Adsorption of Gadolinium (Gd³⁺) Ions on the Dibenzo Crown Ether (DBCE) and Dicyclo Hexano Crown Ether (DCHCE) Grafted on the Polystyrene Surface: Insights from All Atom Molecular Dynamics Simulations and Experiments

Abstract

All atom molecular dynamics simulations and experiments were performed to understand the adsorption behavior of gadolinium (Gd 3+ ) ion on the crown ethers grafted polystyrene (PS) surface. Two different types of crown ethers, viz., dibenzo crown ether (DBCE) and dicyclo hexano crown ether (DCHCE), were grafted separately on the PS surface to understand the adsorption behavior. We investigate the roles of Gd 3+ ion concentration and grafting density (ρ s ) of the crown ether on the adsorption behavior of Gd 3+ ion on the PS surface. The adsorption of Gd 3+ shows an increasing trend with increasing salt concentration, for all cases of crown ether grafting densities. The adsorption behavior follows the Langmuir isotherm model. The maximum amount of Gd 3+ ion adsorption was observed to be 1.83 mg/g for DBCE and 2.02 mg/g for DCHCE at ρ s = 2.07 mol/nm 2 . The maximum amount of Gd 3+ ion adsorption on DBCE coated PS beads, in batch experiments, was found to be 1.76 mg/g, which is in good agreement with the theoretical results. The increase in ρ s from 0.25 to 2.07 mol/nm 2 shows an increase in the q max value by ∼422% and 329% for DBCE and DCHCE, respectively. The optimum value of the crown ether grafting density is found to be 1.25 mol/nm 2 , beyond which the q max saturates. We further investigate the dynamics of the Gd 3+ ion by evaluating the diffusion coefficient (D) and mean residence time (τ). It was found that D decreases with increasing salt concentration for both DBCE and DCHCE. On the contrary, as expected, the τ value of Gd 3+ increases with an increase in salt concentration. Overall, a 3-fold increase in τ was seen with increasing salt concentration. The potential of mean force analysis using umbrella sampling reveals favorable binding energy for higher grafting density of DCHCE compared to that of DBCE.

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

“…25 The details of the generation of an amorphous polymer surface are provided in previous work. 24 However, brief information is provided in Figure S1 in the Supporting Information. The molecular geometry and partial atomic charge calculation of the DBCE and DCHCE are performed by ChelpG scheme using density function theory calculations using Gaussian09 with B3LYP/6-31 G(d,p).…”

“…The Einstein relation is used for calculating the self-diffusion coefficient of UO 2 2+ . 24 3. RESULTS AND DISCUSSION 3.1.…”

“…The optimum crown ether grafting density (ρ s ) mol/ nm 2 on the polystyrene surface is found to be 1.25 mol/nm 2 for the adsorption of Gd 3+ from the aqueous solution as evident from recent molecular dynamics (MD) simulations and verified with the experimental studies. 24 Despite all the literature available on adsorption UO 2 2+ from the aqueous waste, there are a few questions which need to be answered. How the polystyrene surface grafted with crown ethers influence the adsorption of UO 2 2+ from an aqueous medium?…”

“…Interestingly, the optimum ρ s value observed in this work is similar to the earlier work for adsorption on Gd 3+ ions. 24 Figure 7 presents the Langmuir equilibrium parameter (R L ) calculated using eq 3 at different ρ s values. The R L show a decrease from 0.5 to 0.05 with an increase in the salt concentration for both DBCE and DCHCE.…”

Rational fabrication of a new ionic imprinted carboxymethyl chitosan-based sponge for efficient selective adsorption of Gd(iii)

“…Considering the factors such as adsorption capacity and selectivity are the most direct indexes to assess the practicability of adsorbent, the comparison with other adsorbents used to adsorb REM ions was carried out. [35][36][37][38] As summarized in Table 4, it can be observed that the adsorption capacity of PEI-CMC-IIS is not the highest, but PEI-CMC-IIS simultaneously possesses comparable adsorption capacity and excellent selectivity under neutral conditions, which is superior to other adsorbents in some way. Moreover, the main advantages of PEI-CMC-IIS are the cost-effective design, good stability, easy separation, pollution-free for the treated water, satisfactory adsorption capacity and selectivity.…”