Procedia Engineering 148 ( 2016 ) 1141 – 1148

Available online at www.sciencedirect.com

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of ICPEAM 2016doi: 10.1016/j.proeng.2016.06.567

ScienceDirect

4th International Conference on Process Engineering and Advanced Materials

Framework for Ecotoxicological Risk Assessment of Ionic Liquids

Muhammad Ishaq Khana, Dzulkarnain Zainia,*, Azmi Mohd Shariffa, Muhammad Moniruzzamana

aUniversiti Teknologi PETRONAS, Chemical Engineering Department, Bandar Seri Iskandar,32610 Darul Ridzuan, Perak, Malaysia

Abstract

Although ionic liquids (ILs) hold salient features and considered as a replacement of conventional organic solvents, but their possible toxicological and ecotoxicological impacts must be considered equally in the design and development stage. Predicted no-effect concentration (PNEC) plays a vital role in assessing the ecotoxicological hazards in the ILs. A systematic methodological framework is presented for assessing the predicted environmental concentration (PEC) of ionic liquids. Initially, an effect level is predicted with a general quantitative structure–activity relationship (QSAR). Secondly, predicted no-effect concentration (PNEC) is calculated by using assessment factor (AF). The PEC is compared to the PNEC to characterize the risk for the freshwater aquatic organisms. If the ratio continued to exceed one, ionic liquids under investigation will need further aquatic risk assessment procedures, which involve iterative steps to refine the PEC and PNEC. © 2016 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of ICPEAM 2016.

Keywords: Ionic liquid;Aquatic ecotoxicity; Risk Assessment;

1. Introduction

As chemical substances are an important part of our life. The applications of chemical substances range from domestic use to industrial as well as educational and research purposes. It is, therefore, the responsibility of the government and different agencies that chemicals synthesized and marketed should be safer for use, taking into account the advantages of their applications. Common organic solvents are the chemicals that were being used in many processes industries. Apart from the applications in numerous fields, these organic chemicals were toxic towards ecosystem. The introduction of ionic liquids as emerging solvents made a revolution in engineering because of numerous physical and chemical properties. Ionic liquids have low melting points and are liquids at room temperature. Initially, ionic liquids were considered to be the replacement of the commonly used organic solvents [1] As the chemistry of these liquids was studied, many new applications were explored because of the salient feature of these liquids. The solubility of ionic liquids into water makes a potential release into the aquatic ecosystem, which consequently cause problems to aquatic ecosystem [2]. Different literature suggest that human, ecological system and its biotic components may suffer largely due to the exposures of toxic liquids. The impact of such toxic liquids on ecosystem demands a great concern to suggest better solutions which would be better for the ecosystem and its components [3]. Bernd Jastorff and his co researchers in 2003 [4] tried to find out how hazardous are the ionic liquids. They suggested to use SAR and for the desire of cheaper, greener and sustainable Ionic Liquids. Cation-anion interaction plays an important role in

