Environmental Application, Fate, Effects, and Concerns of Ionic

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Environmental Application, Fate, Effects and Concerns of Ionic Liquids: A Review Meseret Amde, Jing-fu Liu, and Long Pang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 7, 2015

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Environmental Science & Technology

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Environmental Application, Fate, Effects and

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Concerns of Ionic Liquids: A Review

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Meseret Amde,†,§ Jing-Fu Liu,*,†,‡ and Long Pang＃

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†

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Environmental Sciences, Chinese Academy of Sciences, P. O. Box 2871, Beijing 100085, China

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‡

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China

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§

State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-

Institute of Environment and Health, Jianghan University, Hubei Province, Wuhan 430056,

College of Resources and Environment, University of Chinese Academy of Sciences, Beijing

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100049, China

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＃

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No. 166, Science Avenue, Zhengzhou 450001, China

Department of Material and Chemical Engineering, Zhengzhou University of Light Industry,

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* Corresponding author: E-mail: [email protected]

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Tel: +86-10-62849192

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Fax: +86-10-62849192

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ABSTRACT

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Ionic liquids (ILs) comprise mostly of organic salts with negligible vapor pressure and low

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flammability that are proposed as replacements for volatile solvents. ILs have been promoted as

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“green” solvents and widely investigated for their various applications. Although the utility of

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these chemicals is unquestionable, their toxic effects have attracted great attention. In order to

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manage their potential hazards and design environmentally benign ILs, understanding their

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environmental behavior, fate and effects is important. In this review, environmentally relevant

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issues of ILs, including their environmental application, environmental behavior and toxicity are

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addressed. In addition, also presented are the influence of ILs on the environmental fate and

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toxicity of other co-existing contaminants, important routes for designing non-toxic ILs and the

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techniques that might be adopted for the removal of ILs.

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1. INTRODUCTION

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Ionic liquids (ILs), which are mostly organic salts made of organic cations and organic/inorganic

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anions that are liquids at room temperature, have gained wide recognition as novel solvents for

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various applications especially as a medium for organic synthesis and catalysis.1-5 Many ILs have

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been synthesized from organic cations like imidazolium, pyridinium, phosphonium,

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pyrrolidinium, piperidinium, morpholinium and cholinium. For instance, over 30,000

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imidazolium salts are collected in the CAS database.6 A potentially large number of ILs could be

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prepared by varying the cations and anions combination.7,8 Nowadays, many ILs covering a wide

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range of properties are now commercially available.

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ILs have unique properties including negligible vapor pressure, good thermal stability,

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non-flammability, a wide electrochemical (conductivity) window, tunable miscibility, and good

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extraction capability for various analytes. These exceptional properties merit their potential

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applications.9-15 ILs are widely applicable in extraction, absorption and degradation processes.16-

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distribution coefficients, which may indicate possible applications in heavy metal pollution

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remediation. As an example, the addition of ILs (1–5 wt%) to a diphenyl (dibutyl)

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carbamoylmethylphosphine oxide solution enhance the extraction coefficients of americium from

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nitric acid.24

Task-specific ILs have been also synthesized for metal extraction in water8,21-23 with higher

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Because of their low volatility, atmospheric pollution due to these chemicals is unlikely.

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However, due to their significant solubility in water,25 ILs may enter into the environment

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through industrial wastewater. Consequently, researchers are more concerned about their

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potential impacts on the aquatic and terrestrial environments. To disclose the possible toxic

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effects of ILs, different model organisms have been considered and the toxicity data have been

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extensively reported along with the traditional solvents,26-31 indicating the possible toxicity of

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ILs to the environment. Even though numerous toxicity studies have been published, only few

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review papers have been published.32-34 More importantly, a lot of works have been reported

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since the publication of the most recent review,34 while environmental factors affecting the

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toxicity of ILs like dissolved organic matter (DOM) and salinity were not addressed in previous

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reviews.

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In this paper, we focus on the environmentally relevant issues of ILs, including (i) the

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environmental applications, processes and toxicity of ILs; (ii) the effects of ILs on the fate and

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toxicity of other contaminants; (iii) approaches for designing “green” ILs; and (iv) techniques

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that might be adopted for the removal of ILs.

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2. ENVIRONMENTAL APPLICATION

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2.1. Enrichment of Environmental Pollutants for Analytical Purpose. ILs have been applied

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in sample treatment and pre-concentration processes like single drop microextraction,35-41 hollow

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fiber liquid-phase microextraction,42,43 dispersive liquid-liquid microextraction44-53 and solid

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phase (micro)extraction.54-59 A recent review15 has provided fundamentals, advances, and

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perspectives of ILs in analytical chemistry with detail information.

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IL-based sample pretreatment methods have been applied for the extraction of organic

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analytes like substituted benzene derivatives,60 biofuels,61 polycyclic aromatic hydrocarbons

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(PAHs),35,55,62,63

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antibiotics.46,50,66 Ordinary and task-specific ILs have also been used for the extraction of

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inorganic pollutants.67-70 The superior roles of ILs in analytical and separation sciences have

phenolic

compounds,64

pesticides,16,41,44,53,56,65


bactericides,48

and

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been well documented elsewhere,15,35,71-77 thus the details are not addressed herein. However, we

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tabulated these applications for quick overview (Table S1).

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2.2. Removal of Environmental Contaminants. Heavy metal pollution has given rise to

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various environmental problems, especially in areas with high anthropogenic stress,78,79 and

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various remedial techniques involving the application of ILs have been reported. 1-Butyl-3-

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methylimidazolium hexafluorophosphate ([C4MIM][PF6]) was found to be effective (80 – 95%

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within ~2 min) for the removal of Cu2+, CuO, and Cu0.80 Kalidhasan et al. proposed the use of

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ultrasound in conjunction with Aliquat 336 IL impregnated Dowex 1×8 resin for effective

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adsorption (efficiency, ≥97%) of Cr (VI).81 Zhang et al.82 suggested the use of ILs for

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biotreatment of uranium. The glucaminium-based ILs was found to be applicable for the removal

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of boron from water.49 1-Dodecyl-3-methylimidazolium chloride ([C12MIM][Cl]) and 1-

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hexadecyl-3-methylimidazolium chloride ([C16MIM][Cl]) were adsorbed on a high charge Ca-

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montmorillonite for the removal of chromate (2.6 mM) from water with high adsorption

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capacity (190 mmol/kg) and efficiency (99.5%).83

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Water soluble organics were separated from produced water, waste water in the extraction

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process of oil/gas, using hydrophobic ILs in aqueous solution.84 Trihexyltetradecylphosphonium

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tetrachloroferrate (III) was proposed for the removal of phenolic compounds efficient extraction

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and removal efficiency.64 ILs were immobilized on to porous ceramic membranes for the

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removal of dioxins from high temperature vapor streams.85 The extraction and removal of

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anionic dyes like methyl orange, eosin yellow and orange G from aqueous phase were achieved

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with imidazolium-based ILs.17



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ILs have also been investigated for the removal of various organic contaminants from soils.

