Porphyrin thin-film composite cation exchange membranes enable high retention of amino acids in electrodialysis

In order to improve the retention of amino acids in desalinating a saline stream using electrodialysis, a novel porphyrin-containing thin-film composite (TFC) cation exchange membrane was prepared by interfacial polymerization (IP) using tetraphenylporphinesulfonate (TPPS), m-phenylene diamine (MPD), and trimesoyl chloride. The separation of a single amino acid [Arginine (Arg), Lysine (Lys), Histidine (His), and Proline (Pro)] and the mixture of four from a NaCl solution was investigated. Results showed that sodium ions transported much faster than the amino acid ions due to its small molar mass and high mobility. Arg migrated much faster than Lys may attribute to its guanidine that increased the polarity of the ion, thus resulting in a higher loss degree of Arg (60%) than that of Lys (32%) in the single amino acid test. In the mixed amino acid test, a competition behavior between Arg and Lys limited their transport over membrane. The loss degree of Arg and Lys was reduced to 32.7% and 9.31% with porphyrin TFC membrane, which could be attributed to combination of the TFC dense layer and the strong electrical interaction between the charge-elevated membrane surface and the amino acids. The porphyrin TFC achieved a loss degree of total amino acids of 13.8%, which was much lower than that of pristine TFC membrane (20.2%) and based membrane (27.25%), respectively. These findings indicated that using porphyrin TFC membrane can efficiently separate salts and amino acids, and may also be beneficial for other membrane separation techniques.