On the long-term pH stability of polyelectrolyte multilayer nanofiltration membranes

Long-term pH stability is critical for nanofiltration membranes in many applications, e.g. dairy and mining industry. We present a systematic study on the long-term pH stability of four different polyelectrolyte multilayer (PEM) nanofiltration membranes. The stability was assessed by comparing their performance before and after exposure to up to 1 M HNO₃ (~pH 0) and 1 M NaOH (~pH 14), in terms of pure water permeance (PWP), salt retention, and molecular weight cut-off (MWCO). Poly(diallyldimethylammonium chloride) (PDADMAC)/poly(styrenesulfonate) (PSS) nanofiltration membranes show excellent stability under extreme acidic and basic conditions for more than 2 months (10.7 L m⁻²h⁻¹bar⁻¹ PWP, 95.5% MgSO₄ retention, 279 g mol⁻¹ MWCO), attributed to the use of strong polyelectrolytes, of which the charge is unaffected by pH. Poly(allylamine hydrochloride) (PAH)/PSS membranes show stable performance when exposed to extreme acidic conditions (9.7 L m⁻²h⁻¹bar⁻¹ PWP, 97.5% MgSO₄ retention, 249 g mol⁻¹ MWCO). Under these conditions, PAH remains charged and therefore a stable multilayer is maintained. PDADMAC/poly(acrylic acid) (PAA) and PAH/PAA membranes are not stable at extreme pH conditions. These results highlight that PEM nanofiltration membranes, especially PDADMAC/PSS membranes, have tremendous potential for use at extreme pH conditions. Compared to most commercially available membranes they have superior long-term stability and very relevant performance.