A new voltammetric method for the measurement of the effect of humic substances in iron speciation. Application to Surface Arctic waters

Abstract

[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.