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

Abstract

Abstract. Competitive ligand exchange–adsorptive cathodic stripping voltammetry (CLE-AdCSV) is used to determine the conditional concentration ([L]) and the conditional binding strength (logKcond) of dissolved organic Fe-binding ligands, which together influence the solubility of Fe in seawater. Electrochemical applications of Fe speciation measurements vary predominantly in the choice of the added competing ligand. Although different applications show the same trends, [L] and logKcond differ between the applications. In this study, binding of two added ligands in three different common applications to three known types of natural binding ligands is compared. The applications are (1) salicylaldoxime (SA) at 25 µM (SA25) and short waiting time, (2) SA at 5 µM (SA5), and (3) 2-(2-thiazolylazo)-ρ-cresol (TAC) at 10 µM, the latter two with overnight equilibration. The three applications were calibrated under the same conditions, although having different pH values, resulting in the detection window centers (D) DTAC > DSA25 ≥ SA5 (as logD values with respect to Fe3+: 12.3 > 11.2 ≥ 11). For the model ligands, there is no common trend in the results of logKcond. The values have a considerable spread, which indicates that the error in logKcond is large. The ligand concentrations of the nonhumic model ligands are overestimated by SA25, which we attribute to the lack of equilibrium between Fe-SA species in the SA25 application. The application TAC more often underestimated the ligand concentrations and the application SA5 over- and underestimated the ligand concentration. The extent of overestimation and underestimation differed per model ligand, and the three applications showed the same trend between the nonhumic model ligands, especially for SA5 and SA25. The estimated ligand concentrations for the humic and fulvic acids differed approximately 2-fold between TAC and SA5 and another factor of 2 between SA5 and SA25. The use of SA above 5 µM suffers from the formation of the species Fe(SA)x (x>1) that is not electro-active as already suggested by Abualhaija and van den Berg (2014). Moreover, we found that the reaction between the electro-active and non-electro-active species is probably irreversible. This undermines the assumption of the CLE principle, causes overestimation of [L] and could result in a false distinction into more than one ligand group. For future electrochemical work it is recommended to take the above limitations of the applications into account. Overall, the uncertainties arising from the CLE-AdCSV approach mean we need to search for new ways to determine the organic complexation of Fe in seawater.

Seasonal dispersal of fjord meltwaters as an important source of iron and manganese to coastal Antarctic phytoplankton

“…2). There is potential to overestimate ligand concentrations using these methods (Gerringa et al, 2021); however, the trends within these data and interpretations are valid. Analysis of the ligands within Andvord Bay shows a down-fjord gradient in both quantity and quality (all ligand data presented in Table 2).…”

Equilibrium calculations of iron speciation and apparent iron solubility in the Celtic Sea at ambient seawater pH using the NICA-Donnan model

“…Figure 3 shows that the relationship between [Fe 3+ ]titration and total Fe was quite well described by A, C and D, but not well described by parameter set B, although some differences between Fe 3+ at 1 µmol L -1 HNN at low total Fe concentrations was evident for all sets at DOC concentrations > 55 µmol L -1 . Accurate determination of Fe 3+ concentrations at low total Fe concentrations appears to be an issue for other voltammetric methods used for analysis of Fe speciation (Gerringa et al, 2021) and could be especially problematic when using HNN as an added ligand because of its lower sensitivity (Ardiningsih et al, 2021). Results of correlation between [Fe 3+ ]titration and [Fe 3+ ]NICA are given in Table 3.…”

Competitive Interactions Between Microbial Siderophores and Humic-Like Binding Sites in European Shelf Sea Waters

“…Nevertheless, our results suggest that at low DFe concentrations, high affinity binding sites in the heterogeneous DOM pool could be effective competitors for siderophores. An important consequence of this is that it would be very difficult to distinguish siderophores from humic substance-like binding sites present in marine DOM by titration techniques typically applied to investigate iron speciation in seawater (Gledhill and van den Berg, 1994;Croot and Johansson, 2000;Buck et al, 2007;Gerringa et al, 2021). The often used categorisation into ligand groups based on conditional stability constants is thus not as well defined as previously suggested (Gledhill and Buck, 2012).…”