Nitrogen Isotopic Fractionation in Ammonia during Adsorption on Silicate Surfaces

Abstract

Adsorption is a fundamental phenomenon that occurs at various interfaces; however, the isotopic fractionation in stable isotopes associated with this process has not yet been well documented for most molecules. In this study, we conducted ammonia adsorption experiments on two silicate minerals, montmorillonite and saponite, to determine the nitrogen isotopic fractionation during the process. Ammonia adsorbed on these minerals is up to +44‰ enriched in 15N relative to initial ammonia. The degree of 15N enrichment has a negative correlation with the adsorption ratio of ammonia. These enrichments are remarkably large compared to those reported in other physicochemical (e.g., evaporation) or biological (e.g., enzymatic reaction) processes. On the basis of these results, we can predict that preferential accumulation of 15NH3 occurs by adsorption on mineral surfaces, which may explain the heterogeneity of the 15N/14N ratio in the solar system.