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.