1
Introduction
In the modern era, industrialization has resulted in an abnormal increase in the amounts
of hazardous wastes in the environment [1]. Uncontrolled deposition of non-biodegradable and hazardous chemical contaminants
into water bodies is harmful to the environment and raises economic concerns [2]. Most of these contaminants are carcinogenic, and they may readily accumulate in
living beings via water bodies, either directly or indirectly. Once ingested, it will
take a long time for them to be broken down, resulting in phenomena such as biomagnifications
[3]. This entire scenario underlines the need for more effective, sustainable, and environmentally
friendly water treatment techniques. Wastewater treatment, in general, refers to the
processes used to remove biogenic contaminants, undesired chemical pollutants, organic
or inorganic particulates, and gases from water [2], [4]. The nature and extent of the purification approach are mainly determined by the
type of contamination and its utility. Even though several existing chemical methods
(e.g., ozone treatment, distillation, ion exchange, neutralizing filtration, sediment
filtration, membrane filtration, reverse osmosis) are already available, their efficacy
in removing trace metals and micro contaminants is not particularly satisfactory [5]
[6]
[7]. Conventional wastewater treatment methods, though widely used, face several limitations
in effectively addressing modern pollution challenges. These techniques in general
are energy-intensive, costly, and generate secondary pollutants, which require further
treatment or disposal [8]. Moreover, they may not be adaptable to the increasing complexity and variability
of industrial waste streams, limiting their overall efficiency and sustainability.
These observations indicate the dire need for the development of novel industrial
wastewater treatment methods that can get around the drawbacks of existing techniques
while offering enhanced environmental sustainability, efficiency, and selectivity.
Extraction techniques serve as an effective means for separation and purification
due to their simplicity, low energy requirements, and great efficiency [9]. LLE, commonly referred to as solvent extraction, is a straightforward and environmentally
sustainable procedure among extraction methods. LLE is a separation process wherein
a solute is moved from one liquid phase to another immiscible liquid phase, typically
utilizing a solvent that selectively separates the desired solute from the initial
solution [10]. LLE provides simplicity, cost-efficiency, and mitigates the creation of secondary
pollutants, rendering it progressively appealing. Nonetheless, the selection of solvents
in
liquid–liquid extraction presents difficulties, particularly with the attainment of
quantitative extraction while maintaining environmental sustainability. Conventional
organic solvents used in liquid–liquid extraction can pose significant risks, highlighting
the need for more environmentally friendly alternatives. Recently, alternative solvents
such as ionic liquids (ILs) and deep eutectic solvents (DESs) have gained significant
attention due to their wide range of potential applications [11]
[12]
[13]
[14]
[15]
[16]. They have benefits like low volatility, high stability, and tunability, rendering
them optimal selections for sustainable extraction methods in industrial wastewater
treatment [17]
[18]
[19]
[20]. Their compatibility with various pollutants and capacity for efficient separation
further highlight their significance in this field [21].
Owing to the distinctive properties of ILs, IL-based liquid–liquid extraction has
attracted considerable attention as a feasible alternative to traditional solvents.
Recent research has examined the application of ILs for the extractive elimination
of diverse contaminants, including micropollutants, medicines, personal care product
residues, dyes, and heavy metal ions from industrial effluents [22]. These findings highlight the effectiveness of IL-based LLE in facilitating efficient
and selective extraction while minimizing environmental impact. A primary obstacle
in advancing IL-based LLE was the complexity of comprehending the fundamental mechanisms
of extraction. The complex interactions between ILs and contaminants sometimes hinder
the accurate prediction of extraction outcomes. Recent advancements in computational
methodologies, such as density functional theory (DFT) and the conductor-like screening
model for real solvents (COSMO-RS), have
provided critical insights into these systems [23], [24]. These computational approaches have enabled researchers to comprehend and forecast
the behavior of ILs in LLE processes, leading to improved solvent design and process
optimization. A fundamental element of IL-based LLE is the recyclability and reusability
of the ILs employed in the procedure. Although ILs have various benefits, their high
costs and possible environmental repercussions demand the development of techniques
for solvent recovery and reuse [25]. Recent studies have begun addressing this difficulty, demonstrating effective solutions
for IL recovery and reusability without considerable loss in extraction efficiency
(EE). This advancement is crucial for guaranteeing the long-term sustainability and
economic feasibility of IL-based LLE in industrial applications.
As a whole, this review aims to offer an extensive overview of IL-based LLE for wastewater
treatment by analyzing the evolution of this approach from its initial stages to its
present state. The emphasis will be on the fundamental components of IL design, the
intricacies of the process, mechanism analysis, recyclability, and sustainability,
offering insights into potential future advancements for this promising technology.
This review aims to identify research gaps and highlight opportunities for further
innovation in the field of IL-based LLE for industrial wastewater treatment.
2
Ionic Liquids
ILs are chemical entities consisting of organic cations combined with organic or inorganic
anions, exhibiting distinctive chemical and structural characteristics. These characteristics
encompass an extensive liquid range, thermal stability, low vapor pressure, a broad
electrochemical window, and the capacity to solubilize various compounds [26]. An ionic liquid is defined as a chemical composed exclusively of ions, generally
existing in a liquid state at temperatures below 100 °C. Categorizing an IL exclusively
as a category of salts with a melting point below 100 °C is not universally endorsed.
ILs can be regarded as a separate class of solvents, alongside water and organic solvents
[27].
In recent times, ILs have deeply influenced different scientific fields and technologies,
displaying notable growth in both research output and practical applications. Their
versatility in properties has engrossed substantial attention from various sectors,
fostering several scientific developments and inventions. The steady increment in
the number of publications based on IL chemistry over the years underlines the increasing
significance and widespread application of ILs across key areas ([Fig. 1]). The highly tunable structure of ILs provides remarkable versatility, allowing
them to be custom-made for a wide array of applications. Additionally, their potential
for reuse makes them a sustainable option in numerous fields [28].
Figure 1 Year-wise publications on ionic liquids from 2006 to 2024.Source: SCOPUS, as of December
13, 2024.
