Some methods are available to assess the biodegradability of an IL, which are not detailed in this chapter. However, Coleman et al. [141] proposed an exhaustive summary; for more than 60 % possible biodegradation, a compound can be classi­fied as biodegradable.

However and beforehand to a biodegradability study, possible biosorption of ILs should be assessed. Experiments using 4 g/L thermically inactivated sludge (autoclaved) showed a constant IL concentration (10 g/L initial amount) throughout the experiments which lasted 30 days, indicating an almost negli­gible biosorption of the considered imidazolium IL, [C4Mim][PF6] and [C4Mim] [NTf2]. Biodegradability tests were then performed on these ILs showing constant IL values throughout the short — and long-term (5 days and 1 month) experiments performed.

These results are in agreement with those of other authors [107, 109]. Indeed, with an imidazolium-based cation, irrespective of the considered anion, such as Br~, Cl~, NTf2~ and BF4~, these authors did not observe any biodegradation using activated sludge. Similarly, Stolte et al. [139] reported that activated sludge is not able, even after 31 days, to assimilate [C4Mim] cations.

Boethling [175] presented some factors used in the design of biodegradable compounds: the presence of potential sites of enzymatic hydrolysis (for example, ester and amides groups in the molecule) or the introduction of oxygen in the form of hydroxyl, aldehyde or carboxylic acid groups or even the presence of unsubstituted linear alkyl chains (especially in molecules with more than four carbon atoms) and phenyl rings [175]. Most of authors who designed ILs follow these factors, showing that some might be or not completely applied to ILs [108, 109, 117, 138, 139]. Therefore, primary biodegradation of ILs have been studied with different anions, cations and side chains configurations.

An absence of biological degradation was found that for imidazolium-based ILs with short alkyl chains (<6 carbon atoms) and short functionalized side chains, and that the introduction of functional groups with a higher chemical reactivity did not improve their biodegradability. For example, butylmethylimidazolium salts were found to be poorly biodegradable, while an impact on the biodegradability was observed related to the counter-ion considered, leading to the following order for their biodegradability [117]:

Octyl-OSO3 > N(CN)2- = CP = BP = PF6- = NTf2-

Gathergood et al. [117] also studied the biodegradability of 3-methyl-1- (propyloxycarbonyl) imidazolium and it varies according to the following order:

Octyl-OSO3 > N(CN)2- > NTf2- > BP > PF6- = BF4-

However, octyl chains in imidazolium cations have shown to be highly biode­gradable and the introduction of — OH and — COOH into the octyl chains improves primary degradation of ILs [108, 138, 139].

In addition, ILs containing an ester or an amide group in the alkyl side chain (>4 carbon atoms) were found to be biodegradable and that biodegradability increases slightly if the alkyl chain length increases for the lowest alkyl esters and then remain nearly invariable [117].

Nevertheless, for pyridinium with long side chains (6-8 carbon atoms), Docherty et al. [138] have observed complete mineralization of ILs, but those with short side chains (<4 carbon atoms) are not mineralized. Therefore, pyridinium-based ILs might be considered as readily biodegradable. A link between the length of the alkyl chain and the metabolization rate of pyridinium compounds is also discussed.

Yu et al. [176] showed that the presence of benzene cycles also increases the biodegradability of ILs, but no information dealing with their toxicity is presented. It should be specified that the less toxic IL is, a priori, more easily biodegradable since it does not attack the microorganisms involved in its degradation [177].

Other structural modifications would allow reducing the biodegradability of ILs: the presence of halogens (particularly chlorine and fluorine), alkyl ramified side chains (trisubstituted nitrogen or quaternary carbons), functional groups such as nitro, amino or arylamino, polycyclical structures (indole, etc.), heterocyclic struc­tures, aliphatic ethers. Concerning the anions, it has been showed that the alkylsulfates increase the biodegradability of ILs [178].

Identification of the biodegradation products of some ILs, such as imidazoliums and pyridiniums, have been the purpose of several studies [118, 119, 139, 140, 179—182]. The related metabolites generally result from a first oxidation reaction on a lateral chain of the cation, followed by water or CO2 extrusion.

