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Extraction of aromatic organic compounds by polyurethane foam
Tolanro, Vol. 34, No. 11, pp. 951-962, 1981 in Great Britain. All rights reserved
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Printed
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0039-9140/87 $3.00 + 0.00 1987 Pergamon Journals Ltd
EXTRACTION OF AROMATIC ORGANIC COMPOUNDS BY POLYURETHANE FOAM L. SCHUMACK and A. CHOW Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada (Received
10 September
1986. Reoised 25 March 1987. Accepted 27 May 1987)
mechanism of extraction of organic compounds by open-cell polyurethane foam has been investigated through a detailed study with simple aromatic compounds. Comparison with identical extractions into diethyl ether suggests that the basic extraction mechanism is an ether-like solvent extraction process. The addition of salt increases the extraction and changing the dielectric constant of the aqueous solution also affects the extraction. For organic compounds which have a group capable of hydrogen bonding, some additional factor appears to influence the extraction. This appears to be hydrogen bonding with the polyurethane foam; it is stronger with polyether foam and reduced by the presence of a strong intramolecular hydrogen-bonding group placed ortho to the hydrogen-bonding group. Thermodynamic studies support these conclusions. Summary-The
Since their origin, polyurethane polymers have been applied to a variety of tasks. In 1970, Bowen’ first used foamed polyurethane for the extraction from aqueous media of a variety of metal species such as mercury(II), gold(III), iron(III), antimony(V), molybdenum(VI), rhenium(II1) and uranium(V1) as well as benzene and phenol. Because of the large sorption capacities (OS-l.5 mole/kg), Bowen deduced that the extraction was not a surface phenomena but that absorption into the bulk of the polyurethane must occur. The possibility of a solvent extraction mechanism and the similarity between extraction by foam and by diethyl ether was reported. Considerable further research has dealt with the extraction of various inorganic compounds and metal species, a summary of which can be found in several reviews” and books.4,5 Only a few papers have discussed the mechanism of the extraction process. Gesser and co-workers6s7 described the extraction of Ga(III), regarding the polyurethane foam extraction as similar to an ethertype solvent extraction in which the neutral HGaCl, species was extracted. Later work by Lo and Chow8 also found the foam-extraction of antimony to be similar to that into an organic solvent. This view of the extraction mechanism was further supported by Korkisch ef aL9 who examined the extraction of uranium and found the salting-out effects to be similar to those in the liquid-liquid extraction of uranium by ethers. More evidence was presented by Braun et al.,” who used Miissbauer spectroscopy to show the similarity between diethyl ether extraction and the extraction by polyurethane foam. An alternative mechanism was postulated by Hamon et al.” for the extraction of cobalt and other metals. This mechanism, known as the “cationconsisted of having the chelation mechanism”, negatively-charged metal complexes solvated within
the polymer matrix, with accompanying cations strongly solvated within “crown ether” type structures within the polymer configuration. Since only the polyether-type foam can form these “crown ether” type structures, it is a much better extractor than the polyester-type polyurethane foam. Further research1”r4 also supports this mechanism. Some work has been done on the extraction of organic compounds by polyurethane foam since Bowen’s publication. Gesser et ~1.‘~ extracted polychlorinated biphenyls from water, by using foam plugs placed in a chromatographic column. It was found that organochlorine pesticides were extracted along with the polychlorinated biphenyls and a detailed study of the factors affecting the extraction and recovery of these compounds was made by Musty and Nickless.16 The uncoated foams gave better extraction than those coated with chromatographic greases, indicating an absorption mechanism. Later, Gough and Gesser ” looked at the extraction and recovery of phthalate esters from aqueous media. They found that a column containing foam plugs could remove these phthalates at the pg/l. level and that these compounds could be recovered by elution with acetone and hexane. There was no attempt to define the mechanism involved. Recent work by Ahmad et aLI looked at extraction by polyurethane foam as a means of separating different carboxylic acids in aqueous solution. Although there has been some speculation as to the mechanism of extraction for inorganic compounds, to date there has been very little work done on the mechanism of extraction of organics by untreated open-cell polyurethane foam. The present investigation looked at the extraction of simple aromatic compounds with the intention of determining whether the extraction by solvent extraction, cationchelation or another mechanism. 957
L. SCHIJMACK and A.
958 EXPERIMENTAL
CHOW
RESULTS AND DISCUSSION
Apparatus
Extraction time
Ultraviolet absorbance measurements were made with a Unicam SPSOO series 2 or a Varian Series 6348 spectrophotometer. Sodium concentration was measured with a Waters Millipore ILC-I Ion/Liquid Chromatonraph with a Wisp 710B, a model 430 conductivity detector and a 740 data module with Waters Millipore IC Pak cation and guard columns. Ethanol concentraiion was measured v&h a Hewlett-Packard 5710A gas chromatograph with a flameionization detector, equipped with a glass column packed with Waters Porapak P. The solutions were equilibrated with the foam in borosilicate glass squeezing-cells mounted within a thermostatically controlled (25.0 + 0.1”) cabinet. This equipment was designed to produce repetitive solution-mixing and foam-squeezing at a rate of 30 f 0.5 cycles/min and is described elsewhere.r9
Preliminary experiments showed that extraction equilibrium was attained within 1 hr. Further experiments indicated that a 5-ppm solution of mnitrophenol in 10% ethanol reached equilibrium after 4 min of squeezing with the foam. The same results were obtained with 0.25-g pieces of either polyether or polyester foam. For 0.40-g pieces of polyester foam the time for equilibrium to be reached increased to 6 min. The extraction with diethyl ether extraction reached equilibrium within 15 min. According to Hamon et al.” the cation-chelation mechanism can take up to 18 hr to reach equilibrium for 0.05-g foam pieces. Thus the extraction time for organics is similar for polyurethane foam and for diethyl ether extraction, and is more rapid than would be expected for a cation-chelation mechanism.
