Measurements and Data for Pressure-Driven Membrane Separations

J. Pellegrino, E.J. Han, and M. Lewis; G. Amy, J. Cho, Y. Yoon, P. Brandhuber, S. Wright, and S. Delagah (Univ. of Colorado); and M. Chapman Wilbert and K. Price (U.S. Bureau of Reclamation)

Objective: To develop improved quantitative characterization techniques and predictive models for the filtration of complex mixtures using commercial membranes based on high quality measurements of streaming potential and water transport coefficients and detailed measurements of filtration results on dilute, complex aqueous mixtures encountered in membrane-based separations.

Problem: The first commercially viable, synthetic membranes suitable for molecular scale separations (reverse osmosis, nanofiltration, and ultrafiltration) were developed over twenty-five years ago. However, research during the intervening years has not developed a systematic approach for matching membranes to complex mixtures and predicting the filtration figures-of-merit: species partitioning into the membrane (rejection), solvent (water) permeability, and permeability decline with time. Improved processes for obtaining specialty chemicals, pharmaceuticals, and advanced monomers using environmentally benign processes, and more economic ways to recover, reuse, and supply water are examples of important industrial and municipal uses of membranes.

Approach: This program has both measurement and modeling components. Meaningful and accurate measurements on both the membrane and the complex mixtures are required in order to develop a systematic correlative approach. These measurements provide a means to combine the effects of chemical, physical, and structural characteristics of the membrane and the mixture, and ultimately, to delineate rational design criteria for separations. Through our collaborations with the U.S. Bureau of Reclamation and the University of Colorado, we are developing new test protocols, refining existing characterization techniques, and developing a database of consistent measurements of filtration figures-of-merit and membrane and mixture characteristics. This database is being compiled to facilitate the development of correlative models for matching membranes to specific applications.

Results and Future Plans: This year we have used our improved protocol for measuring the tangential flow streaming potential of membrane sheets. This technique is commonly used to characterize the relative surface energy and charge at the membrane interface. We are measuring streaming potential as a function of electrolyte composition, concentration, pH, and temperature. These data will be used in a model to calculate the surface potential of the membrane or film. The membrane’s surface potential will then be incorporated into materials research, manufacturing quality control, and engineering design models. We have continued development and testing of a new apparatus to measure the kinetics of solvent diffusion through membranes. This apparatus has a resolution on the order of 10^-8 L/s and may provide an improved method for absolute characterization and monitoring of very subtle structural changes in membrane materials, caused by aging, exposure to chemicals, and mechanical trauma. We were able to successfully use this apparatus to identify small structural changes in reverse osmosis membranes exposed to dilute NaCl solutions versus control samples. We have extended our filtration database measurements beyond natural organic matter filtration to include trace hazardous species, for example, arsenate, arsenite, and perchlorate ions, and colloidal particles. We have also developed a semi-empirical model for modeling and predicting flux decline in macromolecule filtration that is mostly based on parameters that may be measured independently or estimated from physicochemical properties of the solutes.

Publication: