Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: II. Membrane physiochemical properties and their dependence on polyamide and coating layers

The physiochemical properties of 17 widely used commercial RO and NF polyamide (PA) membranes were fully characterized by atomic force microscopy, transmission electron microscopy, contact angle measurement, streaming potential analysis, and flux and rejection performance tests. The surface properties (roughness, hydrophilicity, and surface charge) and bulk properties (permeability and rejection) were demonstrated to be highly inter-dependent, as all these were determined by the polyamide chemistry and any associated surface coating layer. The 1,3-benzenediamine and trimesoyl chloride based fully aromatic membranes had surface roughness on the order of 100 nm, an order of magnitude rougher than the semi-aromatic poly(piperazinamide) membranes. Furthermore, the uncoated fully aromatic membranes were significantly more hydrophobic (contact angles 43–49°) than the semi-aromatic ones (~30°). The presence of a neutral polyvinyl alcohol (PVA) coating layer can significantly enhance hydrophilicity and reduce surface charge and roughness for fully aromatic PA membranes, while its effect was only marginal for semi-aromatic poly(piperazinamide) membranes. The selectivity of a membrane appeared to be inversely related with its permeability. The highly permeable piperazine based membranes were much less selective than the fully aromatic ones. The salt rejection of a membrane was enhanced upon coating with a PVA layer, at the expense of reduced permeability. The current study suggests that the physiochemical properties can be used to diagnose the polyamide and coating chemistry, in addition to the conventional spectroscopic methods. Understanding such dependence of membrane properties and performances on their structure and chemistry might also be important for membrane synthesis, modification, and their applications in water and wastewater treatment.