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Life Cycle Perspectives on Aquatic Ecotoxicity of Common Ionic Liquids Amirhossein Mehrkesh, and Arunprakash Karunanithi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04721 • Publication Date (Web): 24 Nov 2015 Downloaded from http://pubs.acs.org on December 30, 2015
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Environmental Science & Technology
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Life Cycle Perspectives on Aquatic Ecotoxicity of Common Ionic Liquids
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Amirhossein Mehrkesh and Arunprakash T. Karunanithi*
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Center for Sustainable Infrastructure Systems, University of Colorado Denver
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1200 Larimer Street, NC 2008B, Denver, Colorado, 80217
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Abstract
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This study compares the aquatic ecotoxicity impacts of production and use phase release of five
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common ionic liquids (ILs). Integrating toxicity data, physical properties, and fate and transport
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parameters with USEtox model we report, for the first time, the freshwater ecotoxicity
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characterization factors for [Bmim]+[Br]-, [Bmim]+[Cl], [Bmim]+[BF4]-, [Bmim]+[PF6]-, and [BPy]+[Cl]-
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as 624, 748, 823, 927, and 1768 CTUe/kg respectively. IL Production life cycle inventories were
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modeled and utilized to estimate their production side ecotoxicity impacts. Literature on
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environmental aspects of ILs either propagate their green characteristics - no air emissions and
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high recyclability - or their non-green aspects due to toxicity concerns of their release to water.
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This study adds a third dimension by showing that,
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producing ILs could outweigh potential ecotoxicity impacts of direct release during use. Further,
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for the studied ILs, on an average 83% of ecotoxicity impacts associated with their production can
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be linked to chemicals and materials released during the upstream synthesis steps while only 17%
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of ecotoxicity impacts relate to life cycle energy consumption. The findings underscore the need
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to develop sustainable synthesis routes, tight control over chemical releases during production,
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and careful selection of precursor materials and production processes.
the upstream ecotoxicity impacts of
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1. Introduction
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Ionic Liquids (ILs) are salts that possess weak interaction between an organic cation and an
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organic/inorganic anion. They are a fascinating group of chemicals with several interesting
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properties such as negligible vapor pressure, low melting point, good thermal stability,
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nonflammability, and wide electrochemical window. Due to these unique characteristics the
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technological importance of ILs span a wide range of applications such as synthesis1,
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separations2,
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electrodeposition7, biomechanics8, microfluidics9, astronomy10 and many others. Since the field
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of ILs is developing rapidly, it is important to consider their environmental, ecological, and human
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health impacts at the design stage for their long-term acceptance. Despite the large amount of
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attention, ILs are still a fairly new class of materials and several of their environmental
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characteristics have only been studied recently. The non-volatile nature of ILs can greatly limit
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atmospheric pollution and for this reason, they are often promoted as green chemicals with the
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potential to replace volatile organic solvents11. However, in recent years, many studies have
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reported that several ILs possess relatively high level of toxicity towards freshwater organisms.
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While both points are valid, a realistic picture can only emerge through analysis of the life cycle
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ecological impacts that considers the upstream ecological impacts associated with producing
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them as well as the downstream ecological impacts due to their use. The current study addresses
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this research gap by focusing on understanding the production side and end-of-life fresh water
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ecotoxicity impacts related to ILs.
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To date there have been very few Life Cycle Assessment (LCA) studies performed on processes
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that involve ILs.12,13,48,49 These studies indicate that, from a life cycle perspective, IL based
electrolytes3,
carbon
capture4,
cellulose
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processing5,
thermal
storage6,
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processes do not necessarily improve the environmental performance in comparison to
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molecular solvents based processes. One common limitation in all of these studies is that they did
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not consider any possible impacts associated with the direct release of ILs into the environment.
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Therefore, it is clear that the environmental fate as well as toxicity information related to ILs has
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not been incorporated in any LCA study to date. This is because toxicity-based characterization
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factors for ILs are not yet available in impact assessment tools such as TRACI 2.1. One of the main
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objective of this paper is to address this research gap by developing, freshwater ecotoxicity
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characterization factors for the following five common ILs: 1-butyl-3-methylimidazolium bromide
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([Bmim]+[Br]-), 1-butyl 3-methylimidazolium chloride ([Bmim]+[Cl]-), 1-butyl 3-methylimidazolium
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tetrafluoroborate
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([Bmim]+[PF6]-), and 1-butylpyridinium chloride ([BPy]+[Cl]-).