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[C4MIM][PF6] and [C4MIM][Cl] were employed for the extraction of DDT, dieldrin,

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hexachlorobenzene, and pentachlorophenol from glacial till soil and montmorillonite. While

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[C4MIM][PF6] was found to be effective for their extraction from the montmorillonite, both ILs

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were effective for soils possessing abundant organic matter.16 Ma and Hong reviewed the

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potential applications of ILs to control and recycle organic pollutants in waste gas, waste water,

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solid waste and contaminated soils.86

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3. FATE AND TRANSPORT OF ILs IN ENVIRONMENT

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The low volatility of ILs make them an attractive alternative to volatile organic solvents, as they

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are unlikely to act as air contaminants even though some ILs can be distilled at low pressure

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without decomposition.87 However, ILs could contaminate environmental recipients like soils,

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sediments, surface and ground water. Some ILs are relatively stable in environment due to their

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resistant to photodegradation88 and small degree biodegradation,89 though their degradability can

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be modified.90 Therefore, it is essential to have a comprehensive understanding of their fate,

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transport and transformation in terrestrial and aquatic systems (Figure 1).

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3.1. Adsorption Behavior of ILs in Terrestrial System. Some studies have investigated the

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adsorption behavior of ILs to soil and sediments,83,91-95 and proposed different adsorption

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mechanisms. Mrozik et al. reported that for the sorption of imidazolium ILs onto kaolinite, ion-

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exchange and van der Waals interactions are primarily responsible at the beginning of the

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binding process, whereas the later becomes dominant at higher concentrations.92 Another report

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also disclosed that the ionic interaction can affect the sorption and desorption of ILs in soil.93 The


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sorption of [C12MIM][Cl] and [C16MIM][Cl] on montmorillonite was found to be through cation-

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exchange at lower initial concentration (CMC).83 Normally, the sorption mechanism of ILs to soil/sediment depends on their physico-

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chemical properties, including a diversity of sorption related properties of the soil/sediment (e.g.

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organic matter content, accessibility of sorption sites, ion exchange domains) and ambient

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parameters (e.g. temperature and salinity). Markiewicz et al. suggested that processes like

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adsorption of monomers with alkyl chains, formation of small aggregates, and formation of a

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double layer are involved in the adsorption of imadazolium ILs.96

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The sorption strength depends on the physicochemical properties of the IL, which in various

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ways is dependent on their chemical structure. Insignificant effect of side chain length was

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reported by Beaulieu et al. using imidizolium-based ILs ([CnRIM][X] (R=H/CH3; n=4,6 and 10;

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X=Cl/Br)) and four types of aquatic sediments. Rather, the positive charge could cause ILs to

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adsorb onto the sediments via electrostatic interactions. The hydrogen atoms on the imidazolium

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ring can form hydrogen-bonds with the polar moieties in sediment organic matter (SOM).91

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Contrasting to this, hydrophobic long chained ILs were found to adsorb much more strongly than

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hydrophilic ILs92,95 and ILs with short and/or hydroxylated derivatives, which are more mobile in

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soils/sediments and therefore probably could be released more readily to surface/ground

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waters.95 The adsorption of ammonium-, phosphonium- and pyrrolidinium-based ILs with

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single/quaternary substitution were tested on soils by Mrozik et al.97 and at lower concentrations,

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single alkyl chained ILs adsorbed more strongly (especially with soils having higher cation

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exchange capacity) than the quad-substituted. On the other hand, because of the double-layer

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formation and induced stronger dipole interaction with previously sorbed molecules, the quad-



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substituted ILs interacted more strongly at higher concentrations, with sorption coefficients

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between 16.8 mL/g (tetrabutylphosphonium chloride) and 1.1 mL/g ([C4MIM][Cl]). This

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indicates that high levels of substitution can also affect the transport of ILs in soil/sediments.

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Soils and aquifer materials with low pH showed limited availability of negatively charged

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active sites that are responsible for electrostatic interaction with IL cations. Contrarily, low pH

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values promote the sorption of anions by anion exchange owing to the formation of neutral and

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positively charged surface sites.98 Increasing pH leads to the deprotonation of anionic soil

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surfaces, enhancing the cation exchange capacity. However, as reported by Gorman-Lewis et

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al.,99 [C4MIM][Cl] adsorb neither onto Bacillus subtilis nor gibbsite (pH 6 – 10) and may travel

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unimpeded to groundwater in areas dominated with these surfaces. Clay minerals typically

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exhibit both pH-dependent and pH-independent sorptions.100 The pH-dependent sorption occurs

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through site-specific surface complexation reactions involving clay edge sites, similar to

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reactions that occur on oxide surfaces.101 The pH-independent sorption occurs through cation-

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exchange reactions in the interlayer, and form electrostatic interactions from the permanent

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charge on the clay.102 In the work of Mrozik et al., the existence of lower sorption potency in

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lower pH was reported.95

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For soils with higher organic carbon (OC), the strong bonding of ILs to soil matrices can

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reduce the migration of the solutes to the solution.94 In a recent study, high affinity of humic acid

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(HA) towards ILs in aqueous solutions was reported.103 Our group also found that the sorption of

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[C4MIM][Cl] and [C8MIM][Cl] to HA significantly reduce their freely dissolved concentration

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and bioavailability.104

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3.2. Transfer Behavior of ILs in Aquatic System. Alkylimidazolium cation, [CnMIM]+, is the


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most widely used IL cation. It has received much attention because of its amphiphilic properties,

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such as aggregation, which is analogous to short-chain cationic surfactants. The aggregation

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behavior of [CnMIM]+-based ILs in aqueous solution has been investigated by Bowers et al.105

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Their result illustrated that [C4MIM][BF4] can be modelled as a dispersion of polydisperse

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spherical aggregates above critical aggregation concentration (CAC), while ILs with longer-

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chain, [C8MIM][I], can be modeled as a system of regularly sized near-spherical charged

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micelles that form above the CMC. However, varying the anions may affect their aggregation

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behavior. Ghasemian et al.106 investigated the effect of electrolytes on surface tension and surface

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adsorption of [C6MIM][Cl] in aqueous solution. From the surface and bulk properties of ILs,

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they revealed that ILs behave surfactant-like and aggregate in aqueous solution, and the

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electrolytes reduce surface tension and CAC of the ILs. Singh and Kumar107 illustrated that the

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aggregation properties of ILs depend on the aromatic ring, alkyl chain length, counter ions, and

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their interaction with water, which agree with the report of Bowers et al.105 As reported by Sastry

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et al.108 on the aggregation behavior of short chain pyridinium-based ILs in water, CAC values

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and area per adsorbed molecule decreases as the alkyl chain length decreases.

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3.3. Degradation of ILs in the Environment.

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3.3.1. Biodegradation. Microorganism based degradation method seems more friendly to the

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environment.109 Coleman and Gathergood90 presented a comprehensive review on ILs

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biodegradation, including methods for the biodegradation assessment, trends observed for

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structurally related ILs, and applications of biodegradable ILs in synthetic chemistry. ILs are

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classified as “readily biodegradable”, corresponding to Organization for Economic Cooperation

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and Development (OECD) standards, for which ≥60% biodegradation level is required in 28



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days.110 Besides, full biodegradation should yield completely non-toxic products.90 Gathergood

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and Scammells89 reported the first investigation on the biodegradability of dialkylimidazolium

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ILs. Imidazolium-based ILs can also be partially degraded in aerobic aqueous solution inoculated

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with soil-bacteria.111 Roughly, protic ILs exhibited higher biodegradability (57 – 95% in 28 days)

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than ordinary aprotic ILs (0.61 – 1.33%) in water.112 In general, ILs showed weak

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biodegradability in the environment and also determination of the degradation products is not

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straight-forward. Therefore, most studies on this area were carried out based on the active slurry.

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This has been addressed under artificial methods for the removal of ILs in this paper.

