Redox reactions and potentials in natural waters at disequilibrium

Andri Stefánsson; Stefán Arnórsson; Árný E. Sveinbjörnsdóttir

doi:10.1016/j.chemgeo.2005.06.003

Redox reactions and potentials in natural waters at disequilibrium

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Abstract

The study investigated the redox chemistry of natural non-thermal surface waters and 0 to 90 °C ground waters in North Iceland. The waters are dilute with dissolved solid concentrations between 6 and 431 ppm and a pH ranging from 6.3 to 10.4. The redox potential was calculated for the following redox couples based on measured concentration of a given element in a given oxidation state: Fe³⁺/Fe²⁺, O₂/H₂O, H₂S/SO₄, NO₃/NO₂, NO₃/NH₄ and NO₂/NH₄. The redox potentials differed by up to 1200 mV between couples, indicating redox disequilibrium in the waters. The differences were independent of water age, which ranged from modern to 25,000 years, but was dependent on water type. Based on these findings it is concluded that the redox chemistry of natural open aqueous systems are controlled by steady state between supply of the elements to the system and the kinetics of reactions involving these elements. The concentrations of individual elements like O₂ and Fe may be controlled by equilibrium of kinetically fast reactions, whereas an overall redox equilibrium will not be reached until the system is completely closed. The measured in situ electrode potential with a Pt electrode in a disequilibrium system represents a mixed potential. In waters with low concentrations of redox species and low exchange currents the measured potential with the Pt electrode is related to a mixed potential arising from redox reactions on the Pt electrode surface and other redox reactions. We conclude that the measured redox potential with Pt electrodes in dilute natural waters has limited meaning and that it can not be used for quantitative modeling of the transport properties and the aqueous speciation of redox sensitive elements from bulk analysis only. In natural waters at redox disequilibrium, aqueous speciation of redox sensitive elements must be determined by independent measurement of each oxidation state of a given element and by solving separately mass balance and mass action equations for each oxidation state.

Original language	English
Pages (from-to)	289-311
Number of pages	23
Journal	Chemical Geology
Volume	221
Issue number	3-4
DOIs	https://doi.org/10.1016/j.chemgeo.2005.06.003
Publication status	Published - 5 Oct 2005

Bibliographical note

Funding Information: The present study forms a part of a research project on water–rock interaction supported by the Research Fund of the University of Iceland and the Science Fund of the National Research Council, Iceland. We are grateful to B. Gudmundsson and I. Gunnarsson for their help with the database and the aqueous speciation calculations. We thank the two reviewers for their constructive reviews. [LMW]

Other keywords

Aqueous speciation
Disequilibrium
Ground water
Redox potential
Redox reactions
Surface water

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10.1016/j.chemgeo.2005.06.003

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@article{360229e9dd2245d09f0091eb0bcd8702,

title = "Redox reactions and potentials in natural waters at disequilibrium",

abstract = "The study investigated the redox chemistry of natural non-thermal surface waters and 0 to 90 °C ground waters in North Iceland. The waters are dilute with dissolved solid concentrations between 6 and 431 ppm and a pH ranging from 6.3 to 10.4. The redox potential was calculated for the following redox couples based on measured concentration of a given element in a given oxidation state: Fe3+/Fe2+, O2/H2O, H2S/SO4, NO3/NO2, NO3/NH4 and NO2/NH4. The redox potentials differed by up to 1200 mV between couples, indicating redox disequilibrium in the waters. The differences were independent of water age, which ranged from modern to 25,000 years, but was dependent on water type. Based on these findings it is concluded that the redox chemistry of natural open aqueous systems are controlled by steady state between supply of the elements to the system and the kinetics of reactions involving these elements. The concentrations of individual elements like O2 and Fe may be controlled by equilibrium of kinetically fast reactions, whereas an overall redox equilibrium will not be reached until the system is completely closed. The measured in situ electrode potential with a Pt electrode in a disequilibrium system represents a mixed potential. In waters with low concentrations of redox species and low exchange currents the measured potential with the Pt electrode is related to a mixed potential arising from redox reactions on the Pt electrode surface and other redox reactions. We conclude that the measured redox potential with Pt electrodes in dilute natural waters has limited meaning and that it can not be used for quantitative modeling of the transport properties and the aqueous speciation of redox sensitive elements from bulk analysis only. In natural waters at redox disequilibrium, aqueous speciation of redox sensitive elements must be determined by independent measurement of each oxidation state of a given element and by solving separately mass balance and mass action equations for each oxidation state.",

keywords = "Aqueous speciation, Disequilibrium, Ground water, Redox potential, Redox reactions, Surface water",

author = "Andri Stef{\'a}nsson and Stef{\'a}n Arn{\'o}rsson and Sveinbj{\"o}rnsd{\'o}ttir, \{{\'A}rn{\'y} E.\}",

note = "Funding Information: The present study forms a part of a research project on water–rock interaction supported by the Research Fund of the University of Iceland and the Science Fund of the National Research Council, Iceland. We are grateful to B. Gudmundsson and I. Gunnarsson for their help with the database and the aqueous speciation calculations. We thank the two reviewers for their constructive reviews. [LMW]",

year = "2005",

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doi = "10.1016/j.chemgeo.2005.06.003",

language = "English",

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pages = "289--311",

journal = "Chemical Geology",

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TY - JOUR

T1 - Redox reactions and potentials in natural waters at disequilibrium

AU - Stefánsson, Andri

AU - Arnórsson, Stefán

AU - Sveinbjörnsdóttir, Árný E.

