**24. Conclusions**

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 [153–155]. 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 SiO2–PHMG–NRS. The other examples are presented in Table 2.

2-Amino-1-cyclopentene-1-dithiocarboxylic

22 Ion Exchange - Studies and Applications

**Table 2.** Organofunctionalized silicas in pre-concentration of noble metal ions.

**Functional group Metal ion Ref.**

Thiosemicarbazide Pd(II) [159]

N-(3-triethoxysilylpropyl)-4,5-dihydroimidazoleuene Pd(II), Pt(II) [162]

Dithizone Ag(I) [160]

acid Ag(I) and Pd(II) [161]

Amidinothiourea Ag(I), Au(III) and Pd(II) [162]

3-(1-thioureido)propyl Ag(I), Au(III) and Pd(II) [163]

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 impor‐ tance 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.
