Determination of conditional stability constants and kinetic constants for phenolic Fe-Binding ligands in seawater

Abstract

Conditional stability constant and rates of formation and dissociation for the inorganic iron species (Fe′) and for Fe3+ complexation with three model polyphenols,(±) – catechin, sinapic acid and gallic acid were measured in UV-treated seawater by competitive ligand adsorptive stripping voltammetry (CLE-CSV) and kinetic approach using the iron binding ligand TAC (2-(2-thiazolylazo)-p-cresol). Conditional stability constants for the ligands ranged from log K′Fe′L 11.0 to 11.9. The stoichiometry of the complexes formed between dissolved Fe (dFe) and the polyphenols, in UV-seawater, was 3.5:1, 1.5:1 and 1:2.8 for (±) – catechin, sinapic acid and gallic acid, respectively. The formation rate constant varied from 3.82•105 to 4.16•105 M-1s-1 and dissociation rate constant 1.88•10-4– 4.4•10-4s-1. The conditional stability constant (log K′Fe′L) computed from the kinetic approach was from 8.96 to 9.4. The different dFe-binding capacity of polyphenols is highly affected by the interaction with Mg2+ and Ca2+ in seawater. Moreover, the dFe-binding ligands are able to keep Fe in solution for longer between 26 – 61 min for Fe′ species and 0.7 – 1.8 years for Fe3+.