pH Switching for the Selective Extraction of Metal Ions into Supercritical CO 2 John P. Hanrahan, † Kirk J. Ziegler, † Jeremy D. Glennon, † David C. Steytler, ‡ Julian Eastoe, § Audrey Dupont, § and Justin D. Holmes* ,† Department of Chemistry and the Supercritical Fluid Centre, University College Cork, Cork, Ireland, School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom, and School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom Received August 20, 2002. In Final Form: December 23, 2002 Here we describe a unique extraction process utilizing pH switches for the selective removal of metal ions into supercritical CO2 using reversed micelles. Supercritical fluid extraction from an aqueous matrix has traditionally been limited by the inherent low pH of the aqueous phase of the system and the use of costly fluorinated chelating agents. The unique extraction process described herein utilizes a cheap, commercially available hydrocarbon surfactant as a complexing agent that forms reversed micelles in CO2. Subsequently, the pH of the biphasic system is controlled by the addition of buffers allowing facile manipulation of metal ion speciation. The control of the metal ion speciation is shown to have a quantitative effect on the efficiency at which metal ions are extracted from the aqueous phase. Furthermore, we demonstrate that pressure-induced pH switches of the aqueous phase can be utilized for the selective separation of chromium(III), cobalt(II), and gold(III) ions from an aqueous metallic broth. Introduction Over the past decade, an increased awareness of the problems associated with metal ions in water streams and contaminated soils has resulted in demands for more selective and effective separation processes. Current commercial processes for removing metals from environ- mental matrixes are based on liquid-liquid extraction techniques that use either toxic organic solvents or corrosive acids. Supercritical fluid extraction (SFE) 1,2 is an increasingly popular alternative to conventional liquid- liquid extraction from contaminated samples. Supercriti- cal carbon dioxide (sc-CO 2 ) is the most utilized supercritical fluid for SFE as it is nontoxic, nonpolar, nonflammable, inexpensive, and easily recyclable. 3 Also, under the control of pressure and temperature, the solvating power of sc- CO 2 can be readily manipulated providing a tunable medium for the selective extraction of targeted metal ions from the target matrix. Fluorinated chelating agents have been highly suc- cessful in the extraction of metal ions from aqueous matrixes by SFE. 4,5 Fluorinated ligands are highly soluble in CO 2 2 and can be engineered for the targeted extraction of a specific metal ion. 6,7 However, fluorinated molecules are often costly and time-consuming to synthesize. Ad- ditionally, the extraction of a specific metal ion from a broth of metallic ions typically requires the use of multiple chelating agents, limiting their usefulness in SFE. It has been known for some time that hydrocarbon surfactants are soluble in CO 2 ; 8 however, their larger cohesive energy densities compared to those of fluorinated surfactants have made it challenging to form hydrocarbon reversed micelles in CO 2 . Recently, several researchers have utilized hydrocarbon surfactants in CO 2 for stabiliz- ing emulsions and microemulsions by minimizing the cohesive energy density of the hydrocarbon surfactant. 9-12 Beckman et al. 9,10 and Eastoe and Steytler et al. 11,12 have synthesized nonfluorous surfactants that are soluble in CO 2 down to pressures of less than 120 bar. The use of hydrocarbon chelating agents in SFE 13-18 is clearly appealing as they are easier to synthesize and cheaper to produce than their fluorocarbon analogues. For example, Triton X-100 has recently been used to extract cholesterol 17 and copper metal ions 18 into CO 2 . However, contrary to the process described within this paper Triton X-100 was only used as an additive to increase the solubility of the primary chelating agent in the supercritical phase. 18 * To whom correspondence should be addressed. Telephone: +353 21 4903608. Fax: +353 21 4274097. 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Talanta 2001, 53, 1149-1154. 3145 Langmuir 2003, 19, 3145-3150 10.1021/la026438p CCC: $25.00 © 2003 American Chemical Society Published on Web 02/25/2003