A. K. Singha Deb

Dual mode of extraction for Cs+ and Na+ ions with dicyclohexano-18-crown-6 and bis(2-propyloxy)calix[4]crown-6 in ionic liquids: density functional theoretical investigation

The unusually high selectivity of the Cs+ ion over the Na+ ion with bis(2-propyloxy)calix[4]crown-6 (BPC6) compared to dicyclohexano-18-crown-6 (DCH18C6) has been investigated using generalized gradient approximated (GGA) BP86, hybrid B3LYP and meta hybrid TPSSH density functionals, employing split valence plus polarization (SVP) and triple zeta valence plus polarization (TZVP) basis sets in conjunction with the COSMO (conductor like screening model) solvation approach. The calculated theoretical selectivity of the Cs+ ion over the Na+ ion was found to be in accordance with the experimental selectivity obtained using solvent extraction experiments in ionic liquids (IL) and octanol. The distribution constant of the Cs+ ion, DCs with DCH18C6 in the 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIMTF2N) IL phase was found to be significantly large than that in octanol. The experimentally measured DCs value was found to be very large compared to the value of DNa in the IL phase. The presence of the BMIM cation in the recorded UV-visible spectra of the raffinate phase with and without DCH18C6 indicates the BMIM cation exchange with Cs+ and Na+ ions, thus supporting the dual mode of extraction. The dual mode of metal ion extraction observed in the experimental study was complemented by density functional theoretical study. The calculated free energy of extraction, ΔGext, for the metal ion was found to be higher in IL compared to octanol. Further, preferential selectivity of the Cs+ ion over the Na+ ion was established from the free energy difference, ΔΔGext, between the two competing metal ions. The unusually high selectivity of Cs+ over the Na+ ion by BPC6 in IL compared to DCH18C6 is also demonstrated by the free energy difference, ΔΔΔGext, between the two competing ligands which was shown to be free from the complicated metal ion solvation energy.

Adsorption of Eu3+ and Am3+ ion towards hard donor-based diglycolamic acid-functionalised carbon nanotubes: density functional theory guided experimental verification

Structure, bonding, energetic and thermodynamic parameters of trivalent Eu3+ and Am3+ with diglycolamic acid-functionalised carbon nanotubes (CNT–DGA) in gas and solvent phases are reported in order to understand their complexation and extraction behaviour. The calculation was performed with generalised gradient approximated BP86 density functional and hybrid B3LYP functional using SVP/TZVP basis set. The free energy of extraction, ΔGext, of Eu3+ and Am3+ was computed using the Born–Haber thermodynamic cycle in conjunction with conductor-like screening model solvation approach. The calculated free energy of extraction was found to be exergonic using an explicit cluster water model for hydrating the ions and it was found to be higher for Eu3+ ion over Am3+ ion. From the estimated distribution constant using synthesised CNT–DGA, it is indisputably established that the Eu3+ ion is preferentially extracted over Am3+ ion and hence confirms the acceptance of the explicit cluster model for ion solvation free energy and thermodynamic cycle for the evaluation of free energy of extraction in solution phase. This is perhaps the first of its kind that a combined theoretical and experimental study has been performed on the free energy of extraction of Eu3+ and Am3+ using CNT–DGA.

Surface Engineering of PAMAM-SDB Chelating Resin with Diglycolamic Acid (DGA) Functional Group for Efficient Sorption of U(VI) and Th(IV) from Aqueous Medium

A novel chelating resin obtained via growth of PAMAM dendron on surface of styrene divinyl benzene resin beads, followed by diglycolamic acid functionalization of the dendrimer terminal. Batch experiments were conducted to study the effects of pH, nitric acid concentration, amount of adsorbent, shaking time, initial metal ion concentration and temperature on U(VI) and Th(IV) adsorption efficiency. Diglycolamic acid terminated PAMAM dendrimer functionalized styrene divinylbenzene chelating resin (DGA-PAMAM-SDB) is found to be an efficient candidate for the removal of U(VI) and Th(IV) ions from aqueous (pH >4) and nitric acid media (>3M). The sorption equilibrium could be reached within 60min, and the experimental data fits with pseudo-second-order model. Langmuir sorption isotherm model correlates well with sorption equilibrium data. The maximum U(VI) and Th(IV) sorption capacity onto DGA-PAMAMG5-SDB was estimated to be about 682 and 544.2mgg-1 respectively at 25°C. The interaction of actinides and chelating resin is reversible and hence, the resin can be regenerated and reused. DFT calculation on the interaction of U(VI) and Th(IV) ions with chelating resin validates the experimental findings.

