Macroporous Condensate Resins as Adsorbents Irving M. Abrams Diamond Shamrock Chemical Company, Redwood City, California 94064
Highly effective adsorbents are made by condensation of phenol and formaldehyde in the form of maor without modification with amines. Although the macroporous croporous (sponge-like) resins-with structure is required for adsorption of high molecular weight organic substances, chemical composition of the polymer (mainly the presence of polar functional groups) is of greater significance than physical characteristics such as surface area, pore volume, and pore size. A few typical applications of both phenol-formaldehyde and phenol-formaldehyde-amine resins are described briefly.
Introduction Most manufacturers of ion-exchange resins have considered making synthetic resins which can serve as adsorbents, particularly for the removal of organic contaminants from aqueous solutions. Such resins should have high sorption capacity, be capable of elution by simple and economical methods, and have sufficient physical and chemical stability to give long service life. For the past 25 years, the manufacturers of the Duolite (Diamond Shamrock Chemical Co., Redwood City, CA.) ion-exchange resins have provided a series of adsorbents based on macroporous condensates of phenol and formaldehyde. These have been used in a wide variety of applications, principally for the removal of colorants from aqueous media. Although these resins possess very weakly acidic and/or weakly basic groups, this discussion pertains to their use as adsorbents rather than as ion-exchange resins. They are generally used in neutral or slightly acid solutions in which the polar groups show little or no ion-exchange activity. Analogy to Activated Carbon Various forms of carbon are widely used as adsorbents in aqueous media and constitute the closest approach to what might be considered universal sorbents. Bone char, comprising a major ratio of tricalcium phosphate to a minor portion (5-20%) of carbon, is used extensively for decolorizing and partial deashing of cane sugar syrups. Activated carbons, derived from vegetable or coal sources, are also used in the sugar industries. Both powdered and granular forms are in use, the latter gaining in popularity in recent years. Commercially available carbons are characterized by having high surface area which is covered with combined hydrogen and oxygen. Such surfaces, in fine structure, are likely to be present in the form of reactive polar groups. As suggested by Smith (1959), they may be carbonyl, hydroxyl, carboxyl, and lactone groups. A variety of chemical reactivity has been shown to exist in activated carbons (Garten and Weiss, 1955). Furthermore, carbons free of chemical reactivity in the surface adsorb benzene preferentially to polar compounds (Gasser and Kipling, 1960). More recently, quantitative measurements have been made on four commercial activated carbons which show carboxylic acid capacities ranging from 0.30 t o 0.90 mequiv/g and phenolic hydroxyl capacities ranging from 0.05 to 0.70 mequiv/g (Giusti, et al., 1974). In order to make effective adsorbent resins, therefore, it was reasoned that some chemical functionality would be required for good adsorption from aqueous solutions. The ideal adsorbent should be sufficiently porous to allow easy diffusion into and out of 108
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the resin matrix and possess a high enough concentration of polar groups to facilitate chemisorption.
Test Methods Numerous physical and chemical methods have been described for characterizing the adsorptivity of carbons. In the evaluation of adsorbents, it is not possible to determine a “standard capacity” value. It is usually necessary to test the particular solution being decolorized or deodorized to establish relative values. For example, in the evaluation of adsorbents for the decolorization of a sugar syrup, comparisons are made with a “standard molasses solution.” Numerous attempts have been made to find the ideal solute and solvent to indicate adsorptivity for a variety of systems. Iodine in KI has been widely used because of the ease of measuring adsorption by color and thiosulfate titration. Other types of solutes (in water) include organic acids, permanganate solutions, and organic dyes such as methylene blue or ponceau red. The ability to adsorb phenol has been used as a criterion for testing a carbon for the removal of odor and taste from water. However, there seems to be no way of making a direct correlation between the adsorptivity of pure compounds and the potential for color removal. Green Dye Test. In casting about for a dye, some years ago, which might be used routinely to determine relative adsorptivity, the author came upon one used in the food industry called Fast Green FCF (manufactured by H. Kohnstamm and Co., Inc., New York, N.Y.). This dye has the following configuration
OH
This compound has a molecular weight of 808, contains a variety of polar groups, and is amphoteric. In the test developed, 0.50 g of moist resin (-30 + 40 mesh) is stirred in a 250-ml Erlenmeyer flask with 100 ml of a solution made by dissolving 100 mg of dye reagent in 1 1. of 0.1 N HC1. Color measurements are made in the supernatant after 15 min and 1 hr. Although equilibrium was not reached in this time, the 15-min value gives an indication of reaction speed and the 1-hr value indicates the relative quantity of dye which can be adsorbed. The 1-hr values range from 10% removal for poor adsorbents to 90% removal for very good adsorbents. Table I shows some typical values obtained with one phenol-formaldehyde condensate, two weak-base phenolic resins, and one highly
Table I. Description of Decolorizing Resins
Trade name
Matrig
Duolite S-761 Duolite ES-33 Duolite S-37
Functional groyP
-
Physical form
Capacity to HC1, equiv/l.
