Ion Exchange Method for Removal and Separation of Noble Metal Ions

Ion exchange has been widely applied in technology of chemical separation of noble met‐ al ions. This is associated with dissemination of methods using various ion exchange res‐ ins which are indispensable in many fields of chemical industry. Due to small amounts of noble elements in nature and constant impoverishment of their natural raw materials, of particular importance are physicochemical methods of their recovery from the second sources e.g. worn out converters of exhausted gases, chemical catalysts, dental alloys, anodic sludges from cooper and nickiel electrorefining as well as waste waters and run‐ ning off waters from refineries containing trace amount of noble metals. It should be stat‐ ed that these waste materials are usually pyroand hydrometallurgically processed. Recovery of noble metals, from such raw materials requires individual approach to each material and application of selective methods for their removal. Moreover, separation of noble metals, particularly platinum metals and gold from geological samples, industrial products, synthetic mixtures along with other elements is a problem of significant impor‐ tance nowadays. In the paper the research on the applicability of different types of ion exchangers for the separation of noble metals will be presented. The effect of the different parameters on their separation will be also discussed. The examples of the removal of no‐ ble metals chlorocomplexes will also be presented in detail.


Introduction General characteristics of ion exchangers
Ion exchangers are high molecular substances, most frequently solid, organic or inorganic, insoluble in water and many other solvents and capable of exchange of their own active ions into those coming from the surrounding electrolyte. From the chemical point of view, they are polyacids, polybases or both polyacids and polybases (polyampholyte, amphoteric ion exchangers). Those which exchange cations are called cation exchangers and those which exchange anions are called anion exchangers. Generally, those exchanging ions are called ion exchangers. Some ion exchangers prepared by modification of various types of substances, particularly natural ones, besides capability of exchanging ions exhibit distinct sorption properties [1][2][3][4][5]. The cation exchangers occurring most frequently possess functional groups such as -SO 3 H, -COOH and -OH, whereas the anion exchangers possess the primary, secondary and tertiary amine ones and quaternary ammonium ones, quaternary phosphate ones and tertiary sulfone ones. Selective (chelating) ion exchangers and those strongly basic and weakly basic of the polymerization type of the functional trimethylammonium (type 1) and dimethylhydroxyethylammonium (type 2) groups are most widely applied in exchange chromatography. Their affinity mostly depends on the structure, size and change of anion exchanger. Type of functional groups in ion exchangers is decisive about the character of exchange reaction and its applicability.
Besides the general division of ion exchange materials due to the material (organic and inorganic) the skeleton is built, there are many others whose classification is based on the methods of preparation, type of functional groups and skeleton structure. There is still another basis of ion exchangers division conditioned by historical development of this area, i.e. according to their origin -natural, semi-synthetic and synthetic [1][2][3][4][5].

Application of ion exchangers of various types in recovery of platinum metal ions from secondary sources
Small amounts of noble elements in nature and constant impoverishment of their natural resources result in significant importance of physicochemical methods used for platinum metal ions recovery from secondary resources e.g. worn car exhaust gas convertor, chemical catalysts, stomatology alloys as well as waste waters and waters of refinery origin. Noble metals recovery from such raw materials requires individual approach and application of selective methods of their removal. Moreover, worse quality of these raw materials makes removal of pure noble metals more difficult. Determination of noble metals, particularly platinum ones in the above-mentioned materials, geological samples and synthetic mixtures together with other elements is of significant importance nowadays. Liquid-solid phase extraction (SPE) has some advantages compared to liquid-liquid extraction. Among others, it is faster, cheaper, uses small amounts of reagents and above all its automatization is easier. Moreover, its simple performance and high enrichment coefficients decide about its common laboratory application. SPE uses solid sorbents which should be characterized by not only high capacity towards metal ions under determination but also large selectivity and suitable sorption and desorption kinetics.
