Determination of structural and electrical parameters for activated composite membranes containing di-(2-ethylhexyl)dithiophosphoric acid as carrier

Abstract

The paper reports the results obtained on the physicochemical characterisation of activated composite membranes (ACMs). Membrane samples containing different concentrations of di-2-ethyl-hexyldithiophosphoric acid (DTPA) as a carrier were prepared and characterised by different analytical techniques. The amount of the carrier incorporated in ACM appears to be directly proportional to the concentration of carrier in the casting solution. Respective distribution dependencies can serve as calibration graphs applicable for pre-calculation of extractant concentration in the casting solution to manufacture an ACM with a given carrier content. The carrier distribution in the membrane was evaluated by X-ray microanalysis (EDS), which allowed us to determine that only a limited concentration of the carrier can be incorporated into the top polyamide membrane layer, resulting in redistribution of the excess of extractant along the polymeric structure of the membrane. These results also correlate with those obtained by impedance spectroscopy measurements. The equivalent circuits associated with the different membrane samples agree with a two-layer model for the ACM containing a high concentration of carrier ( C DTPA > 0.5 M), which allows for the location of a larger amount of carrier into the polyamide layer. The results obtained by the indicated techniques are in good agreement with each other, and provide for the structural characterisation of the membrane.