Photochemical Transformation of Four Ionic Liquid Cation Structures in Aqueous Solution

Fenton and photo-assisted advanced oxidative degradation of ionic liquids: a review

Ionic liquids (ILs) are the class of materials which are purely ionic in nature and liquid at room temperature. Their remarkable properties like very low vapour pressure, non-inflammable and high heat resistance are responsible for their use as a very appealing solvent in a variety of industrial applications in place of regular organic solvents. Because ILs are water soluble to a certain extent, the industrial wastewater effluents are found to contaminate with their traces. The non-biodegradability of ILs attracts the attention of the researchers for their removal or degradation from wastewater. Numbers of methods are available for the treatment of wastewater. However, it is very crucial to use the most efficient method for the degradation of ILs. Advanced oxidation process (AOP) is one of the most important techniques for the treatment of ILs in wastewater which have been investigated during last decades. This review paper covers the cost-effective Fenton, photochemical and photocatalytic AOPs and their combination that could be applied for the degradation of ILs from the wastewater. Theoretical explanations of these AOPs along with experimental conditions and kinetics of degradation or removal of ILs from water and wastewater have been reported and compared. Finally, future perspectives of IL degradation are presented.

Imidazolium ionic liquids as potential persistent pollutants in aqueous environments: Indirect photochemical degradation kinetics and mechanism

Ionic liquids (ILs) with promising application are likely to become ubiquitous contaminants in water environment for their high hydrophilicity, low biodegradability, and especially its potential toxicity. In this work, we have investigated photochemical transformation of six imidazolium ILs for fate prediction and ecological risk assessment. We found that the reaction rates of the ILs with •OH, CO₃^•─ and ¹O₂ enhanced with their increasing alkyl chain and varied slightly with the paired anions. Meanwhile, modelled results under different scenarios indicate that the primary contributors to transformation of the ILs are triplet-stated dissolved matter (³CDOM*), •OH and CO₃^•-. Besides, the overall half-lives of the ILs can reach 670 days, which indicates persistence of these ILs in the environment. Products for ILs in reaction with •OH and triplet-stated sodium anthraquinone-2-sulfonate (³AQ2S*) were probed by UHPLC-Q-TOF-MS/MS and there is a difference between their products: Products by •OH are likely formed by hydrogen abstraction from the side alkyl chain, followed by dehydrogenation, hydroxylation and carbonylation, while one of the products by ³AQ2S* is formed by dihydroxyl-addition of the imidazolium ring. Furthermore, the ILs and its products were estimated to have toxicity and non-readily biodegradability, suggesting potential eco-risk for the environmental water.

Multi- and trans-generational effects of N-butylpyridium chloride on reproduction, lifespan, and pro/antioxidant status in Caenorhabditis elegans

Ionic liquids (ILs) became emerging pollutants. Their poor degradation and accumulation in organisms urged studies on the long-term effects and also the underlying mechanisms. Currently, 1-butylpyrinium chloride ([bpyr]Cl) was chosen to represent the pyridine-based ILs. Its multi-generational effects were measured on C. elegans for 14 consecutive generations (F1 to F14), and the trans-generational effects were also measured in the great-grand-children (T3 and T3') of F1 and F14. The multi-generational results from F1 to F14 showed that the effects of [bpyr]Cl on the initial and total reproduction and lifespan showed oscillation between inhibition and stimulation. Notably, hormetic effects on reproduction were observed in F7 to F10. The trans-generational effects in T3 and T3' showed different residual consequences between one generational exposure (F1) and multiple generational exposure (F14). Further biochemical analysis showed that the pro/antioxidant status also showed oscillation between inhibition and stimulation. The oscillation levels were greater in superoxide dismutase (SOD), catalase (CAT) and protein carbonyl content (PC) than those in glutathione peroxidase (GSH-Px), reactive oxygen species (ROS) and hydroxyl radical (OH). The pro/antioxidant status contributed to both multi- and trans-generational effects of [bpyr]Cl. Future studies should pay attentions to the long-term influence of ILs and also epigenetic explanations.

Reproductive toxicities of 1-ethyl-3-methylimidazolium bromide on Caenorhabditis elegans with oscillation between inhibition and stimulation over generations

Ionic liquids (ILs) become emerging pollutants and their toxicities earn increasing attentions. Yet, their effects were seldom explored on reproduction which connects generations and also effects across generations. In the present study, reproductive effects of 1-ethyl-3-methylimidazolium bromide ([C₂mim]Br), one representative IL, were studied on C. elegans with 11 continuously exposed generations (F1 to F11). At 8.20E-5 g/L, the effects on the initial reproduction showed oscillatory changes between stimulation (in F1, F3, F4, F6 and F10) and inhibition (in F2, F5, F7, F8 and F11). At 8.20E-3 g/L, the effects on the reproduction over generations also showed such oscillation despite of different stimulation or inhibition levels, and even opposite influences in F4 and F11. The effects of [C₂mim]Br on the total reproduction also showed the concentration-dependent oscillation between stimulation and inhibition over generations, though they had less alteration frequencies than those on the initial reproduction. Biochemical and molecular indicators were further measured in F1, F4, F7 and F11 to explore potential mechanisms. Results showed that the effects on spermatocyte protein 8 (SPE8) showed positive correlation with those on reproduction while the influences on major sperm protein (MSP) and sperm transmembrane protein 9 (SPE9) showed negative correlation with SPE8. Moreover, the dysregulation on expressions of acs-2 and akt-1 indicated the involvement of glucolipid metabolism. The changes in expressions of set-2, met-2, set-25 and mes-4 demonstrated that the long-term reproductive impacts of [C₂mim]Br over generations also involved histone methylation at H3K4, H3K9 and H3K36, which also connected with the glucolipid metabolism.

