Recent advances of CO2-responsive materials in separations

Journal of CO₂ Utilization 30 (2019) 79–99

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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou

Review Article

Recent advances of CO2-responsive materials in separations Ziqi Yang a b c

a,b

a

a,b,c

, Changqing He , Hong Sui

, Lin He

a,b,c,⁎

, Xingang Li

T

a,b,c

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China National Engineering Research Centre for Distillation Technology, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 300072, China

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2-responsive Oil-solid separation Oil-water separation CO2 capture Recycling

Separation is widely involved in many fields, such as materials synthesis, gas purification or removal, water treatment, petroleum exploitation, soil remediation, food and medicine production, etc. Compared with the physical separation, the chemical separation has unique advantages when the functionalized process aids are added in systems. The CO2-responsive materials are considered as promising separating mediums due to their relatively green and easily recyclable properties, which realize the resource utilization of CO2. Up to date, the reported CO2-responsive materials are mainly referred to switchable-hydrophilicity solvents, switchable surfactants, switchable water additives, switchable ionic liquids and CO2-responsive polymers. These CO2-responsive materials can be switched by CO2 trigger at moderate conditions, showing switchable properties, such as hydrophilicity-hydrophobicity conversion, switching “on” or “off” surface-activity and switchable change in ionic strength. These switchable properties can be applied in separations, especially in oil-solid separation, oilwater separation, wastewater treatment and acid gas capture. In this review, the recent advances of CO2-responsive materials in separation fields above have been overviewed. Additionally, the present challenges and future engineering considerations have been touched to provide potential insights and theoretical fundamentals for developing CO2-responsive materials.

1. Introduction

(VOCs) are discharged into air, leading to heavy air pollution [8–11]. The oil/petroleum leakage into soil, during oil production, transportation and storage, brings severe soil pollution [12–14]. The wastewater produced from industry contains metal or organic contaminants, which affect the ecological environment and human health [15]. Either exploiting the alternative energy or remediating the environment requires efficient separation technologies (e.g., extraction, absorption, adsorption, etc.). Some of them require the use of functionalized process chemicals such as organic solvents, surfactants, ionic liquids and porous materials [4,5,8,16–18]. Although these chemicals perform well in separations, some challenges are still undergoing: (i) An unavoidable solvent volatilization in washing, extraction, etc. (ii) A high energy consumption by distillation for the recovery of liquid chemicals. (iii) A

Energy crisis and environmental pollution are two major problems during the modern industrialization [1–3]. The increasing demand for energy causes a strain on energy supply, especially for the conventional petroleum resources. To relieve energy crisis, some countries tried to exploit the emerging sources of energy fuels such as unconventional oils (e.g., oil sand, oil shale, etc.) and renewable energies (e.g., biofuels, solar energy, wind power, etc.) [1,4–7]. Additionally, the industrial production and frequent human activities lead to environmental pollutions to the earth, such as air pollution, water pollution and soil contamination. For example, millions of tons of waste inorganic compounds (e.g., NOx, SO2, H2S, CO2, HCl, HF, NH3, etc.) and volatile organic compounds

Abbreviations: SHSs, Switchable-hydrophilicity solvents; VOCs, Volatile organic compounds; Kow, The octanol–water partition coefficient; pKaH, The strength of conjugate acids; LD50, Median lethal dose; ILs, Ionic liquids; DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene; DEAEMA, Diethyl-amino-ethyl methacrylate; DMAEMA, 2(dimethylamino)ethyl methacrylate; CNC, Cellulose nanocrystal; FAEEs, Fatty acid ethyl esters; HPLC, High Performance Liquid Chromatography; MCHA, N-methylcyclohexylamine; SEM, Scanning electron microscope; DMCHA, N,N-dimethylcyclohexylamine; RT, Room temperature; EBA, N-ethylbutylamine; DPA, N-dipropylamine; TEA, Triethylamine; TEPDA, N, N, N′, N′-tetraethyl-1, 3-propanediamine; TritonX-100, Octylphenylpolyethylene glycol; SDS, Sodium dodecyl sulfate; DUSNa, 11-dimethylaminoundecyl sulfate sodium salt; SDBS, Sodium dodecyl benzene sulfonate; DMEA, N, N-dimethyl ethanolamine; DMDOA, N,N-dimethyl dodecyl amine; DOAPA, N,N-dimethyl oleoaminde-propylamine; TMPDA, N, N, N′, N′-tetramethylpropanediamine; DMBUA, N,N-dimethylbutyl amine; DMHEA, N,N-dimethyl hexadecyl amine; OA, Oleic acid; DSA, Dodecyl seleninic acid; TMEDA, N,N,N’,N’-tetramethyl-1,2-ethylenediamine; LPME, Liquid phase microextraction; PAN, 1-(2-pyridylazo)-2 naphthol; APDC, Ammonium pyrrolidinedithiocarbamate; TEPA, Tetraethylenepentamine; AMP, 2-amino-2-methyl-1-propanol ⁎ Corresponding author. E-mail address: [email protected] (L. He). https://doi.org/10.1016/j.jcou.2019.01.004 Received 1 November 2018; Received in revised form 22 December 2018; Accepted 14 January 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Families of CO2-responsive materials.

molecular design, synthetic routes and self-assembly. However, a systematic summary and evaluation of recent studies on using CO2-responsive materials in separations are not yet reviewed. With the increasing development of CO2-responsive materials, some specific advantages and common problems have arisen when using these materials in separations, which should be considered to guide the design and applications of these smart materials in turn. Herein, from an engineering point of view, our aim is to provide an overview on recent development of CO2-responsive materials in separations, as shown in Fig. 2. Different types of CO2-responsive compounds will be briefly overviewed. More emphasis is placed on the applications of CO2-responsive materials in oil-solid separation and oil-water separation. Other aspects such as metal ions separation from wastewater, preconcentration of trace contaminants and acid gas capture are also summarized and discussed in this review. In addition, some challenges and emerging prospects based on specific characteristics and common problems of materials, especially the engineering suggestions, are proposed for future development of CO2-responsive materials. We hope that this review could help researchers select new materials, extend the applications of CO2-responsive materials in more fields and provide new insights for the utilization of CO2 in oil (e.g., petroleum, bio-oil, etc.) exploitation, gas separation and water treatment.

