Membrane technology has emerged as an attractive approach for water purification, while mitigation of fouling is key to lower membrane operating costs. This article reviews various materials with antifouling properties that can be coated or grafted onto the membrane surface to improve the antifouling properties of the membranes and thus, retain high water permeance. These materials can be separated into three categories, hydrophilic materials, such as poly(ethylene glycol), polydopamine and zwitterions, hydrophobic materials, such as fluoropolymers, and amphiphilic materials. The states of water in these materials and the mechanisms for the antifouling properties are discussed. The corresponding approaches to coat or graft these materials on the membrane surface are reviewed, and the materials with promising performance are highlighted.

Polymeric membranes for wastewater reuse are often fouled by suspended solids and dissolved organic matters, resulting in a dramatic decrease in water flux. Fouling can be mitigated by enhancing hydrophilicity of the membrane surface to avoid favorable interactions between the membranes and foulants. However, the modification of the membrane surface is often complicated and difficult to integrate into industrial membrane production processes. Herein we demonstrate a facile one-step coating of superhydrophilic zwitterions on the surface of ultrafiltration (UF) membranes by codepositing dopamine and sulfobetaine methacrylate (SBMA). In the presence of oxygen, dopamine forms polydopamine (PDA) adhering onto the membrane surface and anchors SBMA via Michael addition to form a robust thin superhydrophilic layer, as confirmed by contact angle measurement and X-ray photoelectron spectroscopy (XPS). The modified UF membranes demonstrate up to 80% higher water flux than the uncoated ones, when tested with water containing bovine serum albumin (BSA) as a model foulant in a crossflow system. This facile approach of membrane modification is also adapted for postmodification of a commercial nanofiltration (NF) membrane module, which demonstrates superior antifouling properties when tested with real wastewater at a wastewater treatment plant.

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Zwitterionic materials have been widely used to modify membrane surfaces to increase surface hydrophilicity and mitigate fouling, which would otherwise decrease water permeance. However, zwitterionic materials are water-soluble, and it is challenging to graft or coat zwitterions for long-term underwater operation. In this work, we demonstrate a facile two-step coating of membrane surface using a novel zwitterionic copolymer that contains thiol groups, which is achieved using bioadhesive polydopamine (PDA) as the intermediate layer. The copolymers were synthesized by reverse addition–fragmentation chain transfer (RAFT) copolymerization of phosphobetaine methacrylate (MPC) and 2-(methacryloyloxy) ethyl lipoate (MAEL), followed by reducing the MAEL units of the resulting copolymers to dithiol-containing units (i.e., DTMAEL). While strongly bound to the membrane surface, PDA reacted with the thiol groups through Michael addition reaction to covalently graft the zwitterionic polymer (p­(MPC160-co-DTMAEL42) or PMD) onto the membrane surface. The modified membrane surface was characterized using X-ray photoelectron spectroscopy (XPS) and water contact angle measurement. The effect of surface modification on pure water permeance was investigated. The modified membranes were challenged with water containing bovine serum albumin (BSA) using a crossflow filtration system. The modified membrane exhibited less flux decline (38% reduction) than the PDA-coated one (45% reduction) or the unmodified one (53% reduction), but lower water flux than the unmodified one.

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