ion exchange resins containing chelating groups show superior selectivity. They may be used to advantage in c hro ma togra phic separations and in removal and recovery of traces of heavy metals from relatively concentrated salt solutions
LLOYD D.
PENNINGTON
Southern Oregon College, Ashland, Ore, MAX B. WILLIAMS Oregon State College, Corvallis, Ore.
Increase selectivity in metal separations by using
...
Chelating Ion Exchange Resins N
SEPARATION procedures employing ion
exchange resins are frequently made more selective by the addition of complexing agents during absorption or elution. Although the most stable and most selective complexes often involve chelating compounds, some of these, because of size and solubility characteristics, are not suitable for the usual ion exchange techniques. T o avoid these difficulties and still take advantage of the selectivity of chelating agents, resins have been prepared which incorporate the chelating compound in the structure of the resin itself.
Experimental
Preparation of Resins. The resins prepared were of the phenol-formaldehyde type. The following standardized procedure was generally satisfactory. The phenolic compound, 0.2 mole, was dissolved in 50 ml. of 6N sodium hydroxide (0.3 mole) and 100 ml. of water. Forty milliliters of 37% formaldehyde
Literature Background Subject Ref. General characteristics of ion exchange resins and absorption process (7, 11) Applications of ion exchange (4-7, 11) Preparation and absorption properties of chelating resins (1-8,8,16)
(0.5 mole) were added? giving a deep red solution. This mixture did not gel immediately but formed a firm gel either on standing at room temperature overnight or on being heated on a steam bath for 20 to 30 minutes. The resulting gel was then usually heated for several hours more at 95' to lldo C. Resins prepared in this way were fairly easily broken up, ground in a mortar, and screened to give a fraction passing the 20-mesh screen and retained on the 32mesh screen. Batches of resins prepared from more concentrated solutions than above or subjected to higher temperatures were tough and difficult to grind up, had a lower moisture content and lower absorption capacity. Resins were prepared by the above standardized procedure from o-aminophenol, resorcinol, P-resorcylic acid (2,4-dihydroxybenzoic acid), and resacetophenone (2,4-dihydroxyacetophenone). In the preparation ofresins from P-resorcylic acid some decarboxylation occurred. I n preparation of the 8resorcylic and resacetophenone resins 0.01 to 0.02 mole of resorcinol was added to assist in cross linking, although actually adequate cross linking occurred with these materials to form good, firm resins without resorcinol. Preparation of the 8-quinolinol resin required modification of the procedure, because this material is incapable of cross linking.
A thick, yellow slurry was formed by addition of 0.2 mole of 8-quinolinol to 30 ml. of 6N sodium hydroxide (0.18 mole) and 100 ml. of water. Forty milliliters of 3770 formaldehyde (0.5 mole) were added to this and slowly reacted to form a deep red solution. After about an hour a solution of 0.2 mole of resorcinol in 30 ml. of GN sodium hydroxide and 100 ml. of water was added, followed by 40 ml. of 37% formaldehyde. A deep red gel formed slowly a t room temperature or in a few minutes when heated in a steam bath. This gel was heated further for several hours at 95' to 110' C. The resin was then ground and screened as in the standardized procedure. The 20- to 32-mesh portions of the resins were washed thoroughly with water, 0.1N sodium hydroxide, water, 1N hydrochloric acid, and finally with water until nearly neutral.
