Ageing under extreme ph conditions, modification and characterization of ultrafiltration and nanofiltration membranes

Abstract

The aim of this work has been the study of the ageing of ultrafiltration (UF) and nanofiltration (NF) membranes under long-term exposure to acidic and alkaline cleaning solutions. A theoretical model has been developed for the interpretation of liquid-liquid displacement porosimetry (LLDP) results. In addition, membranes with an improved fouling resistance have been prepared by graft copolymerization of a polyelectrolyte hydrogel on the top of an UF membrane. A comprehensive literature review, concerning the main foulants, the most common cleaners, the consolidated techniques to determine cleaning effectiveness, the parameters affecting such process and the most suitable cleaning procedures as function of the fouling nature, is also included in this work. The first section of the experimental work is devoted to the ageing of polyethersulfone UF and polyamide NF membranes under long-term exposures to acidic and alkaline conditions. Six commercial UF membranes from Nadir, Koch and GE Osmonics manufacturers and two NF ones from Dow Filmtec were subjected to different concentration nitric acid and sodium hydroxide solutions at several temperatures for long periods of time (up to six months) to simulate the successive cleaning steps throughout their lifetime. Permeability and rejection were used to evaluate membrane performance. Membranes were also characterized in their pristine state and after ageing by physicochemical techniques. Attenuated total reflection Fourier transform infrared and energy-dispersive X-ray spectroscopy were used to analyze surface membrane composition. Scanning electron microscopy, atomic force microscopy and contact angle measurements allowed characterizing structure, morphology and wettability of membrane surfaces, respectively. The second section of the experimental work describes a new approach to LLDP to characterize UF membranes. Four polymeric UF membranes with molecular weight cut-offs ranging from 5 to 300 kDa were employed. A model membrane defined by a log-normal pore number distribution and a given pore number density was fitted to the actual membrane during the LLDP analysis. Model validation was carried out comparing the pore number and permeance distributions as well as typical LLDP parameters (mean radius, asymptotic permeance, etc.) obtained from the proposed model with those from the traditional Grabar-Nikitine method. New NF membranes, characterized by a high charge density and a low roughness, thus, low prone to fouling, have been manufactured using UV-photo initiated co-polymerization of strong polyelectrolyte hydrogels on the surface of PES membranes. The effects of the functional monomer concentration, the cross-linker fraction, the irradiation intensity and the modification time on the degree of grafting, composition, structure and membrane performance of the hydrogel membranes were studied.