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3.3.2. Abiotic Hydrolysis. The abiotic hydrolysis of ILs have been studied in relation to anions,

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and the formation of different products have been reported. Hydrolysis of [PF6]- has been

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reported to form volatiles such as HF, [POF3]-, [PO2F2]- and [PO3F2]-.113 Besides,

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[C4MIM][F·H2O] has been identified as one of the hydrolysis products of [C4MIM][PF6] during

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its purification process.114 Baker and Baker115 studied the relative intrinsic hydrolytic stabilities

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of [C4MIM][BF4], [C4MIM][PF6], [C6MIM][(C2F5)3PF3] and [C4MPyr][Tf2N]. Their results

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showed that [C4MIM][BF4] exhibit the fastest degradation kinetics, presumably due to intimate

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contact with water. In contrast, [(C2F5)3PF3]- and [Tf2N]- have shown excellent hydrolytic

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stabilities because the C-F bond is relatively inert to hydrolysis under mild conditions,116 and

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[(C2F5)3PF3]- performed even better than [Tf2N]-. Both [(C2F5)3PF3]- and [Tf2N]- underwent

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hydrolysis after one week at 50 oC,115 which indicates that the anion decomposition may be

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slowed/halted under low temperature. Besides, Steudte et al. studied the hydrolytic stability of

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[N(CN)2]-, [C(CN)3]-, [B(CN)4]-, [(CF3SO2)2N]-, [(C2F5)3PF3]- and [H(C2F4)SO3]-,117 and their

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half-life was reported to be about 1 year at 25 oC and pH 7 – 9.


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Generally, ILs with good hydrolytic stability are desired in industrial applications. However,

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it is not important from ecotoxicity point of view as abiotic hydrolysis inhibit the transport of IL

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anions in aquatic ecosystem, and decreases their ecotoxicity. Besides, the profile of hydrolysis

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products of each anion should be investigated to manage/control the formation/side effects of

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hazardous metabolites.

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4. ENVIRONMENTAL EFFECTS OF ILs

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The toxicity of ILs depends on their interaction with cellular membranes,118-120 which is mainly

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dependent on the ILs type (alkyl chain length, cation family and anion moiety) and morphology

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of the model organisms.120-122 Hitherto, various biological organisms have been utilized as

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representative model organisms (Table S2 – S7). In this section, we discuss ILs toxicity related

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issues.

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4.1. Traditional Solvents vs ILs. ILs have been considered as “green” solvents relative to

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traditional solvents. However, toxicity studies on bacteria,27,123 invertebrates,26,30 algae28-31 and

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cell lines27,118 indicated that they can be equivalent in toxicity, or even more toxic, than

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traditional solvents.

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The biological effects of alkylimidazolium ILs to Vibrio fischeri (logEC50/µM, -0.182 – 3.94)

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were found to be higher than acetone, acetonitrile, methanol, and methyl tert-butyl ether

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(logEC50/µM, 3.89 – 7), except methyl tert-butyl ether which has similar toxicity (logEC50/µM,

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3.89) with the least toxic IL, [C3MIM][BF4], (logEC50/µM, 3.9).27 [C8MIM][Br], [C8MPy][Br]

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and [C6MIM][Br] were reported to be more toxic (EC50, 1.17 – 6.44 mg/L) to the bacteria than o-

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xylene, phenol, toluene, methyl isobutyl ketone, benzene, ethylene glycol, chloroform,



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dichloromethane, ethyl acetate, acetone and methanol (EC50, 9.25 – 101068.5 mg/L).123 In the

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work of Hernandez-Fernandez et al., the toxic effects of [C4MPheIM][MeSO4], [C4MIM][H2SO4]

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and [C4MPy][BF4] (EC50, 7.60 – 30.93 mg/L) were reported to be comparable to that of toluene

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(EC50, 31.94 mg/L) and higher than that of chloroform (EC50, 1193.80 mg/L).124

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The LC50 (mg/L) values of imidazolium-based ILs towards Daphnia magna indicated that

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the ILs are similar in toxicity (8.03 – 19.91) to ammonium (2.90 – 6.93) and phenol (10 – 17),

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but more poisonous than trichloromethane (29), tetrachloromethane (35), benzene (356 – 620),

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methanol (3289) and acetonitrile (3600).26 Wells et al. also found that the toxicities of ILs

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towards Daphnia magna and Selenastrum capricornutum were about 104 – 106 times higher than

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that of methanol.30

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A toxicity study of [C4MIM][Br], [C6MIM][Br] and [C8MIM][Br] on Scenedesmus

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quadricauda and Chlamydomonas reinhardtii showed that these ILs are more/as toxic (EC50,

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0.005 – 13.23 mg/L) than/as acetone, benzene, toluene and phenol.28 The toxicity of

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[C4MIM][Br], [C4MPy][Br], 1-butyl-1-methylpyrrolidinium bromide, tetrabutylammonium

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bromide and tetrabutylphosphonium bromide (logEC50/µM, 2.35 – 4.09) were also reported to be

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2 – 4 orders of magnitude greater than methanol, dimethylformamide and 2-propanol

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(logEC50/µM, 4.37 – 5.85) towards Selenastrum capricornutum.29 The photosynthesis inhibitory

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effects (EC50, mM) of selected imidazolium ILs (3.47 – 23.99, except [C3MIM][Br] which

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showed a value of >1000) and pyridinium (0.055 – 53.7) on Pseudokirchneriella subcapitata was

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found to be higher than methanol (2570), dimethyl-formamide (2089) and 2-propanol (589).31

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Ranke et al. presented the biological effects of imidazolium ILs in leukemia cells (IPC-81)

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and glioma cells (C6), and their toxic effects were reported to be higher than the toxicity of

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acetone, acetonitrile, methanol, and methyl t-butyl ether.27,118


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4.2. Factors Affecting the Toxicity of ILs

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4.2.1. Effect of Structural Modification. The structural composition of the IL, including the

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cation, alkyl chain length and anion can affect the degree of its toxicity.122,125,126 The summary of

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structural modification effect is presented in Figure 2.

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4.2.1.1. Effect of Cations. Based on LC50 (mg/L) values of [C4MIM][PF6] (19.91) and

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[C4MIM][BF4] (10.68), and their corresponding sodium salts, NaPF6 (9344.81) and NaBF4

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(4765.75), towards Daphnia magna, Bernot et al.26 concluded that the toxicity of ILs is explicitly

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associated to the cation entity. A mathematical model-based study on the toxicity of ILs to

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Daphnia magna by means of quantitative structure activity relationship, also indicated that

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cations contribute significant percentage (12 – 48%) to the total toxicity.127 The trend of IL

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toxicity with cation variation is shown in Figure 2.

