1	Solving for Constants of Membrane Permeability Amalia Leona Dee Anderson Luther College, Decorah IA Advisor: Dr. John Pellegrino Graduate Student: Craig Gorman University of Colorado at Boulder Environmental Engineering REU
2	INTRODUCTION: Semi-permeable membranes are used increasingly for the separation of matter from water in its treatment and purification Membranes are used in water treatment to remove some form of matter from the water There is not a standardized system for membrane testing Meaningful comparison requires standard parameters for membrane testing Currently comparison relies on testing outside of the manufacturer
3	PURPOSE The intent of this research was to take measurements of the diffusion of salt occurring across various membranes in order to extract the diffusion coefficient for each membrane tested. Commercial electrodialysis membranes and reverse osmosis membranes made in the lab were to be analyzed.
4	Membrane Types: Porous membranes used in pressure driven filtration act as a grating and use a pressure gradient that blocks any particles larger than its pores. Examples: microfiltration and nanofiltration Nanofiltration and reverse osmosis membranes also rely on pressure but solvent and solutes move through the membranes by diffusion Electrodialysis membranes use an electrical potential and a sophisticated chemistry in order to transport the ions selectively
5	Electrodialysis Membranes:
6	Membranes Electropure Excellion Cation I-100 Electropure Excellion Anion Ameridia Cation CMX-SB Ameridia Anion AMX-SB Nafion 950 EW (Cation) Cellulose Acetate membranes from a phase inversion process
7	"Transport of matter across a..." Transport of matter across a membrane depends upon diffusion, the random motion of molecules or colloidal-sized particles as a result of kinetic energy; the membrane adds a frictional coefficient to diffusion.
8	The Diffusion Cell Two sections divided by a membrane Stir rods in each section Section 1: Initially 360ml deionized water Section 2: Initially a solution of deionized water and NaCl at 10000 ppm, 15000 ppm, or 20000 ppm; ~840 ml Conductivity readings taken in ten minute intervals Samples removed with a syringe from lid of cell
9	Conductivity Readings Conductivity measurements (mS/cm) were converted to units of concentration (g/L) by a function obtained experimentally and verified by an accepted function.
10	Experimental Data
11	"Dependant upon the surface area..." Dependant upon the surface area and thickness of the membrane, diffusion is described mathematically:
12	RESULTS Mass Transfer Coefficients
13	RESULTS: Permeability Constants
14	Assumptions In the Calculation of the Mass Transfer Coefficients and Permeability Constants the following assumptions were made: The liquid-phase mass transfer coefficients are large Mass balances can be used to find concentration in one section of the cell The volume in each cell changes as a linear function over time according to initial and final volumes of each; the causes of volume change: Diffusion of deionized water across the membrane in both directions Gradual loss of volume due to sampling process
15	Conclusions For Excellion 44e-13< p < 120e-13 For Amerida 8e-13< p < 22e-13 For Nafion 950 1.2e-13< p < 2.8e-13 For cellulose acetate membrane 5e-13< p < 9e-13 For Nafion 450 5.8e-11
16	Future Uses and Modifications: Uses: The mass transfer coefficients and permeability constants will be used for modeling in related projects that deal with electrodialysis membranes. Modifications: Repeat existing experiment Use a dye like blue Dextrin, detectable by spectroscopy, to track diffusion of water across a membrane Test the membranes in a different diffusion cell for comparison of constants Use stir bars that have variable stir rates
17	Acknowledgements: Dr. John Pellegrino Craig Gorman, graduate student Laura Richards Hans Anker, graduate student Dr. JoAnn Silverstein, REU director Joy Jenkins, REU coordinator