Reagents
All chemicals were of reagent grade unless otherwise indicated. Pure water was obtained from a Bamstead Nanopure II water purification system fed with water purified by reverse osmosis. Two types of polyurethane foam were used. The polyester foam was obtained as Dispo T. M. plugs from Canlab, Winnipeg and the polyether plugs were cut from a foam pad obtained from a local department store. The cleaning process for both foam types consisted of soaking for 1 hr in 1M hydrochloric acid with occasional squeezing; rinsing with pure water; soaking for 1 hr in 95% ethanol with occasional squeezing; rinsing overnight in pure water; extracting with acetone for 3 hr in a Soxhlet apparatus; drying overnight in a vacuum desiccator before use. General procedure
Preliminary experiments showed that 1 hr of squeezing was enough for equilibrium to be attained for the compounds studied. Simple aromatic compounds were used because they had several desirable properties such as good ultraviolet absorption for easy detection, enough polarity to dissolve sufficiently in 10% ethanol solution, and reasonable extraction into the foam. The 10% ethanol in water medium was chosen because it was able to dissolve the compounds tested, did not give severe evaporation problems and gave larger extraction values than higher ethanol concentrations did. Sample solutions were prepared by diluting 1ml of an organic stock solution to 100 ml with 10% ethanol solution. The ultraviolet absorbance (A) of the sample solutions was measured before addition of the foam and again after the squeezing period. The values obtained were corrected by means of the values from a blank run simultaneously. The degrees of extraction (E) and the distribution coefficients were then calculated; E = [(A,,,,,,, -
&a,W,n,,ta,Ix 100%
E
D=(lOOx
volume of solution weight of foam
the units used being litres for volume of solution and kg for weight of foam. Several trials were run simultaneously and the standard deviations were calculated. The wavelength used for absorbance measurements was that for the highest absorbance (,l_). They were either taken from the Sadtler HandbookZ or the D.M.S. UV atlas,2’ or determined experimentally by scanning from 500 to 200 nm with the Varian spectrophotometer. For many of the compounds an extraction was performed with diethyl ether and compared with the foam extraction. The ether extraction consisted of extracting the organic compound from 20 ml of sample solution with 10 ml of diethyl ether at 25”.
General survey The results of the general survey and for the diethyl ether extraction are shown in Table 1. In the solvent extraction with diethyl ether, more than 99% extraction was obtained in some cases; because of the relative volumes of the two phases, the amount of solute left in the aqueous phase was too small for accurate measurement in these cases, so the distribution coefficient is recorded as being greater than a value arbitrarily selected as the highest measurable. The distribution coefficients obtained in the foam experiments generally paralleled those of the ether experiments. This supports the suggestion that the extraction mechanism with the polyurethane foam is similar to that with ether. The reason for the negative values of D reported for the pyridine/foam systems is not known; since it means that the absorbance of the aqueous phase is greater after the extraction than before it, it is perhaps due to extraction of a degradation product of the foam into the aqueous phase, but this was not investigated. E$ect of salts on the extraction The extraction of m-nitrophenol by foam from various salt solutions gave the results shown in Table 2. Korkisch et aL9 found that the salting-out efficiency of cations by polyurethane
in the extraction foam was in
of uranium
the order NH: < Ca*+ < A13+. The results in Table 2 show that a singly-charged cation (K+) had less effect on the extraction than a doubly-charged cation (Ca*+). It was found that the effect of the alkali-metal ions on the extraction increased as the charge-density on the ion increased. This is consistent with a solventextraction mechanism, as an ion with a larger charge density should be more strongly solvated, thus reducing the number of solvent molecules available to solvate the organic compound, which would therefore be forced out of the solvent phase into the foam. The solvation numbers for the cations examined are
Extraction of aromatic organic compounds
959
Table 1. Extraction of organic compounds from 10% v/v ethanol by polyurethane and diethyl ether
D f std. devn. Number of trials
Compound
4 8 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
Axobenzene Aniline Benxaldehyde Benzonitrile Toluene Acetophenone m-Dinitrobenzene Nitrobenzene Benxene Biphenyl Benzophenone Ethyl benzoate Phenyl ether* Chlorobenzene Pyridine p-Nitroanailine o-Nitroaniline Triphenylphosphine
&_, nm
Polyether, l.lk
Concentration, ppm
314 230 244 230 261 240 233 251 254 247 252 230.5 258 264 256 382 281 223
5 6 4 6 100 50 5 10 500 8 9 5 50 100 10 5 10 5
Polyester, Wg
(3.8 f 0.6) x IO3 20 + 3 41 & 1 56+ 1 (8 + 5) x lo* 38&2 121+3 124+2 (4.7 f 1.8) x IO* (6.5 f 0.4) x lo3 (1.20 f 0.04) x 103 (2.6 + 0.1) x Id (2.97 + 0.09) x lo3 (6.8 + 1.6) x Id -9*3 275 & 6 264k6 (5.8 f 0.2) x lo2
(3.3 f 0.2) x 10’ 26k6 45 + 1 53 * 2 (8 f 5) x lo2 40*1 167 + 12 116k3 (4.4 &-3.2) x lo* (5.9 f 0.9) x 103 (1.09 * 0.03) x 103 188&2 (2.2 + 0.2) x lo3 (5.6 + 1.1) x IO2 -4*3 193*3 208&- 11 (5.4 + 0.8) x 102
Solvent extraction >500 14 30 74 > 450 58 67 120 >340 2340 2oo >330 2 36
Conditions: 25.0 f 0.1”; polyether 0.25-g pieces; polyester 0.40-g pieces; I hr extraction time. *Extraction from 20% ethanol.