low separating efficiency when using conventional materials, such as aqueous alkanolamine for CO2 absorption. (iv) A potentially secondary pollution due to the addition of process aids, such as salts, surfactants, etc. To alleviate the problems above, one strategy is to design and exploit novel materials with excellent separating efficiency, reversible recovery process and eco-friendly properties. The CO2-responsive materials, as smart systems, have attracted much attention during the past decades [19–23]. These novel compounds can be triggered by CO2 with different functions, such as hydrophilicity-hydrophobicity conversion [24,25], switching “on” or “off” surface-activity [26,27], switchable change in fluorescent [28] and dispersion [29–31]. Using CO2 trigger has unique advantages. The transition process could be conducted in a mild and green condition without additional chemicals (e.g., acid, base, oxidants, etc.) and potentially second pollution. Moreover, CO2 is a cheap, non-toxic and sustainable trigger. The exploitation of CO2-responsive materials also provides a rational utilization of this greenhouse gas. The examples of CO2-responsive materials are shown in Fig. 1. The first CO2-responsive materials was a type of CO2-responsive latex resins, synthesized in 1986 [32]. Subsequently, Jessop’s group reported that ionic liquids (ILs) had switchable polarity in the absence and presence of CO2 [20]. Other CO2-responsive compounds such as switchable surfactants, switchable-hydrophilic solvents and switchable water additives have been successively synthesized, extending the families of CO2-responsive materials [24,25,33,34]. By designing these novel materials, they have dual roles or dual functions in one system, forming a green cycle and showing advantages in separation fields. Some reviews have been published related to CO2-responsive materials [19,35–37]. In 2012, Jessop et al. [35] reviewed the present examples of CO2-responsive materials based on their properties. Later, Lin et al. [36], Darabi et al. [19] and Liu et al. [37] reviewed the development of CO2-responsive polymers, with emphasis on the

2. CO2-responsive materials Different types of CO2-responsive materials have different structures, properties and reactions with CO2. Some examples of CO2-responsive materials are summarized in Table 1. 2.1. Switchable-hydrophilicity solvents The switchable-hydrophilicity solvents (SHSs) are a kind of organic solvents with switchable miscibility between hydrophilicity and 80

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Fig. 2. Major CO2-responsive materials and their applications in separations.

dependent on external parameters, such as pressure, temperature and the addition of water volume [23,39,40]. Additionally, the design of eco-friendly SHSs with low toxicity is a key point before industrialization. Although almost all of the SHSs have much higher boiling points and flashing points, SHSs are considered more hazardous than traditional hydrocarbon solvents (e.g., toluene and n-hexane), in terms of eutrophication potential and other toxicity index (e.g., LD50) [23]. Some researchers suggested that one approach for designing eco-friendly SHSs is to introduce low-toxicity functional groups into their structures [23,42]. However, little knowledge is reported on the detailed effect of functional groups and chain structures.

hydrophobicity [23,38–41]. These novel solvents are mainly nitrogencontaining compounds, including tertiary amines [23,25,42], secondary amines [23] and amidines [23,24]. Generally, they possess high boiling points meaning less volatility [23,35]. Due to the presence of amino groups, this type of chemicals could react with CO2, switching between water-immiscible state and water-miscible salt. This switchablility suggests its potential in energy saving compared with traditional distillation. The synthesis process and reaction rate of SHSs with CO2 is highly dependent on the structures of SHSs. The amidine-structured SHSs usually require moderate reaction rates with CO2, but their synthetic routes are relatively complex [24,25]. The synthetic route of tertiary amine-structured SHSs is much easier than that of amidines [23,25]. Due to the simple structures of tertiary amines, they are regarded as predominant SHSs in separations. Generally, tertiary amines could form bicarbonate salts when bubbling CO2 for about 20–120 min under an ambient condition. Secondary amines could react with CO2 in a faster rate (within 20 min) due to the formation of ammonium carbamate salts. However, the reversed reaction becomes much more difficult and more energy (heating to over 80 ℃) should be input to break down the stabilized carbamate salts [23]. The sterically hindered secondary amines could prevent the formation of stabilized carbamate structures [23,43]. Several secondary amines, containing sec-butyl or isopropyl groups, form bicarbonates rather than carbamate salts when bubbling CO2 within 10 min. The formation of bicarbonate salts, in turn, allow easily reversed reaction, decreasing the heating temperature to 65℃. Some principles are proposed for designing and selecting the ideal SHSs. Generally, the log Kow of SHSs is located within 1.0–2.5 to ensure switchable phase behaviors [23]. The pKaH values of SHSs at room temperature should be from 9.5 to 11 and the C/N ratio should be from 6:1 to 12:1 to ensure switchable reactions [23,44]. In addition to the internal factors above, the switchable behavior of SHSs is highly

2.2. Switchable surfactants The switchable surfactants could switch “on” or “off” their surfaceactive properties triggered by CO2 [35]. These surfactants could adjust emulsions between emulsification and demulsification. In this way, the surfactants are recycled directly for further use, deceasing water pollution. Basically, the switchable surfactants are classified into cationic and anionic surfactants [27,33,45]. The switchable anionic surfactants, containing long-chain phenolates and carboxylates, are surface-active in the absence of CO2 [27,35]. The switchable cationic surfactants are generally long-chain nitrogen compounds including amidines, guanidines, tertiary amines and imidazoles [19,21,35,45–47]. The surfaceactive behavior of switchable cationic surfactants is opposite to the switchable anionic surfactants. Nevertheless, both switchable anionic and cationic surfactants are surface-active (switching on) in their ionic form and can be altered to neutral form (switching off) by CO2 trigger. 2.3. Switchable water additives The switchable water additives refer to the materials that could 81

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Table 1 Some examples of CO2-responsive materials with different properties, structures and reactions with CO2. Name

