Charge-selective separation of amino acids with ultrafiltration and nanofiltration

Polymeric spiral wound membranes with a nominal molecular weight cut-off (MWCO) between 1000-5000 Da were applied to separate single glutamic acid or lysine. The focus of the study was the influence of charge exclusion on the separation. Three pH values were assessed, at which these amino acids are negatively or positively charged, or electrically neutral. The pH slightly affects the permeate flux but significantly influences the transmission of both amino acids. Adding NaCl reduces this effect on transmission at alkaline conditions due to charge screening. The type of membrane material, polyethersulfone or polyamide, also affects the effect of charge on amino acid transmission. The effects can be explained by the number of charges of the amino acids and small ions (H+, Cl-, Na+, and OH-) interacting with the negatively charged membrane surface. Both amino acids were mixed at different concentration ratios of lysine to glutamic acid at pH 8. The presence of glutamic acid in mixtures strongly decreased the lysine transmission compared to solutions of just lysine. This effect vanished after the addition of NaCl. The charge impact from changing solution pH and salt content on amino acid transmission was reduced with larger nominal MWCO values, corresponding to larger membrane pore sizes.