Exploring an organometallic redox complex for the remediation and reclamation of ionic contaminants.
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Ion exchange for the removal of ionic contaminants is typically considered a mature technology. However, recovery of extracted contaminants in a minimal volume of secondary waste and regeneration of ion exchange materials for use in subsequent extraction cycles continue to represent significant challenges for the separation chemist. Substantial progress in overcoming these barriers has been made by employing redox-switchable organometallic complexes for the extraction and recovery of ionic contaminants. As compared with traditional ion exchange, this approach offers an alternative redox-based stripping mechanism and affords quantitative recovery of a target contaminant in a minimal volume of secondary waste. In contrast to previous redox extraction and recovery processes (R2ER) which focus on selectivity and design of an organometallic extractant, we have developed a novel redox process with potential for adaptation to any existing ion exchanger. The lipophilic metal complex Fe(η5-C5H5)(η5-(3)-1,2-C2B9H9(n-C12H25)2) has been used in combination with tetra-n-heptylammonium salts to demonstrate an improved R2ER paradigm for aqueous anions. Liquid-liquid extraction of a generic contaminant was shown to follow a traditional ion-exchange equilibrium. Subsequent recovery of the extracted contaminant as a compact solid was promoted by treatment of the water-immiscible phase with zinc powder, and the spent solvent was regenerated by treatment with an aqueous oxidant. Using this method, 100 L of industrial waste simulant can be reduced to 25 mL of secondary waste in a single reductive step. As compared to previously reported R2ER processes for anions, the new process alleviates potential stability concerns related to the presence of charge dense anions and enables efficient regeneration of a variety of counter ions in tetra-n-heptylammonium ion pairs, including NO3−, Br−, Cl−. Repeatable extraction/regeneration cycles demonstrated no loss in extraction efficiency. Extracted contaminants were recovered in yields ranging from 97% to 100%. R2ER regeneration of OH is also discussed. Liquid-liquid extraction studies demonstrating the feasibility of the approach and extension of this chemistry to a polymer-supported format is presented.