23 Jun 2021

23 Jun 2021

Review status: this preprint is currently under review for the journal BG.

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

Loes J. A. Gerringa1, Martha Gledhill2, Indah Ardiningsih1, Niels Muntjewerf1, and Luis M. Laglera3 Loes J. A. Gerringa et al.
  • 1Royal Netherlands Institute for Sea Research (NIOZ), Department of Ocean Systems, Texel, the Netherlands
  • 2GEOMAR Helmholtz Centre for Ocean Research, 24148 Kiel, Germany
  • 3FI-TRACE, Departamento de Química and ‡Laboratori Interdisciplinari sobre Canvi Climàtic, Universidad de las Islas Baleares, Palma, Balearic Islands 07122, Spain

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 are compared. The applications are: 1) Salicylaldoxime (SA) at 25µM (SA25) and short waiting time, 2) SA at 5µM (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 log D 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 non humic 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 under estimated the ligand concentration. The extent of overestimation and underestimation differed per model ligand, and the three applications showed the same trend between the non humic 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.

Loes J. A. Gerringa et al.

Status: open (until 04 Aug 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-134', Anonymous Referee #1, 18 Jul 2021 reply
  • RC2: 'Comment on bg-2021-134', Dario Omanović, 19 Jul 2021 reply

Loes J. A. Gerringa et al.

Loes J. A. Gerringa et al.


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Short summary
For three decades, competitive ligand exchange-adsorptive cathodic stripping voltammetry was used to estimate the Fe-binding capacity of organic matter in seawater. In this manuscript the performance of the competing ligands is compared trough the analysis of a series of model ligands. The main finding of this manuscript is that the determined speciation parameters are not independent of the application, making interpretation of Fe speciation data more complex than it was thought before.