Worldwide, 1.2 billion people lack access to sufficient amounts of clean water, and 2.6 billion lack adequate sanitation. Also, industry relies on large quantities of water during manufacturing, which is then returned to the environment. Having adequate water supplies, and removing pathogens, chemicals, and other contaminants with high throughput at a low cost is a growing challenge around the world. This issue of MRS Bulletin examines how materials research, through the development of membranes, catalysts, nanoparticles, and other materials, is addressing these needs.

Membrane techniques for water treatment have been growing significantly for the last decade. Reverse osmosis, for example, is the leading seawater desalination technique. Nanofiltration membranes are used more and more for hardness removal and even to desalinate slightly polluted waters. Ultrafiltration and microfiltration membranes are used extensively mainly as membrane bioreactors in wastewater recovery. While this trend is growing, the membranes still may be improved significantly, based on new materials designed to increase the flux of water through the membranes at reduced pressures while maintaining or even improving the rejection of dissolved matter or suspended matter. Better membranes will reduce energy consumption while maintaining affordable separation properties.

Fibrous media in the form of nonwoven filters have been used extensively in water treatment as pre-filters or to support the medium that does the separation. Nonwoven media are composed of randomly oriented micron-size fibers and provide a one step separation as a substitute for conventional processes comprising chemical addition, flocculation, sedimentation, and sand filtration. At present the use of nonwoven filter media is limited to pre-filters and is not used further downstream as high performance filters. However it is expected that by reducing the fiber size in the nanometer range, higher filtration efficiency can be achieved. With the advent of nanotechnology, the ease of producing high quality nano scaled fibers is now a reality. Recent advancements in nanofibrous media through surface modifications have shown that nonwoven media can be used beyond the prefilter stage. Furthermore the pore size of the filter media can be controlled through modification of fiber size and thickness of membranes. These nanofibrous membranes possess high surface area and large porosity leading to high flux, low pressure membranes. This article highlights important opportunities and challenges associated with developing nanofibrous media for water treatment. In addition, we have attempted to capture a snapshot of this rapidly developing new area of fibrous media for water treatment for the benefit of the wider membrane community.

The polyamide active layers of commercial reverse-osmosis and nanofiltration membranes are examples of nanoscale functional materials that challenge the state of the art of materials characterization. The active layer is only ∼100 nm thick, and because the active layer is formed by a process of interfacial polymerization, the structure and composition of the membrane is highly inhomogeneous. Even such basic physical and chemical properties of the membrane as the atomic density, swelling in water, distribution of charged species, and the mobility of water and ions, are poorly understood. In this article, we briefly review progress in the characterization of polyamide separation membranes using transmission electron microscopy, atomic force microscopy, vibrational spectroscopy, positron annihilation, nuclear magnetic resonance, and Rutherford backscattering spectrometry. Advances in the microanalysis methods applicable to these complex materials will advance fundamental understanding of the structure–property relationships of polymer membranes and further the long-term goal of synthesizing membranes with improved performance.

Trace contaminant detection in water represents both a grand challenge and great opportunity for materials scientists and engineers. The recent discovery that functional DNA can be obtained to bind selectively to a wide range of contaminants makes it possible to interface these molecules with nanoscale materials, such as gold nanoparticles and quantum dots, to transform the molecular reorganization between functional DNA and contaminants into physically detectable colorimetric and fluorescent signals. Micro- and nanofluidic devices have also played a critical role in lowering the detection limits of functional-DNA sensors, promoting sensor regeneration and thus improving sensor performance and allowing long-term unattended monitoring of water quality.

A molecular-scale interpretation of interfacial processes is often downplayed in the analysis of traditional water treatment methods; however, such a fundamental approach is perhaps critical for the realization of enhanced performance in traditional desalination and related treatments, and in the development of novel water treatment technologies. Specifically, we examine in this article the molecular-scale processes that affect water and ion selectivity at the nanopore scale as inspired by nature, the behavior of a model polysaccharide as a biofilm, and the use of cluster-surfactant flocculants in viral sequestration.

There has been considerable recent interest in natural bone as a material, due in part to its interesting combination of mechanical properties: bone is stiff and tough but lightweight. This unusual combination of properties results from a nanocomposite structure of approximately equal volumes of mineral and hydrated organic matter. Much recent effort has been focused on the structure, properties, and performance at different length scales relative to the hierarchical organization of bone. Historically, such bone research has emphasized clinical and medical aspects, including engineering materials for bone augmentation or replacement, bone–biomaterial interactions and interfaces, and more recently, scaffolds for bone tissue engineering. However, within the fast-growing biomimetics field, the bone extracellular matrix is taken as a model for materials development. Efforts have been made both to mimic the bony material itself as well as to mimic the process by which bone forms.