2.1
History and Classifications
The voyage of IL research finds its roots in the ground-breaking investigations of
Paul Walden in the early 20th century [29]. Ethylammonium nitrate ([EtNH3][NO3]), the first discovered IL, was prepared by the neutralization of ethylamine by nitric
acid. This IL had a surprisingly low melting point of 12 °C. Nevertheless, the actual
potential of ILs remained mostly unmapped for the next four decades. Hurley and Wier,
in 1951, found that a 2:1 mixture of 1-ethylpyridinium bromide and aluminum chloride,
[C2py]Br-AlCl3, persisted as a liquid at room temperature [30]. This finding led to further analysis, including the development of phase diagrams
and the invention of novel liquid state compositions. Building upon this, the limitations
of the current mixture were later refined. This particular research study was focused
on formulating a broader range of liquid compositions at room
temperature, which ended up in the discovery of 1-butylpyridinium chloride-aluminum
chloride, [C4py]Cl-AlCl3. This compound possessed improved properties as an IL in comparison to the former
one [31]. During this period, the research on IL revolved around halo aluminate-based ILs,
which can be regarded as the first generation of ILs.
A significant milestone in the history of ILs occurred with the introduction of moisture-stable
ILs by Wilkes and Zaworotko in 1992. These ILs replaced aluminum chloride with other
stable anions, rapidly expanding the room-temperature IL family [32]. Following this advent of air and water-stable ILs—characterized by anions such
as tetrafluoroborate, nitrate, methyl sulfonates, trifluoromethane sulfonate, hexafluorophosphate,
and halides—there arose a surge in the concept of designer solvents. This surge was
fueled by the tunable physical and chemical properties inherent in ILs. Beyond the
widely favored imidazolium category, the cationic selection of ILs extended to include
ammonium, phosphonium, triazolium, pyridinium, morpholinium, cholinium, and beyond.
[Fig. 2] depicts different categories of cations and anions generally used in ILs. Task-specific
ILs (TSILs) mark a significant evolution in the field of ILs [33]. TSILs are specifically designed to perform specific tasks or functions with high
efficiency and selectivity. TSILs have found various applications across different
fields. Apart from their role in conventional chemical processes, TSILs have also
been widely explored in biological and environmental contexts [34]. Another noteworthy evolution in the field of ILs was the advent of a new generation
of ILs as mixtures of ILs and molecular solvents [35]. This development from traditional IL formulations signified a paradigm shift in
the understanding of solvent systems.
Figure 2 Pictorial representation of commonly used cations and anions in ILs.
Classifying ILs presents a challenging task. Still, in accordance with the method
of preparation and chemical nature, ILs can be categorized into different groups.
Aprotic ILs (AILs) are typically synthesized through quaternization reactions followed
by anion metathesis (when required). These ILs are usually produced by the alkylation
of phosphine, amine, etc., which produces intermediate salts. Subsequently, the desired
anions are often introduced, resulting in the displacement of those produced during
the intermediate stage [36]. AILs represent a significant portion of publications in the field of IL chemistry
and exhibit superior thermal stability relative to other IL categories [37]. Another major category, protic ILs (PILs) are generally formed through the proton
transfer from a Brønsted acid to a Brønsted base [38]. The process leads to the formation of sites that can donate and accept protons,
thereby promoting hydrogen bonding within the PIL structure. A significant difference
in the pK
a values of the involved acids and bases is really essential, and that governs the
extent of proton transfer during PIL formation. Commonly employed cations in PILs
include phosphonium, ammonium, caprolactam, and imidazolium, combined with anions
such as trifluoroacetate, triflate, and nitrate. In PILs, the cations possess a proton
bonded to either nitrogen or phosphorus. PILs exhibit superior conductivity relative
to other classes of ILs, attributed to the presence of free protons that facilitate
hydrogen bonding interactions [39]. The enhancement in conductivity is accompanied by a trade-off in stability, as
the potential for backward reactions results in back proton transfer, which diminishes
the stability of PILs at higher temperatures.
Pseudoprotic ILs (PPILs) are a distinct subclass of PILs. In contrast to conventional
PILs, there is an incomplete proton transfer among the components, leading to a distinct
ionic composition that may affect their properties and potential applications. This
is due to the relatively low pK
a differences [40]. Despite this, PPILs exhibit many properties typical of ILs [41]. The method of synthesis is easier in comparison with multistage synthesis and purification
needed for commercially available extractants and other Room Temperature Ionic Liquids
(RTILs). Commonly employed cations in PPILs include phosphonium, ammonium, caprolactam,
and imidazolium, with a variety of anions such as salicylate, benzoate, etc [42]. PPILs present a distinctive combination of characteristics, integrating features
of both protic and AILs, and are relevant for multiple applications in extraction,
catalysis, electrochemistry, and materials science [43].
Magnetic ILs (MILs) represent a category of RTILs distinguished by their intrinsic
magnetic characteristics, independent of any magnetic particle incorporation [44]. These magnetic properties are induced by the cation, anion, or their combination.
MILs frequently incorporate transition metal or lanthanide complexes in their anion
frameworks, which imparts paramagnetic properties. MILs containing the [FeCl4]− anion were among the first to be synthesized and have been the subject of extensive
investigation [45]. Recently, MILs incorporating transition metals like Co and Mn, as well as lanthanide
complexes such as Gd or Dy have gathered attention [46]. Commonly employed cations include phosphonium, ammonium, and imidazolium, paired
with a variety of anions such as tetrachloroferrate, tetrachloromanganate, etc. MILs
generally show potential for different kinds of extraction, optical,
and catalytic applications [47].
2.2
Unique Properties and Applications of ILs
ILs are distinguished from conventional solvents by their numerous distinctive characteristics.
Their intrinsic tunability enables tailored traits, which makes them compatible with
many domains and applications [48]. Due to their wide solubility range and highly tunable structures, ILs are widely
utilized in solubilization applications across several fields. To enhance the bioavailability
of poorly dissolvable drugs in pharmaceuticals they are employed [49]. Additionally, ILs are used in the dissolution of biomass, including lignin [50]. Their distinctive features make them efficient solvents for decomposing lignocellulosic
biomass into its constituent components, such as cellulose, hemicellulose, and lignin
[51]. The dissolution process is vital in numerous biomass conversion technologies, such
as the generation of biofuels, biochemicals, and biomaterials [52].
