Determination of Labile Fe(II) Species Complexed with Seawater Extractable Organic Matter in a Seawater Environment

A fertilizer composed of a mixture of steel slag and compost was used to restore seaweed beds in barren coastal areas. Complex species of Fe(II) from steel slag are supplying Fe(II) to coastal areas and stimulating seaweed growth. Seawater extractable organic matter (SWEOM) from a compost can serve as a chelator of Fe(II) in the fertilizer. Among the Fe (II)-SWEOM complexes, readily indissociable and undissociable complex species are defined as “labile” and “inert,” respectively. The bioavailability of Fe(II)-SWEOM complexes is higher in the labile species. In the present study, a novel method for analyzing labile species of Fe(II)-SWEOM complexes under conditions of seawater (pH 8.0, I = 0.7) was developed. The method is based on a ligand-exchange reaction between SWEOM and ferrozine, in which the ferrozine-exchangeable Fe(II) in the Fe(II)-SWEOM complex corresponds to the labile Fe(II) species. Fe(II) is readily oxidized to Fe(III) in seawater. Thus, to avoid the oxidation of ferrous ions, ascorbic acid was added as an antioxidant, a situation that had no effect on the ligand-exchange reaction between SWEOM and ferrozine. A comparison of the kinetic curve for the formation of the Fe(II)-ferrozine complex in the absence and presence of SWEOM permitted the free [Fe(II)] and labile [Fe(II)-SWEOM] to be determined. In a variety of total Fe(II) concentrations, conditional binding constants for labile Fe(II)-SWEOM complexes and binding capacities of labile sites in SWEOM to Fe (II) were evaluated.