reported**
to be 5.4, 8.4 and 14.0 for K+, Na+ and
Li+ respectively, the reference ion, Cl-, being assumed to have a solvation number of 4. The degree of extraction also increased with the amount of salt added, which is also characteristic of a solventextraction mechanism, since the amount of “free” solvent molecules will decrease with increase in amount of preferentially solvated ions. The decrease in extraction when sodium perchlorate is added to the system is interesting, but the reason for it has not been discovered. Both ion-chromatography and gravimetric analysis showed that no sodium perchlorate is extracted. The effect of pH on the extraction Compounds such as benzoic acid, m-nitrophenol and aniline, which can be involved in equilibria with protons, were extracted into the foam only at pHvalues at which they were in the neutral form. For compounds such as nitrobenzene, which exist as the
neutral species irrespective of pH, change in the acidity had no effect on the degree of extraction. Thus the extraction mechanism involves neutral species and there is no evidence for a mechanism requiring ionic species. This observation is also consistent with a solvent-extraction mechanism. Ef/ect of ethanol concentration on the extraction It was found, as shown in Table 3, that as the aqueous phase was made less polar by increasing its ethanol content, the degree of extraction decreased. When a compound of low dielectric constant is distributed between a phase which has a low dielectric constant and another which has a high dielectric constant, the degree of extraction should increase with increase in the polarity of the polar phase. A compound with a high dielectric constant such as ethanol should not be extracted at all, and this was confhrned experimentally. These observations are
Table 2. Effect of salts on the extraction of 5 nom m-nitronhenol
D f std. devn., I./kg salt LiCl NaCl NaCl NaCl NaCl NaCl KC1 NaClO, None CaCl, Na, So,
Concentration, M 3.00 0.50 1.00 1.50 2.00 3.08 3.00 3.00 0.00 1.00 1.00
Polyether (1.94 f 0.03) x lo3 (6.5 +O.ll) x lo2 (8.0 f 0.1) x 10r (9.3 f 0.3) x lo2 (1.26 f 0.1) x 10’ (1.55 f 0.06) x 10’ (8.7 f 0.2) x 102 (4.1 iO.1) x 102 (5.5 f 0.1) x 102 (1.11 f 0.06) x IO3 (1.53 f o.oij x lo3
Polyester (6.0 + (2.6 + (2.9 + (3.6 f (4.9 f (5.4 f (3.4 f (2.0 f (2.3 f (3.7 f (4.7 f
0.5) x lOr 0.12) x lo2 0.04) x IO* 0.12) x lo2 0.3) x I@ 0.3) x 102 0.1) x 102 0.1) x 102 0.06) x 102 0.1) x 102 0.2) x 102
Conditions: 25.0 f 0.1”; polyether = 0.25-g pieces; polyester = 0.40-g pieces; 1 hr extraction time; three trials for each value.
L.
960
%XUMACK
Table 3. Effect of ethanol concentration on extraction
Compound
Ethanol concentration, % v/v
~Nitrophenol
Benzoic acid
Phenol
0 10 20 30 0 10 20 30 0 10 20
D f std. devn., I./kg Polyether
Polyester
550 f 12 441*10 282 f 16 145 k 10 100 * 12 111 k6 60+8 10 * 10 88 + 1 79 * 1 47&4
229 + 6 180&6 143 + 12 17 k 6 29.6 + 0.9 30.4 + 2.3 8*5 0 * 0.3 40+2 34 * 1 26 + 1
25.0 f 0.10; pieces; Conditions: polyether = 0.25-g polyester = 0.40-g pieces; 1 hr extraction time; mnitrophenol(5 ppm) I, = 229 nm; benzoic acid (10 ppm) I = 228 nm; phenol (40 ppm) 1 = 270 nm.
again consistent anism.
with a solvent-extraction
mech-
Hydrogen bonding It was found that for many compounds an additional factor was involved in the extraction mechanism. The compounds listed in Table 4 all contain a phenolic or a carboxylic group, and unlike the compounds listed in Table 1 they all have larger distribution coefficients for extraction by polyether foam than by polyester foam, though for onitrophenol, o-methoxyphenol and salicylaldehyde the difference was small. This effect may be due to hydrogen bonding between the phenolic or carboxylic group and the foam. Such bonding would be stronger with the polyether foam and very weak with the polyester. This bonding is prevented by the presence of a strongly electron-donor group ortho to the
and A. CHOW
hydrogen-bonding group, as in o-nitrophenol, salicylaldehyde, and o-methoxyphenol, where intramolecular hydrogen bonding can take place. For the para and meta nitrophenols the distribution coefficients are much greater for the polyether foam than for the polyester foam, whereas for the ortho compound the coefficients are similar for the two types of foam. This cannot be attributed to a difference in dipole moment, because the dipole moment gradually increases for the compounds in the order ortho c meta cpara. Induction effects due to the position of the nitro group should be minimal owing to the distance between that group and the phenol group, so it may be concluded that the strong intramolecular hydrogen bonding is responsible for preventing formation of a hydrogen bond with the polyurethane foam. The hydrogen bonding is probably stronger with the polyether foam, since ethers are much better than esters at forming hydrogen bonds. This is reasonable, because the ether group is easier to protonate than an ester group23 and the ability of a group to form hydrogen bonds is indicated by its protonation wnstant. Effect of temperature on the extraction
The degree of extraction of azobenzene and mnitrophenol was measured at various temperatures, by use of a jacketed temperature-controlled squeezing cell. For the equilibrium Organic(,, &
Organico,,
the equilibrium constant, K, with standard enthalpy change AH” and standard entropy change AS” is given by In K = - AH”/RT + AS”IR
Table 4. Extraction of compounds containing an -OH
group
D + std. devn., I.lka Compound Benzoic acid Phenol Salicylic acid Pyrogallol Hydroquinone Catechol Resorcinol p-Nitrophenol ~Nitrophenol o-Nitrophenol 2,CDinitrophenol 2.4Dinitroohenol 2&Dihydroxybenzoic acid 3,5-Dihydroxybenzoic acid o-Tert-butylphenol 2,6-Di-tert-butylphenol 2,4-Di-tert-butylphenol Salicylaldehyde o-Methoxyphenol
Number of trials 8 8 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
L, nm 228 270 235 267 293.5 277.5 275 311 229 272 253 258 256 241 271 270 258 255 274.5
Concentration, ppm 10 40 10 10 40 10 10 8 10 10 10 10 10 10 30 50 10 2;
Polvether
Polyester
112*7 30*2 19 f 1 34+1 21*3 206 f 15 54 + 30 -3*5 8+8 21 + 10 60+9 26 k 5 28*2 53*4 407 * 7 165+3 470 + 10 183 & 6 128k4 122 f 2 213 + 16 179 * 12 382 + 23 276 + 13 292 f 12 36+6 142*8 12*3 (4.84kO.15) x 10’ (1.32 kO.1) x 10’ (2.63 f 0.09) x 10’ (1.90 f 0.25) x 10’ (2.15 f 0.14) x lo4 (9.8 + 0.4) x lo3 104*3 82 + 5 47 f 3 33*3
Conditions; 25.0 f 0.1”; polyether = 0.25-g pieces; polyether = 0.40-g pieces; 1 hr extraction time.
