2023

Soto-Salcido L.A.; Pihlajamäki Arto; Mänttäri Mika

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.