Typical moisture Typical retention, Green Dye % sorptionb
P-F P-F
Phenolic hydroxY l Granules 48 70 Tertiary amine Granules 0.7 58 98 P-F Secondary and Granules 1.4 55 15 tertiary amine Duolite ES-111 PS Quat. ammoniunn Beads 1.o 67 I2 a P-F = phenol-formaldehyde condensate. PS = cross-linked pol)istvrene. The “Green Dve” test is used to determine mlat.ive ndaornt.ivi. tyof decolorizing resins. It is based on the uptake of a food dye (FD &L C porous strong-base anion exchanger. Over the years, the results of this test have provided reasonably good correlation with decolorization of a variety of aqueous solutions. Physical Measurements. In characterizing rigid adsorbents, measurements such as surface area and pore volume are often made. In such tests, the adsorhent is dried first, then equilibrated with a known amount of inert gas. The amount adsorbed is measured by change in pressure and calculated as in the Brunauer-Emmett-Teller method (1938). However, since the macroporous condensate ... gels ana nave Dotn ,. .. . pores resins are nyaropninc aiscrete and a gelular structure, there is some shrinkage on drying. Therefore, measurements of surface areas and pore volumes are not as significant as they are with relatively hydrophobic materials such as activated carbon or less polar addition copolymers. Some typical values of surface area and pore volume obtained with condensate resins are given io Table n. These were dried at 50°C under high vacuum for 3 hr. The adsorbate gas was nitrogen. Scanning electron microscope photos of phenolic condensate resins show pores ranging from a few Angstrom units to over 100,000 b, (See Figures 1-4.) Since the particles are granular (resulting from a grinding operation), the larger “holes” are probably due to gross cracks and discontinuities. In most of the photos, the larger macropores appear to he in the range of 1000 to 5000 A. The major portion of the pores which show good resolution a t magnifications up to 25,OOOX are in the range of 125 to 600 A. Mercury porosimeter measurements indicate average pore diameters in the range of 200 to 300 A. Again it should be emphasized that the surface area, pore volume, and pore size data were all obtained with air-dried samples and therefore do not accurately reflect the parameters which would apply to the fully hydrated conditions in which the resins would be used.
.. .
.
..
.
Table 11. Surface Area and Pore Volume Measurements on Dried Condensate Resins (BET-Nitrogen)
Sample no.
Resin composition
DS-74132 DS-02674 DS-73054
P-F-Amine P-F-Amine P-F
Surface area, m2/g
Pore vol., ml/g
Pore vol., ml/ml
168 123 295
0.78 0.73 1.10
0.42 0.47 0.72
Factors Affecting Adsorptivity Some of the factors which have been investigated include (a) composition of the polymer matrix, (b) nature of the functional groups, (c) porosity and surface area, (d) degree of polarity, (e) hydrophilicity, and (f) particle shape. Composition of Polymer Matrix. Two types of polymer structures have been investigated: (a) polymers of styrene and divinylhenzene and (b) condensates of phenol and formaldehyde. As might he expected, the hydrophobic polystyrenes were far less reactive in aqueous solutions than the relatively hydrophilic phenol-formaldehyde condensates. Some early work (Abrams and Dickinson, 1949) showed that such a resin could he highly effective in removing color from both a clarified beet juice and a corn syrup “hydrol” (molasses produced in the refining of starch hydrolysate). Over the years, the phenolic conden-
Sate resin, Uuolite Y-Yo, nas proven particularly effective industrially for the reversible sorption of high molecular weight organic substances. Functional Groups. Experience has shown that a high degree of polarity is necessary in making an effective resinous adsorhent. Because the colorants present in both sugar extracts and contaminated water supplies are predominantly acidic in character, resins having basicity have proven more effective than those containing acid functional groups. Thus a wide variety of anion-exchange resins containing primary, secondary, and tertiary amines Ind. Eng. Chem., Prod. Res. Dev., VoI. 14. No. 2. 1975
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Figure 2. Duolite S-761; magnification, 10,OOOX.
