Atom-Scale Understanding the Adsorption Mechanism of Uranyl in the Interlayer of Montmorillonite: Insight from Dft+U Calculation

Abstract

In this work, the adsorption mechanism of uranyl with varied water ligands (UO22+·xH2O; x = 3, 4 and 5) in the interlayer of montmorillonite (MMT) was studied theoretically based on the first-principles DFT + U calculation. The adsorption energy, adsorption geometry configuration and electronic properties (PDOS, electron density difference, Bader charge, ELF) of MMT-UO22+·xH2O systems were investigated systematically. It was found that UO22+·5H2O and UO22+·3H2O are more likely adsorbed above OM (the bridging oxygen on the six-membered ring of SiO4 tetrahedra sheet, signed as T site) and UO22+·4H2O is preferentially adsorbed between OM atoms (signed as B site). DFT+U calculations showed that the migration of uranyl in the interlayer of MMT can be inhibited by forming hydrogen bonds through OM and water ligands. In addition, with the decrease of water ligands, the positive charge of UO22+ increases and is directly exposed to the internal surface of MMT. Resultantly, the U atom in UO22+ can form weak chemical bond with OM, thus blocking the migration of UO22+. In summary, this work can provide insights into the adsorption of uranyl in the interlayer of MMT at the microscopic atom-scale, which contributes to improving the experimentally based internal surface complexation model.

Keywords: Montmorillonite, Uranyl, Interlayer, Adsorption mechanism, DFT+U