Preparation and analytical properties of a chelating resin containing cysteine groups

Ekevier Scientific publishing Company, Amsterdam - Printed in The Netherlands

PREPARATION AND ANALYTICAL PROPERTIES OF A CHELATING RESIN CONTAINING CYSI’EINE GROUPS

CHUEN-YING IJU and PENG-JOUNG SUN*

Department of Chemistry, National Taiwan University. Taipei, Taiwan (Republic of China)

(Received 21stMay1981)

SUMMARY

A macroporous, cross-linked polyacrylonitrile copolymer was synthesized, the nitrile groups were converted to carboxylic acid by hydrolysis, and these carboxylic acid groups were treated with L-cysteme and 1,6-hexanediol (binding agent). Studies of the basic characteristics of this resin showed that it was highly selective for silver(I), mercury@), gold(III) and platinum(IV) in aqueous acidic solution, the maximum capacities being 0.97, 0.65, 1.22 and 0.39 mm01 g' of dry resin, respectively_ These four metal ions can be separated from each other, or concentrated from very dilute solutions, on a short column of the resin. The effects of different acids and of various common metal ions are reported.

The chemical behavior of an ion-exchange resin is determined by the nature

of the functional groups attached to the hydrocarbon skeleton. Highly selec-

tive chelating resins can be synthesized by attaching appropriate ligand

groups to the resin

interesting ligand: at low pH values, only the sulfhydryl group is used in

complex formation [l]

Studies have shown that sulfur-containing ligands

exhibit better selectivity for the noble and heavy metals than their oxygen

and nitrogen analogs [ 21.

many complexes of cysteine and its

alkyl esters, both in solution and in the solid state with a variety of metals

have been described [3-lo],

less information is available about the com-

plexes of noble metals with cysteine [ 11, 121. No information on resins con-

taining cysteine groups seems to have been reported. This paper describes

the synthesis and basic characteristics of a resin with cysteine ligands and its

analytical application in the chromatography of silver and some noble

metals.

EXPERIMENTAL

Instrumentation and reagents

A Radiometer pH meter was used with saturated calomel (Type K401)

and glass (Type G202B) electrodes, which were calibrated against Beckman

standard buffer solutions of pH 4.00 and 7.00. A Hitachi model 624 digital

spectrophotometer connected to a Hitachi model QDls recorder and IO-mm

acrylonitrile and divinylbenzene as described by Vernon and Eccles [13]

The copolymer was air-dried, ground and sieved. The 60-100

mesh fraction

was used for the further synthesis after being washed with 12 M HCI, water

and acetone. The copolymer (200 g) was stirred at 80°C with 1 1 of 37%

(w/v) sodium hydroxide solution until ammonia evolution ceased. The

hydrolyzed polymer was cooled to room temperature, collected by filtration

under suction and washed with 12 M HCl, water and acetone.

For the first esterification, the carboxylic acid resin (200 g) was mixed

with 600 g of molten 1,6-hexanediol (m-p. 41°C) containing 50 ml of 18 M

sulfuric acid as catalyst. The mixture was kept at 70°C for 30 h for esterifica-

tion. The product was collected from the hot solution by filtration under

suction, and washed with boiling methanol_ For the second esterification, a

mixture of 55 g of this product and 300 g of r_,-cysteine was added to 500 ml

of dioxane containing 20 ml of 18 M sulfuric acid. The mixture was heated

at 90°C for 30 h. The fmal product was collected by filtration under suction

and washed sequentially with water, 12 M HCl, water and acetone.

Characterization of resin

In

order to verify the presence of cysteine groups in the synthesized resin,

the infrared spectrum of the resin was obtained with KBr pellets after each

step in the synthesis. The i-r. spectrum of the polyacrylonitrile-divinyl-

benzene copolymer showed bands at 2260-2240

cm-’ (--CN),

whereas the

spectrum of the hydrolyzed product exhibited bands at 3500-2500,

1720,

and 920 cm-’ (-COOH). The spectrum of the diol ester intermediate (Fig. la)

exhibited bands at 3450 (-OH),

1735, 1163, and 1064 cm-’ (-COOR)_

The spectrum of the final product (Fig. lb) showed bands at 2545 (-SH),

2960,1610,

and 1513 (-NH;),

and 1735,1163

and 1064 cm-’ (--COOR).

The nitrogen and sulfur contents, the hydrogen ion capacity, the capacities

for ..4g(I), Hg(II), Au(II1) and Pt(IV) at pH 1, and the acid ionization con-

stants, were determined with the results shown in Table 1.

The distribution coefficients

(mm01 of metal/g of dry resin)/(mmol of metal/ml of solution), were

determined by using the batch equilibrium method. For each equilibrium

experiment, 25 ml of a mixture consisting of various amount.. of acid and

0.3 mmol of the metal ion in question was treated with 0.2 g of fresh resin

and the solution was stirred for 8 h at room temperature (25°C). Thesolution

4000 2800 1800 1200 600

Wave Number km-‘)

Fig. 1. Infrared spectra: (a) RCOO(CH,),OH intermediate; (b) resin with cysteine group. was

filtered to remove the resin and the metal ion content of the filtrate was

determined by conventional spectrophotometric procedures. The results are

shown in Tables 2 and 3.

On the basis of these experiments, the order of selectivity of the resin

was Au > Ag > Hg > Pt > MO. The results also show that ‘he resin readily

retains Au(III), Ag(I), Hg(I1) and Pt(IV) from either 0.1 M HCIOd or 0.1 M

KCl. Molybdenum is also retained by the resin from 0.1 M HCl. Negligible

adsorption was shown by the alkali metals, alkaline earths, iron(III), cobalt-

(II), nickel(II), zinc(II), cadmium(II), and lead(I1) in 0.1 M acid. This sug-

gested that column chromatographic separation and concentration of metal

ions (Pt(IV), Hg(II), Ag(I), Au(II1)) should be possible with this resin.