The objective of most of the available biodegradability studies is to develop ‘green’ ILs and hence biodegradable [183]. However and owing to their high cost, for possible implementation in a TPPB the subsequent recycling of the IL should be considered in view of reducing cost process, and thus the objective is an absence of biodegradability (and toxicity) facing the considered microorganisms.

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.

Prof. Dr. Zhen FANG is the leader and founder of biomass group, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences. He is also an adjunct full Professor of Life Sciences, University of Science and Technology of China. He is the inventor of “fast hydrolysis” process. He is specializing in thermal/biochem — ical conversion of biomass, nanocatalyst synthesis and its applications, pretreatment of biomass for biorefineries. He obtained his PhDs from China Agricultural University (Biological and Agricultural Engineering, 1991, Beijing) and McGill University (Materials Engineering, 2003, Montreal).

Richard L Smith, Jr. is Professor of Chemical Engineering, Graduate School of Environmental Studies, Research Center of Supercritical Fluid Technology, Tohoku University, Japan. Professor Smith has a strong background in physical properties and separations and obtained his Ph. D. in chemical engineering from the Georgia Institute of Technology (USA). His research focuses on developing green chemical processes, especially those that use water and carbon dioxide as the solvents in their supercritical state. He has expertise in physical property measurements and in sepa­ration techniques with ionic liquids and has more than 200 scientific papers, patents and reports in the field of chemical engineering. Professor Smith is the Asia Regional

Z. Fang et al. (eds.), Production of Biofuels and Chemicals with Ionic Liquids, Biofuels and Biorefineries 1, DOI 10.1007/978-94-007-7711-8,

© Springer Science+Business Media Dordrecht 2014

Editor for the Journal of Supercritical Fluids and has served on editorial boards of major international journals associated with properties and energy.

Xinhua Qi is Professor of Environmental Science, Nankai University, China. Professor Qi obtained his Ph. D. from the department of environmental science, Nankai University, China. Professor Qi has a strong background in environmental treatment techniques in water and in chemical transformations in ionic liquids. His research focuses on the catalytic conversion of biomass into chemicals and biofuels with ionic liquids. Professor Qi had published more than 50 scientific papers, books and reports with a number of papers being in top-ranked international journals.

[1] As an example see pioneer work of [7].

[2] As an example see pioneer work of Richard et al. [10].

[3] Lindman et al. [11] and references cited therein.

[4] As a recent selected example see [13].

12.3.1 Solubility

There is a lack of investigations on VOCs affinity towards ILs. Some results are available dealing with the partition coefficient between an ionic liquid and water [18]:

Polar organic compounds, such as dichloromethane and trichlorobenzene, are soluble in ILs. Huddleston [18] determined the partition coefficient between a given ionic liquid and water, specific to these compounds (log D) (Eq. 12.1):

For instance, a value of 1.8 and 2.4 were found for toluene — water [18] and [C6Mim][PF6] — water [26]. Huddleston [18] showed that ILs can solubilize apolar or lowly polar compounds. The literature dealing with the solubility of organic compounds in ionic liquids remains limited [27—31]. However, a trend seems to emerge showing that polar compounds are more easily soluble than apolar ones. Huddleston [18] also explored the replacement of usual solvents by ionic liquids in the liquid-liquid extraction of VOCs; they found that the affinity of a solute (the VOC) for ILs increases with the augmentation of the log POW value of the considered VOC.

Regarding the Henry’s constants, the scarce available results concern toluene and sulfur VOC, dimethyl sulfide (DMS), dimethyl disulfide (DMDS); they were measured for two ILs, [C4Mim][PF6] and [C4Mim][NTf2] and are compared in Fig. 12.3 to some other solvents [4, 32]. For instance, the Henry’s constants of DMDS and toluene in water are 123 and 615 Pa. m3.mol_1, respectively; they were found to be significantly lower in ILs, with ratio of the partition coefficients in water and the considered IL, [C4Mim][PF6] and [C4mim][NTf2], of 284 and 448 for toluene and 33 and 37 for DMDS, respectively [4]. If compared to the often used silicone oil (polydimethylsiloxane, PDMS), partition coefficients were similar for DMDS, while toluene showed a higher affinity for [C4Mim][NTf2] if compared to [C4Mim] [PF6] and PDMS [4].

