[eng] Understanding of iron cycling in ocean waters is one of the most
challenging tasks in oceanographic studies and requires of new analytical
strategies. Despite being the most abundant element on Earth, iron is limitant
to phytoplankton growth in at least one third of the ocean surface waters, due
to the incredibly low solubility in seawater of its thermodynamically stable
redox species, Fe (III). The solubility of this element is slightly increased by
organic complexation with a variety of natural ligands that due to their low
concentration and heterogeneity have not been currently characterized. The
list of natural ligand candidates, such as siderophores and polysaccharides,
also includes humic substances (HS). Although most of natural iron ligands do
not form iron complexes electrolabile on mercury working electrodes, Fe-HS
complexes can be detected by cathodic voltammetry if a strong oxidant such
as bromate is added for a catalytic reoxidation of Fe(II) back to Fe(III). This
property was used in the past to set up a new direct analytical method to
determine the concentration of humic substances in seawater.
In this Master’s Thesis is proposed a rearrangement and extension of
the original analytical protocol by Laglera et al. (2007) for the determination of
the concentration of humic substances in seawater. This rearrangement
extends the possibilities of the previous protocol, including the determination
of the percentage of iron-binding groups of humic substances that are
originally bound to iron. The combined knowledge of the binding capacity of
HS and the saturation percentage permits the quantification of the
concentration of iron present in the sample as Fe-HS complexes. The method
modifications consist briefly in saturating with iron the humic standard
prepared in ultrapure water, preventing the underestimation of the sensitivity
during calibration and analyzing the sample twice, before and after iron
saturation. This rearrangement has been recently published (Sukekava et al.
2018).
As a result of the analytical development presented here, quantification
of the contribution of a determined type of natural ligands (concentration of FeHS complexes) to the organic speciation and solubility of iron is achieved for
the first time. This Master’s Thesis made a first application of the method to
samples collected in surface Arctic Ocean waters which are characterized by
high iron concentrations and a high content of terrigenous organic matter.