Molecular sieving effects of disk-shaped molecules on reverse osmosis and nanofiltration separation

• Published in: Separation and purification technology 2017-02, Vol.173, p.286-294
• Main Authors: Kiso, Yoshiaki, Oguchi, Tatsuo, Kamimoto, Yuki, Makino, Kazuya, Takeyoshi, Yuki, Yamada, Toshiro
• Format: Article
• Language: English
• Subjects: Alcohols; Crown ethers; Disk shaped molecule; Disks; Mathematical models; Molecular sieving; Nanofiltration; Porosity; Rejection; Reverse osmosis; Separation; Steric partition factor
• ISSN: 1383-5866; 1873-3794
• DOI: 10.1016/j.seppur.2016.09.031

[Display omitted] •The shape of crown ethers was approximated to be a disk.•The calculated rejections corresponded well to the observed rejection.•The calculated rejection of boric acid was markedly smaller than the observed one. The solute separation in reverse osmosis/nanofiltration (RO/NF) membrane processes is mainly controlled by both the diffusivity in a pore and the steric partition factor, and the latter is defined by the geometrical probability at which a solute can be accessible into a pore. The rejection of alcohols has been estimated semi-empirically by using the steric partition factor derived by the approximation of molecular shape as a rectangular parallelepiped. However, the approach was not suitable for crown ethers, and in this work the shape of crown ethers was approximated as a disk: the disk radius and disk thickness were developed as new shape parameters. The calculated rejections of crown ethers by using the disk-shaped model corresponded well to the observed rejections, where the used pore radius was calculated on the basis of the rejections of alcohols. Boric acid in non-dissociated form is also assumed to be a disk-shaped molecule, but the calculated rejection by the disk-shaped model was smaller than the observed one. The results suggest that boric acid transports through membrane pores with hydrated water molecules.