In conclusion, compared to the most often used non-aqueous phase liquid (NAPL), namely silicone oils, especially polydimethylsiloxane, for hydrophobic VOCs removal [10], which have been often implemented in TPPB [11—13], ionic liquids appear especially promising owing to their solvent capacities.

Molecular diffusion is the transport phenomena caused by a concentration gradient [33]. It must not be mistaken for convection which is caused by the bulk motion.

The diffusive flux can be described by the Fick’s first law at steady state. It postulates that the flux (J in mol. m2.s-1) is proportional to the spatial concentration gradient (from large to low concentration areas):

dC

J = — D‘j dX (12.2)

Dj j is the diffusion coefficient (or diffusivity in m2.s-1) of the compound i in the solvent j.

In gas-liquid mass transfer operations, the removal efficiency depends directly on the mass transfer rate, the absorption rate (which accounts for the solubility) and on the contact time. According to various mass transfer theories (Higbie or Danckwerts theories for example), the mass transfer rate in both phases are pro­portional to the square root of the diffusion coefficient in each phase [34]. A low mass transfer rate in the liquid phase can significantly hamper the VOC removal and is therefore a key point to control for a liquid ionic selection.

With traditional solvents (water and organic solvents), the diffusion coefficient of non ionic solute increases with the temperature and decreases with the solvent dynamic viscosity (jxj) and the solute molar volume (Vm i):

Several theories are currently applied to calculate diffusion coefficients in traditional solvents (Stokes-Einstein, Wilke-Chang, Arnold, Hayduk-Laudie, Schreibel, etc.) [27, 33]. Except the Arnold theories, a = в = 1 and 1/3 < у < 0.6. The solvent molecule size is also sometime taken into account. Thereby, it is admitted that the diffusion coefficient increases with the loading of the solvent in solute. For mass transfer operations, the infinite dilution diffusion coefficient, with an order of magnitude of 10~9 m2.s-1, is generally used in the computation [35].

Several experimental techniques have been applied for the determination of the diffusion coefficients of various solutes (CO2, alkanes, alkenes and hydrofluor­ocarbons) in ionic liquids. All these techniques are based on two methods: the thermogravimetric method or the manometric method. These methods enable to investigate at the same time the solubility and the diffusion properties of a solute in a given solvent. They are usually conducted in static mode to avoid convection contributions. The manometric method is based on the measurement of the pressure decay in a thermo-controlled cell chamber which contains a layer of the investigated solvent. An accurate amount of gas, often provided by a pressur­ized feed chamber, is introduced rapidly at the beginning of the measurement. The diffusion coefficient is determined by fitting the pressure decay to a one-dimensional diffusion model for solute uptake into the liquid [28, 36, 37]. An alternative manometric method consists to immobilize the solvent in a membrane which separates the feed and the cell chambers [27, 29, 30]. This technique is often called the two-cells methods or the lag-time technique. The thermogravimetric method is a relatively recent method based on the measure­ment of the solute uptake in the investigated solvent using a microbalance [31, 38—40]. Buoyancy corrections are necessary to take into account the expansion of the solvent due to the solute absorption during the experiment. The feed gas is usually pure.

For both techniques, vacuum is applied to the cell chamber before the gas sample introduction. Depending on the thickness of the solvent layer, several dimensional diffusion models have been used to determine by fitting the diffusion coefficient (thin-film model, semi-infinite model, etc.).

Diffusion coefficients of small solutes such as VOCs in ionic liquid are usually one or two order of magnitude lower than in traditional organic solvents (in the range 10~n-10~10 m2.s-1) (Table 12.1) mainly because of the high viscosity of the RTILs. Diffusivity drops more significantly in RTILs than in traditional solvents when the temperature decreases (lower molecular agitation and larger viscosity). Several studies demonstrate that this evolution follows the Arrhenius law, with activation energies typically larger than traditional solvents in the range 10-25 kJ. moP1 [27, 36, 38].

