Degradation of ionic liquids by Fenton reaction; the effect of anions as counter and background ions E.M. Siedlecka *, M. Gołe ˛ biowski, Z. Kaczyn ´ ski, J. Czupryniak, T. Ossowski, P. Stepnowski Faculty of Chemistry, University of Gdan ´sk, ul. Sobieskiego 18/19, PL 80-952 Gdan ´sk, Poland 1. Introduction Ionic liquids (ILs) are a class of organic salts that are liquid at or near room temperature. They serve as good solvents for various chemical and electrochemical reactions, are nonvolatile and nonflammable. Therefore, they have been proposed as ‘‘green solvents’’ for chemical reactions and separation processes [1–4]. This wide applicability of ILs is mainly based on their beneficial physico-chemical properties, e.g. high thermal and electrochemi- cal stability, high conductivity, extraction behavior, etc. The optimization of the properties necessary for the defined technical application of ILs is determined by the high structural variability of cations and corresponding anions. However, regard- ing the hazard assessment of ILs, this structural variability represents an almost insurmountable problem, as it would be necessary to generate a profound knowledge of the effects on man and on the environment for every single compound. Several authors have already mentioned ecotoxicological influences of ILs on some selected organisms, for example bacteria [5,6], algae [7,8], Q1 waterfeals [9] and zebrafish [10]. The impact of ILs on aquatic ecosystems is highly important as some ILs have a high solubility in water. The solubility of these ionic liquids in water is strongly dependent on both the length of the alkyl chains attached to the cation and the kind of anion used. Generally, two different types of IL anions are used: fluorous anions such as CF 3 SO 3  , BF 4  , and non-fluorous anions such as Cl  , AlCl 4  , N(CN) 3  ; and there are four main groups of cations: dialkylimidazolium, dialkylpyridi- nium, dialkylpyrrolidinium and tetraalkylamonium. However, polluted industrial effluents usually contain different organic ions, as well as considerable concentrations of common inorganic ions, which in particular may affect, e.g. the efficiency of their utilization. Perfluorinated acids (PFCA) and their salts have been widely used in industry as surfactants, lubricants and corrosion inhibitors; they are used as emulsifying agents in polymer synthesis and as surface treatment agents in photolithography. The widespread application, environmental persistence and bio-accumulative potential of perfluorinated compounds results in the global occurrence of these substances in air, sediment, water, animals and humans [11,12]. Perfluorooctanoic acid and its salts, regarded as terminal degraded products, are the chemicals that have frequently been detected in environmental samples and very often occur in high concentrations [13]. Ionic liquids may be used as the solvents in fluorination of organic compounds as well as in the extraction of organic pollutants from water. These facts indicate that ionic liquids could appear in wastewater together with perfluoric compounds, Applied Catalysis B: Environmental 91 (2009) 573–579 ARTICLE INFO Article history: Received 24 March 2009 Received in revised form 25 June 2009 Accepted 27 June 2009 Available online 3 July 2009 Keywords: Ionic liquids Perfluoric acids Fenton reaction Oxidation of organic matter ABSTRACT The influence of counter ions (Cl  , C(CN) 3  and CF 3 SO 3  ) and background ions (C 6 F 11 O 2  ,C 8 F 15 O 2  and C 10 F 19 O 2  ) on the degradation rates of 1-butyl-3-methylimidazolium cation (bmim + ) in a Fenton-like system were investigated. The degradation rate of bmim + was influenced by the particular anions as follows: Cl  > C(CN) 3  > CF 3 SO 3  . The inhibition of imidazolium moiety oxidation was observed in the case of counter anions in ionic liquid which entered the competition for hydroxyl radicals with bmim + or which formed complexes with ferric and ferrous ions. The presence of perfluoric acids as a background in solution also had significance for the bmim + degradation process. These organic anions can inhibit bmim + oxidation through strong interactions between positively charged bmim + and negatively charged perfluocarboxylic anions (PFCA). Additionally, bmim + seemed to be more resistant to oxidation in the presence of a longer n-perfluoric alkyl chain anions. However, it was observed that the inhibition of bmim + oxidation decreased when the concentration of PFCA in solution exceeded a critical micellar concentration. This fact can be explained by weaker interactions between bmim + and perfluoric anions, stronger associated with aggregated PFCA in these experimental conditions (pH 3). Our investigations also indicated the possibility of PFCA interactions with ferric and ferrous ions. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +48 58 5235437; fax: +48 58 5235572. E-mail address: ewas@chem.univ.gda.pl (E.M. Siedlecka). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.06.029