Exchange of heavy metal ions in titanosilicate Na-ETS-10 membrane from molecular dynamics simulations

The exchange of heavy metal ions Pb2+, Cd2+ and Cu2+ in a titanosilicate Na-ETS-10 membrane is investigated using molecular dynamics simulations. The simulation system consists of Na-ETS-10 membrane and ionic solution in a dielectric continuum. Ion exchange is observed from the temporal variation of ion density profiles. The residence time distributions, mean square displacements and radial distribution functions are determined to characterize the dynamic and structural properties of exchanged ions in the membrane. Pb2+ ions are highly coordinated with Ti atoms and strongly bound with the membrane. Consequently, Pb2+ ions reside in the membrane exclusively and have smallest mobility, while Cd2+ and Cu2+ ions exchange with their counterparts outside the membrane and exhibit relatively larger mobility. With increasing concentration of Pb2+ ions, the favorable binding sites in the membrane tend to be saturated and exchange occurs for the excess Pb2+ ions. The presence of competitive ions has negligible effect on the residence of Pb2+ ions in the membrane. The exchange efficacy of Na-ETS-10 for the three heavy metal ions follows the order Pb2+ > Cd2+ > Cu2+, consistent with experimental observation.