Bull. SOC. Chim. Belg. vol. 98 / n43 / 1989 0037-')64(, / 89 / $2.00 + 0.00

0 1989 Cornit6 van Beheer van het Bulletin V.Z.W.

COORDINATIVE PROPERTIES STUDY AND PREPARATION OF A RESIN CONTAINING POLYSTYRENE AND THIOUREA

* * Ricardo Schmidt , Juan E. Forster, Sergio A. Moya

Departamento de Qulmica, Facultad de Ciencia, Universidad de Santiago de Chile, Casilla 5659, Santiago 2, Chile.

Received : 02/09/1988 - Accepted : 20/01/1989

ABSTRACT. A matrix formed by reaction of styrene-divinylbenzene copolymer with thiourea has been prepared and studied. The thiourea is covalently bound to the polymer matrix through the sulfur atom. The properties of the prepared resin have been studied with respect to the uptake of some ionic species such as: Cu(II), Zn(II), Fe(III), Au(III), Re(VI1) and As(II1).

INTRODUCTION. The use of polymeric resins in industrial processes for recovery of metal

ions from aqueous solutions has currently become a method of great interest. Direct applications of this method are found either in the recovery of metallic species from hydrometallurgical processes or in the elimination of harmful metal ions from industrial efluents1-8. A large number of resins have been prepared using polystyrene divinylbenzene copolymer as the matrix, because it is commercially available and because of the long durability of the obtained product^^-^^.

In this work, the preparation and characterization of the resin developed by reaction of thiourea and chloromethylated polystyrene are reported, as well as the coordinative properties shown by the prepared resin with a number of ionic species such as Cu(II), Zn(II), Fe(III), Au(III), Re(VI1) and As(II1).

EXPERIMENTAL. General procedures. X-ray fluorescence spectra were recorded with an EDAD PV- 9100 X-ray spectrophotometer (energy dispersion type). The pH of the solutions was adjusted in the acidic region with potassium chloride and hydrochloric acid and in the basic pH region with sodium bicarbonate and sodium hydroxide. The final pH was controlled with a Carl-Zeiss pH meter. Depending on each element, metal ion concentrations were determined, either with a Perkin-Elmer Model 403 atomic absorption/flame spectrophotometer or by using standard volumetric procedures. IR spectra were recorded with a Perkin Elmer model 735- B spectrophotometer. IR(KBr) (resin 11) v(NH2), 3020 ( s ) , 2900 ( s ) , 2840 ( s ) ,

1600 ( s ) , 1490 ( s ) cm-l, v(S-C-(NH~)~), 1450 ( s ) , 1370 (m), 1190 ( s ) , 1160 ( s ) ,

1080 ( s ) , 1040 ( s ) , 970 (m), 910 ( s ) .

Reagents. Polystyrene-2%-divinylbenzene chloromethylated (200-400 mesh ASTM, ca 5 mM Cl/resin gram) was obtained from Merrifield Polymer Fluka.

The solutions of the studied species were prepared by using chemicals (p.a.) obtained from Merck and Molymet-Chile.

Preparation of the resin. Polystyrene chloromethylated ( 5 g, ca 25 mM) and thiourea (2.85 g, 37.5 mM) were added to a 100 ml flask containing tetrahydrofuran (THF) (60 ml). The mixture was refluxed under argon in a dry

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atmosphere for 2 4 hours. The thiourea excess after the reaction was thoroughly extracted with THF via Soxhlet extraction. The trace amount of solvent trapped in the resin was eliminated by heating at 5OoC under vacuum.

The resulting resin (6.5 9) presents 3 . 4 9 mM and 3.56 mM of sulfur and chlorine, respectively.

