Transport mechanism of uranyl nitrate in nanopore of geopolymer with different Si/Al ratios: Molecular dynamics simulation

Geopolymer is an amorphous cementitious material produced by geopolymerization using minerals or industrial waste as the main raw material under the action of activators. It can be used for safe treatment of heavy metal and radioactive pollution. This study applied molecular dynamics simulation to investigate the transport behaviour of uranyl nitrate in nanochannel of geopolymer, and focused on the influence of changes in the Si/Al ratio of geopolymer on the adsorption and diffusion of ions and water molecules. The simulation results described by density distribution, radial distribution function (RDF), hydrogen bonding network structure, mean square displacement (MSD) and diffusion coefficient. The density distribution shows that geopolymer has the strongest adsorption capacity for UO22+, followed by NO3−, and finally H2O. The results also indicate that the changes in molecular composition of geopolymer have a significant impact on kinetic characteristics of UO22+, NO3− and H2O. As the Si/Al ratio decreases, the adsorption capacity of geopolymer shows an upward trend, with the most significant improvement in the adsorption of UO22+. In addition, when the Si/Al ratio reduces, the migration rate of UO22+ and NO3− slows down. It could hinder the diffusion of ions in nanopores of geopolymer. The work could optimize the preparation of geopolymer adsorbent, and reveal the transport mechanism of uranyl nitrate in nanopore of geopolymer with different Si/Al ratios at the atomic scale.