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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

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journal contribution
posted on 2019-08-05, 12:36 authored by Praveenkumar Sappidi, Jayant K. Singh
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 ρs (mol/nm2) of DBCE and DCHCE on the adsorption behavior with increasing UO22+ ion concentration Cs (M). The amount of adsorption (qe) (mg/g) increases with an increase in UO22+ salt concentration and follows the Langmuir adsorption isotherm model. The maximum amount of adsorption (qmax) (mg/g) increases with increasing grafting density for both DBCE and DCHCE. The DCHCE shows higher qmax values compared to DBCE over the entire range of ρs: 0.25 mol/nm2 < ρs < 2.07 mol/nm2. Overall, qmax show a 5-fold increase for DBCE and DCHCE with an increase in ρs from 0.25 mol/nm2 to 2.07 mol/nm2. The optimum ρs 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 ρs, with an increase in Cs. Similarly, the τ value shows more than a 5-fold increase irrespective of grafting densities for both DBCE and DCHCE with increasing Cs. 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.