Figure3. Duolite A-7; magnification, 2OOOx. or quaternary-ammonium groups have been useful for the removal of colors, odors, and organic contaminants in general. Although strongly basic anion exchangers remove organic contaminants from aqueous solutions, they have two basic deficiencies in serving as reversible adsorhents. First, the affinity of certain organic compounds for qua110
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Figure 4. Duolite A-7; magnification, 5000X ternary ammonium groups is so high that sorption is essentially irreversible. This is particularly true when the contaminants consist of aromatic acids or the salts of aromatic acids (Abrams, 1969; Ward and Edgerley, 1966). Secondly, the presence of ions in solution other than chloride serves to reduce the capacity for organic anions. The predominant sorption mechanism with strong-base resins is ion exchange. In the use of weak-base resins, adsorption of organics occurs with little change in ionic composition. Porosity, Surface Area. During the past 10 years, a number of patents and papers have described methods for making macroporous ion-exchange resins. This is achieved, in general, by conducting the polymerization under conditions which result in phase separation. High cross-linked copolymers of styrene and divinylbenzene are relatively rigid structures, even after introduction of the polar groups. Lower cross-linked polymers possess elasticity as well as macroporosity. Less well known is the fact that macroporous structures can also he made with condensate resins. For example, phenol and formaldehyde can he condensed under conditions which form two-phase, sponge-like structures. These are macroporous in the same sense as the copolymers of styrene and divinylhenzene. By varying the ratio of formaldehyde to phenol, resins can he made which range from a very loose gel to highly rigid polymers. The macroporous phenolic condensates, both unmodified and modified by amination, have proven highly effective as adsorbents. Some of the physical characteristics have been indicated in Tables I and 11. High surface area is frequently equated with good adsorptivity. Although this may he true with completely rigid inorganic adsorhents, it has not been the case with synthetic polymers. The data in Table 111 show surface area measurements and relative adsorptivity (as indicated by color removal from a pond water) with a variety of adsorbents having various degrees of macroporosity. The highest relative adsorptivity was obtained with a
Table 111. Influence of Surface Area on Relative Adsorptivity (Removal of Color from Pond Water)
Adsorbent Nuchar CEE Duolite 5-30 ES-140
Duolite A-7D Duolite A-30B ES-111
Composition Carbon P-F condensate S-B An. Exr. P-F-amine cond. Epoxyamine cond. S-B An. Exr.
Sur face Relative area, adsorpm2/g tivity 740 128 110 24 1.4 1.0
1.o 1.3 2.4 3.4 5.0 2.8
gel resin having a surface area (as measured by BET) of only 1.5 m2/g. Least sorption among the resins was shown with the product having the highest surface area; lowest adsorptivity was observed with the carbon which had the highest surface area of any of the adsorbents tried. Although a good resinous adsorbent should have adequate porosity to allow diffusion into and out of the structure, an open-lattice gel can be as effective as discrete intraparticle pores. Furthermore, surface area measurements on resins cannot be used as a criterion for adsorptivity. Polarity. The degree of polarity, in this case, is related to the nitrogen content in a series of homologous phenolformaldehyde-amine condensates. Dickinsan and Barrett (1969) have shown a “complementary effect” when finely divided resins are mixed with activated carbon. In one series of experiments, in which a mixture of resin and carbon in a ratio of 1 : l O was equilibrated with “process greens,” a linear relationship was obtained when the percent nitrogen was plotted against the “complementary effect” (see Figure 5 ) . From this series, one might surmise that adsorptivity increases with increasing polarity of the resin. Hydrophilicity. In testing a wide variety of weak-base and strong-base resins as adsorbents, best overall correlation was obtained between moisture retention capacity and adsorptivity-as measured by the Green Dye test. Figure 6 shows such a plot with six different types of anion exchangers having widely varying compositions