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In the recent past, several studies have provided new insights into IL chemistry, environmental
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fate and toxicity. Many ILs exhibit significant solubility in water15 and even water immiscible ILs
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show limited water solubility and stability.16 This, combined with the fact that they are non-
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volatile, makes wastewater discharge as the most likely route through which ILs will eventually be
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released into the environment Once discharged, ILs will interact with aquatic ecosystems through
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a variety of mechanisms and can cause damage to aquatic species.17 To evaluate the ecotoxicity
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of ILs, a broad range of testing models (bacteria, fungi, algae, plants, and animals) have been
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used. 18 Standard ecotoxicological tests show that many ILs have high toxicity towards freshwater
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organisms (e.g. algae and D. magna).19 Wells et al.19 examined four different classes of ILs, which
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all had relatively high ecotoxicity impacts (EC50 < 100 mg L-1). Certain ILs, such as those with
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shorter side-chain 1-alkyl-3-methyl imidazolium and pyridinium, show only moderate ecotoxicity
([Bmim]+[BF4]-),
1-butyl-3-methylimidazolium
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hexafluorophosphate
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to bacteria, algae, and invertebrates, but when the side chains
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ecotoxicity profiles become significantly worse. This is also true for phosphonium and
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ammonium-based ILs, which have higher molecular weights. The environmental fate of ILs
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depend on several biotic and abiotic factors. Only in the past few years have the major abiotic
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mechanisms of ILs (e.g. their sorption in different type of soils) been understood.20,21 The extent
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of the final toxicity impact of ILs will depend heavily on their physicochemical properties,
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interactions with surrounding environmental media, and chemical and biological transformations.
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This information is essential to understand how these compounds are transformed and how long
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they will persist in the environment.
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On the other end of the spectrum, there is still very little understanding of the environmental and
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ecological impacts associated with the production of ILs. This is because ILs are emerging
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materials and are not yet produced in commercial scales. Consequently, no primary data is
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available on material/energy consumption. Further, very little data is available about the
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precursors needed to produce these ILs. This study integrates the existing12,13 and newly
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developed life cycle inventories for the production of the above mentioned five ILs with an aim to
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understand their production side upstream freshwater ecotoxicity impacts.
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Through an in-depth analysis, we compare the relative contributions of the different IL life cycle
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stages (production phase and use phase loss) towards ecotoxicity impacts and discuss their
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significance. We would like to point out that similar studies on other materials, such as carbon
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nanotubes (Eckelman et al.
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and use phase impacts.
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are longer than C8 their
) have revealed important information on comparing production
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2. Methods
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2.1 Goal, Scope, System Boundary
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The main goal of this study was to understand and compare the relative freshwater ecotoxicity
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impacts of ILs related to their production phase and direct release to the environment during use
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phase. The scope of this study includes: 1) building cradle-to-gate life cycle inventory for
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production of five common ILs; 2) developing characterization factors for their freshwater
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ecotoxicity impacts; and 3) comparing the potential ecotoxicity impacts of the production and
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use phase release. A functional unit of 1 kg of the five ILs was considered.. The system boundary
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(shown in Figure S1 in the supplementary information) includes all upstream steps necessary for
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the production of ILs, treatment, and recovery as well as the direct release of IL to the
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environment during use.
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2.2 Life Cycle Inventory of Ionic Liquid Production and Data Sources
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Inventory of upstream steps include raw material inputs and emissions related to the production
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of reactants, precursors, reagents and ancillary materials as well as the extraction, conversion
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and delivery of energy inputs. The emissions from the construction and maintenance of chemical
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plants are assumed to be relatively low13 and hence neglected. Data related to electricity,
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thermal energy (steam), transportation systems, and chemical production were derived either
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from Ecoinvent,46 or
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permitting, for consistency purposes we assumed that all life cycle steps involved in the
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production of ILs to be within the United States. Whenever non-U.S. data had to be used, every
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effort was made to adjust the data to corresponding US energy mix. The inventory of all materials
USLCI database integrated in SimaPro software.47 Data availability
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(e.g. precursors, reagents, reactant etc.) and energy used for the production of the ILs along with
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the data sources are listed in Table S2 of supplementary information. Please note that data for
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chemicals that were not available in standard LCI databases (ILs and some of their precursors)
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were assembled using mass and energy balances derived from chemical process simulations
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(CPS)13,22 supplemented with theoretical calculations based on sound judgement and good
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engineering practice. A detailed description of the procedure adopted to generate the mass and
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energy balance is presented in supplementary information (Section 1).
