title: Chemical speciation of iron with humic ligands in estuarine and coastal waters
Creators: name: Abu Alhaija, Mahmoud
Publisher: name: University of Liverpool Repository
Year: 2016

description: Iron in coastal and sea water is readily complexed with organic matter (ligands) causing the formation of different species of iron, which affects its biogeochemistry. For this reason it is of interest to determine iron speciation in addition to its concentration. The chemical speciation of iron in coastal and sea water is typically determined by cathodic stripping voltammetry (CSV), making use of ligand competition between an electroactive ligand added to obtain the CSV signal and the natural ligand to determine the complex stability of the natural species. This thesis aims to identify the main ligand responsible for binding iron. Thereto, I first set out to improve the existing method of iron speciation in seawater using salicylaldoxime (SA) (as an added competing ligand) and subsequently used this method to determine the complex stability and concentration of the unknown Fe-binding ligands. Using another CSV method the concentration of humic substances (HS) was determined in samples from estuarine, coastal, shelf and ocean origin. Particular attention was given in this dissertation to the comparison between humic substances and natural Fe-binding ligands in all samples. The improved SA method has a much better sensitivity due to two effects: 1) the use of dissolved oxygen (DO) as an oxidant was found to give a catalytic effect; this improved the sensitivity greatly, and also stabilized the pH. 2) the analytical conditions of using SA were re-optimised and best sensitivity is now obtained at 5 µM SA (~5 x less than the pre-existing method). Interpretation of the experiments showed that the complex responsible for adsorption on the mercury drop electrode is FeSA, whereas the FeSA2 species does not adsorb. The complex stability for complexes of Fe with SA (FeSA and FeSA2), in pH 8 seawater, were calibrated over a range of SA concentrations and salinities. The similarity of the stability constants (log K’Fe’SA and log B’Fe’SA2) determined via calibration against EDTA to those calibrated without EDTA shows that the speciation of Fe with SA and EDTA is well understood. The improved method is applied to a mixed depth Celtic Sea sample, and two GEOTRACES samples from the Atlantic, at a SA concentration of 5 µM. Ligand concentrations were 1.47 and 1.49 nM in the GEOTRACES water (log K’Fe’L values of 11.1 and 11.9) and 2.53 nM in the Celtic Sea water (log K’Fe’L =11.5). Application of the method to ligands added to seawater gave log K’Fe’L values of 11.6  0.1 for humic acid (Suwannee River) and 12.2 0.3 for a siderophore (desferrioxamine B). The improved SA method was also applied to samples from the Mersey estuary and Liverpool Bay (salinity between 19 and 32). The concentration of unknown Fe-binding ligands was virtually the same as that of Fe-binding humics and both are greater than [Fe] in all samples, indicating that there was an excess of the ligand concentration and that almost the entire ligand concentration (95%) could be ascribed to humic substances. The average complex stability (log K’Fe’L) was 11.2  0.1, the same as for the iron-humic species (log K’Fe’-SRHA). Copper additions demonstrated competition between Cu and Fe for the natural ligands. Multiple analytical windows (MAWs) were used to determine the iron speciation in the Mersey estuary by varying the concentration of salicylaldoxime (SA). Data fitting of the individual titrations was compared to fitting of all data simultaneously (MAW fitting) giving good agreement. Individual titration fitting as well as the MAW fitting demonstrated the presence of only one ligand class in all samples (which was identified as humic substance in the previous work). Measurement of the composition of dissolved organic carbon (DOC) indicates the presence of terrestrial as well as microbial sources of organic matter in the estuary. The fraction of HS in the DOC amounted to between 47 and 25 % between salinities of 19 and 31. Application of the improved method to samples from the shelf and the Atlantic ocean (vertical profile) shows that the Fe-binding ligands concentration is greater than Fe concentration in all shelf and ocean samples. The complex stability of the Fe species in the Atlantic and over the shelf is identical within a narrow range of log K’Fe’L values of 11.2  0.1 (Atlantic profile) and 11.2  0.2 for the shelf samples. Comparison of the iron-binding humics and complexing ligands shows that the humic substances account for 95 to 98 % of the total ligand for iron in the shelf and ocean waters. This revolutionary finding challenges the paradigm that iron-binding ligands largely originate from in-situ production by microorganisms and much facilitates the modeling of the background ligand concentration that controls the global oceanic distribution of Fe. Data fitting of the complexometric titrations in all samples from the estuarine, coastal, shelf and ocean origin demonstrated the presence of only one ligand class.