Piotr CYGANOWSKI, Dorota JERMAKOWICZ-BARTKOWIAK, Paweł WILKOWSKI ? Department of Polymer and Carbon Materials, Wrocław University of Technology, Wrocław, Poland

Abstract:
As part of this research, A, B and C ion exchangers were obtained and their capacity for sorbing chloro complexes of precious metals PtCl6 -2 Pt(IV), PdCl4 -2 Pd(II), AuCl4 – Au(III) were discussed. Anion exchangers marked as A, B and C were obtained by modifying suspension copolymer of vinylbenzene chloride and divinylbenzene VBC/DVB (2% by weight) with the following pyridine derivatives: 4-aminopyridine (A ion exchanger); 2-(aminoethyl)pyridine) (B ion exchanger); 2,6-diaminopyridine (C ion exchanger). Sorption capacity for chloro complexes of precious metals: Pt(IV) PtCl6-2, PdCl4-2 Pd(II), AuCl4- Au(III) was assessed using the statistical method in three- omponent model solutions (Au+Pt+Pd) in 0.1 M HCl within the range of initial concentrations of metals from 10.44 to 919.0 mg/dm3 by drawing adsorption isothermal lines. A ion exchanger turned out to be the most effective for platinum sorption (184.5 mg Pt/g), and B ion exchanger was the most effective for gold and palladium sorption (295.3 mg Au/g, 104.6 mg Pd/g).

Please cite as: CHEMIK 2013, 67, 4, 317-324

 Introduction
Precious metals Metals (raw or partially processed) are the fundamental material to produce all kinds of tangible goods. Many countries export metals and their market prices influence the global financial stability [1]. In the years 2001÷2008, the International Monetary Fund reported that the stock exchange indexes for non-fuel raw materials increased by 66%. This trend was just halted by the financial crisis in the United States of America in 2008; however it did not have an adverse impact on the prices of precious metals. During the next year, their price was increasing regarding other non-fuel raw materials by just 135% making them, besides energy carriers, a driving force for the advanced economies [2]. Diversification of extracted natural resources is in the interest of each country, and diversified types of exported raw materials provides the opportunity for eliminating a negative impact when the price of any of them collapses [3].

Precious metals belong to the group of most readily extracted raw materials because, apart from their ?direct? function (for producing coins, jewellery or catalysts), they are also employed (particularly gold and platinum) to assess the financial capacity of countries. It results from the fact that gold has properties characteristic for money, but it is more resistant to lose its value because gold itself has a purchasing power. Metallic raw materials are used by governments and investors as their capital protection [4].

Precious metals are not affected by economic destabilisation caused by the financial crisis, which is confirmed by excellent share price performance of companies involved in extracting metallic raw materials. It is becoming more and more popular to invest in metal market as the mechanisms influencing this market are transparent. Increasing prices caused by the continuous growth in demand on metals incline investors to invest their assets into this market during the crisis. Such a phenomenon is likely to induce the further increase in prices of precious ores [5]. An increasing demand, depleting resources, financial crisis and the need for the development of civilisation extorts the search for alternative methods for extracting metals. The recovery of precious metals on ion exchangers belongs to such a method.

Ion exchangers
Ion-exchange polymers are synthetic ion-exchange resins based on cross-linked organic copolymers containing functional groups built into a copolymer matrix due to a chemical modification. Such functional groups are capable of dissociating and exchanging ions for ions originating from the surrounding solution of electrolyte (Fig. 1). They are usually obtained in the form of spherical grains that are insoluble in water and in organic solvents. Ion-exchange resins are classified regarding a nature of the functional groups attached to the polymer matrix and their capability for dissociation, that is, ion-exchange resins exchanging cations are called strong-acid or weak-acid cation exchangers, while ion-exchange resins exchanging anions are called strong-base or weak-base anion exchangers; and regarding the porosity of the polymer matrix, that is, gel ion-exchange polymers, expanded and porous gels. Chelating resins belong to a separate group. They are complexing ions with a semi-polar bond formed as a result of covering atomic orbitals of functional groups or ligands populated with lone electron pairs.

High affinity to gold and platinum metals is characteristic for modified polymers with ion-exchanging and chelating properties whose functional groups introduced into the polymer matrix contain donor atoms of nitrogen, sulphur or phosphorus. Thus, the selection of a polymer matrix and a type of a built-in functional group is very important to obtain the adequate sorption selectivity of metals. Ion exchangers have already played an important role in the history of humanity and their capability for exchanging ions was known in times of Aristotle even though 300 years BC the term of ion exchange was not used, but salt water was demineralised using sand or soil. The first synthetic ion exchanger was developed in 1903 by two German chemists Harm and Rümpler [6] who used it for industrial purification of sugar beet syrup. And in 1905, Gans softened water on synthetic and natural aluminosilicates which he named zeolites [7]. The search for other materials was caused by their limited application in the industry. Long-term plans of Gans were related, among other things, to recover gold from the seas and oceans [6] and to conduct investigations on gold colloids [8].