Determination of the retention capacity of the resin in the ionic species studied. In a 250 ml reactor provided with two necks, one for placing the pH electrode and the other for obtaining samples, the resin (100 mg) was suspended in the metal or no-metal ion aqueous solutions (100 ml). A stoichiometric ionic concentration excess in relation to the thiourea contained in the resin was used. Sampling was carried out after pre-established intervals to determine the concentration of the studied ionic species in the solution. The amount of metal or no metal ion in the solution was determined with an atomic absorption spectrometer or by a standard volumetric method.

RESULTS AND DISCUSSION.

Preparation and characterization of the polystyrene thiourea resin. Reaction of chloromethylated polystyrene coDolymer I with thiourea in THF leads, according to the following equation, to the formation of resin 11, after 2 4

hours : (- C H - C H2- In

__c ,NH2 + s - c 'NH2

k

- (-CH-CH2-

H-5- Q, 5- C ~

H @

( 1 )

I I1 The suggested structure for I1 summarizes the following four resonant

structures for the molecule.

The X-ray fluorescence spectra of resin I1 show two strong peaks centered at 0 . 2 3 and 0.26 KeV, which correspond to K-alpha lines for sulfur and chlorine, respectively.

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The reaction of a suspension of resin 11 with a silver nitrate solution slowly produces a silver chloride white precipitate which increases and darkens with time, as chemical equation 2 shows :

1" The infrared spectrum of the prepared resin I1 can be interpreted by

comparison with the thiourea infrared spectrum. Both spectra display similar patterns. The IR spectrum of the resin I1 shows several bands, some of which can be assigned to NH2 stretching and others to the S-C(NH2)2 moiety. The observed band at 1490 cm-l in the thiourea has been assigned to the NH2 rocking vibration, C-N stretching or C=S stretching motion^'^'^^'^^. This band undergoes an appreciable change of frequency and intensity or no change at all depending on the fact that the thiourea is coordinated through the nitrogen atom or through the sulfur atom, respectively. Because the band centered at 1490 cm-' for resin I1 did not show any change either in intensity or position, there are good reasons to believe that the coordination of the thiourea to the matrix occurs via sulfur atom. More support for this belief comes from the analysis of the band centered at 1383 cm-' which has been assigned in the free thiourea to a NH2 rocking, C=S stretching or C-N stretching motion. When thiourea is coordinated through sulfur atom the band is shifted to a frequency of 972 cm-l with a considerable decrease in intensity16. In resin 11, the band is observed at 970 cm-l with a clear decrease in intensity.

Additional support for our belief that thiourea is coordinated to the polystyrene matrix using its sulfur atom comes from the reaction of the same matrix with thiourea mono and bisubstituted. Experiments carried out with N- ethylthiourea and N,N'-diethylthiourea showed no change for both analyzed bands. However, modifications in position and intensities were observed for NH2 stretching vibration modes.

Besides showing the bands corresponding to thiourea, the resine I11 IR spectrum shows the nitrate group bands. The chloride and sulfur quantitative analysis obtained from resine I1 gave 346 and 349 mM per g of resin, respectively, which indicates an equimolar presence of these elements in the resin 11.

The chemical information and the spectroscopic results obtained for resin I1 suggest that this resine is formed when the thiourea sulfur atom attacks nucleophilically the carbon of the methylene group of the polystyrene, which leads to the chlorine substitution.

Capacity of the prepared resin for element uptake. Since resin I1 has hydrophilic properties all the studies tending to establish the capacity of the resin for ion uptake were performed in aqueous solutions. The resin was suspended in water and magnetically stirred in a batch reactor. The aqueous solutions were prepared using a stoichiometric excess of ion..related to the

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thiourea content in the resin. The capacity of resin I1 was evaluated for the uptake of As(III), Au(III), Fe(III), Re(VI1) and Zn(I1) ion as a function of pH and for a stirring period of 24 hours. In general, the resin is able of retaining either cationic or anionic species from aqueous solution. The retention mechanisms might involve different extents of complexing or ionic exchange of the chemical species with the resin. In more effective in acidic media, therefore showing a anionic species. The maximum capacities of resin I1 given in table 1.

