Clean water is a universal need for prosperity in human societies. Removing harmful chemicals from water and wastewater is necessary to protect public health and the environment. Membrane separation processes such as reverse osmosis and nano-filtration are quickly replacing conventional treatment methods due to their high efficiency, high automation and compact design. It is reported that the market size for reverse osmosis and nanofiltration is projected to reach USD 13.5 billion and 950 million in 2025, respectively. Nevertheless, the performance of these membranes has improved significantly in the past several decades, a trade-off between permeability (water transfer rate per membrane area) and selectivity exists in most membrane materials, i.e., increasing permeability of target species leads to a decrease in selectivity between target species over competing species. Increasing global water stress, more stringent regulations, and rising expenses for wastewater disposal demand higher water recovery, lower treatment cost, and greater value generation–all pointing to the need for high selectivity membranes.
A recent collaboration between NEWT institutes and Tsinghua University has comprehensively analyzed this problem and highlighted the potential of advanced materials to improve membrane selectivity for the recovery of target species over various competing species.
Collaborative cross-campus efforts (left to right): Kuichang Zuo (Rice), Kunpeng Wang (Tsinghua), Ryan M. DuChanois (Yale), Qiyi Fang (Rice), Eva M. Deemer (UTEP), Xiaochuan Huang (Rice), Ruikun Xin (Rice), Ibrahim A. Said (Rice), Ze He (Rice), Yuren Feng (Rice), W. Shane Walker (UTEP), Jun Lou* (Rice), Menachem Elimelech* (Yale), Xia Huang* (Tsinghua) and Qilin Li* (Rice).*Corresponding author.
In this review, the authors discuss the drivers, fundamental science, and potential enabling materials for high selectivity membranes, as well as their applications in different water treatment processes. Membrane materials and processes that show promise to achieve high selectivity for water, ions, and small molecules¾as well as the mechanisms involved¾are highlighted. In particular, approaches are used to construct membranes with well-defined pore sizes and structures using novel building blocks, including two-dimensional nanomaterials, vertically aligned carbon nanotubes (CNTs), metal/covalent organic frameworks (MOFs/COFs), and liquid crystal(LC) polymers; unique chemistries are utilized to develop coatings and extractants for ion exchange membranes(IEMs) and liquid membranes(LMs) to increase the selectivity of target ions or molecules; biological water and ion channels and their synthetic equivalents are incorporated in membranes to achieve highly selective water or chemical transport. Besides the materials aspect, the article further identifies practical needs, knowledge gaps, and technological barriers in both material development and process design for high selectivity membrane processes. Finally, the authors discuss research priorities in the context of existing and future water supply paradigms.
Table 1. Advanced materials enable the synthesis of different types of membranes endowing selectivity for different targets.
As an associated project of NEWT’s Thrust 3, this effort provides readers guidelines on how to develop selective membrane for high rate, precise separation of chemicals from water, which is necessary not only for pollution control, but also for minimization of chemical and energy consumption, and recycling and reuse of resources. These capabilities will enable high energy efficiency, low-cost, and environmentally sustainable treatment processes necessary for future water systems.