A New Paradigm in Separations : Recognition Membranes

The objectives of this project were to understand the molecular-level mechanisms that govern transport of ions and molecules through a semipermeable membrane and to develop a new class of robust molecular recognition membranes. The approach used recent advances in nanotechnology to fabricate thin membrane assemblies with specific receptors for molecular recognition incorporated into the pore structure. The new membrane technology will strengthen the Laboratory’s core competencies in separation science and technology, nuclear and advanced materials, and advanced manufacturing by providing new methods to minimize wastes and increase safety and efficiency of nuclear materials operations. Significant progress toward the objectives was made during the lifetime of the project, and a solid foundation for progress toward important new separation technology was laid. Background and Research Objectives A membrane is a semi-permeable structure separating two phases that can operate as an active or passive barrier to the transport of matter between the phases. The membrane can discriminate between the components of the phases based on differences in one or more properties of the components such as size, shape, electrical charge, volubility, and diffusion rate. The resulting separation achieved by a membrane system is a function of both thermodynamic partitioning and kinetics. Control of the interaction of these features provides a wide range of separation possibilities with membranes, but is difficult to achieve in practical applications. The wide spectrum of separation possibilities encompassed by membranes can be illustrated by the contrast between the gravity-driven filtration of ground coffee beans from fresh-brewed coffee to the selective transport of sodium ions through cell membranes driven by the cell’s metabolic machinery. Filtration in all its forms is a very important separation technology with industrial spending of about $75 billion per year. Molecular recognition membranes that accomplish precise separations based on chemical properties would allow membrane separation systems to reduce separation costs for a much wider range of industrial needs, including many separation challenges faced by the U.S. Department of Energy. *Principal Investigator, e-mail: gjarvinen @lanl.gov . . .. . .,, ,...=T .j..,. -. ,-’.. =-, ,,/ ,; ;.. .. .=:. ~.LeJ; .ti, & ,: -J cc-f 3 ‘: ::3 ~~~~ Our focus in this project was on membranes for selective separation of metal io from solution. Supported liquid membranes (SLMS) have been investigated for many years to selectively separate metal-ions and other species from aqueous solutions. The preparation of an SLM essentially involves placing a liquid-liquid extraction system into the pores of a thin membrane support. A carrier molecule dissolved in the organic layer contained in the pores acts as a shuttle for metal ions between an aqueous feed solution on one side of the membrane and an aqueous receiving solution on the other side. The SLM systems have illustrated that the selective chemistry developed for liquid-liquid extraction of metal ions could be applied in a membrane format, but the poor long-term stability of the SLMS has inhibited their industrial application. A number of approaches to mitigate the stability problem have been attempted, but none have yet been a significant commercial success. Our grand challenge in this project was to develop very stable thin membrane structures containing ionic recognition sites that facilitate the selective transport of target metal ions. The objective of our efforts was to understand the molecular-level mechanisms that control transport of ions and molecules through membrane pores and use this knowledge to develop a new class of molecular recognition membranes. Importance to LANL’s Science and Technology Base and National R&D Needs The Department of Energy faces many challenges in the 21s’ Century that require advances in separation technology. The continuing defense mission will require purification and processing of nuclear materials with increased safety and reduced wastes. Cleaning up the legacy of over 50 years of defense nuclear material production operations will be much less costly with improved separation technologies. This legacy cleanup also extends to aiding efforts in the former Soviet Union where the impacts of nuclear operations have often been more severe than in the U.S. Nuclear energy is likely to grow in importance because of the negative global environmental impact of fossil fuels and the increasing worldwide energy demand. Major improvements in the nuclear power fuel cycle are possible with new approaches to separating the components of spent fuel. Thus the Laboratory needs to maintain a strong core competency in separation science and technology. Membrane separation technology can be a significant contributor to meeting these challenges, Industry also faces significant separation challenges. Separation processes are often a significant part of the cost of industrial production operations.