A XAFS study on the mechanism of isotopic fractionation of molybdenum during its adsorption on ferromanganese oxides

Abstract

Due to its potential as a paleoredox proxy, there have been many studies on Mo isotopic fractionation during adsorption onto ferromanganese oxides in seawater. However, the mechanisms of both adsorption and isotopic fractionation are still under debate due to the lack of structural information on the adsorbed species. In this study, XAFS analyses were performed to reveal the mechanism, based on structural information at the molecular level, of Mo isotopic fractionation during adsorption onto ferromanganese oxides. Molybdenum L 3 -edge XANES and K-edge EXAFS revealed that Mo species adsorbed on the surface of ferrihydrite was a tetrahedrally coordinated outer-sphere complex, while that on δ-MnO 2 was an octahedrally coordinated inner-sphere complex. Additionally, it was also revealed that δ-MnO 2 was the dominant host phase of Mo in the hydrogenetic ferromanganese nodules from the comparison of their XAFS spectra. Previous studies reported that lighter isotopes of Mo were preferentially incorporated into ferromanganese oxides from seawater. This fractionation can be explained based on the structural difference between tetrahedral MoO 4 2-(=a major species in seawater) and the octahedral species adsorbed on the Mn oxide phase in ferromanganese nodules. In contrast, little change in Mo local structures during its adsorption onto ferrihydrite also suggested the little or no fractionation of Mo isotopes in the presence of Fe hydroxides without Mn oxides. These facts imply that the Mo isotopic composition in ancient marine sediments can distinguish redox boundaries of Fe 2+ /Fe(OH) 3 and of the more oxic Mn 2+ /MnO 2 .

The molecular mechanism of Mo isotope fractionation during adsorption to birnessite

“…It is not possible to assess how closely the Kashiwabara et al (2009) fit results matched their own data, since the fits, fitting methods, and statistical assessments of fits were not provided in that publication. When we apply the full model of Kashiwabara et al (2009) to our data, including the two shells of oxygens and the single shell of Mn atoms at 3.04 Å , the result is a poor fit to our BigMo04 spectrum, as shown in Figs. 2 and 3.…”

“…We infer that the difference in coordination environment for Mo between the predominant dissolved species (tetrahedral MoO 2À 4 ) and predominant adsorbed species (polynuclear complex with Mo in distorted octahedral coordination) drives the large Mo isotope effect observed in this system. Kashiwabara et al (2009) independently collected an EXAFS spectrum for Mo sorbed on Mn oxyhydroxide and inferred monomeric complexes with a Mo-Mn distance of 3.04 Å . It is not possible to assess how closely the Kashiwabara et al (2009) fit results matched their own data, since the fits, fitting methods, and statistical assessments of fits were not provided in that publication.…”

Molybdenum isotope fractionations observed under anoxic experimental conditions

“…The importance of the Fe cycle as an additional control on anoxic sediment Mo fractionation, through interactions with either pyrite or iron oxides, is particularly noteworthy as Mo is thought to interact with these phases (Huerta- Diaz and Morse, 1992;Helz et al, 1996Helz et al, , 2004Zheng et al, 2000;Goldberg et al, 2009Goldberg et al, , 2012Kashiwabara et al, 2009). Experimental investigations of molybdate adsorption to Fe-oxides generated a range of Mo isotope fractionations, from 0.8 to 2.2‰ depending upon the Fe-oxide mineralogy (Goldberg et al, 2009;Fig.…”

Molybdenum isotope fractionation in soils: Influence of redox conditions, organic matter, and atmospheric inputs

“…Specific mechanisms that may cause fractionation of Mo isotopes in weathering and soil environments include changes in coordination from tetrahedral to octahedral, kinetic effects during metal-ligand bond dissociation, ion specific diffusion rate differences, and equilibrium effects due to varying bond strengths between metal complexes (Malinovsky et al, 2007a;Goldberg et al, 2009;Kashiwabara et al, 2009). Experimental work on Mo adsorption on Mn and Fe oxyhydroxides, as well as observations in nature, point to the preferential adsorption of light isotopes (Siebert et al, 2003;Barling and Anbar, 2004;Malinovsky et al, 2007b;Wasylenki et al, 2008;Liermann et al, 2011;Goldberg et al, 2012).…”

“…Experimental work on Mo adsorption on Mn and Fe oxyhydroxides, as well as observations in nature, point to the preferential adsorption of light isotopes (Siebert et al, 2003;Barling and Anbar, 2004;Malinovsky et al, 2007b;Wasylenki et al, 2008;Liermann et al, 2011;Goldberg et al, 2012). The molybdate oxyanion adsorbs to the solid phase through an inner sphere bond (ligand-exchange) (Goldberg et al, 1996;Fontes and Coelho, 2005), possibly accompanied by a coordination change from tetrahedral to octahedral (Bibak and Borggard, 1994;Kashiwabara et al, 2009 (Archer and Vance, 2008;Pearce et al, 2010;Scheiderich et al, 2010;Neubert et al, 2011). …”