Birendra Babu Adhikari

Selective recovery of gold using some cross-linked polysaccharide gels

Effective recovery of gold from aqueous acidic chloride media was achieved with different kinds of polysaccharides with different structures by treating with concentrated sulfuric acid. The gels were found to be highly selective for Au(III) over other precious and base metals at varying hydrochloric acid solution. The adsorption isotherms of Au(III) followed the Langmuir type of adsorption and the maximum adsorption capacities for Au(III) was evaluated as high as 7.57, 7.20, 5.64 and 4.80 mmol g−1 for the gels of cellulose, dextran, alginic acid and pectic acid, respectively. Analysis of kinetic data shows that the uptake of gold(III) on celulose gel at the initial stage follows pseudo-first-order kinetics. Crystalline structure of the cellulose before and after the treatment with concentrated sulfuric acid was measured by XRD analysis. Furthermore, Au(III) was found to be reduced to elemental form by all types of polysaccharides gels, which was confirmed by the formation of clearly visible elemental gold particles and by means of the XRD-spectra of the adsorbents after adsorption. Infrared spectrum studies provided supporting evidence for the reduction of Au(III) to Au(0) with a suitable mechanism of Au(III) adsorption followed by reduction using various polysaccharide gels.

Methylene crosslinked calix[6]arene hexacaarboxylic acid resin: a highly efficient solid phase extractant for decontamination of lead bearing effluents.

Calixarene-based cation exchange resin has been developed by methylene crosslinking of calix[6]arene hexacarboxylic acid derivative and the resin has been exploited for solid phase extraction of some toxic heavy metal ions. The selectivity order of the resin towards some metal ions follows the order Pb(II) > Cu(II)> Zn(II), Ni(II), Co(II). The maximum lead ion binding capacity of the resin was found to be 1.30 mmol g(-1) resin. The loaded lead was quantitatively eluted with dilute acid solution regenerating the resin. Mutual separation of Pb(II), Cu(II) and Zn(II) was achieved by using the column packed with the resin.

Multiple proton ionizable calixarene derivatives with different ring sizes and complexation ability as efficient ionophores for complexation and solvent extraction of trivalent indium

The solvent extraction behavior of multiple proton ionizable p-tert-butylcalix[4]arene and [6]arene carboxylic acid derivatives towards indium has been investigated along with an acyclic monomeric analogue from weakly acidic media into chloroform. The extraction mechanism is ion exchange and carboxylic acid groups are adequate ligating sites for extraction. The cyclic structure of calixarene ligands to accommodate the potential guest species and the cooperativity effect of multifunctional groups significantly affect the complexation behavior and calixarene derivatives are found to be excellent extractants over the monomeric analogue. The composition of the extracted complex depends on the solution pH and attempts to determine the composition of the extracted complex for the extraction of indium have been stymied by complications arising from the formation of polynuclear species of indium and bridged polymeric species of calixarene carboxylic acid derivatives. One mole of calix[4]arene derivative extracts 2.5 moles of indium whereas the calix[6]arene derivative tends to extract 4.0 moles of indium. The loaded indium is back extracted with 1 mol dm(-3) hydrochloric acid solution. Though quantitative back extraction of indium was achieved from the fully loaded calix[6]arene derivative, it was only achieved up to 85% in the case of the calix[4]arene derivative.

Extraction of Pb2+ with p-tert-butylcalix [4]-, [5]-, [6]Arene Carboxylic Acid Ligands and their Monomeric Counterpart: A Thermodynamic Approach