Life stage-dependent toxicities of 1-ethyl-3-methylimidazolium bromide on Caenorhabditis elegans

Ionic liquids (ILs) are considered as extracting solvents in soil remediation. However, they can be pollutants themselves, and their own toxicities are of concerns. Notably, organisms were exposed to pollutants at random life stages in actual environmental exposure scenario, which is different from the set-up of one uniform life stage in usual experiment designs. The influence of life stages on ILs toxicities will provide essential information on their actual environmental risks. In the present study, effects of 1-ethyl-3-methylimidazolium bromide ([C₂mim]Br) were measured on C. elegans with egg exposure and adult exposure. In egg exposure, [C₂mim]Br increased the lifespan, stimulated initial reproduction and inhibited the total reproduction. Biochemical indices including oxidative stress, antioxidant responses and oxidative damage were further measured to explore the toxicity mechanisms. Results showed that [C₂mim]Br significantly stimulated O₂^-· as the oxidative stress and superoxide dismutase (SOD) as the antioxidant defense. In adult exposure, [C₂mim]Br inhibited initial reproduction, total reproduction and lifespan. Biochemical results showed that [C₂mim]Br significantly stimulated H₂O₂ and oxidized glutathione (GSSG). The overall findings demonstrated that [C₂mim]Br caused life stage-dependent toxicities on C. elegans. Future studies are still needed for the detailed mechanisms.

Photochemical fate of quaternary ammonium compounds in river water

Quaternary ammonium compounds (QACs) are not completely removed during wastewater treatment and are frequently detected in surface waters and sediments. The photochemical transformation of QACs has not been thoroughly investigated as a potential degradation pathway affecting their fate in the environment. Kinetic studies of common QACs with and without aromatic groups under simulated and natural sunlight conditions were performed with model sensitizers and dissolved organic matter to estimate photochemical half-lives in the aquatic environment. All QACs investigated react with hydroxyl radicals at diffusion-controlled rates (∼2.9 × 109 to 1.2 × 1010 M-1 s-1). Benzethonium reacted via direct photolysis (ΦBZT,outdoor = 1.7 × 10-2 (mol Ei-1)). Benzethonium also reacted with the triplet excited state model sensitizer 2-acetylnaphthalene, but evidence suggests this reaction pathway is unimportant in natural waters due to faster quenching of the triplet 2-acetylnapthalene by oxygen. Reactivity with singlet oxygen for the QACs was minimal. Overall, reactions with hydroxyl radicals will dominate over direct photolysis due to limited spectral overlap of sunlight emission and QAC absorbance. Photolysis half-lives are predicted to be 12 to 94 days, indicating slow abiotic degradation in surface water.

Alteration in concentration-response curves of four N-alkylpyridinium chloride by exposure concentration, time and in their mixtures by uniform design

The concentration-response curves (CRCs) of chemicals are important in extrapolating their effects from laboratory studies to their risk assessment in the field. Yet, the CRCs can be altered by exposure concentration and mixture conditions, and also by exposure time in recent reports. Presently, ionic liquids (N-alkylpyridinium chloride, [apyr]Cl) were used for CRC-alteration studies. In individual effects on Vibrio qinghaiensis sp. Q67 (Q67) from 0.25 to 24 h, the CRCs of [epyr]Cl and [bpyr]Cl changed from S- to J-shaped with decreases in inhibition and increases in stimulation, while the CRCs of [hpyr]Cl changed from S- to flat-shape with decreases in inhibition but without stimulation. In mixture effects on Q67, the CRCs all changed from S- to J-shaped from 0.25 to 24 h. By means of the variable selection and modeling method based on the prediction (VSMP), the CRC-alterations of mixtures were positively contributed by [epyr]Cl but negatively contributed by [bpyr]Cl. Furthermore, a parameter was developed by the area of a triangular that combined acute inhibition (EC_50,0.25h) and chronic stimulation (Zero-effect Point, i.e., ZEP_24h and the minimum inhibition effect, i.e., E_min,24h). This parameter successfully evaluated the CRC-alterations in both individual and mixture effects over time, and indicated potential interactions in CRC-alteration in mixtures.

