Flux enhancement by hydrophilization of thin film composite reverse osmosis membranes

Abstract

Protic acids, including hydrofluoric, hydrochloric, sulfuric, phosphoric and nitric acids, are studied as hydrophilizing agents for modifying the surfaces of thin film composite (TFC) reverse osmosis membranes. HR95PP and HR98PP membranes on exposure to various concentrations of these acids increase in flux up to an order of magnitude without any loss in ion-rejection. The flux enhancement is about an order of magnitude in HR98PP and a factor of two in high flux HR95PP membranes. Surface characterization with contact angle measurements indicate an increase in hydrophilicity of the membrane surface. At solvatable sites along the polymer chain, reactions causing partial hydrolysis may be responsible for the increase in hydrophilicity resulting in the observed flux increase. Exposure to mild solvents like ethanol and 2-propanol (isopropyl alcohol, IPA) also increases the flux with no loss in rejection; in fact a significant increase in rejection is obtained in some cases. Selective dissolution and elimination of defects probably are responsible for the increased rates of transport. Exposure to a mixture of acid and ethanol caused an increase in flux with no loss in rejection within experimental error. This method of treatment with protic acids and alcohols could be used as a viable post treatment method in developing high flux and high rejection membranes of the future.