and porosities. Most of the resins showed the trend toward increased sorption of green dye with decrease in the dry matter of the resin. In these tests, the weak-base resins were used in the free-base form and the strong-base resins in the chloride form. Moisture retention values were obtained on these same forms. These results clearly indicate the high degree of adsorptivity obtained with the relatively low moisture phenolic weak-base macroporous resins. The four resins indicated by solid, upright triangles all gave adsorption values in excess of 80% with moistures ranging from 44 to 52%. The practical significance of this observation lies in the fact that highly effective adsorbents need not be resins containing high moisture and having relatively low physical stability. The phenolic weak-base macroporous structures have proven to be highly durable when used either as working anion exchangers or as organic scavengers. Particle Shape. Condensate resins, particularly those made from phenol and formaldehyde or m-phenylenediamine and formaldehyde, are usually made in granular (nonspherical) forms. Bead-form resins can also be made by suspending the aqueous solutions in nonaqueous solvents during condensation. Experience has shown that the granular resins are always more effective sorbents than the spherical resins having the same composition. This is
1.0
I
I
I
1
I
I
I
I
0
I
2
3
4
5
6
7
X NITROGEN IN RESIN
Figure 5. Effect of nitrogen content in resin on adsorptivity. KEY 0
PS, 5-6
I , Gel
0 PS. 5-61, MP @ P 5 , 5-8E. (Gel
A
PhenaINc,
A
E p o x y - O m # n C , W-B, G e l
8 MPI
PS,
W-8, M P
W 8 , MP
100 1
20
/
40
45
50 MOISTURE
55
60
65
R E T E N T I O N , 7.
Figure 6. Relationship between moisture retention capacities of various anion exchange resins and adsorptivity (as measured by “Green Dye Test”).
probably due to the higher surface area of the granules and a “skin effect” obtained with the beads. Applications In the following paragraphs, a few typical applications of both the phenol-formaldehyde (PF) and the phenolformaldehyde- amine (PFA) adsorbent resins will be described briefly. PF Adsorbents. 1. Sugar Refining. Colorants are removed from dilute or concentrated sugar solutions derived from cane, beets or corn (Abrams, 1971; Abrams and Dickinson, 1949; Diamond Shamrock Chemical Co., 19’72). Duolite $30 removed 80 to 100% of the color from decationized second carbonation juice (beet sugar) and from decationized “hydrol,” the molasses of starch hydrolysates. This resin is especially effective in adsorbing caramel and melanoidin pigments from solutions in the pH range of 0 to 6. Colorants are effectively eluted from the resin by dilute alkali, generally 1 to 4% sodium hydroxide. The resin is then restored to the hydrogen form by rinse with dilute acid. 2. Glycerol. A purified grade of glycerol is produced from crude glycerol obtained as a by-product in the manufacture of soap and fatty acids with the aid of a PF adsorbent resin (Stromquist and Reents, 1951). More than 95% color removal is obtained by a combination of ion-exchange and adsorbent resins. Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 2, 1975
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3. Wines. Phenol-formaldehyde condensate resins have been authorized in the treatment of wines "for removing excessive oxidized color, foreign flavors and odors, and for stabilizing wines by the removal of . . . proteinaceous material." Although results of such treatment have not been published in the U.S., the removal of brown color from wine by Duolite S-30 was reported in Japan (J. Ferment. Technol , 1968). 4. Fermentation Broths. P-F adsorbent resins have been used to decolorize fermentation broths. The isolation of Vitamin BIZ by this type of resin has been patented (Shafer and Holland, 1955). P F A Adsorbents. The weak-base adsorbents are used to remove color and organic substance from a variety of aqueous solutions. Optimum pH for such applications is generally in the range of 4 to 7. These resins are particularly effective for the reversible adsorption of simple and polymeric aromatic acids-including flavonoid pigments which are frequently present in extracts of plant materials. 