ANALYST, DECEMBER 1985, VOL. 110 1501

Separation of Lead and Copper on a Series of Chelating Ion-exchange Resins. Part I

Ajay Shah and Surekha Devi* Department of Chemistry, Faculty of Science, M.S. University of Baroda, Baroda 390 002, India

The physico-chemical and chelating properties of aromatic poly( hydroxamic acid) ion-exchange resins are described. The resins are used for the quantitative separation of the heavy metals lead and copper, which are often present in industrial effluents. These resins show excellent kinetic characteristics for copper and lead ions. The effect of metal concentration on the cation-exchange capacity is also reported. Even after 20 loading - acid washing cycles the resins show greater stability than the hydroxamic acid resins reported earlier. Quantitative separation of the two metals is achieved by using different eluting reagents.

Keywords: Poly(h ydroxamic acid) resin; column chromatography; ion-exchange resin; copper; lead

The study of polymeric hydroxamic acids was pioneered by Deuel and co-workers, who prepared hydroxamic acid deriva- tives of Amberlite IRC-50 and other poly(methacry1ic acid) resins.1-3 During the last 30 years attempts have been made by researchers to synthesise polymeric resins containing hydrox- amic acid functional groups.4-10 Amongst these contributions those of Vernon and Eccles are significant.

A number of chelating resins containing amino carboxylic acid groups,11-14 such as the phenylalanine group,15 have been synthesised and their analytical properties have been studied. The commercially available chelating resin Dowex A-1 containing the iminodiacetic acid group shows a relatively high affinity towards metals but insufficient selectivity. 16-18 Duolite CS-346, which contains amide oxime groups, forms stable metal complexes but breaks down in acidic solutions and hydrolyses to hydroxamic acid. So far no commercially available resin has been reported that contains a hydroxamic acid functional group.

The hydroxamic acid resins reported in the literature are mainly used for recovery and separation of uranium from simulated sea water. No hydroxamic acid resin has been reported for the quantitative removal and separation of copper and lead. As industrial effluents are often rich in copper and lead, removal of these metals is important to industry. Therefore we have studied the effect of electrophilic and nucleophilic substitution at the nitrogen in poly- (hydroxamic acid) ion exchangers on the removal and separation of heavy metals. This paper discusses the condi- tions for the separation of the commonly occurring metal ions copper and lead in industrial effluents. We found effective separation and removal of copper and lead is possible using different eluting systems. The resins show good thermal and chemical resistance. The effect of the concentration of the metal solution on the cation-exchange capacities of the resins was also studied.

Experimental Reagents

The chemicals and metal salts used were of analytical-reagent grade. The monomer acrylonitrile was obtained from Fluka (Buchs, Switzerland). Solvents were distilled prior to use.

Procedure

Monomer acrylonitrile was passed through a column of activated alumina to remove any inhibitor and was co- polymerised using divinylbenzene (5%) by the pearl poly- merisation technique. The resulting polyacrylonitrile was hydrolysed by 50% V/V H2SO4 at 70-80 "C for 16 h. The resulting poly(acry1ic acid) was treated with different hydroxy-

lamines in the presence of sodium methoxide at 75-80 "C for 18 h. The hydroxylamines used were the simple aliphatic hydroxylamine, and N-phenylhydroxylamine, p-chloro- phenylhydroxylamine , rn-chlorophenylhydroxylamine, p-tolylhydroxylamine, rn-tolylhydroxylamine and 3-chloro-4- tolylhydroxylamine, all of which were synthesised by partial reduction of the corresponding nitrobenzene derivatives. Partial reduction was carried out in the presence of ammo- nium chloride using zinc powder at 60 "C. The hydroxylamines were then recrystallised from a mixture of light petroleum and benzene. The simple aliphatic hydroxylamine was prepared from hydroxylamine hydrochloride by the addition of an excess of sodium hydroxide.

Caution-Benzene is highly toxic and appropriate precau- tions should be taken.

The resulting polymers were washed thoroughly with methanol then with 2 N HCl and finally with de-ionised water until free from chloride. Samples were vaccuum dried prior to elemental analysis but the bulk polymer was stored in the fully swollen form for further study.

The resins are insoluble in benzene, chloroform, carbon tetrachloride, acetone, alcohol, DMF, DMSO, toluene and all acids and alkalis of high concentration. All the resins show high swelling properties in acids of higher concentrations.