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Stock et al. presented the inhibition effect of imidazolium, pyridinium and phosphonium ILs

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to acetylcholinesterase, and cations bearing positively charged nitrogen and certain lipophilicity

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inhibited the test organism (Table S2). Specifically, pyridinium-based ILs, [C4MPy][BF4] (EC50,

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34 µM) and [C4MPy][PF6] (EC50, 28 µM), were found to be more toxic than their corresponding

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imidazolium-based ILs, [C4MIM][BF4] (EC50, 105 µM) and [C4MIM][PF6] (EC50, 140 µM).128

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Pyridinium-based ILs were also reported to be slightly more toxic than imidazolium-based ILs

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towards Vibrio fischeri.123 Dicationic cholinium-based ILs showed significantly lower toxicity to

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Vibrio fischeri than monocationic counterparts.129 Similarly, inferior toxicity due to cholinium-

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based ILs (LC50, 2.896 – 9.517 mM) towards Artemia salina was reported in comparison to the

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toxicities of imidazolium and pyridinium ILs (LC50, 0.079 – 0.117 mM).125 The pyrrolidinium-



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based ILs exhibited lower toxicity (EC50, 4588.85 – >29130 mg/L) than imidazolium- and

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pyridinium-based ILs (EC50, 7.6 – 505.64 mg/L) towards Vibrio fischeri.124

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Petkovic et al. assessed the toxicity of sixteen ILs having various head groups towards fungi

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and the imidazolium-based ILs were found to be the most toxic, followed by pyridinium-,

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pyrrolidinium- and piperidinium-based ILs, while cholinium-based ILs were the least in

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toxicity.130 However, similar toxicity of [C4MIM][Cl] (EC50, 930 – 3742 mg/kg) and [C4MPy][Cl]

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(EC50, 588 – 2890 mg/kg) towards Allium cepa, Lolium perenne and Raphanus sativus plants

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have been reported.131 This indicates that the toxicity is also affected by susceptibility of the

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model organisms.

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Bado-Nelles et al.132 reported lower toxicity of imidazolium-based ILs (EC50, 17.3 – 300.8

308

mg/L) than phosphonium-based (EC50, 0.053 – 130.7 mg/L) to Daphnia magna. In the work of

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Costello et al., pyridinium-based (LC50, 21.4 – 901 mg/L) and imidazolium-based (LC50, 21.8 –

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1290 mg/L) ILs were found to have similar toxicities towards Dreissena polymorpha.133 Choline-

311

based ILs (LC50, 2.896 – 9.001 mM) exhibited lower toxicity on Artemia salina than

312

imidazolium (LC50, 0.079 – 0.114 mM) and pyridinium (LC50, 0.086 – 0.117 mM) ILs.125 A

313

similar toxicity trend was obtained using human cell HeLa.125

314 315

4.2.1.2. Effect of Side Chain Length. The toxicity of ILs has strong correlation with its

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lipophilicity which may affect their interaction with the surface of the model organisms.128,134 In

317

the work of Stock et al.128 IL with longer alkyl chain length, [C10MIM][BF4] (EC50, 13 µM),

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showed stronger inhibition to acetylcholinesterase than that of [C3MIM][BF4] (EC50, 189 µM).

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The proposed toxicity mechanism involves the choline cation binding to the anionic site of the

320

enzyme, such that longer side chain results in an improved fit. Similarly, 105 and 46 µM EC50


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values were reported for the inhibitory effects of [C4MIM][BF4] and [C8MIM][BF4] ILs,

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respectively towards acetylcholinesterase.134

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Considering bacterial model organisms (Table S3), the direct correlation between toxicity

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and lipophilicity of ILs was validated by high susceptibility of the Gram-positive bacterial strains

325

compared to the Gram-negative strains, largely because the former has thicker and more

326

hydrophobic cell wall.135 However, while assessing the antimicrobial activity of imidazolium ILs,

327

Docherty et al. noticed that Gram-positive was both the most and the least resistant strain, but the

328

toxicity nevertheless increased with alkyl chain length.123 In a recent study, Gram-positive

329

Listeria monocytogenes was also found to be more tolerant towards ILs induced toxicity than

330

Gram-negative Escherichia coli. Such strong distinctions in terms of susceptibility might be

331

attributed to the bacterial strategies like efflux pumps, cell membrane variations and increased

332

osmolyte production against stress.136 Increase in ILs toxicity towards Vibrio fischeri was

333

observed with alkyl chain length from [C3MIM][BF4] (logEC50/µM, 3.94) to [C10MIM][BF4]

334

(logEC50/µM, -0.182) except for [C5MIM][BF4] (logEC50/µM, 3.14) and [C6MIM][BF4]

335

(logEC50/µM, 3.18) which had similar toxicity.27 The toxicities of [C4MIM][Cl] (logEC50, 3.39

336

µM), [C6MIM][Cl] (logEC50, 2.18 µM) and [C8MIM][Cl] (logEC50, 0.94 µM) to the Vibrio

337

fischeri supports the trend of direct association between side chain length and ILs toxicity.137 In

338

the work of Peric et al., a long chain IL, [C8MIM][Cl] (EC50, 0.5 mg/L), also exhibited greater

339

toxicity than [C4MIM][Cl] (EC50, 278 mg/L) to Vibrio fischeri.112 Markiewicz et al. also reported

340

elevated ILs toxicity towards activated sludge communities with the elongation of the alkyl

341

chain.138 Toxicities of cholinium-based ILs and its derivatives towards Vibrio fischeri was found

342

to exacerbate with the alkyl/linkage chain length, the number of hydroxyethyl groups and the

343

insertion of carbon–carbon multiple bonds.129 Using bioluminescent bacteria, Ventura et al. also



Page 16 of 55

344

showed the increase in toxicity of imidazolium- and phosphonium-based ILs with the alkyl chain

345

length.139

346

Using

algae,

the

effect

of

side

chain

length

on

ILs

toxicity

has

been

347

investigated.28,30,31,112,121,140-142 According to the toxicity data, the cation lipophilicity was found

348

to be a dominant factor influencing the overall toxicity (Table S4). The work of Kulacki and

349

Lamberti indicated increase in the toxicity of imidazolium-based ILs to Scenedesmus

350

quadricauda (EC50, 0.005 – 13.23 mg/L) and Chlamydomonas reinhardtii (EC50, 4.07 – 2138

351

mg/L) with alkyl chain length.28 A high correlation (R2 ≥0.9837) between the EC50 values of

352

[CnMIM]+ ILs towards Chlorella vulgaris and Oocystis submarina with the number of carbons in

353

the alkyl chain was reported.121 The toxicity of [CnMIM][Cl] (n=2,4,6,8 and 10) towards

354

Bacillaria paxillifer (EC50, 0.99 – 34.4 µM) and Geitlerinema amphibium (EC50, 0.02 – 30.9 µM)

355

was also positively affected by the alkyl chain length.143 Similarly, Chen et al. investigated the

356

toxicity of [C4MIM][Cl] (EC50, >1000 µM), [C6MIM][Cl] (EC50, 118.78 µM), [C8MIM][Cl]

357

(EC50, 12.69 µM) and [C10MIM][Cl] (EC50, 0.34 µM) towards Scenedesmus obliquus in which

358

the trend of chain length effect on the toxicity can be easily explored from the EC50 values.142

359

An increase in ILs toxicity towards human carcinoma with IL side chain length was also

360

reported.144 The toxicity of [C4MIM][PF6], [C4MIM][BF4], [C4MIM][Br], [C4MIM][Tf2N],

361

[C5MIM][Tf2N], [C7MIM][Tf2N] and [C10MIM][Tf2N] towards a fish cell line was obtained to

362

be moderate to high (EC50, >10 – 4400 µmol/kg) on Folsomia candida also indicated the relationship of an increasing

376

alkyl chain length on ILs toxicity.134 Swatloski et al. employed Caenorhabditis elegans to

377

examine the putative toxicity of [C4MIM][Cl], [C8MIM][Cl] and [C14MIM][Cl], and it was

378

observed that an increase in the alkyl side chain increased lethality. When exposed to 1.0 mg/L,

379

the lethality went from 0% with [C4MIM][Cl] to 11% with [C8MIM][Cl], and then 97% with

