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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: jfliu@rcees.ac.cn
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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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20
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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.
80 81
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
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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).
94 95
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
121 122
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).
130 131
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.
241 242
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
270
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
279
structural modification effect is presented in Figure 2.
280 281
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
284
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
286
cations contribute significant percentage (12 – 48%) to the total toxicity.127 The trend of IL
287
toxicity with cation variation is shown in Figure 2.
288
Stock et al. presented the inhibition effect of imidazolium, pyridinium and phosphonium ILs
289
to acetylcholinesterase, and cations bearing positively charged nitrogen and certain lipophilicity
290
inhibited the test organism (Table S2). Specifically, pyridinium-based ILs, [C4MPy][BF4] (EC50,
291
34 µM) and [C4MPy][PF6] (EC50, 28 µM), were found to be more toxic than their corresponding
292
imidazolium-based ILs, [C4MIM][BF4] (EC50, 105 µM) and [C4MIM][PF6] (EC50, 140 µM).128
293
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
295
Vibrio fischeri than monocationic counterparts.129 Similarly, inferior toxicity due to cholinium-
296
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
300
Petkovic et al. assessed the toxicity of sixteen ILs having various head groups towards fungi
301
and the imidazolium-based ILs were found to be the most toxic, followed by pyridinium-,
302
pyrrolidinium- and piperidinium-based ILs, while cholinium-based ILs were the least in
303
toxicity.130 However, similar toxicity of [C4MIM][Cl] (EC50, 930 – 3742 mg/kg) and [C4MPy][Cl]
304
(EC50, 588 – 2890 mg/kg) towards Allium cepa, Lolium perenne and Raphanus sativus plants
305
have been reported.131 This indicates that the toxicity is also affected by susceptibility of the
306
model organisms.
307
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
309
Costello et al., pyridinium-based (LC50, 21.4 – 901 mg/L) and imidazolium-based (LC50, 21.8 –
310
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
316
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),
318
showed stronger inhibition to acetylcholinesterase than that of [C3MIM][BF4] (EC50, 189 µM).
319
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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321
values were reported for the inhibitory effects of [C4MIM][BF4] and [C8MIM][BF4] ILs,
322
respectively towards acetylcholinesterase.134
323
Considering bacterial model organisms (Table S3), the direct correlation between toxicity
324
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
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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
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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
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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]-
19 ACS Paragon Plus Environment
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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
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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
21 ACS Paragon Plus Environment
Environmental Science & Technology
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
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Environmental Science & Technology
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
23 ACS Paragon Plus Environment
Environmental Science & Technology
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
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Environmental Science & Technology
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,
25 ACS Paragon Plus Environment
Environmental Science & Technology
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
26 ACS Paragon Plus Environment
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Environmental Science & Technology
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
27 ACS Paragon Plus Environment
Environmental Science & Technology
620
Na+.201 Another researchers also investigated the existence of pH independent adsorption of
621
[C4MIM][Cl] on to Na-montmorillonite.99
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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,
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Environmental Science & Technology
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;
33 ACS Paragon Plus Environment
Environmental Science & Technology
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.
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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: jfliu@rcees.ac.cn
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.
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781 782
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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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TOC Art
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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
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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
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Figure 4.
Advanced Oxidation
Methods for Removal of ILs
Degradation
Adsorption
Biodegradation
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Enhanced Photodegradation Ultrasonic based degradation Electrolytic based degradation