Separation of heavy metals by parametric pumping with variation of pH -: Part I:: Application of cation exchangers in binary systems

Parametric pumping with variation of the pH is an innovative ion-exchange process which allows heavy metals from mixed waste solutions to be recovered as single components and, therefore, to be recycled. The process is based on the addition of complexing agents to a mixture, on its separation into two half volumes and a periodic contact of an ion exchanger with both liquid phases. Optimum conditions for the separation with respect to the kind of complexing agent, its amount relative to the concentration of heavy metals and to the amount of ion exchanger material have been deduced in preliminary investigations. Using citric acid as complexing agent the separation of binary mixtures of divalent/divalent and trivalent/divalent metal ions has been investigated. The kinds of metal species forming more stable complexes are concentrated in the high-pH solution, the other ones in the low-pH solution. Separation is more successful the greater the difference of the ratio of non-complexed metal cations at the two operating pH values becomes. As a consequence, systems with copper as one component can be well separated. Less effective separations were found for mixtures of other divalent metals. The separation can be emphasized by applying a second separation step to the two half volumes. (C) 2000 Elsevier Science B.V. All rights reserved.