Reverse osmosis of binary organic solute mixtures in the presence of strong solute–membrane affinity

Reverse osmosis performance for certain low molecular weight organic solutes in water, such as phenol and its derivatives, is markedly different from that of simple aqueous salt solutions. The unusual solute–membrane affinity reverse osmosis behaviour of phenol–water–cellulose acetate system has been observed. This strong solute– membrane affinity has caused the separation decrease with increasing pressure and permeate flux reduction in the system. This is due to the strong solute–membrane affinity through cellulose acetate (CA) membranes. This work deals with the separation of a mixed organic solute system. A systematic study has been conducted to demonstrate the applicability of reverse osmosis fractionation of diluted aqueous solutions, namely phenol, p-nitrophenol and 3-chlorophenol by asymmetric cellulose acetate membranes. Besides, the effects of operating pressure and feed concentration on the performance of separation are investigated. Results show that the separation can be positive, zero, or negative, depending on the operating pressure, solute concentration as well as the coupling solutes.

The separation behaviour of the phenolic compounds and a cellulose acetate membrane showed that strong solute–membrane affinity exists in this system and could be explained by preferential sorption, acidity (polar and non-polar) mechanism and the coupling effect of the solute mixture. In the presence of strong solute–membrane affinity, the separation of mixed solutes may be positive, negative or zero depending on the operating conditions. As the system pressure was increased the separation decreased whereas the separation increased as the feed concentration was increased. This phenomenon is typical for strong solute– membrane affinity systems. Due to higher acidity p-nitrophenol shows higher affinity towards the membrane and the poorest rejection among the solutes. The effect of the solute size and diffusivity was less important compared to the acidity factor in this separation process.