Nanofiltration Performance in Single and Binary Solutions for the Purification of Glycine and Triglycine with Nacl

Nanofiltration can be used as the last step in the separation and purification of biomolecules that are present in many industrial by-products, such as biological protein hydrolysates. The present study explored the viability of two nanofiltration membranes in the separation of a low-molecular-weight peptide and an amino acid in binary solutions with NaCl at different feed pH values. First, membrane performance with single solutions was studied and the experimental data were fitted with the Donnan steric pore model with dielectric exclusion (DSPM-DE). Glucose rejection was assessed to estimate the membrane pore radius of the MPF-36 membrane and a pH dependence was observed. For a tight membrane (Desal 5DK), glucose rejection was close to unity and the membrane pore radius was estimated from the glycine rejection in the feed pH range from 3.7 to 8.4. The highest permeate flux and lowest solute rejection were observed at the corresponding membrane isoelectric points. In binary solutions, triglycine and glycine rejections decreased with NaCl concentration, especially in the MPF-36 membrane. The variations in solute rejections with feed pH and NaCl concentration were attributed to changes in the membrane pore radius and effective thickness-porosity ratio by the DSPM-DE. Triglycine rejection was always higher than NaCl rejection for the MPF-36 membrane. Therefore, it can be concluded that solutions with low-molecular-weight peptides can be purified from salts using a nanofiltration membrane with a high molecular weight cut-off.