Photolytic degradation of chlorinated phenols in room temperature ionic liquids

Abstract

The photolytic degradation of 2-chlorophenol (2-CP), 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), 2,3,4,5-tetrachlorophenol (2,3,4,5-TeCP) and pentachlorophenol (PCP) in 1-butyl-3-methylimidazolium hexafluorophosphate {[bmim +]PF 6 −} and 1-ethyl-3-methylimidazolium bis(perfluoroethylsulfonyl)imide {[emim +]beti} room temperature ionic liquids (RTILs) has been investigated using UV radiation of 253.7 nm. Stability study revealed that these RTILs are relatively resistant to phototransformation when used as pure phases. At low concentrations (less than 1.57 mM), chlorinated phenols could be degraded in these RTILs following pseudo-first-order kinetics. Intermediate product identification using Electrospray TOF MS, GC–MS and HPLC revealed the formation of phenol among the stable phototransformation intermediates of 2-CP. It is proposed here that two possible pathways, via formation of phenoxyl radical or carbene, could lead to phenol generation. In these two mechanisms, {[bmim +]PF 6 −} may participate in the reaction as H-donor. The increase of the chlorine atoms in the phenolic ring generally resulted in a decrease in the phototransformation rates, with exception of 2,4,6-TCP. UV absorbing impurities present in the ionic liquids as received from the manufacturer had a significant negative effect on the phototransformation rate of chlorophenols. Purification of RTILs using activated carbon enhanced the photodegradation rates. On the other hand, these impurities could, at some extent, protect the ionic liquid from photolysis and enhance the stability of the solvent. Recyclability study indicated that the recycling of the solvents was hindered at some extend by the presence of organic impurities that were resistant to photolysis or by the generation of byproducts that were resistant to photolytic degradation and compete with the chlorophenols for photon absorption. While further research is necessary to examine all aspects of this system, including the use of non-toxic RTILs, it is feasible that this technology can be further developed into a two-step process for extraction of organic pollutants from solid matrices, such as contaminated soils or dredged sediments, using ionic liquid solvents followed by in situ photodegradation of the organic contaminants in the ionic liquid extractant phase while achieving simultaneously regeneration of the solvent.