Cr(III)/Al(III)/Fe(III) ion binding on mixed bed ion exchangers. Synergistic effects of the resins behaviour

Ion exchange, as a conservative technology, allows for removal and recycling of metals from liquid effluents. Chelating exchangers may be used to the purpose due to their specificity toward metal species. The specificity, related to the metal-functional group affinity and thus the high stability of the complexes formed, is paid in terms of regeneration levels necessary to reverse the exchange reaction by mass action. Moreover, the liquid-phase speciation of the metals with organic and inorganic ligands, commonly present in industrial effluents, determines the active metal concentrations at the binding sites and thus the removal efficiency of the chelating resins which is, in general, more affected by stereochemical factors during the interaction of the active species at the functional groups. In this context, we investigated the behaviour of a mixed bed weak electrolyte anion (amino) and cation (carboxylate) conventional ion-exchangers as an alternative to chelating resins. The synergistic effects of the two functional groups to selectively remove, separate and recover Cr(III), Al(III), Fe(III) from segregated industrial effluents are discussed. The exhaustion behaviour of the carboxylate resin toward chromic and aluminum species is synergized by the cooperative presence of the anion resin, this latter being able to selectively retain ferric species. Difficult regeneration behaviour of the metal-form carboxylate resin is also favored by the presence of the weak base amino functional groups in the free base form.