In this study, a pristine UiO-66 metal–organic framework (MOF) was employed as the membrane material due to its outstanding physicochemical properties, particularly its exceptional mechanical robustness and chemical stability in aqueous environments. The membrane was systematically investigated for the simultaneous separation of two heavy metal cations, Pb²⁺ and Cu²⁺, from wastewater streams under an applied pressure range of 3–12 MPa. To capture the system dynamics, molecular dynamics simulations were performed using the LAMMPS package, consisting of a 0.5 ns equilibration stage followed by a 6 ns production run. Overall, the water flux exhibited an increasing trend with increasing applied pressure. However, the increase in pressure from 7.5 MPa to 12 MPa did not result in a pronounced enhancement in water flux over the simulation timescale. This behavior can be attributed to strong water-membrane interactions as well as water–water hydrogen bonding within the confined pore environment, which promotes the formation of structured water clusters inside the membrane channels. The transport and separation behavior of the cations was analyzed in detail using density profiles, mean square displacement (MSD), and radial distribution function (RDF) calculations. These atomistic simulations were further supported and validated by density functional theory (DFT) calculations, which provided additional insight into the dominant interaction mechanisms governing ion rejection. The results indicate that water–cation interactions, membrane-cation interactions, and the applied hydraulic pressure collectively govern the separation performance of the system, yielding removal efficiencies of 90.31–90.63% for Pb²⁺ and 88–97% for Cu²⁺ under the studied conditions.
