Effect of pore characteristics on carrier-facilitated transport of Am(III) across track-etched membranes

Abstract

Track-etched membranes (TEMs) with well-defined pore characteristics were prepared by bombarding polycarbonate plastics with 161 Dy ions of 13.0 MeV per nucleon energy and by bombarding polycarbonate plastics with fission fragments from a 252 Cf source. In order to optimise the properties of TEMs for their applications as model supports in supported liquid membrane (SLM) studies, the transport of Am(III) across the SLM with TRUEX extractants (0.2 M n-octylphenyl ( N, N-diisobutyl carbamoyl methyl) phosphine oxide (CMPO)+1.2 M tributyl phosphate (TBP) in dodecane) as carrier has been examined. The latent tracks formed by heavy-ions in the plastics were enlarged into well-defined pores by chemical etching. The geometry of the pores formed in the plastics was obtained by the track-etched model, based on the dynamics of pore etching. The TEMs were converted as supports for SLM studies by impregnating them with the carrier (0.2 M CMPO+1.2 M TBP in dodecane). The SLMs with different supports were used for studying the facilitated transport of Am(III). The permeation across the SLM formed by using TEM with 17% porosity was found to be comparable to those SLMs formed by using commercial membranes with much higher porosities. The comparison of permeability coefficients ( P) obtained by using TEMs with 29–30 μm and 10 μm thickness indicated that P decreases linearly with thickness and hence transport of Am(III) is diffusion controlled, and reaction kinetics of extraction and stripping steps are fast. The variation of P as a function of porosity of the TEM support indicated that P decreased with increase in porosity upto 10% and thereafter it remained constant. The data on variation of P as a function of porosity were used to obtain the thickness of the aqueous diffusion layer and the membrane diffusion coefficient of the SLM system studied in the present work. The TEMs with very small pore size (0.07 μm) and very large pore size (3 μm) were found to deviate from the expected trend and gave anomalously low P values. Such low P values were also reported elsewhere with membranes of small pore size. The low P values with small pore size support may be due to hindered diffusion due to aggregation of the permeant (Am–CMPO complex in this case), but no satisfactory explanation was found for low P values in the case of supports with large pore size.