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2.3 Fresh Water Ecotoxicity Impacts of Ionic Liquid Production
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The life cycle impact assessment methodology, Tool for the Reduction and Assessment of
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Chemical and other environmental Impacts (TRACI 2.1.) developed by U.S. Environmental
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Protection Agency utilizes characterization factors derived from the USEtox model26. Using these
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characterization factors, emissions of organic and inorganic materials released during the
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upstream life cycle steps of IL production was translated into total freshwater ecotoxicity
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impacts.
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2.4 Development of Ecotoxicity Characterization Factors for Ionic Liquids
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Characterization factors are used to quantify the extent to which a given pollutant contributes to
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environmental impacts. In this work we utilized the USEtox model26, which is a state-of-the-art
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modeling framework based on scientific consensus for characterizing ecotoxicity impacts of
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chemicals, to develop freshwater ecotoxicity characterization factors for the five ILs. In this
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approach, the aquatic ecotoxicity characterization factors are estimated as a product of fate
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factor (FF), exposure factor (XF) and effect factor (EF) as shown in Eqn. 1.
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= ∗ ∗
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EF relates to the inherent toxicity of the substance of interest, FF relates to the fate and transport
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of the substance, and XF relates to potential routes of exposure and intake of the substance. In
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the following section, we describe the procedure for the development of each of these factors for
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ILs.
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Effect Factor: EF reflects the relationship between the concentration of an IL and potentially
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affected fraction (PAF) of aquatic organisms. It is defined as the slope of the concentration
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response relationship up to the point when the PAF reaches 50% (Eqn. 2)
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EF = =
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HC50 corresponds to the geometric mean of species-specific EC50 data and EC50 represents the
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effective concentration of the IL of interest at which 50% of the population of a particular species
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experiences a response. Table 1 summarizes the aquatic toxicity data (EC50) of the five ILs
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collected from the literature. It can be observed that, there is a wide distribution of EC50 values
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associated with ecotoxicty of ILs towards different species. In the next step, as suggested by
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USEtox, the geometric mean of EC50 values for each IL was calculated. Based on the guidelines
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provided for the use of USEtox model acute toxicity data were converted to chronic data using
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acute-to-chronic ratio of two. Similarly, when no observed effect concentration (NOEC) was
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reported we extrapolated NOEC to EC50 by a factor of nine. Next, the geometric mean of
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individual EC50 data for each IL, HC50, was used to estimate the effect factor.
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Fate Factor: Multimedia fate and transport models are used to derive the environmental fate
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factors. In these models the environment is represented as a number of homogeneous
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compartments (e.g. air, water, and soil). The intermedia transfer of chemical substances between
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different
.
compartments
Eqn. 2
is
modeled
as
a
set
of
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mass
balance
equations.
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Table 1: Toxicity values of selected Ionic Liquids
Reported toxicity values (mg/L) Type
Species
Geometric mean of EC50s [Chronic] (mg/L)
Test type
Photobacterium
growth inhibition
phosphoreum
(Acute)
Bacteria
Ref [Bmim] [Br]
[Bmim] [Cl]
[Bmim] [BF4]
[Bmim] [PF6]
[BPy] [Cl]
[Bmim] [Br]
[Bmim] [Cl]
[Bmim] [BF4]
[Bmim] [PF6]
[BPy] [Cl]
408.020
---
---
---
---
204
---
---
---
---
27
256-2250
429-897
799-802
334-929
257-440
491.6
216.5
399.5
278.5
168.1
28,29,16
229-5260
504
568
375
63.9
393.1
252
284
187.5
32
30-31
20
10.7
9.5
5.4
16.9
10
17
Luminescence Bacteria
Aliivibrio fischeri (Acute) growth of algal Pseudokirchneriella biomass or
Algae subcapitata
photosynthesis O2 evolution (Acute) first brood number Daphnia
of neonates &
(Crustacean)
NOEC= >3.2 NOEC = >3.2
Daphnia magna immobilization
NOEC= >3.2 10.7
EC50= 6.3
EC50= 19.9
EC50 = 8.03 (Acute)
Animalia
egestion rate &
NOEC = 4
death (Acute)
LC50 = 229
----
Physella acuta
death & frond fish
LC50 = 123
---
64.2
---
---
61.5
---
32
LC50>=100
LC50>=100
LC50>=100
50
38.6
50
50
50
45
105.2
66.9
74.2
77.0
40.5
LC50>=100 LC50>=100
Danio rerio
---
number (Acute)
EC50=59.5
HC50
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The fate factor, which represents the persistence of a substance (residence time in days) in a
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particular environmental compartment, depends on physicochemical properties, partition
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coefficients, and biodegradation rates of the substance of interest. Most of the IL fate and
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transport parameters needed as input to the model were gathered from literature, and are listed
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in Table 2. A small value of 1 x 10-20 for biodegradation rate was assigned to all ILs that were
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categorized as non-biodegradable but had no quantitative values43.