380

[C14MIM][Cl].149 Bernot et al. studied the effects of imidazolium- and pyridinium-based ILs on

381

survivorship and behavior (movement and feeding rates) of Physa acuta, and it was found that

382

the LC50 values with Br- and PF6- counter ions ranged from 1 to 325 mg/L. High toxicity was

383

reported for the ILs with eight-carbon alkyl chains and weakened for shorter alkyl chains,

384

indicating a positive relationship between alkyl chain length and toxicity.150 Similarly, the effect

385

of imidazolium- and pyridinium-based ILs on the mortality and feeding of Dreissena

386

polymorpha was reported to cause acute mortality (LC50, 21.4 – 1290 mg/L), and longer alkyl

387

chained ILs were more toxic.133

388

Various aquatic and terrestrial plants have been also used in investigations of the relationship

389

between ILs toxicity and its side chain length (Table S7). Peric et al. investigated the



Page 18 of 55

390

comparative terrestrial eco-toxicities of protic and aprotic ILs towards Allium cepa, Lolium

391

perenne and Raphanus sativus, in which ILs with longer anion and cation chain length were

392

observed to exhibite higher toxicity. Namely, [C8MIM][Cl] (EC50, 150 – 561 mg/kg) shown high

393

toxicity than [C4MIM][Cl] (EC50, 930 – 3742 mg/kg).131 In the study of [CnMIM][BF4] toxicity

394

towards Triticum aestivum and Lepidium sativum, increment in growth inhibition with alkyl

395

chain length was observed.151 A similar toxicity trend was reported for [CnMIM][BF4] ILs

396

towards Lemna minor and Lepidium sativum plants.152 [CnMIM][Cl] also affected the growth of

397

Lemna minor112 and Lepidium sativum153 plants, and ILs with longer alkyl chain exhibited higher

398

toxicity. Inhibitory effects of imidazolium-based ILs on Hordeum vulgare growth was found to

399

depend on hydrophobicity, whereby the most toxic was [C10MIM][Br], followed by [C7MIM][Br]

400

and [C4MIM][Br].126

401

Generally, increase in chain length escalates the deleterious effect of ILs (Figure 2). This

402

may be due to the increase in the interaction with the organism, or since short chain ILs are the

403

most soluble, giving rise to less sorption to enzymes and hence more rapid excretion. However, a

404

“cut-off effect” was observed with elongation.30,129,141,154 Further elongation of the side chain154

405

or symmetrical chains128,155 resulted in lower activities, since high steric effect may affect ILs

406

interaction with the cell surface.154

407 408

4.2.1.3. Effect of Anions. IL toxicity is mainly affected by the cations and the side chain length.

409

However, in case of less toxic cations, anions have significant contribution to the overall toxicity.

410

Mainly, more lipophilic/unstable anions play a major role in the toxicity of ILs.134,156

411 412

Matzke et al., reported high toxicity of [(CF3)2N]- (EC50, 40 µM) on acetylcholinesterase compared to Cl-, [BF4]-, [C8OSO3]- and [(CF3SO2)2N]- (EC50, 80 – 100 µM) using [C4MIM]+.134


Page 19 of 55


413

Ranke et al.27 assessed the influence of [BF4]-, Br-, and p-toluenesulfonate on the toxicity of

414

[C4MIM]+ towards Vibrio fischeri and reported 3.55, 3.07, and 3.52 logEC50/µM values,

415

respectively. Similarly, [C10MIM]+ derivatives of Cl- and [BF4]- ILs exhibited 0.5 and -0.18

416

logEC50/µM, respectively.27 Romero et al. reported the presence of anion effect in IL toxicity

417

with logEC50/µM values of 2.18, 2.11, 0.94 and 0.70 for [C6MIM][Cl], [C6MIM][PF6],

418

[C8MIM][Cl] and [C8MIM][PF6], respectively, indicating the higher toxicity of [PF6]- than Cl-.137

419

Monoatomic anions (Br- and Cl-) were found to contribute less effect than large sized anions.157

420

Similarly, Br- (EC50, 3.27 µM) and Cl- (EC50, 3.34 µM) derivatives of [C4MIM]+ exhibited lower

421

toxicity than [BF4]- (EC50, 3.1 µM) and [PF6]- (EC50, 3.07 µM) to Photobacterium

422

phosphoreum.158 In the work of Mester et al., chaotropic anions were reported to affect the

423

chaotropicity of ILs to Listeria monocytogenes and Escherichia coli by enhancing the surfactant

424

like behavior of cations and chaotropicity itself represents cation independent toxicity of ILs.136

425

Markiewicz et al.138 examined the influence of [B(CN)4]-,

426

[(C2F5)3PF3]- on the toxicity of [C2MIM]+ towards activated sewage sludge (municipal WWTP

427

and industrial WWT). Among the anions, [(C2F5)3PF3]- (logIC50/µM, 3.24 and 3.26 for municipal

428

WWTP

429

(logIC50/µM, >5.00 and 4.39 for municipal WWTP and industrial WWT, respectively) were the

430

least toxic.

and

industrial

WWT,

respectively)

were

the

[N(CN)2]-, [(CF3SO2)2N]- and

most

toxic

and

[N(CN)2]-

431

Cho et al. employed Selenastrum capricornutum to figure out the contribution of Br-, Cl-,

432

[BF4]-, [PF6]-, [CF3SO3]-, [C8H17SO4]- and [SbF6]- to the toxicity of [C4MIM]+-based ILs. Except

433

for [PF6]- (EC50, 1318 µM) and [SbF6]- (EC50, 135 µM), all have low effect on the toxicity (EC50,

434

2137 – 2884 µM). The high toxicity of [SbF6]- may be associated to its ability to undergo

435

hydrolysis in water.159 Among [BF4]-, [DCNA]-, [TFMS]-, [MeSO4]- and [MPEGSO4]-



Page 20 of 55

436

derivatives of [C4MIM]+, [BF4]- exhibited higher toxicity (EC50, 425.33 and 707.81 µM,

437

respectively) towards Chlorella vulgaris and Oocystis submarina than others (EC50, 930.81 –

438

2650.98 µM and 897.04 – 3292.98 µM, respectively).121 The high toxicity of [BF4]- based IL

439

might be ascribed to fluoride formation during the hydrolysis of [BF4]- which enhanced the toxic

440

effect.121

441

Bernot et al. employed Daphnia magna to evaluate the toxicity of [C4MIM]+-based ILs with

442

Cl-, Br-, [PF6]-, and [BF4]-. Though the toxic effects were comparable (LC50, 8.03 – 19.91 mg/L),

443

the slight toxicity difference is attributed to the anions.26 Garcia et al.158 also reported difference

444

in [C4MIM]+ toxicity with the counter anions using Daphnia magna.

