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

Kechen Zhu; Antony J. Birchill; Angela Milne; Simon Ussher; Matthew P. Humphreys; Nealy Carr; Claire Mahaffey; Maeve C. Lohan; Eric P. Achterberg; Martha Gledhill

doi:10.1016/j.marchem.2021.104038

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

Kechen Zhu, Antony J. Birchill, Angela Milne, Simon Ussher, Matthew P. Humphreys, Nealy Carr, Claire Mahaffey, Maeve C. Lohan, Eric P. Achterberg, Martha Gledhill^*

^*Corresponding author for this work

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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)_i³⁻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 (Fe³⁺) 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.

Original language	English
Article number	104038
Number of pages	15
Journal	Marine Chemistry
Volume	237
Early online date	6 Oct 2021
DOIs	https://doi.org/10.1016/j.marchem.2021.104038
Publication status	Published - 20 Dec 2021

Keywords

intrinsic binding constants
ocean acidification
trace metals
UKRI
NERC
NE/L501840/1
NE/K001779/1
NE/K002007/1
NE/K001973/1

UN SDGs

This output contributes to the following UN Sustainable Development Goals (SDGs)

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10.1016/j.marchem.2021.104038

Zhu_et_al _manuscript_2021_AAMAccepted author manuscript (Post-print), 517 KBLicence: CC BY-NC-ND

https://pearl.plymouth.ac.uk/handle/10026.1/18030

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Zhu, K., Birchill, A. J., Milne, A., Ussher, S., Humphreys, M. P., Carr, N., Mahaffey, C., Lohan, M. C., Achterberg, E. P., & Gledhill, M. (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, Article 104038. https://doi.org/10.1016/j.marchem.2021.104038

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

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)i3−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 (Fe3+) 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.",

keywords = "intrinsic binding constants, ocean acidification, trace metals, UKRI, NERC, NE/L501840/1, NE/K001779/1, NE/K002007/1, NE/K001973/1",

author = "Kechen Zhu and Birchill, {Antony J.} and Angela Milne and Simon Ussher and Humphreys, {Matthew P.} and Nealy Carr and Claire Mahaffey and Lohan, {Maeve C.} and Achterberg, {Eric P.} and Martha Gledhill",

note = "Funding Information: The authors would like to thank Bert-Jan Groenenberg for his help with PEST-ORCHESTRA. We thank the captain and crew of the RSS Discovery. KZ was supported by a scholarship from the China Scholarship Council . The project was funded by the UK Natural Environment Research Council (NE/L501840/1 (A.B.), NE/K001779/1 (M.L., S.U., A.M.), NE/K002007/1 (N·C and C.M.), NE/K001973/1 (E.A. and M.G.) and the Helmholtz Association. The authors declare no competing financial interest. All data that supports the findings of this study have been submitted to the British Oceanography Data Centre. Raw titration data can be downloaded from dx.doi.org/10.17504/protocols.io.brc4m2yw . Publisher Copyright: {\textcopyright} 2021 Elsevier B.V.",

year = "2021",

month = dec,

day = "20",

doi = "10.1016/j.marchem.2021.104038",

language = "English",

volume = "237",

journal = "Marine Chemistry",

issn = "0304-4203",

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TY - JOUR

T1 - Equilibrium calculations of iron speciation and apparent iron solubility in the Celtic Sea at ambient seawater pH using the NICA-Donnan model

AU - Zhu, Kechen

AU - Birchill, Antony J.

AU - Milne, Angela

AU - Ussher, Simon

AU - Humphreys, Matthew P.

AU - Carr, Nealy

AU - Mahaffey, Claire

AU - Lohan, Maeve C.

AU - Achterberg, Eric P.

AU - Gledhill, Martha

N1 - Funding Information: The authors would like to thank Bert-Jan Groenenberg for his help with PEST-ORCHESTRA. We thank the captain and crew of the RSS Discovery. KZ was supported by a scholarship from the China Scholarship Council . The project was funded by the UK Natural Environment Research Council (NE/L501840/1 (A.B.), NE/K001779/1 (M.L., S.U., A.M.), NE/K002007/1 (N·C and C.M.), NE/K001973/1 (E.A. and M.G.) and the Helmholtz Association. The authors declare no competing financial interest. All data that supports the findings of this study have been submitted to the British Oceanography Data Centre. Raw titration data can be downloaded from dx.doi.org/10.17504/protocols.io.brc4m2yw . Publisher Copyright: © 2021 Elsevier B.V.

PY - 2021/12/20

Y1 - 2021/12/20

N2 - 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)i3−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 (Fe3+) 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.

AB - 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)i3−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 (Fe3+) 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.

KW - intrinsic binding constants

KW - ocean acidification

KW - trace metals

KW - UKRI

KW - NERC

KW - NE/L501840/1

KW - NE/K001779/1

KW - NE/K002007/1

KW - NE/K001973/1

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U2 - 10.1016/j.marchem.2021.104038

DO - 10.1016/j.marchem.2021.104038

M3 - Article

AN - SCOPUS:85116823970

SN - 0304-4203

VL - 237

JO - Marine Chemistry

JF - Marine Chemistry

M1 - 104038

ER -

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

Abstract

Keywords

UN SDGs

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