The remarkable solubility range, tunability, and efficient phase-separation properties
of ILs are crucial in many extraction and separation processes. ILs are extensively
utilized in the extraction and separation of bioactive compounds [53], valuable metals from electronic waste [54], pollutants from wastewater [55], as well as lanthanides and actinides from spent nuclear waste [56]. Their adaptability in various applications arises from their capacity to effectively
dissolve substances of interest while facilitating straightforward separation.
The wide electrochemical potential windows and high conductivity of ILs are utilized
in electrochemical applications [57]. They are extensively utilized in energy storage devices, such as fuel cells, supercapacitors,
and batteries [58]. The versatility of ILs comprises semiconductor applications, metal electrodeposition,
and the revolutionizing of electroplating processes [59]. In addition, ILs are included in various electrolyte systems, including polymer
or gel polymer electrolytes, and utilized as additives [60]. The unique characteristic that makes ILs suitable for sensing applications is their
ability to alter physicochemical and biological properties in response to specific
conditions. This attribute enables ILs to interact with target analytes in a controlled
manner, allowing for their detection with heightened precision and selectivity [61].
Consequently, they have substantial applicability in numerous sensing devices [62]. In addition to sensing, ILs are utilized for the absorption of greenhouse gases.
The low volatility and gas absorption capacity of ILs make them appealing candidates
for greenhouse gas capture [63]. Specifically formulated ILs have successfully captured various greenhouse gases,
including CO2, CH4, and N2O [64].
Industrial chemical processes like catalysis, which require high temperatures, greatly
benefit from the exceptional thermal stability of ILs. In catalysis, ILs can serve
as solvent media, catalysts, or co-catalysts. Many studies reported that when ILs
are employed in reaction mediums, catalyst exhibits considerable selectivity and activity.
Catalytic reactions facilitated by ILs encompass various processes, including Diels-Alder
cycloadditions, polymerizations, acylation of isobutylbenzene, biomass dissolving,
and olefin dimerization [65]. Furthermore, ILs have been employed as additives to enhance the efficiency of various
industrial materials, including paints [66]. A widely used feature of ILs is their ability to adjust the viscosity and rheological
properties of the product. The flow properties of substances such as paints and shampoo
can be modified by tuning the flow properties of ILs. The overall durability and shelf
life of the product are improved due to its thermal and chemical stability. Additionally,
ILs can protect against degradation from heat, oxidation, and other chemical processes,
thereby extending the lifespan of the manufactured product.
Recent research has explored the potential of ILs as alternatives to traditional refrigerants
used in refrigeration systems [67]. As hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs) gain popularity as refrigerants
for refrigeration and cryogenic applications, concerns regarding their significant
environmental impacts are growing. Due to their high gas absorption capacity, negligible
vapor pressure, and outstanding thermal stability, ILs present a promising alternative
for reducing the environmental impact of refrigeration systems. Another commercialized
application of ILs is their use in dispersing crude oil spills [68]. The emergence of magnetic ILs possessing inherent magnetic properties has opened
up a new range of potential applications [69]. ILs can be applied to the surface of an oil spill, where they interact with the
oil molecules because of their amphiphilic nature. The ILs create a
stable emulsion with the crude oil, breaking it into smaller droplets and inhibiting
its coalescence into large slicks [70].
4
IL-Based LLE for Wastewater Treatment
Conventional methods for treating industrial wastewater have several drawbacks. They
often lack specificity, meaning they may not effectively target-specific contaminants.
These methods can also be time-consuming, require large amounts of chemicals, and
sometimes create secondary pollutants as a byproduct. The extraction of various micropollutants,
dyes, and heavy metal ions using IL-based LLE has emerged as a highly efficient and
environmentally benign technique. This method offers enhanced extraction performance,
reduced solvent loss, and potential recyclability, making it an attractive alternative
to conventional methods in wastewater treatment applications. In some cases, phase-separation
promoters are utilized to segregate the water-IL combination into two immiscible aqueous
phases employing hydrophilic ILs. [Fig. 4] shows a schematic diagram depicting the optimal techniques for IL-based LLE in wastewater
treatment.
Figure 4 Schematic diagram explaining the IL-based LLE for wastewater treatment.
The treatment of wastewater via ILs commences with the acquisition of real wastewater
samples or the creation of simulated samples in a laboratory setting. Researchers
then develop ILs particularly formulated for the contaminants of concern, using their
tunable characteristics to efficiently target pollutants. These pollutants include
micropollutants, pharmaceutical residues, personal care product waste, dyes, and heavy
metals, which are frequently present in industrial effluent. Subsequently, the most
suitable type of LLE is determined, and extensive optimization is conducted to ascertain
the optimal extraction conditions. Parameters such as temperature, pH, extractant
concentration, and extraction duration are optimized to enhance pollutant removal
efficiency. A crucial phase entails devising methods to recycle the ILs postextraction.
Solvent recycling is often accomplished by suitable stripping techniques, facilitating
numerous extraction cycles. A comprehensive review of
the current literature on IL-based LLE techniques in wastewater treatment is carried
out, emphasizing recent developments in EE, solvent reusability, and overall environmental
impacts.
4.1
Extractive Removal of Organic Contaminants
Initially, scientists employed IL-based conventional and dispersive liquid–liquid
microextraction (DLLME) techniques for the extractive removal of organic pollutants
from wastewater, subsequently advancing to more sophisticated liquid–liquid microextraction
(LLME) methods utilizing temperature and ultrasound. The predominant research in IL-based
water treatment of organic impurities concentrated on the extraction of phenol and
phenolic pollutants. Numerous industries have made extensive use of phenols and their
derivatives. The main ingredients in personal care products are also phenol derivatives,
such as bisphenol-A, naphthol, resorcinol, and catechol.