445

Using IPC-81, Stolte et al. reported the contribution of anions to the total toxicity of

446

imidazolium-based IL [C4MIM]+, showing that the anion [CF3SO3]- exhibited more cytotoxicity

447

(EC50, 1000 µM) than [CH3SO3]- (EC50, 3200 µM). While, because of its vulnerability to

448

hydrolysis, [SbF6]- had the highest cytotoxicity (EC50, 180 µM) than [BF4]- (EC50, 1700 µM) and

449

[PF6]- (EC50, 1300 µM)156 that was also observed by Cho et al. using Selenastrum

450

capricornutum.159 The toxicities of [C8MIM]+ and [Choline-Cn]+ ILs exhibited toxicity variation

451

with [FeCl4]-, [GdCl6]3-, [CoCl4]2- and [MnCl4]2- counter ions on human cell lines. Namely,

452

[CoCl4]2- and [MnCl4]2- derivatives are more prone to generate cytotoxicity.160 Compared to Cl-

453

(EC50, 0.74 µM) with the same alkoxymethyl chain, lower cytotoxicity of saccharinates (EC50,

454

4.2 µM) and acesulphamates (EC50, 3.1 µM) to IPC-81was reported161

455

Anions can also affect IL toxicity to plants (Table S7). Biczak et al. reported the presence of

456

anions could influence the [C3MIM]+-based IL toxicity towards spring barley and common

457

radish plants though regularity within the effect was not observed.162 In the work of Bubalo et al.

458

on the effect of [C4MIM]+-based ILs on the growth of Hordeum vulgare, the effect of counter


Page 21 of 55


459

anions was reported to be in the order of Br->[CH3CO2]->[BF4]-.126 Furthermore, toxicity of

460

[NTf2]- to Triticum aestivum was found to be higher than [BF4]-, Cl- and [HSO4]-, and

461

independent of the soil composition.163

462

Generally, some fluorinated anions like [BF4]-, [PF6]- and [SbF6]- usually induce IL toxicity

463

which might be ascribed to their ability to undergo hydrolysis and yield toxic fluoride containing

464

products.121,134,159 Stable anion, [NTf2]-, exhibits over-additive toxicity effects,156 due to its high

465

lipophilic nature could enhance its ability to destruct phospholipid membranes.164 More or less, it

466

can be said that anions do have some contribution to the toxicity of ILs, particularly for shorter

467

alkyl chained ILs.

468 469

4.2.2. Environmental Factors.

470

4.2.2.1. Dissolved Organic Matter. The more an IL is sorbed to a mineral or organic component

471

of soil or (pore)water, the lower the amount of freely-dissolve ILs is present,104 which implies

472

that less of the ILs within the soil or water system will be bioavailable or able to exert toxic

473

effects. The effect of soil organic matter on the toxicity of [C4MIM][BF4] and [C8MIM][BF4] to

474

Triticum aestivum151 and Lepidium sativum151,153 has been reported and the toxicity effect

475

decreases as the amount of organic matter increases. An increase in the total organic matter by 5%

476

was found to reduce by about 50% of [C4MIM][BF4] and [C8MIM][BF4] toxicity at 500

477

mg/kg.151 Similarly, natural DOM was found to slightly reduce the toxicity of imidazolium

478

cations to Lemna minor.165 Our group104 studied the sorption of ILs to DOM and its effects on

479

toxicity of ILs in the presence and absence of HA, and showed that the freely dissolved

480

concentration of [C4MIM][Cl] and [C8MIM][Cl] apparently decreases in the presence 11 µg/mL

481

DOM (the free fraction of the ILs was decreased to 0.85 and 0.79, respectively). This reduction



482

of freely dissolved concentration gave rise to remarkable reduction of bioavailability and

483

therefore toxicity of the ILs, indicating that DOM may play an important role in determining the

484

environmental fate and toxicity of ILs. Some ILs such as [C8MIM][Cl] can form complexes with

485

DOM below the CMC, which not only affects the solubility and bioavailability of IL, but also the

486

solubility and bioavailability of other organic compounds in soil pore water.166 Therefore, the

487

effects of DOM in the system should be taken into account while assessing the fate and potential

488

effects of ILs in environment.

Page 22 of 55

489 490

4.2.2.2. Salinity. Salinity should also be considered while investigating the toxic effects of ILs.

491

There are different mechanisms for the effect of salinity on IL bioavailability and toxicity,

492

including its effect on the solubility of ILs through salting-in or salting-out effects, competition

493

of IL ions with other ions for the interaction with ionic sorption sites on soils and in

494

tissues/enzymes, the roll of IL on micelle (aggregate formation) by the screening effect, and the

495

presence of ion-pairing environment for the IL cations which prevent their interaction with

496

cellular structures. Latala et al.167 investigated the effect of salinity variations on the toxicity of

497

imidazolium ILs towards Oocystis submarina, Chlorella vulgaris, Geitlerinema amphibium and

498

Cyclotella meneghiniana. Their report indicated that increasing the salinity significantly

499

decreases ILs toxicity (eight–ten times in 0 – 32 practical salinity unit (PSU)), which might be

500

due to the reduced permeability of IL cations through the algal cell walls. Similarly, the control

501

cell density of Oocystis submarina was reduced by 50% after 3 days exposure to [C6MIM][Cl],

502

and the cell growth inhibition was only 30% and 10% at salinity of 8 and 16 – 32 PSU,

503

respectively. For [C4MIM][Cl], 30% and 10% inhibition was observed in fresh water and 16 PSU,

504

respectively, while it was unaffected at 32 PSU.168 In contrast, the toxicity of [C4MIM][Cl] to


Page 23 of 55


505

Skeletonema marinoi was reported to be insignificant at 35 (EC50, 0.12 mM), 25 (EC50, 0.1 mM)

506

and 15 PSU (EC50, 0.14 mM) salinities.169 Kulacki and Lamberti28 also reported opposing result

507

to the work of Latala et al.167,168 as difference in IL effects on Scenedesmus quadricauda was not

508

observed between modified water (375 µS) and ground water (742 µS) media. The difference in

509

these studies might be due to the initial conditions, origin and type of the algal strains and the

510

experimental media.

511 512

4.3. Impacts on the Fate and Toxicity of Co-Existing Environmental Pollutants. ILs may

513

affect the fate and transport of co-existing environmental pollutants. Due to the hydration layer

514

on their surface, metal oxides and clays are not effective sorbents for nonionic organic

515

compounds in aqueous system. However, they can adsorb ionic surfactants of opposite charge

516

and therefore neutralize the surface, which enhance the hydrophobic interactions and thus

517

increase the affinity for non-ionic organic compounds.170-173

518

Pino et al.174 investigated the partitioning behavior of aliphatic hydrocarbons, PAHs, phenols

519

and esters to imidazolium-based IL aggregates (partition coefficients, 30 – 5200). Hydrophobic

520

analytes (KOW >300) such as aliphatic hydrocarbons, esters and PAHs are preferably extracted. In

521

a recent study, the release of PAHs and DOM from soil to water by [C8MIM][Cl] was thoroughly

522

investigated and enhanced release of the materials by sub-CMC IL concentration was reported.

523

In addition, due to the dissolution of soil organic matter, high concentration of DOM was also

524

observed upon addition of sub-CMC IL concentrations.166

525

As mentioned earlier, ILs behave as traditional short-chain surfactants and are prone to sorb

526

to minerals and organic matter. Thus, in the study of subsurface transport of contaminants, they

527

can be sorbed to immobile aquifer media and decrease groundwater pollution. Conversely, ILs



528

aggregate and form colloids in aqueous phase, which could sorb organic and inorganic

529

contaminants and increase amount of the contaminants in groundwater.175

Page 24 of 55

530

In the environment, there can be various multi-contaminant cocktails with different

531

compositions and concentrations. Since it is impossible to study the combined effects of all

532

chemical mixtures, two basic concepts, concentration addition (CA) and independent action (IA),

533

have been used to investigate the effect of ILs on toxicity of various pollutants.176-178 CA is a

534

model based on dilution principle and designed for compounds which share similar functional

535

sites, exhibit similar interaction or similar chemical structures. IA is designed for a mixture of

536

dissimilarly acting compounds. In recent reports, other techniques like mixture information179

537

equipartition ray design180 and integrated CA with IA based on multiple linear regression

538

(ICIM)178 models have been proposed.

