Experimental investigation of the effects of temperature and ionic strength on Mo isotope fractionation during adsorption to manganese oxides

Laura E. Wasylenki, Bryan A. Rolfe, Colin L. Weeks, Thomas G. Spiro, Ariel Anbar

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Abstract

The Mo stable isotope system is being applied to study changes in ocean redox. Such applications implicitly assume that Mo isotope fractionation in aqueous systems is relatively insensitive to frequently changing environmental variables such as temperature (T) and ionic strength (I). A major driver of fractionation is the adsorption of Mo to Mn oxyhydroxide surfaces [Barling J. and Anbar A. D. (2004) Molybdenum isotope fractionation during adsorption by manganese oxides. Earth Planet. Sci. Lett. 217(3-4), 315-329]. Here, we report the results of experiments that determine the extent to which Mo isotope fractionation during adsorption of Mo to the Mn oxyhydroxide mineral birnessite is sensitive to T and I. The results are compared to new predictions from quantum chemical computations. We measured fractionation from 1 to 50 °C at I = 0.1 m and found that Δ97/95Modissolved-adsorbed varies from 1.9‰ to 1.6‰ over this temperature range. Experiments were also performed at 25 °C in synthetic seawater (I = 0.7); fractionation at this condition was the same within analytical error as in low ionic strength experiments. These findings confirm that the Mo isotope fractionation during adsorption to Mn oxyhydroxides is relatively insensitive to variations and T and I over environmentally relevant ranges. To relate these findings to potential mechanisms of Mo isotope fractionation, we also report results for density functional theory computations of the fractionation between MoO4 2 - and various possible structures of molybdic acid as a function of temperature. Because no plausible species fractionates from MoO4 2 - with a magnitude matching the experiments, we are left with three possibilities to explain the fractionation: (1) solvation effects on the vibrational frequencies of aqueous species considered thus far are significant, such that our calculations in vacuo yield inaccurate fractionations; (2) a trace aqueous species not yet considered fractionates from MoO4 2 - and then adsorbs to birnessite; or (3) a surface complex not present in solution forms on birnessite in which Mo is not tetrahedrally coordinated. Our findings help validate assumptions underlying paleoceanographic applications of the Mo isotope system and also lead us closer to understanding the mechanism of isotope fractionation during adsorption of Mo to Mn oxyhydroxides.

Original languageEnglish (US)
Pages (from-to)5997-6005
Number of pages9
JournalGeochimica et Cosmochimica Acta
Volume72
Issue number24
DOIs
StatePublished - Dec 15 2008

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manganese oxide
Fractionation
Ionic strength
Isotopes
fractionation
isotope
adsorption
Adsorption
temperature
birnessite
Temperature
effect
experiment
Experiments
Molybdenum
Solvation
Vibrational spectra
Planets
molybdenum
Seawater

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

Experimental investigation of the effects of temperature and ionic strength on Mo isotope fractionation during adsorption to manganese oxides. / Wasylenki, Laura E.; Rolfe, Bryan A.; Weeks, Colin L.; Spiro, Thomas G.; Anbar, Ariel.

In: Geochimica et Cosmochimica Acta, Vol. 72, No. 24, 15.12.2008, p. 5997-6005.

Research output: Contribution to journalArticle

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AU - Weeks, Colin L.

AU - Spiro, Thomas G.

AU - Anbar, Ariel

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N2 - The Mo stable isotope system is being applied to study changes in ocean redox. Such applications implicitly assume that Mo isotope fractionation in aqueous systems is relatively insensitive to frequently changing environmental variables such as temperature (T) and ionic strength (I). A major driver of fractionation is the adsorption of Mo to Mn oxyhydroxide surfaces [Barling J. and Anbar A. D. (2004) Molybdenum isotope fractionation during adsorption by manganese oxides. Earth Planet. Sci. Lett. 217(3-4), 315-329]. Here, we report the results of experiments that determine the extent to which Mo isotope fractionation during adsorption of Mo to the Mn oxyhydroxide mineral birnessite is sensitive to T and I. The results are compared to new predictions from quantum chemical computations. We measured fractionation from 1 to 50 °C at I = 0.1 m and found that Δ97/95Modissolved-adsorbed varies from 1.9‰ to 1.6‰ over this temperature range. Experiments were also performed at 25 °C in synthetic seawater (I = 0.7); fractionation at this condition was the same within analytical error as in low ionic strength experiments. These findings confirm that the Mo isotope fractionation during adsorption to Mn oxyhydroxides is relatively insensitive to variations and T and I over environmentally relevant ranges. To relate these findings to potential mechanisms of Mo isotope fractionation, we also report results for density functional theory computations of the fractionation between MoO4 2 - and various possible structures of molybdic acid as a function of temperature. Because no plausible species fractionates from MoO4 2 - with a magnitude matching the experiments, we are left with three possibilities to explain the fractionation: (1) solvation effects on the vibrational frequencies of aqueous species considered thus far are significant, such that our calculations in vacuo yield inaccurate fractionations; (2) a trace aqueous species not yet considered fractionates from MoO4 2 - and then adsorbs to birnessite; or (3) a surface complex not present in solution forms on birnessite in which Mo is not tetrahedrally coordinated. Our findings help validate assumptions underlying paleoceanographic applications of the Mo isotope system and also lead us closer to understanding the mechanism of isotope fractionation during adsorption of Mo to Mn oxyhydroxides.

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