Scovazzo and coworkers investigated by the lag-time technique the solubility and diffusion of CO2 and several VOCs (ethylene, propylene, 1-butene, butadi­ene, methane, butane) at 303 K in five imidazolium, five phosphonium and nine ammonium based RTILs, covering a large range of viscosities (from 10 to 3,000 cP) [27, 29, 30]. They found that the value of a depends on the kind of cation (0.66 for the imidazolium, 0.35 for the phosphonium and 0.59 for the ammonium based RTILs) whereas the Stokes-Einstein and the Arnold theories predict respectively 1 and 0.5. у was equal to 1.04 for imidazolium, 1.26 for phosphonium and 1.27 for ammonium based RTILs. Therefore, diffusivity in RTILs is less dependent on viscosity and more dependent on solute size than predicted by the conventional Stokes-Einstein model. As mentioned by Morgan, the deviation between RTILs and traditional organic solvents may result from the physical situation of small solutes diffusing in an universe of large IL molecules solvents [27].

At identical viscosities, diffusivities are larger in phosphonium, then in ammo­nium, then in imidazolium based RTILs. This may be explained by the increased molar volume of the phosphonium based ILs («600 cm3.mol-1 compared to approximately 400 and 200 cm3.mol_1 for respectively ammonium and imidazolium based ILs) which can allow for faster diffusion rates. Skrzypczak and Neta suggest that this trend is due to the increased number and length of the aliphatic chains present on the phosphonium cation [41]. Because the chains are flexible and can move more rapidly than the whole cation, they enable a more rapid diffusion of solutes from one void to another in the phosphonium-based ILs [41]. Therefore, the amount of free volume in an anionic liquid could be a better indicator of diffusivity than viscosity.

1,3-butadiene diffuses faster for an identical molar volume than the other alkene due to a possible weak complexation of the conjugated double bonds with the positively charged cation which facilitates the transport [30]. All these conclusions suggest that finding a universal correlation to determine diffusivities for all classes of RTILs and solute is utopist, even if similar trends are observed for different kind of RTILs. Moreover, it is also important to consider the effect of other impurities and large solute concentrations on diffusion. It has been shown that water and other co-solvents increase very significantly the viscosity and consequently the diffusiv — ity. Therefore, the diffusivities should be measured for any couple RTIL-solute to assess the mass transfer rate.

Camper et al. measured the diffusivities of several VOCs (ethane, ethane, propane, propene) and CO2 on [C2Mim][NTf2] for various temperatures using the manometric method and a semi-infinite model [28]. They confirmed the same trends and the order of magnitude found by Morgan et al. with the same solute-RTIL couple, even if the values of Morgan were 20 % lower (Table 12.1).

Shiflett and Yokozeki investigated the solubility and the diffusion of hydrofluorocarbons in several RTILs (mainly imidazolium based RTILs) by the thermogravimetric method in isothermal mode for pressure up to 20 bar [31, 39, 40]. The effective measured diffusion coefficients increase with the pressure and the temperature applied in the chamber. Indeed, a larger pressure applied increases the solute uptake in the solution. Therefore, the determined diffusion coefficients are not determined at infinite dilution. The values found for the various diffusion coefficients are all included between 2 • 10~n and 8 • 10_11 m2.s-1 at 298.15 K.

Except for the few VOCs presented before, the most investigated solute remains CO2. Even if this compound is out of the scope of this review, this study is worthwhile since it enables a comparison between the different mea­surement techniques. Hou and Baltus investigated CO2 solubility and diffusivity in five imidazolium based RTILs by the manometric method and using a transient thin-liquid film model [36]. Their results are consistent with the results of Camper et al. and Morgan et al. deduced with manometric methods (Table 12.2) [27, 28]. However, the values found using the thermogravimetric method were considerably smaller (five times) [38]. Hou and Baltus suggest that the thermogravimetric method present a larger uncertainty due to several buoy­ancy corrections necessary and they questioned the fact that the measurements

were performed at large pressure which might influence the density and the viscosity of the RTIL [36]. Whatever, the difference between the two methods remains controversial and poorly commented in the literature. Some systematic and/or random errors would be responsible for the discrepancy among research groups. Further investigations to try to understand the difference between both methods would be interesting.

