Journal
MARINE CHEMISTRY
Volume 84, Issue 1-2, Pages 35-47Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/S0304-4203(03)00101-4
Keywords
iron; Fe-EDTA chelation; photolysis; ferric hydrolysis constants; pH; temperature
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We used a novel technique, adsorption of dissolved labile ferric hydrolysis species (Fe(III)') onto C(18)-silica cartridges, to measure concentrations of Fe(Ill)' 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 degreesC, 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 EDTA(4-), Fe(3+), Ca(2+), Mg(2+), and H(+) to compute ratios of [Fe(III)']/[Fe(3+)] as a function of pH at 20 degreesC. Modeling of this data yielded ferric hydrolysis constants for formation of Fe(OH)(2)(+) (log beta(2)* = -6.40 +/- 0.15), Fe(OH)(3) (log beta(3)* = -15.1 +/- 0.8), and Fe(OH)(4)(-) (log beta(4)* = -22.70 +/- 0.08) that were consistent with other published values. Light increased steady-state Fe(Ill)' 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. (C) 2003 Elsevier B.V. All rights reserved.
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