Solvent extraction 18 37 7 5 10 6 100 170 120 57 93 6 2 >500 >700 70 24
961
Extraction of aromatic organic compounds Table 5. Thermodynamic Compound Azobenzene
Foam type Polyether
AH”, kJ/mole
AS”, J.mole-‘.deg-’
-20+2 -23+1 -19k2 -22* 1 -30*1 -31+1 -21*2 -28f5
5*5 -3*5 2*4 -4*4 -so+3 -55+2 -21*1 -28&2
Polyester m-Nitrophenol
data for extraction
Polyether Polyester
Conditions: extraction from 10% ethanol; azobenzene (5 ppm) d = 314 nm; m-nitrophenol (10 ppm) 1 = 230 nm; all values f standard deviation calculated from slope; temperature range 25”-75”.
Assuming no chelation or precipitation and that the compounds exist only as neutral species, then K is equivalent to D. Thus by plotting In D vs. l/T, the values of AH” and AS” can be obtained (Table 5). The AS” values for azobenzene are almost the same for extraction into either polyether or polyester foam. Thus the organic compound has approximately the same degree of freedom in the polymer as in the aqueous solution, which is consistent with a solventextraction mechanism. With m-nitrophenol there is a decrease in entropy for both the polyether and polyester foam extractions, with the change larger for the polyether system. This entropy change is believed to be due to hydrogen bonding reducing the freedom of movement of the organic compound in the polymer. The bonding of the organic compound with the polyether foam is apparently much stronger than reported previously. The strength of the hydrogen bond with the foam can be estimated as about 10 kJ/mole by subtracting the enthalpy change AH’ for the polyester system from that for the polyester system. Although this calculation is only valid if it can be assumed that there is no hydrogen bonding between the organic compound and the polyether foam, it at least gives a rough idea of the value. The calculation is useful as the intramolecular hydrogen binding in o-nitrophenol, salicylaldehyde and omethoxyphenol has been calculated to have a bond strength of 30,24 30,25and 1O,26kJ/mole, respectively, which is greater than or equal to the estimated polyurethane hydrogen-bonding strength. The hydrogen bonding between the foam and the organic compound should occur for groups other than -OH which are capable of hydrogen bonding. A few amines were surveyed but any hydrogen bonding was not strong enough to be detected within the experimental error of the work. Future work with thiol compounds could be used to look for evidence of this hydrogen bonding. CONCLUSIONS
The extraction of organic compounds by polyurethane foam appears to occur generally by an ether-like solvent-extraction mechanism. This conclu-
sion is supported by the short time required for extraction equilibrium to be achieved and the evidence of the salting-out phenomenon. It was found that the addition of inert salts increases the extraction efficiency and that changing the polarity of the raffinate phase also affects the extraction. Varying the pH showed no evidence of a mechanism requiring an ionic species. All observations were consistent with a solvent-extraction mechanism. However, when the compound contained a group capable of hydrogen bonding, such as -OH, an additional factor appeared to be involved. This factor is probably hydrogen bonding between the polyurethane and the group on the organic compound. Strong intramolecular hydrogen bonding in the organic compound will negate this hydrogen bonding, and occurs whenever a group such as -NO2 is o&o to the -OH group. Because ester groups form weaker hydrogen bonds than ether groups, this bond is weaker in the polyester foam system. The solvent-extraction mechanism, modified by hydrogen bonding wherever applicable, may explain the extraction of all organic compounds by polyurethane foam. Future studies on aliphatic organic compounds will be used to examine this. Acknowledgement-This work was supported by the Natural Sciences and Engineering Research Council of Canada. REFERENCES
1. H. J. M. Bowen, J. Chem. Sot. A, 1970, 1082. 2. T. Braun and A. B. Farag, Anal. Chim. Acta, 1978, 99, 1. 3. T. Braun, Cellular Polym., 1984, 3, 81. 4. T. Braun, J. D. Navratil and A. B. Farag, Polyurethane Foam Sorbents in Separation Science, CRC Press, Boca Raton, 1985. 5. G. J. Moody and J. D. R. Thomas, Chromatographic Separation and Extraction with Foamed Plastics and Rubbers, Dekker, New York, 1982. H. D. Gesser, E. Bock, W. G. Baldwin, D. W. McBride and W. Lipinsky, Sepn. Sci., 1976, 11; 317. H. D. Gesser and G. A. Horsfall. J. Chim. Phvs. .~ Phys.-Chim. Biol., 1977, 74, 1072. V. S. K. Lo and A. Chow, Anal. Chim. Acta, 1979,106, 161. J. Korkisch, I. Steffan and J. D. Navratil, Radioact. Waste Manage., 1982, 6, 349.
L. SCMUMACK and A. CHOW
962
10. M. N. Abbas, A. Vertes and T. Braun, Radiochem. Radioanal.
Lett.,
1982, 54, 17.