The history of science is full of accidental and simultaneously landmark discoveries which have led to a breakthrough in many fields and everyday life, e.g. hardening rubber by Goodyear in 1839 [9], the Baum tests developed by Koch in 1890, a discovery that potassium dicyanoaurate inhibits the development of tuberculosis bacteria [10] and development of cellophane by Brandenberg in 1908 [11]. The discovery of two English chemists ? Adams and Holmes, in 1935 was a similar coincidence and at the same time a spectacular achievement. They discovered that crashed phonograph records, made of synthetic, condensed phenol formaldehyde resins exhibited ion-exchange properties [12]. From that moment, the works on synthetic ionexchange resins became more intensive [13]. Since 1939, the American consortium Rohm and Haas Company has been producing ion-exchange polymer resins of AMBERLITE? brand [14].

In the years 1942÷1943, Boyd demonstrated that ion exchange was useful for the sorption of trace amounts of products of uranium cleavage [15]. After declassifying the results from the Manhattan Project, Tompkins (1947) described the process of separating lanthanides on Amberlite IR-1 cation exchanger with methyl sulfonic and carboxylic groups [15]. After the World War II and publishing the Manhattan Project experiment, large scale research works on new ion-exchange resins began. D?Alelio research works in the field of suspension polymerisation of vinyl monomers, such as styrene and divinylbenzene, and particularly in the field of introducing strong-acid sulfonic groups into the crosslinked polystyrene were a breakthrough in the studies on obtaining synthetic ion exchangers [16, 17].

A wide treatment of this subject, the related theory and the application of ion exchange was described for the first time by Helfferich in his book Ion Exchange in 1959 [6], and Kunin presented a practical aspect of applying ion exchangers in Ion Exchange Resins [18]. Many researchers, including Helfferich and Millar, called the 1950s as the ?Gold Age? in research and discoveries of synthetic ion exchangers [19]. In the meantime, ion exchangers from the period of intense research works on their synthesis entered into the period of a wide application in many fields of economy.

Experimental part
This paper is aimed to synthesise ion exchangers based on suspension copolymer of vinylbenzene chloride (VBC) cross-linked with divinylbenzene (DVB) and to determine their relevance to recover precious metals from model solutions in HCl. The obtained
ion-exchange resins were based on expanded copolymer gel (2% by weight). The polymer matrix was modified with amines by substituting chlorine atom in VBC units to obtain A, B and C resins. A scheme for synthesising VBC/DVB copolymer-based resins is illustrated in Figure 2.

Ion exchangers were characterised by specifying the sorption capacity of 0.001 M HCl, ion exchange capacity determined according to the Hecker?s method [20], chlorine content determined according to the Schöniger?s method [21], and nitrogen content determined according to the Kiejldahl method [22]. Characteristics of obtained ion-exchange resins are presented in Table 1.

Sorption was tested in three-component model solutions in 0.1 M HCl containing chloro complexes of gold (III), platinum (IV) and palladium (II). Figures 3 ÷ 5 illustrate the structures of these complexes AuCl4 ?, PtCl6 2?, PdCl4 2?.

Sorption in the range of initial concentrations of metals from 10.44 to 919.0 mg/dm3 was assessed using the statistical method by drawing sorption isothermal line (Figs 5 ÷ 10). The initial concentrations of used solutions S1-S5 are shown in Table 2. Centrifuged ion exchangers were weighed and put to glass containers of 25 cm3 capacity, and then 20 cm3 of an adequate solution was added.

Such prepared samples were centrifuged at an ambient temperature (ca. 20°C) for 48 hours. After reaching the equilibrium state, each sample with ion exchanger was filtrated through a sintered glass funnel to separate ion exchanger grains from the solution. Filtrate was collected in test-tubes. The concentrations of particular metals in the filtrate were determined using Perkin Elmer AANALYST 400 Atomic Absorption Spectrophotometer. The value of S sorption was calculated on the basis of the performed measurements of concentrations of adequate metals in the filtrate. The value of sorption was the amount of metals absorbed on ion exchangers and defined as: S=C0-Ce/ms, where: C0 ? initial concentration of solution; Ce ? equilibrium concentration; ms ? dry matter of ion exchanger. The results from measured concentrations calculated as the values of sorption expressed as mg/g and partition coefficients lgK are presented in Table 3.