The water regain, sodium exchange capacity, rate of exchange for sodium ion, metal ion capacity, equilibration rate, resin stability, effect of metal ion concentration on 9, effect of different eluting reagents, true density, void volume fraction and apparent density were determined according to literature methods.19920

Ion-exchange Columns The glass columns (length cu. 15 cm, i.d. cu. 0.7 cm) were packed with 5 g of the resins (H+ form). After the complete washing procedure the metal ion solutions were passed down the resin columns at flow-rates of 1 ml per 5 min. During elution a flow-rate of 1 ml min-1 was employed, Copper and lead ions were determined spectrophotometrically21 and complexometrically.22

Results and Discussion The physico-chemical properties of the poly(hydroxamic acid) resins are summarised in Table 1. According to Vernon and Ecclescs hydroxamic acid groups are incapable of reacting with sodium hydrogen carbonate and the resins we prepared did not show any exchange capacities when treated with NaHC03, indicating the presence of hydroxamic acid groups and the absence of free carboxylic acid groups in the resins.

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1502 ANALYST, DECEMBER 1985, VOL. 110

Table 1. Physico-chemical properties of the hydroxamic acid resins

Property Unsubstituted N-Phenyl Moisture content, % . . . . . . 3.36 2.38 Truedensity/gcm-3 . . , , . . 1.35 1.37

Sodium-exchange capacity/mmol per g

Copper ion capacity at pH 5.O/mmol

Lead ion capacity at pH 4.0/mmol per

Apparent density/g cm-3 . . . . . . 0.22 0.20

of dry resin . . . . . . . . . . 8.71 6.13

per g of dry resin . . . . . . . . 5.6 5.5

g of dry resin . . . . . . . . 5.2 4.9

p-Chloro- phenyl

4.96 1.42 0.45

5.24

3.9

3.5

m-Chloro- phenyl

5.61 1.38 0.17

5.16

3.4

3.2

3-Chloro- p-Tolyl m-Toly 1 4-tolyl

4.81 6.67 7.52 1.27 1.40 1.38 0.64 0.48 0.64

5.71 5.92 5.44

5.3 5.7 3.9

4.6 5.3 3.1

Table 2. Effect of pH on sodium-exchange capacity (mmol per g of dry resin) on the hydroxamic acid resins

p-Chloro- rn-Chloro- 3-Chloro-4- pH Unsubstituted N-Phenyl phenyl phenyl p-Tolyl m-Tolyl tolyl 10.5 1 .oo 11.0 2.02 1.91 2.17 2.51 11.5 3.68 2.06 3.23 3.73 4.76 5.51 5.14 12.0 6.52 4.18 4.68 5.01 4.95 5.61 5.23 12.5 8.71 6.13 5.24 5.17 5.70 5.92 5.44

This is confirmed by IR spectral evidence. The reported sodium-exchange capacity of the N-phenyl substitution in hydroxamic acid resin is 3.3 mmol g-1fj-8 whereas by modifying the experimental conditions we could achieve 6.18 mmol g-1 capacity for N-phenyl substitution and 8.7 mmol g-1 for the unsubstituted poly(hydroxamic acid) resin. Further substitution of different electrophilic and nucleophilic groups at nitrogen in the hydroxamic acid group decreases the sodium-exchange capacity by up to 5.16 mmol g-1 (Table 1) depending upon the position of the substitution, its size and electrophilic and nucleophilic character. No other such comparative studies of the effects of substitution at nitrogen in hydroxamic acid resins have been reported. We observed that the decrease in sodium-exchange capacity and metal capacity is greater if an electrophilic group is substituted and less when it is a nucleophilic group. This may be due to the electron donating character of the nucleophilic group, which makes the hydroxamic acid chelating group more basic in character and facilitates the exchange and chelating process.

In addition the decrease in the capacity of the resin on substitution is due to steric hinderence of the bulky group compared with that of the unsubstituted hydroxamic acid resin. Therefore, the order for the exchange capacity for sodium, copper and lead is as follows: the unsubstituted resin > N-phenyl > rn-tolyl = p-tolyl = 3-chloro-4-tolyl > p-chlorophenyl = rn-chlorophenyl substituted resins. However, the position of substitution ( i e . , either para or rneta) has very little effect on the exchange capacities. The experimental values for the cation-exchange capacities of the resins are in good agreement with the theoretical ones. The experimental values for moisture contents (Table 1) are in good agreement with those calculated from thermogravi- metric studies. The column densities of the prepared resins are low in comparison with commercial resins. This may be because of differences in the polymer backbone. The high value of the void volume fraction facilitates the diffusion of ions in the resins.

