Environ. Sci. Technol. 2009, 43, 2018–2021
Removal of Common Organic Solvents from Aqueous Waste Streams via Supercritical CO2 Extraction: A Potential Green Approach to Sustainable Waste Management in the Pharmaceutical Industry J O H N N I E L . L E A Z E R , J R . , * ,† S E A N G A N T , ‡ ANTHONY HOUCK,† WILLIAM LEONARD,† AND CHRISTOPHER J. WELCH† Department of Process Research, Merck Research Laboratories, Rahway, New Jersey 07065, and Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109
Received September 16, 2008. Revised manuscript received November 14, 2008. Accepted December 15, 2008.
Supercritical CO2 extraction of aqueous streams is a convenient and effective method to remove commonly used solvents of varying polarities from aqueous waste streams. The resulting aqueous layers can potentially be sewered; whereas the organic layer can be recovered for potential reuse. Supercritical fluid extraction (SFE) is a technology that is increasingly being used in commercial processes (1). Supercritical fluids are well suited for extraction of a variety of media, including solids, natural products, and liquid products. Many supercritical fluids have low critical temperatures, allowing for extractions to be done at modestly low temperatures, thus avoiding any potential thermal decomposition of the solutes under study (2). Furthermore, the CO2 solvent strength is easily tuned by adjusting the density of the supercritical fluid (The density is proportional to the pressure of the extraction process). Since many supercritical fluids are gases at ambient temperature, the extract can be concentrated by simply venting the reaction mixture to a cyclone collection vessel, using appropriate safety protocols. Supercritical fluids are valuable candidates for removing organic chemicals from solid matrices due to the high diffusivity, low viscosity, and temperature-pressure dependence of solvent strength of the fluid. Supercritical carbon dioxide (scCO2) is the most studied solvent for this technology. CO2 is a low molecular weight gas which has a low critical temperature (31 °C) and pressure (73 atm). Additionally, CO2 is inexpensive and readily available in bulk in highly pure form. Taylor has shown that SFE can be successfully used to extract a variety of organic compounds from aqueous streams (3). SFE extraction of organic residues from aqueous streams has potential implications in the remediation of contaminated drinking water (3), removal of commercial chemicals * Corresponding author phone:
[email protected]. † Merck Research Laboratories. ‡ University of Michigan. 2018
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 6, 2009
from waste streams (4), and the removal of pesticides/ insecticides from freshwater runoff (5). An abundance of patent literature focuses on the use of SFE to extract organic compounds from aqueous streams (6). Hardman made use of the fact that CO2 forms a hexahydrate which serves to separate the aqueous phase from an organic solute (acrylic acid) (6a); and Shimshick was able to recover carboxylic acids from aqueous solutions of alkali metal salts using scCO2. Under these reaction conditions, the salt reacted with CO2 to form an acid which dissolved in the supercritical phase (6b). Bhise developed a process to recover ethylene oxide from a dilute aqueous stream using either scCO2 or near scCO2 (6c). In a subsequent patent Bhise extracted ethylene oxide from an aqueous solution with scCO2 in the presence of a catalyst which caused the substrate to react with CO2 to form ethylene carbonate. The carbonate was subsequently hydrolyzed to ethylene glycol (6d). DeFilippi described a device that is a supercritical extractor, distillation unit, and vapor recompressor, an invention which seems well suited for recovering organic residues from aqueous media (6e). Pujado reported a process to remove trace quantities of hydrocarbons from aqueous streams using SCE techniques (6f); and both Hagen and Victor used SFE techniques in the separation of ethanol from aqueous streams (6g, 6h). Based on the abundant historical precedents presented above, we reasoned that scCO2 extraction could be effective in removing organic solvents from aqueous waste streams in the pharmaceutical industry, thereby providing a green approach to remediation of nonsewerable wastewater. If so, this could potentially lead to greener manufacturing processing by reducing water waste. Waste water disposal issues have become more prevalent in recent years as many manufacturing industries have embraced the principles of green chemistry (7). Strict locally mandated guidelines must be adhered to regarding proper disposition of aqueous waste streams contaminated with organic solvents in the U.S. In 1990 the U.S. pharmaceutical industry spent $59.3 M to properly dispose of aqueous streams from manufacturing processes (8). Adjusting for a 3% per annum inflation rate only, this translates to >$100 M in today’s terms. In an effort to reduce our overall environmental footprint, we have embarked on a research program aimed at investigating novel approaches for cleanup of solvent-laden aqueous streams so that the resulting aqueous bulk may be rendered sewerable and the solvents potentially recovered for subsequent reuse. (It is generally accepted that 75-80% of the waste associated with the production of active pharmaceutical ingredients is solvent related (cf., ref 7b). We herein present the preliminary findings on the evaluation of scCO2 SFE for the removal of the commonly used organic solvents of varying polarities including toluene, tetrahydrofuran (THF), isopropyl acetate (IPAc), ethyl acetate (EtOAc), N-methyl-2-pyrrolidone (NMP), methyl tert-butyl ether (MTBE), and acetonitrile (MeCN) from artificial aqueous waste streams as an alternative green methodology in waste management in the pharmaceutical industry.
Experimental Section A Thar R100CW system was used for all SFE experiments. A detailed diagram of the unit is shown in Figure 1. The CO2 is obtained from a 10 ton tank as a high pressure liquid. Liquid CO2 leaves the bulk tank and enters a pressure regulator and then a heat exchanger. It is then pumped into a mixing chamber where it can potentially mix with a 10.1021/es802607a CCC: $40.75
2009 American Chemical Society
Published on Web 02/06/2009
FIGURE 1. Schematic of Thar R100CW supercritical system used in studies.
TABLE 1. SFE of Common Organic Solvents from Aqueous Waste Streams under Single-Pass Equilibrium Conditionsa
a
entry
solvent
°C
bar CO2
1 2 3 4 5
sat’d toluene
22 30 35 40 50
0 100 100 100 100
536 ( 3
