Using interfacial tension measurements to analyze the mechanism of zinc extraction with D2EHPA

By measuring the distribution ratio of zinc between an aqueous sulphate solution and n-dodecane containing di(2-ethylhexyl)phosphoric acid (D2EHPA) the different species of possible zinc complexes have been identified. An analysis of the distribution ratio in the case of three constant pH-values illustrates the existence of at least two species in the organic phase, particularly ZnR2 and ZnR2RH. Considering the known dominance of the dimeric form of D2EHPA in aliphatic solvents, the existence of another species – ZnR2(RH)2 – is likely.Studies of the equilibrium interfacial tension show that the zinc complex without ligands is lowering the interfacial tension in addition to the well-known interfacial activity of the monomer and its anion. The zinc complex is only adsorbed at the interface when the concentrations of the other two interfacial active species are low. This is why the solvation of the zinc complex follows the desorption in the organic phase.Measuring the dynamic interfacial tension demonstrates the absence of the sorptive barriers and explains the transport limitation caused by mass transport from the bulk to the interface.Based on these new investigations there follows a description of the zinc extraction mechanism using the extractant D2EHPA.

► The zinc complex exists in different solvated forms depending on zinc loading.
► There are three complex variations: without, with one or with two monomeric ligands.
► The solvated forms of the zinc complex are not interfacially active.
► There are different mechanisms for the zinc transfer reaction.
► The equilibrium near the interface must be performed by adsorption isotherms.