Synthetic ion exchange resins are widely applied in platinum metal ions enrichments. Among them of particular interest are cation exchangers, chelating ion exchangers and anion exchang-ers of different basicity of functional groups. Ion exchange selectivity depends on the kind and number of functional groups of ion exchanger as well as cross-linking and composition of external electrolyte solution composition from which concentration proceeds. Of these types of ion exchangers, the most effective are monofunctional ones, which ensure the same strength of bonding ions with the ion exchanger surface due to the presence of one type of groups which does not make equilibrium establishment difficult. Complexity of platinum carrying samples for enrichment makes it necessary to separate a component under determination due to large interference of components present in the analyte. In the chloride systems, ion exchange enables platinum metal ions separation not only from their mixtures but also from other metals. In the hydrogen chloride acid solutions, most platinum metal ions are present in the form of anion chloride complexes; therefore they are retained by strongly basic anion exchangers. Anion exchangers enable selective removal of platinum metal ions from solutions of other metals; however, there appear some problems with their elution due to strong sorption of platinum metal ions. It can often occur that recovery is not quantitative or rendered difficult. Such situation arises because of stable ion pairs formation between anionic complexes and the quaternary ammonium groups of the anion exchanger. Reduction of noble metal ions can proceed in the ion exchanger phase which also affects incomplete recovery. Some difficulties with determination can result from different behaviour of new and 'old' solutions which is connected with the hydrolysis process in the solution. This reflects mainly to rhodium or iridium chloride complexes because they have the greatest tendency towards hydrolysis. Using cation exchangers, rare earth elements, transient metals as well as alkali metals and alkaline earth family form weakly anionic or stable cationic complexes therefore they are retained on the cation exchange deposit whereas platinum metals go through the column not being retained by the cation exchangers making separation of the above-mentioned metal ions possible.

Application of cation exchangers in concentration and removal process of platinum metals
There are numerous reports in the literature about cation exchangers application in concentration and separation of trace amounts of platinum metal ions. Ion exchange is widely applied also for control of bound and free platinum contents in the serum added in the cis-platinum form. After ultrafiltration, ethylenediamine was added to form complexes which are sorbed on cation exchange disks. Platinum ions were desorbed from the disks by means of 5 M HCl and determined using the AAS method. The detection limit was 35 μg/dm 3 [6].
Besides the above-described procedures, there are separation methods using cation exchangers. As commonly known platinum metals tend to form anion complexes in the chloride systems, therefore partition coefficient values should not be high [7][8][9]. Much higher values of partition coefficients of platinum metal ions can be obtained by the addition of thiourea which results in cation complexes formation [9].
The polystyrenesulfone cation exchanger Dowex 50Wx8 in the hydrogen form was used for the determination of metals from the platinum group in ores and concentrates. Before ion exchange stage, noble metals in metallic copper were collected. The obtained alloy was digested in aqua regia. The pH of the solution was made 1 using hydrochloric acid. Ions of metals such as Cu(II), Ni(II) and Fe(III) were sorbed on the cation exchangers, whereas noble metals were not retained. The deposit was washed in HCl solution of pH = 1. The separated noble metals were determined gravimetrically and spectrophotometrically [10][11][12][13][14]. A similar technique was applied for the determination of noble metals and for the collection of Cu-Ni-Fe and ferronickel alloys [15][16][17][18][19][20][21][22][23][24]. Also the cation exchanger Dowex 50Wx8 was used for noble metal ions removal. Before the ion exchange stage, collection was made using fused tin. The obtained alloy was digested in the HCl-H 2 O 2 solution. Tin(IV) was removed by evaporation from the HCl-HBr solution, and the obtained solution was evaporated dry several times in the presence of 12 M HCl [25]. The removal of noble metals from ores, the cation exchange followed by anion exchange method was applied. Cu(II), Ni(II) and Fe(III) ions are adsorbed at pH = 1.5 on the cation exchanger Dowex 50Wx8, whereas the noble metal ions pass on the column filled with the strongly basic anion exchanger in the chloride form Amberlite