Dynamic features of speciation analysis by adsorptive stripping techniques

Abstract

Environmental context.The environmental fate and bioavailability of metal ions in natural waters is determined by their thermodynamic stability and kinetic features, both of which are distributed. Competing ligand exchange – adsorptive stripping (CLE-AdS) is a technique that measures a certain portion of these complexes as determined by the stability of the selected competing ligand and the dynamic features of the sample complexes that remain following ligand exchange. Exploitation of CLE-AdS to determine a spectrum of sample complexes requires insight into its thermodynamic and kinetic windows. Abstract.The kinetic features of the accumulation step of competing ligand exchange – adsorptive stripping (CLE-AdS) in metal speciation methodology are elaborated. During the adsorptive accumulation process, the flux of the surface active complex MLad towards the electrode may be modified by the coupled conversions of ML and M into MLad. An immediate consequence is that the accumulation flux of MLad can be greater than that corresponding to its mere bulk concentration: a labile ML contributes fully to the MLad accumulation, and a further flux enhancement can arise if ML is more mobile than MLad (DML > DMLad). Applying the conventional lability criterion, we present a framework for interpretation of CLE-AdS measurements in the presence of kinetic contributions from sample ML to the adsorptive accumulation of MLad. Measured accumulation fluxes for a kinetic case are well described by the presented theoretical framework. The dynamic analysis provides the basis for exploitation of CLE-AdS over a wider kinetic window than has been used to date. Consideration of the dynamics of sample species during the adsorptive accumulation step is fundamental for interpretation of metal speciation by CLE-AdS in complex natural systems that contain a distribution of complexes of different stability, lability, and mobility.