Iron Speciation in Fram Strait and Over the Northeast Greenland Shelf: An Inter-Comparison Study of Voltammetric Methods

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.

Northern High-Latitude Organic Soils As a Vital Source of River-Borne Dissolved Iron to the Ocean

“…CLE-AdCSV is based on competition for Fe 3+ of an added ligand (AL) with the natural ligands. Ardiningsih et al (2021) compared 2-(2-thiazolylazo)-p-cresol (TAC), salicylaldoxime (SA), and 1-nitroso-2-napthol (NN) in order to analyze seawater samples from Fram Strait and northeast Greenland shelf region. The three methods provided significantly different results for log K cond,FeL .…”

Influence of pH and Dissolved Organic Matter on Iron Speciation and Apparent Iron Solubility in the Peruvian Shelf and Slope Region

“…65 The CLE-AdCSV method has been widely applied in seawater, 32,34,66 and the limit of detection is 0.13 nmol L −1 Fe when using HNN. 67 We used a single window titration ([HNN] = 2 μmol L −1 ) with six different pH NBS values (7.0, 7.3, 7.6, 7.9, 8.2, and 8.5) and five different Fe concentrations (0, 0.5, 1.5, 5, 7.5, and 10 nmol L −1 ) for each pH NBS value, resulting in a 2D pH-Fe titration matrix with a total of 30 titration data points per sample (an example titration is shown in Figure S1 in the Supporting Information). We observed no significant difference in voltammetric sensitivity over the pH NBS range 7.0−8.5 in seawater that had been irradiated with ultraviolet (UV) light to remove DOM (Figure S2 and Table S2).…”

“…Fe­(III) speciation was determined by CLE-AdCSV at room temperature (20 °C) using 1-nitroso-2-naphthol (HNN) as the added ligand, after a 16 h equilibrium period . The CLE-AdCSV method has been widely applied in seawater, ,, and the limit of detection is 0.13 nmol L –1 Fe when using HNN . We used a single window titration ([HNN] = 2 μmol L –1 ) with six different pH NBS values (7.0, 7.3, 7.6, 7.9, 8.2, and 8.5) and five different Fe concentrations (0, 0.5, 1.5, 5, 7.5, and 10 nmol L –1 ) for each pH NBS value, resulting in a 2D pH-Fe titration matrix with a total of 30 titration data points per sample (an example titration is shown in Figure S1 in the Supporting Information).…”

Comparing CLE-AdCSV applications using SA and TAC to determine the Fe binding characteristics of model ligands in seawater

“…Humic substances are ubiquitous in seawater (Laglera et al, 535 2009;Whitby et al, 2020;Yamashita et al, 2020) and potentially more representative of the dominant fraction of dissolved organic matter actually present in seawater than the model A ligands tested here, since although siderophores are detected in seawater, they are typically only present at pM concentrations (Mawji et al, 2008(Mawji et al, , 2011Velasquez et al, 2016;Boiteau et al,2018). The different results between the three applications could explain a major part in the offset between the TAC and SA methods in natural waters (Buck et al, 2012(Buck et al, , 2015Slagter et al, 2019;Ardiningsih et al, 2021). Moreover, the deviation 540 in obtained by TAC from the other two applications ( / up to 1.08 and / up to 1.1) is greater than for most model A ligands (Table 3).…”

“…As far as we know, four publications describing an intercomparison exist. Three of these compared SA25 and TAC (Buck et al, 2012(Buck et al, , 2016 al., 2019), one compared SA5 and TAC (Ardiningsih et al, 2021). In all publications, [LSA] is larger than [LTAC] when the data is fitted with a one ligand model).…”