For conventional solvents, molecules solubilization is generally expressed in terms of dielectric constant of solvent. However, these criteria could not be applied to ionic liquids. Several studies have been conducted to establish a specific ranking based on interactions between ILs and solutes [43]. This model considered inter­actions involving hydrogen bond and polarizability. According to these authors, ionic liquids are polar solvents but they have completely different behaviors from conventional solvents, so it is impossible to compare these two kinds of solvents only on their polarity.

Most of the time, the n interactions between aromatic cations and solutes are prevailing. Imidazolium ring is a good electron acceptor and due to the strong electron delocalization, the nitrogen does not form H bonds easily. On the contrary, a pyridinium cation is a good electron donor [44].

Some ionic liquids have been functionalized (TSILs: Task Specific Ionic Liq­uids) for a specific purpose. For example, ureas or thioureas groups were introduced in few ionic liquids in order to capture CO2, H2S or heavy metals [45]. RTILs (Room Temperature Ionic Liquids) have also been developed to perform extraction of metals [46] or elements such as Uranium [47]. Bifunctionalized ionic liquids can be used to optimize the extraction of Europium [48]. The [C4Mim] [NTf2] was proved to be effective in extraction of inorganic acids (HNO3, HCl, HReO4, HClO4) from aqueous phases [49].

Several authors have studied the distribution of organic compounds, such as aniline, benzene derivatives and organic acids or organic anions (phenolates) between an ionic liquid phase (usually [CxMim] [PF6, BF4 or NTf2]) and an aqueous phase. As expected, the most lipophilic compounds are more soluble in the ionic liquid phase [50—61]. In addition, it has been shown that aromatic compounds were more soluble in ionic liquids than their aliphatic counterparts, probably due to п-stacking interactions [43, 62—64].

Some studies have reported that the physical and chemical properties of ionic liquids are due to intra-molecular hydrogen interactions and Van Der Waals interactions. These studies explain the particular geometries adopted by ILs [65, 66]. Hydrogen bonds define the cation position versus the anion but also the distance between the two poles of an ionic liquid [67]. The structural organization of an IL can be explained by a combination of both types of interaction in the liquid phase, Coulomb and Van der Waals interactions [68, 69]. We observe that these interactions have a direct influence on the melting point, viscosity and ionic liquids enthalpy of vaporization. These inter­actions between the anion and the cation can also explain the variable hydro — phobicity observed. It is essentially depending on the nature of the anions. The self-organization of ionic liquids may also influence their potential extraction and gas absorption [65, 66, 70, 71].

The [CxMim] [PF6] were applied to the extraction of anionic dyes [72], to the identification or the extraction of organic pollutants in soil or in water [54, 73, 74] or to extract and separate bioactive molecules from plants [75—77]. They are known to solubilize some natural polymers (cellulose, BSA, etc.), sugars or amino acids [46, 78—82]. They constitute the overwhelming majority of the ionic liquids used for the extraction of organic molecules.

N-methylimidazoliums functionalized by carboxylic chains (CH2CO2H) and associated with fluorinated anions such as BF4~ or PF6~ were synthesized in order to trap organic compounds (chlorophenyl, amines) [83]. Some ionic liquids (mainly alkylimidazoliums) are described for the absorption of volatile organic compounds such as benzene, toluene, phenols, anilines or of sulphur heterocycles [84, 85]. Some studies report the solubility of several hydrocarbons (benzene, toluene, xylene, heptane, hexadecane, methanol, acetonitrile or chlo­roform) in ionic liquids such as [CxMim] [PF6] [86]. This work has shown that aromatic hydrocarbons, methanol and acetonitrile are soluble or partially soluble in [CxMim] [PF6], while aliphatic hydrocarbons are immiscible. It has been shown that for a fixed cation, the anion could affect the interaction between ILs and VOCs. Hard anions (NO3~,MeSO3~, etc.) are worse hydrogen bonds acceptor than softer anions such as B(CN)4~ and offer low affinities with VOCs [87, 88].

