In the last decades, reverse osmosis (RO) has become the leading technology for water treatment. Microfiltration and ultrafiltration (UF) membranes are the key features of this technology, as they have demonstrated being very efficient and long-lasting for the treatment of a wide range of different water sources, such as underground water, surface water wastewater and seawater. During the last years, capillary membranes (CMs), a subclass of UF membranes, have attracted increasing attention for water purificaton applications. The CMs are currently used in some purification plants, providing up to 60,000 m3/day drinkable water. The CMs consist in a tube with the inner part working as membrane and the outer part as shell. The raw water, referred to as a feed solution, flows inside the inner part and the purified water is filtrated out through tangential crossflow. The purified water is then collected from the outer shell part of the tube. The main advantage of this technology is the possibility to pack hundreds of CMs in a large catridge-like container and make them work in tandem. Nevertheless, the bottleneck of CM technology is the low removal efficiency of nanoscale-size pollutants, such as heavy metal ions and small molecules.
The GRAPHEME project aimed to enhance the filtration capability of CM technology from microscopic to nanoscopic pollutants, such as metal ions and small molecules. This goal could be achieved thanks to the integration of graphene and related materials (GRMs)-based CMs. This combination led to a new hybrid material, GRM–CM, which has high removal efficiency, low production cost and long lifetime. The oxidized homologue of graphene, also known as graphene oxide (GO), proved to be the best candidate as GRMs because of its availability and well-known chemical tunability. Moreover, GO displays numerous oxygen functional groups (OFGs) such as carbonyls, epoxides, and hydroxides that trap the pollutant. The functionalization of CMs surfaces with GO enabled to control both the water permeability and the ions sieving behaviors. The functionalization technique required only a flow deposition of GO on the inner part of the CM and a low-temperature thermal treatment of the functionalized membranes under atmospheric condition. It made our approach economically convenient and easy to be scaled up. This hybrid material obtained showed a filtration efficiency towards heavy metal ions 78% higher than the unfunctionalized CMs, working with a water flow of 20 mL/min. In view of the low cost of all components and scalability of all processes used, our methodology holds potential to estiablish itself as a new and revolutionary water purification technology.
