Biological processes play an important role in the treatment of VOC. According to the compound or the family of compounds to be removed, the biomass does not contain the same microbial species. At lab-scale, bacteria are mainly used; com­mercial or strains isolated for their potential can be implemented. Some recent applications also involve fungi, which are more tolerant than bacteria to low water activities and acidic pH and their important enzyme complex [184], especially aromatic compounds [185]. However, owing to the low resistance of pure strains facing actual effluents variability, industrial applications involve mainly activated sludge. The use of multiphase systems for VOCs degradation has been the subject to numerous studies. These systems have been tested to treat various VOCs, including toluene [186], benzene [187], hexane [11] etc., using mainly bacteria belonging to various species, like Pseudomonas or Mycobacterium [186, 188, 189], microbial consortium [190] or activated sludge [191, 192]. Various organic phases have been implemented for this purpose, including hexadecane [187], dodecane [193] or silicone oil [13, 194, 195].

There are scarce results regarding the use of ILs as NAPLs for VOCs absorption. To our knowledge, only the imidazolium salts, [C4Mim][PF6] and [C4Mim][NTf2], have been implemented in a TPPB for toluene and DMDS biodegradation [4, 5]; activated sludge was considered, which was beforehand acclimated or not to the target VOC.

In the absence of activated sludge acclimation, there was a clear toxic effect of both ionic liquids, especially in the presence of [C4Mim][NTf2], since higher biodegradation rates were recorded in the absence of IL; the trend was especially pronounced in the case of toluene. Cell acclimation was therefore needed, which clearly improved biodegradation rates for both VOCs in the control and in the presence of IL. The improvement was especially significant for toluene, in the presence of both ionic liquids, and the most striking result was observed after acclimation, since biodegradation rates were nearly similar for the control deprived of IL and in the presence of 5 % [C4Mim][PF6], 0.49 and 0.48 g. m~3.h_1 respectively.

Neither biosorption nor biodegradability has been observed for these two ILs. However, [C4Mim][NTf2] has a toxic effect since even after cell acclimation, biodegradation rates remained lower than those observed in the absence of this IL. Promising results have been recorded for toluene in the presence of [C4Mim] [PF6] after cell acclimation, while both ILs appeared toxic regarding microorgan­isms involved in DMDS assimilation. From this, more complex strategies, includ­ing acclimation to IL, should be subsequently considered.

12.5 Conclusion

Hydrophobic ILs show interesting specific properties which make them attractive for the development of chemical and biochemical processes. These properties can be designed by selecting appropriates anion and cation. Regarding their synthesis, the general rule is a short and simple process, even if various possible functiona­lizations can lead to complex synthetic scheme. Among the wide number of structure, alkylimidazoliums are the most studied ILs.

In the case of an implementation in a bioreactor, the RTIL parameters to control are viscosity, safety towards the microorganisms contained in the reactor and an absence of ecotoxicity, an absence of biodegradability and a high affinity for the targeted VOC. They should display a high hydrophobicity to allow an easy sepa­ration from the aqueous phase and hence an efficient recycling (low losses during successive recycling cycles for a low water solubility, below 2 %). Furthermore, a synthesis at a moderate cost and in high amounts is also required for the selected ILs.

Accordingly and among the tested ILs, some promising results have been obtained using [C4Mim][PF6] and [C4Mim][NTf2], showing that they can be an alternative to the most often implemented organic phase, silicone oil. Indeed, the partition coefficients for some model VOCs, toluene and DMDS, appear similar to those observed with silicone oil. Inhibitory tests for glucose consumption have shown that after 1 day lag phase, biodegradation rates are comparable to those observed in the absence of IL. In addition, neither biosorption nor biodegradation by activated sludge have been observed for [C4Mim][PF6] and [C4Mim][NTf2].

In addition, significant rates of toluene biodegradation have been found in the presence of [C4Mim][PF6], similar to those observed in the absence of IL; however only after activated sludge acclimation. Contrarily, [C4Mim] [NTf2] shows a toxic effect even after an acclimation phase, since low degradation rates have been observed. At the opposite, a toxic effect of IL has been observed regarding DMDS, even after an acclimation time.

This toxic effect can limit the use of ILs in multiphasic bioreactors, but the promising results recorded for toluene suggest that more investigations are needed, especially regarding the acclimation strategy, and in particular to the considered IL in addition to VOC acclimation.

In conclusion, these compounds can be an alternative to silicone oils, but further works are needed to confirm their relevance for implementation in multiphase bioreactors.

Acknowledgement The authors want to thanks the French National Research Agency (ANR — Blank program) for the financial support of this work.