Chroma tographic application

The resin column (6 mm i.d., 55

long) was conditioned with 30

ml of 0.1 M HCl at 0.5 ml

A sample that contained 0.4-1.5

pmol

each of Ag(I), Hg(II), Au(II1) and Pt(IV) in 0.1 M acid was added to the

column and the liquid level was allowed to drop to the top of the resin

bed. The wall of the reservoir was rinsed with 0.1 M hydrochloric acid

and then 10 ml of 0.1 M hydrochloric acid was used to elute any other metal

ions from the column while the above metals were retained. The retained

TABLE

Physical and chemical characterization of the cysteine-containing resin (SO-100 mesh) Percent cross-linkiig 5.8% Gold capacityb 1.22 mm01 g-’ Nitrogen content? 1.16 mmol g-’ Mercury capacity’

Silver capacityb

0.65 mmol g“

Sulfur content 1.15 mm01 g-1 0.97 mm01 g-*

Hydrogen ion capacity 2.85 mm01 g-’ Platinum capacityb 0.39 mm01 g-1 pK, (-COOH, -NH,+, -SH) 5.65, 6.59, 7.16 Molybdenum capacityb 0.22 mm01 g’ The nitrogen content of the original copolymer was 25.0% (17.9 mmol g-I). All weights refer to dry resin. OAt pH 1.0.

& 95 102 104 104 117 96 AU 1360 2430 13300 18000 24300 28300 I-& .54 43 40 38 22 21 Pt 39 46 55 75 68 50 Hydrochloric acid _4U 220 315 337 442 473

Hg

58

Hydra bromic acid

Au 112 195 165 282 374 326 195 - - 473 0 4 12 20600 2800 1150 21 17 13 34 34 - 537 - - - - - - - - 16 26 -

Pt(IV), Hg(II), and Ag(1) were eluted sequentially with 0.5 M HCI, and with

a 6 M HCI-2 M HC104 solution. The metals were determined spectrophoto-

metrically as their chloride complexes at 262,229 and 213 nm, respectively_

The gold(II1) retained by the resin was eluted with 0.1% thiourca in 0.1 M

and determined spectrophotometrically at 269 nm as the thiourea

complex. -4 separation curve for the 4component

mixture is shown in

Fig. 2.

The effects of other metal ions on the recoveries of Pt, Hg, Ag and Au are

listed in Table 4. Samples containing gold and various other metal ions were

separated on the column by elution with 6 M HCl; good recoveries of gold

were obtained in all cases (Table 5).

TABLE 3

Distribution coefficients in mixtures of hydrochloric acid and perchloric acid Ion Final concentration (M)

HCIO, 1.5 2.0 2.5 3.0 3.5 4.0

HCI 7.0 6.0 5.0 4.0 3.0 2.0

Au 498 548 577 578 578 676

Ht.z 10 0 12 12 12 12

6 40 60 120 160

Eluent Volume (ml)

Fig. 2. Separation of Pt(IV), Hg(II), Ag(1) and Au(III) with the resin column. Column 55 x 6 mm i-d.; flow rate 0.5 ml min-‘; 0.45 ctmol each of Ag, Hg and Pt; 1.50 pmol of Au.

Concentration procedure

‘..

_{The resin column (6 mm i.d., 25 mm long) was conditioned with 0.1 M}

HCl; then 500 ml of a very dilute solution of the metal ion tested was passed

through the column at a flow rate of 0.5 ml mm-‘. The sorbed metal ions

were eluted and determined spectrophotometrically. The results are shown

in Table 6.

DISCUSSION

The

infrared frequencies for the cysteine-containing resin are in good

agreement with those for the cysteine monomer [ 51. A potentiomet.ric titra-

tion curve obtained when the synthesized resin in the acid form was titrated

with 0.1 M KOH at ionic strength 0.1 showed three breaks, only the first of

which was quite distinct, corresponding to pK values of 5.65, 6.59 and 7.16

which are assignable to the residual carboxylic acid, amino and the sulfhydryl

Effect of 5.0 pmol of various metal ions on the recovery of 0.45 Mmol of Pt(IV), Hg(II) and Ag(1) and 1.50

pmol of

Metal ion Recovery (%)

I% Hg Ag Au

cu

1.0, which was sufficiently acidic to avoid

any interaction of the metal ions

with other coordinated

groups in the synthesized resin, and ensured that

only the sulfhydryl group was involved in complexation.

The maximum capacity of the resin, determined by the batch method in perchloric acid medium, was 1.10 mm01 g-’ for silver, 0.65 mm01 g-’ for mercury, 1.34 mm01 g-’ for gold and 0.59 mmol g-l for platinum (Fig.

3A).

The

content of

1.15 mm01 g-l, so that 1:l complexes are

indicated for the resin functional group with silver and gold while

in the hydroch!oric

acid medium compared with in perchloric acid medium.

Collection of metal ions from dilute solutions Metal

ion Amount added* (pmol)

&

1.6 c 1.2 z g 0.8 i 0.4 P G 0.0 0 2 4 6 8 Acid (Ml ,_._.-.d_._. 0.8 t Acid (M)

Fig. 3. Total capacity of the cysteine-containing resin for metai ions versus molarity of (A) perchloric acid and (B) hydrochloric acid: (0) Au(II1); (A) Ag(1); (I) Hg(I1); (x) Pt(IV).

The lower capacities for mercury and platinum in hydrochloric acid than in

perchloric acid at the same hydrogen concentration can be ascribed to com-

petition of the chloride ion with the resin ligand.

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