* Corresponding author. Tel.: +605-3687588; Fax: +6053656176

E-mail address: dzulkarnain.zaini@petronas.com.my

© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of ICPEAM 2016

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determining the properties of ionic liquids. Variations in cat ion and anion can produce a large number of ionic liquids. Hence, the properties of ionic liquids such as physical and solvent properties as well as toxic properties are controlled by cat ion anion interactions. Researchers started to find the effects of cat ion, anion, alkyl chain length and the functional groups attached with ionic liquids on the ecosystem. These studies mainly started in the start of last decade. Green aspect of Ionic liquids was challenged when many of research evidence showed the adverse effects of ionic liquids on the ecosystem. Dochrty and Kulpa studied different alkyl chain length and compared with other alkyl chains [5].Their research provided toxicity and antimicrobial information about ionic liquids and reported that ionic liquids with the octyl and hexyl alkyl chains were more toxic than benzene and toluene. This means that toxicity of ionic liquids is more than that of conventional organic solvents. Also, from their study, it was concluded that the increasing alkyl chain length adds to the toxicity of Ionic Liquids. In their study Stasiewicz and co researchers [6] they carried out cellular toxicity testing towards the IPC-81 rat promyelocytic leukaemia. Not too many differences were observed when the anion in the enzymatic assay was varied. The cytotoxicity data showed EC50 for acesulphamates and saccharinates to be higher than the values for the chloride analogues. Results show that modification on both cat-ion and an-ion can decrease toxicity and biodegradability of these ionic liquids. Ventura and co researchers used Cholinium based ionic liquids to investigate the toxic effects on vibrio fischeri. The Cholinium based ionic liquids were generically considered as environmentally harmless and hence non-toxic. Their work provided new ecotoxicological data for ten Cholinium based salts and ionic liquids. The results suggested that not all the Cholinium tested can be considered harmless towards the test organism adopted. Furthermore the results suggested that the Cholinium family exhibits a different mechanism of toxicity as compared to the imidazolium ionic liquid. Cholinium based ionic liquids which have a higher affinity for water (lower Kw0) present lower toxicity (higher Log E50). Because for Cholinium based ionic liquids, the toxicity correlates with anion hydrophobicity. The mechanism for this is related to permeation through membrane [7]. Polarity in the cationic ring may also be responsible for the toxicity of ionic liquids, especially in imidazolium and pyridinium based ionic liquids (Vibout et. al., 2012). A possible reason could be the aromatic cation has lower steric hindrance which may favour interaction with lipid membrane with greater potential. F. Hernández and co-workers suggested that alkyl group of imidazolium could be modified by adding hydroxyl group which might cause a decrease in toxicity. In their work, the HOPMIM derivatives showed less toxicity than BMIM because of the presence of the oxygenated group in cation which could cause a decrease in EC50 toxicity (Guerra et,al, 2011).It was also concluded from their work that hydrophobic character had a direct relation to toxicity, i.e. higher the hydrophobicity, higher the toxicity [8]. In their research Susana and co researchers discussed toxicity results considering the structural particularities of selected ionic liquids toward vibrio fischeri. Imidazolium, choline, pyridinium, and ammonium head groups were used in the toxicity evaluation. The impact of anions to V. fischeri was assessed through the analysis of five groups commonly used in ionic liquids methanesulfonate, bistriflimide, acetate, chloride, and Tetrafluoroborate. It was reported that ionic liquids with non-aromatic groups present generally lower toxicity than the ones incorporating aromatic rings. Quaternary ammonium incorporating polar hydroxyl group showed less toxicity which is evidence that polarity of the cationic core is a key factor relating to the toxicity of ionic liquids [9]. Matzke and co researchers investigated the effect of anion and the side chain by a flexible (eco) toxicological test for effectively testing the hazardous effects of the selected ionic liquids. Six different anions and varying alkyl chain length moieties were tested for imidazolium ionic liquids. For all tests dose response models were used to evaluate EC50 values. It was found that mostly cat-ionic part is responsible for ionic liquid toxicity to aquatic life. From their experiments, it was shown that lipophilicity (tending of combining with or dissolving in lipids or fat) is correlated with toxicity. Lipophilic and reactive anion were shown to have a strong effect on the toxicity of the selected ionic liquids [10]. Romero and co researchers carried out several bioassays to analyse the toxicity and biodegradability of several imidazolium ionic liquids in the aqueous phase. The synthetized compounds consist of an imidazolium cation different anions (Cl−, PF6, and XSO4−). Acute toxicity and EC50 values of each compound in the aqueous solution were determined by using the Microtox standard procedure. It was found that the shorter the chain length of side chain, the lower the toxic effect is. The anion had a little effect on the ionic liquid toxicity [11]. In their research Pretti and co researchers studied three most commonly used ionic Liquids (Imidazolium, pyridinium, and pyrrolidinium) using acute toxicity test according to the OECD Guideline No. 203. From the selected ionic liquids Ammonium salts were reported to be toxic to freshwater algae, crustaceans, and fish. The substitution of an alkyl chain with a hydrogen atom reduced toxicity but this was only proved in the case when imidazolium based ionic liquids were tested. Furthermore, it was also reported that substitution of one or more carbon atoms of alkyl chain length with a more electronegative atom can also minimize the toxicity for imidazolium based liquids. Also, it was investigated that introduction of Cl and O2 in alkyl chain can reduce the toxicity. A possible reason could be the more electronegative behaviour of Cl and O2 but the molecular weight kept same. This could be thought that electronegativity can play an important role in the cationic part of ionic liquids. Anion also plays an

