Engineering carbon nanotubes enhanced hydrophobic membranes with high performance in membrane distillation by spray coating

Abstract

The commonly used commercial hydrophobic membranes are facing great challenges in wetting and low permeation when used in membrane distillation (MD) because of the limited materials and structures. To address these problems, we herein fabricated a kind of carbon nanotubes (CNTs) enhanced membrane with high flux and stable performance in membrane distillation, where a porous hydrophobic network of multi-walled carbon nanotubes (MWCNTs) was spray-coated onto the commercial flat-sheet hydrophobic membrane surface. Then, we systematically investigated the impact of the CNTs loading on the substrate surface properties, membrane structures and MD performance (flux and rejection), and experimentally illustrated the role of CNTs towards improving MD performance. Results illustrate that there is an optimal match of the CNTs spray amount with the substrate pore size and the substrate materials to achieve the maximum specific surface area of membrane, and thus the maximum flux and salt rejection of the resultant CNTs-enhanced membrane, which is governed by the interfacial energy of the CNTs and the substrate, and thus the CNTs packing density. Results find that the CNTs packing density decreases with the decrease of the substrate free surface energy, and the loosest CNTs packing is formed on the polytetrafluoroethylene (PTFE) membrane with the lowest free surface energy. Under the optimized conditions, the CNTs enhanced PTFE membrane exhibited salt rejection of >99.9% and water flux of 33.2 LMH in MD, which was three times as high as that of the pristine PTFE membrane under the temperature difference of 40 °C between hot and cold streams when operated in direct contact membrane distillation. Our research here provides an efficient method to enhance the permeation and stability of commercial membranes by spray-coating CNTs and also supplies some insights on the design of next-generation MD membranes.