Sulfur Ligands

In the quest for high affinity and high selectivity, many sulfur-containing ligands have been investigated, including thiol, thioether and thiocarbonyl groups, as well as sulfur-nitrogen heterocycles. Thiol resins are available from several manufacturers. Although falling outside the above definition of chelat-ing resins, they can be regarded as the prototype of resins containing thiocarbonyl, thioether or dithiocar-boxylate groups, or multiple ligands such as thiol/amino combinations. Thiol resins have been used for adsorption of mercury(II) and are effective in the presence of competing ligands such as Cl~. However, regeneration is difficult, requiring concentrated acid, and the thiol group is prone to oxidation.

Sulfur-containing ligands tend to have high affinity for platinum group metals (PGM). Isothiuronium resins (Figure 9), first prepared in the 1960s by Koster and Schmuckler and subsequently commercialized in both gel (Srafion) and macroporous (Monivex) versions, adsorb PGM from strongly acidic solution and require complexing agents for regeneration. Adsorption is believed to be by simple ion exchange when the functional groups are fully protonated, but this not

Figure 9 Isothiuronium group.

achievable in the gel resin, and adsorption then involves both electrostatic and complexing interactions.

Isothiuronium resins have been used in laboratory-and pilot-scale PGM separations from HCl leach solutions, with thiourea or thiocyanate as regenerant. However, new sulfur-nitrogen resins are still being reported for heavy metal and PGM applications. Polybenzimidazole functionalized with dithiooxam-ide was found to be strongly selective for Pd(II) and Pt(IV) over first-row transition metals in acidic solution. Other researchers report similar behaviour with resins containing, for example, thiosemicar-bazide, dithizone, thiadiazole or thiotetrazolium substituents.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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