ALKALI ADSORPTION BY SYNTHETIC RESINS E. I. AKEROYD'
AND
GEOFFREY BROUGHTON
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts Received December 3, 1987
In spite of the great industrial importance and common use of the phenol-formaldehyde resins, it was not until 1935 that attention was drawn to their adsorptive properties and possible use as water softeners (1). This appears somewhat surprising, since it might be expected that some, at least, of the phenolic hydroxyl groups will remain active after condensation, serving as active centers for adsorption. Thus, although attention had been called to the amphoteric properties of phenol-formaldehyde resins (9), and Shono (11) had examined the behavior of metallic salts reacting with phenolic resins to give colored products, the first systematic adsorption experiments were made by Adams and Holmes (l), using a flow method. The solution under examination was allowed to pass down a column packed with the resin, and, although difficulties due t o channelling made the results only semiquantitative in nature, strong selective adsorption in acid solution was shown by catechol, resorcinol, quinol, pyrogallol, and catechin resins and general adsorption by phloroglucinol and quebracho tannin resins. In alkaline solution marked adsorption of cations occurred, varying considerably with the nature of the phenol present. Amine-formaldehyde resins showed adsorption in acid solution. Ellis (4) has stated that a wide variety of phenols of high molecular weight and of natural origin may be condensed with formaldehyde to give products which will remove cations from dilute aqueous solution. Complete removal of dissolved salts, therefore, may be accomplished by treatment with a phenolic resin to remove cations followed by an amine-aldehyde resin to remove anions (1). Since many of the resins can be produced a t economical prices, their application to water treatment is obvious and appears to have commercial possibilities. The present investigation was an attempt to elucidate the mechanism of the adsorption process and the relation of the molecular structure of the resins to their adsorptive power. For this purpose the adsorption and rates of adsorption of calcium hydroxide and three other bases by simple mono-, di-, and tri-hydroxy phenol resins were measured. The results should be 1
Fellow of the Salters' Institute of Industrial Chemistry, London, England.
343
344
E. I. AKEROYD AND GEOFFREY BROC'GIITON
of value in the study of resins made from more complex, naturally occurring phenols, which are of greater utility in commercial water softening. PROCEDURE
A . Preparation of resins Two parts2 of phenol, 20 parts of water, and 4 parts of formalin (40 per cent) were heated to boiling and 1 part of concentrated hydrochloric acid in 2 parts of water was added. The resin separated almost immediately and was filtered and washed, first with boiling water, then with aqueous calcium hydroxide solution until the filtrate became colorless. The resin was regenerated by washing several times with 5 per cent hydrochloric acid, followed by water, alcohol and ether. After drying a t 100°C. it mas crushed and graded through 48 mesh on to 200 mesh. In most preparations the resin was precipitated on 2 parts of kieselguhr, added to the mixture before the catalyst. Subsequent procedure was the same as for the pure resin, but analyses were made for water and organic content. The presence of kieselguhr resulted in a smaller loss in fines on grading the resin.
B. Adsorption measurements A known quantity of resin was weighed out into a bottle and a known amount of standard calcium hydroxide or other solution added. After agitation for a measured time on a rotary shaker a t room temperature (25' i.2'C.), the solution was filtered and analyzed. Blanks were run to determine any adsorption by the kieselguhr. This was small, of the order of 0.012 g. of calcium per gram of kieselguhr. This was allowed for when calculating the amount of adsorption. Results were reproducible, within 3 per cent, on different samples of a resin as well as on the same sample and are expressed as inillimoles per gram of resin in all cases. VARIATION OF ADSORPTION WITH ALKALINITY
Adams and Holmes (1) found that the simple phenolic resins did not appear to adsorb cations from neutral solution. Phenol, quinol, catechol, and resorcinol resins behaved similarly with calcium sulfate solution. Even after agitation for three months the adsorption by catechol resin from 0.01 M calcium sulfate was only 0.15 niillimole per gram of resin. RATE O F ADSORPTION
Adsorption is relatively slow, apparently becoming almost complete after about seven days, but probably proceeding very slowly over a period of months. I n figure 1 are represented the results obtained for various 2
All parts are parts by weight
345
ALKALI ADSORPTION BY SYNTHETIC RESINS
FIQ.1. Rate of adsorption of calcium hydroxide by various resins
HOURS
FIG.2. Adsorption of calcium hydroxide by various resins TABLE 1 Rates of adsorption by differant resins REBIN
EQUILIBRIUM ADSORPTION
lo
millimoles per gram of resin
Resorcinol .......................... Quinol .............................. Catechol, ........................... Phloroglucinol A , . . . . . . . . . . . . . . . . . . . Phloroglucinol B . . . . . . . . . . . . . . . . . . . . Phenol. ............................