Moisture Content of Resins Dry Resin in
Resin
3
Moisture
0.500-Gram Sample,
Resorcinol Resacetophenone 0-Resorcylic acid o-Aminophenol 8-Quinolinol Amberlite IRC-50 Amberlite IR-120
65.2 50.8 71.8 81.4 78.8 56.4 51.5
0.174 0.246 0.141 0.093 0.106 0.218 0.242
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The resins were air-dried slightly to remove surface moisture and stored in tightly stoppered bottles. Moisture content was determined by drying to constant weight over phosphorus pentoxide in a vacuum desiccator. Results are shown in the table. For comparison, two commercial resins Amberlite IR-120 and IRC-50, were also used. These resins were not screened but were already of a size comparable to the resins prepared above. These commercial resins also were thoroughly washed as described above. Batch Absorption Studies. To determine whether the chelating resins showed specific absorption of certain cations undei certain conditions, a wries of batch absorption tests was run. Gregor and others ( 3 ) examined some of their resins by a similar procedure in which they varied pH, ionic strength, and initial concentration of cation. I n the present investigation ionic strength and initial concentration were held constant, to permit study of more resins and a wider range of pH values. T o measure the absorption of cations at a definite pH it was necessary to buffer the resins and the solutions used. The resins themselves, being weak acids or bases, affected the acidity of the solution, and in many cases absorption of a metal ion released hydrogen ion; hence buffers of fairly high capacity were required. I n preparing the buffers it was desirable to use materials which would be absorbed as little as possible by the resins, and in basic solutions it was necessary to use a medium which would keep the various metal ions in solution. I t was also desirable to introduce as few different materials as possible into the various buffers. For these reasons all buffers were prepared by starting with ammonium acetate (0.25 mole per liter
of completed buffer for p H 3 to 10, 0.24 mole for p H 2 , and 0.15 mole for p H 1) and adding hydrochloric acid or potassium hydroxide as needed to reach the proper pH. These quantities were chosen to keep the ionic strength of the buffers substantially constant at 0.25. Buffer solutions prepared in this way did not have equal buffering capacity. I n practice, however, they were satisfactory. Buffer solutions, also prepared in the same way, contained the various metal ions whose absorption was being studied. I n all of these the concentration of metal ion was 0.010M. Dificulties were encountered in preparation of the ferrous ion solutions due to air oxidation and precipitation of the hydroxides. Addition of hydroquinone and displacement of air with nitrogen made possible the preparation of solutions stable enough to test. T o avoid precipitation in the cobalt solutions at p H 9 and 10 it was necessary to use ammonium hydroxide instead of potassium hydroxide in the preparation of these two buffers. The following procedure was used in all batch absorption tests. A 0.500-gram sample of the moist resin was placed in a stoppered 125-ml. Erlenmeyer flask, 50 ml. of the desired buffer solution were added to the flask, and the flask was mechanically shaken for 24 hours. The buffer solution was then decanted and discarded. Fifty milliliters of the same buffer were again added, the flask was shaken again for 24 hours, and the solution was again decanted and discarded. This treatment brought the resin to the desired p H prior to the absorption test. A 50.0-ml. volume of the desired metal ion buffer solution of the same pH was then added and the flask was shaken for 24 hours. I n all cases the initial metal ion concentration was 0.01OM. At the end of
4 Carboxylic resin, Amberlite IRC-50, shows some selectivity toward copper
b 8-Quinolinol resin shows the greatest selectivity of resins tested
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INDUSTRIAL AND ENGINEERING CHEMISTRY
24 hours the solution was decanted from the resin into a clean, stoppered bottle and analyzed as soon as convenient for the metal ion. As there was some change in the pH in spite of the buffering, the pH of each solution was also determined after the absorption. Although the 24-hour absorption period did not achieve complete equilibrium, the very slight additional absorption obtained with longer periods did not justify the additional time required. The various metal ions were determined by colorimetric methods, using a Klett-Summerson photoelectric colorimeter with test tube type cells. Copper was determined by the o-phenanthroline method (74, 76), nickel by dimethylglyoxime (72, 7 4 , magnesium by Titan Yellow (72, 7 4 ) ,iron by o-phenanthroline (76),cobalt by Nitroso-R salt (72- 74, 76) and calcium by murexide ( 7 7 ) . Chromatographic Separations. A 60- to 100-mesh sample of the 8-quinolinol resin was used in columns 10 mm. in internal diameter and 30-cm. long. Resin depth was about 12 cm. The resin was backwashed at a rate of about 3 ml. per minute. Flow rate during absorption, elution. regeneration, rinsing, and buffering was maintained at 1 to 1.5 ml. per minute. One of the most interesting possibilities appeared to be the separation of copper, nickel, and cobalt. Separations were attempted starting at pH 8, 5, and 2, and eluting with 0.01M hydrochloric acid. Copper was held on the resin so tightly that it did not appear in the eluate under any of these conditions. Starting at pH 8 some cobalt and nickel were separated, but it appeared that a longer column or slower elution would be required and this separation was not studied further. At p H 2, copper was quantitatively separated from cobalt.
