Abstract
Competitive ligand exchange – adsorptive cathodic stripping voltammetry (CLE-AdCSV) is a widely used technique to determine dissolved iron (Fe) speciation in seawater, and involves competition for Fe of a known added ligand (AL) with natural organic ligands. Three different ALs were used, 2-(2-thiazolylazo)-p-cresol (TAC), salicylaldoxime (SA) and 1-nitroso-2-napthol (NN). The total ligand concentrations ([Lt]) and conditional stability constants (log K′Fe’L) obtained using the different ALs are compared. The comparison was done on seawater samples from Fram Strait and northeast Greenland shelf region, including the Norske Trough, Nioghalvfjerdsfjorden (79N) Glacier front and Westwind Trough. Data interpretation using a one-ligand model resulted in [Lt]SA (2.72 ± 0.99 nM eq Fe) > [Lt]TAC (1.77 ± 0.57 nM eq Fe) > [Lt]NN (1.57 ± 0.58 nM eq Fe); with the mean of log K′Fe’L being the highest for TAC (log ′KFe’L(TAC) = 12.8 ± 0.5), followed by SA (log K′Fe’L(SA) = 10.9 ± 0.4) and NN (log K′Fe’L(NN) = 10.1 ± 0.6). These differences are only partly explained by the detection windows employed, and are probably due to uncertainties propagated from the calibration and the heterogeneity of the natural organic ligands. An almost constant ratio of [Lt]TAC/[Lt]SA = 0.5 – 0.6 was obtained in samples over the shelf, potentially related to contributions of humic acid-type ligands. In contrast, in Fram Strait [Lt]TAC/[Lt]SA varied considerably from 0.6 to 1, indicating the influence of other ligand types, which seemed to be detected to a different extent by the TAC and SA methods. Our results show that even though the SA, TAC and NN methods have different detection windows, the results of the one ligand model captured a similar trend in [Lt], increasing from Fram Strait to the Norske Trough to the Westwind Trough. Application of a two-ligand model confirms a previous suggestion that in Polar Surface Water and in water masses over the shelf, two ligand groups existed, a relatively strong and relatively weak ligand group. The relatively weak ligand group contributed less to the total complexation capacity, hence it could only keep part of Fe released from the 79N Glacier in the dissolved phase.
Highlights
Organic Fe-binding ligands allow dissolved Fe (DFe) to be present at concentrations above the inorganic solubility of Fe in seawater, increase the residence time of DFe and enable DFe to be transported by ocean currents (van den Berg, 1995; Hunter and Boyd, 2007; Boyd and Ellwood, 2010; Gerringa et al, 2015)
Atlantic Water (AW) is present at depths shallower than ∼500 m in our stations
The assessment of TAC, SA and NN as added ligand (AL) with their specific detection windows in CLE-AdCSV titrations resulted in different ligand characteristics
Summary
Organic Fe-binding ligands allow dissolved Fe (DFe) to be present at concentrations above the inorganic solubility of Fe in seawater, increase the residence time of DFe and enable DFe to be transported by ocean currents (van den Berg, 1995; Hunter and Boyd, 2007; Boyd and Ellwood, 2010; Gerringa et al, 2015). The concentration and conditional binding strength of the bulk ligands in seawater are typically determined with an electrochemical technique, using the competitive ligand exchange–adsorptive cathodic stripping voltammetry (CLEAdCSV), where the conditional binding strength is used to infer a broad ligand group In this technique, a sample aliquot is titrated with Fe to saturate natural Fe-binding ligands. Using the Langmuir isotherm, the data is fitted with non-linear regression to calculate the ligand concentration [Lt] and conditional binding strength expressed as a log value of the conditional stability constant, log K Fe L (Hudson et al, 2003; Gerringa et al, 2014; Omanovicet al., 2015)
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