Diglycolamic Acid Functionalized CNTs for Preferential Selection of Eu(III) over Am(III) Ion: Density Functional Theoretical Modelling Validated by Experiments

We report the structure, bonding, energetic, and thermodynamic parameters of Eu3+ and Am3+ with diglycolamic acid functionalized CNTs at BP86 and B3LYP functional level of theory using SVP and TZVP basis set. The free energy of extraction, ΔGext, of Eu3+ and Am3+ was computed using standard thermodynamical procedure in conjunction with COSMO (conductor like screening model) model. The value of ΔGext for Eu3+ ion was found to be higher than that of Am3+ as observed in the extraction experiments. The HOMO-LUMO analysis indicates that Eu3+ ion is harder than Am3+ ion leading to stronger interaction with hard donor based DGA-CNT.

Molecular modeling guided isotope separation of gadolinium with strong cation exchange resin using displacement chromatography

ABSTRACT Molecular modeling was carried out using DFT to identify the suitable displacing agent for carrying out Gd isotope separation using displacement chromatography. EDTA was identified as the best eluting agent among EDTA, malic acid and citric acid. Displacement chromatography of Gd adsorption band in cation exchange resin was performed to observe the isotope effects in the Gd ion exchange processes involving complex forming reagent – EDTA. The heavier isotope of 160Gd was found to be enriched at the front boundary of Gd adsorption band, while the lighter isotopes of 155Gd and 157Gd were enriched at the rear boundary.

Molecular engineering of functionalized crown ether resins for the isotopic enrichment of gadolinium: from computer to column chromatography

Density functional theoretical modelling was performed to design and screen suitable macrocyclic crown ether functionalized resins for the isotopic enrichment of gadolinium. Theoretical calculations predict the complexation stability order of Gd3+ ion as follows: di-cylohexano-18-crown-6 (DCH18C6) > dibenzo-18-crown-6 (DB18C6) > benzo-15-crown-5 (B15C5), which was experimentally verified. The calculated isotopic separation factor value was shown to be the highest for DB18C6. From the theoretical analysis of both the stability and isotopic separation factor, DB18C6 is predicted to be the most promising candidate for isotopic separation of gadolinium. Hence, DB18C6 was functionalized with chloromethylated polystyrene (CMPS) resin. Subsequently, CMPS-grafted DB18C6 resin was synthesized and characterized. Furthermore, isotopic enrichment of gadolinium was carried out by performing column chromatographic experiments using CMPS-DB18C6 resin. The absorption capacity of the novel CMPS-DB18C6 resin for gadolinium was found to be 1 mg g−1. The separation coefficient, e × 103, was found to be 6.3, 8.9, 3.4, and 9.7 for Gd-155/158, Gd-156/158, Gd-157/158, and Gd-155/160 isotopic pairs, respectively, and thus hold promise for future isotopic enrichment technology.

Conformational effect of dicyclo-hexano-18-crown-6 on isotopic fractionation of zinc: DFT approach

Generalized gradient approximated BP86 density functional employing triple zeta valence plus polarization (TZVP) basis set has been used to compute the reduced partition function ratio and isotopic separation factor for zinc isotopes. The isotopic separation factor was found to be in good agreement with the experimental results. The isotopic separation factor was found to depend on the conformation of the crown ether ligand. The trans-trans conformation shows the highest fractionation compared to cis-cis conformer. The present theoretical results can thus be used to plan the isotope separation experiments.