Effect of pH on the ageing of reverse osmosis membranes upon exposure to hypochlorite Original Research Article

The declining functionality associated with membranes ageing includes changes in physical parameters, such as, thickness, roughness and density of defects, but also in chemical structure. All these factors impact synergistically on the major performance indicators: permeability and salt rejection. In this study, three types of commercially available RO membranes were statically exposed to hypochlorite solutions and analysed by Fourier transform infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) in conjunction with performance tests. IR data support the chemical structure alteration for samples aged at pHs 7 and 4, where hypochlorous acid (HOCl) is the main oxidant. For two of the membranes, AFM results indicate increasing roughness at pH 4. Performance tests show a reduction of de-ionised (DI) water and brackish water permeability at pH 7 and pH 4, while at pH 10, where hypochlorite ion (ClO−) is abundant, permeability increases. Salt rejection results vary in a narrow interval of 5% and depend on the type of membrane. Based on these results the ppm∙h concept appears to fail to express a simple ageing kinetic over the entire range of pH, owing to the competing mechanisms of ring chlorination and surface hydrolysis of amide groups.