Switchable properties

Structures

Switchable-Hydrophilicity Solvents

Miscibility

Amidines

Reactions with CO2

Secondary amines

Tertiary amines Switchable Water Additives

Ionic strength

Polyamines

Switchable Ionic Liquids

Polarity

Amidines/alcohols

Alkanolguanidines

Switchable Cationic/Anionic Surfactants

Surfactivity

Amidines

Imidazoles Tertiary amines Phenolates

Carboxylates CO2-responsive polymers

Fluorescent

important type of switchable ILs is the switchable polarity ILs, which can switch between molecular liquid state and ionic state [35,50]. These switchable ILs behave as low-polarity liquids in the molecular state and become high-polarity ILs after bubbling CO2. Generally, these switchable ILs are made of two-component or single-component systems such as amidine/alcohol [20], guanidine/alcohol [52], amidine/ amine [53], alkanolguanidine [54], secondary amine [55] and primary amine [56]. The two-component system (e.g., alcohol/DBU) usually is water-sensitive because it would thermodynamically prioritize to form solid bicarbonate salts with water rather than alkylcarbonate salts with alcohol [35]. In most cases, forming solid bicarbonate salt is not desirable because it will undermine the recyclability of switchable ILs [57]. Therefore, these alcohols associated with switchable ILs are suitable for non-aqueous extraction with a low polarity. In addition, the evaporability of alcohols in the process may influence the switchable performance and narrow the practical application of two-component switchable ILs. The single-component switchable ILs, which are made through the combination of base and alcohol structures into one molecular, may avoid this problem. The combination of two structures increases the molecular weight of switchable ILs (meaning lower volatility), but leads to the high viscosity of ILs. In addition to the switchable polarity, ILs with other switchable properties are developed, such as switchable basicity, switchable miscibility like SHSs (e.g., [C4DIPA][Im] and [P4444][Tf-Leu]) and switchable structural transitions (e.g., [C16MDEA][2-Pyr]) [58–61].

switch between low and high ionic strength [34]. These novel additives are generally amine-structured compounds with excellent water solubility. In the absence of CO2, the non-ionic state of switchable water additives only behave as water-soluble organic compounds and cannot change the solubility of solute in aqueous solution. When bubbling CO2 into aqueous solution, they could react with the carbonic acid, forming ionic state (i.e., salt form). With the growth of ionic strength, these additives behave as a “switching on” salting-out effect. Therefore, by simply controlling the absence and presence of CO2, the switchable water additives can be applied to separate compounds from aqueous solution and be reused in further recycle. Generally, the ionic strength of solution is highly dependent on the number of protonated sites in additives [22]. Increasing the number of nitrogen atoms of switchable water additives could improve the ionic strength of solutions, suggesting a much more complete salting-out process. Based on the principles above, some new polyamines have been designed and synthesized [22]. Other designing principles of switchable water additives, such as the selection of suitable pKaH values and C/N ratios, could be found elsewhere [22,23]. 2.4. Switchable ionic liquids The switchable ionic liquids are a new type of functional ionic liquids (ILs) with CO2-responsive properties. The switchable property of ILs can be used in the recovery of ILs in an easy way, showing wider applications in extraction and materials synthesis [48–51]. One 82

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optimized [79]. However, due to the consumption of ethanol in transesterification, the stoichiometric balance between DBU and ethanol will be disrupted, decreasing the recyclability of DBU/ethanol [50,78]. Recently, some examples of monoamine-structured and diamine-structured SHSs have been proved to extract lipids from soybeans [80]. It was interesting to note that the diamine-structured SHSs remained less in soybean oil (with concentration of 1 g/L) than the monoamines (with concentration of 20 g/L). This is attributed to the unique structure of diamines allowing more complete reaction with CO2, though it usually requires much more bubbling time.

2.5. CO2-responsive polymers CO2-responsive polymers have switchable properties including changes in fluorescent [28], sol–gel state [62,63], vesicular state [64,65] and dispersion [29–31]. Generally, the selection of suitable monomers with CO2-responsive properties are critical to polymerizations. The diethyl-amino-ethyl methacrylate (DEAEMA) and the 2-(dimethylamino)ethyl methacrylate (DMAEMA) are main CO2-switchable monomers because their pKa values are in a suitable range that can be controlled by CO2 trigger [19]. Some fundamental work have been reviewed elsewhere on the structure, chemical composition, preparation and self-assembly of these CO2-responsive polymers [19,36,37]. The recent advances in CO2-responsive polymers could be found in the relevant literatures [31,66–69].

3.1.1.2. Algae extraction. Compared with edible feedstocks, algae is considered as an excellent candidate in producing biofuels due to its rapid growth speed, high oil productivity and less land usage [77,81]. However, due to the high water content in algae, a low lipid yield is obtained in extraction when using the hydrophobic organic solvent (e.g., hexane) [82]. One way to increase the lipid yield is to dry algae samples before extraction. However, the drying pretreatment may increase the operational cost. Some researchers also proposed that the application of hydrophilic solvent (e.g., methanol) assisted with hydrophobic solvent could improve the lipid yield [83]. Whereas, a huge energy input is required to recover the mixed solvents from extraction process. To alleviate the problems above, great efforts have been made in algae extraction. Recently, the switchable solvents have been tested for algae extraction, summarized in Table 2. These switchable solvents include switchable ILs [51,84] and SHSs [42,76,85–88]. For example, compared with a traditional solvent (i.e., n-hexane), Samorì et al. [51] observed that the lipid yield was doubled when using DBU/1-octanol for extracting both freeze-dried (16 wt%) and liquid algal samples (8.2 wt%). Due to the water sensitivity of this switchable IL, the DBU may react with water to form undesired solid bicarbonate salts, destroying its recyclability [35,81]. To avoid this problem, Du et al. [88] used a water-insensitive SHS (i.e., N-ethylbutylamine) for algae extraction. After the optimization of operational conditions, they found that the maximum lipid yield (61.3 wt% based on dry algae mass) was obtained after four-stage extractions [126]. Later, Yang et al. [82] used an amphiphilic amine (i.e., N-methylcyclohexylamine, MCHA) to directly extract a wet algal slurry (containing 85% water) and obtained a high lipid yield of 85% at 200 rpm within 1 h. This is because the amphiphilic MCHA increases the interaction between water and lipid, accelerating the lipid to dissolve in the MCHA [82]. To recover this amphiphilic amine from mixtures, a hydrophobic IL named [C4-mim] [PF6] was added to extract MCHA from water (recovery rate at 66%), as shown in Fig. 4. However, during the recovery of MCHA, some of the lipids and the MCHA are still entrained in the IL. To confirm whether the application of switchable solvents is potential energy-saving, an energy evaluation in switchable solvent extraction was made by Du et al. [81]. Their results showed that, compared with traditional methods (e.g., n-hexane extraction and supercritical CO2 extraction), switchable solvent extraction could save 50–70% of energy during the extraction of lipids from algae, as shown