Khachatryan et al. used an RTIL, 1-butyl-3-methyl imidazolium hexafluorophosphate,
[BMIm][PF6], to recover phenolic chemicals from industrial wastewater. Specifically, phenols,
nitrophenols, and naphthols were the main compounds targeted. LLE was employed in
all operations at room temperature, maintaining a 1:3 volume ratio at pH values between
1 and 14. Nitrophenols and naphthols recovered more than 90%, whereas phenols and
polyphenols recovered much lesser amount. The variation in extraction capacity with
different ILs was explained by the process’s pH dependence [78]. Using a similar collection of ILs, Vidal et al. have also tried to extract phenolic
chemicals from aqueous solutions. They achieved this by employing hexafluorophosphates
with extended alkyl chains and 1-(n-alkyl)-3-methylimidazolium tetrafluoroborates. Tyrosol, phenol, and p-hydroxy benzoic
acid were among the phenolic compounds that were more successfully extracted
using tetrafluoroborate-based ILs. An EE of about 90% was observed, and the pH shift
was found to have no effect on it. Conversely, the effectiveness of ILs based on hexafluorophosphate
was comparatively low and was sensitive to pH changes [79]. Egorov et al. investigated the extraction of phenols and aromatic amines using
novel quaternary ammonium ILs, comparing their efficiency to that of previously reported
imidazolium-based ILs. Ammonium-based RTILs, such as tetrahexylammonium dihexylsulfosuccinate
and trioctamethylammonium salicylate, were found to have considerably higher solute
distribution ratios than imidazolium-based RTILs [80]. Similarly, Cesari et al. have developed choline bis(trifluoromethylsulfonyl)imide
[Ch][NTf2] as an extractant medium for the extraction of phenolic compounds from aqueous solution
[81].
Emilio et al. investigated the extraction of phenolic compounds from wastewater using
aromatic and nonaromatic ILs. They used ILs based on pyrrolidinium and imidazolium
with bis(trifluoromethanesulfonyl) imide as the anion to extract phenol, o-cresol,
and resorcinol. In addition, they analyzed the extraction process computationally
using COSMO-RS. It was observed that pyrrolidinium-based ILs show improved extraction
performance and the efficiency increases in the order for o-cresol > phenol > resorcinol
[82]. Sas et al. established an LLE method for extracting phenolic compounds using 1-ethyl-3-methylimidazolium
bis(trifluorosulfonyl) imide. The phenolic derivatives extracted include o-cresol,
resorcinol, 2-chlorophenol, and phenol, from aqueous solutions, with initial concentrations
varying from 3 to 1000 mg L−1. Furthermore, the extraction of four phenolic compounds from aqueous solutions was
evaluated within the identical
concentration range. All chemicals, except for resorcinol, were extracted at rates
exceeding 90%, whereas resorcinol was removed at approximately 78%. Most notably,
they successfully regenerated the IL by using NaOH solution [83]. Subsequently, the same group refined their research by developing pyridinium-based
ILs including bis(trifluoromethanesulfonyl) imide and bis(trifluorosulfonyl) imide
as anions. Upon analyzing the ionic structure of the extracting agent, it was determined
that the anion is of paramount importance. The impact of the structure of extracted
phenolic contaminants was also examined. Substituted phenols were extracted more efficiently
by ILs than by phenol itself. Hydrophobic interactions were identified as essential
in the transfer of phenols from the aqueous phase to the IL-rich phase [84].
Identifying the presence of active pharmaceutical ingredients (APIs) and their removal
from water bodies is imperative. In recent years, IL-based liquid–liquid extraction
(IL-based LLE) has been widely investigated for the same. Seven unique functionalized
ILs were designed by Yao et al. to extract 14 organic contaminants, including APIs
such as acetaminophen and sulfisomidine from aqueous solutions. In their study, the
IL 1-(6-amino-hexyl)-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate,
demonstrated significant selectivity and sensitivity in the extraction of molecules
with tertiary amine functionality [85]. Hou et al. employed a temperature-regulated liquid–liquid extraction approach to
isolate eight distinct tetracycline antibiotics from environmental water samples.
The antibiotics were initially transformed into hydrophobic complexes utilizing La(III)
as a chelating agent, and these complexes were subsequently extracted employing
1-alkyl-3-methylimidazolium hexafluorophosphate ILs. Trace concentrations of antibiotics
were quantified via ultra-high-pressure liquid chromatography [86]. Parrilla et al. investigated imidazolium-based ILs with hexafluorophosphate anion
for the extraction of nine distinct medicines, including paracetamol, naproxen, and
bisoprolol. Subsequent to extraction, the samples were analyzed utilizing a high-performance
liquid chromatograph-quadrupole-linear ion trap mass spectrometer [87]. A recent study by Padinhattath et al. explored the removal of APIs from wastewater
using novel n-benzyl ethanolamine-based ILs. Four novel N-benzylethanolamine-based
hydrophobic ILs were designed for the extractive removal of top-priority pharmaceutical
micropollutants from wastewater samples, with a special focus on diclofenac sodium.
The structural optimization of the ILs was performed using DFT studies with the B3LYP
method and a 6-311++G(d,p)
basis set, using the Gaussian 16 suite of programs. The interactions between the ILs
and diclofenac medium were explored using the integral equation formalism polarizable
continuum model (IEFPCM) of solvation. Experimental studies employed the LLE method,
with extraction parameters being optimized to ensure efficient extraction. The reusability
of the most efficient IL was also assessed. Computational interaction studies and
FT-IR analysis were conducted to determine the primary factors driving the extraction
process. The primary driving forces of extraction were determined to be hydrophobicity,
hydrogen bonding, van der Waals interactions, and π–π interactions between the IL
and the pollutant. Moreover, the study’s aim was extended to encompass the extractive
removal of additional micropollutant pharmaceuticals, including ciprofloxacin and
metronidazole [88].
