ION-EXCHANGE OF AMINO-ACIDS AND DIPEPTIDES ON CATION RESINS WITH VARYING DEGREE OF CROSS-LINKING .2. INTRAPARTICLE TRANSPORT

The ion exchange kinetics of alanine, a-aminobutyric acid, leucine, phenylalanine, phenylalanylalanine, and phenylalanylphenylalanine with sulfonated polystyrene-divinylbenzene resins having a degree of cross-linking from 2 to 10% is studied under alkaline desorption conditions. Desorption in a shallow-bed contactor with NaOH concentrations in the range of 50-200 mM is shown to occur in the absence of equilibrium limitations and of external mass-transfer resistance. For these conditions, the desorption rate is controlled by the slow diffusion of amino acid or peptide cations, which appears to be severely hindered in the cross-linked polymer mesh of the resins. The diffusivity values are found to be correlated with a decreasing exponential function of the effective degree of cross-linking of the resin and of the apparent molar volume of the diffusing solute. Intraparticle effective diffusivities obtained from these alkaline desorption measurements are shown to provide an adequate simulation of the rate of uptake of these solutes from acidic solutions.