Customizing the surface charge of thin-film composite membranes by surface plasma thin film polymerization
Introduction
The surface properties of reverse osmosis (RO) thin-film composite (TFC) membranes are of fundamental importance in order to produce high water quality and to control both surface interactions and deposition mechanisms [1]. The ability of the membrane to reject dissolved salts is dependent on the charge of the surface and, therefore, on the polarizability of the functional groups present at the liquid/solid interface [2], [3]. The pH of the solution in contact with the membrane will dictate the hydronium and hydroxyl ion concentrations and the charge environment within the Stern layer, thus affecting which ions are repelled or attracted by electrostatic forces [4], [5]. The charge density, polarity and isoelectric points (IEP), as well as the roughness and exposed surface area of the membrane, therefore play a significant role in species rejection [6], [7]. Control of the surface charge and polarizability parameters is regularly highlighted as critical for suitable wastewater treatment or management in order to maintain membrane performance during operation [8]. In addition to the native poly(amide)(PA) material, commercial reverse osmosis (RO) membranes also often incorporate a protective proprietary hydrophilic coating, to limit the adverse effect of fouling on the long term operation of these materials [9].
Plasma polymerization technologies, typically performed with low pressure plasma systems, offer advanced platforms for a rapid surface functionalization, allowing for the simultaneous tuning of surface energy and morphology [10], [11]. The morphology of TFC membranes makes them, however, vulnerable to surface reactions and degradation. New routes to engineer customized surfaces properties of TFC membrane materials for specific applications and effluents, without compromising the integrity of the selective layer, are required. To date, most of the plasma surface modifications of membrane materials have involved the use of radio frequency (RF), low pressure, plasma generation to induce chemical polymerization and deposition [12], [13]. However RF plasma generators are highly energetic and yield large densities of activated radical species, thus generating harsh conditions that can result in degradation of both substrate and monomer molecules [14], [15]. The drying process induced at low pressure may cause capillary stresses and collapse across the pores of the PA or underlining layers within the membrane material and ultimately compromise liquid permeability [16], [17], [18]. The ultra-thin PA layer across the TFC membranes was previously shown to be compromised by surface etching mechanisms even at low working powers and plasma glow intensities [10] and, in such conditions, the water flux was previously shown to be severely reduced by up to 70% [19]. Besides, polymerization - deposition mechanisms typically lead to a high degree of branching and crosslinking of the monomer molecules and, therefore, to low control of the free volume density and overall microstructure of the material [20]. Low frequency alternating current (AC) systems on the other hand offer higher control in terms of both deposition kinetics and chemical activation since they are typically operated with much lower carrier gas pressures [20]. Monomer molecules may therefore be primed and activated by the AC plasma during the polymerization process leading to better defined macromolecular structures and degrees of cross-linking [21]. The control and investigation of polymerized layers across the membrane surfaces may, however, be controlled down to the nanoscale and shows promise for the customization of the charge density and polarizability of TFC membranes [22].
In this work, an advanced strategy following our previous work on plasma polymerization was investigated for the surface modification of commercial RO membranes, which led to (i) enhanced performance in terms of permeability/selectivity and (ii) unprecedented control over the surface charge of the material. The application of an AC low pressure plasma generator was explored to control the deposition of functional moieties across the commercial TFC membrane surface without the need for washing off the nascent coating preservative layer prior to plasma treatment. The monomers, maleic anhydride (MA) and vinylimidazole (VIM), were employed to generate negative or positive charged ultra-thin coatings, respectively.
Section snippets
Materials and reagents
BW30 TFC membranes were purchased from Dow Filmtec Corp. (IMCD Australia Limited). The flat sheet membranes used in this work were collected from the same internal area of the element and stored in a dry environment at 4 °C. Analytical grade sodium chloride (NaCl) was purchased from Sigma Aldrich (Sydney, NSW, Australia) and used for the preparation of saline feed solution to give a concentration of 2000 ppm. Milli-Q water was used for the preparation of all aqueous solutions. The monomers used
The influence of polymerized coating layers on performance of the membranes
The performance of the two series of membranes were evaluated without any pre-conditioning treatment in a cross-flow filtration system.
As shown in Fig. 1A, the water flux after plasma polymerization with the VIM monomer was initially increased to 47.2 and 49.2 L m−2.h−1 after 5 and 9 min of treatment corresponding to 5% and 10% increase, respectively, compared to the control membrane (44.9 L m−2 h−1). This increase may be attributed to combined effects simultaneously happening across the surface of
Conclusions
This work investigated the impact of plasma polymerization on the performance of TFC membranes using two monomer models. The surface charge of the membranes was customized by altering the nature of the monomers and plasma durations used, in otherwise fixed plasma treatment and pre-conditioning conditions. It is found, compared to previous studies, that AC plasma provide better deposition conditions, while preservatives present across the surface could be etched within minutes of treatment
Acknowledgement
Dr. DUMEE acknowledges Deakin University for his Alfred Deakin Post-Doctoral Fellowship (ADPDF2015). The support of the Australian Synchrotron for the beamtime M6419 on the IR spectroscopy beamline.
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