Abstract
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.
Highlights
Nanofiltration (NF) membranes are used in a wide range of appli cations, such as water treatment, in the food industry, biotechnology and in the textile industry [1,2]
The first part deals with the performance of the four different polyelectrolyte multilayer (PEM) membranes in terms of pure water permeance (PWP), salt retention and molecular weight cut-off (MWCO) prior to exposure to extreme pH
In the second part we show how the performance of the membranes evolves when they are exposed to different extreme pH conditions
Summary
Nanofiltration (NF) membranes are used in a wide range of appli cations, such as water treatment, in the food industry, biotechnology and in the textile industry [1,2]. The formation of multilayers is mainly driven by the release of counter ions, leading to an entropic gain [6,7,8] Though, other effects such as electrostatic effects, van der Waals forces, hydrogen bonding or hydrophobic interactions may have an influence [9]. A great advantage of PEMs is that their properties can be finely tuned by varying a number of parameters, such as ionic strength, types of poly electrolyte, pH and number of layers [6,10]. In this way, good control over the PEM material properties and over the final membrane properties can be achieved
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