IRA-400. Before sorption on the anion exchange column, the solution is evaporated dry in the presence of sodium chloride, then it is dissolved in 12 M HCl and diluted to weakly acidic reaction. Under such conditions, rhodium(III) ions pass through the column, and the ions of other noble metals are retained on the anion exchanger bed [26]. A similar method was applied for the determination of iridium in the flotation concentrate [27]. The application of cation exchange for the determination of platinum metals on the meteorites is quite interesting. In this method, twostage adsorption on the cation exchanger was used. Non-noble metal ions were adsorbed at pH 1.5 on the first column. Then they were desorbed by means of 3 M HCl. The obtained eluant was evaporated dry and the residue was dissolved in diluted HCl and the pH value was brought to 1.5. The other stage of non-noble metal ions adsorption was conducted in the same way as the first one. Application of the second stage allows avoiding errors connected with co-adsorption of noble metal ions [28,29]. For determination of platinum and palladium in the copper and nickel stone, there were used two ion exchangers: the cation exchanger Dowex 50 in the hydrogen form and the anion exchanger Amberlite IRA-400 in the chloride form. Before the ion exchange process, the sample was melted with SnO 2 to collect noble metals. Tin(IV) was evaporated from the mixture of HCl and HBr acids. After bringing the solution to pH 1.5, Cu(II), Ni(II) and Fe(III) cation were adsorbed on the cation exchanger bed. The obtained eluant containing Pt(IV) and Pd(II) was evaporated from NaCl and dissolved in HCl, next it was passed through the anion exchanger bed. At first, there were eluted Pd(II) ions by means of 12 M HCl and then Pt(IV) ions using 2.4 M HClO 4 . Both elements were determined using the spectrophotometric method [30]. The residue in the copper-nickel stone was determined using the cation exchanger Bio-Rad AG50W-x8 and the chromatographic column Porasil C impregnated by means of TBP (tri-n-butyl phosphate). The sample was melted with Na 2 O 2 , then digested in HCl and the acid concentration was brought to 0.1 M. Non-noble metal ions were separated on the cation exchanger and platinum metals were sorbed on the chromatographic column. Pt(IV) and Pd(II) ions were eluted by means of TBP in toluene but Rh(III) and Ir(IV) ions using water. Noble metals were determined gravimetrically and by means of AAS [31,32]. Platinum alloys were determined in a two-stage separation process of noble metal ions. The sample was digested in aqua regia, next it was evaporated dry and the residue was diluted with HCl up to concentration about 0.1 M. The weakly acidic cation exchanger Amberlite IRC-50 of carboxylic groups in the sodium form was used in the first column. Palladium(II) ions were sorbed (probably precipitated in the hydrated oxide form [33]) on this ion exchanger, whereas Rh(III) and Pt(IV) passed to the eluant which was next put through the column with the strongly basic anion exchanger Dowex 2 in the chloride form. The adsorbed Rh(III) and Pt(IV) ions were eluted with 2 M HCl and 7 M HCl, respectively. The ion exchange technique was also applied for the determination of trace amounts of noble metals in common metals of high purity such as Fe, Ni, Cu, Mn and Al. The sample can be digested or melted with alkalis depending on its kind. The two-column cation-anion exchanger system was used for separation of noble metal ions. On the cation exchanger Dowex 50x8, there were adsorbed ions of metals such as Fe(III), Ni(II), Mn(II), Cu(II) and Al(III) from the 90% v/v ethanol + 10% v/v 1 M HCl solution. Then by evaporation, the medium changed from chloride to nitrate(V) one. In the other column on the anion exchanger Dowex 1x 8 in the nitrate form, there were sorbed noble metal ions from the aqueous solution of pH 6. Noble metals were analyzed in the resin phase by means of the radioisotope technique [34]. The similar method was used for the determination of noble metals in atmospheric dusts melting them with Na 2 O 2 [35]. The cation exchanger Dowex 50x8 was used for the separation of copper(II) from the 0.03 M hydrochloric acid solution from the noble metal ions such as Pt(IV), Pd(II), Au(III) and Rh(III) [36,37]. Ion exchange combined with extraction was applied for the determination of noble metals present in uranium alloys which can be uncoupled. Uranium can be separated from rhodium by extraction with 30% solution of TBP in CCl 4 . Then after complete removal of chlorides, fluorides and nitrates by evaporation with chloric acid, rhodium(III) cations were sorbed by cation exchanger Dowex 50Wx8 from 0.3-0.9 % HClO 4 solution. Rhodium(III) was eluted form the cation exchanger by washing the column with 6 M hydrochloric acid solution and then determined spectrophotometrically using the method with SnCl 2 [38].
