Synthesis of modified calix[4]arene derivatives and their applications in the removal of toxic metals from water

Abstract

Calixarenes have been proven to be very useful precursors for the synthesis of host molecules for the recognition of ionic as well as neutral guests. During the last three decades, they have attracted much attention as key receptors in ‘Supramolecular Chemistry’. Immense interest in these compounds was stimulated by their simple large- scale synthesis and the different ways in which they can be selectively functionalized at the narrow or wide rim. Keeping in view the excellent role of calixarenes in separation and purification science; we have designed new schemes to synthesize calix[n]arene based ionophores and some polymeric matrices incorporated with these compounds. After the synthesis and characterization of these synthetic materials, they were applied in the extraction/complexation of toxic metal ions from aqueous media. Thus, the present study comprises strategies, which include following three phases. In the first phase of study, the work was carried out for the synthesis of p-tert- butylcalix[8]arene (I) followed by Williamsons’ etherification to obtain octaethyl-p-tert- butylcalix[8]arene octaacetate (II). The treatment of (II) with N-(2-aminoethyl)- piperazine afforded, p-tert-butyl-49,50,51,52,53,54,55,56-octa-(2-piprazinoethylamino)- carbonylmethoxycalix[8]arene (III). Compound III was impregnated onto Amberlite XAD-4 resin to get resin (IV). Compound III was utilized to explore its extraction affinity toward selected transition metals, i.e (Co(II), Cd(II), Ni(II), Pb(II) and Cu(II)). From the results it has been observed that III has preferably higher extraction ability for Pb(II) and Cu(II) as compared to other metal ions. Both ions were extracted up to 95% and 91%, respectively. These interactions can be attributed to the size, geometry and nature of these metal ions, which are compatible with ligand III. Similarly, the extraction efficiency of its impregnated resin (IV) toward oxoanions (dichromate/chromate, arsenite and arsenate) has been evaluated. From the experimental results it has been deduced that IV is highly effective for the removal of Cr(VI) and As(V) at a wide range of pH. Sorption data was also evaluated by applying various isotherm models. Langmuir and Temkin isotherms revealed that Cr(VI) sorption onto resin IV is favorable and physisorption in nature. Similarly the data obtained from Flory-Huggins and Temkin models revealed that the sorption of As(V) onto resin IV is feasible, spontaneous and exothermic in nature. The resin IV was also applied to remove the arsenic from real water samples. The resin has shown quite remarkable efficiency (up to 90%) for the removal of total arsenic from real samples that proves its utility in industrial field. In the second phase of the study, 5,11,17,23-p-tert-butyl-25,26,27,28- tetrahydroxycalix[4]arene(V) was synthesized followed by its de-butylation from p- position by the treatment of V with AlCl3 and phenol in dry toluene to obtain 25,26,27,28-tetrahydroxycalix[4]arene (VI). Compound VII and VIII were synthesized by the treatment of compound VI with pipridine and morpholine separately using Mannich reaction pathway. Afterwards, the compounds VII and VIII were anchored onto Merrifield resin to develop calixarene based resins IX and X. The compound VIII was then used to explore its metal ligating/sensing property toward selected transition metals, i.e. Co(II), Cd(II), Ni(II), Pb(II) and Cu(II); while resin IX and X were used for the removal of arsenic from water. It has been observed that VIII is a versatile compound showing chemosensing property toward Co(II), Ni(II) and Cu(II). Moreover, the liquid-liquid extraction studies reveal that VIII prefers Cu(II) more than the other metal cations. However, it exhibits good extraction ability toward the metal cations in the order Cu(II) >Ni(II) >Co(II). The increased extraction of essential metal cations with VIII is because of the presence of soft binding sites (i.e. nitrogen containing ligating groups). The solid-liquid extraction studies of IX and X were carried out for the removal of arsenic from aqueous environment and it has been noticed that both resins are effective sorbents for arsenic in acidic conditions. X possesses greater interaction for arsenite with maximum sorption (97% at pH 1) as compared to arsenate (89% at pH 7). Similarly, in case of resin IX, it has been noticed that its behavior is opposite to the resin X. Its interaction with arsenate is greater, (95% sorption at pH 5) than for arsenite (68% sorption at pH 1). Concerning regeneration ability, resin IX was found more efficient than resin X that could be reused repeatedly for the sorption of both arsenate and arsenite effectively. Best regeneration was achieved by using 0.1 M NaOH solution. In the third and last phase of the study, two more calix[4]arene based ionophores (XI and XIII) were synthesized by the reaction of VI with 2-ethoxyethyl-4- methylbenzenesulfonate and 2-ethylsulfanyl)ethoxybenzenesulphonate (d) in dry dimethylformamide (DMF) in the presence of sodium hydride (NaH). The compounds XI and XIII were grafted onto Amberlite XAD-4 to obtain DE-4 (XII) and TS-4 (XIV) resins. The resins DE-4 and TS-4 have been found to sorb As(III) effectively up to 88% and 95%, respectively in acidic conditions. Column sorption was tested at different bed heights, flow rates and inlet concentrations. The maximum sorption capacity obtained through column sorption for both DE-4 and TS-4 resins was found to be 5.5 and 0.13 mmolg-1, respectively. The resin could be regenerated by using 5% NaOH solution. Both DE-4 and TS-4 resins were found thermally stable up to 400 K and due to this quality both resins find their applicability in various industrial processes where high temperature water purification is required. In short it can be concluded that all the developed materials will provide an efficient stuff for Engineers working in designing water filters, which can be applied for the removal of toxic metals from water both on low as well as industrial scale water purification systems. The important features of this study are that the work done in this contest may help not only to recover the particular targeted metals from the industrial effluents for reuse, but also to restore the polluted sites in their original state. Moreover, the resins are easy to synthesize from cheap materials and their regeneration offers another advantage in this contest, i.e. the resins could be reused several times after regeneration.