11.8
0.12
11.2
17.0 18.1 12.0 10.8 2.7
0.042 0.054 0.081 0.068 0.29
10.5 12.1 9.6 7.9 6.25(approx.)
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E. I. AKEROYD AND GEOFFREY BROUGHTON
resins and calcium hydroxide solution. In all these experiments a quantity of sorbent, corresponding t o 0.27 g. of resin after correcting for the kieselguhr content, and 500 cc. of 0.0198 N calcium hydroxide were taken. The curves, as seen from figure 2, can be represented by an equation of the usual type ( 6 ) x / m = kin where x / m is the adsorption in millimoles of calcium per gram of resin, t is the time of adsorption, and k and n are constants. The values of the TABLE 2 Adsorption of calcium hydroxide b y a resorcinol resin 0.5175g. of resin (0.270 g. of organic content) agitated with 500 cc. of calcium hydroxide solution for three months INITIAL CONCENTRATION OF CALCIUM HYDROXIDE
millimoles o j Ca per lite7
99.0 148.5 198.0
1
FINAL CONCENTRATION OB CALCIUM SYDROXIDE
xlm
millimoles of Ca per liter
26.2 74.5 125.0
13.5 13.7 13.6
FIG.3
constants k and n and the equilibrium adsorptions (Le., the adsorptions at infinite time read from figure 1) for the various resins are tabulated in table 1. Results obtained for a resorcinol resin (table 2) appear to indicate that slow adsorption does occur even after three hundred hours. The equilibrium adsorption value is raised somewhat and is independent of concentration.
347
ALKALI ADSORPTION BY SYNTHETIC RESINS 20
L
6
e
8
t
8 az
10
m
1 ='
0
0
2M)
100
300
HOURS
FIG.4.
Adsorption of calcium, barium, sodium, and trimethylbenzylammonium hydroxides on a resorcinol resin containing kieselguhr
FIQ.5.
Adsorption of calcium, barium, sodium, and trimethylbenzylammonium hydroxides on a resorcinol resin containing kieselguhr
HOURS
TABLE 3
Adsorption of diferent bases b y a resorcinol resin containing kieselguhr
I BABE
.
l
I
Calcium.. .%. . . . . . . . . . . . . . . . . . . . Barium ..... . . . . . . . . . . . . . . . . . . . Sodium ........................... Trimethylbenzylammonium hydroxide ......... i;. . . . . . . . . . . . . . .
n
RATE8 OF AD0ORPllON
1 0 E D ; S N T
l
n
l
1 EzR?; I DIAMETER OF ION I N
EQUILIBRIUM ADSORPTION
millimoka per uram
11.9 9.17 5.09
0.12 0.16 0.17
1.02 1.37 0.98
11.5 11.3 14.0
2.92
0.21
9.0'
8.6
348
E. I. AKEROYD AND GEOFFREY BROUGHT03
I t is surprising to find that incorporation of kieselguhr by direct formation of the resin on it has little effect on the rate of adsorption or the final value attained. Thus, up to about twenty-five hours the curves in figure 3 for resins, with and without kieselguhr, appear to be identical. Above this point they run parallel with a spread of roughly 5 per cent. This may be compared with the finding of Bhatnagar, Hapur, and Puri (2) that different modes of activation had little or no effect on the adsorptive properties of a resorcinol-formaldehyde resin. A series of experiments were also made with calcium, barium, sodium, and trimethylbenzylammonium hydroxides on a resorcinol resin containing kieselguhr. As before, 500 cc. of 0.0198 N base with 0.27 g. of resin (after correcting for kieselguhr content) was used. The results are shown in figures 4 and 5 and are summarized in table 3. z
210 (c
8 3,
zi
5m $5
2
HOURS
FIG.6 Adsorption of calcium hydroxide by phloroglucinol resiris EFFECT O F CATALYST OK ADSORPTIVE POWER
The equilibrium adsorption found above for the phloroglucinol resin, in contrast to the results of hdams and Holmes (l),is considerably lower than the equilibrium values for the dihydroxy resins. For this reason samples of the resin were prepared, using different catalysts and conditions. It was found that a phloroglucinol resin prepared using ammonia as catalyst had an even lower adsorptive power than the acid-catalyzed resin (figure 6). ADSORPTION ISOTHERMS
Bhatnagar, Hapur, and Puri (2) determined the adsorption' isotherms of a number of substances on resins of this type. The values they found for a resorcinol resin, apparently prepared by the same method, are, however, very much lower than those found in the present investigation. For
ALKALI ADSORPTION BY SYNTHETIC RESINS
349
adsorption from 0.1 N sodium hydroxide solution their adsorption maximum was approximately 5 millimoles per gram of resin as compared with 14.0 in the present experiments. This may possibly be due t o differences in the resins or t o insufficient time being allowed by them for their solution to come to equilibrium. Thus they state “the systems were agitated for half an hour and allowed to remain until equilibrium was attained.” Their values, in fact, check roughly with those interpolated at two hours from the present rate curves (figure 4). I n figure 7 are plotted “adsorption IS
z
:: 10 I
. a
u *
v)
