Separation of amino acid mixtures using multilayer polyelectrolyte nanofiltration membranes

Amino acids such as l-glutamine are usually obtained commercially from microbiological fermentation broths. However, such broths contain many amino acids that must be separated to obtain a pure product. Several research groups recently tried to separate amino acid mixtures using nanofiltration (NF) and found that pH must be carefully controlled to obtain fractionation because charge effects (repulsion of coions and attraction of counterions), rather than sieving effects, dominate the separation. This study examines the potential of a new class of NF membranes (polyelectrolyte multilayers) for the separation of neutral, zwitterionic amino acids without adjusting pH. Membranes composed of six to seven bilayers of poly(styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH) on porous alumina successfully allowed selective transport of glycine from a mixture of glycine, l-alanine, l-serine, l-glutamine, and l-lysine. These amino acids differ in size and yet have similar pI values, excepting l-lysine. In nanofiltration of mixtures of the four neutral amino acids using (PSS/PAH)7 membranes, the selectivity of glycine over l-glutamine was about 50 with a solution flux of 1.3 m3/(m2 day) at 4.8 × 105 Pa (4.8 bar). This flux is equal to that with the best commercial NF membranes.