The Role of Dissolved Organic Matter (DOM) in Biogeochemical Cycling of Iron within Estuarine and Coastal Waters
Iron (Fe) is an essential element for the biochemical and physiological functioning of terrestrial and oceanic organisms. Speciation describes the chemical forms in which Fe is present in an aquatic environment. It is important to understand Fe speciation due to its existence with organic matter in a complexed form. Those complexes are causing the formation of different species of Fe, which affect its bioavailability and hence, biogeochemistry. The samples used for Fe-speciation in the present study were collected from freshwater (Whau River), estuarine (Mersey River estuary and Whau River estuary) and coastal waters (Liverpool Bay) and were measured for Fe-speciation using competitive ligand exchange–adsorptive cathodic stripping voltammetry (CLE-AdCSV). The technique measures the Fe-binding complexes and conditional stability constants of those complexes in the presence of an electroactive artificial ligand, which is added to obtain the CSV signals and the natural ligand to determine the complex stability of the natural species. The strength of the so-called detection window is a function of stability constant and the concentration of the artificial ligand. Usually, different optimised windows are used to measure weaker or stronger natural Fe-binding ligands, respectively. For most CLE-AdCSV methods, high concentrations of artificial ligands are added to measure strong ligands. However, for salicylaldoxime (SA), used as the artificial ligand for the determination of natural organic Fe-ligands, this rule does not apply. This is because electrochemical inactivity of the FeSA2 complex and the analytical sensitivity decreases with an increase in added ligand concentration. Despite this technical complication, this thesis was aimed at identifying multiple Fe-binding natural ligand classes by performing speciation analysis at more than one analytical window. For this purpose, multiple analytical windows (MAWs) were obtained by varying the concentration of added ligand (SA). The data obtained from different analytical windows applied to the same sample were analyzed individually and simultaneously by Excel-based KINETEQL multiwindow Solver (KMS). Within, both estuaries, individual and MAW titrations agreed and demonstrated the presence of only one ligand class dominating in all samples and was in excess of the iron concentration throughout the salinity range tested. The ligand concentration co-varied with that of iron-binding humic substances (Fe-HS) only in Mersey River estuary whereas a moderate correlation was observed between dFe-complexing ligand and Fe-HS, UVA and UVC humic-like fluorophore, suggesting that humic-type ligands account only a partial pool for dFe-complexing ligands within Whau River and Whau River estuary. Measurement of the composition of dissolved organic carbon (DOC) using 2-dimensional fluorescence scans indicated the presence of terrestrial as well as microbial sources of organic matter within the estuaries. The fraction of HS in the DOC amounted to between 4 and 46 % whereas in freshwater 80 %. Phytoplankton productivity endorses the presence of high nutrient loads within the Whau River estuary. Dissolved organic matter (DOM) characterization was attained at molecular levels for samples collected from Mersey River estuary and Liverpool Bay using electrospray ionization coupled to Fourier transform ion cyclotron resonance mass spectrometry technique (ESI FT-ICR MS). The complexity of molecular level data was resolved with the aid of advance multivariate statistical tools. The presence of heteroatomic containing molecular formulae in high abundance among total molecular formulae was evident for impact from anthropogenic inputs onto DOM composition. The detection and chemical characterization of Fe-complexes are only slowly progressing due to the high diversity of natural DOM in the natural aquatic environment. The chemical identity of naturally occurring Fe-binding complexes can only be determined by the direct analysis of their physical and chemical characteristics that require sufficient quantities of natural Fe-complexing ligands (milligram to gram quantities of isolated material). Besides large sample volume owing to low concentration of Fe-binding chelators (picomolar to nanomolar), technical and economic reasons are the big hurdles toward the isolation of those Fe chelators and subsequently, compound-specific characterizations. In the present study, we have statistically combined the data obtained from molecular characterization of dissolved organic matter with voltammetric data. The knowledge on metal ligands and DOM molecular composition in the same system are essentially absent in the literature, and it is very valuable to the community by improving our knowledge towards trace metal cycling and subsequently, to better predict the responses to projected global alteration due to a changing climate.
Advisor: Sander, Sylvia; Hunter, Keith A.; van den Berg, Stan
Degree Name: Doctor of Philosophy
Degree Discipline: Department of Chemistry
Publisher: University of Otago
Keywords: Dissolved iron and dissolved iron speciation by voltammetry; Multiple analytical windows; Dissolved organic matter characterization; Humic substances; Mersey River estuary and Liverpool Bay; Whau River and Whau River estuary Auckland; ESI FT-ICR MS
Research Type: Thesis