Influence of sorption on removal of tryptophan and phenylalanine during nanofiltration

Under conditions when amino acids were effectively neutral and the membrane was near its point of zero charge, crossflow nanofiltration experiments revealed an extended duration before steady-state permeate concentrations were attained for tryptophan and phenylalanine compared with glycine and alanine. Valine showed an intermediate behavior compared with Trp and Phe on one hand and Gly and Ala on the other. Additionally, steady-state rejections of Trp and Phe were lower than that expected from predominantly steric and electrostatic considerations (Gly, Ala, and Val), consistent with enhanced diffusion across the active layer of the membrane due to partitioning onto the polymeric matrix (polymer phase diffusion plus pore diffusion). Batch tests substantiated the unsteady-state removals during crossflow nanofiltration by revealing significant uptake of Phe and Trp, limited uptake of Val, and no measurable uptake of Gly and Ala on the polymeric membrane. Hence, sorption can lead to the overestimation of Trp and Phe, (and possibly Val) rejection capabilities of nanofiltration membranes in the short-term. In other words, even sorption of solutes with low octanol–water partition coefficients (log K_ow < 0) such as Trp and Phe requires more careful long-term measurements since it substantially increases the time to achieve steady-state conditions.