Comprehensive screening of quaternary ammonium surfactants and ionic liquids in wastewater effluents and lake sediments

Quaternary ammonium compounds (QACs) are widely applied as surfactants and biocides in cleaning and personal-care products. Because of incomplete removal during wastewater treatment, QACs are present in wastewater effluents, with which they are discharged into natural waters, where they accumulate in sediments. To assess the levels of QACs in aquatic environments, a liquid chromatography high-resolution mass spectrometry method using both target and suspect screening was developed. The water and sediment sample preparation, measurement, and data analysis workflow were optimized for 22 target compounds with a wide range of hydrophobicity, including ionic liquids that have potential use as solvents and QACs common in personal-care and sanitizing products. In wastewater effluents, average concentrations of all target and suspect QACs combined ranged from 0.4 μg L-1 to 6.6 μg L-1. Various homologs of benzylalkyldimethylammonium (BAC) and dialkyldimethylammonium (DADMAC) as well as the ionic liquid butylpyridinium and 15 suspect QACs were detected in at least one wastewater effluent sample. A spatial profile of sediment samples in a lake demonstrated potential inputs from both municipal wastewater effluent and agricultural sources for BACs. In sediment cores, two distinct trends of temporal QAC accumulation were observed. In lakes with large watersheds and mixed domestic and industrial wastewater sources (Lake Pepin and Duluth Harbor), peak concentrations of QACs were found at depths corresponding to deposition in the 1980s and decreases after this time are attributed to improved wastewater treatment and source control. In a smaller lake with predominantly domestic wastewater inputs (Lake Winona), concentrations of QACs increased slowly over time until today.

Study on Degradation of Benzothiazolium-Based Ionic Liquids by UV-H2O2

Ionic liquids (ILs) are considered to be a new type of solvent due to their low vapor pressure, some unique properties and are expected to be used widely in the following days. Given their water solubility, stable chemical structure, and biological toxicity, ILs probably reach aquatic environments and become long-term pollutants. We studied the fate of benzothiazolium-based ionic liquids catalyzed by UV-H2O2 in water. Due to the synergistic effect of UV and H2O2, the degradation efficiency can be significantly improved compared to any of them alone. Fitting results showed that the degradation reaction follows pseudo-first-order kinetics and reaction rate constants with hydroxyl radicals ranging from 0.0005 s−1 to 0.0529 s−1. The experimental data showed that satisfactory results can be obtained at a temperature of 45 ℃, an initial concentration of ionic liquid of 0.1 M, and a hydrogen peroxide concentration of 0.1 M. We also explored the effects of chloride ions and pH on degradation reactions. In an appropriate situation, the degradation yield of [C4Bth]PF6 could achieve 97% in 60 min and 99% in 90 min. The UV-H2O2 combination process could provide an effective technique for the treatment of benzothiazolium-based ionic liquids in wastewater.

N-Nitrosamine formation from chloramination of two common ionic liquids

Ionic liquids (ILs) are a class of solvents increasingly used as "green chemicals." Widespread applications of ILs have led to concerns about their accidental entry to the environment. ILs have been assessed for some environmental impacts; however, little has been done to characterize their potential impacts on drinking water if ILs accidentally enter surface water. IL cations are often aromatic or alkyl quaternary amines that resemble structures of previously confirmed N-nitrosamine (NA) precursors. Therefore, this study has evaluated two common ILs, 1-ethyl-3-methylimidazolium bromide (EMImBr) and 1-ethyl-1-methylpyrrolidinium bromide (EMPyrBr), for their NA formation potential. Each IL species was reacted with pre-formed monochloramine under various laboratory conditions. The reaction mixtures were extracted using liquid-liquid extraction and analyzed for NAs using high performance liquid chromatography tandem mass spectrometry. At low concentration of IL (250 μmol/L), the yields of NAs (NMEA or NPyr) increased with increasing doses of monochloramine from both IL species. The total NA yield was as high as 2.5 ± 0.3 ng/mg from EMImBr, and as high as 8.6 ± 0.8 ng/mg from EMPyrBr. At high concentration of IL (5 mmol/L), the NA yield reached a maximum at 2.5 mmol/L NH₂Cl, and then decreased with subsequent increases in the reactant concentrations, demonstrating ILs' solvent effects. This study re-emphasizes the importance of preventing discharge of ILs to water bodies to prevent secondary impacts on drinking water.

A streamlined workflow to study direct photodegradation kinetic and transformation products for persistence assessment of a fragrance ingredient in natural waters

Photodegradation can be an important abiotic degradation process to consider for the fate and persistence assessment of chemical substances in the environment. In this work, using a fragrance ingredient (FI, (E)-4-(2,2,3,6-tetramethylcyclohexyl)but-3-en-2-one) as an example, we developed a streamlined workflow to investigate direct photodegradation of chemicals in the aquatic environment, including laboratory investigation of kinetics and transformation products and estimation of its aquatic environmental half-lives. Direct photodegradation was determined to be the dominant photodegradation process for FI with a quantum yield of 0.25, which was supported by photodegradation experiments conducted in natural sunlight. Accounting for light attenuation by dissolved organic matter in natural waters of different depths resulted in aquatic half-lives of <31 days even at polar latitudes. Photoisomerization was shown to be a major photodegradation pathway along with the formation and subsequent degradation of constitutional isomers and photooxidation products. These results contributed to FI being assessed as non-persistent in the environment.