Some studies have demonstrated that the ability of the anion to accept hydrogen bonds was related to the distribution of organic compounds between the ionic liquid phase and an aqueous phase [89]. A study was published by

Milota, and describes a process for absorption of gaseous VOCs (MeOH, formaldehyde, phenol, acrolein, acetaldehyde and propionaldehyde) in an ionic liquid absorption column (Tetradecyl (trihexyl) phosphonium dicyanamide) [90, 91].

Predictive models have been developed by Chen, to improve understanding of interactions between ionic liquids and organic compounds [92]. Oliferenko, conducted a study based on 48 ionic liquids and 23 industrial gases (alkanes, alkenes, fluoroalkanes…). The most soluble compounds described in this study are butadiene and butene (high polarizable molecules) [93]. The absorption value of gas was measured for several ILs. The CO2 absorption is the most widely described [94], as also discussed above (see Sect. 12.3.2). Thus, Jalili, found that H2S was better absorbed than CO2 in IL [95]. Henry constants of few gases absorbed by [C4Mim] [PF6] were published by Safamirzaei, and the solubility of gases (CO2, ethylene, ethane, CH4, Ar, O2, CO, H2, N2) has been reported [96, 97]. It seems that it is the nature of the anion which is crucial for the absorption of gas by ILs [98]. Thus, supported imidazoliums are well known to have a good absorption capacity for CO2 and this characteristic is exacerbated when the cation is functionalized with an amino acid or an amine [99].

It also appears that the solubility of CO2 increase with the molecular weight of the ILs [94]. These authors have studied the selectivity of absorption of different gas versus CO2 (H2, N2, O2, CH4, H2S, etc.). By reducing the size and molecular mass of ionic liquids, it is possible to trap the most volatile gases. Increasing the pressure also improves the absorption of gases in ILs [96]. Many analyzes can be conducted to study the interaction between VOCs and ionic liquids but the most common is the FTIR spectroscopy to visualize the characteristic peaks of the functions present on VOCs (alcohols, aldehydes, etc.) and potential shifts induced by the ionic liquid/VOC interactions [100].

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].

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].

These criteria have to be considered as soon as possible in the ILs structure development. First studies concerning ILs toxicity were reported in the 1950s and since the 1990s many publications have dealt with this topic as ILs have become popular in green chemistry [110, 140, 141]. A good knowing of IL toxicity is necessary to develop an industrial application [110].

Pham et al. published a quite exhaustive report summarizing many of the results described in literature [110]. In 2010, another review treated of ILs ecotoxicity (in water and soils) and toxicity on humans [142]. It is crucial to consider that toxicity is dependent of the biological target’s nature. Indeed, an IL can be harmless towards a specific microorganism and very toxic towards another one.

Ecotoxological

Fig. 12.5 Biological targets used for ILs toxicity evaluation

To assess regarding ILs toxicity towards microorganisms, microbial activity in the presence or not of ILs can be compared. For instance, Zhang et al. [143] showed that the activity of Aureobasidium pullulans was not affected by the presence of [C4Mim][PF6], contrarily to some other solvents like hexane or dibutylphtalate. Matsumoto et al. [52] examined the toxicity of 1-R-3-methylimidazolium hexafluorophosphate ILs with R = butyl, hexyl or octyl, on a lactic acid bacteria, Lactobacillus delbruekii; they observed that lactic acid production decreases with the augmentation of the alkyl chain. However, the study realized on various lactic acid bacteria showed that the IL tolerance is related to the acid production (micro­bial activity in the presence of IL < microbial activity in the presence of water).

Figure 12.5 presents the biological targets used for ILs’ toxicity evaluation [110].

A lot of these evaluations have been practiced on Danio rerio (zebrafish) or Caenorhabditis elegans with concentrations between 1 and 100 mg. L-1. Before proceeding, ILs must be dried (water quantity is currently determined by a Karl — Fischer titration in ppm) and highly pure (determination by HPLC, NMR,…) [144].