539

The study on the mixture of ILs and pesticides showed that all binary mixtures exhibited a

540

similar toxicity action rule: a synergistic interaction (more toxic than expected) in high

541

concentration region; an additive action in medium concentration region; and an antagonistic

542

interaction (less toxic than expected) in low concentration region.177,181 Matzke et al. employed

543

CA and IA concepts to investigate the toxic effect of different IL mixtures ([C4MIM][BF4],

544

[C8MIM][BF4] and [C14MIM][NTf2]), and cadmium to Scenedesmus vcuolatus and Triticum

545

aestivum. The authors obtained underestimated toxicity with both CA and IA, which illustrated

546

the presence of interactions among the compounds or the compounds and other constituents.176

547

In a recent study, the combined toxicity of heavy metals and ILs on Vibrio qinghaiensis was

548

investigated using CA, IA and ICIM models.178 As was found by Matzke et al.,176 the combined

549

toxicities were underestimated by CA and IA models but effectively predicted by ICMA.

550

Accordingly, the mixtures exhibited synergism, and ICIM was proposed as appropriate model.178


Page 25 of 55


551

These findings imply that to objectively assess the ecotoxicological risk of ILs, the complex

552

scenarios of mixture toxicity and pre-pollution assessment should be made.

553 554

5. APPROACHES FOR DESIGNING “GREEN” ILs.

555

Structural modification-based approaches, which yield non-toxic and biodegradable ILs can be

556

followed to design more “green” ILs (Figure 3). As initially proposed by Gathergood et al.,182

557

introduction of polar functional groups, to alkyl side chain significantly decreases the

558

toxicity.33,124,139,141,182-185 Besides, substitution of alkyl group with hydrogen in 1-position of

559

imidazole,185 can reduce its toxicity. However, incorporation of a methyl group or a hydroxyethyl

560

group in an imidazolium ring enhances its antimicrobial activity.186 Furthermore, cytotoxicity is

561

strongly dependent on the position of the polar functional group in the side chain (less toxic as

562

distant from the ring).187

563

Benign ILs can also be synthesized using proper cations (e.g. cholinium)129 and anions (e.g.

564

saccarinate/acesulphamate).161,188 The use of aromatic containing cations148,189 and fluorine

565

containing anions189 increase the toxicity. Egorova et al.190 studied the cytotoxicity of several

566

amino acid-containing ILs, with amino acid-based cations and anions towards cell cultures and

567

compared their toxicity with imidazolium-based ILs. Even though functionalization of natural

568

amino acids was considered to reduce the toxicity, the result of this study gave new insight into

569

biological effects of amino acid-containing ILs and showed that an amino acid residue may make

570

ILs more biologically active. Therefore, attention should be paid to the plausible synergetic

571

effect of ILs combination with biologically active molecules.

572

The inhibitory effect of aprotic ILs (EC50, 8.59 – 14.4 mg/L) were reported to be more

573

significant than protic ILs (EC50, 302 – 8912 mg/L) towards acetylcholinesterase.112 Hence,



574 575 576

Page 26 of 55

protic ILs are recommended as environmentally safer ILs. Based on these findings and recommendations, we have compiled important routes of synthesizing relatively less toxic and more biodegradable ILs (Figure 3).

577 578

6. ARTIFICIAL METHODS FOR THE REMOVAL OF ILs

579

To manage the environmental hazards of ILs, various techniques have been proposed as adequate

580

solution for the issue of disposal of ILs. A summary of methods investigated to remove of ILs is

581

presented in Figure 4 and presented below.

582 583

6.1. Adsorption. Adsorption separation technology based on the accumulation of target entities

584

on to solid surfaces has been used in environment cleanup.109,191 Likewise, adsorption has been

585

employed to remove ILs from aqueous solutions using appropriate adsorbents such as activated

586

carbon (AC),192-198 bacterial biosorbents,199 clays,93,99,200-202 ion-exchange resin,203 and

587

aluminum-based salts.204

588

AC has been used as effective, environmental friendly and non-destructive adsorbent for the

589

removal of various ILs in aqueous solution.192-198 Structural and chemical properties of AC can

590

be conveniently modified for efficient adsorption192,195,198,205 via chemical treatments (acidic and

591

basic), impregnation of foreign materials and modification of its physical characteristics. 206 A

592

study on kinetic aspects of ILs adsorption onto AC showed that the relatively low adsorption rate

593

can be efficiently enhanced by decreasing the adsorbent particle size.197 For ACs, adsorbent

594

porosity, pH, temperature and IL entities (cation, chain length and anion) can affect the

595

adsorption efficiency.194,195 The larger average pore diameter facilitates easy diffusion and

596

sorption of ILs. On the other hand, microporous/narrow mesoporous ACs (with high content of


Page 27 of 55


597

pores and small diameter) presented highest adsorption capacities (up to 1 g/g of imidazolium-

598

based ILs).195 In basic media, main interactions shift from dispersive to electrostatic, which

599

significantly increases the adsorption process.194 The kind of interactions between organic

600

cations and the carbon surface depends on the amount of oxygenated groups and IL type, and the

601

presence of oxygen groups promote electrostatic interaction which is stronger for more

602

hydrophilic cations.194,198,205 Generally, ACs containing high polar functional groups in their

603

surface and with low polarity are recommended for effective adsorption of hydrophilic and

604

hydrophobic ILs, respectively.195,205

605

Won et al.199 employed Escherichia coli biomass for biosorption of [C2MIM]+ from aqueous

606

solution. At optimal pH (7 – 10), fast (10 min) and efficient adsorption (72.6 mg/g) of the cation

607

was reported. Moreover, acetic acid can easily desorb [C2MIM]+ from the biosorbent. Hence,

608

such non-destructive, environmental friendly and cheap adsorbents seem promising but require

609

extensive research for identification of proper biosorbents for the numerous ILs.

610

Choi et al.203 reported the adsorption of [C2MIM]+ by ion-exchange resins possessing

611

different functional groups. Resins with sulfonic acid functional groups showed the highest

612

sorption abilities (578.2 to 616.2 mg/g). Large bead size led to lower kinetics of [C2MIM]+

613

adsorption, and the bead size and degree of cross-linking of the resins insignificantly affected the

614

sorption performance.

615

Pioneering work on adsorption of [C4MIM][Cl], [C8MIM][Cl], 1-allyl-3-methylimidazolium

616

chloride ([AMIM][Cl]), [C4Py][Br] and [C8Py][Br] to Na-montmorillonite (pH 7, at 25 oC) was

617

reported by Reinert et al., and adsorption capacity was found to closely related to nature of the

618

cation and alkyl chain length ([C4Py][Br] > [C8Py][Br]~[AMIM][Cl]~[C4MIM][Cl] >

619

[C4MIM][Cl]). The involved adsorption mechanism is via cation exchange with the interfoliar



620

Na+.201 Another researchers also investigated the existence of pH independent adsorption of

621

[C4MIM][Cl] on to Na-montmorillonite.99

Page 28 of 55

622 623

6.2. Artificial Degradation Methods of ILs. It is likely ILs will entry into the environment,

624

once they are utilized in industrial application. Thus, degradation methods could play essential

625

role to overcome their potential impacts on the environment, and artificial methods (Figure 4)

626

would be an alternate option.

