Silver-Sodium Ion Exchange Dynamics in LTA Zeolite Membranes

Because of their high ion exchange capacity, silver exchanged LTA (Ag-LTA) zeolites can be incorporated in polymeric materials as antibacterial agents for various purposes such as preventing biofouling in water treatment membranes due to the formation of microorganisms like bacteria, algae. Ag-LTA can be prepared by exchanging extra-framework Na+ ions in an aqueous solution containing Ag+ ions. Since the release amount and rate of Ag+ ions from the zeolite are crucial factors determining the antibacterial efficiency of these materials, it is important to understand the ion-exchange dynamics to develop membranes featuring enhanced antibacterial activity. In this work, quantum mechanical calculations and molecular dynamics (MD) simulations were employed to elucidate the effects of several parameters on Ag+/Na+ ion exchange rates in LTA-type zeolites. Two different membrane systems in which Ag-LTA crystal in contact with Na+ containing electrolyte solution and Na-LTA crystal in contact with Ag+ containing electrolyte solution were modeled. The electrostatic charges of the framework and the surface atoms at the crystal/solution interface were determined via periodic DFT calculations at PW91/DNP level. The ion exchange dynamics between the LTA crystal and the electrolyte solution was followed by long MD simulations. First order kinetics was assumed for ion-exchange rates and the ion concentration in and out of the membrane, temperature and crystal thickness were considered as the principal parameters affecting the release rate. The changes in the ion release rates and ion-exchange ratios were analyzed by varying these parameters. Our simulations reproduced experimentally established relationships between considered parameters and ion exchange dynamics.