3. Separating applications using CO2-responsive materials 3.1. Oil-solid separation Oil-solid separation is a common topic in many fields, including the unconventional petroleum exploitation, oil-contaminated soil remediation, bio-oil extraction, food industry, material synthesis and cleaning, etc. [4,70–73]. Because of the wide adaptability and high efficiency, solvent extraction has been considered a promising choice for oil-solid separation. Distillation is a common method for the recovery of solvent from mixtures [74]. However, an unavoidable problem is the high energy input for evaporating and condensing materials, especially for obtaining a high purity of products [57,75]. Although the processing is enclosed, the volatile property of traditional solvents (e.g., toluene, n-hexane, etc.) still leads to the loss of solvents to the environment [8]. To reduce the operational energy consumption and solvent evaporation, some primary attempts have been made by applying the SHSs and switchable ILs in oil-solid separation, based on two oil sources, bio-oils and fossil oils. 3.1.1. Bio-oils and solid separation 3.1.1.1. Soybean oil extraction. As a type of renewable energy, biofuels have aroused worldwide attention [76]. During the production of biofuels, the lipid extraction and transesterification are two important steps [6]. Soybean oil, the first-generation of liquid biofuels, is generally extracted from edible feedstocks using organic solvents followed by catalytic conversion to fatty acid ethyl esters (FAEEs) [77]. To avoid the high energy consumption during solvent recycling, the DBU/ethanol was used to extract soybean oil from soybean flakes [24,78]. However, the recovery rate of using such switchable IL was only 45 wt% at 25 ℃ for 60 min [78]. Further increasing temperature even decreased the oil recovery rate. The HPLC analysis suggested that the transesterification occurred during extraction, resulting in lower oil recovery rate [78]. This is ascribed to the presence of ethanol in the system, in which the DBU plays two roles: as solvent in extraction and as catalyst in transesterification, as shown in Fig. 3 [50,79]. The kinetics of DBU-catalyzed transesterification was investigated and

Fig. 3. The transesterification of soybean oil (TG) and ethanol with DBU. Reproduced from Ref. [79] with permission of the Royal Society of Chemistry. 83

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Table 2 Extraction of lipids from algae using switchable solvents. No.

System

1 2 3 4 5 6 7 8 9 10

Note:

Two components Two components Two components One component One component One component One component One component One component One component a

Solvent

DBU/octanol DBU/octanol MCHA/[C4-mim][PF6] DMCHA DMCHA DMCHA DMCHA EBA DPA EBA

Algae species

Botryococcus braunii Botryococcus braunii Docosahexaenoic acid algae Botryococcus braunii Nannochloropsis gaditana Tetraselmis suecica Desmodesmus communis Desmodesmus sp. Desmodesmus sp. Neochloris oleoabundans

Expressed on the basis of algal dry weight.

b

Algae conditions

Freeze-dried samples Suspended algal Wet slurries with 85% water Freeze-dried samples Wet samples with 80% water Wet samples with 80% water Wet samples with 80% water Non-broken algae with 95% water Non-broken algae with 95% water Fresh non-broken algae

Extraction conditions Time (h)

Temperature (℃)

4 24 1 18 24 24 24 24 24 18

60 RT RT 80 RT RT RT RT RT RT

Lipid yield (wt%)a

Ref.

16 8.2 85 22 57.9 31.9 29.2 16.8 15.4 61.3b

[51] [51] [82] [76] [85] [85] [85] [87] [87] [88]

After four-stage extractions.

switchable solvent (i.e., DMCHA) could disrupt the thick cell walls of microalgae without additional cell disruption. The wet microalgae samples are directly used in extraction process, requiring no drying pretreatment. Results showed that the maximum extraction yield of using DMCHA was 87.2%. 3.1.2. Fossil oils and solid separation Compared with conventional crude oils, fossil oils have much higher polarity, viscosity and more complex compositions [4,73]. In addition, some mineral solids coexist with oils, resulting in much more difficulty in oil-solid separation. 3.1.2.1. Unconventional oil extraction. Unconventional oils, such as oil sands and oil shale, are types of emerging sources of petroleum fuels. Due to their high viscosity and coexistence with mineral solids, traditional water flooding method work poorly in separating oil from unconventional oil ores, especially from the carbonated asphalt rocks [4]. Solvent extraction, therefore, is proposed as a promising method for unconventional oil separation due to its universal applicability, low operating temperature, high extraction rate [17,73]. However, this traditional solvent extraction method is limited to some extent due to the high energy consumption of solvent recovery and solvent loss by volatilization. Recently, the switchable solvents are primarily applied for extraction in place of the conventional solvents. Basically, the switchable solvents act as two roles in extracting unconventional oil from ores: as solvent to dissolve heavy oils and as interfacial modifier to enhance oil-solid separation [17,74,90]. In 2012, Holland et al. [74] proposed the application of SHS (i.e., DMCHA) in extracting bitumen from Canadian oil sands, as shown in Fig. 7. They found that more than 94% of the bitumen could be

Fig. 4. The algal lipid extraction using an amphiphilic amine. Reproduced from Ref. [82] with permission of the Elsevier.

