Speciazione del ferro nell'acqua di mare: sviluppo, ottimizzazione e caratterizzazione di un nuovo metodo di Competitive Ligand Equilibration - Cathodic Stripping Voltammetry (CLE-CSV)

Iron speciation in seawater is of the utmost importance in understanding the biogeochemical cycle of this element, in view of its central role in regulating the primary productivity of the oceanic environment and its connection to global planetary cycles. The Competitive Ligand Equilibration-Cathodic Stripping Voltammetry (CLS-CSV) methods, often with the catalytic enhancement of the signal, fit for this purpose, as they allow to study the organic speciation of the metal by determining the organic iron binding ligands concentration and their stability constant for iron. Moreover, they avoid any sample separation or pretreatment step showing acceptable detection capabilities and robustness towards the saline matrix. Aim of the present PhD project was the development, optimization and characterization of a new method based on this technique characterized by optimal detection capabilities (sensitivity and limit of detection) and by an easy to use instrumental configuration. In fact, several methods experienced a development during the last 20 years, but each of them was characterized by the presence of important drawbacks as poor sensitivity, too high time analysis requirement and also, in some cases, by the use of a suspected carcinogenic species as catalytic enhancer. In particular, 2,3-dihydroxynaphthalene and atmospheric oxygen were employed as iron chelator and catalytic enhancer, respectively, as they ensure the best analytical performances. A new hardware configuration was firstly developed, leading to an overall simplification of the system: a silver wire pseudoreference was installed replacing the traditional Ag/AgCl 3 M KCl reference electrode and air was directly introduced into the close cell by an air pump. An extensive optimization procedure was accordingly performed, and the detection capabilities were carefully evaluated: fully satisfactory analytical performances for trace and ultratrace iron determination were reached. A comprehensive approach to the characterization of an adsorptive cathodic stripping voltammetry with catalytic enhancement followed. The focus of this part was on the understanding of the chemical features and processes involved by method and their consecutive effect on the analytical signal to appreciate possible limitations and future directions for improvements. Accordingly, the ligand degradation was study and its consequences on the analytical sensitivity and the speciation procedure evaluated. The stoichiometry of the FeDHN complex was also determined as it plays a major role in the speciation data treatment. Furthermore, the thermodynamics of ligand and complex adsorption onto the electrode surface was considered to evaluate a possible competition between the two species. As the final step of this part of the work, the electron transfer kinetics (k0 ) and catalytic enhancement (k’cat) were study to completely characterize the electrode reaction, i.e., to give full insight into the intimate mechanism responsible for the high sensitivity. The new CLE-CSV method was subsequently developed. Firstly, the side reaction coefficients for the FeDHN complex were calibrated against EDTA and the conditional stability constant calculated. The method was successfully validated in UV digested seawater using diethylenetriaminepentaacetic acid (DTPA) deferoxamine (DFO), and protoporphyrin IX (PPIX). Furthermore, good detection capabilities also for humic acid (HA) were proved. The analysis of six seawater samples from a Ross Sea water column (Antarctica) was then performed, demonstrating the fit for purpose for the detection of trace amounts of iron ligands in seawater. Lastly, as a further development of the technique, the pH buffer usually employed to perform voltammetric analysis in seawater was removed. The latter innovation was enabled by the natural pH buffer capacity of seawater due to the presence of the CO2/HCO3 - /CO3 2- system. The application of the unbuffered method resulted in not statistically different values for both the side reaction coefficients and conditional stability constant with respect to the buffered method. Again, the unbuffered method was successfully validated in UV digested seawater using both an artificial (diethylenetriaminepentaacetic acid) and a natural ligand (deferoxamine), and then applied to the same six seawater samples from the Ross Sea water column. The proposed unbuffered approach may demonstrate relevant to already existing speciation procedures for iron, resulting in a correct determination of complexing capacity and stability constant of organic iron binding ligands. The new method, both in its buffered and unbuffered version, achieved a tenfold sample size reduction and a tenfold increase in the analytical sensitivity compared with other methods employing 2,3- dihydroxynaphthalene. Furthermore, the analysis time was halved with respect to the fastest method reported in the literature as half an hour resulted enough to measure a twelve points titration.