Separation of noble metals can be conducted on the cation exchangers from the thiourea systems. Separation of microquantities of various pairs of noble metal ions was made using the polystyrene sulfone cation exchanger Bio-Rad AG 50Wx4 in the hydrogen form. Pd(II) and Au(I), Pd(II) and Pt(II) ions as well as the Rh(III), Au(I), Pt(II) and Ag(I) mixture were separated using the acetate-thiourea solutions in the hydrochloric or hydrobromic acid medium [39]. Platinum(II) and palladium(II) ions were separated from aluminium ions using also the cation exchanger Bio Rad AG50Wx4 in the hydrogen form. Aluminium ions as well as Fe, Zn, Pb, U, Ni, Co and Sr ones do not form cationic complexes under experimental conditions, therefore only noble metals are sorbed from 0.1 M thiourea solution in 1.5 M hydrochloric acid solution. 2% Br 2 and 1.5 M HCl solution was used for elution of Pt(II) and Pd(II) ions. 0.87 M HBr-0.01 M thiourea solution in 90% acetone proved to be the best eluant towards Pt(II) ions. The presence of Cu(II) and Hg(II) ions is not recommended because of possible co-adsorption with noble metals [40]. It is relatively difficult to separate rhodium(III) ions from platinum(IV) ones in the chloride medium, therefore in some cases it is necessary to change the medium into the nitrate one. Before the separation of platinum(IV) and rhodium(III) ions on the polystyrenesulfone cation exchanger Varion KS in the hydrogen form, the chloride complexes were in contact with sodium hydroxide at pH 13 for four hours. Then the obtained solution was acidified with 4 M HNO 3 to pH 2. Under such conditions, rhodium(III) ions occur in the cation form and platinum(IV) ions in the anion form. Platinum(IV) ions are not retained by the cation exchanger. Rhodium(III) ions can be eluted with 1 M hydrochloric acid from the cation exchanger [41].

Application of chelating ion exchangers for concentration and removal of platinum metal ions
Chelating ion exchangers also called complexing ion exchangers are formed by building organic reagents containing organic groups into the ion exchange resin skeleton. Owing to that they possess active chemical groups capable of selective/specific interactions with metal ions in the solution forming chelating complexes when a metal ion can bind with two or a larger number of donor atoms of their functional groups. These ion exchangers are characterized by high selectivity and their sorption capacities depend, among others, on the kind of functional groups, their reciprocal position and spatial configuration (steric effects) and also on physicochemical properties of the polymer matrix [42,43].
On the huge number of chelating ion exchangers, on a large laboratory and industrial scale, there are produced ion exchangers of functional dithizone, thiourea, isothiourea, aminophosphonic, phosphonic, thiol, amidooxime, aminoacetate, dithiocarbamate, iminodiacetate, thiosemicarbamate groups as well as chelating ion exchangers containing triisobutylphosphine sulfides .
Grote and Kettrup [45][46][47][48], by conversion of the ion exchanger of functional dehydrodithizone groups, prepared a chelating resin containing dithizone groups. It was used on both sorption and separation of 27 noble and non-noble ions from acids (HCl, HNO 3 ).They showed very high values of partition coefficients of noble metal ions of the order 10 4 -10 6 (Pd(II)-7.7×10 5 ; Pt(IV)-3. Satisfactory results were obtained using the chelating ion exchanger with dithizone functional group in concentration and recovery of Au(III), Pt(IV) and Pd(II) ions originating from the extraction of sulfide ores, stones and enriched ores. Elution of the above-mentioned ions was conducted by means of 2 M chloric(VII) acid and 5% thiourea solution [44]. There was also made a thorough analysis of desorption of single noble metal ions and their mixtures from the resin with the dithizone functional groups using the following eluents: HCl, HClO 4 , NH 4 NO 3 , NaSCN, (NH 2 ) 2 CS. Palladium(II) and platinum(IV) ions retained on this resin can be qualitatively desorbed using the thiourea solution [46]. Similar investigations using polyvinylpyridine resin of functional dithizone groups in Pd(II) and Pt(IV) ions concentration in the presence of Au(III), Ni(II) i Hg(II) ions were carried out by Shah and Devi [49]. The values of maximal ion exchange capacities towards palladium and platinum ions were 100 and 250 mg/g of resin, respectively. Separation of the above-mentioned ions from nickel, gold and mercury ions (Pd(II)-Ni(II); Pt(IV)-Au(III); Pt(IV)-Ni(II); Pd(II)-Pt(IV)-Ni(II); Pt(IV)-Au(III)-Hg(II)) was conducted using various eluants 0.1 M HCl + 1% (NH 2 ) 2 CS (elution of Pd(II)), 0.1 M HCl + 5 % (NH 2 ) 2 CS (elution of Pt(IV)), 0.2 M CH 3 COOH (elution of Ni(II)), 5 M HCl + 1 M HNO 3 (elution of Au(III)) and 0.5 M HNO 3 + 2 % NH 4 NO 3 (elution of Hg(II)).
