Partitioning of organic solutes between water and polyamide layer of RO and NF membranes: Correlation to rejection

The presence of organic compounds such as pesticides, herbicides, hormones and other toxic substances in raw surface and ground water is one of the emerging problems in the drinking water production. Membrane processes such as reverse osmosis (RO) and nanofiltration (NF) are considered promising candidates for removal of organics from water, however, recent reports indicate that many organic compounds, such as amines, phenols and amides, pass the membranes. Sorption in the membrane was assumed to be one of the factors that govern the selectivity of membranes towards small organic molecules and quantitative sorption data are crucial for understanding this effect. In this paper we report results of sorption measurements in the polyamide active layer (skin) of thin film composite membranes (SWC1, ESPA1 and NF200) of a group of organic compounds that pass these membranes significantly. The method is based on the analysis of attenuated total reflection FTIR spectra of the isolated active layer and is thus free of possible artifacts related to sorption by the support. Knowledge of the skin thickness allowed us to calculate the partition coefficient of the organic solute between the polyamide and water. Comparison of the sorption and rejection data for different membranes and solutes reveals that partitioning may significantly modify and even reverse the general rejection-size trend, as was demonstrated for a homologous series of alcohols. For some solutes, different rejection was found for similar values of both partitioning coefficient and size, suggesting that yet unclear effects related to molecular friction could be involved.