(757h) Reaxff Reactive Force Field Study of Cesium Adsorption in Micaceous Clay Minerals

Authors: 
Raju, M., The Pennsylvania State University
van Duin, A., RxFF_Consulting, LLC
van Duin, D., RxFF_Consulting, LLC
Nakamura, H., Japan Atomic Energy Agency
Okumura, M., Japan Atomic Energy Agency
Machida, M., Japan Atomic Energy Agency
Among the radionuclides released into the environment during the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, 137Cs is the most significant long-term contributor to environmental contamination due to its high release rate (~ 1.3 × 1016 Bq) and longer half-life (30.1 years). High affinity of Cs+ absorption in micaceous clay minerals through ion-exchange with native interlayer cations is well known. Knowledge of Cs binding energetics to various clay mineral surfaces is critical to developing effective decontamination strategies. We perform metadynamics simulations to study the adsorption of Cs on the (001), (010) and (110) surfaces of kaolinite, vermiculite and muscovite clay minerals as a function of temperature in pure and marine water under acidic/basic conditions, using a recently developed ReaxFF reactive force field. Our results demonstrate that the adsorption of Cs on various clay mineral surfaces depends critically on the surface corrugation, coordination of the surface atoms and their mutual separations and arrangement in space. The free-energy profiles from the metadynamics simulations were used to calculate the solid-liquid partition coefficient Kd for Cs on the various clay mineral surfaces. Cs was found to adsorb the strongest on the (110) surfaces, followed by the (001) and (010) clay mineral surfaces. Marine solution at pH 7 is the most energetically favorable environment to remove Cs from the studied clay surfaces. Weathered micaceous clay minerals are known to play a crucial role on adsorption and retention of cesium. They have special adsorption sites, called frayed edge sites (FESs), which adsorb cesium selectively and irreversibly. We investigate Cs adsorption at FESs in muscovite surface at various interlayer separations using metadynamics simulations to obtain the Cs/K exchange energies. We observe that Cs adsorption at FESs is significantly more energetically favorable (up to a factor of 10) compared to adsorption on the unweathered clay mineral surface. A critical interlayer separation is necessary for Cs uptake in clay minerals. At lower interlayer separation Cs/K exchange is unfavorable and at wider interlayer separations the FESs have adsorption sites suitable for Cs ions that have a larger radius than the K ions. We reveal the adsorption mechanism of Cs at FESs and show that multiple distinct intermediate states can occur during Cs uptake in clay minerals.