Modification of the commercially available polyacrylate matrix Diaion HP-2MG with dithizone resulted in the preparation of the selective sorbent towards Pd(II) and Pt(IV) ions. Chwastowska et al. [51] used the above-mentioned sorbent for removal of Pd(II) and Pt(IV) ions from the environmental samples, among others, from road dusts, soil and grass collected from fast traffic routes. After proper preparation, among others, drying (673 K, 1 h) and digestion in aqua regia, the geological samples were analyzed using the GF AAS technique. The detection limit (LOD) for the determined metal ions was 1 ng/g for Pt(IV) and 0.2 ng/g for Pd(II). Metal ions desorption was run in two ways: using thiourea solution (possibility of determination of both elements in the eluant by the GFAAS technique) and concentrated HNO 3 solution (possibility of directed determination of only Pd(II) ions in the eluant). The obtained ion exchange capacity towards both metal ions was about 0.16 mmol/g of resin.

Ion exchangers of functional (amino)pyridine groups
Resins of pyridine [81] and α-aminopyridine groups on the polyphenylethylene support [82] were

Ion exchangers of functional amine and guanidine groups
Resins of functional amine and guanidine functional groups were successfully used in sorption of Pd(II), Pt(IV) and Au(III) ions from chlorides solutions (initial concentration of each metal  Ge et al. [86] proved high selectivity of the P-NHZ resin towards noble metal ions and possibility of its exploitation for separation and concentration of trace amounts of Pd(II) and Pt(IV) contained in road dust samples. Sorption of the above-mentioned ions on the P-NHZ resin can be conducted also from the HNO 3 , HF and H 3 BO 3 solutions of the concentration 0.08-1.2 M.

Polyorgs ion exchangers
Polyorgs type chelating ion exchangers [58,[99][100][101][102][103] are used for separation, concentration and removal of palladium(II) ions and other noble metals from deposits, rocks, ores, minerals and industrial waste waters. The Polyorgs sorbents were prepared by introducing e.g. imidazole, pirazole, mercaptobenzothiazole, amidooxime groups to the macroporous copolymers (polystyrene, polyvinyl, polyacrylonitrile) and other matrices. Sorbents of this type are characterized by high chemicals stability in strong acid and alkaline solutions as well as high thermal resistance and can be applied in the whole pH range. The sorbents of Polyorgs type (11-n, 15-n, 17-n and 33-n) were also used for filling fibres e.g. polyacrylonitrile (PAN), cellulose, polyvinyl (PVA) ones. In the literature, such sorbents are called filled fibrous sorbents (FFS) [99,102,103]. Main advantages of FFS are good kinetics of sorption and ease in their separation from solution which make them more attractive than sorbents in the form of powder or grains.

Ion exchangers of functional iminodiacetate groups
Recovery of palladium ions from chloride and chloride-nitrate(V) solutions using the ion exchanger Amberlite IRC-718 of functional iminodiacetate groups and polystyrene skeleton was conducted by Hubicki et al. [104].

Chelating fibres
It is worth presenting also the studies of using chelating fibres for the removal of platinum elements, for example, those Gong [115] and Li et al. [116] on application of fibres of functional Ion Exchange -Studies and Applications imidazole groups in noble metal ions sorption. Bilba et al. [117] used chelating polyacrylamidoxime fibres in concentration and recovery of Pd(II) ions from chloride solutions. The attention should be also paid to the investigation by Gong and Wang [118,119] as well as Chang et al. [120] on concentration of trace amounts of Au(III), Pd(II), Pt(IV) and Ir(IV) by means of chelating polyacrylacylaminothiourea fibres. Poly (acrylamidrazonehydrazide) [121] and poly (acryl-p toluenesulfonamideamidine-p-toluenesulfonylamide) [122] fibres were applied in quantitative concentration and separation of Au(III) and Pd(II) as well as Ru(III), Rh(III), Au(III) and Pd(IV) ions in the column system.