in Fig. 5 [81]. This finding suggests that the switchable solvents are promising candidates for extracting algae from biomass. Recent studies also showed the ability of switchable solvents in extracting astaxanthin from wet microalgae (i.e., Haematococcus pluvialis) [89]. The SEM images in Fig. 6 confirmed that using the

Fig. 5. Energy consumptions for lipid extraction with different extraction methods. Reproduced from Ref. [81] with permission of the Elsevier. 84

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Fig. 6. The SEM images of Haematococcus pluvialis (a) before and (b) after 24 h of extraction using DMCHA. Reproduced from Ref. [89] with permission of the American Chemistry Society.

carbonate asphalt rocks. The one-staged bitumen recovery rate could reach 90% and the two-staged recovery rate could reach 96%. The results indicated that most SHS acted as a solvent in its non-ionic state to soften and dissolve the heavy oil. Meanwhile, a small amount of SHS in its ionic form acted like a surfactant in reducing the oil-water interfacial tension and further enhancing the oil–water separation [17]. Shale oil and shale gas drilling process usually produce oil-based drill cuttings, which contain lots of oil fractions such as diesel [91]. To extract diesel efficiently, Wang et al. [91] screened different types of SHSs based on the CO2 absorption rate and the solubility parameter model. Additionally, they studied the SHS recovery process based on the influence factors such as temperature and gas bubbling rate. The dynamic analysis suggested that the SHS recovery process confirmed to the first-order kinetic. Based on the rate constant, the activation energy of SHS recovery was calculated with 30.37 kJ/mol, which provided a guidance for the optimization of energy consumption in SHS fields. Different from the dissolution effect of solvents, Duan et al. [92] used switchable water additive solutions to separate diesel from oily cuttings. In this process, the role of switchable water additives (a kind of

recovered from the oil sand ores. However, about 16 wt% of DMCHA were entrained in the recovered water and 12 wt% of DMCHA were entrained in the bitumen product [74]. This is because the protonation and deprotonation of DMCHA are not sufficient, resulting in an insufficient separation. Additionally, the viscous bitumen increases the separating difficulty between bitumen and DMCHA. Different from the dissolution role, the SHS in its hydrophilic form serves as a process aid by interfacial modification [90]. Sui et al. [90] protonated the SHS into its ionic state, forming a homogenous SHS solution. This solution worked in conjunction with organic solvents (e.g., toluene) to extract bitumen from Canadian oil sands. As shown in Fig. 8, the SHS (e.g., trimethylamine, TEA) in its protonated form acted as a surfactant in replacing the bitumen components on mineral solid surfaces and thereby improving the bitumen recovery to 98% [90]. Later, on basis of the principles above, Li et al. [17] combined the two roles of SHSs (i.e., dissolution effect and interfacial modification) in one extraction process using a diamine-structured SHS (i.e., N, N, N′, N′tetraethyl-1, 3-propanediamine, TEPDA), as shown in Fig. 9. They developed a SHS-water hybrid process to separate bitumen from Indonesia

Fig. 7. The application of DMCHA in extracting bitumen from Canadian oil sands. 85

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Fig. 8. The mechanism of the protonated SHS in enhancing bitumen liberation from solid surface. Adapted from Ref. [90] with permission of the Elsevier.

remediation and then screen qualified materials.

alkaline compounds) is proved to activate the naturally surface-active acids (e.g., naphthenic acids) in diesel, helping them become indigenous surfactants in samples. The oil-solid separation is enhanced with the help of stable oil-water emulsion.

3.2. Oil-water separation Oil-water separation is commonly involved in different fields, such as petroleum production, food industry, material synthesis and medicine production [21,107,108]. Quantities of methods have been proposed such as membrane separation, adsorption and chemical demulsification [106,109,110]. Herein, our focus is on the application of CO2-responsive polymer membranes and functionalized demulsifiers for oil-water separation.

3.1.2.2. Soil remediation. Soil contamination occurs in most countries including the USA, the United Kingdom, Japan and China, etc. [12,93–96]. The organic-contaminated soil is one of the major polluted soil, which lead to serious risk to the environment and human health [12]. During the past decades, great attempts have been made to exploit advanced technologies and chemicals for soil remediation, such as bioventing, thermal desorption, leaching, microwave frequency heating [12,27,97–100]. Soil leaching is a common method on the basis of washing liquids, such as surfactant aqueous solutions and solvents [101–104]. The cationic surfactant solutions perform poorly in soil washing due to the adsorption of cationic surfactants on the anionic sites in soil [105]. Recently, several switchable anionic surfactants have been proposed for the remediation of oil-contaminated silica sand. The results showed that one of the phenolate salt surfactants in the absence of CO2 performed better in removing light crude oil (82%) than conventional surfactants, such as Triton X-100 (68%) and SDS (40%) [27]. Xu et al. [106] also found that one of the sulfate salt surfactant (i.e., 11-dimethylaminoundecyl sulfate sodium salt, DUSNa) worked well in soil washing with only 3.2% of mineral oil (D80) entrained on residual sands. However, few attention was paid on the comparison of sand type or oil components for affecting the soil remediation and selecting the surfactant candidates. For example, when removing extremely complex heavy oils from carbonate sands, things would be totally different. Therefore, future studies should consider comprehensive factors in affecting soil

3.2.1. Oil-water mixture separation Because of the difference in hydrophility between oil and water, the oil-water mixture can be separated by super-wettability membranes [111–113]. Among these membrane materials, the CO2-responsive polymer membranes can reversibly change their wettability. With this property, these membranes can be applied in two types of oil-water separation with oil density higher or lower than water density [109,114,115]. As shown in Fig. 10, the PDEA-based membrane was hydrophobic in its initial state and oil molecules (higher density than water) could selectively penetrate the membrane [114,115]. In the presence of CO2, the membrane switched from hydrophobicity to hydrophilicity due to the protonation of DEA components and water (higher density than oil) could penetrate the membrane [114,115]. In the absence of CO2, the membrane could switch back to hydrophobicity. Lei et al. [114] observed that the smart polymer with 30 wt% of DEA components obtained a high efficiency in both chloroform/water and water/hexane separation (> 96 wt%). This DEA-based membrane showed a good