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important role as the also increase the toxicity. Lower toxicity can also be characterized as a result of cyclic and acyclic compounds. Solubility also plays an important role in the toxicity of ionic liquids. [12]. Although the cations have been assumed as the main driver of toxicity, the importance of the anion cannot be neglected. Anion effect on ecotoxicity was studied by Stepehen and co researchers (2012). Based on qualitative structure–activity relationships it was showed that with increasing hydrophobicity, a trend of increasing toxicity was found [13]. Zhao and co researchers investigated to the toxicity of ionic liquids by QSAR using support Vector Machine. They found that increasing number on alkyl chain length increases the lipophilicity which causes the cell membrane to be damaged. When the number of carbon in side chain increased from 14 or decreases from 6, this general trend was not obtained. Also, it was reported that increasing relative number of oxygen in anion the toxicity decreased. Anion has an inverse relation to toxicity with a number of the oxygen atom. The reason for this is may be Hydrophobicity directly depends on the charge density of anion. Since in NTf2 charge is more delocalized than other anions used so NTf2 is more hydrophobic than other anion studied [14]. In their research of Hernández-Fernández et.al used sixteen different ionic liquids to evaluate toxicity. Results suggested that Methyl imidazoles should be preferred over phenyl imidazole because the lipophilicity of phenyl is greater than methyl. Pyrrolidinium cation was reported to be less toxic than imidazolium and pyridinium because it has non aromatic cat ion ring. This is evidence that aromaticity may also be considered as one variable that affects the toxicity of ionic liquids [8].

2. Methodological framework

Ecotoxicological risks are typically assessed by exposure assessment and effect assessment. This research proposed a new frame work for ecotoxicological risk assessment of ionic liquids. The framework is shown in fig.1.The methodological frame-work starts from selection of ionic liquids. This research focused on Imidazolium ionic liquids with chloride and halide anions as these ionic liquids are being studied and investigated for many potential industrial applications.

2.1. Effect Assessment

Effects are directly related to the type of the liquids used. In the case of ionic liquids, exposure effects can be minimized by selecting greener ionic liquids for processing within a plant. A detailed discussion is undergoing since last decade regarding the toxicity of ionic liquids and ecotoxicological assessment of these liquids. Cationic part of ionic liquids is said to be responsible for the toxicity. Much literature evidenced that ionic liquids with long side chain (alkyl chain) are more toxic as compared with that containing shorter chain length. The anion is also a factor responsible for the toxicity. As ionic liquids are a new invention in the field of engineering, initially thought to be green solvents, are under investigation due to the toxic behaviour. Ionic liquids are still not being used so far in major industrial processes. Therefore, there is no historical industrial data is available regarding the hazards of ionic liquids. Experimental laboratory toxicity data of more than 250 ionic liquids is reported in recent research studies. These studies show that selection of cations, anions, functional group and alkyl chain length may lead to synthesize green liquids as for ecosystem is concerned. A simple and logical question is still to be answered that why ionic liquids are toxic. When ionic liquids are exposed living organisms they permeate into their cell by cell membranes and cell wall resulting in the molecular initiating event which causes disruption and malfunctioning of the cell, tissues and organs respectively. Ionic liquids are soluble in water because of polarity. As ionic liquids are composed of cations and anions, due to ionic nature many functional groups can be attached to either the part. Mostly there is the addition of functional groups and alkyl chain on cationic part for enhancing the properties of the core ionic liquids. This makes these ILs feasible for a number of applications e.g. absorptions application, especially recent focused is being on the CO2 capturing property of ionic liquids. As an example, the CO2 absorption capacity of imidizolium ionic liquids increases with increase in alkyl chain length [15]. But unfortunately increasing alkyl chain length makes interactive forces between cations and anions weaker which in result makes ionic liquids feasible to penetrate into the bodies of living organisms through the cell wall and cell membrane. Toxicity cannot be minimized until greener solvent are synthesized because toxicity is an inherent property of these types of chemical which is due to the components of the chemicals and the chemistry of their structure.

