Recent developments in the preparation of improved nanofiltration membranes for extreme pH conditions

Conventional commercially available nanofiltration (NF) membranes show limited stability at extreme pH conditions, whereas commercially available NF membranes that can cope with these conditions often show (too) low permeance or are relatively open. Since NF applications in many industrial sectors require pH stable NF membranes with a range of molecular weight cut-offs (MWCO), including tight NF membranes, substantial research and development efforts are being devoted to the research and development of these membranes. This review discusses these developments as reported in open literature, with a focus on the period between 2016 and 2021. Most developments relate to the use of interfacial polymerization to produce thin film composite membranes. Polyamine and polyurea membranes with good chemical stability for extreme pH conditions have been prepared. For polysulfonamide membranes indications for good chemical stability at low pH have been shown. These membrane types show improved stability compared to developed poly(aryl cyanurate), polyesteramide, poly(amide-sulfonamide), and polyamide membranes, which are more susceptible to hydrolysis and therefore less chemically stable at extreme pH. Furthermore, layer-by-layer coating using strong cation - and anion polyelectrolytes has led to new pH stable NF membranes. Despite the extension of the pH stable NF membrane portfolio, most of the developed membranes still do not meet the performance characteristics of the best commercially available NF membranes applicable for the common pH range between 2 and 11. Only a few layer-by-layer coated polyelectrolyte membranes combine high permeance, relatively low MWCO and stability at extreme pH. However, these membranes are not yet commercially available for full-scale applications requiring high pressure operation. Consequently, there is still room for research and development to further improve pH stable NF membranes.