Multiphase bioreactors for environmental applications are known as two-phase partitioning bioreactors (TPPB — Fig. 12.4). TPPB are based on the addition of a non-aqueous liquid phase (NAPL) offering a high affinity for the target pollutants to be removed [123] in order to improve pollutant mass transfer from the gaseous to the liquid phase, and hence to improve the subsequent biodegradation kinetics. In most cases, pure microorganism strains, microbial consortium or activated sludge are implemented in this kind of reactors, with some possible drawbacks, namely possible NAPL toxicity towards microorganisms or on the contrary NAPL assim­ilation by the microorganisms.

As a consequence, among the only classes of solvents fulfilling the required characteristics, silicone oils and ionic liquids, ILs appear especially promising. It is worth noting that possible ILs toxicity has been reported as a key drawback for whole-cell applications [106, 107], even if there is not a general agreement regard­ing toxicity. The non-biodegradability of the NAPL is a required characteristic, owing to its continuous reuse [9]. However, reports on ILs biodegradability are controversial in the literature. These apparently contradictory reports clearly indi­cate that ILs structure selection is a key step in process development since it influences all preponderant properties.

All TPPB studies are based on the addition of a NAPL, either a liquid solvent or a solid polymer, with a high affinity for the target pollutants to be removed [123] in order to overcome two main limitations encountered during pollutant destruction: (i) the high toxicity of some pollutants resulting in cell inhibition and (ii) a limited substrate delivery to the microbial community in the case of pollutants with low affinity for water [1, 9]. NAPL addition improves the transfer of the target com­pounds from the gaseous phase to the liquid phase, and hence enhances their subsequent biodegradation. In most cases, pure microorganism strains, microbial consortium or activated sludge are implemented in this kind of reactors, with some possible drawbacks, namely possible NAPL toxicity towards microorganisms or on the contrary NAPL assimilation by the microorganisms.

VOC biodegradation performances depend on the presence and the assimilation potential of various microbial agents such as bacteria, micro-algae, fungi or yeasts contained in the aqueous phase [124]. there is a selective partitioning of the pollutant between water and NAPL in a TPPB; hydrophobic (or toxic) compounds are delivered to the aqueous phase at sub-inhibitory levels for microorganisms in case of toxic compounds or at the solubilisation limit in case of hydrophobic compounds.

For Deziel et al. [7] three mechanisms can be involved in the consumption of hydrophobic or toxic compounds:

1. VOC consumption in the aqueous phase. The degradation rate depends on the

mass transfer rate from the organic to the aqueous phase, reported as lower than

the VOC biodegradation rate [125, 126].

2. VOC removal takes mainly place at the liquid-liquid (organic-aqueous) inter­face, since VOC can be directly assimilated at the interface after biofilm development between both liquid phases [127], or microorganisms accumula­tion at the interface [11, 125].

3. If the microorganisms are surfactant or emulsifier-producers, the formation of small droplets or micelles can lead to a reduction of the surface tension of the aqueous phase and to an increase of the interfacial area until microemulsion formation [128]. This phenomenon improves substrate availability for the micro­organisms mainly located at the organic-aqueous interface.

There is a general agreement that the major part of hydrophobic VOC uptake is achieved at the interface of both liquid phases [7, 129].

TPPB involving ILs as NAPL can take advantage of the high absorption capacity of ILs for a wide number of pollutants [46]. As already specified, there is a lack of data dealing with the use of ILs for pollutants removal; Baumann et al. [3] showed efficient phenol biodegradation rates in the presence of ILs in multiphase reactors [9]. Regarding hydrophobic VOCs, microorganisms acclimation led to interesting results for toluene, while further work is needed to optimise DMDS degradation in the presence of ILs [5].

ILs characteristics (non-inflammability, low vapor pressure, etc.) make them attractive solvents for chemical processes [130] and several kind of bioreactors, such as multiphase bioreactors, involve the use of ILs, especially for enzymatic transformations [2]. Biphasic devices involving IL/water systems are available; but they are mainly used for organic compounds extraction (or the extraction of compounds formed in situ during chemical transformations) or macromolecules (proteins) from the aqueous phase thanks to ILs [114, 131—133]. For instance, ILs can be used for fuel desulfurization [134], biofuels production [135], the extraction of food waste [73] or can be also included as part of solar cells [136].

To separate valuable molecules or to develop vectors for molecules solubiliza­tion, membrane processes have been described involving membranes modified with ILs [137].

In view of the implementation of ILs in TPPB and their subsequent recycling owing to their cost, their non-biodegradability and their biocompatibility are important parameters. Biodegradability is discussed thereafter, while toxicity towards microorganisms is discussed in the following sub-section (cf. 4.2).

Recently, some works have been conducting dealing with ILs biodegradation by a bacterial consortium. Docherty et al. [138] have examined the various steps and the degradation products of some imidazolium — and pyridinium-based ILs. It was shown a capacity of the microorganisms to degrade such ILs substituted by an octyl or a hexyl group. Contrarily, imidazolium — and pyridinium-based ILs with a butyl substi­tution seem to be not assimilated by activated sludge. Docherty et al. [138] also observed that the removal rate of these compounds increases with the length of the carbon chain, and hence they also highlight the ease and the rapidity of the assimi­lation of the pyridinium-based ILs with a long carbon chain. Contrarily, no biodeg­radation by activated sludge of the imidazolium-based ILs was observed in the case of [C4Mim][Br], [C4Mim][PF6] and [C4Mim][BF4] anions. However, the addition of an ester group in the alkyl chain results in the biodegradation of the liquid ionic [138].

The biodegradability of the basic constituents of ILs was examined by Stolte et al. [139]. For example, they showed that 1,2-dimethylimidazole is completely assimilated by a bacterial consortium after 31 days, while the 1-methylimidazole and the 1-butylimidazole are not degraded. These authors also reported that [C4Mim][Cl] is not degraded after 31 days of contact time with active sludge, while 11 % degradation has been observed for [C6Mim][Cl].

In conclusion, the alkyl chain added to the cation has a non-negligible impact on the degradation, while the anion impact appears not clear; and finally a total absence of biodegradability was only shown for [C4Mim][PF6] [109].