We characterized the fully aromatic polyamide (PA) active layers of six commercial reverse osmosis (RO) and nanofiltration (NF) membranes and found that in contrast to their similar elemental composition, total concentration of functional groups, and degree of polymerization, the ionization behavior and spatial distribution of carboxylic (R-COOH) groups within the active layers can be significantly different. We also studied the steric effects experienced by barium ion (Ba2+) in the active layers by determining the fraction of carboxylate (R-COO-) groups accessible to Ba2+; such fraction, referred to as the accessibility ratio (AR), was found to vary within the range AR=0.40-0.81, and to be generally independent of external solution pH. Additionally, we studied an NF membrane with a sulfonated polyethersulfone (SPES) active layer, and found that the concentration of sulfonate (R-SO3-) groups in the active layer was 1.67 M, independent of external solution pH and approximately three times higher than the maximum concentration (approximately 0.45+/-0.25 M) of R-COO- groups in PA active layers. The R-SO3- groups were found to be highly accessible to Ba2+ (AR=0.95+/-0.01).

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In this work, a novel process for lactic acid purification based on ion exchange is proposed. It consists of two stages: first, a strong cation resin was used to reduce the broth pH below the pK a of lactic acid (3.86) and remove the cations present in the solution; then lactic acid was separated from the other anions in the broth using a weak anion exchanger. The ion exchangers selected were Lewatit S2568H and Lewatit S3428 (Bayer, Germany). The Lewatit S2568H resin retained the cations in the broth with an apparent capacity of 1.7 equiv/ L, reducing the broth pH below 3.86. It was able to decrease the pH of the broth to as low as 1.5, generating 4 bed volumes (BV) of acidified and partial demineralized broth. In the second step, the acidified broth was purified by the Lewatit S3428 resin, which retained the other anions present in the broth, producing an aqueous solution of lactic acid. This resin was able to produce 25 BV of purified lactic acid with selectivity higher than 99.9%. The final purity of lactic acid was higher than 99%.