Separation of amino acids by nanofiltration and ultrafiltration membranes

Abstract

Abstract The aim of this work was to determine the amino acid retention as a function of concentration and the ionic composition of the feed for various polymeric membranes. Results from the literature and the insights gained from our amino acid experiments were then applied to the separation of amino acid mixtures using nanofiltration and ultrafiltration membranes. Attention was paid to the charge and size properties of the amino acids and the membranes and their effects on the separation. The separation was severely influenced by size exclusion and Donnan exclusion effects, as expected. The size exclusion effect was predominant with the nanofiltration membrane tested, while Donnan exclusion effects were dominant with the ultrafiltration membranes. In the case of charged amino acids, a dependence of their retention on their concentration and the salt composition was found. Amino acids carrying no net charge acted as neutral species and were practically not influenced. Trends observed with pure amino acids were also found with mixtures of amino acids and it was shown that the selectivity of the separation of amino acids can be manipulated by modifying the charged state of the amino acid by changing the pH, or by modifying the salt composition of the feed solution. The highest selectivity of separation was obtained when Donnan exclusion was the dominating mechanism.