627 628

6.2.1. Activated Sludge Microorganisms Based Degradation. To reduce incineration and landfill

629

wastes, readily biodegradable chemicals should be utilized. Docherty et al.207 examined the

630

biodegradability of imidazolium-based ([C4MIM][Br], [C6MIM][Br], and [C8MIM][Br]) and

631

pyridinium-based ([C4MPy][Br], [C6MPy][Br], and [C8MPy][Br]) ILs using OECD standard test

632

method. In the report, only [C8MPy][Br] can be classified as readily biodegradable (96%

633

degraded within 25 days), and both butyl substituted cations ([C4MIM][Br] and [C4MPy][Br])

634

exhibited 0% degradation in 43 days. However, the ability of microorganisms to degrade

635

[C4MPy][Br] at low concentration (46.7 µM) was reported.208 Another group, Stolte et al.209 also

636

reported the good biodegradability of pyridinium-based ILs bearing an ester containing

637

substituent and longer alkyl chain length. Liwarska-Bizukojc and Gendaszewska evaluated the

638

biodegradability of [C2MIM][Br], [C6MIM][Br] and [C10MIM][Br], and it was found the

639

degradation was inadequate (90%),

729

[C4MPyr][Br] (>80%) and N-butyl-N-methylmorpholinium bromide (>76%) in aqueous solution

730

by US-ZVI/AC with micro-electrolysis system was reported by Zhou et al.,226 and the

731

degradation pathways were suggested based on the detected intermediates.

732

The efficiency of ILs degradation in Fenton/Fenton like systems can be affected by the type

733

of IL, H2O2 concentration and the background ions. The degradation efficiency decreases as the

734

side chain elongates (e.g., >93% for [C2MIM][Br] and 73.7% for [C10MIM][Br]).228 Similarly,


Page 33 of 55


735

lengthening the alkyl chain [C4MIM][Cl] to [C8MIM][Cl] lowered the ILs degradation from

736

~100% to ~70%, respectively, under similar conditions.230 Change in H2O2 concentration from

737

100 to 400 mM enhanced [C4MIM][Cl], [C6MIM][Cl], [C8MIM][Cl] and [C4MPy][Cl] ILs

738

degradation from 57 – 84% to 87 – 100% within 60 min.230 Stoichiometric H2O2 dose is

739

recommended for effective conversion of imidazolium-based ILs and to avoid toxic effluents

740

from the system.227 Counter anions may affect the degradation efficiency due to their ability to

741

compete with the target cation towards •OH or form complex with the catalyst which hinder the

742

•

743

the ILs cation, increasing the stability of cations and interacting with the catalyst. Siedlka et al.

744

investigated the influence of counter ions (Cl-, [C(CN)3]- and [CF3SO3]-) and background ions

745

([C6F11O2]-, [C8F15O2]- and [C10F19O2]-) on the degradation of [C4MIM]+-based ILs in H2O2/Fe3+

746

system, and the effect of counter anions on the degradation rate followed Cl->[C(CN)3]-

747

>[CF3SO3]- order.225

OH formation. The background anions may interfere the degradation process by interacting with

748 749

7. SUMMARY AND FUTURE RESEARCH DIRECTIONS

750

ILs have been considered as “green” solvents and have gained numerous environmental

751

applications, though studies on their environmental fate and toxic effects have brought a question

752

on their greenness. The toxic effects of ILs vary considerably across their type, test conditions

753

and morphology of the model organisms. On the whole, it is recommended to create a database

754

of environmentally benign ILs based on their toxicological and biodegradation data, which

755

should play as crucial role in manufacturing non-toxic and degradable ILs. In addition, the

756

following directions typifies the research gap should be worked on. (i) Systematic study on the

757

effects of ILs on the transport and environmental processes of co-existing pollutants like POPs;



758

(ii) Fundamental understanding on the toxicity mechanism (mode of action) of ILs to various

759

organisms; (iii) Conducting the toxic effects and environmental processes of ILs in real

760

environmental conditions rather than controlled laboratory conditions; (iv) The species and

761

toxicity of degradation products and intermediate products of various ILs; (v) Extensive research

762

on the development of techniques for the removal of ILs; (vi) Design and synthesis of

763

environmentally benign ILs from the green chemistry point of view.

Page 34 of 55

764 765

■ ASSOCIATED CONTENT

766

Supporting Information

767

Additional information Tables S1 to S7 have been cited in the text. This material is available free

768

of charge via the Internet at http://pubs.acs.org.

769 770

■ AUTHOR INFORMATION

771

Corresponding Author

772

*Phone: +86 10 62849192; Fax: +86 10 62849192; e-mail: [email protected]

773

Notes

774

The Authors declare no competing financial interest

775 776

■ ACKNOWLEDGEMENT

777

This work was supported by the Strategic Priority Research Program of the Chinese Academy of

778

Sciences (XDB14020101), and the Chinese Academy of Sciences (YSW2013A01,

779

YSW2013B01). Meseret Amde acknowledges the support of CAS-TWAS President’s

780

Fellowship for his PhD study.


Page 35 of 55


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1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442



1443 1444

Figure Captions

1445

Figure 1. The transport and transformation of ILs in the environmental system

1446

Figure 2. Effects of structural modifications on toxicity of ILs

1447

Figure 3. Some important routes to synthesize less toxic and more biodegradable ILs

1448

Figure 4. Methods for removal of ILs

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1466 1467

TOC Art

1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 51 ACS Paragon Plus Environment


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1483 1484

Figure 1.

CO2

Air Water

Aerobic degradation

Suspended particle IL

IL

IL

IL

IL

ILs

IL

1485

IL IL

Suspension IL

IL

IL IL

Sediment/Soil

IL

Uptake

Deposition IL

IL

IL

IL IL

IL

IL

IL

Dissolved NOM

Sorption

Anaerobic degradation

1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496


IL

IL IL

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1497

Anion effect

Increases

• Side chain length (up to “cut off” effect) • Hydrophobicity of the side chain

Cation type

Side chain

Figure 2.

Cation effect

1498

Increases

• Anion lipophilicity • Anion lnstability

Toxicity Increases 1499 1500 1501 1502 1503 1504 1505 1506



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1507 1508

Figure 3.

• • • • •

Use of protic ILs Salts of organic acids Short alkyl side-chains Use of cholinium cations Polar group to the side chain (away from the ring) • Use of saccharinate and acesulphamate anions • Methyl group in 1-position of the imidazole

• Use of protic ILs • Salts of organic acids • Long hydrophobic alkyl sidechains • Use of pyridinium cations • Use of anions like: • Alkyl sulphates • Alkyl sulphonates • Alkyl benzene sulphonates

Recommended

ILs

Less Toxic

More Biodegradable Side chain

Cation

Anion

• Adding methyl or hydroxyethyl in to the imidazolium ring

• Introduction of polar functional groups

• Use of aromatic containing cations

• Fluorine containing ILs

• Using [BF4]-, [PF6]- and [SbF6]anions • Methylation of the pyridinium ring

• Short alkyl side chains

Conflicts/trade-offs

1509 1510 1511 1512 1513 1514 1515 1516 1517 54 ACS Paragon Plus Environment

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1518 1519

Figure 4.

Advanced Oxidation

Methods for Removal of ILs

Degradation

Adsorption

Biodegradation

1520


Enhanced Photodegradation Ultrasonic based degradation Electrolytic based degradation

Environmental Application, Fate, Effects, and Concerns of Ionic

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