Abstract
Superoxide radical produced during algal metabolism is one of the most important agents that contribute to the transformation of the Fe(III)-organic ligand complex to more bioavailable Fe(II). In this work, we investigated the kinetics of superoxide-mediated Fe(II) production from Fe(III) complexed by citrate (Fe(III)Cit) in coastal seawater on the basis of a laboratory experiment and a kinetic model. To simulate a coastal environment, Fe(III)Cit samples with various ratios of citrate concentration to iron concentration (ligand/Fe ratio) were incubated for five minutes to one week in seawater medium. Then, Fe(II) production rate was spectrophotometrically measured by detecting the ferrous-ferrozine complex. During the measurement, superoxide was constantly generated by oxidizing xanthine. Fe(II) production rate generally decreased with incubation time, since Fe(III)Cit as a source of Fe(II) gradually dissociated to form less reactive iron oxyhydroxide in coastal seawater. However, when the ligand/Fe ratio was sufficiently high, the dissociation of Fe(III)Cit was suppressed and Fe(II) was formed at a relatively higher rate over one week. These results demonstrated that Fe(II) production rate is largely influenced by ligand/Fe ratio and incubation time. The kinetic model developed was in good agreement with the experimentally determined Fe(II) production rates.
