The pH effect in the diffusion coefficient of humic matter: influence in speciation studies using voltammetric techniques

Abstract

The dynamic aggregation properties of humic matter and the influence of intramolecular electrostatic interaction lead to an increase of polydispersity at pH > 5 in two humic samples studied (“Purified Peat Humic Acid”, PPHA and “Laurentian Fulvic acid”, LFA). The smaller entities formed at pH increasing above 5 are responsible for the increase of the diffusion coefficient as “seen” by voltammetry. The voltammetric results show that the diffusion coefficient of the humic complex inmplex increases from 5 × 10 −12 m 2 s −1 (pH≤5) up to 2 × 10 −11 m 2 s −1 (pH=6) for Cd PPHA and from 6 × 10 −11 m 2 s −1 (pH < or =5) up to 1.2 × 10 −10 m 2 s −1 (pH=5.5) for Pb LFA . Systematic shifts in voltammetric results with changing ligand concentrations often attributed to adsorption of humic matter on the electrode and/or heterogeneity effects, may also be due to incorrect diffusion coefficient values used in speciation calculations. The aim of this paper is to show that the diffusion coefficient of metal-humic complexes is a function of pH. It is necessary to take this into account to obtain more accurate speciation data from voltammetric results and interpret the physicochemical background of the metal humic interaction.