Effect of pH, light, and temperature on Fe–EDTA chelation and Fe hydrolysis in seawater

We used a novel technique, adsorption of dissolved labile ferric hydrolysis species (Fe(III)′) onto C18-silica cartridges, to measure concentrations of Fe(III)′ in equilibrium with ethylenediaminetetraacetate (EDTA) in UV-treated seawater. Effects of temperature, pH, and light on steady-state Fe(III)′ concentrations and resultant conditional dissociation constants for Fe–EDTA chelates were determined. Measured dissociation constants in the dark were similar at 10 and 20 °C, but increased by 600-fold between pH 7.7 and 9.0, due largely to the formation of mixed EDTA-hydroxy chelates with more rapid dissociation kinetics. The conditional dissociation constants for Fe–EDTA chelates were combined with thermodynamic constants for equilibria among EDTA4−, Fe3+, Ca2+, Mg2+, and H+ to compute ratios of [Fe(III)′]/[Fe3+] as a function of pH at 20 °C. Modeling of this data yielded ferric hydrolysis constants for formation of Fe(OH)2+ (log β2*=−6.40±0.15), Fe(OH)3 (log β3*=−15.1±0.8), and Fe(OH)4− (log β4*=−22.70±0.08) that were consistent with other published values. Light increased steady-state Fe(III)′ concentrations (and resultant steady-state Fe–EDTA dissociation constants) due to the photo-reductive dissociation of Fe–EDTA chelates. This effect decreased at higher temperature and pH due to a larger influence of these parameters on dark (thermal) rates for Fe–EDTA dissociation and association than on Fe–EDTA photo-dissociation rates. Similar temperature effects should occur for iron-chelates with natural organic ligands, which could enhance the importance of photo-dissociation at colder ocean temperatures. This effect may increase iron availability to phytoplankton in cold water regions by increasing concentrations of biologically available dissolved inorganic Fe(II) and Fe(III) species.