I O N EXCHANGE RESINS 4 Absorption of resorcinol resin in acid range is higher than would be expected from weakly acidic phenolic groups, may involve chelation with free methylol groups
3.5 -
3.0-
E 2.5-
z
P
Resacetophenone and 8resorcylic acid resins show similar absorption curves
0
2
5 2.0-
E E
b 10
Quantities used were 0.100 mmole of each metal ion. The column was buffered to p H 2.0, a pH 2.0 solution of the metal ions was added, and the column was eluted with 0.010M hydrochloric acid. Under these conditions all the cobalt appeared in the first 150 ml. of eluate. No copper appeared in 450 ml. of eluate. T o remove copper quantitatively from the resin 1M hydrochloric acid was required. Nickel was not completely separated from copper by this procedure, only 88% of the nickel being recovered in the first 250 ml. of eluate. The resins were regenerated many times with no apparent loss in activity. Isolation of Trace Constituents. A second type of application investigated was the isolation and concentration of trace constituents. While the ordinary ion exchangers have been very successful in removing traces of ionic constituents from solutions having an over-all low ionic concentration, their limited selectivity has hindered their use for separation of a trace of one ion from a high concentration of another. Riches (70) was able to absorb copper, cadmium, nickel, zinc, and manganese a t concentrations of 4 to 8 mg. per liter from 0.1 N ammonium chloride or ammonium phosphate solutions, but not from 1.ON solutions. Using the 8-quinolinol resin it has been possible to absorb cupric ion a t a very low concentration from high concentrations of sodium chloride. A column 21 mm. in inside diameter
o-Aminophenol resin shows good selectivity and capacity
was used with a 100-mm. depth of 20to 32-mesh 8-quinolinol resin. About 20.5 liters of 0.500M sodium chloride were prepared from ACS grade sodium chloride and distilled water. This solution was run through the above column at an average rate of 5 ml. per minute. The purpose of this treatment was to remove all impurities in the solution which could be absorbed by the resin. The column was then regenerated with 1M hydrochloric acid by reverse flow (upward through the column) and rinsed with distilled water until the pH of the effluent reached 5.0 (nearly the same as the p H of the distilled water). T o 20 liters of the 0.500M sodium chloride solution which had been put through the column were added 10.0 ml. of 0.010M cupric chloride (0,100 mmole of cupric ion). This solution was stirred thoroughly and then run through the column at the rate of 5 ml. per minute. The column was then eluted with 1M hydrochloric acid by upward flow at the rate of 1 ml. per minute. Samples of eluate were collected and analyzed. No copper appeared in the first 50 ml. of eluate, but in the next 100 ml. the entire 0.100 mmole was recovered. No remaining trace of copper could be detected in the sodium chloride solution which had been put through the column. The original concentration of copper ion in the sodium chloride solution was 5 x 10-6 M or 0.32 p.p.m. (0.32 mg. per l i t e r L Sodium ion concentration was 0.5M or about
12,500 p.p.m. This amount of copper would amount to an impurity of 0.0011% in the sodium chloride. The ratio of total electrolyte concentration to copper ion concentration in this test was about 100 times as great as that reported by Riches (70).
Discussion of Results The amount of absorption by the resins prepared in this investigation is comparable to that of the commercial resins. This quantity does not represent the total capacity of the resins, but rather an equilibrium absorption. In comparing the amount absorbed, two conditions must be considered. First, although the absorption is reported in millimoles per gram of dry resin, the samples used in each case were 0.500 gram of moist resin. As the moisture content of the resins differed, the amount of dry resin involved was different for each resin (see table). Second, in some cases most of the metal ion present in the sample solution was absorbed. In these cases the absorption was limited by the quantity of metal ion rather than the capacity of the resin, particularly of the Amberlite IRC-50 resin. When equal dry weights of this resin and the 8-quinolinol resin were present, the amounts of cupric ion absorbed by the two resins were approximately equal. A second conclusion evident from the curves is that the chelating resins prepared in this investigation exhibit VOL. 51, NO. 6
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2 ,o
4
Sulfonic acid resin Amberlite IR-120, shows E relatively slight effect of 1.5 pH except as the nature v) of the ionic species present is affected
E
E 1.0
c-
0 .-+J
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$0.5
2
,
0,o
Resacetophenone resin
greater selectivity than the commercial cation exchangers used. This may be seen in the greater spread of the absorption curves. The absorption of the heavy metals on the chelating resins greatly exceeds that of calcium and magnesium, whereas on the carboxylic and sulfonic acid resins only slight differences in capacity are shown for the various metals. The absorption of all the resins except the sulfonic acid resin is strongly affected by the pH, as would be expected from consideration of their structures. The resorcinol, resorcylic acid, and resacetophenone resins show very similar curves, even to the extent of some very minor fluctuations. T h e o-aminophenol and 8-quinolinol resins are also similar in absorption properties, except that the 8-quinolinol resin shows greater absorption at low pH. The 8-quinolinol resin shows the greatest selectivity of all the resins tested. I n all the chelating resins described here the relative affinities of the metal ions are in approximately the same order. These resins compare well also with respect to capacity and selectivity with the rn-phenylenediglycine chelating resin described by Gregor ( 3 ) . Gregor’s resin shows especially high absorption a t low p H values, although not greater than that of the o-aminophenol or 8quinolinol resins. The order of absorption of the metals reported for the mphenylenediglycine resin is different from that shown for the resins prepared in this study. Four of the five resins studied here were prepared, at least in part, from monomers which are themselves chelating agents. The one resin which does not involve a chelating monomer is the resorcinol resin. Although resorcinol itself is not capable of chelation, the initial products of its reaction with formaldehyde in the presence of strong base-Le., the phenol alcohols-possess strong chelating tendencies. Many of the methylol groups first formed would necessarily be tied u p in the subsequent condensation reaction to form the resin.