Fig. 9. The schematics of the SHS-water hybrid process in separating bitumen from Indonesia carbonate asphalt rocks. Adapted from Ref. [17] with permission of the Elsevier. 86

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Fig. 10. Switchable wettability of poly(St-co-DEA)-HIPE membrane in oil/water separation. Reproduced from Ref. [114] with permission of the American Chemistry Society.

separating capacity in the gravity-driven oil–water separation. Recently, Shirin-Abadi et al. [116] found that electric potential could replace the CO2 trigger to adjust the hydrophilicity and hydrophobicity of membrane materials in oil-water separation. They synthesized the PMMA-co-PDEAEMA for fabricating CO2-responsive membrane. It was interesting to note that when putting the membrane at cathode or anode, the materials could switch from hydrophilic to hydrophobic state or hydrophobic to hydrophilic state, respectively. The conversion efficiency was much faster than traditional bubbling CO2 or N2. This multi-trigged membrane would widen the applications in water treatment. 3.2.2. Destabilization of emulsions Generally, oil-water emulsions could be stabilized by surface-active materials, such as chemical surfactants, nanoparticles and amphiphilic mineral particles, at a specific pH and ion strength [4]. In some cases, the emulsions should be broken with the addition of demulsifiers or polymers, which increase the wastewater treatment loading. To alleviate the tailings, great efforts have been made to synthesize novel surfactants which could combine the two steps above (emulsification and demulsification) by one single chemical. Because CO2-responsive materials could switch between two different states, researchers try to establish switchable emulsion using these smart chemicals. In this review, different methods will be discussed on how to establish switchable emulsion and how to break the surfactant-stabilized emulsion.

Fig. 11. The schematics of emulsification and demulsification with the sulfate surfactant (DUSNa). Reproduced from Ref. [106] with permission of the American Chemistry Society.

CO2 [21]. This is ascribed to the presence of naturally surface-active materials (e.g., asphaltenes, naphthenic acids) in heavy oils that influence the behaviors of switchable materials [21]. As mentioned before, the acidic components could help to stabilize the heavy oil-water emulsion synergistically with the alkaline compounds (e.g., amidinestructured surfactants). Therefore, the stable O/W emulsion was formed in the beginning, as shown in Fig. 12. Once bubbling CO2, these indigenous surfactants were deprotonated and deactivated to their initial states, resulting in oil and water separation. However, the aqueous solutions after bubbling CO2 still contained some residual crude oils (1 wt% of oil in water). Similar studies related to the interaction between acid components in oil samples and alkaline switchable materials were confirmed by Liu et al. [117] and Duan et al. [92]. In Liu’s work, a small amount of switchable solvents (i.e., DMCHA) formed stable O/W emulsion [117]. Bubbling CO2 in this low oil content sample showed a high demulsification efficiency (approximately 96–97%). However, for a high oil content sample, the emulsion could not be completely broken by CO2 with the demulsification efficiency of only 56.29%. The authors believed that a large amount of oil could cover the upper layer and thereby increased the difficulty for CO2 to escape from the system. In a recent work, the graft-modified cellulose nanocrystals (i.e., CNC-g-P(DMAPMAm-co-S) and CNC-g-P(DEAEMA-co-S)) were reported as pickering emulsifiers in controlling emulsification and demulsification process, as shown in Fig. 13 [118]. The amphiphilic modified CNCs firstly accumulated at the oil–water interface to stabilize the oil-water emulsion. When bubbling CO2, the polymer chains of the modified CNCs transformed into hydrophilic forms and the modified CNCs shifted into aqueous solution. This transformation of CNCs broke the stable emulsion. The emulsification and demulsification were reversible

3.2.2.1. Directly using switchable surfactants or other switchable emulsifiers. The switchable anionic surfactants (i.e., the phenolate salt and sulfate salt) could be applied in soil remediation, obtaining a high oil-removal efficiency from solids [27,106]. After oil-solid separation, the washed emulsions containing switchable surfactants should be demulsified to separate oil from aqueous solution. Ceschia et al. [27] found that, when the emulsion was bubbled CO2 at room temperature for 10 min, the deactivated phenol acid generated, separating 55–81% of oil from the aqueous solution. Increasing temperature to 50 ℃, the oil-removal efficiency was significantly increased up to 95%, similar to that of using traditional SDS (98%) with salts as demulsifier. [27]. The emulsion stabilized by the sulfate salt surfactant (i.e., DUSNa) could also be broken with the help of CO2 trigger due to the formation of the precipitated DUS, as shown in Fig. 11 [106]. Almost 90.8–95.2% of DUSNa were recovered for recycling and 92.1–94.1% of oil were retrieved after emulsion breaking [106]. Similarly, using switchable cationic surfactants could form switchable emulsion. Liu et al. [33] used the amidine-structured cationic surfactant to stabilize light crude oil emulsions in the presence of CO2. Once CO2 was removed from the system, the cationic surfactant switched back to a non-ionic demulsifier. Subsequently, the stable emulsion was separated into two layers within 30 min [33]. However, different things occur in heavy oils. Liang et al. found that the tested amidines performed poorly in forming stable O/W emulsions in the presence of 87

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Fig. 12. The schematic illustration of emulsification and demulsification in a crude oil sample (containing indigenous surfactants, HA) with the addition of amidine (B). Reproduced from Ref. [21] with permission of the American Chemistry Society.