2.1.1. Estimation of the lowest toxicity values A wide range of toxicity data is available in the literature. Dose response relationship is plotted for the toxicity assessment of

different target species. The toxicity level of ionic liquids is usually assessed by using tests batteries covering all trophic level of the aquatic environment. [16]. As fresh water is representative of three taxonomic groups, therefore in this study we have focused on reporting the toxicity for these three groups. Fish, daphnia manga and algae represent the different tropic levels of the aquatic environment. Three important tropic level of aquatic environment are Fish, Algae and daphnia magna. Fish acute toxicity tests are usually conducted to find the LC50 , Algal growth tests are conducted to calculate growth inhibition

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concentrations EC50 and Daphnia immobilization test are conducted to determine acute toxicity in terms of immobilization to determine EC50.

Fig 1. Methodological framework of the study.

The literature reported toxicity data of imidazolium chloride and bromide ionic liquids is presented in table 1 and table 2.

Table 1: Toxicity data for Imidazolium bromide ionic liquids D.Manga Algae Algae

Ionic liquids LC50(µg/L) Reference Algae EC50(µg/L) Reference Algae EC50(µg/L) Reference [C4mim]Br 15330 [17] P. Subcapitata 500 [18] S.Quadricauda 47600 [19] [C6mim]Br 2852 P. Subcapitata 85 S.Quadricauda 78 [C8mim]Br 2840 P. Subcapitata 11 S.Quadricauda 5

[C10mim]Br 152 P. Subcapitata 4 S.Quadricauda 0.1

1145 Muhammad Ishaq Khan et al. / Procedia Engineering 148 ( 2016 ) 1141 – 1148

Table 2: Toxicity data for Imidazolium chloride ionic liquids Algae (S. vacuolatus) Guppy Fish

Ionic liquids LC50(µg/L) Reference LC50(µg/L) Reference [C4mim]Cl 504000 [20] 97720 [21] [C6mim]Cl 242 78520 [C8mim]Cl 0.401 62370

[C10mim]Cl 0.007 44312

2.1.2. Calculation of PNEC using assessment factors Environmental and eco-toxicological effects of ionic liquids can be characterized by extrapolating PNEC based on mean EC50

or LC50 values obtained from a set data of acute toxicity tests of usually three taxonomic groups fish, algae and Daphnia. Assessment factors are very useful in the calculation of predicted no effect concentrations for a standard class of ecosystem for example freshwater compartment. The predicted no effect concentration (PNEC) is calculated by applying an assessment factor (AF) to the lowest toxicity values from relevant effect studies. The AF is an expression of the degree of uncertainty in the extrapolation from the test data on a limited number of species to the actual environment. Usually, the assessment factor 1000 is used for acute toxicity data for available toxicity data of three taxonomic groups. [22]. As a worst case scenario, we take the mean geometric values of the reported lowest toxicity values for Imidazolium ionic liquids from three endpoint values i.e. from fish, algae and daphnia manga. Taking the imidazolium ionic liquids with alkyl chain length containing 10 Carbons we calculate PNEC as following

PNEC= 14.82 µg/L This value shows that it is the concentration of Imidazolium (Chloride and bromide) ionic liquids which are expected to be no effect concentration on three tropic level of the aquatic ecosystem. Environmental concentration must be less than this value for the safer aquatic compartment.

2.2. Exposure Assessment

Risks are minimized by minimizing the exposure i.e minimize the likelihood of toxic release of ionic liquids. This demand for release information. As ionic liquids are not being used in industries on a large scale, therefore, it is difficult to completely find release information and likelihood data [23]. Allowable concentrations of ionic liquids to aquatic compartment need to evaluate from exposure assessment.

