An investigation of the solvent extraction kinetics of Germanium by 7- alkylated.-8-hydroxyquinoline extractants.
Foster, Stephen James.
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Equilibrium and kinetic data for the solvent extraction of germanium by three impure commercial 7-alkylated-8hydroxyquinoline extractants which vary in structure at the 7alkyl group, are evaluated in order to elucidate an holistic kinetic extraction model which accounts for the various reactions and partition effects occurring during the metal-ion chelation process. It is proposed that for the extraction process, which is first order in germanium concentration, by the ligand reagents Lix 26, TN 01787 and TN 02181, the rate-determining step, on stereochemical grounds, is the attachment of either a neutral ligand or a protonated ligand species-to the biligand intermediate GeL2 2+ (L:ligand) at the interface. In high speed shaking/mixing assemblies the extraction process was observed to occur in two discrete reaction regimes : a fast initial rate for which the orders with respect to ligand reagent are 1,06, 2,10 and 1,77 for TN 02181, Lix 26 and TN 0178-7 respectively, and a slower subsequent rate for which the apparent reaction orders with respect to ligand concentration are 1,12, 2,70 and 3,08 for TN 02181, Lix 26 and TN 01787 respectively. For the slower reaction regime, orders between 1 and 3 are explicable if the steady state approximation is invoked for the intermediate germanium species GeL3+ and GeL2 2+ formed at the interface. In the fast reaction regime, it is proposed that the accelerated extraction rates are a function of (i) the speciation of germanium and (ii) participation in the rate-determining step by the protonated ligand moiety H2L+HS04- which is rapidly formed after phase contact. At low ligand concentration, the following order of ligand efficacy has been observed : TN 01787 < Lix 26 < TN 02181 whereas at high concentration ligand efficacies are similar because the interface is saturated with ligand. Orders with respect to [H+] for the reagents vary from -1 to - 3 during the course of reaction, indicating complex mixedorder behaviour. The effects upon extraction of ionic strength, temperature, the addition of organic modifiers and diluent nature are investigated as well as the kinetics of germanium stripping by aqueous hydroxide. The physical effects of interfacial tension, viscosity and relative dielectric constant are also reported and suggestions are made as to their effect upon the extraction characteristics. Computer modelling of the extractants has been used as an aid in describing size, structure and stereochemical considerations of the ligands and the chelate products.