11. R. F. Hamon, A. S. Khan and A. Chow, Tafanta, 1982, 29, 313.
12. G. J. Moody, J. D. R. Thomas and M. A. Yarmo, Anal. Proc., 1983, 20, 132.
13. R. Caletka, R. Hausbeck and V. Krivan, Talanta, 1986, 33, 219.
14. I&m, ibid., 1986, 33, 315. 15. H. D. Gesser, A., Chow, F. C. Davis, J. F. Uthe and J. Reinke, Anal. I.&r., 1971, 4, 883. 16. P. R. Musty and G. Nickless, J. Chromatog., 1974, 100, 83.
17. K. M. Gough and H. D. Gesser, ibid., 1975, 115, 383. 18. S. R. Ahmad, H. S. Rathore, I. Ali and S. K. Sharma, J. Indian Chem. Sot.,
1985, 62, 786.
19. L. J. Schumack, M.Sc. Thesis, University of Manitoba, 1986. 20. The Sadtier Handbook of Uliraviolei Spectra, W. W. Simons (ed.), Heyden, London, 1978. 21. D.M.S. U.V. Atlas of Organic Cornpow& Verlag Chemie, Weinheim, 1966. _ 22. E. S. Amis and J. F. Hinton. Solvent Effects on Chemical Phenomena, Vol. I, p. 52. Academic Press, New York, 1973. 23. J. March, Advanced Organic Chemistry, 3rd Ed., p. 220. Wiley, New York, 1985. 24. T. Schaefer and T. A. Wildman, Can. J. Chem., 1979, 57, 450. 25. T. Schaefer, R. Sebastian, R. Laatikainen and S. R. Salman, ibid., 1983, 62, 326. 26. T. Schaefer, J. Phys. Chem.. 1975, 79, 1888.
Copyright
Printed
0
0039-9140/87 $3.00 + 0.00 1987 Pergamon Journals Ltd
EXTRACTION OF AROMATIC ORGANIC COMPOUNDS BY POLYURETHANE FOAM L. SCHUMACK and A. CHOW Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada (Received
10 September
1986. Reoised 25 March 1987. Accepted 27 May 1987)
mechanism of extraction of organic compounds by open-cell polyurethane foam has been investigated through a detailed study with simple aromatic compounds. Comparison with identical extractions into diethyl ether suggests that the basic extraction mechanism is an ether-like solvent extraction process. The addition of salt increases the extraction and changing the dielectric constant of the aqueous solution also affects the extraction. For organic compounds which have a group capable of hydrogen bonding, some additional factor appears to influence the extraction. This appears to be hydrogen bonding with the polyurethane foam; it is stronger with polyether foam and reduced by the presence of a strong intramolecular hydrogen-bonding group placed ortho to the hydrogen-bonding group. Thermodynamic studies support these conclusions. Summary-The
Since their origin, polyurethane polymers have been applied to a variety of tasks. In 1970, Bowen’ first used foamed polyurethane for the extraction from aqueous media of a variety of metal species such as mercury(II), gold(III), iron(III), antimony(V), molybdenum(VI), rhenium(II1) and uranium(V1) as well as benzene and phenol. Because of the large sorption capacities (OS-l.5 mole/kg), Bowen deduced that the extraction was not a surface phenomena but that absorption into the bulk of the polyurethane must occur. The possibility of a solvent extraction mechanism and the similarity between extraction by foam and by diethyl ether was reported. Considerable further research has dealt with the extraction of various inorganic compounds and metal species, a summary of which can be found in several reviews” and books.4,5 Only a few papers have discussed the mechanism of the extraction process. Gesser and co-workers6s7 described the extraction of Ga(III), regarding the polyurethane foam extraction as similar to an ethertype solvent extraction in which the neutral HGaCl, species was extracted. Later work by Lo and Chow8 also found the foam-extraction of antimony to be similar to that into an organic solvent. This view of the extraction mechanism was further supported by Korkisch ef aL9 who examined the extraction of uranium and found the salting-out effects to be similar to those in the liquid-liquid extraction of uranium by ethers. More evidence was presented by Braun et al.,” who used Miissbauer spectroscopy to show the similarity between diethyl ether extraction and the extraction by polyurethane foam. An alternative mechanism was postulated by Hamon et al.” for the extraction of cobalt and other metals. This mechanism, known as the “cationconsisted of having the chelation mechanism”, negatively-charged metal complexes solvated within
the polymer matrix, with accompanying cations strongly solvated within “crown ether” type structures within the polymer configuration. Since only the polyether-type foam can form these “crown ether” type structures, it is a much better extractor than the polyester-type polyurethane foam. Further research1”r4 also supports this mechanism. Some work has been done on the extraction of organic compounds by polyurethane foam since Bowen’s publication. Gesser et ~1.‘~ extracted polychlorinated biphenyls from water, by using foam plugs placed in a chromatographic column. It was found that organochlorine pesticides were extracted along with the polychlorinated biphenyls and a detailed study of the factors affecting the extraction and recovery of these compounds was made by Musty and Nickless.16 The uncoated foams gave better extraction than those coated with chromatographic greases, indicating an absorption mechanism. Later, Gough and Gesser ” looked at the extraction and recovery of phthalate esters from aqueous media. They found that a column containing foam plugs could remove these phthalates at the pg/l. level and that these compounds could be recovered by elution with acetone and hexane. There was no attempt to define the mechanism involved. Recent work by Ahmad et aLI looked at extraction by polyurethane foam as a means of separating different carboxylic acids in aqueous solution. Although there has been some speculation as to the mechanism of extraction for inorganic compounds, to date there has been very little work done on the mechanism of extraction of organics by untreated open-cell polyurethane foam. The present investigation looked at the extraction of simple aromatic compounds with the intention of determining whether the extraction by solvent extraction, cationchelation or another mechanism. 957