Amphoteric ion exchangers and anion exchangers
Of a large group of ion exchangers, anion exchangers of different basicity (strong, average and weak basic) of functional groups are applied in ion exchange chromatography of noble metal ions. Strongly basic anion exchangers possessing well-dissociated functional groups capable of anion exchange of even weak acids, e.g. quaternary ammonium groups, are widely applied in the whole pH range. This group includes types 1 and 2 strongly basic anion exchangers of functional trimethylammonium groups (type 1) and dimethylhydroxyethylammonium groups (type 2). Weakly basic anion exchangers possess poorly dissociated functional groups i.e. primary-, secondary-and tertiary amine groups. There is also a group of amphoteric ion exchangers which, depending on solution pH, are able to exchange anions or cations. They are polyacids and polybases so-called polyampholites, e.g. of COOand -N + (CH 3 ) 3 groups (snake in cage polymers). The amphoteric vinylpiridine ion exchangers VP-14K, ANKF-5 and the anion exchanger AN-251M were used for recovery of Pd(II) ions from spent car exhaust gas convertors subjected to extraction with the NaCl (2-2.3 M) solution acidified with hydrochloric acid (0.5-2 M) at 353 K, the extraction time was 4h. These ion exchangers were characterized by high affinity towards palladium(II) ions and their recovery was 98-99%. The sorption capacity of the anion exchanger AN-251M towards Pd(II) ions and the aminophosphonic ion exchanger ANKF-5 was comparable (2.4-2.5 mmol/g) and much larger than that of the ion exchanger VP-14K (1.4 mmol/g) so the ion exchangers AN-251M and ANKF-5 can be recommended for this type of application.
The strongly basic gel anion exchanger Dowex 1x10 (Clform, grain size 100-200 mesh) was successfully applied for removal of Pd(II) and Pt(IV) ions from the dust collected in Germany from street and fast traffic roads (Saarbrücken, motorway A-1, A-61, road B-262). Quantitative desorption of sorbed metal ions took place using the 0.1 M thiourea solution in 0.1 M HCl at the increased temperature 333 K enabling reduction of the eluent volume by half. The matrix ions, i.e. Cd, Cu and Fe, were not retained on the anion exchanger but Ni, Pb and Zn sorbed at 8-15 %. Elimination of interferences during noble metals determination was achieved by using the reagents masking the matrix ions even before the sorption process, e.g. xylene orange (C 31 H 32 N 2 Na 4 O 13 S) [126]. Application of ion exchange technique for the determination of platinum(II) ions in biological tissues gives interesting results. The tissues with the cisdichloro-diamineplatinum(II) were irradiated with neutron in the reactor. The sample irradiation was mineralized by means of HNO 3 -H 2 SO 4 -H 2 O 2 mixture. Then platinum ions were sorbed on the anion exchanger Dowex 1x8 in the chloride form with 6 M hydrochloric acid solution. Platinum was determined using the radiometric method [127]. Platinum and rhodium contained in ores were determined after separation on the anion exchanger Dowex 1x8 in the chloride form. The ore was digested in aqua regia. Next the solution was passed through two columns. In the first one, platinum ions were sorbed from 9 M HCl solution. In the other one, rhodium was sorbed in the form of a complex with zinc(II) chloride from 0.5 M HCl solution. 104 Rh was determined directly in the ion exchange phase using the radiometric method [128].
For removal and determination of platinum from geological materials, a technique using the anion exchanger Rexyn 201 was proposed. Sorption was performed from 0.5 M of hydrochloric acid solution containing Ir(IV), Pt(IV), Pd(II) and Au(III) ions. Elution was carried out by means of 0.1 M solution of thiourea in 0.1 M HCl. Ir(III) was eluted using 6 M HCl. Platinum metals and gold were determined radiometrically [129]. The same methods were applied for the determination of platinum in carbons [130]. Somewhat modified technique was used for the determination of platinum metals in meteorites. Modification consisted in the change of the anion exchanger Rexyn 201 on Deacidite FF in the chloride form [131].
Bio-Rad AG1x8 (100-200 mesh) was characterized by high selectivity for Pd(II), Pt(IV) and Au(III) ions (the partition coefficient values were 10 6 , 10 4 , 10 3 for Au(III), Pt(IV) and Pd(II) ions, respectively), and therefore it could be applied for removal of noble metal ions from the environmental and geological samples, among others, from rocks, ores as well as dust and road dust [132].
Among weakly basic anion exchangers of special interest is the macroporous polystyrenedivinylbenzene anion exchanger of functional dimethylamine groups Amberlite-93 used for recovery of Pd(II), Pt(II) and Rh(III) ions from spent car exhaust gas convertors. Rhodium(III) was desorbed from the anion exchanger as the first using 6 M hydrochloric acid solution, then palladium(II) was desorbed using 1% ammonia solution at room temperature. Platinum(II) was washed out with the ammonia solution of the concentration 5% (at increased temperature). Separation of palladium from platinum from the eluant solution can be achieved reducing to the metallic form or precipitating (NH 4 ) 2 PdCl 4 and (NH 4 ) 2 PtCl 6 using hydrochloric acid. The presented method of selective removal of platinum metals using Amberlite IRA-93 can be regarded as an effective technique for separation of these ions on a laboratory and commercial scale [139].