i: 5
5
-I
1
l l l l l l l l 0 0
01 N
02 N
03 N.
FINAL CONCENTRATION OF Ca(OH),
FIG.7. Adsorption of calcium hydroxide by resorcinol and catechol resins
FINAL CONCENTRATON OF Ca(OH12
FIG.8. Adsorption of calcium hydroxide by catechol and resorcinol resins during short times of agitation
isotherms” of resorcinol and catechol resins in calcium hydroxide solution, determined after agitation for one day and seven days, respectively. Points for a resorcinol resin after three months’ stirring are also shown. Hence for short times of agitation typical adsorption isotherms are obtained (figure 8), but for long times a curve indicative of chemical combination is observed. The isotherms resemble those for calcium hydroxide and silica gel ( 7 ) , where there is undoubtedly chemical combination to give calcium silicate.
350
E. I. AKEROYD AND GEOFFREY BROUGHTON INTERPRETATION AND DISCUSSION O F RESULTS
Adsorption of cations in any appreciable quantity is peculiar to the phenol-formaldehyde resins; natural resins and glyptal, vinyl, and ketoneformaldehyde resins show inappreciable adsorption (1). Hence adsorption would seem to be bound up with the phenolic hydroxyl groups and might be expected to be chemical in nature. The present work supports this view. There is now considerable evidence (7, 10, 8) for the following mode of formation of phenol-formaldehyde resins:
+ CHzO -+ HO-R’-CHzOH + nCHzO -+ HO--RZ-(CH~OH)X + R-OH HO-R’-CHz-R’-OH
R-OH or
R-OH HO--R’-CHzOH
--+
+ HzO
CHdll
HO-R’-CHi-R”(OH) ROH
1
CHzOH
HO-R’-CH~-R”(OH)-CHz--R’-OH
i
HO-R’-CH~-(R’’(OH)-CH~).-R’-OH The linear condensation products may further give complex three-dimensional structures by the formation of cross links (4),but in both linear and three-dimensional polymers the phenolic hydroxyl groups are not involved. Formation of cross links leads to modification of the physical properties of resins, which become harder and more insoluble. Insoluble resins are always formed in the presence of alkaline catalysts although acid catalysts, in the absence of excess formaldehyde, produce soluble Novolak resins, which can only be converted into the insoluble form with excess formaldehyde (6, 1). The resins under investigation are of the insoluble, crosslinked type, excess formaldehyde always being used in their preparation. It is interesting to compare the adsorptions obtained with the values which might be expected on the basis of the above structure, assuming chemical combination to occur between the hydroxide and the base. Taking the basic unit of the dihydroxy phenol resin as C~H(OH)Z(CHZ)~,~, its molecular weight will be 128, there being two hydroxyl groups per unit. Steric considerations indicate that the most probable compound would be
OCaOH OCaOH
i
CH2
I
ALKALI ADSORPTION BY SYNTHETIC RESINS
351
giving a saturation adsorption of 15.5 millimoles of calcium per gram of resin.3 The corresponding figures for mono- and tri-hydroxy phenol resins are 8.9 and 20.8 millimoles per gram, respectively. For a monobasic hydroxide and a dihydroxy phenol resin the saturation adsorption would similarly be 15.5. Tables 1, 2, and 3 show that the resins, with the exception of the phloroglucinol resin, which will be discussed later, approach these maxima. This confirms the predominantly chemical character of the adsorption. Viewing the adsorption as a chemical reaction, it is necessary to explain the different equilibrium adsorptions obtained for the various resins. Table 3 and figure 4 would also seem to indicate that the size of the adsorbed ion is important, the equilibrium adsorptions being in inverse order to the ionic diameters. This suggests steric hindrance as a factor; different resins may contain hydroxyl groups of varying degrees of accessibility. Again, the calcium ions as they become adsorbed on the resin may affect accessibility of the remaining hydroxyl groups. Models of the resins were constructed; they indicated that the hydroxyl groups in the catechol resin, being in the ortho position, have a tendency to fall