One of the most pervasive problems afflicting people throughout the world is inadequate access to clean water and sanitation. Problems with water are expected to grow worse in the coming decades, with water scarcity occurring globally, even in regions currently considered water-rich. Addressing these problems calls out for a tremendous amount of research to be conducted to identify robust new methods of purifying water at lower cost and with less energy, while at the same time minimizing the use of chemicals and impact on the environment. Here we highlight some of the science and technology being developed to improve the disinfection and decontamination of water, as well as efforts to increase water supplies through the safe re-use of wastewater and efficient desalination of sea and brackish water.

Science and technology for water purification in the coming decades

Metal–Organic Frameworks for Separations

Kenji Sumida, David L. Rogow, Jarad A. Mason, Thomas M. McDonald, Eric D. Bloch, Zoey R. Herm, Tae-Hyun Bae, Jeffrey R. Long

Carbon Dioxide Capture in Metal–Organic Frameworks

This critical review provides a processing-structure-property perspective on recent advances in cellulose nanoparticles and composites produced from them. It summarizes cellulose nanoparticles in terms of particle morphology, crystal structure, and properties. Also described are the self-assembly and rheological properties of cellulose nanoparticle suspensions. The methodology of composite processing and resulting properties are fully covered, with an emphasis on neat and high fraction cellulose composites. Additionally, advances in predictive modeling from molecular dynamic simulations of crystalline cellulose to the continuum modeling of composites made with such particles are reviewed (392 references).

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Cellulose nanomaterials review: structure, properties and nanocomposites

In recent years, numerous large-scale seawater desalination plants have been built in water-stressed countries to augment available water resources, and construction of new desalination plants is expected to increase in the near future. Despite major advancements in desalination technologies, seawater desalination is still more energy intensive compared to conventional technologies for the treatment of fresh water. There are also concerns about the potential environmental impacts of large-scale seawater desalination plants. Here, we review the possible reductions in energy demand by state-of-the-art seawater desalination technologies, the potential role of advanced materials and innovative technologies in improving performance, and the sustainability of desalination as a technological solution to global water shortages.

http://science.sciencemag.org/content/sci/333/6043/712.full.pdf

The Future of Seawater Desalination: Energy, Technology, and the Environment

Reverse osmosis desalination: Water sources, technology, and today's challenges

BACKGROUND Synthetic membranes are used for desalination, dialysis, sterile filtration, food processing, dehydration of air and other industrial, medical, and environmental applications due to low energy requirements, compact design, and mechanical simplicity. New applications are emerging from the water-energy nexus, shale gas extraction, and environmental needs such as carbon capture. All membranes exhibit a trade-off between permeability—i.e., how fast molecules pass through a membrane material—and selectivity—i.e., to what extent the desired molecules are separated from the rest. However, biological membranes such as aquaporins and ion channels are both highly permeable and highly selective. Separation based on size difference is common, but there are other ways to either block one component or enhance transport of another through a membrane. Based on increasing molecular understanding of both biological and synthetic membranes, key design criteria for new membranes have emerged: (i) properly sized free-volume elements (or pores), (ii) narrow free-volume element (or pore size) distribution, (iii) a thin active layer, and (iv) highly tuned interactions between permeants of interest and the membrane. Here, we discuss the permeability/selectivity trade-off, highlight similarities and differences between synthetic and biological membranes, describe challenges for existing membranes, and identify fruitful areas of future research. ADVANCES Many organic, inorganic, and hybrid materials have emerged as potential membranes. In addition to polymers, used for most membranes today, materials such as carbon molecular sieves, ceramics, zeolites, various nanomaterials (e.g., graphene, graphene oxide, and metal organic frameworks), and their mixtures with polymers have been explored. Simultaneously, global challenges such as climate change and rapid population growth stimulate the search for efficient water purification and energy-generation technologies, many of which are membrane-based. Additional driving forces include wastewater reuse from shale gas extraction and improvement of chemical and petrochemical separation processes by increasing the use of light hydrocarbons for chemicals manufacturing. OUTLOOK Opportunities for advancing membranes include (i) more mechanically, chemically, and thermally robust materials; (ii) judiciously higher permeability and selectivity for applications where such improvements matter; and (iii) more emphasis on fundamental structure/property/processing relations. There is a pressing need for membranes with improved selectivity, rather than membranes with improved permeability, especially for water purification. Modeling at all length scales is needed to develop a coherent molecular understanding of membrane properties, provide insight for future materials design, and clarify the fundamental basis for trade-off behavior. Basic molecular-level understanding of thermodynamic and diffusion properties of water and ions in charged membranes for desalination and energy applications such as fuel cells is largely incomplete. Fundamental understanding of membrane structure optimization to control transport of minor species (e.g., trace-organic contaminants in desalination membranes, neutral compounds in charged membranes, and heavy hydrocarbons in membranes for natural gas separation) is needed. Laboratory evaluation of membranes is often conducted with highly idealized mixtures, so separation performance in real applications with complex mixtures is poorly understood. Lack of systematic understanding of methodologies to scale promising membranes from the few square centimeters needed for laboratory studies to the thousands of square meters needed for large applications stymies membrane deployment. Nevertheless, opportunities for membranes in both existing and emerging applications, together with an expanding set of membrane materials, hold great promise for membranes to effectively address separations needs.

