(244c) Steric and Electrostatic Interactions Govern Nanofiltration of Amino Acids

Crossflow nanofiltration experiments were performed to investigate the factors influencing the removal of amino acids by a polymeric thin-film composite membrane.

The first part of the presentation will focus on the removals of five monoprotic (Ala, Val, Leu, Gly, and Thr), one diprotic (Asp), and one dibasic (Arg) amino acids in a range of permeate fluxes, feed pH values, and ionic strengths. Transport of these hydrophilic amino acids was analyzed using the phenomenological model of Kedem and Katchalsky. At any given pH and ionic strength, reflection coefficients (rejection at asymptotically infinite flux) of monoprotic amino acids increased with molar radius demonstrating the role of steric interactions on their removal. Additionally, consistent with Donnan exclusion, higher reflection coefficients were obtained when the membrane and the amino acids both carried the same nature of charge (positive or negative). In other words, both co-ion repulsion and molecular size determined amino acids removal. Importantly, the removal of effectively neutral amino acids were significantly high than neutral sugars and alcohols of similar size demonstrating that even near their isoelectric point, zwitterionic characteristics preclude them from being considered as strictly neutral.

The second part of the presentation will demonstrate the importance of sorption in determining the dynamic nanofiltration of two hydrophobic amino acids, viz. phenylalanine and tryptophan. Batch tests showed that these amino acids having hydrophobic side chains were substantially absorbed on the polymeric membrane, which was quantitatively modeled using the Freundlich isotherm. Crossflow nanofiltration revealed that convective flow makes adsorption sites more accessible to amino acids. Hence, the permeate concentration changed more rapidly at the initial stages of filtration compared with batch tests.

In summary, co-ion repulsion and steric interactions govern hydrophilic amino acids removal by ?tight? nanofiltration membranes whereas sorption plays an important role during the initial filtration stages for hydrophobic amino acids.