Solvent choice is the first issue to be considered since it will determine the whole process. The selected non-aqueous phase liquid (NAPL) should not add pollution, must be non-flammable, and its chemical and thermal properties must fulfil those required with the aim of its recycling [6]. The NAPL must be liquid and not very viscous in a range of temperature between 5 and 40 °C. To make the separation from water after biodegradation step feasible, the considered NAPL should be water-immiscible and should not lead to a stable emulsion.

Several NAPL have been previously used either in a two-phase partitioning bioreactor or as absorbents in gas-liquid contactors (scrubber, airlift, bubble column, etc.). Most of them display very low degradation rates or are refractory towards microorganisms and are described as bio-recalcitrant. However, recalcitrance is not enough for the proposed process, since it means a biodegradation of the considered NAPL after an acclimation time. According to some authors [7], five classes of NAPL are potentially non-biodegradable: HMN (2,2,4,4,6,8,8-Heptamethylnonane) owing to the presence of terminal methyl groups, fluorocarbon FC 40, some polymers like the polyisobutylene which contains many terminal methyl groups, silicone oils, especially polymethylsiloxane, and ionic liquids. However, HMN seems biodegrad­able by some acclimated microbial communities [8]; fluorocarbons are toxic toward humans or the environment [9], while polymer viscosity may induce a too high energy consumption in the TPPB (stirring).

Among the available solvents, only silicone oils and ionic liquids appear there­fore really relevant [1, 10]. Even if silicone oils are interesting NAPL candidates, especially polydimethylsiloxane, for hydrophobic VOCs removal, owing to their biocompatibility, their non-biodegradability [10], and have often been implemented in TPPB [11—13], ionic liquids seem promising.

Ionic liquids have been recognized for about a century, but have only started receiving closer attention in the last two decades. Historically, an ionic liquid is an organic salt with a melting point below 100 °C [14]. They are composed by an association between an organic cation containing one or more hetero-atom(s) (nitro­gen, phosphorus or sulfur) and an inert anion or Lewis acid [15], namely the counter­ion, leading to aneutral compound [16]. Since the first chloroaluminate “molten salt”, many efforts have been made about ionic liquids to lower their melting points (development of RTILS, “Room-Temperature Ionic Liquids”) and to improve their stability towards air and water.

Their low vapor pressure and non-flammability [17, 18] makes them particularly interesting class of solvents for ‘green chemistry’ or absorption. However, based on recent data, these assumptions have been progressively reconsidered [19—23]. In addition, they are generally thermically stable (decomposition temperatures >150-200 °C), chemically or electrochemically inert.

Their interest is not only due to their remarkable physicochemical properties (lipophilicity, viscosity, density, etc.) but also for their recyclability. However, these properties are usually presented as applicable to all ILs are not so “universal” and the large number of possible combinations of a structural point of view suggests that some ILs are not as harmless [24].

So, ILs can be designed for specific applications [2, 18]. Hence, it is possible to fine-tune IL physicochemical properties by means of modifying the substituent groups or the identity of the cation/anion pair [17, 18].

One of the first reviews on ILs (synthesis, applications, etc.) was published in 1999 and related the general methods of synthesis and the first applications of ILs based chloroaluminates [25]. Below are shown the structures of most common ILs (Fig. 12.2).

As they are readily tunable, ILs could be selected as NAPL for TPPB. The physico­chemical characteristics of the ionic liquid (viscosity, hydrophobicity toxicity, etc.), as well as possible biodegradability, vary according to the considered radical.