http://science.sciencemag.org/content/sci/356/6343/eaab0530.full.pdf

Maximizing the right stuff: The trade-off between membrane permeability and selectivity

Gas separation properties of polymer membrane materials follow distinct tradeoff relations:  more permeable polymers are generally less selective and vice versa. Robeson1 identified the best combinations of permeability and selectivity for important binary gas pairs (O2/N2, CO2/CH4, H2/N2, etc.) and represented these permeability/selectivity combinations empirically as αA/B = βA/B , where PA and PB are the permeability coefficients of the more permeable and less permeable gases, respectively, αA/B is selectivity (=PA/PB), and λA/B and βA/B are empirical parameters. This report provides a fundamental theory for this observation. In the theory, λA/B depends only on gas size. βA/B depends on λA/B, gas condensability, and one adjustable parameter.

Basis of Permeability/Selectivity Tradeoff Relations in Polymeric Gas Separation Membranes

Energy-efficient polymeric gas separation membranes for a sustainable future: A review

The permeability of poly(dimethylsiloxane) [PDMS] to H2, O2, N2, CO2, CH4, C2H6, C3H8, CF4, C2F6, and C3F8, and solubility of these penetrants were determined as a function of pressure at 35 °C Permeability coefficients of perfluorinated penetrants (CF4, C2F6, and C3F8) are approximately an order of magnitude lower than those of their hydrocarbon analogs (CH4, C2H6, and C3H8), and the perfluorocarbon permeabilities are significantly lower than even permanent gas permeability coefficients This result is ascribed to very low perfluorocarbon solubilities in hydrocarbon-based PDMS coupled with low diffusion coefficients relative to those of their hydrocarbon analogs The perfluorocarbons are sparingly soluble in PDMS and exhibit linear sorption isotherms The Flory–Huggins interaction parameters for perfluorocarbon penetrants are substantially greater than those of their hydrocarbon analogs, indicating less favorable energetics of mixing perfluorocarbons with PDMS Based on the sorption results and conventional lattice solution theory with a coordination number of 10, the formation of a single C3F8/PDMS segment pair requires 460 J/mol more energy than the formation of a C3H8/PDMS pair A breakdown in the geometric mean approximation of the interaction energy between fluorocarbons and hydrocarbons was observed These results are consistent with the solubility behavior of hydrocarbon–fluorocarbon liquid mixtures and hydrocarbon and fluorocarbon gas solubility in hydrocarbon liquids From the permeability and sorption data, diffusion coefficients were determined as a function of penetrant concentration Perfluorocarbon diffusion coefficients are lower than those of their hydrocarbon analogs, consistent with the larger size of the fluorocarbons © 2000 John Wiley & Sons, Inc J Polym Sci B: Polym Phys 38: 415–434, 2000

Benny D. Freeman

Papers

Reverse osmosis desalination: Water sources, technology, and today's challenges

Maximizing the right stuff: The trade-off between membrane permeability and selectivity

Basis of Permeability/Selectivity Tradeoff Relations in Polymeric Gas Separation Membranes

Energy-efficient polymeric gas separation membranes for a sustainable future: A review

Gas sorption, diffusion, and permeation in poly(dimethylsiloxane)