Roles of metal ion complexation and membrane permeability in the metal flux through lipophilic membranes. Labile complexes at permeation liquid membranes

The various physicochemical factors that influence the flux of carrier-transported metal ions through permeation liquid membranes (PLM) are studied systematically. Understanding PLM behavior is important (i) to optimize the application of PLM as metal speciation sensors in environmental media and (ii) because PLM may serve as bioanalogical devices that help to elucidate the environmental physicochemical processes occurring at the surface of biological membranes. Diffusion of free and complexed metal ions in solution, as well as diffusion of the metal carrier complex in the membrane, is considered. The respective roles of diffusion layer thickness, ligand concentration, complex stability, carrier concentration, and membrane thickness are studied experimentally in detail and compared with theory, using various labile complexes, namely, Pb(II)-diglycolate, Cu(II)-diglycolate, and Cu(II)-N-(2-carboxyphenyl)glycine. Conditions where either membrane diffusion or solution diffusion is rate limiting are clearly discriminated. It is shown in particular, that, by tuning the carrier concentration or membrane thickness, either the free metal ion concentration or the total labile metal species are measured. PLM can thus be used to determine whether models based on the free ion activity in solution (such as BLM or FIAM models) are applicable to metal uptake by microorganisms in a real natural mediu