Discussion on results

The largest total sorption expressed as mg/g was observed for A ion exchanger containing the group derived from 4-aminopyridine (537.6 mg/g) and the lowest one was observed for C ion exchanger containing the group derived from 2,6-diaminopyridine (415 mg/g) (Tab. 3, Figs 6, 10) which is also demonstrated in the ion exchange capacities (Tab.1). The smallest ion exchange capacity of C ion exchanger (1.07 mmol/g) can be explained by the presence of two groups of primary amines in the groups derived from 2,6-diaminopyridine used to modify the copolymer (Fig. 1). The modification of copolymer with the use of such amine can be accompanied by the process of additional cross linking of the matrix. This theory was confirmed by the lowest sorption of 0.34 g/g demonstrated by the solvent for C ion exchanger, among all tested ion exchangers (Tab.1).

A and C ion exchangers exhibited higher sorption of mg/g Pt (IV) (A ion exchanger: 184.5 mg Pt/g, C ion exchanger: 90.9 mg Pt/g) than of Pd (II) (A ion exchanger: 107.7 mg Pt/g, C ion exchanger: 70.2 mg Pd/g) (Figs 6, 10; Tab. 3) and regarding a molar aspect, sorption of A and C ion exchangers expressed as mmol/g of Pt (IV) and Pd (II) was comparable (Figs 7, 11). For B ion exchangers, the sorption of Pt (IV) and Pd (II) was different (1.0 mmol Pd/g, 0.5 mmol Pt/g) (Fig. 9). Such a phenomenon could also arise from the differences in shapes of metal chloro complexes (Figs 3 ÷ 5). The complexes of AuCl4 -, PdCl4 2- have quadrilateral flat structures which provide their easier migration in the polymer lattice of ion exchanger in comparison to the octahedral complex of PtCl6 2-. This regularity leads to higher sorption of Au (III) and Pd (II) expressed as mmol/g than the sorption of Pt (IV) (Fig. 9). As such a phenomenon was not observed for A and C ion exchangers (Figs 7, 11), this could be explained by the differences in interactions between chloro complexes and the modified polymer matrix, and consequently, the diffusion rates of metal complexes in the polymer lattice of particular ion exchangers [23, 24].

According to Bernardis [25], Gibbs energy of hydration (?Ghyd) is a decisive parameter for the migration rate of metal chloro complexes in the polymer lattice of ion exchanger. This value decides on the size of hydration shell appearing around the resin grain, which can hinder the access of metal complexes to the functional groups of an ion exchanger. Gibbs energy of hydration is strongly associated not only with the size and shape of ion exchanger grains, but also with a type of functional groups introduced onto its surface. Thus, it is possible to state that the correlation between molecules of Pt (IV) chloro complexes and the hydration shell of B ion exchanger has a negative impact on the quantity of moles of sorbed platinum [25] (Fig. 9).

All tested preferential anion exchangers can sorb gold (Tab.3). No adsorption isothermal line plateau of gold was observed on adsoprtion isothermal lines of A, B and C ion exchangers (Figs 6 ÷ 11). The above could be explained with the Hard and Soft Acid Theory and the Pearson Principles [26]. Precious metals are considered as ?soft acids? capable of forming stable complexes with ligands containing ?soft? atoms possessing a lone electron pair. Lone electron pairs, derived from nitrogen present in 4-aminopyridine ligands (A ion exchanger), 2-(aminoethyl)pyridine ligands (B ion exchanger) and 2,6-diaminopyridine ligands (C ion exchanger) can coordinate gold chloro complex. The potential capacity of tested resins used to chelate Au (III) can be a reason for the lack of adsorption isothermal line plateau.

Conclusions
The best ion exchanger for Pt (IV) sorption turned out to be A ion exchanger (modified with 4-aminopyridine) with the sorption capacity of 184.5 mg Pt/g; for Au (III) and Pd (II) sorption ? B ion exchanger (modified with 2-(aminoethyl)pyridine) exhibiting the sorption capacity of 295.3 mg Au/g and 104.6 mg Pd/g.

The highest total sorption of metals was observed for A ion exchanger ?SA mg/g = 537.6, and the lowest one for C ion exchanger ?SC mg/g = 415.4. It suggests that the ion exchange capacity of ion exchanger is adversely affected by the cross linking process occurring during the modification (caused by the presence of two primary amines in the structure of 2,6-diaminopyridine). A high sorption capacity of Au (III) on A, B and C ion exchangers can be caused by complexing gold with lone electron pairs in ligands derived from pyridine.

The lowest sorption of Pt (IV) on B ion exchanger (expressed as mg/g and mmol/g) probably results from the differences in interactions between hydration shells of ion exchanger grain and sorbed metal chloro complexes.

This research work was financed by the Ministry of Science and Higher Education as part of the statutory activities of the Faculty of Chemistry at the Wrocław University of Technology S20096/Z0309.

