Constraining silicon isotope exchange kinetics and fractionation between aqueous and amorphous Si at room temperature

Xin Yuan Zheng, Brian L. Beard, Clark M. Johnson

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Abstract

Silicon (Si) isotopes are useful tracers for the modern and ancient Si cycle, but their interpretation is limited by inadequate understanding of Si isotope exchange kinetics and fractionation factors at low temperature. This study investigated Si isotope exchange and fractionation between aqueous and amorphous Si at circumneutral pH and room temperature through a series of 29 Si-spiked isotope-exchange experiments. Four different amorphous Si solids with varied surface areas were reacted with aqueous Si solutions of high ionic strength similar to seawater, or low ionic strength typical of freshwater, under conditions close to chemical equilibrium with respect to amorphous Si solubility. In contrast to the common perception of negligible Si isotope exchange at low temperature, ∼50–85% isotope exchange was achieved between aqueous and amorphous Si within ∼60 days. Larger solid surface areas and higher aqueous ionic strength generally promoted Si isotope exchange. Drying/aging of Si gel, however, impedes Si isotope exchange between amorphous and aqueous Si relative to freshly prepared Si gels. Excluding the experiments that used the aged Si gel, temporal trajectories of Si isotope evolution of the two phases from all other experiments showed significant curvature in three-isotope space ( 29 Si/ 28 Si and 30 Si/ 28 Si). These results can be best explained by a model that comprises two Si isotope exchange processes with different exchange rates and fractionation factors during the interactions between aqueous and amorphous Si towards isotope equilibrium. The faster exchange is associated with surface sites, and slower exchange occurs between exterior and interior Si atoms of the solid. Exchange with surface sites tends to partition heavy Si isotopes in the aqueous phase relative to the solid surface, whereas exchange between surface and interior sites in the solid tends to enrich heavy Si isotopes in the interior. Two experiments that achieved >80% isotope exchange provided the best estimates of equilibrium Si isotope fractionation factors between bulk amorphous Si solid and aqueous monomeric silicic acid H 4 SiO 430 Si amorphous–aqueous ) at 23 °C: +0.52‰ (±0.15‰, 1sd) at seawater ionic strength, and −0.98‰ (±0.12‰) at freshwater ionic strength. The observed “salt effect” on Si isotope exchange kinetics and fractionation factor is interpreted to reflect an influence of cations on Si speciation of solid surfaces. This work highlights the value of three-isotope method in studying both reaction kinetics and isotope fractionation mechanisms. The observed Si isotope exchange between amorphous and aqueous Si at low temperature implies that Si isotope re-equilibration, a previously neglected process, may be important in controlling Si isotope compositions of natural samples.

Original languageEnglish (US)
Pages (from-to)267-289
Number of pages23
JournalGeochimica et Cosmochimica Acta
Volume253
DOIs
StatePublished - May 15 2019
Externally publishedYes

Bibliographical note

Funding Information:
This work was supported by the NASA Astrobiology Institute under grant NNA13AA94A . We thank Mike Spicuzza for providing the “Old Mike” gel (Solid–a) used in the experiments, Eric Roden for providing access to the orbital shaker in his lab, Seungyeol Lee for help with the BET-analyzer, and Franklin Hobbs for the XRD analysis. Three anonymous reviewers are thanked for their helpful comments and we thank Professor Anthony Dosseto for editorially handling this manuscript.

Keywords

  • Si isotopes
  • fractionation factors
  • isotope exchange kinetics
  • low-temperature isotope experiments
  • three-isotope method

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