Studies on extraction equilibrium constants at different temperatures and thermodynamic parameters of solvent extraction of Pb2+ ion with carboxylic acid derivatives of different ring size calixarenes and structure related monomeric compound have been carried out. The extraction equilibrium constants corresponding to calix[n]arene (n = 4, 5, 6) derivatives decrease in the order [5]arene > [6]arene > [4]arene. In all cases, the complexation process is primarily enthalpy driven. The favorable enthalpic contribution for extraction of Pb2+ is in the order hexamer ≈ monomer > tetramer > pentamer. However, the unfavorable entropic loss follows the order: monomer > hexamer > tetramer > pentamer. Overall stability of the host-guest complex is the function of entropy-enthalpy compensation and the free energy of complexation is minimum for the pentamer, followed by tetramer ≈ hexamer and monomer. Although the carboxylic acid derivative of calix[4]arene is more preorganized than the calix[5]arene derivative, extraction of Pb2+ ion with the tetramer passes through greater entropic loss than that with the pentamer and the degree of preorganization of calix[4]arene derivative is far from perfect for the complexation and extraction of Pb2+ ion. As compared to tetrameric and hexameric counterparts, the structural features of the carboxylic acid derivative of calix[5]arene prior to complexation contribute much to interact with the Pb2+ ion and form a thermodynamically stable complex. Supplementary materials are available for this article. Go to the publishers online edition of Solvent Extraction and Ion Exchange to view the supplemental file.

Highly selective and efficient extraction of two Pb2+ ions with a p-tert-butylcalix[6]arene hexacarboxylic acid ligand: an allosteric effect in extraction

Solvent extraction behavior of p-tert-butylcalix[6]arene hexacarboxylic acid towards Pb2+ and other divalent transition metal ions has been studied to investigate the selectivity, extraction mechanism, stoichiometry of complexation and composition of the complex in the organic phase. Extractability and selectivity of the cyclic ligand were analyzed in comparison with the acyclic analog. The macrocyclic effect was genuinely operative and the size-match effect was the governing factor for cation selectivity. Accordingly, the cyclic ligand selectively extracted the Pb2+ ion in a pH range fairly more acidic than the acyclic ligand. The cations were extracted by ion exchange mechanism, and the electroneutral complexes were resulted by the exchange of two protons for a divalent cation. A molecule of p-tert-butyl calix[6]arene hexacarboxylic acid ligand extracted two Pb2+ ions. One Pb2+ was extracted due to the size-fit effect inside the hydrophilic cavity of calix[6]arene composed by phenoxy and carbonyl oxygen atoms. The macrocyclic ring inversion was obstructed due to guest encapsulation, and the encapsulated Pb2+ ion acted as a template for aggregation of the carboxyl groups. This binding event caused a positive allosteric effect and led to the extraction of the second Pb2+ ion at the aggregated functional group site.

Intramolecular synergism for group separation extraction of trivalent rare earths by a cross type calix[4]arene with phosphonic and carboxylic acid bifunctionality

A well organized calix[4]arene derivative in the cone configuration with carboxylic and phosphonic acid groups positioned at distal pairs has been prepared in order to investigate the extraction behavior of trivalent rare earths together with selected divalent metals. This product exhibited high extraction ability for the rare earths over the divalent metals investigated due to the complementarity of these trivalent ions with the above two functional groups without chelation to the phenoxy oxygen atoms. The extraction of the rare earths takes place by one of ion-exchange, where three protons are replaced by one Ln(III) cation. The stoichiometry of rare earth complexation was also determined by slope analysis, loading test and the continuous variation procedures. The extraction ability of the above reagent towards the respective rare earth ions was constant and their group separation over other metal ions proved possible. It was concluded that the presence of the two different functional groups on the calixarene structure results in a favorable synergistic interaction with the lanthanides. The results of stripping experiments for the loaded metals are also reported.

Adsorptive Recovery of Palladium and Platinum from Acidic Chloride Media Using Chemically Modified Persimmon Tannin

Novel bio-adsorbent gel was prepared by immobilizing functional groups of glycidyltrimethyl ammonium chloride, a kind of quaternary ammonium chloride, onto polymer matrices of persimmon extract for the recovery of precious metals such as palladium(II) and platinum(IV) from chloride media. It was found that this adsorption gel exhibits high selectivity to gold(III), palladium(II) and platinum(IV) over base metals such as copper(II) and iron(III) while crude persimmon extract, the feed material, exhibits high selectivity only for gold(III). From the adsorption isotherms study, the maximum adsorption capacities for gold(III), palladium(II) and platinum(IV) were evaluated as 3.30, 1.67 and 1.00 mol/kg, respectively. Those for palladium(II) and platinum(IV) are higher than those of simple quaternary ammonium chloride type of chemically modified persimmon tannin gel and commercially available anion exchange resin.