Abstract
Membrane based separation processes represent a sophisticated way to limit industrial wastes in the natural environment. Crown ethers have been recognized as very effective class of compounds to achieve selective separation of heavy metal ions from aqueous solutions. Lead (Pb) is a well known toxic metal and is heavily used in industry. Therefore, finding ways to selectively remove Pb(II) from aqueous solutions is important. In the present paper, different crown ethers of varying cavity size, subsituent groups, donor atoms, and ring number are compared for their ability to transport Pb2+ through a supported liquid membrane. All experiments were carried out in a laboratory scale membrane reactor with crown ether solution immobilized on a polypropylene porous sheet interposed between feed and strip solutions. It was observed that when 0 atoms of the same sized macrocycle are replaced with N or S atoms, percentage recovery of Pb(II) increases significantly. By substituting a benzene ring on the same sized macrocycle, the percentage of Pb(II) transport increased from 78 to 86%. The cavity size of the crown ether seems to affect the rate of transport. After different trials, it was found that transport is maximal where the ionic radius of Pb(II) matches maximum with the cavity diameter of the macrocycle. These interactions of crown ethers with Pb(II) are explained on the basis of metal-ligand coordination chemistry.