The weakly basic Amberlite IRA 67 is applied for selective removal of microquantities of platinum(IV) ions from the acid solution containing CuCl 2 , FeCl 3 , NiCl 2 , AlCl 3 and ZnCl 2 . In chloride solutions, the above components can partly form anions, which reduces the sorption capacity of weakly basic anion exchangers. The effect of the above-mentioned macrocomponents on decrease of sorption capacity towards platinum(IV) ions can be presented in the series: CuCl 2 ≈ FeCl 3 ≈ NiCl 2 < AlCl 3 < ZnCl 2 [140,141]. A similar series can be determined for the anion exchanger Duolite S 37, which contains secondary and tertiary functional groups added to the phenol-formaldehyde skeleton [142].

Low-cost sorbents
In the literature, there are many examples of the alternative sorbents for noble metals removal produced from renewable and low-cost resources [143]. Among them, those based on bacteria, fungi and algae as well as agriculture and seafood wastes (coffee, green tea, tea, yuzu, aloe, wheat and barley straw, maize crop, coconut shell, rise husk, etc.) have been investigated. One of the low-cost sorbents is chitosan (CS) [144][145][146][147][148][149][150][151][152][153][154][155]. It is a kind of abundant natural polysaccharide. Chitosan is produced by the alkaline deacetylation of chitin, the most abundant biopolymer in nature after cellulose. It is extracted from shrimp and crab shells. Due to large availability of functional groups such as amino and hydroxyl ones, CS has been proved to be very efficient for the recovery of several toxic metal ions such as Cu(II), Cd(II) and Pb(II) and strategic metal ions such as Pt(IV) and Pd(II) [144]. It should be mentioned that sorption properties of CS are due to its composition and presence of active and functional groups. CS is characterized by its high percentage of nitrogen present in the form of amine groups, which are responsible for metal ion binding through chelation mechanisms. Due to the fact that it is protonated in acidic solutions, it is also capable of sorbing metal ions through anion exchange mechanisms. It should be mentioned that chitosan protonation in acidic solution causes the polymer to dissolve (except in sulphuric(VI) acid solutions). For the sorption of some metal ions (for example noble metals), sulphuric(VI) acid cannot be used for pH control due to reduction of sorption efficiency [145].
In the case of chitosan derivatives obtained by glutaraldehyde cross-linking (GA), poly(ethyleneimine) grafting through glutaraldehyde linkage (PEI) or thiourea grafting (T), noble metal ions can be sorbed. The reasons for grafting new functional groups are (i) to increase the density of sorption sites, (ii) to change the pH range for metal ions sorption and (iii) to change the sorption sites and/or the uptake mechanism in order to increase sorption selectivity for the noble metals [146]. Such derivatives were used for palladium and platinum removal [147]. It was found that the maximum adsorption capacity occurred at pH 2.0 for both Pt(IV) and Pd(II) species. The material selectively adsorbs Pt(IV) and Pd(II) from binary mixtures with Cu(II), Pb(II), Cd(II), Zn(II), Ca(II) and Mg(II). The isotherm adsorption equilibrium was well described by the Langmuir isotherms with the maximum adsorption capacity of 129.9 mg/g for Pt(IV) and 112.4 mg/g for Pd(II), which was relatively high compared with the glycine chitosan derivative (122 mg/g for Pt(IV) and 120 mg/g for Pd(II)). The results show that 0. In the paper [143], rubeanic acid was grafted on chitosan through the reaction with glutaraldehyde as the linker to obtain the sorbent for Au(III) with the thiol functional groups. It was found that the maximum sorption capacity was high and equal to 600 Au(III) mg/g. The speciation of gold in the chloride and hydroxide chloride systems appears to be a predominant parameter influencing the removal process. The optimum pH range was between 2 and 3 for glutaraldehyde cross-linked chitosan. However, the sorption capacity strongly decreases with the increasing pH. It was found that in the case of the grafting of sulfur compounds on chitosan derivatives, the partial change in the sorption mechanism occurs. In this case, metal ion chelation with sulfur compounds is weakly sensitive to the pH change [148]. Sorption capacity in the HCl system reaches 2 mmol/g (180 mg/g) and is slightly lower than for the chitosan derivatives obtained by grafting of pyridyl groups (6 mmol/g). Increasing chloride concentration involves a significant decrease in sorption capacity.
