Effect of sodium hypochlorite exposure on polysulfone recycled UF membranes and their surface characterization

The present work aims to investigate the effect of chlorine on the surface of the polysulfone (PSF) layer uncovered in the transformed ultrafiltration (UF) membranes (recycled from end-of-life reverse osmosis (RO) membranes). With this purpose, 6 end-of-life RO membranes used previously during their useful life to treat brackish water or seawater were transformed using two higher exposure doses of free chlorine (50,000 and 300,000 ppm h). On one hand, the end-of-life membranes were first characterized in terms of the type of fouling identified by TGA, ICP and bacteria detection. On the other hand, to determine the stability of the PSF layer, the transformed PSF UF membranes were then characterized by means of its permeability and molecular weight cut-off (MWCO). In addition, membrane surface characterization was performed by ATR-FTIR, SEM and AFM. The results show that all the end-of-life RO membranes with organic and inorganic fouling were effectively transformed to PSF UF membranes at the ppm∙h values studied. However, one of the analysed sea water membrane model (HSWC3) showed more resistance to be transformed and the pores of the uncovered PSF layer only appeared at 300,000 ppm h. This membrane showed different surface appearance, with bigger pores after 300,000 ppm h exposure dose that might have affected the PSF UF membrane performance. This study indicates that a complete characterization of transformed PSF UF membrane surface is crucial for a deep understanding of the recycling process of end-of-life membranes.