N1 - Funding Information: The present study forms a part of a research project on water–rock interaction supported by the Research Fund of the University of Iceland and the Science Fund of the National Research Council, Iceland. We are grateful to B. Gudmundsson and I. Gunnarsson for their help with the database and the aqueous speciation calculations. We thank the two reviewers for their constructive reviews. [LMW]

PY - 2005/10/5

Y1 - 2005/10/5

N2 - The study investigated the redox chemistry of natural non-thermal surface waters and 0 to 90 °C ground waters in North Iceland. The waters are dilute with dissolved solid concentrations between 6 and 431 ppm and a pH ranging from 6.3 to 10.4. The redox potential was calculated for the following redox couples based on measured concentration of a given element in a given oxidation state: Fe3+/Fe2+, O2/H2O, H2S/SO4, NO3/NO2, NO3/NH4 and NO2/NH4. The redox potentials differed by up to 1200 mV between couples, indicating redox disequilibrium in the waters. The differences were independent of water age, which ranged from modern to 25,000 years, but was dependent on water type. Based on these findings it is concluded that the redox chemistry of natural open aqueous systems are controlled by steady state between supply of the elements to the system and the kinetics of reactions involving these elements. The concentrations of individual elements like O2 and Fe may be controlled by equilibrium of kinetically fast reactions, whereas an overall redox equilibrium will not be reached until the system is completely closed. The measured in situ electrode potential with a Pt electrode in a disequilibrium system represents a mixed potential. In waters with low concentrations of redox species and low exchange currents the measured potential with the Pt electrode is related to a mixed potential arising from redox reactions on the Pt electrode surface and other redox reactions. We conclude that the measured redox potential with Pt electrodes in dilute natural waters has limited meaning and that it can not be used for quantitative modeling of the transport properties and the aqueous speciation of redox sensitive elements from bulk analysis only. In natural waters at redox disequilibrium, aqueous speciation of redox sensitive elements must be determined by independent measurement of each oxidation state of a given element and by solving separately mass balance and mass action equations for each oxidation state.

AB - The study investigated the redox chemistry of natural non-thermal surface waters and 0 to 90 °C ground waters in North Iceland. The waters are dilute with dissolved solid concentrations between 6 and 431 ppm and a pH ranging from 6.3 to 10.4. The redox potential was calculated for the following redox couples based on measured concentration of a given element in a given oxidation state: Fe3+/Fe2+, O2/H2O, H2S/SO4, NO3/NO2, NO3/NH4 and NO2/NH4. The redox potentials differed by up to 1200 mV between couples, indicating redox disequilibrium in the waters. The differences were independent of water age, which ranged from modern to 25,000 years, but was dependent on water type. Based on these findings it is concluded that the redox chemistry of natural open aqueous systems are controlled by steady state between supply of the elements to the system and the kinetics of reactions involving these elements. The concentrations of individual elements like O2 and Fe may be controlled by equilibrium of kinetically fast reactions, whereas an overall redox equilibrium will not be reached until the system is completely closed. The measured in situ electrode potential with a Pt electrode in a disequilibrium system represents a mixed potential. In waters with low concentrations of redox species and low exchange currents the measured potential with the Pt electrode is related to a mixed potential arising from redox reactions on the Pt electrode surface and other redox reactions. We conclude that the measured redox potential with Pt electrodes in dilute natural waters has limited meaning and that it can not be used for quantitative modeling of the transport properties and the aqueous speciation of redox sensitive elements from bulk analysis only. In natural waters at redox disequilibrium, aqueous speciation of redox sensitive elements must be determined by independent measurement of each oxidation state of a given element and by solving separately mass balance and mass action equations for each oxidation state.

KW - Aqueous speciation

KW - Disequilibrium

KW - Ground water

KW - Redox potential

KW - Redox reactions

KW - Surface water

UR - https://www.scopus.com/pages/publications/25144432344

U2 - 10.1016/j.chemgeo.2005.06.003

DO - 10.1016/j.chemgeo.2005.06.003

M3 - Article

SN - 0009-2541

VL - 221

SP - 289

EP - 311

JO - Chemical Geology

JF - Chemical Geology

IS - 3-4

ER -

Redox reactions and potentials in natural waters at disequilibrium

Abstract

Bibliographical note

Other keywords

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