Extraction of Indicator Dyes into Imidazolium-Based Ionic Liquids

Abstract

Among the major classes of water pollutants, dyestuffs have proven to be particularly problematic, both in terms of the quantities released to the environment and the difficulties involved in their recovery from aqueous solution. Each year, textile mills worldwide discharge millions of gallons of dye-laden effluents, leading to significant adverse environmental impact as a consequence of the high chemical oxygen demand and toxicity of these effluent streams. For this reason, there has been considerable interest in the development of a means by which to remove dyes from aqueous waste streams. To accomplish this, textile mills generally employ a combination of physical, biological, and chemical methods, many of which are inefficient and expensive. Liquid-liquid extraction employing ionic liquids (ILs), a novel class of organic solvents typically comprising a bulky asymmetric organic cation in combination with any of a wide variety of anions, has the potential to overcome these problems. In this work, a variety of imidazolium-based ionic liquids were evaluated for their ability to extract representative cationic and anionic dyes from aqueous solution. UV-visible spectrometry was employed to determine the extent to which each dye is extracted from aqueous solution as a function of pH. Ionic liquids incorporating small (i.e., less hydrophobic) cations were found to extract cationic dyes more efficiently than ILs incorporating larger (i.e., more hydrophobic) cations. Conversely, ILs incorporating large cations extracted anionic dyes more efficiently than those based on small cations. Unfortunately, recovery of the extracted dyes (and thus reuse of the IL) proved difficult. Thus for ionic liquids to realize their full potential as media for dye removal from wastewater, efficient methods for dye stripping must be identified. In addition, more environmentally benign ILs are required to address their inevitable loss to the aqueous phase during the extraction process.