Rejection properties of alkyl phthalates with nanofiltration membranes

We have examined the rejection properties of nanofiltration (NF) membranes for alkyl phthalates and other solutes such as mono-substituted benzenes, alcohols, and saccharides. In this study, four kinds of NF membranes were used, each with nominal desalting degree (NaCl rejection) of 91, 60, 51, and 15%, respectively. The membrane separation experiments were carried out with a bench scale batch type cell at room temperature. The rejection of alcohols and saccharides was mainly controlled by steric hindrance. We have found that molecular width is a useful descriptor to represent this effect. On the other hand, the rejection of aromatic compounds, alkyl phthalates and mono-substituted benzenes, was strongly affected by their hydrophobic properties (log P). Higher hydrophobic alkyl phthalates (log P>4.7) were rejected at more than 99% even by the lowest desalting membrane. The highest desalting membrane rejected almost all alkyl phthalates at more than 96%. However, p-dimethyl phthalate and p-diethyl phthalate showed significantly lower rejection, from which it can be concluded that their molecular widths are close to those of mono-substituted benzenes, i.e. smaller than those of the other alkyl phthalates.