Influence of Adsorption of Pharmaceuticals onto RO/NF Membranes on Their Removal from Water

Adsorption together with size exclusion and charge attraction/repulsion has to be taken into account when considering removal of pharmaceuticals as emerging contaminants from water by reverse osmosis and nanofiltration membranes. Glucocorticosteroids (hydrocortisone (HYDRO), dexamethasone (DEXA)), anesthetics (procaine, lidocaine) with relatively weak hydrophobicities (1 < log KO/W < 3), and membranes (XLE, LFC–1, CPA3, SWC1, NF90, and NF270) have been investigated in this study. Adsorption was studied by measuring the concentration of compounds in feed and permeate and by monitoring changes in membrane flux in the batch mode operation during 24 h. A decrease in the feed concentrations for HYDRO and DEXA (log KO/W < 2) was observed. The loss of these compounds in feed was associated with irreversible adsorption onto an NF270 and a CPA3 membrane. Therefore, when considering removal of pharmaceuticals with lower hydrophobicity, adsorption has to be particularly taken into account for membranes with bigger pores in the selective layer. Also, a high dipole moment and low water solubility affected adsorption on the membranes. For smaller and slightly more hydrophobic pharmaceuticals (log KO/W > 2), an increase in the feed concentration was obtained. Firstly, these compounds instantly adsorbed to the membrane. Secondly, the compounds diffused through the polymer matrix and desorbed to the permeate side after equilibrium had been reached.