Development of Porous Materials and their Application in Separation of Ions

Abstract

The deleterious effect of toxic metals on the environment is an area of extreme focus recently. Many materials were proposed previously to counteract the toxic metal contamination issue; however, still search for new and better material with improved metal complexation qualities is continued. Hence, in the present study two types of advanced porous materials i.e. calixarene derivatives and nanofiltration membranes are presented here as alternative techniques for this purpose. The first phase of the study includes the synthesis of N-(2-tosylato)ethylpiperazine (3) and 5,11,17,23-tetra-tert-butyl-25,27-bis-(2-piprazinoethyl)-26,28-dihydroxycalix[4]arene (4). The confirmation of newly synthesized compounds (3, 4) were carried by their characterization through elemental analysis, FT-IR, 1H NMR and/or 13C NMR studies. Furthermore, the liquidliquid extraction study was carried to evaluate the efficiency of compound 4 for Cd2+, Co2+, Cu2+, Ni2+, and Hg2+ metal picrates and CrO42-/Cr2O72- anions. The compound 4 showed selective extraction ability for Hg2+ among transition metals and also found efficient for chromate/dichromate anions at low pH. Comparison between extraction properties of compound 4 with previously reported 5,11,17,23-tetra-tert-butyl-25,27-bis(isoniazidylcarbonylmethoxy)26,28-dihydroxy-calix[4]arene (iv) and protonated pyridinium form of iv (v) was also described. During the second phase of the study, the porous materials (calixarene derivatives) were applied in bulk liquid membrane (BLM) to investigate their efficiency towards metal transport. For this purpose, calix[6]arene hexaester derivative (6) for Pb(II), 5,11,17,23 tetrakis[(propylthio)methyl]-25,26,27,28-tetrahydroxycalix[4]arene (8) for Hg(II) and pmorpholinomethylcalix[4]arene (9) for Cu(II) transport were utilized as carriers in BLM in separate studies. Influence of various kinds of kinetic parameters on the metal transport such as time, carrier concentration, solvent type and temperature was examined in terms of flux (Jdmax, Jamax), rate constants (k1, k2), Rmmax, and tmax values. The kinetic analysis was accomplished by following the laws of two consecutive irreversible first order reactions. Mostly it was observed that metal transport through BLM increase with increase in concentration of carrier, rise in temperature and at high stirring speed. Activation energy values were also calculated in each BLM study, to check the metal transport is either diffusionally or chemically controlled. In the case of Cu(II) transport, initially complexation and decomplexation ability of carrier 9 towards Cu(II) was investigated through conventional liquid-liquid extraction study. In alkaline conditions i.e. pH=12, maximum Cu(II) was decomplexed in the acceptor phase. Moreover, binary solvent types were utilized along with single solvent in membrane phase to augment the metal transport. Thus, the rate constant and flux values for these solvent types increases in the order as follows: chloroform< dichloromethane-ethylacetate< dichloromethane< dichloromethane-diethylether< dichloromethane-hexane. But dichloromethane made stable membrane phase relative to other solvent types. During the third phase of the present study, the porous materials (calixarene derivatives) were also utilized in supported liquid membrane (SLM) for metal transport. In SLM, 5,11,17,23 tetrakis[(propylthio)methyl]-25,26,27,28-tetrahydroxycalix[4]arene (8) and pmorpholinomethylcalix[4]arene (9) were employed as carriers in separate work for Hg(II) and Cu(II), respectively. Influence of different parameters such as solvent, (chloroform, xylene, diphenyl ether, and toluene), membrane dipping time, support membrane (Celgard 2500 and 2400 membranes), type of co-anions (  Cl /  3NO ), donor and acceptor pH, and carrier concentration on metal transport was checked. Danesi mass transfer model and Fick’s first law of diffusion were followed for determining permeability (P) and flux (J) values. Higher metal permeability was observed in diphenyl ether, Celgard 2500 as support membrane and Cl- as coanion in both cases. However, in Cu(II) transport diffusion coefficients (D) and extraction constant (K) were also calculated additionally using Reinhoudt’s model, lag time measurements as well as by Wilke-Chang relation and compared. D and K values for Cu(II) transport were calculated as 1.54×10-10 m/s and 1.19×10-5 m/s, respectively. Thus, it was concluded that transport of Cu(II) in SLM was accelerated under diffusion realm. Consequently, in the fourth and last phase of the study thin film composite nanofiltration (TFC-NF) membranes were developed following interfacial polymerization (IP) technique for desalination purpose. Ethylene diamine (EDA) and terephthaloyl chloride (TPC) were employed as aqueous and organic phase monomers, respectively whereas Celgard 2400 membrane was taken as support. The thin, active polyamide layer on the surface of Celgard 2400 membrane was characterized after IP through fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface polymerization was also confirmed by estimating molecular weight cutoff (MWCO) of TFC-NF membranes. The MWCO was calculated below 342 Da using dextrose, sucrose and raffinose solutions, which is in the range of NF. The economical home-made NF Cell was utilized for salt rejection and solvent permeation experiments. For optimization of IP technique, effect of various parameters such as monomer concentration, residence time in each monomer and curing temperature on the performance of TFC-NF membrane was evaluated. Under optimum conditions, water flux and MgCl2 rejection for these membranes were found as 33 L/m2h and 90%, respectively at 7 bar applied pressure. Finally, the pore size of the TFC-NF membrane was estimated to be ~ 0.45 nm following Hagen-Poiseuille equation.