Electrostatic effects on ligand-assisted transfer of metals to (bio)accumulating interfaces and metal complexes (bioavai)lability

Abstract

The electric charge at a reactive interface influences the diffusion rate of ionic species towards the interface as well as their local concentration profiles. For metal ions (M) and their complexes (ML) formed with ligands (L), the interfacial electric field at a metal-consuming interface will lead to coupling between reaction layer and electric Debye layer, interfacial polarization of species concentrations and, thereby, so-far unexplored changes in ML contribution (lability) to sustain or not metal accumulation fluxes. A formalism and computational approach -based on coupled steady-state Nernst-Planck equations corrected for interfacial electrostatics and chemical kinetics- are here elaborated to obtain relevant metal surface flux and spatial distributions of M, L and ML. ML lability is subsequently derived and evaluated for practical settings of ML chemodynamics and macrosurface electrostatics under metal sink boundary condition. The extent to which the interfacial electric field affects lability of metal complexes depends on the charges of the reactive species and macrosurface. For example, repulsive M/ML-macrosurface interactions lead to significant loss of lability. Mass transfer features of ligands, even when present in large excess in bulk solution, are also found to impact on lability, even for moderately charged reactive surfaces. Predictions are in qualitative agreement with the available experimental data.