762
However, with the excess of alkali and of formaldehyde and the very mild cure conditions used in preparation of these resins it is probable that many of the methylol groups remain the final resin. The high capacity of the resorcinol resin, even in the acid range, and the preferential absorption of metals which readily form chelates cannot be explained by the weakly acidic properties of resorcinol itself. Further work is in progress to determine the free methylol groups in this resin and to correlate this with conditions of preparation and absorption capacity. In preparing the solutions to be used for the batch absorption tests it was not possible to cover the range of p H values with the metals used without formation of some complex ions. Presence of these complexes, of course, affects the absorption of the metals involved. In the higher p H ranges stable ammonia complexes are formed with copper, nickel, and cobalt. These metal-ammine complexes may be absorbed as such by ordinary cation exchange processes, but could not be taken u p by a process of chelation. The strong absorption of these metals by the phenolic resins at high p H couId be due either to absorption of the complex by the weakly acidic phenolic group or to transfer of the metal from the ammonia complex to the chelate complex. Both processes probably occur t o different extents a t different pH values. T h e decrease in absorption of copper by the sulfonic acid resin, Amberlite IR-120, in the p H 5 to 6 range may be due to formation of anionic acetate complexes. This is suggested also by the fact that copper is readily absorbed from these solutions by several anion exchange resins, Amberlites IR-4B and IR-45, and Dowex 3 (9).
Conclusions Chelating ion exchange resins have been produced which show superior selectivity in their absorption of various metallic cations and possess high absorp-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2
4
PH
6
8
10
tion capacity. These may advantageously be applied to chromatographic separations of certain metals and to isolation of traces of certain metallic ions from high concentrations of other electrolytes. Literature Cited
(1) Chem. Eng. News 32, 1896-8 (1954). (2) Gregor, H. P., Taifer, M., Becker, E.,
Abstracts of Papers, 117th Meeting, ACS, Houston, Tex., 1950. (3) Gregor, H. P., Taifer, M., Citarel, L., Becker, E., IXD.END.CHEM.44, 2834-9 (1952). (4) Kunin, Robert, McGarvey, F. X., Anal. Chem. 26, 104-9 (1954). (5) Kunin, R., SfcGarvey, F. X., Farren, A. L., IND. ENG. CHEM. 48, 540-6 (1956): (6) Ibid.,49, 507-13 (1957). (7) Kunin, R., Myers, R . J., “Ion Exchange Resins,” New York, M‘iley, 1950. (8’1 . , Parrish. J. R.. Chem. G’ Ind. (London) 1955, No’. 14, 386-7. (9) Penninpton. L. D.. Ph.D. thesis. Oregon gtate College, ’1956. (10) Riches, J. P. R., nhture 158, 96 (1946). (11) Samuelson, Olof, “Ion Exchangers in Analytical Chemistry,” New York, Wiley, 1953. (12) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., New York, Interscience, 1950. (13) Shipmen, W. H., Feti, S. C., Simon, W., Anal. Chem. 27, 1240-5 (1955). (14) Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysis,” 3rd ed., vol. 2, New York, Van Nostrand, 1949. (15) Soldano, B. A., Boyd, G. E., Gordon Research Conference on Ion Exchange, h-ew London, N. H., Jul 1950; Ann. Rev. Phys. Chem. 2, 309-42 6 9 5 1 ) . (16) Welcher, F. J., “Organic Analytical Reagents,” New York, Van Nostrand, 1947. (17) Williams, M. B., Moser, J. H., Anal. Chem. 25, 1414-17 (1953). I
,
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RECEIVED for review November 23, 1957 ACCEPTED February 24, 1959 Section of Analytical Chemistry, Pacific Northwest Regional Meeting, ACS, Spokane, Wash., June 13 and 14, 1957. Based on a thesis presented by Lloyd D. Pennington to Oregon State College in partial fulfilIment of the requirements for the Ph.D. degree. Approved for publication by the Oregon State College Monograph Committee. Research Paper 351, Department of Chemistry, School of Science.
