br Conclusion br Introduction Platinum group metals PGMs are

Conclusion
Introduction Platinum group metals (PGMs) are applied in several different industries, hence the interest in developing methods for their separation (Kolekar and Anuse, 2002). Rhodium, for example, has been in demand in recent years since it is indispensable in automotive catalytic converters, chemical production plants, electrical protease inhibitor cocktail generation and in the glass making industry (Sivrikaya et al., 2011; Shaikh et al., 2011; Kononova et al., 2011; Kramer et al., 2004; Akkaya et al., 2013; Muşina et al., 2014). Iridium is most commonly used with platinum to impart additional corrosion resistance, for example in Pt–Ir crucibles. It is also applicable in high temperature applications such as in spark plugs of jet engines due to its very high melting point. PGMs occur as complex species in solution and this has contributed to the difficulty in developing methods for their separation (Fayemi et al., 2015). The separation of rhodium(III) and iridium(III) is known to be difficult due to their similar chemical properties, for example, the behaviour of the anionic chlorido complexes of Rh(III) and Ir(III) is very similar. However, the kinetics of ligand exchange of Ir(III) compounds are slower than their Rh(III) analogues (Du Preez et al., 2004). The separation of the two metal ions is based on the differences in anionic Ir(IV) and Rh(III) chlorido complexes. [IrCl6]2− with iridium in oxidation state +4 can be achieved with ease relative to its rhodium analogue and it is readily phase transferable with a suitable cation to a non-polar medium than [RhCl5(H2O)]2− (the rhodium(III) chlorido complex that is formed under the same conditions for the formation of [IrCl6]2−). Thus, it is essential that the iridium is fully oxidized and chloridated before one can execute the separation step (Kolekar and Anuse, 2002; Du Preez et al., 2004). Various techniques have been studied for rhodium(III) and iridium(IV) separation, recovery and recycling from the ore samples (primary resources) or waste materials (secondary resources), including ion exchange, solvent extraction and precipitation (Xiong et al., 2011; Muslu and Gulfen, 2011). Hydrometallurgical processes have been applied for metal recovery, which involve a leaching stage to dissolve the metal ions followed by further stages to separate the target metal ion from other impurities. Solvent extraction is an important technology applied to hydrometallurgical processes of metal separation. Inspite of the various successes in the industrial processes, solvent extraction retains inherent limitations such as insufficient enrichment efficiency, use of toxic and/or flammable organic solvents, and difficulty in phase separation. Adsorption processes using chelating resins presented an attractive alternative for the recovery of low concentrations of PGMs due to high enrichment efficiency, ease in phase separation and they do not require organic solvents. However, the slow kinetics of complex formation with PGMs limits the use of solid phase chelating resins for their separation despite the attempts to improve uptake through heating or microwave irradiation (Yousif et al., 2012). Reports describing the use of solid adsorbents through exploitation of anion–cation interactions are also becoming prominent in the literature (Nabi et al., 2011). For these reasons, ion exchange is preferred over solvent extraction and other separation methods for PGMs. The selective separation of these metal ions with the ion exchange technique is based on the strong base anion exchange capacity derived from the quaternary ammonium site(s) (Hubicki et al., 2008). The conclusion was suggested that doubly charged quaternary ammonium cation is likely to lead a higher loading of iridium than that of single charge cation. In recent years, increasing attention has been paid to silica gel as support material due to its excellent thermal and mechanical stability, large surface area, and with a possibility for functionalization to form materials which do not swell or shrink (Qu et al., 2008; Liu et al., 2010). Moreover, the silica-based materials can be regenerated for many cycles of usage (Radi et al., 2014). Generally, it is difficult for organic functional groups to bond to silica gel directly because of the relative inertness of the original surface of silica gel. However, bonding of organic functional groups to the silica gel surface can be achieved after surface activation and modification. As an amorphous inorganic polymer, silica gel is composed of internal siloxane groups (Si–O–Si) with a large number of silanol groups (Si–OH) distributed on the surface. The most common method for modification of the silica gel surface involves the reaction of the surface hydroxyl groups with organosilanes which act as precursors for further immobilization of functional organic groups. It has been found that the behaviour of these solids, when used as adsorbents, are mainly dependent on the presence of active donor atoms, such as O, S and N, of the incorporated organic moieties (Yin et al., 2011). The functionalized silica-based materials have received a great deal of attention recently because of their excellent performance in the field of chromatography, adsorption, ion exchange, solid phase extraction, metal ion preconcentration, catalysis and other applications.