The extraction of aromatic and nonaromatic hydrocarbons from aqueous samples is a
compelling topic. The hazardous traits of specific hydrocarbons were identified as
a concern, and the IL-LLE methods surfaced as the most effective solution to address
it. The initial attempts of Liu et al. were of great importance. They successfully
illustrated the distribution ratios of specific polycyclic aromatic hydrocarbons (PAHs)
in water and ILs. Fifteen specific PAHs were identified as target compounds, with
imidazolium-based ILs containing hexafluorophosphate anion utilized as extractants.
log D values were recorded between 3.34 and 4.36, exhibiting a gradual increase with the
molar mass of PAH [89]. Fan et al. conducted a noteworthy study on the extraction of aromatic amines from
river water, wastewater, and tap water utilizing dispersive LLE. The targeted aromatic
amines were 1-naphthylamine, 2-methylaniline, 4-aminobiphenyl, and 4-chloroaniline,
whereas the IL employed was 1-butyl-3-methylimidazolium hexafluorophosphate. They
have optimized extraction parameters including extraction duration, pH of the aqueous
solution, and amount of IL. Good sensitivity and repeatability were achieved under
optimal conditions [90]. Pena et al. introduced an IL (1-octyl-3-methylimidazolium hexafluorophosphate)-based
technique for the extraction of 18 unique PAHs from various water samples. The extraction
method employed was IL-DLLME. This technique leverages the chemical affinity between
the IL and the target analytes, facilitating the extraction and preconcentration of
PAHs from the sample. Various parameters influencing EE (%), including IL type and
volume, dispersion solvent type and volume, extraction duration, centrifugation duration,
as well as ionic strength, were optimized. The EE (%) of the optimized method exceeds
that of conventional LLE techniques. The present method proved effective in analyzing
PAHs in water samples [91]. Saleem et al. conducted a significant study on the IL-based extraction of various
halogenated hydrocarbons (HHCs), including CCl4, CHCl3, and CHBr3, from wastewater. ILs comprising piperidinium, pyrrolidinium, and ammonium cations,
with bis(trifluoromethanesulfonyl) imide as the anion, were utilized for this purpose.
The applied extraction method was conventional liquid–liquid microextraction. The
ILs were chosen for their capacity to solubilize significant pollutants. Moreover,
their hydrophobicity, viscosity, and stability in the presence of superoxide ions
would be employed to decompose HHCs. The chosen ILs successfully removed harmful HHCs
from the aqueous phase, with extraction efficiencies ranging between 83% and 100%.
The study demonstrated that ILs with octyltriethyl-ammonium and pyrrolidinium cations,
along with bis(trifluoromethylsulfonyl) imide anion, efficiently extract particular
HHCs. The influence of various parameters, such as the properties of the components
(HBA and HBD), temperature, pH, polarizability, and octanol/water partition coefficient
on the EE, was thoroughly investigated [92].
Research into the extraction of pesticides and insecticides from water bodies is equally
crucial. Pesticides and insecticides are frequently employed in agriculture, and most
of them have significant toxicity and cause substantial damage to aquatic systems.
Many of these are not directly extractable using conventional methods due to their
low concentrations. As a result, the usage of IL-based LLME gave the study in this
field a new perspective. Lijun et al. investigated the extraction of organophosphorus
pesticides from tap, well, rain, and yellow river water samples. The pesticides extracted
were parathion, phoxim, phorate, and chlorpyrifos, while the extracting agent was
1-alkyl-3-methylimidazolium hexafluorophosphate ILs. The procedure was persuaded by
the development of a cloudy solution, which consisted of tiny droplets of IL dispersed
completely into the sample solution using the disperser solvent methanol. The extraction
solvent volume, the dispersion solvent volume,
extraction time, centrifugation time, the influence of salt addition, extraction temperature,
and sample pH were all studied and optimized. Because of its higher EE, 1-octyl-3-methylimidazolium
hexafluorophosphate was found to be the best among ILs [93]. Liu et al. used a similar approach and the same category of ILs to extract and
identify four heterocyclic insecticides, namely, fipronil, chlorfenapyr, buprofezin,
and hexythiazox, from water samples [94]. Zhang et al. investigated the feasibility of extracting benzoylurea insecticides
(BUIs) from water samples using 1-alkyl-3-methylimidazolium hexafluorophosphate ILs.
The team experimented with a rapid dispersive LLE strategy, followed by magnetic retrieval
of the ILs utilizing unmodified magnetic nanoparticles (MNPs). Fine IL droplets produced
in aqueous samples functioned as an extractant for the extraction of BUIs. The suggested
method’s repeatability and reproducibility
were found to be satisfactory, and it was successfully employed for the rapid determination
of BUIs in real water samples [95].
Several additional organic pollutants were extracted and identified using various
classes of ILs. Zhao et al. attempted to extract bactericides from natural water samples
using DLLME. Triclosan and triclocarban were the pollutants chosen. They combined
hexafluorophosphate and tetrafluoroborate anions with imidazolium-based cations to
form ILs. The extraction experiment was highly efficient, with a profound recovery
rate [96]. Later, the same group investigated the extraction of hexabromocyclododecane (HBCD)
diastereomers in environmental water samples using the same set of ILs. They performed
experiments on lake water, river water, rainfall, and snow water to determine the
presence of α, β, γ-HBCD. The EE using 1-octyl-3-methylimidazolium hexafluorophosphate
was found to be higher than that of the other ILs [97]. Bhosale et al. were able to extract energetic materials from industrial effluents
using IL-based LLE. TNT (2, 4,
6-trinitrotoluene), RDX (hexahydro-1,3,5trinitro-1,3,5-triazine), and tetryl(2, 4,
6-trinitro-phenyl methylnitramine) were the materials targeted. For this purpose,
five different imidazolium-based ILs containing [NTf2] − and [PF6] − anions were used. They observed that EE improves with a change in anionic moiety
from [NTf2] − to [PF6] −, pH drop, and an increase in phase volume ratio. After numerous washes with diethyl
ether, ILs were regenerated and reused for further cycles [98].
