Application of negative retention in organic solvent nanofiltration for solutes fractionation

Abstract

For the first time, the utilization of negative retention in organic solvent nanofiltration (OSN) for solute fractionation was proposed and realized. Using membranes made of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) and ethanol as a solvent, this principle was successfully proved for two different pairs of dyes: (i) Orange II (anionic; 350Da) and Solvent Blue 35 (neutral; 350Da), (ii) Remazol Brilliant Blue R (anionic; 626Da) and Oil Red O (neutral; 408Da). The solutes ratio in the permeate after one step filtration with 80% of liquid recovery was achieved as follows (initial ratio in the feed – 1:1): 1:10 for Orange II to Solvent Blue 35 and 1:26 for Remazol Brilliant Blue R to Oil Red O. With compromising of liquid recovery, the solutes ratio in the permeate can be significantly improved; for example, 15% liquid recovery may provide the ratio in favor of neutral solute as 1:260 and 1:340, respectively. Pronounced difference in solutes retention was interpreted by membrane–solute interaction expressed in terms of equilibrium solute distribution coefficient K between the membrane and liquid and difference in solubility parameters δ. Low values of solute distribution coefficient K for Orange II and Remazol Brilliant Blue R (less than 1) revealed that both anionic dyes have a tendency to be concentrated in liquid phase resulting in high solute retention (more than 90%). Better rejection of Remazol Brilliant Blue R (1258Å3) in contrast to Orange II (297Å3) was explained by difference in solute size. High K values (more than 10) observed for Solvent Blue 35 and Oil Red O indicated that these two neutral solutes have a high affinity to membrane material and can be easily accommodated within interconnected free volume elements of PTMSP. Such solubility determined solute transport resulted in negative retention. Analysis of solubility parameters δ of components showed that neutral solutes have a stronger affinity to the membrane material rather than ethanol; meanwhile, PTMSP has a preferential affinity to solvent molecules over selected anionic dyes. These experimental observations allowed to conclude that the principle of one step solutes fractionation based on combination of negative and positive retention could be realized with PTMSP membranes for OSN applications.