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Exposure Factor: The environmental exposure factor for freshwater ecotoxicity is defined as the
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fraction of IL dissolved in freshwater and is determined using Eqn. 3
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XF = ((
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Where, +,-- , represent the partition coefficient between water and suspended solids and
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+012 represents the partition coefficient between water and dissolved organic carbon
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respectively. BAF represents the bio-concentration factor in fish. SS (15 mg L-1), DOC (5 mg L-1),
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and BIOmass (1 mg L-1) are default values for suspended matter, dissolved organic carbon and
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biomass concentrations, respectively.36 The exposure factor (XF) for each IL was also calculated
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based on parameters listed in Table 2. We would like to note that there are certain limitations in
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using the proposed approach to the case of ILs. The USEtox model is suited for relatively small
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organic and inorganic materials while ILs typically have high molecular weights. However, other
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studies have reported results using the USEtox model for complex materials such as carbon
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nanotubes.36 In addition, even though we assume that ILs can be treated as neutral entities, the
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ionic nature of these compounds may influence the results. Despite these limitations, we believe
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that, as per current state of knowledge, the approach of using the USEtox model represents the
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best possible way towards ecotoxicity impact assessment of ILs.
)* . .!"#.#$"%&''(. (
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Eqn. 3
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Table 2: Environmental Properties of the studied Ionic Liquids Value Parameter
Unit +
Molecular weight
g mol
-1
Octanol-water partition coeff. KOW -1
Organic carbon-water partition coeff. KOC
L kg
a
Henry’s law constant at 25 ℃, (KH)
Pa kg mol
Water solubility (25 ℃)
mg L b
Dissolved carbon-water partition coeff. (KDOC) b
Suspended solids partition coeff. (KpSS)
b
Sediment-water partition coeff. (Kpsd) b
Soil-water partition coeff. (Kpsl) Biodegradation rate in air
Biodegradation rate in water
Biodegradation rate in sediment Biodegradation rate in soil
d
+
-
+
-
[Bmim] [PF6]
[BPy] [Cl]
219.12
174.67
226.03
284.18
171.67
0.0033
0.0029
0.0030
0.0218
0.0020
398.11
398.11
398.11
398.11
141.61
-20
1×10
18.27
18.27
18.27
6.50
-1
18.27
18.27
18.27
18.27
6.50
-1
18.27
18.27
18.27
18.27
6.50
-1
18.27
18.27
18.27
18.27
6.50
-1
1×10
-1
1×10
-1
1×10
-1
1×10 -1
L kg
-20
1×10
-20
3.47×10
-20
1×10
-20
1×10
-4
1.58×10
-20 -8
-20
1×10
-8
1×10
-20
1×10
-20
1×10
1×10
-20
1×10
1×10
-20
-20
-20
-20 -4
1×10
1×10 1×10
1×10
-4
8.94×10
4
-20
1.36×10
-20
8.55×10
4
-20
1×10
18.27
L kg
6
-20
1×10
-1
L kg
5
-20
1×10
4.32×10
L kg
6
-20
1×10
1.9×10
s
Bioaccumulation factor in fish (BAFfish)
-
1×10
s
c
+
[Bmim] [BF4]
5.5×10
s c
-
1×10
s c
+
[Bmim] [Cl]
-1
L kg
c
-1
-
[Bmim] [Br]
-3
5.73×10
-20 -20 -20 -20
a) The value 1×10 for KH was assigned to the ionic liquids, as all ILs have very small (negligible) vapor pressures. b) The values for soil-water partition coefficient of all ionic liquids were calculated using the formula of Kd = KOC*fOC, in which foc represents the fraction of organic carbon in the soil ≅ 0.0459 and KOC is the organic carbon-water partition coefficient. This value of Kd was assigned to all four parameters KDOC, KpSS, Kpsd, and Kpsl -20 c) The small value of 1×10 was used for all non-biodegradable ionic liquids which had a biodegradability rate