As previously mentioned, various advanced modifications of LLE have been developed,
including in situ dispersive liquid–liquid microextraction, temperature-controlled
LLE, and ultrasound-assisted LLE. These innovative techniques have recently gained
attention among researchers for their effectiveness in removing organic pollutants
from wastewater samples. Yao et al. conducted a noteworthy demonstration of in situ
liquid–liquid microextraction, where they designed an experiment to extract aromatic
compounds, including biphenyl, 3-tert-butyl phenol, and polyaromatic hydrocarbons,
from natural water samples. In situ metathesis was employed to develop a water-immiscible
imidazolium-based IL that preconcentrated aromatic compounds in water samples. They
observed that, in comparison to conventional IL-based DLLME, the combined extraction
and metathesis process in the IL-based extraction phase significantly reduced extraction
time while providing higher enrichment factors [99]. In the following years, Darias et al [100]. and Zhong et al [101]. used similar methods to recover a range of organic pollutants from natural water
samples. The method of temperature-controlled LLE was effectively utilized by Zhou
et al., where they focused on the detection of organophosphorus pesticides in environmental
samples using IL-based temperature-controlled LLE. The extraction solvent was 1-hexyl-3-methylimidazolium
hexafluorophosphate, and the factors influencing the EE (%), like IL volume, pH of
solutions, extraction and centrifugation duration, temperature, and salt effect, were
optimized [102]. Later, the team used the same IL to extract phenols from water samples using temperature-controlled
IL-based DLLME [103].
Recently, IL-based ultrasonic-assisted dispersive liquid–liquid microextraction techniques
have emerged as a promising alternative for the separation of organic contaminants
from water samples. In this technique, sonication will thoroughly distribute IL into
the aqueous sample solution. The analytes would be transferred into tiny IL droplets,
resulting in a high enrichment in performance. Zhang et al. employed a similar approach
to extract benzophenone-type UV filters from water samples, utilizing 1-alkyl-3-methylimidazolium
tris(pentafluoroethyl)trifluorophosphate ILs. They have successfully isolated four
types of UV filters, identifying 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate
as the optimal IL due to its lower viscosity [104]. Wang et al. performed a study in this field, combining ultrasound-assisted technique
with in situ solvent formation microextraction and solidification of sedimentary ILs.
This technique was used
in conjunction with HPLC to identify various triazole pesticides in water and juice
samples. In this technique, the tributyloctylphosphonium hexaflurophosphate [P4448][PF6] is the microextraction solvent, which was synthesized from tributyloctylphosphonium
bromide [P4448]Br and potassium hexaflurophosphate. Various parameters influencing the EE (%) like
the amount of [P4448]Br, the molar ratio of [P4448]Br to KPF6, salt addition, centrifugation rate and time, and sample pH were all investigated.
The recovery rates for these four triazole insecticides range from 85% to 91%[105]. Zeeb et al. applied a similar method to detect trace amounts of five PAHs in environmental
water samples. They added 1-butyl-3-methylimidazolium tetrafluoroborate (hydrophilic),
to the sample solution along with an ion-pairing agent (NaPF6), which generated a hydrophobic IL, 1-butyl-3-methylimidazolium
hexafluorophosphate. The PAHs were extracted into the IL phase, with ultrasonic radiation
dispersing the microextraction solvent throughout the sample. By combining the advantages
of both techniques, this innovative method demonstrated high efficiency and potential
for broader applications [106].
4.2
Extractive Removal of Dyes
The extraction and separation of dyes from aqueous solutions utilizing ILs have become
widely popular since the early 21st century. Vijayaraghavan et al. put forward a study
on the extraction and recovery of acid blue and acid red dyes—azo dyes utilized in
the leather industry using N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)amide.
A similar experiment was repeated using an actual tanning effluent dye sample. The
ILs were back-extracted from the IL–dye combination utilizing a 1:1 isopropyl alcohol–water
solution and subsequently reused [107]. Li et al. have put forward a method for isolating acidic dyes (acid yellow RN and
brilliant blue RAW) and reactive dyes (reactive black KN-G2RC, reactive yellow M-5R)
using 1-butyl-3-methylimidazolium hexafluorophosphate. The IL effectively extracted
acidic dyes [108]. However, the extraction of reactive dyes was improved by the incorporation of dicyclohexyl-18-crown-6.
The pH significantly affected the partition coefficient values in the case of acidic
and reactive dyes, but it did not impact the extraction of the weak acid dye [108]. Moreover, Othman et al. illustrated the IL-based extraction of remazol brilliant
orange 3R from textile effluent utilizing tetrabutyl ammonium bromide. Multiple parameters
influencing EE were analyzed. The research indicated that extraction utilizing dichloromethane
and chloroform as diluents was more efficacious than using toluene, kerosene, n-dodecane,
and xylene. The influence of pH on the extraction ratio was determined to be minimal.
Following the extraction, a 1:1 ratio of salicylic acid to NaOH was employed for the
stripping process [109].
Zhang et al. conducted significant work utilizing temperature-controlled dispersive
liquid–liquid microextraction to examine the extraction of malachite green and crystal
violet with 1-octyl-3-methylimidazolium hexafluorophosphate. The examination into
the effects of NaCl salinity demonstrated a direct proportionality between the extraction
coefficient and NaCl concentration up to 20%, followed by a subsequent decline afterward
[110]. In the subsequent year, Chen et al. conducted a study on the extraction of methyl
orange and methylene blue dyes with quaternary ammonium-based ILs. The influence of
the phase ratio on EE (%) was determined to be minimal. The endothermic nature of
the procedure resulted in an increased distribution coefficient for the extraction
of methyl orange with rising temperature. Conversely, the extraction of methylene
blue was an exothermic process, and the distribution coefficient diminished with increasing
temperature. The
research on the influence of pH on the extraction coefficient determined that pH does
not significantly affect the extraction of methylene blue, but it does have a pronounced
effect on the extraction of methyl orange. Methylene blue was subsequently extracted
using 0.1 molar HCl and recovered from IL with chloroform [111].
Talbi et al. conducted another intriguing investigation on the removal of cationic
dyes from aqueous solutions utilizing ILs and nonionic surfactant-IL systems, with
the results subjected to empirical fitting analysis. The study utilized 1-butyl-3-methylimidazolium
hexafluorophosphate for the extraction of methylene blue dye. The results indicate
that EE decreased with increasing temperature and enhanced with the addition of the
nonionic surfactant, Triton X-114. The dye’s ability to easily dissolve into micelles
at high pH levels is associated with its improved EE in alkaline settings. The addition
of K2CO3 salt negatively affected EE [112]. Fan et al. initiated a study on the influence of imidazolium-derived ILs on the
extraction of azo dyes. The distribution ratios (D) were found to be constant with
increasing phase ratio after 40 minutes of extraction at pH 1.25 and 10.21, whereas
a little decrease was detected at pH 4.32.
