Rejection of pharmaceuticals during forward osmosis and prediction by using the solution–diffusion model

Abstract

Two commercial forward osmosis (FO) membranes (HTI-ES and HTI-NW) were employed to study the rejection performance of 24 pharmaceuticals (PhACs) using NaCl as the draw solute. The PhAC permeability coefficient (B value) was determined for each PhAC by using both the reverse osmosis (RO) mode method and the diffusion cell method. The B values were used to predict the rejection ratios in the FO mode. The rejection ratio increased with the increase of draw solute (NaCl) concentration for each PhAC. Under a NaCl concentration of 1mol/L, all PhACs were highly rejected by >90%, except for a few including nalidixic acid, gemfibrozil, carbamazepine and sulfamethoxazole, which were rejected by 80–90% when HTI-ES membrane was used. The HTI-NW membrane could reject PhACs better than the HTI-ES membrane; however, the PhACs followed almost an identical sequence in terms of the rejection ratios. Results showed that the B values for several charged PhACs of relatively low molecular weight obtained by the diffusion cell method could be substantially larger than that determined by the RO mode method. In comparison with the experimental data, the B values obtained by the diffusion cell method were more appropriate to be used to predict the rejection ratios of the PhACs by the solution–diffusion model during FO operation. The underestimation of the B values by using the RO mode method might be primarily due to the ion exchange mechanism caused by reverse draw solute permeation during FO operation. Compared with the hydrophobicity and the charge properties, the molecular weight of PhAC was a more important factor in determining its B value. Very low B value is expected if the molecular weight is higher than 300Da. Exceptions, however, were found including clofibric acid, gemfibrozil and sulfadiazine. The solute-membrane affinity should also be taken into consideration when trying to link the B values with physicochemical properties of the PhACs.