L. SCHIJMACK and A.
958 EXPERIMENTAL
CHOW
RESULTS AND DISCUSSION
Apparatus
Extraction time
Ultraviolet absorbance measurements were made with a Unicam SPSOO series 2 or a Varian Series 6348 spectrophotometer. Sodium concentration was measured with a Waters Millipore ILC-I Ion/Liquid Chromatonraph with a Wisp 710B, a model 430 conductivity detector and a 740 data module with Waters Millipore IC Pak cation and guard columns. Ethanol concentraiion was measured v&h a Hewlett-Packard 5710A gas chromatograph with a flameionization detector, equipped with a glass column packed with Waters Porapak P. The solutions were equilibrated with the foam in borosilicate glass squeezing-cells mounted within a thermostatically controlled (25.0 + 0.1”) cabinet. This equipment was designed to produce repetitive solution-mixing and foam-squeezing at a rate of 30 f 0.5 cycles/min and is described elsewhere.r9
Preliminary experiments showed that extraction equilibrium was attained within 1 hr. Further experiments indicated that a 5-ppm solution of mnitrophenol in 10% ethanol reached equilibrium after 4 min of squeezing with the foam. The same results were obtained with 0.25-g pieces of either polyether or polyester foam. For 0.40-g pieces of polyester foam the time for equilibrium to be reached increased to 6 min. The extraction with diethyl ether extraction reached equilibrium within 15 min. According to Hamon et al.” the cation-chelation mechanism can take up to 18 hr to reach equilibrium for 0.05-g foam pieces. Thus the extraction time for organics is similar for polyurethane foam and for diethyl ether extraction, and is more rapid than would be expected for a cation-chelation mechanism.
Reagents
All chemicals were of reagent grade unless otherwise indicated. Pure water was obtained from a Bamstead Nanopure II water purification system fed with water purified by reverse osmosis. Two types of polyurethane foam were used. The polyester foam was obtained as Dispo T. M. plugs from Canlab, Winnipeg and the polyether plugs were cut from a foam pad obtained from a local department store. The cleaning process for both foam types consisted of soaking for 1 hr in 1M hydrochloric acid with occasional squeezing; rinsing with pure water; soaking for 1 hr in 95% ethanol with occasional squeezing; rinsing overnight in pure water; extracting with acetone for 3 hr in a Soxhlet apparatus; drying overnight in a vacuum desiccator before use. General procedure
Preliminary experiments showed that 1 hr of squeezing was enough for equilibrium to be attained for the compounds studied. Simple aromatic compounds were used because they had several desirable properties such as good ultraviolet absorption for easy detection, enough polarity to dissolve sufficiently in 10% ethanol solution, and reasonable extraction into the foam. The 10% ethanol in water medium was chosen because it was able to dissolve the compounds tested, did not give severe evaporation problems and gave larger extraction values than higher ethanol concentrations did. Sample solutions were prepared by diluting 1ml of an organic stock solution to 100 ml with 10% ethanol solution. The ultraviolet absorbance (A) of the sample solutions was measured before addition of the foam and again after the squeezing period. The values obtained were corrected by means of the values from a blank run simultaneously. The degrees of extraction (E) and the distribution coefficients were then calculated; E = [(A,,,,,,, -
&a,W,n,,ta,Ix 100%
E
D=(lOOx
volume of solution weight of foam
the units used being litres for volume of solution and kg for weight of foam. Several trials were run simultaneously and the standard deviations were calculated. The wavelength used for absorbance measurements was that for the highest absorbance (,l_). They were either taken from the Sadtler HandbookZ or the D.M.S. UV atlas,2’ or determined experimentally by scanning from 500 to 200 nm with the Varian spectrophotometer. For many of the compounds an extraction was performed with diethyl ether and compared with the foam extraction. The ether extraction consisted of extracting the organic compound from 20 ml of sample solution with 10 ml of diethyl ether at 25”.
General survey The results of the general survey and for the diethyl ether extraction are shown in Table 1. In the solvent extraction with diethyl ether, more than 99% extraction was obtained in some cases; because of the relative volumes of the two phases, the amount of solute left in the aqueous phase was too small for accurate measurement in these cases, so the distribution coefficient is recorded as being greater than a value arbitrarily selected as the highest measurable. The distribution coefficients obtained in the foam experiments generally paralleled those of the ether experiments. This supports the suggestion that the extraction mechanism with the polyurethane foam is similar to that with ether. The reason for the negative values of D reported for the pyridine/foam systems is not known; since it means that the absorbance of the aqueous phase is greater after the extraction than before it, it is perhaps due to extraction of a degradation product of the foam into the aqueous phase, but this was not investigated. E$ect of salts on the extraction The extraction of m-nitrophenol by foam from various salt solutions gave the results shown in Table 2. Korkisch et aL9 found that the salting-out efficiency of cations by polyurethane
in the extraction foam was in
of uranium
the order NH: < Ca*+ < A13+. The results in Table 2 show that a singly-charged cation (K+) had less effect on the extraction than a doubly-charged cation (Ca*+). It was found that the effect of the alkali-metal ions on the extraction increased as the charge-density on the ion increased. This is consistent with a solventextraction mechanism, as an ion with a larger charge density should be more strongly solvated, thus reducing the number of solvent molecules available to solvate the organic compound, which would therefore be forced out of the solvent phase into the foam. The solvation numbers for the cations examined are
Extraction of aromatic organic compounds
959
Table 1. Extraction of organic compounds from 10% v/v ethanol by polyurethane and diethyl ether
D f std. devn. Number of trials
Compound
4 8 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
Axobenzene Aniline Benxaldehyde Benzonitrile Toluene Acetophenone m-Dinitrobenzene Nitrobenzene Benxene Biphenyl Benzophenone Ethyl benzoate Phenyl ether* Chlorobenzene Pyridine p-Nitroanailine o-Nitroaniline Triphenylphosphine
&_, nm
Polyether, l.lk
Concentration, ppm
314 230 244 230 261 240 233 251 254 247 252 230.5 258 264 256 382 281 223
5 6 4 6 100 50 5 10 500 8 9 5 50 100 10 5 10 5
Polyester, Wg
(3.8 f 0.6) x IO3 20 + 3 41 & 1 56+ 1 (8 + 5) x lo* 38&2 121+3 124+2 (4.7 f 1.8) x IO* (6.5 f 0.4) x lo3 (1.20 f 0.04) x 103 (2.6 + 0.1) x Id (2.97 + 0.09) x lo3 (6.8 + 1.6) x Id -9*3 275 & 6 264k6 (5.8 f 0.2) x lo2
(3.3 f 0.2) x 10’ 26k6 45 + 1 53 * 2 (8 f 5) x lo2 40*1 167 + 12 116k3 (4.4 &-3.2) x lo* (5.9 f 0.9) x 103 (1.09 * 0.03) x 103 188&2 (2.2 + 0.2) x lo3 (5.6 + 1.1) x IO2 -4*3 193*3 208&- 11 (5.4 + 0.8) x 102
Solvent extraction >500 14 30 74 > 450 58 67 120 >340 2340 2oo >330 2 36
Conditions: 25.0 f 0.1”; polyether 0.25-g pieces; polyester 0.40-g pieces; I hr extraction time. *Extraction from 20% ethanol.
