The effect of Dunaliella tertiolecta organic exudates on the Fe(II) oxidation kinetics in seawater.

Abstract

The role played by the natural organic ligands excreted by the green algae Dunaliella tertiolecta on the Fe(II) oxidation rate constants was studied. The concentration of dissolved organic carbon was measured for each day of growth demonstrating that the concentration increased over time. The oxidation kinetics of Fe(II) was studied under the different physico-chemical conditions such as dissolved organic carbon produced by cells (2.1-7.1 mg L−1), pH (7.2-8.2), temperature (5-35ºC) and salinity (10-36.72). The rate constant always decreased in the presence of organic exudates respect to that in the control seawater. The Fe(II) rate constants were also studied in the context of Marcus theory, where ∆Gº was 39.31-51.48 kJ mol−1. A kinetic modelling approach was applied for computing the equilibrium and rate constants for Fe(II) and exudates present in solution, the Fe(II) speciation and the contribution of each Fe(II) species to the overall oxidation rate constant. The best fit model took into account two types of ligands with pKa,1 = 9.45 and pKa,2 = 4.9, capable of complexing Fe(II). The Fe(II) complexing constants were KFe(II)-LH = 3×1010 and KFe(II)-L = 107 and the corresponding computed oxidation rates were 68 ± 2 and 36 ± 8 M−1 min−1 respectively.