Here are enunciated some microorganisms used to evaluate ILs toxicity: Danio rerio (zebrafish), Scenedesmus vacuolatus (green algae), Lemna minor (marine plant), Caenorhabditis elegans (earthworm), Physa acuta (aquatic snail), Oocystis submarine (Baltic algae), Cyclotella meneghiniana (Baltic algae), Daphnia magna (marine plant), Lactobacillus (bacteria), L. rhamnosus (bacteria), Vibrio fischeri (bacteria), E. Coli (bacteria), Pichia pastoris (bacteria), Bacillus cereus (bacteria) [51, 52, 140, 144, 145].

A high quantity of tests has been practiced on Vibrio fischeri, a marine bacteria [146—148]. In general, ILs are more toxic than acetonitrile, an organic solvent known for its toxicity [149]. They also have observed that pyridiniums are among the most toxic ILs and morpholiniums seems to be the less toxic [150]. Ammoniums are toxic towards Zebrafish [145] and other structures as choline-based ILs, morpholiniums, tropiniums, quinuclidiniums or alkylpyrrolidiniums were devel­oped to improve biocompatibility (in association with bromide anion) [149]. Alkyl chain length and the number of carbon atoms (>4 carbons) play an important role in toxicity. Longer chains exhibit higher toxicities [107, 108, 140, 144, 151]. The chain’s position on the ring does not influence the toxicity (e. g. methyl’s position on dimethylimidazoliums) [146].

Other evaluations on aquatic microorganism Dreissena polymorpha, a mollusc [152, 153], and Daphnia magna, a crustacean [154], showed similar tendencies (long alkyl chains increase lipophilicity and intensify toxicity).

Some functional groups like esters increase toxicity [141] whereas other func­tional groups like nitriles, hydroxyles or ethers reduce it [119, 149, 155]. Some 1-methyl-3-alkoxyalkylimidazoliums have been evaluated on bacterial targets have been found harmless, except if the cation wears long alkyl chains (>6-7 carbons) [106]. Moreover, aromatic groups like phenols show higher toxicity than a butyl chain [149].

Alvarez-Guerra et al. established a model comparing 30 anions and 64 cations on Vibrio fischeri [147].

Perfluoroanions as PF6~ in water at 50 °C can produce HF, via hydrolysis, and this acid can be very harmfull towards microorganisms [18, 119, 156—158]. The hydrolysis of PF6~’s hydrolysis was studied by Swatloski et al. and these authors proposed anions in principle non toxics but they confer an hydrophilic profile [159]. It was shown that RfBF3~ anions were very stable towards hydrolysis and produce less quantity of HF [160]. Ignat’ev et al. reported the synthesis of Rf3PF3~ anions (Rf is perfluoroalkyl chain containing 2 or 3 carbon atoms) less sensitive towards hydrolysis [156] and these ILs exhibit lower viscosities but higher toxicities [161].

All these studies demonstrate that the lipophilicity of the anion is a preponderant criteria [115]. It can be expected that the association with an appropriate cation could modify the global toxicity of the IL and balance the intrinsic anion toxicity [150].

Metabolites produced by degradation of ILs could be toxic too and a metabolisation route by cytochrome P450 of C4Mim cation has been proposed in the literature [44].

To conclude, all these studies show that lipophilicity of ILs is the determinant criteria related to toxicity and a hypothesis is that lipophilic cations can interact with lipidic membranes of microorganisms [150]. Considering this fact, measuring the Log POW parameter could be a relevant indicator to estimate on ILs toxicity [51, 52,162]. As we have evocated earlier, toxicity also depends on the biological target [119], so some results in appearance contradictory were reported in literature. As an example, NTf2~ is not toxic towards some bacteria whereas it is toxic towards some microorganisms (Scenedesmus vacuolatus and Vibrio fischeri) [150].

The environmental parameters must be considered because it was established that they can influence toxicity values. Indeed, it was published that NaCl content in water has a protective effect in high concentrations. Microorganism’s size and the cell membranes’ composition also play an important role since they are implicated in interactions between the IL and the biological target [142].

In aquatic environment, interactions with DOM (Dissolved Organic Mattercould) influence toxicity of ILs. Evaluations on Lemna minor demonstrated that association with high concentrations of DOM increase ILs toxicity [163].