Molecular Dynamics Study on the Adsorption of UO₂²⁺ from an Aqueous Phase: Effect of Grafting Dibenzo Crown Ether and Dicyclohexano Crown Ether on the Polystyrene Surface

Atomistic molecular dynamics (MD) simulations are performed in order to understand the adsorption behavior of UO₂²⁺ ions from an aqueous medium. The dibenzo crown ether (DBCE) and dicyclo hexano crown ether (DCHCE) grafted on the polystyrene surface is used as the adsorbent. We investigated the role of grafting density ρ_s (mol/nm²) of DBCE and DCHCE on the adsorption behavior with increasing UO₂²⁺ ion concentration C_s (M). The amount of adsorption (q_e) (mg/g) increases with an increase in UO₂²⁺ salt concentration and follows the Langmuir adsorption isotherm model. The maximum amount of adsorption (q_max) (mg/g) increases with increasing grafting density for both DBCE and DCHCE. The DCHCE shows higher q_max values compared to DBCE over the entire range of ρ_s: 0.25 mol/nm² < ρ_s < 2.07 mol/nm². Overall, q_max show a 5-fold increase for DBCE and DCHCE with an increase in ρ_s from 0.25 mol/nm² to 2.07 mol/nm². The optimum ρ_s for the maximum adsorption is found to be 1.25 mol/nm² for both DBCE and DCHCE. The dynamical behavior of the adsorbed UO₂²⁺ is also investigated by calculating the self-diffusion coefficient (D) and mean residence time (τ). The D value decreases by ∼47% for DBCE and ∼61% for DCHCE, for the entire range of ρ_s, with an increase in C_s. Similarly, the τ value shows more than a 5-fold increase irrespective of grafting densities for both DBCE and DCHCE with increasing C_s. In the end, we investigate the UO₂²⁺ binding structure with DBCE and DCHCE. The potential of mean force (PMF) analysis shows a favorable free-energy for the binding of UO₂²⁺ to DCHCE compared to DBCE.