Uphill permeation model of gold(III) and its separation from base metals using thiourea derivatives as ionophores across a liquid membrane

This work deals with carrier-facilitated membrane transport of Au(III) from chloride media across a polymer-immobilised liquid membrane (PILM) using a s organic reagents N-(thiocarbamoyl)benzamide derivatives and N-benzoylthio urea derivatives, denoted as 2a-c and 3a-f, respectively. Both the composit ion of the organic membrane solvent and the type of carrier have a marked e ffect on gold permeation. Recovery and permeability of gold using 2a-c and 3a-f across a PILM proceed in the following order: 3e approximate to 3d app roximate to 3e much greater than 3f > 3b approximate to 3a approximate to 2 a approximate to 2b approximate to 2c, In view of the performance of these carriers, 3c was selected as a metal receptor for detailed studies of Au(II I) in permeation. A model is presented for the permeation of Au(III) (61 mu M) in 0.5 M Cl- at pH 2.5 using 3c as a membrane carrier. The mathematical equations describing the rate of permeation are derived to correlate the m embrane permeability coefficient with diffusional and equilibrium parameter s. The mass transfer coefficient was calculated from the described model as 1.1 x 10(-5) m s(-1), and the thickness of the aqueous boundary was later calculated to be 65 mu m Several polymeric supports were tested for impregn ation of the organic extractant, and Durapore (Millipore) afforded the maxi mum flux for Au(III), yielding a value of 1.1 x 10(-14) mol m(-2) s, The re lationship between flux and support characteristics is derived and a mathem atical equation is presented, Of the several diluents used, cumene had the most satisfactory performance in terms of FILM stability and metal transpor t. Of the different reagents used, 0.5 M sodium thiocyanate in 0.5 M NaCl a t pH 2.5 served most efficiently as the stripping agent. More than 80% of t he Au(III) could be readily separated using 3c in the presence of various m etals such as Cu(II), Fc(III) and Zn(II). (C) 1998 Published by Elsevier Sc ience B.V. All rights reserved.