Atomic layer deposition and electrospinning for membrane surface engineering methods for water treatment: a short review

With the development of water purification, the membrane process has drawn attention because of its separation efficiency. However, low rejection efficiency and flux decline driven by membrane fouling remain as challenges. Accordingly, the surface modification of membranes has been widely adopted to enhance the membrane process. For example, the conventional method of sol–gel coating has been mostly used to employ the functional top layer that can prevent the accumulation/adsorption of foulants. Such a conventional method limits to designing the physico-chemical properties of membranes in a precise manner to achieve an enhanced membrane performance. One advanced method is the atomic layer deposition, in which a uniform metal oxide layer is coated in an atomic level. Because of its precise control of the metal oxide layer thickness, it has been mainly applied for tuning the pore structure and/or surface hydrophilicity of membranes. It consequently enhances the rejection efficiency and alleviates fouling by engineering the membrane surface chemistry. The fabrication of an electrospun nanofiber via electrospinning is another approach that has been applied to modify the physico-chemical properties of membranes. It engineers the surface chemistry of membranes (e.g., hydrophilicity/hydrophobicity and surface roughness) by integrating a specific nanomaterial into the electrospun nanofiber. Electrospinning enables porosity tuning by controlling the variations of the fabrication protocol. Despite many studies on membrane surface modification, review articles on the technological progress and challenges encountered by these functionalization methods are lacking. This review article provides the following: i) an introduction of the three methods for surface modification; ii) a systematic summary of their application to the membrane process for water treatment; and consequently iii) an insight of the technological challenges and future perspectives for their wide application to the membrane industry.