Preparation of antifouling TFC RO membranes by facile grafting zwitterionic polymer PEI-CA

Highlights

• A zwitterionic polymer PEI-CA was synthesized and grafted onto the RO membrane surface via facile LbL-IP method.

• The water flux of tailored membrane M-PEICA reached 3.92 L m⁻² h⁻¹ bar⁻¹ and with high NaCl rejection of 98.93 %.

• The M-PEICA membrane presented much enhanced antifouling performances towards typical model foulants LYZ, BSA and DTAB.

• The mixed reagents modified membrane M-mixed presented excellent antifouling performances, even better than the CR100.

Abstract

In this work, we synthesized a zwitterionic polymer PEI-CA for the first time and then grafted it onto the surface of polyamide RO membrane via layer-by-layer interfacial polymerization (LbL-IP). XPS and ATR-FTIR techniques were used to verify the successful modification with PEI-CA. The zwitterionic hyperbranched polymer PEI-CA endowed the modified membrane with much improved surface hydrophilicity and decreased surface negative charge. The tailored M-PEICA exhibited a high water permeance up to 3.92 L m⁻² h⁻¹ bar⁻¹ (increased by 62 %) while holding a good NaCl rejection (98.93 %). The dynamic fouling experiments proved that grafting of PEI-CA can help to effectively elevate its antifouling performance against most organic foulants except small molecular negatively charged surfactant SDS. Therefore, we further employed the mixed reagents modification strategy with arginine and PEI-CA, the resulting RO membrane M-mixed presented obviously improved antifouling performance towards SDS, the overall antifouling performances are even better than the well-known RO membrane CR100 of DOW FILMTEC™, that well illustrated multiple reagents modification strategy could be an efficient way that can meet all critical standards. Overall, the novel polymer PEI-CA and as-fabricated RO membranes with outstanding antifouling performances in this study have vast application potential in field of water treatment.

Introduction

Due to the increase in the global population and the vigorous development of industry, the problem of water shortage needs to be solved as soon as possible. As an advanced water treatment technology, reverse osmosis technology has been widely used in water desalination and reclamation [1], [2]. Polyamide thin-film composite (PA TFC) membranes fabricated via interfacial polymerization on polysulfone substrates are the most widely used reverse osmosis membranes [3], [4], [5], [6]. The dense and thin active layer of the reverse osmosis membrane provides it with high efficiency and high quality of eluents [7], [8]. However, PA RO membranes still have a few challenges, such as relatively low permeance and high fouling propensity, which will increase the cost of RO processes in practical applications [9], [10]. Therefore, the development of advanced reverse osmosis membranes with high permeability and anti-fouling properties are still on highly demand [11], [12].

As known, hydrophilic modification of the membrane surface and regulation of membrane surface charge are the two effective strategies to enhance the RO membranes' antifouling properties [13], [14], [15]. Firstly, the dense hydration layer produced by these hydrophilic materials can make it difficult for foulants to approach and adsorb on membrane surface since most organic pollutants are hydrophobic [16], [17], [18]. For example, Liu et al. [19] grafted PVA onto the polyamide membrane surface to improve their surface hydrophilicity, thereby greatly improved its antifouling property towards protein and surfactant pollutants. Piatkovsky et al. [9] coated the alternative lysine–glutamic acid polypeptide onto the RO membrane to construct a quasi-zwitterionic surface, that vastly enhanced the antifouling performance of the RO membrane for municipal wastewater treatment. However, although the above-mentioned examples all could effectively improve the antifouling properties of membranes. The incorporation of these linear polymers often leads to extra water transport resistances, which in turn reduces the permeability of the membranes [20].

The unique 3D structure of hyperbranched polymers will provide relatively loose deposition layer when anchoring on the membrane surface thus not to import extra resistance for water transportation [21], [22], [23]. Liu et al. [24] grafted hPG onto forward osmosis membranes, which meanwhile improved the membrane's water permeability and antifouling performances. Xu et al. [25] grafted PEI of different molecular weights onto the RO membrane surface, which greatly improved their antifouling performances to positively charged foulants, whilst with little impact on theirs water flux. Zwitterionic materials have already been proved as an efficient tool in membrane antifouling modification due to their super-hydrophilicity and near-neutral charge properties [26], [27], [28], [29], [30]. Lü et al. [31] grafted PEI onto the PA surface, and then performed the zwitterionic functionalization by reacting with 1,4-butanesultone. The modified NF membranes significantly improved the antifouling performances in filtration of cationic and anionic dyes solutions. Deng et al. [32] firstly grafted PEI to the PA NF membrane surface, followed by amino methylation and then zwitterionic functionalization by 3-bromopropionic acid, which exhibited improved water permeance and greatly enhanced antifouling properties. Lin et al. [1] constructed a zwitterion-like ultrathin layer on RO membrane surface based on PEI through grafting and subsequent Michael addition reaction, the tailored membrane showed strong antifouling performances towards most of organic foulants except anionic surfactant SDS. However, the above-mentioned three examples based on PEI polymer all adopted complex and time-consuming modification method, which make them difficult to be practically applied in industrial production [32]. Specifically, the exact molecular structure and/or zwitterionic ratio of the PEI-based hydrophilic polymer is also an important factor to influence their antifouling performance on the tailored membranes. Thus, it is necessary to further expand the diversity of the PEI-based zwitterionic polymer. Moreover, to the best of our knowledge, the combination of the other reagents with zwitterionic PEI for the further elevation of RO membranes' antifouling performance have not been disclosed yet.

Thus, herein a PEI-based zwitterionic polymer PEI-CA was prepared for the first time and then anchored it onto the polyamide RO surface via the facile layer-by-layer interfacial polymerization (LbL-IP) strategy, also named as second interfacial polymerization (SIP) [33], [34]. Here we utilized the residual acyl chloride groups on the nascent polyamide surface to react with the tailored molecules by the way of pouring grafting aqueous solution onto it. The PEI-CA was synthesized by a facile one-step reaction of 3-bromopropionic acid (3-BPA) with PEI in aqueous phase and purified by dialysis. The physical and chemical properties of the pristine and tailored membrane named M-PEICA were investigated substantially, which exhibited super-hydrophilic membrane surface with water contact angle (WCA) down to ~21.6°. Meanwhile, we performed dynamic fouling experiments with four typical model foulants to assess the effect of PEI-CA grafting on the antifouling performance of the RO membrane, together with comparison of the advanced RO membrane CR100 of DOW FILMTEC™. Moreover, we further studied the mixed reagent grafting strategy applying PEI-CA and arginine to further improve their antifouling performance especially towards small anionic surfactant SDS. Overall, the novel polymer PEI-CA and the as-fabricated RO membranes with outstanding antifouling performances in this study have vast application potential in the field of water treatment.