Ion separation characteristics of nanofiltration was investigated related to the recovery of lactic acid from fermentation broth. Ion separation characteristics of a nanofiltration membrane, NTR-729 was studied using solutions of monovalent and/or divalent ions. A method for determining membrane surface charge from the rejection of NaCl, $Na_2$SO_4$, Mg$Cl_2$ and Mg$SO_4$ solutions was proposed. It was found that NTR-729 membrane was negatively charged by this method. The rejection and the flux of a certain ion on the membrane were influenced by its concentration and the ionic strength. Rejection of divalent anions was higher than that of monovalent anions due to the repulsion between anions and negative surface charge of the membrane. Thus, cations paired with monovalent anions were more permeable to the membrane than those paired with divalent anions. It was also found that cations in a mixed-solution containing monovalent and divalent anions were more permeable than those in a solution containing only divalent anions. As the concentration of a particular ion increased, its rejection significantly decreased. It was experimentally found that the major factor for the rejection change with concentration was not the shielding of membrane by coexisting cations as reported previously but an increase in solute permeability. Similar characteristics were observed for sodium lactate. Especially, pH had a significant effect on the rejection and the flux of sodium lactate. In dilute solutions, the rejection increased and the flux decreased as the pH increased. It was due to the increased repulsion between membrane charge and lactate ions as more lactate ions existed in the solution at a higher pH. In concentrated solutions, however, the increase of pH decreased the rejection. It was considered that the effect of the change of dissociation degree with pH was dominated by the increase of permeability by high concentrations of sodium ions added to adjust pH. In the case of a fermentation broth containing 90g/L lactic acid, the rejection and the flux of sodium lactate were about 5% and 2.8L/㎡h, respectively. An irreversible thermodynamics model and a solution-diffusion model which usually used for reverse osmosis systems were applied for the estimation of the flux. However, both models could not accurately predict the flux for a broad range of ion concentrations. Incorporation of the dependency of permeability on concentration to the solution-diffusion model was found to improve its accuracy.
