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

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

We used a combined ion pairing - organic matter speciation model (NICA-Donnan) to predict the organic complexation of iron (Fe) at ambient pH and temperature in the Celtic Sea. We optimized our model by direct comparison with Fe speciation determined by Adsorptive Cathodic Stripping Voltammetry using the added Fe-binding ligand 1-nitroso-2-naphthol (HNN) in the presence and absence of natural organic matter. We compared determined Fe speciation with simulated titrations obtained via application of the NICA-Donnan model with four different NICA parameter sets representing a range of binding site strengths and heterogeneities. We tested the assumption that binding sites scale to dissolved organic carbon (DOC) concentrations in marine waters. We found that a constant low DOC concentration resulted in an improved fit of our titration data to the simulated titrations, suggesting that inputs of autochthonous marine DOM may not increase the heterogeneity or concentrations of Fe binding sites. Using the optimal parameter set, we calculated pFe(III)´ (−log(∑Fe(OH) i 3−i)) and apparent Fe(III) solubility (SFe(III) app) at ambient pH and temperature in the water column of the Celtic Sea. SFe(III) app was defined as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to DOM formed at a free Fe (Fe 3+) concentration equal to the limiting solubility of Fe hydroxide (Fe(OH) 3(s)). SFe(III) app was within range of the determined dissolved Fe concentrations observed after winter mixing on the shelf and in waters >1500 m depth at our most offshore stations. Our study supports the hypothesis that the ocean dissolved Fe inventory is controlled by the interplay between Fe solubility and Fe binding by organic matter, although the overall number of metal binding sites in the marine environment may not be directly scalable to DOC concentrations.

Intrinsic binding constants, Ocean acidification, Trace metals
0304-4203
Zhu, Kechen
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Birchill, Antony J.
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Milne, Angela
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Ussher, Simon
32006952-df4b-4364-8aae-7b9be1f39745
Humphreys, Matthew P.
5edc45a6-2f03-4916-a9f5-4bef28108da7
Carr, Nealy
946bac0c-259f-4fc3-adae-56363ce615ea
Mahaffey, Claire
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Lohan, Maeve C.
6ca10597-2d0f-40e8-8e4f-7619dfac5088
Achterberg, Eric P.
685ce961-8c45-4503-9f03-50f6561202b9
Gledhill, Martha
da795c1e-1489-4d40-9df1-fc6bde54382d
Zhu, Kechen
12818c71-a8ff-49a0-9738-afcdb082d6b2
Birchill, Antony J.
c922fcef-e0f9-48e4-a9b0-69a1b76c505a
Milne, Angela
39b44fbd-8f5e-40ea-80c9-244c20998020
Ussher, Simon
32006952-df4b-4364-8aae-7b9be1f39745
Humphreys, Matthew P.
5edc45a6-2f03-4916-a9f5-4bef28108da7
Carr, Nealy
946bac0c-259f-4fc3-adae-56363ce615ea
Mahaffey, Claire
6c868a01-2c9b-48b0-ae08-9ecf6b870032
Lohan, Maeve C.
6ca10597-2d0f-40e8-8e4f-7619dfac5088
Achterberg, Eric P.
685ce961-8c45-4503-9f03-50f6561202b9
Gledhill, Martha
da795c1e-1489-4d40-9df1-fc6bde54382d

Zhu, Kechen, Birchill, Antony J., Milne, Angela, Ussher, Simon, Humphreys, Matthew P., Carr, Nealy, Mahaffey, Claire, Lohan, Maeve C., Achterberg, Eric P. and Gledhill, Martha (2021) Equilibrium calculations of iron speciation and apparent iron solubility in the Celtic Sea at ambient seawater pH using the NICA-Donnan model. Marine Chemistry, 237, [104038]. (doi:10.1016/j.marchem.2021.104038).

Record type: Article

Abstract

We used a combined ion pairing - organic matter speciation model (NICA-Donnan) to predict the organic complexation of iron (Fe) at ambient pH and temperature in the Celtic Sea. We optimized our model by direct comparison with Fe speciation determined by Adsorptive Cathodic Stripping Voltammetry using the added Fe-binding ligand 1-nitroso-2-naphthol (HNN) in the presence and absence of natural organic matter. We compared determined Fe speciation with simulated titrations obtained via application of the NICA-Donnan model with four different NICA parameter sets representing a range of binding site strengths and heterogeneities. We tested the assumption that binding sites scale to dissolved organic carbon (DOC) concentrations in marine waters. We found that a constant low DOC concentration resulted in an improved fit of our titration data to the simulated titrations, suggesting that inputs of autochthonous marine DOM may not increase the heterogeneity or concentrations of Fe binding sites. Using the optimal parameter set, we calculated pFe(III)´ (−log(∑Fe(OH) i 3−i)) and apparent Fe(III) solubility (SFe(III) app) at ambient pH and temperature in the water column of the Celtic Sea. SFe(III) app was defined as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to DOM formed at a free Fe (Fe 3+) concentration equal to the limiting solubility of Fe hydroxide (Fe(OH) 3(s)). SFe(III) app was within range of the determined dissolved Fe concentrations observed after winter mixing on the shelf and in waters >1500 m depth at our most offshore stations. Our study supports the hypothesis that the ocean dissolved Fe inventory is controlled by the interplay between Fe solubility and Fe binding by organic matter, although the overall number of metal binding sites in the marine environment may not be directly scalable to DOC concentrations.

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Equilibrium Calculations of Iron Speciation - Accepted Manuscript
Restricted to Repository staff only until 11 May 2023.
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More information

Accepted/In Press date: 17 September 2021
e-pub ahead of print date: 11 November 2021
Published date: 20 December 2021
Keywords: Intrinsic binding constants, Ocean acidification, Trace metals

Identifiers

Local EPrints ID: 454115
URI: http://eprints.soton.ac.uk/id/eprint/454115
ISSN: 0304-4203
PURE UUID: 038288cc-d88d-4f1c-b5b0-319a3cfa3708
ORCID for Maeve C. Lohan: ORCID iD orcid.org/0000-0002-5340-3108

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Date deposited: 31 Jan 2022 17:49
Last modified: 28 Apr 2022 02:08

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Contributors

Author: Kechen Zhu
Author: Antony J. Birchill
Author: Angela Milne
Author: Simon Ussher
Author: Matthew P. Humphreys
Author: Nealy Carr
Author: Claire Mahaffey
Author: Maeve C. Lohan ORCID iD
Author: Martha Gledhill

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