It is well-known that the increase of ionic strength leads to the destabilization of most surfactant-stabilized emulsions [120,124,125]. When bubbling CO2, the switchable water additives (i.e., N, N-dimethyl ethanolamine, DMEA) or the SHSs (i.e., DMCHA) transform to their protonated forms, leading to the increase of ionic strength in system [120,124]. The increased ionic strength condenses the diffuse electrical double layer of emulsion particles, resulting in the reduced repulsion force between each particles [92]. Therefore, the stable emulsion could be broken, as shown in Fig. 14 [120]. Compared with using switchable water additives, the advantage of using SHSs is their easier separation from water phase after being used. Whereas, the switchable water additives will still dissolve in water phase and cannot be separated easily [120].

and recyclable with the help of CO2 and N2. The CNC based emulsifiers could be easily recovered through centrifugation, making it potential candidates for oil recovery. Additionally, a CO2-pickering emulsifier based on hydrophilic silica nanoparticles and a switchable surfactant (myristylamidopropyl amine oxide, C14PAO) was reported by Zhang et al. [119]. In this work, the ionic form of surfactant modified the surface property of silica nanoparticles through an electrostatic attraction [119]. More importantly, this emulsification or demulsification process could be simply triggered after bubbling CO2 for 5 min at 30 ℃ or bubbling N2 for 30 min at 30 ℃, respectively, much milder than most surfactant-stabilized process. 3.2.2.2. Association of conventional surfactants and CO2-responsive materials. Due to the costly synthesis and unknown performance in practical applications, the direct applications of switchable surfactants are still limited at present. The conventional surfactants are chosen as alternatives and applied synergistically with CO2-responsive compounds, such as switchable solvents [120], long-chain amines [121–123] and switchable water additives [124]. Recent studies related to conventional surfactant-stabilized switchable emulsions were summarized in Table 3. Due to the presence of conventional surfactants (e.g., SDS, SDBS), the system could form stable emulsions before bubbling CO2. This emulsion could be demulsified when the CO2 was injected into system. According to the presence of different types of CO2-responsive materials, several explanations of demulsification mechanisms were proposed as follows:

• Decreasing concentration of surfactants The decreased concentration of surfactants in system could demulsify the stable emulsions. It was found that, after bubbling CO2, the protonated form of CO2-responsive materials could bind with the anionic SDS or anionic SDBS (e.g., C12H25SO4−) through an electrostatic interaction [121–123]. This interaction may form oil-soluble or watersoluble compounds, as shown in Fig. 15, leading to the consumption of surfactants (e.g., SDS and SDBS). Consequently, the decreased concentration of single surfactants result in the increase of oil–water interfacial tension [121–123]. The emulsions become less stable or even be broken, forming a phase separation. Recently, Dai et al. [126] studied the effects of alkane carbon numbers of CO2-responsive compounds on emulsification and

(i) Increasing ionic strength

Fig. 13. The photographs and schematics of the emulsion stabilized by either CNC-g-P(DMAPMAm-co-S) or CNC-g-P(DEAEMA-co-S) after bubbling N2 or bubbling CO2. Reproduced from Ref. [118] with permission of the Royal Society of Chemistry. 88

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[124] [122] [121] [120] [123] [126] [126]

Ref.

3.2.2.3. Forming CO2-responsive surfactants through an interaction between acid and base. The interaction between particular acid and base could generate surface-active compounds, which stabilize the emulsions. Once bubbling CO2, these compounds would dissociate, leading to the phase separation. Xu et al. [26] synthesized a CO2responsive gemini superamphiphile (D-OA) by assembling Jeffamine D230 with oleic acid (OA) through an electrostatic interaction [26]. This superamphiphile could stabilize the emulsion over two weeks. As bubbling CO2 for 20 s, the OA− switched from ion to molecular state (i.e., HOA), leading to the dissociation of D-OA (Fig. 18(a)). Because of the absence of this surface-active superamphiphile, the phase separation appeared. When the oil phase was changed into paraffin oil diluted crude oils, the emulsion was still stable without water separation after 24 h, as shown in Fig. 18(b). As bubbling CO2 for only 10 s, the phase separation appeared and completed after 24 h, as shown in Fig. 18(c). This finding suggests that the superamphiphile could form the CO2-responsive emulsions with crude oil samples. Similarly, Fan et al. [127] developed a stable O/W emulsion with the help of an assembled CO2-responsive anionic surfactant (DSATMEDA). The DSA-TMEDA was combined with dodecyl seleninic acid (DSA) and N,N,N’,N’-tetramethyl-1,2-ethylenediamine (TMEDA) through an electrostatic interaction (Fig. 19). Using CO2 trigger, the DSA–TMEDA was dissociated to form the precipitated DSA and the water-soluble TMEDA bicarbonate, without any residues in oil phase. Therefore, the recovery and recycling of such surfactant after emulsion breaking tend to be much easier than those of oil-soluble surfactants.

Stable > 36h Stable Stable / Stable / Not stable 65 50 50 / 50 50 50 Stable > 36h Stable Stable Stable > 24 h Stable Stable Stable

3.3. Metal ions separation from wastewater Heavy metals ions, such as Cd2+, Cu2+ and Ni2+, are frequently involved in industrial water, producing large amount of wastewater. Although sometimes their concentrations are not too high, they could exert significant influence on the ecological environment and human physiological health when being discharged [128,129]. Particularly, heavy metals cannot be biodegraded from water and tend to accumulate in organisms and environments [128]. Some newly developed methods, such as precipitation, adsorption and ion exchange, have been proposed for the removal of metal contaminants from wastewater [130–132]. Adsorption is an efficient method for wastewater treatment because of its high separating efficiency and potential recyclability of adsorbents [128]. Polymers with PDMAEMA or PDEAEMA components can form chelate bonds with some metal ions, such as Cu2+ and Ni2+ [128,133,134]. Additionally, these monomers (e.g., DMAEMA) also

1 2 3 4 5 6 7

SDS SDS and n-butanol SDS and n-butanol SDBS SDS and n-butanol SDBS SDBS

Dodecane n-heptane n-hexane Paraffin oil n-hexane Dodecane Dodecane

DMEA DMDOA DOAPA DMCHA TMPDA DMBUA DMHEA

120 45 15 30 30 30 30

Separation Near-complete separation Complete separation Separation after 24 h Separation Not complete separation Separation

1 4 3 / 3 5 5

After Before Surfactant

Oil phase

Additive

Bubble time (min)

After

Bubble time (h)

Heat (℃)

Increasing ionic strength Decreasing SDS content Decreasing SDS content Increasing ionic strength Decreasing SDS content Decreasing SDBS content /

Mechanisms of demulsification Bubbling N2/ Heat Bubbling CO2 Compositions of emulsions No.

