An investigation of the solvent extraction kinetics of Germanium by 7- alkylated.-8-hydroxyquinoline extractants.
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Date
1990
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Abstract
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.
Description
Thesis (Ph.D.)-University of Natal, Durban, 1990.
Keywords
Theses--Chemistry., Extraction (Chemistry), Solvents., Germanium.