the surrounding fluid to maintain the concentration of diffusant a t the capillary mouth as near zero as possible. After a period of time, t , the capillary is withdrawn and C, the average concentration of diffusant remaining, is determined. The diffusion coefficient is then computed from the following equation.
The total differential of R , assuming no interdependence Present address, Chemistry Department, Southwest Texas State University, San Marcos, Texas 78666. 228
ANALYTICAL CHEMISTRY, VOL. 4 8 , NO. 1, JANUARY 1976
of errors, is dR = (aR/aD)dD
+ (aR/at)dt + (aR/al)dl
(2)
The partial derivatives in Equation 2 are obtained from Equation 1 to give aR/aD = -(r2t/412)P aR/at = -(r2D/412)P aRlal = (r2Dt/213)P
(3)
where m
P = 8/r2
exp[- (2n n=O
+ 1)2~2Dt/412]
(4)
Rearranging Equation 2 followed by substitution from Equation 3 gives the following expression for the relative error in D. dD/D = 2dl/l
- dt/t
- (412/r2Dt)dR/P
(5)
From Equations 1 and 4 note that
R I P
(6)
Therefore, replacing P with R in Equation 5 gives an expression for the upper bound on the relative error in D as dDID I I2dW
+ I dtltl + 1 (412/~2Dt)dRIRI
(7)
A rough first-order approximation to the value of the error bound can be obtained by substitution of finite values for the infinitesimals in Equation 7. For example, let I At1 = 1800 sec, 1 AI\ = 0.03 cm, and 1 AR/RI = 0.01. Then for an experiment that lasts 4.3 X lo6 sec (120 hr), in 6-cm long capillaries and for which D is 1.5 X 10-5 cm2 sec-', the approximate value of the upper bound is 4%.