Table 2 summarises the total cation-exchange capacities for all the hydroxamic acid resins studied at different pH values. It was noted that the resins exhibit cation-exchange capacities only at high pH i .e., above 10.5. Thermal stability data indicate high thermal stability of the resins. The capacities of the resins increase with increasing temperature, making the exchange of ions easier. A differential scanning calorimetry study shows that these resins decompose above 385 "C.

Fig. 1 shows the rate of exchange of the cation-exchange

100

80 8 pi

5 60 0)

r X u1

40

20

0 10 20 30 40 50 60 Ti me/m in

, 1 1 1 1 1 1

6 9 12' 15 18 21 24 Time/h

Fig. 1. Rate of exchange for sodium ion using different cation exchangers. U, Unsubstituted hydroxamic acid resin; x , N-phenyl- hydroxamic acid resin; 0, p-chlorophenylhydroxamic acid resin; 0, rn-chlorophenylhydroxamic acid resin; A , p-tolylhydroxamic acid resin; A m-to1 lhydroxamic acid resin; and 17, 3-chloro-4-tolyl- hydroxamic acidrresin

resins. It can be seen that complete exchange of Na+ takes place within 24 h for all of the resins. The time required for 50% exchange is 5,7,8,3,6,3 and 6 min for the unsubstituted, N-phenyl, p-chlorophenyl, rn-chlorophenyl, p-tolyl, rn-tolyl and 3-chloro-4-tolyl substituted hydroxamic acid resins, respectively. The initial fast rate of exchange can be explained on the basis of the law of mass action. Continuous stirring is necessary for efficient exchange. The resins showed better 50% exchange values than those of the hydroxamic acid resins reported by Vernon and Eccles.68

Metal Ion Capacities The effect of pH on the Kd values of the resins for copper and lead was studied by equilibrating 400 p.p.m. metal solutions at different pH values with known amounts of resins for 24 h. Fig. 2(a) and ( 6 ) shows that both the metals chelate above pH 2.0. All the resins show a high affinity to form chelates with copper and lead between pH 4.0 and 5.0. Kd values for copper are found to be from 200 to 24000 while for the lead they are from 10 to 900 at different pH values for the different resins. Fig. 3(a) and (b) shows the rate of exchange for copper and

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ANALYST, DECEMBER 1985, VOL. 110 1503

lead ions. The complete exchange takes place within 24 h for all resins for both metals. A 50% exchange of copper requires 8, 11, 16, 8, 1, 11 and 3 min and lead requires 12, 18, 5, 2, 4, 13 and 6 min for the unsubstituted, N-phenyl, p-chlorophenyl, rn-chlorophenyl, p-tolyl, m-tolyl and 3-chloro-4-tolyl substi- tuted hydroxamic acid resins, respectively. From the study of different metal ion concentrations on ti it is observed that ti is

25 OOC

20 000

15 000

10 000

5 000

Q, - 2 1000

9

80 0

600

400

200

0 1 2 3 4 5 6 PH

Fig. 2. Effect of pH on the Kd values of the resins for (a) copper and (b) lead. Resins as given in Fig. 1

inversely proportional to the metal ion concentration. The total capacities of the resins for the copper and lead are listed in Table 1.

Both the metal ions studied form chelates with the resins under similar conditions. Therefore different eluting reagents were necessary for complete elution and separation. Different concentrations (1-6 N) of HC1, H2S04, HN03, CH3COOH

100

80

60

40

20 8 a- m

c

Lu

5 0 y 100

80

60

40

20

0

I c)

I I I l I l ,

10 20 30 40 50 60 Ti me/m in

3 6 9 12 15 18 21 24 Time/h

Fig. 3. Rate of exchange for (a) and (b) copper and (c ) and ( d ) lead using different cation exchangers. Resins as given in Fig. 1

I I I 1

10 20 30 40 50 60 70 I 0 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70

8- C

.g 40 - al

10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 Volume of eluent/ml

Fig. 4. Separation of copper and lead in a concentration ratio of 1 + 1 using different cation exchangers. (a) Unsubstituted hydroxamic acid rksin; (b) N-phenylh droxamic acid resin; (c) p-chlorophenylhydroxamic acid resin; ( d ) rn-chlorophenylhydroxamic acid resin; (e) p-tolylhydroxamic acidrresin; cr) rn-tolylhydroxamic acid resin; and (g) 3-chloro-Ctolylhydroxamic acid resin

