More Information on Aqueous Biphasic Systems in general can be found on the Aqueous Biphasic Partitioning Community Home Page: http://bama.ua.edu/~rdrogers/aq2phase/
ABSTRACT
In aqueous biphasic systems (ABS), the major component in each of the two immiscible phases is water, and thus a liquid/liquid extraction technology can be envisioned which completely eliminates the use of volatile organic compounds (VOCs). Elimination of VOCs has the potential to revolutionize many industrial processes by drastically reducing potential downstream pollution while increasing safety. This presentation will discuss our research efforts to obviate the need for VOCs in many separation and waste remediation technologies by the development of ABS into useful systems for the selective batch or chromatographic removal and recovery of solutes and particulates.
INTRODUCTION
"The goal of Technology Vision 2020 is to enable the (chemical) industry to continue to lead in technology development, manufacturing and profitability, while optimizing health and safety and ensuring environmental stewardship"... One area of opportunity for new chemical science and engineering technology which will help meet this goal is the development of new separations technologies that eliminate the use of flammable, toxic VOCs as solvents. Used in conjunction with, or instead of appropriate current manufacturing processes, such technologies would help to prevent pollution and increase safety. New separations technologies developed for pollution prevention may also find application in pollution remediation, helping to clean up already contaminated sites.
Traditional solvent extraction employs partitioning of a solute between two immiscible phases, typically an organic solvent and an aqueous solution. The ability to utilize a number of different diluents, extractants, and aqueous phases makes solvent extraction a powerful separations method possessing a number of favorable characteristics including rapid extraction kinetics for many separations, the adaptability of the method to a wide variety of solutes, and back extraction or stripping of the solute and recycling of the solvent and/or diluent are often feasible. Further, liquid/liquid extraction is capable of large volume throughput and is amenable to large-scale separations, and can be engineered for high selectivity and efficiency by the use of multistage contactors.
In spite of these numerous advantages, there are several drawbacks to traditional oil/water solvent extraction. Even with today's environmental standards a number of extraction systems still utilize toxic and flammable organic diluents. When the diluent is coupled with a highly selective extractant the cost of the solvent system can become very expensive. (Not to mention the costs of safely designing a system to operate with a volatile or flammable diluent and the high costs of disposal.)
Aqueous biphasic systems consist of two immiscible phases formed when certain water soluble polymers are combined with one another or with certain inorganic salts in specific concentrations. As two-phase systems they are suitable for carrying out liquid/liquid separations of various solutes such as biomolecules, metal ions, and particulates (1-3). In ABS the major component in each of the two phases is water, and because of this non-denaturing environment these systems have been widely employed in biological separations for over 40 years. It was surprising to us, however, that a liquid/liquid extraction technology that holds the possibility of completely eliminating the use of VOCs has been virtually ignored for all separations except biomolecules.
ABS retain all of the practical advantages of liquid/liquid extraction and also have a number of unique advantages due, in large part, to their aqueous nature. Polyethylene glycol (PEG)-based ABS are virtually nontoxic and nonflammable, all components are commercially available in bulk quantities and are inexpensive, and the systems have reasonable phase separation characteristics and can even be used with traditional solvent extraction equipment. In addition, the PEG-rich phases in PEG-ABS appear to be tunable; their phase characteristics can be changed to match the hydrophobicity and water content of a number of organic solvents.
SUMMARY
The long range goal of this project is to obviate the need for VOCs in many industrial separation and waste remediation technologies by the development of ABS into useful systems for the selective batch or chromatographic removal and recovery of solutes and particulates. This goal includes: a) development of a fundamental understanding of the factors governing solute partitioning in ABS, b) understanding phase behavior in ABS in order to attain the ability to fine-tune the PEG-rich phase and thus solute partitioning, c) expanding the uses of ABS by targeting applications suited to this technology, d) gaining an understanding of the relationships between liquid/liquid ABS separations and solid-supported chromatographic ABS separations, and e) full adaptation of ABS into both liquid/liquid and solid-supported aqueous biphasic extraction technologies.
We have had several successes during the course of our research in this area (4). These include: a) the separation and recovery of TcO4- from MoO42- in a variety of salt solutions, b) the separation and recovery of TcO4- from simulated Hanford tank wastes (Figure 1), c) the separation and stripping of soft metal halide complex anions from sulfate solutions, d) proving that extraction with ionophores capable of metal ion recognition is possible in PEG-ABS, and e) successful adaptation of many of these metal ion separations with PEG-ABS to an Aqueous Biphasic Extraction Chromatographic (ABEC) mode (5). During the course of these studies many correlations between system variables and partitioning behavior have led to a better understanding of the factors governing partitioning in ABS (6). More recently, we have adapted ABEC separations to the removal of color from textile wastes (Figure 2). Many dyes and metal dye complexes partition from salted effluents to the ABEC resins. The dyes can typically be recovered by washing the resins with water.
FIGURE 1. Elution curve for Indigo Carmine on ABEC-5000.
FIGURE 2. Chromatographic separation of 95mTcO4- from Hanford Tank NCAW waste simulant using ABEC-5000.
This presentation will review our results to date and discuss the challenges in implementing ABS or ABEC technologies. Target industries for these technologies will also be discussed in terms of pollution remediation and pollution prevention. Realization of our long range goals will lead to cleaner, cheaper, safer, and better separations technologies than are currently available for a variety of applications. Elimination of VOCs has the potential to revolutionize many industrial processes by drastically reducing potential downstream pollution while increasing safety. In addition, certain separations where traditional oil/water techniques are not applicable or perform poorly, may be successfully carried out utilizing ABS.
ACKNOWLEDGEMENTS
The portions of this research dealing with metal ion separations are funded by the National Science Foundation (Grant CTS-9522159). The research directed toward organic molecule separations is supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy (Grant No. DE-FG02-96ER14673).
REFERENCES
(1) Partitioning in Aqueous Two-Phase Systems. Theory, Methods, Uses, and Applications to Biotechnology; Walter, H.; Brooks, D. E.; Fisher, D., Eds.; Academic Press: Orlando, 1985.
(2) Aqueous Two-Phase Systems; Walter, H.; Johansson, G., Eds.; In Methods in Enzymology; Abelson, J. N.; Simon, M. I., Eds.; Academic Press: San Diego, 1994; Vol. 228.
(3) Aqueous Biphasic Separations: Biomolecules to Metal Ions; Rogers, R. D.; Eiteman, M. A., Eds.; Plenum: New York, 1995.
(4) Rogers, R. D.; Zhang, J. "New Technologies for Metal Ion Separations: Polyethylene Glycol Based-Aqueous Biphasic Systems and Aqueous Biphasic Extraction Chromatography," In Ion Exchange and Solvent Extraction; Vol. 13, Marcus, Y.; Marinsky, J. A., Eds.; Marcell Dekker, New York, 1997; In press.
(5) Rogers, R. D.; Bond, A. H.; Griffin, S. T.; Horwitz, E. P. "New Technologies for Metal Ion Separations: Aqueous Biphasic Extraction Chromatography (ABEC). Part I. Uptake of Pertechnetate," Solvent Extr. Ion Exch. 1996, 14, 919.
(6) Rogers, R. D.; Bond, A. H.; Bauer, C. B.; Zhang, J.; Griffin, S. T. "Metal Ion Separations in Polyethylene Glycol-Based Aqueous Biphasic Systems: Correlation of Partitioning Behavior with Available Thermodynamic Hydration Data," J. Chromatogr., B: Biomed. Appl. 1996, 680, 221.