The removal of Au(III), Pt(IV) and Pd(II) onto the glycine modified cross-linked chitosan resin was investigated in the paper by Ramesh et al. [146]. The results show that the optimum pH appeared to be 1.0-4.0, and the maximum percentage removal was obtained at pH 2.0 for Au(III), Pt(IV) and Pd(II). The pH ZPC was found to be 5.1. At pH < pH PZC , the surface of modified chitosan resin is positively charged, whereas at a pH > pH PZC , the surface of modified chitosan resin is negatively charged. Due to the positive surface charge of sorbent at pH lower than pH PZC , it attracts the chlorocomplexes of platinum, palladium and gold, resulting in the greater amounts of adsorption at low pH. The authors proposed the following mechanism of the sorption process: The results also demonstrated that the amount of adsorption was decreased with the increasing chloride ion concentration. This is because of strong interaction between the chloride ions and precious metal ions to form chlorocomplexes. The 0.7 M thiourea-2 M HCl solution was the most effective for the desorption of Au(III), Pt(IV) and Pd(II).
In the case of chitosan derivatives, noble metal ions are sorbed according to several kinetic models based on pure sorption, pure reduction and dual sorption-reduction mechanisms [149,150]. Moreover, the optimum acid pH for noble metal ions sorption depends on the metal. For platinum and palladium, it was equal to 2. For CS cross-linked by glutaraldehyde (CS-GA) for hydrochloric acid solutions of palladium at pH 2, sorption reached the same level as achieved at pH 1 (capacity strongly decreased) [151]. However, in the sulphuric(VI) acid solutions, the sorption capacity remains almost unchanged. It is well known that pH has a critical effect on the speciation of the metal in solution because the distribution of metal species depends on pH. Other parameters which affect the sorption efficiency are connected with the nature of the sorbent (ionic charge), chemistry of the metal ion: ionic charge, ability to be hydrolysed as well as metal concentration and the composition of the solution and the form of polynuclear species [151]. Sorption kinetics is controlled by particle size, cross-linking ratio and palladium concentration. In hydrochloric acid solutions, equilibrium is achieved at 24 h contact. For chitosan-cellulose fibres, it was found that incorporation of cellulose fibres improves the binding efficiency of chitosan towards Ag(I). The sorption capacity was close to 220 mg/g. This is much higher than for the pure chitosan (140 mg/g) [152]. The cellulose fibres contribute to dispersion of the chitosan chains that are more accessible and available for silver. It is also possible to modify the chitosan structure by introducing cross-linking structure, blending chitosan with synthetic polymers such as poly(vinyl alcohol) (PVA) -a non-toxic, watersoluble synthetic polymer with good physical and chemical properties and film-forming ability. It is also possible to apply the sol-gel process to develop organic-inorganic hybrid materials [153][154][155]. For this aim, clays and silicas are frequently used. Clays are composed of silicate layers which form three-dimensional structures after hydrated in water. They have negative charge and can interact with chitosan. Also silicas are characterized by several advantages which are among others surface stability in the acidic medium and highly developed surface, acceptable kinetics, thermal stability, resistance to microbial attack and low cost should be mentioned [156]. Chemically modified silicas (CMSs) with the functional groups covalently bound to the surface such as polyamines, particularly, linear polyhexamethylene guanidine (PHMG) with convenient amine group configurations and nitroso-R salt (NRS) were used in palladium sorption [157,158]. Complex of palladium(II) with the ratio Pd:NRS = 1:2 formed the SiO 2 -PHMG-NRS. The other examples are presented in Table 2.

Conclusions
Ion exchange has been widely applied in the technology of chemical separation of noble metal ions. This is associated with the dissemination of methods using various ion exchange resins which are indispensable in many fields of chemical industry. Due to small amounts of noble metals in nature and constant impoverishment of their natural sources, of particular importance are physicochemical methods of their recovery from the secondary sources as well as waste waters.
Recovery of noble metals, from such raw materials, requires individual approach to each material and application of selective methods for their removal. Moreover, separation of noble metals, particularly platinum metals and gold from geological samples, industrial products and synthetic mixtures along with other elements, is a problem of significant importance nowadays.