Stability of NF membranes under extreme acidic conditions

Two commercial nanofiltration (NF) membranes (FilmTec NF-45 and Desal-5 DK) and two new NF-1 membranes made by BPT (Bio Pure Technology) for the purpose of a European Union funded research project (RENOMEM) were tested under extreme acidic conditions. The polyethersulphone (PES) ultrafiltration (UF) supports used for casting the BPT-NF-1 membranes were also tested under similar conditions. The 006 and 015 UF supports were found to be stable in 5% nitric acid at 20 and 80 °C for 4 and 3 months, respectively. Both supports (006 and 015) showed a significant reduction in flux after immersion in sulphuric acid at both temperatures. The BPT-NF-1 membranes showed excellent resistance to 20% sulphuric acid for up to 4 months at 20 °C but were attacked by the nitric acid solution. The resistance of the two commercial membranes in 20% sulphuric acid at 20 °C was generally lower than that of the BPT-NF-1 membranes. The NF-45 membrane was slightly more stable in 5% nitric acid at 20 °C. Degradation of the membrane occurred only after 2 months while both the Desal-5 DK and BPT-NF-1 membranes degraded during the first month. At the higher temperature of 80 °C in 5% nitric acid all membranes degraded in the first month.The cause of membrane degradation was attributed to oxidation of the thin NF selective skin layer in nitric acid and to acid-catalysed hydrolysis of this layer in sulphuric acid. Knowing the cause of membrane degradation is a step forward in developing a better and more stable nanofiltration membrane.