COMPLEXATION OF FE(III) BY NATURAL ORGANIC-LIGANDS IN THE NORTHWEST ATLANTIC-OCEAN BY A COMPETITIVE LIGAND EQUILIBRATION METHOD AND A KINETIC APPROACH

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 compl exing ligand to study Fe(III) speciation and the kinetic interaction o f Fe3+ with naturally occurring organic ligands. On the continental sl ope and at the shelf/slope front of the Northwest Atlantic ocean, the total dissolved (< 0.4 mu m) Fe was predominantly 1N2N nonlabile, with 60% nonlabile at the mouth of Delaware Bay. The exact chemical specia tion of this nonlabile Fe is not known; although some of this Fe is li kely in strong organic complexes with a K-FeL > 10(23.22) as determine d by a competitive ligand equilibration/cathodic stripping voltammetry (CLE/CSV) method. To obtain kinetic information on Fe3+ uptake by nat ural ligands in excess of ambient Fe3+ concentrations in seawater, Fe3 + was added to UV irradiated seawater samples with and without the mod el ligands, EDTA and enterobactin. Measurable Fe in UV seawater with m odel ligands decreased with time much faster than that without model l igands. Similar faster decreases of measurable Fe with time were obser ved 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 suggest ed that there were different organic ligands in different oceanographi c regimes. Two classes of ligands are distinguished from these studies ; one class which binds to Ca and Mg and another which does not. The l atter assessment could not be unambiguously determined by a CLE/CSV me thod. Our kinetic experiments indicate that equilibrium is not achieve d at the same rate in all natural samples studied and that different l igands occur in different water masses. The maximum second-order rate constant determined for Fe3+ uptake by natural ligands in our field sa mples is > 10(6) M(-1) s(-1). Using a CLE/CSV method at pH 6.9 with co mplete equilibrium established for the samples, a KFeLCL value of < 10 (12.04+/-0.09) is calculated for the excess ligands in these samples. These data are in agreement with our detailed kinetic analysis using t he steady state approximation based on natural ligand dissociation fro m FeL and recovery of that Fe by 1N2N at pH 8. K-FeL (10(20.6) M(-1)) estimated from kinetic data is within the range of estimates made by r esearchers using only the CLE/CSV approach.