Aggregation vs. breakup of the organic phase complex

Extraction of zinc and other metals from aqueous solutions using D2EHPA as an extractant in organic phases has been investigated extensively in the past. Under the conditions of low loading of metal in organic phase, equilibrium behaviour is adequately explained through an interfacial reaction with constant stoichiometric coefficients. Experiments show that as the loading of metal in the organic phase increases, increasingly smaller number of molecules of D2EHPA are required to extract a metal ion. Reaction mechanisms which explain this behaviour through breakup of complexes formed on the interface to release free extractant for further extraction are shown to be untenable as they fail to explain experimental observations on viscosity variation and dependence of apparent equilibrium constant on loading ratio alone. A new reaction mechanism is proposed in this work. It considers that complex formed on the interface aggregates in the organic phase and releases free extractant for increased extraction. The proposed mechanism successfully explains the above hitherto unexplained features of metal–D2EHPA extraction systems. It also quantitatively predicts the extraction equilibria not just for zinc but also for cobalt and nickel, and may characterize other extraction systems as well.