Literature
1. Mei-Hsiu C.: Understanding world metal prices-returns, volatility and diversification. Res. Pol. 2010, 35, 127-140.
2. Baffes J.: Oil spills on other commodities. Res. Pol. 2007, 32, 126-134.
3. Wang K.M., Lee Y.M.: The yen for gold. Res. Pol. 2011, 36, 39-48.
4. Kuan-Min W., Yuan-Ming L., Thanh-Binh N.T.: Time and place where gold acts as an inflation hedge. Econ. Mod. 2011, 28, 806-819.
5. Morales L., Andreosso-O?Callaghan B.: Comparative analysis on the effects of the Asian and global crises on precious metal market. Research in International Business and Finance 2011, 25, 203-227.
6. Helfferich F.: Ion Exchange. McGraw Hill, New York, 1962.
7. Gans R.: Jahrb. Preuss. Geol. Landesanstalt (Berlin), 1905, 26, 179-221.
8. Gans R.: Ann. Physik. 1915, 47, 27.
9. HYPERLINK ?www.goodyear.com/corporate/history/history_story.html? www.goodyear.com 06-04-2012.
10. Koch R.: Report of address at 10th International Medical Congress, Berlin, August 4, 1890. Deut. Med. Wochenschr. 16.
11. Marshall Cavendish: Inventors and Inventions 2008, 1, 165.
12. Adams B.A., Holmes E.L.: J. Soc. Chem. Ind. (Lond.) 1935, 54, 1.
13. Myers R.I., Eastes J.W., Myers F.J.: Ind. Eng. Chem. 1941, 33/6, 697.
14. Boyd G.E., Schubert J., Adamson A.W.: J. Am. Chem. Soc. 1947, 69, 2818.
15. Tompkins E.R., Khym J.X., Cohn W.E.: J. Am. Chem. Soc. 1947, 69, 2769.
16. Zgłoszenie patentowe nr 2 366 007 (1945), USA.
17. Zgłoszenie patentowe nr 2 593 417 (1952), USA.
18. Kunin R.: Ion exchange resins. 2nd ed. By Robert Kunin. John Wiley & Sons, Inc., New York, 1958.
19. Hendricks D.: Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological, Taylor & Francis, 2010.
20. Hecker H.: Beitrag zur basizitätsprüfung von stark basischen anionenaustauschern. J. Chromatogr. 1974, 102, 135-138.
21. VogelI A.I.: Handbook of Quantitative Inorganic Analysis, Longman, London, 1978.
22. Kjeldahl J.Z.: Anal. Chem. 1883, 22, 366.
23. Guibal E., Von Offenberg Sweeney N., Zikan M.C., Vincent T., Tobin J.M.: Competitive sorption of platinum and palladium on chitosan derivatives. Int. J. Biol. Macromol 2001, 28, 401-408.
24. Garke G., Hartmann R., Papamichael N., Deckwer W-F., Anspach F.B.: Expanded-bed chromatography in primary protein purification. Sep. Sci. Technol. 1999, 34, 2521.
25. Bernardis F.L., Grant R.A., Sherrington D.C.: A review of methods of separation of the platinum-group metals through their chloro-complexes. React. Funct. Polym. 2005, 65, 205.
26. Pearson R.G.: J. Am. Chem. Soc. 1963, 85, 3533-3543.
27. Pilsniak M., Trochimczuk A.: Synthesis and characterization of polymeric resins with aliphatic and aromatic amino ligands and their sorption behavior towards gold from ammonium hydroxide solutions. React. Func. Pol. 2007, 67, 1570-1576.

Piotr CYGANOWSKI ? M.Sc., graduated from the Faculty of Chemistry at the Wrocław University of Technology in 2012. At present, he is doing his postgraduate studies at the Department of Polymer and Carbon Materials at the Wrocław University of Technology. Research interests: polymer chemistry, functional materials for the recovery of precious metals.
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Dorota JERMAKOWICZ-BARTKOWIAK ? Ph.D., (Eng), graduated from the Faculty of Chemistry at the Wrocław University of Technology in 1981. She defended her doctoral thesis at the Wrocław University of Technology in 2000. She received the awards of the Minister of National Education in 1999, of the Minister of National Education and Sport in 2003, of the Rector of the Wrocław University of Technology in 2007 and the Dean of the Faculty of Chemistry at this University in 2001 for her research works in the field of synthesis and assessment of functional polymers in separation processes. Research interests: recovery of precious and expensive metals of gold, platinum, palladium and rhenium from waste materials using the methods of sorption on polymer resins.
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Paweł WILKOWSKI ? M.Sc.,(Eng), graduated from the Faculty of Chemistry at the Wrocław University of Technology. His diploma thesis focused on functionalised copolymers for gold sorption.