Transport properties of ion-exchange membrane systems in LysHCl solutions

Abstract

Electromembrane methods of amino acids purification and separation are very attractive. They do not need any additional chemicals, minimize the desired product loss and are energy efficient and manufacturable. Detailed study of amino acid transfer mechanisms inside membranes and at their surfaces provides a basis to improve these methods. For these purposes, concentration dependencies of cation-exchange (CMX, MK-40) and anion-exchange (AMX, MA-41) membranes electrical conductivity in 0.025–1 M LysHCl and NaCl solutions as well as current–voltage curves (CVC) and chronopotentiograms (ChP) of CMX membrane in 0.01 M solutions are obtained. It is shown that in the case of CMX and MK-40 cation-exchange membranes (Fig. 1a) the replacement of NaCl to LysHCl decreases values of the membranes electrical conductivity (k) more than 10 times. CMX and MK-40 membranes electrical conductivity grows with dilution of LysHCl solution at C < 0.1 M, whereas in the case of NaCl it keeps decreasing. This behavior of membranes is caused by two reasons: (1) the value of LysH counter-ion diffusion coefficient is less than that of Na (in solution: DNa+= 1.33 10 cm/s, DLysH+= 0.667 10 cm/s); (2) when diluting external solution, the cation-exchange membrane internal solution is acidified due to the Donnan exclusion of OH ions; as a consequence, an increase in LysH2 2+ fraction in this solution occurs as a result of LysHCl hydrolysis. The Donnan exclusion of OH ions is proved by the increase in pH during equilibration of these membranes in LysHCl solution if they were initially equilibrated with NaCl solution. In the case of anion-exchange membranes (Fig. 1b), their electrical conductivity is mainly determined by the counter-ion (Cl); the type of co-ion (LysH or Na) influences weakly. This influence is also weak as for the value of the volume fraction ( f2) of the intergel spaces filling with the equilibrium electroneutral solution; f2 being obtained using the microheterogeneous model. For a homogeneous AMX membrane f2 is equal to 0.11 in NaCl solutions and to 0.07 in LysHCl ones. Some decrease in the AMX intergel phase fraction may be caused by a reduction *Corresponding author.