into more accessible positions; the links between the benzene rings are on the opposite side to the groups active in adsorption. On the other hand, resorcinol, having a point between the hydroxyl groups a t which linkage can occur, forms a structure in which the possibility of shielding the hydroxyl groups is much more likely. Consideration of figure 7 shows that a t seven days the catechol resin possesses higher adsorption values at all concentrations than the resorcinol resin a t three months. Furthermore the adsorption measured, 14.5 millimoles per gram, is 94 per cent of the theoretical adsorption maximum. There remains the anomaly of the phloroglucinol resin, with an adsorption maximum of 19.2 compared to 41.7 theoretically required, This may be attributed, partially a t least, to a high degree of polymerization. I n phloroglucinol the 2-, 4-,and 6-positions are free, each one allowing substitution in the position ortho to two hydroxyl groups, so that it might be expected that polymerizationwould occur to a higher degree than in the mono- and di-hydroxy phenols. When the maximum number of cross links are formed the resin offers the maximum steric hindrance to the reaction of the hydroxyl groups; thus a hardened phenol-formaldehyde resin of the Bakelite type does not adsorb cations. Such a pronounced cross-linkage effect would account for the low maximum of the phloroglucinol resin.4 A more active catalyst in the resinification might be If there were two -CHz links per benzene ring, the adsorption maximum would be 33. These figures might be lowered somewhat if some calcium atoms were able t o satisfy two hydroxyl groups simultaneously. T h e difference might also be accounted for by resinification occurring through the hydroxyl groups, b u t this appears unlikely (7, 10, 8).
352
E. I. AKEROYD AND GEOFFREY BrlOUGHTON
expected to increase the number of cross linkages and hence decrease the adsorptive power of the resin. A resin prepared with ammonia, known t o be a very active catalyst (po1ymerizat)iontakes place at room temperature instead of in a' boiling solution as with other catalysts), gave a lower adsorption maximum than the acid catalyst resin (figure 6). This observation, that the total possible adsorption is dependent upon the degree of polymerization, is of considerable importance from the commercial standpoint. The negligible effect of kieselguhr on the rate of adsorption might well be explained by the fact that the amount of kieselguhr employed, 50 per cent by weight', corresponds to only 28 per cent by volume; a larger difference might' be detected by the use of a considerably higher volume per cent of kieselguhr. However if, in keeping with the above discussion, it is assumed tJhat the number of available interior hydroxyl groups rather than the number of surface hydroxyl groups govern adsorption, an explanation is at once obvious. Non-adsorpt,ion of calcium sulfate by the resins, t'he unique behavior of phenolic resins compared with the glypbal, vinylite, and other resins, the results of the adsorption experiments at three months, and the general character of the curves all make it difficult to describe the adsorption as other than chemical. Indeed, the present data can all be explained as a consequence of chemical interaction between the base and the phenolic hydroxyl groups, varying in accessibility in each resin and from resin to resin. Accessibility of these groups, as shown by the amount of base adsorbed, is governed by the structure of the phenol concerned and by the degree of polymerization of the resin. SUMMARY
The rates of adsorption of calcium hydroxide by some simple mono-, di-, and tri-hydroxy phenol-formaldehyde resins have been determined. The results obtained can be explained on the basis of chemical reaction of the calcium hydroxide m-ith phenolic hydroxyl groups of various degrees of accessibility rather t'han as a pure adsorption phenomenon. REFERESCES (1) ADAMS. ~ N DHOLMES: J. Soc. Chem. Ind. 54, 1T (1935). (2) BHATNAGAR, I