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1504 ANALYST, DECEMBER 1985, VOL. 110

and NaCl and 5% HC104 were used for eluting the metal ions; 1 N HN03 and 6 N NaCl solution were found to be satisfactory for the elution of copper and lead, respectively. A study of a series of poly(hydroxamic acid) ion-exchange resins showed that electrophilic or nucleophilic substitution at either the para- or meta-position affects the physico-chemical properties, such as the true density, apparent density, void volume fraction, sodium-exchange capacity and metal capacity. This can be explained on the basis of stabilisation and delocalisa- tion of the ring due to the substitution at nitrogen, which makes the ring acidic or basic in character. The results obtained confirm the electrophilic and nucleophilic substitu- tion theory. However, such electrophilic and nucleophilic substitution does not have much effect on the separation of copper from lead or vice versa. In all instances separation was achieved using 1 N HN03 and 6 N NaCl for elution of copper and lead, respectively.

A study of the physico-chemical properties, effect of pH, effect of metal ion concentration on exchange rate and separations was carried out in four sets. The coefficient of variation calculated from the mean deviation for all results was <2.5%, indicating that the method is precise and accurate.

Applications

Separations were carried out on solutions of copper and lead with concentration ratios of 1 + 1, 1 + 10 and 10 + 1, respectively, at pH 5.0. The solutions were passed down the resin columns at a flow-rate of 1 ml per 5 min, followed by a thorough wash with water. A solution of 6 N NaCl was then passed through the column at a rate of 1 ml min-1 to elute the lead followed by a thorough wash with water. To elute copper, 1 N HN03 was passed down the column at a rate of 1 ml min-1. Copper was determined spectrophotometrically using diethyl- dithiocarbamate22 while lead was determined using an EDTA method.23 From Fig. 4 it can be seen that there is no cross contamination in the separation of lead and copper, and the method gives clear separations for lead and copper at different concentration levels. Different concentration ratios of copper to lead were used and satisfactory results were observed. Graphs for the separation of copper and lead at a concentra- tion ratio of 1 + 1 using the different resins are given in Fig. 4.

We are grateful to C.S.I.R. for supporting the research project. We also thank Professor P. K. Bhattacharya for providing the necessary facilities.

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References Cornaz, J. P., and Deuel, H., Experientia, 1954, 10, 137. Cornaz, J. P., Hutschneker, K., and Deuel, H., Helv. Chim. Acta, 1957, 40, 2015. Deuel, H., and Hutschneker, K., Chimia, 1955, 9, 49. Kern, W., and Schulz, R. C., Angew. Chem., Int. Ed. Engl., 1957, 69, 153. Petrie, G., Locke, D., and Meloan, C. E., Anal. Chem., 1965, 37, 919. Vernon, F., and Eccles, H., Anal. Chim. Acta, 1976, 82, 369. Vernon, F., and Eccles, H., Anal. Chim. Acta, 1976, 83, 187. Vernon, F., and Eccles, H., Anal. Chim. Actu, 1977, 94, 317. Philips, R. J., and Kritz, J. S., Anal. Chim. Actu, 1980, 721, 225. Vernon, F., Pure Appl. Chem., 1982, 54, 2151. Moyers, E. M., and Frits, J. S., Anal. Chem., 1977, 40, 418. Greger, H. P., Taifer, M., Citarel, L., and Becker, E. I., Znd. Eng. Chem., 1952,44,2834. Marhol;M., and Cheng, K. L., Tulantu, 1974, 21, 751. Tomoshige, S. , Hirai, M., and Ueshima, H., Anal. Chim. Acta, 1980, 115, 285. Sugil, A., Ogawa, N., and Katayama, I., Talanta, 1982, 29, 263. “Dowex A-1 Chelating Resin Form No. 164-80, 64,” Dow Chemical Co., Midland, MI, February 1964. Van Grieken, R. E., Bresseleers, C. M., and Venderborght, B. M., Anal. Chem., 1977, 49, 1326. Colella, M. B., Siggia, S., and Barnes, R. M., Anal. Chem., 1980, 52, 2347. Helfferich, F., “Ion Exchange,” McGraw-Hill, New York,

Kunin, R., “Ion Exchange Resins,” Wiley, London, 1958, p. 324 . Sandell, E. B., “Colorimetric Determination of Traces of Metals,” Third Edition, Interscience, New York, 1959, p. 444. Vogel, A. I., “Quantitative Inorganic Analysis,” Fourth Edition, Longman, London 1978, p. 325.

1962, pp. 72-94.

Paper A41428 Received December 5th, 1984

Accepted July 15th, 1985

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