Analysis, Distribution, and Oxidation-Reduction Chemistry of Fe(II) in Seawater

Abstract

Dissolved Fe(II) was isolated and preconcentrated in situ in the euphotic waters of the equatorial Pacific Ocean, using complexation by Ferrozine previously adsorbed on a C18 hydrocarbon substrate and then determined spectrophotometrically. The distribution of Fe(II) in the surface waters varied temporally and spatially, exhibiting maxima near the surface and often at depths with higher chlorophyll a. Laboratory photochemical experiments with equatorial Pacific seawater demonstrated photochemical reduction of Fe(ill) which may be an important source for Fe(II) in these waters, altering the speciation and bioavailability of iron. A highly sensitive stopped-flow chemiluminescence method was developed for the analysis of Fe(II) and reducible iron at subnanomolar levels in seawater. Oxidation of Fe(II) by O2 in the absence of H2O2 as used to catalyze luminol chemiluminescence. Interference studies were conducted with Cr(III), Fe(III), Cu(II), Mn(II), Zn(II), Co(II), and Ni(II). The detection limit for Fe(II) with a 200 μ l sample injection volume is 0.06 nmole/kg in open ocean waters and 0.15 nmole/kg in coastal waters. A series of high resolution profiles of iron was determined across the oxic/anoxic transition zone (OATZ) in the Pettaquamscutt Estuary. Selective chemical treatments and multiple analytical methods were used to determined the oxidation state and lability of iron across the OATZ. Well developed maxima of total dissolved iron occurred within the OATZ. Analysis of Fe(II) by the Ferrozine method indicates that more than 95% of the dissolved iron determined by atomic absorption spectroscopy within the maximum was Fe(II). Thermodynamic speciation calculations indicated that the dominant species of Fe(II) in the anoxic waters was the Fe(HS)+ complex. The Fe(II) concentration in the anoxic zone appeared to be controlled initially by precipitation of amorphous FeS, and for deep waters the ion activity product agreed with the solubility product of mackinawite. A one-dimensional vertical, eddy diffusion model was developed that incorporates redox reactions of iron, sulfide, and oxygen. This model predicted a distribution of Fe(II) that was in good agreement with the observations.