Thus, the pH of the aqueous phase was recognized as a crucial factor influencing the
distribution ratio. An examination of the influence of chemical structure on the D value was conducted. The variation in the hydration capacity of the additional salts
resulted in considerable changes to the D values [113]. Ana et al. investigated IL-based ABS for the extraction of chloranilic acid, Sudan
III, and indigo blue dyes from water. Phosphonium-based ABS exhibited superior EE
compared to imidazolium-based counterparts. Utilizing the appropriate IL and salt,
they accomplished superior dye elimination in a single step and effectively recycled
the IL via filtration [114]. Beatriz et al. employed a liquid–liquid extraction procedure utilizing trihexyltetradecylphosphonium
decanoate to remove three textile dyes from water samples. This IL showed exceptional
extraction capability while necessitating reduced quantities
relative to comparable experiments. The IL showed significant efficacy in removing
dyes from contaminated water; nonetheless, the work raises questions about the toxicity
of phosphonium ILs and their appropriateness for wastewater treatment [115].
A recent study by Padinhattath et al. investigated the use of pseudoprotic ILs (PPILs)
for the efficient removal of various dye classes from both simulated and real industrial
wastewater samples. The study focused on the effective extraction of cationic dyes,
including crystal violet, malachite green, methylene blue, and rhodamine, from neutral
aqueous solutions using nonstoichiometric PPILs, comprising tri-octyl amine and octanoic
acid as the extraction medium. LLE was employed at pH 7 ± 0.1 and 303 ± 1 K, with
parameters such as diluent choice, extractant concentration, equilibration time, interference
effects, stripping agents, and stripping phase ratios systematically optimized. The
dyes were effectively back-extracted from the PPIL-rich phase using dilute citric
acid as the stripping agent, allowing for solvent regeneration and reuse in successive
extraction cycles. Through pH, conductivity, and titrimetric analyses, proton exchange
was identified as the extraction
mechanism. The method achieved quantitative extraction and stripping (>99%) of all
cationic dyes, and its circular process design demonstrates significant potential
for real-world wastewater treatment applications [116].
4.3
Extractive Removal of Heavy Metal Ions
The initial experiment utilizing ILs for the LLE of metal ions from wastewater took
place in the late 1990s. Since then, the methodology for metal ion extraction from
wastewater utilizing ILs has evolved in three specific pathways: (i) ILs functioning
as both extracting agents and organic phases, (ii) ILs working as diluents to dissolve
extractants, and (iii) functionalized or task-specific ILs used for targeted extraction
processes.
The pioneer studies of alkali and alkaline earth metal extraction appeared in 1999,
which laid the foundation of volatile organic solvents (VOSs) replacement with ILs,
which were carried out by Dai and co-workers. They reported extraction of Sr2+ with a combination of dicyclohexane-18-crown-ether-6 (DCH18C6) and a series of imidazolium-based
ILs [117]. Sr is a fission product, and until now, there has been no efficient extraction
method available for its removal from radioactive waste sites, particularly for samples
with a distribution ratio greater than 1. They analyzed the distribution ratio of
metal ions with and without the presence of crown ether and found that even without
the presence of crown ether, some of the ILs were able to provide a distribution ratio
of around 0.9. IL combined with crown ether provided a large hike in the distribution
ratio, which was way beyond the conventional results. The selection of IL anions was
also
very much relevant with [NTf2]− and [PF6]− based ILs showed dashing efficiency in the extraction process. The efficiency of
the ILs was also compared with conventional VOSs such as chloroform and toluene as
well [117]. Later, it was found that the EE of the same process was significantly improved
by the presence of a second ligand, tri-n-butyl phosphate (TBP), because of the formation of a synergistic adduct. These studies
have become the stepping stone for a slower transition toward the IL era in metal
extraction chemistry [118].
Following this, Luo and co-workers synthesized 16 protic amide-based ILs derived from
N,N-dimethylformamide and other amide derivatives with bis(trifluoromethanesulfonyl)imide
as conjugated anions [119]. These ILs were tested as extraction solvents using DCH18C6 an extractant for the
separation of Sr2+ and Cs+ ions from aqueous solutions. Excellent extraction efficiencies were found for a number
of these ILs in comparison with other imidazolium and ammonium-based ILs. In general,
it was observed that without the addition of ILs to these compounds, they did not
extract M2+ cations. The effects of solution acidities, anions, and alkyl chain lengths of cations
of ILs in the EE were also thoroughly investigated. Similar studies were carried out
by Turanov et al., Toncheva et al., and Dukov et al., which affirm the role of ILs
as active reagents in synergic extraction systems [120]
[121]
[122]. One of the major factors that influence the alkali and alkaline earth cations from
an acidic aqueous phase into the IL phase by a crown ether was the hydrophobicity
of both the IL anion and cation. The universality of this discovery was verified by
carrying out extraction tests with different families of ILs [123]. The extraction behavior of Sr2+ ion from high-level liquid waste was examined by Takahashi et.al using [C1C
n
im][Tf2N] (n = 2, 4, 6) and dichloromethane as diluents [124].
In 2017, another group developed task-specific ILs for Li-ion capture. They synthesized
tetrabutylphosphonium bis(2,4,4-trimethylpentyl)phosphinate ([P4444][BTMPP]) along with two nonfluorinated compounds-tetrabutylammonium/tetraoctylammonium
bis(2-ethylhexyl)phosphate ([N4444][DEHP] and [N8888][DEHP]) and evaluated their performance in extracting Li+ ions in comparison to molecular ligand analogues. The synthesized ionic compounds
demonstrated superior EE compared to their molecular counterparts due to an intrinsic
synergistic effect. Notably, the [N4444][DEHP] compound exhibited the highest EE [125], [126].
Although limited, the extraction of p-block metals from water using ILs is noteworthy.
