Complexation of Fe(III) by natural organic ligands in the Northwest Atlantic Ocean by a competitive ligand equilibration method and a kinetic approach

Abstract

Total and labile Fe measurements, and Fe3+ titrations were carried out both at sea and in the laboratory with adsorptive cathodic stripping voltammetry (CSV) methods using 1-nitroso-2-naphthol (1N2N) as a complexing ligand to study Fe(III) speciation and the kinetic interaction of Fe3+ with naturally occurring organic ligands. On the continental slope and at the shelf/slope front of the Northwest Atlantic ocean, the total dissolved (< 0.4 μm) Fe was predominantly 1N2N nonlabile, with 60% nonlabile at the mouth of Delaware Bay. The exact chemical speciation of this nonlabile Fe is not known; although some of this Fe is likely in strong organic complexes with a KFeL1023.22 as determined by a competitive ligand equilibration/cathodic stripping voltammetry (CLE/CSV) method. To obtain kinetic information on Fe3+ uptake by natural ligands in excess of ambient Fe3+ concentrations in seawater, Fe3+ was added to UV irradiated seawater samples with and without the model ligands, EDTA and enterobactin. Measurable Fe in UV seawater with model ligands decreased with time much faster than that without model ligands. Similar faster decreases of measurable Fe with time were observed for field samples, suggesting the existence of Fe(III) complexing organic ligands. The reaction kinetics of the added Fe3+ with natural organic ligands and of the Fe(III)-organic complexes with 1N2N suggested that there were different organic ligands in different oceanographic regimes. Two classes of ligands are distinguished from these studies; one class which binds to Ca and Mg and another which does not. The latter assessment could not be unambiguously determined by a CLE/CSV method. Our kinetic experiments indicate that equilibrium is not achieved at the same rate in all natural samples studied and that different ligands occur in different water masses. The maximum second-order rate constant determined for Fe3+ uptake by natural ligands in our field samples is 106 M−1s−1. Using a CLE/CSV method at pH 6.9 with complete equilibrium established for the samples, a KFeLCL value of < 1012.04 ± 0.09 is calculated for the excess ligands in these samples. These data are in agreement with our detailed kinetic analysis using the steady state approximation based on natural ligand dissociation from FeL and recovery of that Fe by 1N2N at pH 8. KFeL (1020.6 M−1) estimated from kinetic data is within the range of estimates made by researchers using only the CLE/CSV approach.