reported**
to be 5.4, 8.4 and 14.0 for K+, Na+ and
Li+ respectively, the reference ion, Cl-, being assumed to have a solvation number of 4. The degree of extraction also increased with the amount of salt added, which is also characteristic of a solventextraction mechanism, since the amount of “free” solvent molecules will decrease with increase in amount of preferentially solvated ions. The decrease in extraction when sodium perchlorate is added to the system is interesting, but the reason for it has not been discovered. Both ion-chromatography and gravimetric analysis showed that no sodium perchlorate is extracted. The effect of pH on the extraction Compounds such as benzoic acid, m-nitrophenol and aniline, which can be involved in equilibria with protons, were extracted into the foam only at pHvalues at which they were in the neutral form. For compounds such as nitrobenzene, which exist as the
neutral species irrespective of pH, change in the acidity had no effect on the degree of extraction. Thus the extraction mechanism involves neutral species and there is no evidence for a mechanism requiring ionic species. This observation is also consistent with a solvent-extraction mechanism. Ef/ect of ethanol concentration on the extraction It was found, as shown in Table 3, that as the aqueous phase was made less polar by increasing its ethanol content, the degree of extraction decreased. When a compound of low dielectric constant is distributed between a phase which has a low dielectric constant and another which has a high dielectric constant, the degree of extraction should increase with increase in the polarity of the polar phase. A compound with a high dielectric constant such as ethanol should not be extracted at all, and this was confhrned experimentally. These observations are
Table 2. Effect of salts on the extraction of 5 nom m-nitronhenol
D f std. devn., I./kg salt LiCl NaCl NaCl NaCl NaCl NaCl KC1 NaClO, None CaCl, Na, So,
Concentration, M 3.00 0.50 1.00 1.50 2.00 3.08 3.00 3.00 0.00 1.00 1.00
Polyether (1.94 f 0.03) x lo3 (6.5 +O.ll) x lo2 (8.0 f 0.1) x 10r (9.3 f 0.3) x lo2 (1.26 f 0.1) x 10’ (1.55 f 0.06) x 10’ (8.7 f 0.2) x 102 (4.1 iO.1) x 102 (5.5 f 0.1) x 102 (1.11 f 0.06) x IO3 (1.53 f o.oij x lo3
Polyester (6.0 + (2.6 + (2.9 + (3.6 f (4.9 f (5.4 f (3.4 f (2.0 f (2.3 f (3.7 f (4.7 f
0.5) x lOr 0.12) x lo2 0.04) x IO* 0.12) x lo2 0.3) x I@ 0.3) x 102 0.1) x 102 0.1) x 102 0.06) x 102 0.1) x 102 0.2) x 102
Conditions: 25.0 f 0.1”; polyether = 0.25-g pieces; polyester = 0.40-g pieces; 1 hr extraction time; three trials for each value.
L.
960
%XUMACK
Table 3. Effect of ethanol concentration on extraction
Compound
Ethanol concentration, % v/v
~Nitrophenol
Benzoic acid
Phenol
0 10 20 30 0 10 20 30 0 10 20
D f std. devn., I./kg Polyether
Polyester
550 f 12 441*10 282 f 16 145 k 10 100 * 12 111 k6 60+8 10 * 10 88 + 1 79 * 1 47&4
229 + 6 180&6 143 + 12 17 k 6 29.6 + 0.9 30.4 + 2.3 8*5 0 * 0.3 40+2 34 * 1 26 + 1
25.0 f 0.10; pieces; Conditions: polyether = 0.25-g polyester = 0.40-g pieces; 1 hr extraction time; mnitrophenol(5 ppm) I, = 229 nm; benzoic acid (10 ppm) I = 228 nm; phenol (40 ppm) 1 = 270 nm.
again consistent anism.