Post-transition p-block metals including Al, Ga, In, Sn, and Pb exhibit a wide range
of physical and chemical properties. Clio et al. developed a new methodology for the
purification of Indium using cyphos IL 101 and aliquat 336. In3+ ions showed a strong affinity for the IL phase, resulting in extraction percentages
exceeding 95% across the HCl concentration range of 0.5–12 M. An extraction mechanism
was proposed based on the relationship between the viscosity of the IL phase and the
loading with In3+ ions. Indium can be easily recovered as In(OH)₃ through precipitation stripping using
NaOH solution. This new IL-based extraction avoids the use of VOSs [127]. Eguchi et.al utilized 1-alkyl-3-methylimidazolium bis(trifluromethylsulfonyl) Imide
to study IL-chelate-based extraction of group 13 metals. 8-Quinolinol was the chelate
used and they
have varied the alkyl chain length to study the effect. The efficiency of extraction
was maximum with the most hydrophobic IL [128]. Subsequently, Luo et al. [129] investigated the simultaneous leaching and extraction of indium from waste LCDs
using the functionalized IL, betainium bis(trifluoromethylsulfonyl)imide. The IL phase
was transferred into an In-rich solution with the aid of oxalic acid, allowing the
IL to be recovered. The regenerated IL maintained stable properties, making it suitable
for reuse [129]. Recently, many other research groups have focused on IL-based methodologies to
extract p-block elements from acidic solutions as well as from aqueous media [130], [131].
IL-based extraction studies of metals predominantly focus on d-block elements and
heavy metal ions due to their environmental and industrial significance. The mechanisms
underlying these extraction processes are typically driven by preferential coordination
between the functional groups in the IL and the metal ions, which is often governed
by the Hard and Soft Acids and Bases (HSAB) principle. Ion exchange is also cited
as one of the key driving forces in various studies related to the particular process.
In the former, softer acids like transition metals tend to coordinate with softer
bases present in the IL, while harder acids prefer coordination with harder bases.
In ion exchange mechanisms, the metal ions are replaced by ions from the IL, facilitating
the extraction process.
Papaiconomou et al. carried out a study to extract metal ions using task-specific
ILs. The targeted molecules were metal ions like Cu, Hg, Ag, and Pd. They found that
Hg and Cu extraction is more efficient with the use of ILs having disulfide functional
groups, whereas Ag and Pd could be efficiently extracted using ILs with nitrile functional
groups. They have also figured out that the distribution coefficients of metal ions
were higher in IL with pyridinium cations and trifluoromethyl sulfonate than imidazolium
cation and bis[trifluoromethyl]sulfonyl imide [132]. Kogelnig et al. performed a thorough investigation on the extraction of Cd2+. Three hydrophobic ILs have been produced from tricaprylmethylammonium chloride through
reaction with suitable Brønsted acids. Among these, tricaprylmethyl ammonium thiosalicylate
exhibited the highest EE for Cd2+ from both ultrapure and natural river water [133].
Egorov et al. investigated trioctylmethylammonium salicylate as an extractant for
the extraction of transition metal ions Fe3+, Cu2+, Ni2+, and Mn2+. The extraction efficiencies of Fe3+and Cu2+ were 99% and 89%, respectively. In contrast, Ni2+and Mn2+ exhibited lower extraction yields [134]. Subsequently, Rajendran et al. recovered metals, including Ni, Zn, Pb, Fe, and
Cu, from tannery effluents via task-specific ammonium-based ILs [135]. The scientific investigation of extraction of Ag+ and Pb2+ using imidazolium-based ILs by Domanska et al. is worth mentioning, where dithizone
[DTZ] was used as the metal chelator as well as organic extractant. The method adopted
was classical liquid–liquid extraction of metal-DTZ complex and that was a pH-dependent
process. 1 butyl-3-ethylimidazoliumbis [trifluoromethyl sulphonyl]
imide [BEIM][NTf2]− showed an efficiency of 99.3%, which was much greater than conventional organic solvents
like chloroform. Nonetheless, re-extraction was also carried out which indicated that
ILs can be recycled and reused which ensures sustainability [136].
Fetouhi et al. carried out a study on the extraction of heavy metals 1-butyl-3-methylimidazolium
hexafluorophosphate with extracting ligand N-salicylideneaniline. Metals like Cu2+, Co2+, Ni2+, and Pb2+ were extracted. The stoichiometry of these metal complexes with ligands was found
to be 1:2. Cu2+ extraction was found to be independent of pH, while others depend on changes in pH
[137]. Thasneema et al. recently conducted the extraction of hazardous heavy metal ions
from their respective standard solutions. The study utilized metal ions As3+, Cr3+, Cd2+, Cu2+, Zn2+, Pb2+, and Hg2+ together with ILs containing phosphonium cations and hydrophobic anions. UV–visible
spectroscopy and ICP-MS analysis were employed to assess the EE. The extraction performance
was observed to be elevated, and this group of ILs was also
determined to be successful in HMI extraction [138].
Pseudoprotic ILs have lately been investigated for their efficacy in removing heavy
metal ions. Matsumoto et al. broadened the utilization of PPILs comprising trioctylamine
and decanoic acid for the extraction of rare earth elements [139]. Janssen et al. devised a technique for the extraction of heavy metal ions, including
Ni2+, Cu2+, and Co2+, from saline aqueous solution. The PPILs employed in this investigation were trihexylammonium
octanoate, trioctylammonium benzoate, and trioctylammonium salicylate at equimolar
doses. Their investigation revealed that these ILs facilitate the extraction of heavy
metal ions from concentrated sodium chloride brines while minimizing the co-extraction
of sodium ions [140]. ILs with carboxylate anions have recently been explored for the removal of HMIs
[141]. The study was initially restricted to ILs with aliphatic protic carboxylate
anions, which was later extended to both protic and pseudoprotic ILs with cyclic carboxylate
anions [142]. These ILs provide the benefit of adjustable coordination centers, allowing for
customization that is tailored to the specific properties of the targeted heavy metal
ions. Padinhattath et al. recently reported a series of hydrophobic ILs with varying
coordinating atoms in their anions for the extractive removal of toxic heavy metal
ions from wastewater. The reported ILs were able to extract the metal ions even from
their mixtures, mimicking real industrial conditions [143]. These findings further validate the potential of tailored hydrophobic ILs as efficient
and selective extractants for heavy metal ion removal in complex wastewater matrices.
