Reuse of end-of-life membranes through accelerated polyamide degradation

Abstract

End-of-life (EoL) thin-film composite (TFC) reverse osmosis membranes were converted into ultrafiltration-like (UF) membranes in an accelerated degradation process of the polyamide (PA) using an oxidant (NaOCl) in the presence of either MgCl₂ or CaCl₂. The PA degradation was evaluated by measuring pure water permeability (PWP), MgSO₄ passage and molecular weight cut-off; the more PWP increased, and the less MgSO₄ was retained after treatment, the more the PA was degraded. By adding 10 mM of metal ions, PWP increased 2.1 (MgCl₂) and 3.1 (CaCl₂) times compared to the increase achieved with hypochlorite alone (2560 ppm∙h of free chlorine). Changes in the membranes after treatment were analyzed by Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), and by measuring their surface charge and contact angle. FTIR and FE-SEM confirmed the PA layer degradation. FE-SEM micrographs showed that full removal of the PA layer can be achieved by using an oxidation dose of 12,700 ppm∙h when Ca²⁺ is used but doses as high as 300,000 ppm*h are needed without catalyst. The results proved that by controlling the oxidation process it was possible to control the cut-off (MWCO) value of the membrane from 16,100 g∙mol^-1 to 27,100 g∙mol^-1. Before treatment, EoL membranes showed a MWCO of approximately 1200 g∙mol^-1, meaning that molecules with that size could be retained in a 90%. In summary, the presented method enables reducing waste by the conversion EoL membranes into tailored UF-like membranes and by decreasing the amount of oxidant used in the conversion process.

Keywords: Accelerated polyamide degradation; End—of—life; Reverse osmosis; Thin—film composite; Ultrafiltration—like conversion.