with a solvent-extraction
mech-
Hydrogen bonding It was found that for many compounds an additional factor was involved in the extraction mechanism. The compounds listed in Table 4 all contain a phenolic or a carboxylic group, and unlike the compounds listed in Table 1 they all have larger distribution coefficients for extraction by polyether foam than by polyester foam, though for onitrophenol, o-methoxyphenol and salicylaldehyde the difference was small. This effect may be due to hydrogen bonding between the phenolic or carboxylic group and the foam. Such bonding would be stronger with the polyether foam and very weak with the polyester. This bonding is prevented by the presence of a strongly electron-donor group ortho to the
and A. CHOW
hydrogen-bonding group, as in o-nitrophenol, salicylaldehyde, and o-methoxyphenol, where intramolecular hydrogen bonding can take place. For the para and meta nitrophenols the distribution coefficients are much greater for the polyether foam than for the polyester foam, whereas for the ortho compound the coefficients are similar for the two types of foam. This cannot be attributed to a difference in dipole moment, because the dipole moment gradually increases for the compounds in the order ortho c meta cpara. Induction effects due to the position of the nitro group should be minimal owing to the distance between that group and the phenol group, so it may be concluded that the strong intramolecular hydrogen bonding is responsible for preventing formation of a hydrogen bond with the polyurethane foam. The hydrogen bonding is probably stronger with the polyether foam, since ethers are much better than esters at forming hydrogen bonds. This is reasonable, because the ether group is easier to protonate than an ester group23 and the ability of a group to form hydrogen bonds is indicated by its protonation wnstant. Effect of temperature on the extraction
The degree of extraction of azobenzene and mnitrophenol was measured at various temperatures, by use of a jacketed temperature-controlled squeezing cell. For the equilibrium Organic(,, &
Organico,,
the equilibrium constant, K, with standard enthalpy change AH” and standard entropy change AS” is given by In K = - AH”/RT + AS”IR
Table 4. Extraction of compounds containing an -OH
group
D + std. devn., I.lka Compound Benzoic acid Phenol Salicylic acid Pyrogallol Hydroquinone Catechol Resorcinol p-Nitrophenol ~Nitrophenol o-Nitrophenol 2,CDinitrophenol 2.4Dinitroohenol 2&Dihydroxybenzoic acid 3,5-Dihydroxybenzoic acid o-Tert-butylphenol 2,6-Di-tert-butylphenol 2,4-Di-tert-butylphenol Salicylaldehyde o-Methoxyphenol
Number of trials 8 8 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
L, nm 228 270 235 267 293.5 277.5 275 311 229 272 253 258 256 241 271 270 258 255 274.5
Concentration, ppm 10 40 10 10 40 10 10 8 10 10 10 10 10 10 30 50 10 2;
Polvether
Polyester
112*7 30*2 19 f 1 34+1 21*3 206 f 15 54 + 30 -3*5 8+8 21 + 10 60+9 26 k 5 28*2 53*4 407 * 7 165+3 470 + 10 183 & 6 128k4 122 f 2 213 + 16 179 * 12 382 + 23 276 + 13 292 f 12 36+6 142*8 12*3 (4.84kO.15) x 10’ (1.32 kO.1) x 10’ (2.63 f 0.09) x 10’ (1.90 f 0.25) x 10’ (2.15 f 0.14) x lo4 (9.8 + 0.4) x lo3 104*3 82 + 5 47 f 3 33*3
Conditions; 25.0 f 0.1”; polyether = 0.25-g pieces; polyether = 0.40-g pieces; 1 hr extraction time.
Solvent extraction 18 37 7 5 10 6 100 170 120 57 93 6 2 >500 >700 70 24
961
Extraction of aromatic organic compounds Table 5. Thermodynamic Compound Azobenzene
Foam type Polyether
AH”, kJ/mole
AS”, J.mole-‘.deg-’
-20+2 -23+1 -19k2 -22* 1 -30*1 -31+1 -21*2 -28f5
5*5 -3*5 2*4 -4*4 -so+3 -55+2 -21*1 -28&2
Polyester m-Nitrophenol
data for extraction
Polyether Polyester
Conditions: extraction from 10% ethanol; azobenzene (5 ppm) d = 314 nm; m-nitrophenol (10 ppm) 1 = 230 nm; all values f standard deviation calculated from slope; temperature range 25”-75”.
Assuming no chelation or precipitation and that the compounds exist only as neutral species, then K is equivalent to D. Thus by plotting In D vs. l/T, the values of AH” and AS” can be obtained (Table 5). The AS” values for azobenzene are almost the same for extraction into either polyether or polyester foam. Thus the organic compound has approximately the same degree of freedom in the polymer as in the aqueous solution, which is consistent with a solventextraction mechanism. With m-nitrophenol there is a decrease in entropy for both the polyether and polyester foam extractions, with the change larger for the polyether system. This entropy change is believed to be due to hydrogen bonding reducing the freedom of movement of the organic compound in the polymer. The bonding of the organic compound with the polyether foam is apparently much stronger than reported previously. The strength of the hydrogen bond with the foam can be estimated as about 10 kJ/mole by subtracting the enthalpy change AH’ for the polyester system from that for the polyester system. Although this calculation is only valid if it can be assumed that there is no hydrogen bonding between the organic compound and the polyether foam, it at least gives a rough idea of the value. The calculation is useful as the intramolecular hydrogen binding in o-nitrophenol, salicylaldehyde and omethoxyphenol has been calculated to have a bond strength of 30,24 30,25and 1O,26kJ/mole, respectively, which is greater than or equal to the estimated polyurethane hydrogen-bonding strength. The hydrogen bonding between the foam and the organic compound should occur for groups other than -OH which are capable of hydrogen bonding. A few amines were surveyed but any hydrogen bonding was not strong enough to be detected within the experimental error of the work. Future work with thiol compounds could be used to look for evidence of this hydrogen bonding. CONCLUSIONS
The extraction of organic compounds by polyurethane foam appears to occur generally by an ether-like solvent-extraction mechanism. This conclu-
sion is supported by the short time required for extraction equilibrium to be achieved and the evidence of the salting-out phenomenon. It was found that the addition of inert salts increases the extraction efficiency and that changing the polarity of the raffinate phase also affects the extraction. Varying the pH showed no evidence of a mechanism requiring an ionic species. All observations were consistent with a solvent-extraction mechanism. However, when the compound contained a group capable of hydrogen bonding, such as -OH, an additional factor appeared to be involved. This factor is probably hydrogen bonding between the polyurethane and the group on the organic compound. Strong intramolecular hydrogen bonding in the organic compound will negate this hydrogen bonding, and occurs whenever a group such as -NO2 is o&o to the -OH group. Because ester groups form weaker hydrogen bonds than ether groups, this bond is weaker in the polyester foam system. The solvent-extraction mechanism, modified by hydrogen bonding wherever applicable, may explain the extraction of all organic compounds by polyurethane foam. Future studies on aliphatic organic compounds will be used to examine this. Acknowledgement-This work was supported by the Natural Sciences and Engineering Research Council of Canada. REFERENCES
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