Desalination membranes from pH-controlled and thermally-crosslinked layer-by-layer assembled multilayers

Abstract

We introduce a novel and facile approach to improve the desalination performance of pressure-driven layer-by-layer (LbL) assembled membranes. Electrostatic LbL multilayers composed of weak polyelectrolytes (PEs), e.g., cationic poly(allylamine hydrochloride) (PAH) and anionic poly(acrylic acid) (PAA), were prepared on commercial polysulfone substrates. In order to measure the ion rejection and permeate flux of these membranes, the ionic concentration of the feed solution and operating pressure were fixed at 2000 ppm NaCl and 20 bar, respectively. It was observed that the crosslinked (PAH pH 7.5/PAA pH 3.5) n¼10,20 multilayers, which were assembled at the pH conditions allowing a low charge density of the respective PEs, show relatively high ion rejection compared to other multilayers. This result suggests that the optimal structures for desalination membranes should contain a large amount of freely charged groups with densely-packed structures via crosslinking. Finally, the recycling process was employed to further improve the desalination performance. In this case the (PAH pH 7.5/PAA pH 3.5) n¼10,20 multilayers exhibited the ion rejection up to 99.8%. This pH-controlled and thermal crosslinking method suggests a new route for the design of well-defined desalination reverse osmosis membranes based on LbL multilayers.

Confinement Effects on Cross-Linking within Electrostatic Layer-by-Layer Assemblies Containing Poly(allylamine hydrochloride) and Poly(acrylic acid)

“…Amide cross-linking within PAH/PAA LbL films has been reported elsewhere by others, 41,[52][53][54] where temperatures ranging from 130 to 350°C were employed. Substrates used ranged from gold-coated silicon wafer to ITO and polysulfone membranes.…”

Layer-by-layer preparation of polyelectrolyte multilayer membranes for separation

“…[145] The degree of ionization and the presence of charged groups in the PEMM had a significant effect on ion rejection. [146] Thermal crosslinking helped to induce densely packed PEM structures, which increased the physicochemical stability of the PEMMs. [146] The RO membranes based on thermally cross-linked PEMMs with 10 to 20 bilayers of (PAH pH 7.5/PAA pH 3.5) showed an outstanding performance when adjusted to the pH of seawater (8.1) by maint aining a stable and a highly charged state with >99 % ion rejection, which is comparable to a commercial RO membrane.However, annealing didn't favour the permeability.…”

“…[146] Thermal crosslinking helped to induce densely packed PEM structures, which increased the physicochemical stability of the PEMMs. [146] The RO membranes based on thermally cross-linked PEMMs with 10 to 20 bilayers of (PAH pH 7.5/PAA pH 3.5) showed an outstanding performance when adjusted to the pH of seawater (8.1) by maint aining a stable and a highly charged state with >99 % ion rejection, which is comparable to a commercial RO membrane.However, annealing didn't favour the permeability. [146] At the above shown pH, these multilayers comprise non-ionized carboxylic acid and amine groups together with a high degree of internal charge pairing within the multilayers, which yields efficient ion rejection.…”

Functional polyelectrolyte multilayer membranes for water purification applications

“…Promising surface modification methods include polymerization [10][11][12][13][14], polyelectrolyte adsorption, and plasma modification [15]. In particular, the top selective layer is very important because the permeate flux and solute rejection in desalination membranes strongly depends on the physiochemical properties (e.g., surface charge, surface roughness, selectivity and permeability) of the selective layer [16]. Polymers and molecules with a net positive charge are able to kill microorganisms upon attachment or adsorption to surfaces [17][18][19][20][21][22].…”