Degradation of 1-butyl-3-methylimidazolium chloride ionic liquid in a Fenton-like system

Abstract

The study examined the usefulness of a Fenton-like system for the degradation of ionic liquid residues in water. The ionic liquid was oxidized in a dilute aqueous solution of 1-butyl-3-methylimidazolium chloride (bmimCl). The ionic liquid decomposes readily and rapidly in aqueous solution by chemical degradation in a Fenton-like system. Under chosen conditions the initial bmimCl solution was reduced by a factor of 0.973 within 90 min. Additional results showed that bmimCl degradations in a Fenton-like system in excess H₂O₂ could be interpreted as a combined oxidation–reduction mechanism. Preliminary investigations of the mechanism of such degradations have indicated that initial OH radicals can attack any one of the three carbon atoms on the imidazolium ring. The intermediates of this reaction may be mono- di- or amino- carboxylic acids.

Introduction

Room temperature ionic liquids (RTILs) are a new class of semi-organic salts that are liquid below 100 °C. The most attractive characteristic of RTILs is their potential to be ‘designer solvents’: properties such as moisture stability, viscosity, density or miscibility with other solvents can be tailored by the appropriate selection of cation and anion. RTILs already in common use typically involve nitrogen- or phosphorus-containing organic cations such as alkylimidazolium, alkylpyridinium, alkylpyrrolidinium or alkylphosphonium, and anions like chloride, bis(trifluoromethanesulfonyl)imide, hexafluorophosphate or tetrafluorophosphate [1], [2], [3]. Current research indicates that replacing an organic solvent with an RTIL can bring about remarkable improvements in well-known processes. Ionic liquids have been applied not only as alternative solvents in synthesis and as electrolytes, but also in separation techniques, as alternative lubricants or in tissue preservation applications [4], [5], [6], [7]. The wide applicability of ionic liquids as well as advances in the variety of applications will soon result in their production and use on an industrial scale.

Ionic liquids are often uncritically regarded as intrinsically “green” due to their negligible vapor pressure, and therefore a good alternative to the emissions of toxic vapors from conventional molecular organic solvents. A low vapor pressure, however, is generally not enough to justify calling a process or even a whole technology “greener”. Certain amounts of ionic liquids will soon be present in technological wastewaters where, because of their high stability, they could become persistent pollutants and break through classical treatment systems into natural waters.

In the past decade, advanced oxidation processes (AOPs) have involved the increased use of in situ and ex situ chemical oxidation of contaminated soils and groundwater. Among them, oxidation by several agents such as UV radiation, Fenton's reagent, or ozone, have been used with success [8], [9], [10]. Fenton's reaction is the catalytic decomposition of dilute hydrogen peroxide by iron (II), which results in the generation of hydroxyl radicals.

Fe²⁺ + H₂O₂ → Fe³⁺ + OH + OH⁻

The hydroxyl radical is a powerful and relatively non-selective oxidizing agent that reacts rapidly with alkanes and aromatic compounds, but is unreactive toward perhalogenated alkane compounds with a high degree of nitro-substitution and other highly oxidized organic compounds [11]. Fenton's reaction is usually modified for full-scale environmental applications. Modifications of the standard Fenton procedure include the use of alternative catalysts such as iron(III) (which promotes a superoxide-driven reaction) [12], iron oxyhydroxide minerals [13], [14], [15], iron chelates [16] and addition of excess H₂O₂ [17]. Modifications lead to propagation reactions that generate a perhydroxyl radical (HO₂), a superoxide radical anion (O₂⁻), and a hydroperoxide anion (HO₂⁻) [18]. Moreover the modified Fenton reaction is more vigorous than the standard Fenton system, because the groundwater or the soil contaminants are oxidized, reduced or attacked by nucleophiles in a single system. The use of Fenton-like reactions for soil and groundwater remediation was first reported by Watts et al. [19]. Tyre et al. [15] proposed that naturally occurring iron minerals in the soil could catalyze the decomposition of hydrogen peroxide and promote a Fenton-like reaction.

When dissolved in water, most organic priority pollutants react rapidly with a hydroxyl radical, but sorbed contaminants and non-aqueous phase liquids exhibit minimal reactivity with the hydroxyl radical [20]. Alternatively, an OH and a non-OH mechanism may co-exist in vigorous Fenton-like reactions that promote the desorption of contaminants with subsequent transformation in the aqueous phase.

Ionic liquids were also a subject of interest in advanced oxidation processes, not as target solutes however but as a novel reaction media for decomposition of various contaminants including CCl₄, benzophenone, quinones or chlorinated phenols [21], [22], [23], [24]. It was found that formation of stable radiolysis intermediates of ionic liquids may lead to their resistance towards oxidation, thus making them privileged for type of applications. Moreover, it was also found that there is a clear difference in the mechanism of oxidation reaction in ionic liquid (e.g. pentachlorophenol) if compared to aqueous or aqueus – solvent media [25].

The purpose of this study was to verify the usefulness of a Fenton-like system in the degradation of ionic liquid residues in water, in particular, to determine the effect of hydrogen peroxide concentration on degradation rates of an exemplary ionic liquid – 1-butyl-3-methylimidazolium chloride (bmimCl) – oxidized in a Fenton-like system, and to determine the mechanism of its oxidation in a vigorous Fenton-like reaction successfully used for contaminated soil remediation.