Dynamic dissolution of Cm3+ ions incorporated at the calcite-water interface: an ab initio molecular dynamics simulation study.

The stability of actinide-mineral solid solution in a water environment is critical for assessing the safety of nuclear-waste geological repositories and studying actinide migration in natural systems. However, the dissolution behavior of actinide ions incorporated at the mineral-water interface is still unclear at the atomic level. Herein, we present metadynamics simulations of the reaction pathways, thermodynamics and kinetics of trivalent curium ions (Cm3+) dissolving from calcite surfaces. Cm3+ ions incorporated in different calcite surfaces (i.e., terrace and stepped surfaces) with distinct coordination environments have different reaction pathways, free energy barriers and free energy changes. We found that Cm dissolution from a stepped surface is more favorable than that from a terrace surface, both thermodynamically and kinetically. In addition, water molecules seem to promote the detachment of curium ions from the surface by exerting a pulling force via water coordination with Cm3+ and a pushing force via proton migration to the surface layer and water diffusion in the vacant Cm site. Thus, the findings from this work prove to be a milestone in revealing the dynamic dissolution mechanism of trivalent actinides from minerals and would also help predict the dissolution behaviors of other metal ions at the solid-water interface in chemical and environmental sciences.

Theoretical insights into the coordination structures, stabilities and electronic spectra of Cm3+ species at the gibbsite-water interface: A computational study combing ab initio molecular dynamics and wave function theory.

The information of structure and stability of actinide species is key to understand the sorption mechanism of actinides at mineral-water interface. Such information is approximately derived from experimental spectroscopic measurements and needs to be accurately obtained by a direct atomic-scale modelling. Herein, systematic first-principles calculations and ab initio molecular dynamics (AIMD) simulations are carried out to study the coordination structures and absorption energies of Cm(III) surface complexes at gibbsite-water interface. Eleven representative complexing sites are investigated. The most stable Cm3+ sorption species are predicted to be a tridentate surface complex in weakly acidic/neutral solution condition and a bidentate one in the alkaline solution condition. Moreover, luminescence spectra of the Cm3+ aqua ion and the two surface complexes are predicted based on the high-accuracy ab initio wave function theory (WFT). The results give a gradually decreasing emission energy in good agreement with experimental observation of a red shift of peak maximum with pH increasing from 5 to 11. This work is a comprehensive computational study involving AIMD and ab initio WFT methods to gain the coordination structures, stabilities, and electronic spectra of actinide sorption species at the mineral-water interface, thus providing important theoretical support for geological disposal of actinide waste.

Two-dimensional Zr/Hf-hydroxamate metal-organic frameworks.

Novel two-dimensional kagome metal-organic frameworks with mononuclear Zr4+/Hf4+ nodes chelated by benzene-1,4-dihydroxamate linkers were synthesized. The MOFs, namely SUM-1, are chemically robust and kinetically favorable, as confirmed by theoretical and experimental studies. SUM-1(Zr) can be readily made into large (∼100 µm) single crystals and nanoplates (∼50 nm), constituting a versatile MOF platform.