TABLE 1 Maximum capacity of resin I1 for some ionic species with a stirring period of 24 hours.

any case the retention is clear preference for for the specified ions are

as a function of pH and

Ionic species uptake mole of ionic element mg of ionic PH by the resin u take by mole of element ion

tgiourea contained in the resin

by g of resin

As(II1) Au(II1) Cu(I1) Fe (111) Re(VI1) Zn(I1)

87 320 63 17

765 42

23 1.5 218 1.0-4.0 14 2.0 3 2.5

490 4.5 10 2.0

~ ~~

From these results it seems that the order of affinity of the ions for the resin is Re(VII)>> Au(III)>> As(III)> Cu(III)> Zn(II)> Fe(II1). It is important to observe the large affinity that the resin shows for Re and Au anions (490 mg and 218 mg per gram of resin, respectively). The ratio pH/ reteption can be used to find the selectivity of the resin. This can be seen in table 1 which indicates the maximum retention of the ionic species for certain pH. Higher values could be obtained by increasing the concentration of the thiourea in the resin, which can be achieved using an original matrix with a larger chloromethylated grade.

Thus, for rhenium, uptaken as perrhenate, the retention study for the resin was carried out in terms of the solution pH. The results for the reaction of the resin with the rhenium solution after a stirring period of 24 hours are shown in fig. 1. It can be observed that the retention of Re(VI1) ion increases together with the pH reaching a maximum in the 3-5.5 pH range. Over this range the capacity of the resin decreases reaching its minimum at pH = 10. After finding out the pH where the retention for rhenium is the largest the kinetics for the retention under these conditions was analyzed. The results obtained at pH 4.5 are summarized in fig. 2 (curve 1). If the amount of rhenium uptaken in 24 hours is considered to be loo%, the rhenium retention speed is very high during the first hour of reaction reaching 70%. After the first hour the absorption speed clearly decreases and the remaining 30 percent is retained during the following 23 hours.

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Re uptn ken, mg I resin,g

FIG. 1. Rhenium retention by the prepared resin in terms of pH.

The capacity of the resin to coordinate gold (111) ions is pH independent in the range 1-4 showing a maximum retention of 320 nun01 of gold ion per mol of thiourea contained in the resin.

The gold retention kinetics follows a similar pattern, as can be observed in fig. 2 (curve 2). The retention speed of resin I1 for Au(II1) at pH=l is high during the first 3 hours of reaction, period during which almost 90% of the gold ion uptaken in a period of 24 hours is adsorbed. After 3 hours of treatment the speed becomes rather slow, because the remaining 10 percent is retained in the following 23 hours.

Element upluken.mglresin,g I

400-

FIG. 2 . Curve 1, Re(VI1) ion 300- uptaken, pH 4.5; Curve 2, Au(II1) ion uptaken, pH 1; Curves 3 and 4 element uptaken from a mixture of Au(II1) 101 200- and Re(VI1) [ O ] ions, pH 1.5.

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AS described above, both retention kinetics clearly show that the resin retention speed is always high during the first hours of treatment and slow afterwards which is in accordance with the probably availability of active sites.

According to the amount of metal ion uptake per resin gram, rhenium is most easily uptaken, even at pH 1 . 5 . However, when the resin is added to an equimolar mixture of Au(II1) and Re(VI1) at pH 1 . 5 the behaviour of the resin is quite different, as can be observed in fig. 2 (curves 3 and 4 ) .

The retention kinetics in the first hour of reaction is different for each ion with the resin showing a preferent affinity for gold, that is, the retention for qold is faster than for rhenium. Nevertheless in the 1-5 hour range the retention kinetics is quite similar.

ACKNOWLEDGEMENTS.

We thank FONDECYT (Proyecto 1 0 8 4 - 8 8 , S.A.M.) and DICYT - Universidad de Santiago de Chile for the financial support they gave us. S.A.M. would also like to thank Molymet-Chile for a generous loan of rhenium.

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