2.2.1. Predicted Environmental Concentrations Ecotoxicological risk assessment for ionic liquids focuses on the aquatic environment. Exposure is expressed as a PEC in the

water, where waste water effluent is diluted with surface water (e.g., stream or river). This concentration is determined by ionic liquids usage volume, removal during waste water treatment, and dilution factor [24]. Chemical and processing plants use ionic liquids for specific applications and effluent streams may contain untreated or treated ionic liquids which are a concern for environmental risk. There is a need to monitor the concentration of these toxic liquids being released to aquatic and terrestrial environmental. It should be the responsibility of the process and product engineers to monitor and control the toxic release into the environment.

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3. Discussion

The goal of this study was to construct a framework for the risk assessment of ionic liquids to the ecosystem, especially aquatic ecosystem. Previous studies provide the ecotoxicological studies of ionic liquids which resulted in the estimation and evaluation of toxicity values only. Specifically, we proposed the complete framework for the risk assessment of ionic liquids towards different aquatic species. Only the determination and evaluation of effect concentrations (EC50, LC50, LD50) are not sufficient for risk assessment. Toxicity data of Imidazolium Chloride and Bromide ionic liquids is shown in figure

As environmental compartments comprise of different trophic levels so it is not the realistic approach to just conduct the toxicity of these liquids and prioritize the ionic liquids on the basis of median concentration values only. Toxicity rating is based on EC50 values and only toxicity values are not sufficient to determine the risk of the specific liquids towards the target species. For example, the toxicity value of C10mimBr towards algae is 0.1 µg/L which lies in the super toxic ranking of the toxicity provided

0

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Toxicity of Alga

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Figure 2. Toxicity of Imidazolium chloride and bromide ionic liquids towards D. Manga, Algae and Guppy fish

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by smith and Passino [25] but still it does not mean that there will be more risk until we don’t know that which environmental compartments is being exposed and how much the concentration is released to it. It depends upon the priority of risk assessors that which compartment they want to protect and up to which level the risk assessment is needed. The assessment of the ecotoxic risks of ionic liquids by risk quotient methods it covers both the exposure and effect assessment. Firstly, the median effect concentrations (EC50) are converted into predicted no observed effect concentrations (PNEC) by using appropriate assessment factors. There are different tropic levels in aquatic ecosystem which are representative of different species. Keeping in mind the priority of aquatic level representative of more important tropic levels, risk assessment is done. In this research work we have selected the assessment factor of 1000 for fresh water which is representative of three taxonomic groups and additional two marine taxonomic groups. Secondly, the values of PNEC are compared with the measure or predicted environmental concentrations. EC50 was divided by an assessment factor of 1000 to get PNEC. This makes it important when assessing the ecotoxicological risks rather than the ecotoxicity of ionic liquids. For example the PNEC calculated for Imidazolium chloride and bromide ionic liquids is 14.82 µg/L. This means the environmental concentration must be less than that value to get acceptable risk value. In this case when the PNEC is more than measured or predicted environmental concentrations, it is recommended to refine PNEC, PEC or both. PNEC could be refined by using alternative QSAR with reduced assessment factor or using laboratory data containing lower assessment factor. PEC is refined by minimizing the exposure concentrations. As ILs are still not being used in industries on the large scale, therefore still there is a potential to find the more techniques for minimizing exposure of ILs to the environment. However, the refinement of PNEC could also be done by many other statistical methods. Similarly, PEC could be refined by many degradation methods. In this framework, the PEC is proposed to be refined by chemical degradation of ionic liquids because chemical degradation can be done to get 99 % conversion. PEC or MEC should be less than PNEC for the safe aquatic ecosystem.

4. Conclusion

In this paper, a framework is described that can be used for ecotoxicological risk assessment of ILs. The methodology for this framework is simple and applicable to all of the ionic liquids. In the present study a framework is proposed which could be helpful in defining the ecotoxicological risk guidelines for ionic liquids. Refinement in both PNEC and PEC could lead to a best ecotoxicological risk assessment approach.

Acknowledgement

The author would like to acknowledge Universiti Teknologi PETRONAS for providing research fund, necessary facilities and encouragement to carry out this work.

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