A Molecular Basis for Advanced Materials in Water Treatment
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for Advanced Materials in Water Treatment
Randall T. Cygan, C. Jeffrey Brinker, May D. Nyman, Kevin Leung, and Susan B. Rempe Abstract 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.
Introduction The past 20 years have seen only nominal improvements in the flux of polymeric reverse-osmosis (RO) membranes, in which, by exceeding the osmotic pressure of a saltwater solution, water is transported against concentration gradients through a semipermeable membrane that removes most salts. These improvements have relied on engineering solutions (for example, corrugation of the membrane to enhance its effective surface area) rather than improving material properties to increase flux and selectivity of the membrane itself. The same can be said for ion exchangers, a related class of materials that are proven for both RO membranes and brackish water desalination through ion exchange and can be tuned for specific toxic elements such as lead, selenium, and radionuclides. Further progress in desalination demands a scientific understanding of the water–membrane interface and the transport of water molecules in confined geometries. Only then can membranes be intelligently engineered for both high chemical selectivity and fast trans-
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port of pure water. Several examples of water treatment studies that incorporate a molecular-based strategy are discussed in the following sections.
Novel Nanoporous Biomimetic Silica-Based Membranes The permeation and exclusion of salt in nano-confined aqueous media are central to both selective ion transport and purification/desalination of seawater and brackish water. Recently, highly ordered, nanoporous, silica thin films with tailormade interior pore surfaces have been synthesized.1–3 These membranes can be exploited as experimental platforms that help elucidate the roles of pore surface chemistry and confinement on salt hydration and permeation. The design of efficient ion-exclusion membranes may benefit from the study of biology. For example, the aquaporin transmembrane channels present in our kidneys utilize their unique structural motifs to achieve high water flux and quantitative rejection of Na+ and Cl− ions with
only small pressure gradient and energy input.4 Other transmembrane channels, such as the potassium channels of nerve and muscle, are designed to permit rapid permeation of only select ions.5 Harnessing these basic physical and biomimetic principles will potentially lead to robust solid-state membranes in RO
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