1. Water Deionization. Humic and fulvic acids present in surface water supplies often interfere with the production of high purity water by deionization, especially by fouling strong-base anion exchangers. A PFA condensate resin (Duolite S-37) removes these organic acids prior to deionization (Abrams, 1969) and thus allows the ion exchangers to perform their normal function of removing inorganic ions completely for a longer time. 2. Sugar Refining. The PFA resins function in much the same way as activated carbon or bone char in sugar extracted from cane, beets or pineapple wastes (Diamond Shamrock Chemical Co., 1972). 3. Dye Wastes. The effluent from a dyestuff synthesis plant is being treated by a novel two-step adsorption-regeneration process (Montanaro and Moreau, 1974). The first adsorbent is a nonionic addition copolymer which is solvent-regenerated while the second adsorbent is a condensate amine resin which is alkali-regenerated and acid rinsed. 4. Pulp and Paper Mill Effluents. Large-scale Kraft pulp mills are currently in operation in which a weak-base condensate resin is being used to remove color and other organic matter from the highly colored caustic bleach plant effluent (Anderson et al., 1973; Broddevall et al., 1973). According to published results, this method removes 95% of the color, 90% of the COD, and 60% of the BOD. The organic matter adsorbed by the resin is eluted with caustic soda and burned in the recovery furnace. Since nearly all of the inorganic chloride goes into the
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decolorized effluent, there is little corrosion hazard in the eluant. 5. Adsorption of Acids from Hydrocarbons. It has been demonstrated that an amine condensate resin (Duolite A-2) is capable of adsorbing more than half its weight of low molecular weight fatty acids from a hydrocarbon solvent (Robinson and Mills, 1949). In fact, the adsorption of acetic, propionic, and butyric acids from Stoddard solvent was greater than from water. Although this fact was disclosed some years ago, the principle has not been used commercially. 6. Others. Other actual and potential applications include reduction in color of municipal water supplies, organic removal from domestic and industrial wastewaters (Abrams, 1966, 1970; Rebhun and Kaufman, 1967), and color reduction in effluents from cotton dyeing plants (Rebhun et al., 1970). Literature Cited Abrams, I . M., "Adsorbent Resins for Color and General Organics Removal", Second Annual Sanitary Eng. Res. Lab. Workshop, Tahoe City, Calif., June 1970. Abrams, I . M., Chem. Eng. Progr., 65 (97). 106 (1969). Abrams. I. M., SugaryAzucar, 31-34 (May 1971). Abrams. i . M . , U.S. Patent 3,232,867 (1966). Abrams, I . M . , Dickinson. 8 . N.. lnd. E n g . Chem., 41, 2521 (1949). Anderson, L. G., Broddevall, B., Lindberg. S., "Color Removal from Effluents in Forest Industries by Ion Exchange". 59th Annual Meeting of the Canadian Pulp and Paper Association, Jan 1973. Broddevall, B.. Lindberq, S.. Liunqqvist K. J., W. Germ. Patent Appln. . 2,243,141 (1 973). Brunauer, S , Emmett. P. H., Teller, E., J. Am. Chem. SOC, 60, 309 (19381 ---, Dickinson, 8. N . , Barrett, H. E., Jr.. Ind. Eng. Chem., Prod. Res. Dev., 8, 199 (1969). "Duollte Ion Exchange Resins in the Treatment of Sugar Solutions". Diamond Shamrock Chemical Co., Redwood City, Calif. 94064, (1972). Garten, V . A., Weiss, D. E., Aust. J . Chem., 8 , 68 (1955). Gasser, C. G., Kipiing. J. J., "Proceedings of the Fourth Conference on Carbon", p 55. Pergamon Press, London, 1960. Giusti. D. M., Conway, R . A,, Lawson, C. T., J . Water Poll. Control Fed., 46, (5),947 (1974). Montanaro, R . A,, Moreau. H. B.. U.S. Patent 3,803,030 (1974). "On the Removal of Color from Wine", J. Ferment. Technol.. ( J a p a n ) 46 ( 3 ) ,153-157 (1968). Rebhun, M., Kaufman, W. J., "Removal of Organic Contaminants: Sorption of Organics by Synthetic Resins and Activated Carbon", SERL Report No. 67-9, Dec 1967. Rebhun, M . , Weinberg. A,, Narkis, M., "Treatment of Wastewater from Cotton Dyeing and Finishing Works for Re-Use", 25th Industrial Waste Conference, Purdue University, May 1970. Robinson, D. A., Mills, G. F.. lnd. E n g . Chem.. 41, 2221 (1949) Shafer. H. M . , Holland, A. J., U.S. Patent, 2,702,263 (1955). Smith, R. N.. Q. Rev. Chem. SOC.,13, 287 (1959). Stromquist, D. E., Reents. A. C.. lnd. Eng. Chem.. 43, 1065 (1951). Ward, R . F., Edgerley, E., Robert College Research Center Publications, Series No. 304, 1-31, June 1966. I
Received for reuieu: N o v e m b e r 20,1974 Accepted F e b r u a r y 18,1975
