Modification of commercial UF membranes by electrospray deposition of polymers for tailoring physicochemical properties and enhancing filtration performances

Abstract

The main challenge for a widespread use of nanoporous membranes in the removal of ionic contaminants lies in the adjustment of their physicochemical properties to allow adequate ion rejection and mitigate fouling based on the targeted application. Most of the commercial membranes are negatively charged and their use is thus not necessarily relevant for divalent cation rejection. The main objective for researchers is therefore to provide novel tailored membranes by developing specific synthesis or modifying available membranes. It is proposed here to tailor physicochemical properties of a commercial low molecular weight cut-off ultrafiltration membrane by electrospray deposition of polyethylenimine (PEI) and polystyrene sulfonate (PSS). In this study, it is highlighted that, with adequate conditions, it is possible to adjust the charge of the membrane surface, which can reach values from −40 to +40 mV (compared with −20 mV for pristine membrane). Surface hydrophilicity has also been increased with a contact angle decreased from 60 to 30° with a PSS surface layer. In terms of filtration performances, it is shown that the permeation flux is not reduced by the electrospray deposition of polymer and can even be slightly enhanced in specific conditions. When polymer concentration is sufficient, the deposit is able to face the shear stress induced by cross-flow filtration, probably due to viscosity effect. The positive PEI surface layer leads to a strong enhancement in the rejection of divalent cations whereas that of divalent anions is notably decreased due to electrostatic interactions between the charge of divalent ions and that of the membrane. The weaker impact of electrostatic interactions on monovalent ions allows adjustment of the separation selectivity between cations and anions as well as between mono- and divalent ions. Finally, it is also demonstrated that rejection performances are mostly governed by the surface layer, even if the underlying deposit layers and membrane also have a non-negligible impact on ion rejection.