Microorganisms are able to assimilate a wide range of the available solvents, particularly alkanes, ketones or hydroxylated solvents (carboxylic acid, aldehydes, etc.). Solvents containing long alkyl chains or alcohol, ester or carboxylic groups are also biodegradable, and are precursors of beta oxidation. Other solvents like phthalates or plasticiser compounds (for example adipates) can also be degraded by various microorganisms such as Rhodococcus or Sphingomonas [101, 102]. How­ever, various authors showed that biodegradability decreases with the presence of long alkyl chains or hydroxyl, ester and acid groups on the molecule [7, 11]. Pre­viously, Alexander [103] reported that high molecular weight compounds, with lot of ramifications, are biologically recalcitrant. Besides, the type, the number, and the position of the substitutes on simple organic molecules influence their biodegrad­ability. Various compounds having very low degradation rates or totally refractory

towards microorganisms are described as bio-recalcitrant. However, a total absence of biodegradation, even after an acclimation time, is required for the proposed process.

Therefore, among the available solvents only silicone oils and ionic liquids [1] comply with the non-biodegradability criterion; while these latter appear especially promising owing to their solvent capacity and their low volatility [17,18], as well as the possibility of IL tailoring to fit the characteristics required for specific applica­tions [2]. In addition to fine-tuning their physicochemical properties, other ILs properties such as microbial toxicity are also related to the IL structure, showing that suitable ILs for microbial application can likely be designed. ILs have been therefore selected for implementation in the proposed process.

ILs applications in biotechnology have mainly focused on enzymatic catalysis [104]; a versatile battery of reactions being successfully performed in the presence of ILs, including transesterification, perhydrolysis, enantioselective reduction of ketones, and ammoniolysis [16, 105]. On the other hand, ILs toxicity has been reported as a key drawback for whole-cell applications [106, 107]. Studies on ILs toxicity (acute toxicity tests) are usually based on bioluminescence using microor­ganisms such as Vibrio fischeri or Photobacterium phosphoreum [108—110]; how­ever, these microorganisms are not representative of the microbial cells commonly used in bioprocesses. Recently, Azimova et al. [111] observed that the IL toxic concentration for a Pseudomonas strain was up to 700 times higher than those for V. fischeri. Regarding microorganisms commonly used in biotechnology, there are contradictory reports in the literature. Successful whole-cell processes in the pres­ence of ILs have been reported (e. g. synthesis of ketones and alcohols, lactic acid and antibiotic production) [52, 112—114], but also toxic effects of ILs towards yeasts and bacteria can be found [115, 116].

In addition, ILs biodegradability is a fundamental aspect that must be addressed before applying ILs in a whole-cell process. The non-biodegradability of the solvent is a required characteristic during a biotechnological process; being this particularly important when the solvent must be continuously reused [9]. Reports on ILs biodegradability are controversial in the literature. Most of the authors, working on imidazolium-based ILs, reported that ILs are not biodegradable [107, 109, 117]; while some authors observed ILs biodegradation by bacteria and fungi [118, 119]. These apparently contradictory reports clearly indicate that further evidences are necessary to a better understanding of ILs toxicity and biodegrad­ability, which constitute the base for whole-cell biotechnological applications.

Based on these considerations, the regeneration of the ILs can be envisaged by biodegradation. For this purpose, activated sludge can be used to assimilate the VOC absorbed in the IL, enabling subsequent IL recycling to allow its use for a new cycle of VOC absorption. Multiphase bioreactors are frequently encountered in environmental biotechnology; several configurations involving gas/solid/liquid or gas/liquid/liquid phases have been reported. The use of such multiphase systems for the biodegradation of numerous pollutants (e. g. ethylacetate, phenol, toluene, benzene, xylene, and volatile organic contaminants) is well documented in the literature [120—122].