Atomistic Insights into the Interlayer Cation and Water Structures of Na-, K-, and Cs-Birnessite

Abstract

Birnessite is a key mineral involved in the geochemical cycles of trace metals through its high cation sorption capacity. Information about cation and water structures in the interlayer regions, as well as about the layer spacing and associated changes, is essential to understanding the cation exchange mechanism. However, this fundamental knowledge is lacking for birnessite due to the complexity of its structure and the insufficient ability to probe the interlayer species, water molecules in particular. In this study, we performed molecular dynamics (MD) computer simulations of Na-, K-, and Cs-birnessite as a function of the interlayer hydration state and cation exchange ratio. As the monolayer hydrate state transitioned to a bilayer hydrate state, the d-spacing of Na-birnessite increased from ∼7 to ∼10 Å, Na+ cations maintained a position at the midplane of the interlayer, and water molecules were reoriented to form intermolecular H-bonds rather than H-bonds with birnessite surfaces. In the bilayer hydrate states of K- and Cs-birnessite, the interlayer cations were positioned near the birnessite surface rather than the midplane of the interlayer. As the K+ ratio increased in Na0.29–xKxMnO2·0.75H2O, the d-spacing contracted in the initial stage of cation exchange but increased in the subsequent stage. In Na0.29–xCsxMnO2·0.75H2O, the d-spacing expanded continuously with an increasing Cs+-to-Na+ ratio. In our MD simulations, interlayer water molecules needed to be deintercalated during cation exchange to reproduce the experimentally observed trends in d-spacing changes of Na-birnessite during cation exchange. These results suggest the important role of interlayer water in the cation sorption of birnessite that is often neglected in understanding the birnessite reactivity.