Table 3 Switchable emulsions associated with conventional surfactants and CO2-responsive materials.

demulsification. As shown in Fig. 16, 5 types of tertiary amines with different alkane carbon numbers (i.e., 4, 8, 10, 12 and 16, respectively) were chosen as CO2-responsive additives associated with SDBS for developing switchable emulsions. The stabilization of emulsions increased with increasing carbon numbers of tertiary amines. The most stable emulsion was found when the carbon numbers was 12. The demulsification also associated with the carbon numbers of tertiary amines. Generally, amines with longer alkyl chains showed a much easier phase separation because of their stronger affinity with oil molecules. The N,N-dimethyl hexadecyl amine (DMHEA) with 16 alkane carbons took 20 s to achieve a phase separation. Using the N,N-dimethyl dodecyl amine (DMDOA) with 8 alkane carbons required 50 s to achieve the similar result. The N,N-dimethylbutyl amine (DMBUA) with only 4 alkane carbons could not achieve a complete phase separation within 2 h because of its relatively hydrophilicity [126]. Whereas, Liu’s work provided an exception of short-chain tertiary amines. The N,N,N’,N’tetramethyl-1,3-propanediamine (TMPDA) showed not only a good water-solubility but also a rapid demulsification within 30 min, as shown in Fig. 17 [123]. This is primarily attributed to the double cationic state of TMPDA in the presence of CO2, which promotes the formation of certain pseudo-gemini structures associated with SDS (i.e., SDS-TMPDA-SDS) in water phase.

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Fig. 14. (a) The schematic of SHS-assisted emulsification and demulsification in the SDBS-stabilized emulsion; (b) Conductivity and pH of emulsion as a function of CO2 bubbling time. Reproduced from Ref. [120] with permission of the Elsevier.

Pb2+ [135]. Whether the different structures of materials would lead to the selectivity of metals are unknown and require consideration further.

have CO2-responsive property, which helps to release the adsorbed metals in an easy way [135]. Madill et al. [134] prepared a modified chitosan, CTS–g–GMA–PDEAEMA, which could adsorb Ni2+. Unfortunately, the adsorption capacity of this material was out of step with the initial design, slightly lower than that of the unmodified chitosan. However, it was interesting to note that, in the carbonated water, the adsorption capacity of the modified chitosan was improved, higher than that of the unmodified chitosan. This is attributed to the improved dispersity of the modified chitosan in the carbonated water [134]. Compared with linear polymers, the multi-dentate polymers have higher adsorption capacity in wastewater treatment. Bai et al. [135] recently synthesized an octopuslike structure, POSS-PDMAEMA octopus, which could adsorb and desorb 6 types of heavy metals (i.e., Cu2+, Cd2+, Zn2+, Pb2+, Cr3+ and Ni2+) in wastewater with high efficiencies, contributing to purification of water, as shown in Fig. 20. As illustrated in Fig. 20(a), the material showed different adsorption capabilities in terms of different metals. The author indicated that the excellent efficiency in Pb2+ adsorption was attributed to the large ionic size and abundant vacant orbitals of

3.4. Pre-concentration of trace contaminants from water Limited to the detection sensitivity of current instrumental techniques, some of the contaminants in solution are beyond determination at trace level [136]. These contaminants should be separated and preconcentrated before determination [137]. Several separation-preconcentration techniques have been developed including solid phase extraction [138], cloud point extraction [139] and liquid-liquid extraction [140]. However, some of them require complex post-treatments and the addition of large amounts of organic solvents. The liquid phase microextraction (LPME), developed in 1996 by Jeannot and Cantwell, is a novel eco-friendly preconcentration technique [141]. A high efficiency could be obtained in LPME process with the addition of trace extracting solvents. Furthermore, this process combines sampling, extraction and concentration, saving much of time and avoiding posttreatment process.

Fig. 15. Electrostatic interactions between CO2-responsive materials and surfactants using (a) DMHEA [126], (b) DOAPA [121] and (c) TMPDA [123], respectively. 90

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Fig. 16. The molecular structures of tertiary amines tested in establishing switchable emulsion with SDBS [126].

Fig. 17. The schematics of CO2-switchable micro-emulsions based on TMPDA and SDS. Reproduced from Ref. [123] with permission of the Royal Society of Chemistry.

physically induced methods (e.g., bubbling N2 or heating) and chemically induced methods (e.g., adding acid, base or salts). They found that using traditional bubbling N2 or heating was not a complete or quantitative method in recovering SHSs. Only adding NaOH showed the best result of phase separation within 5 min and a simple post-treatment process. Actually, almost all of the studies used NaOH to recover SHSs in LPME process though it introduced ions into system. In addition to SHSs, the switchable water additives could also facilitate the separation of trace contaminants from water [147]. During the separation of trace cadmium from real water and food samples using acetonitrile as an extractant, Naeemullah et al. [147] observed that the addition of the switchable water additive (i.e., tetraethylenepentamine, TEPA) increased the contact area between acetonitrile and aqueous solutions, obtaining a high extraction efficiency. Moreover, it increased ionic strength in system, helping acetonitrile to switch from water-miscible to water-immiscible form [147]. Consequently, the water-immiscible acetonitrile and contaminants could be separated from water samples with the help of the switchable water additive.

Table 4 shows recent studies related to contaminant separation from water using LPME with switchable solvents or switchable water additives. To increase the contact area between different phases, the selected extractants should completely dissolve in water samples [142]. Therefore, when choosing SHSs as extractants, they should be firstly switched to hydrophilic forms in microextraction step [129,142–146]. The schematic of extraction process is shown in Fig. 21. With regard to the removal of heavy metal ions from aqueous solutions, some chelating agents such as 1-(2-pyridylazo)-2 naphthol (PAN) and ammonium pyrrolidinedithiocarbamate (APDC) should be synergistically added into extraction system, forming the hydrophobic complex with heavy metals (e.g., Cd-PAN and Cd-APDC). Followed by inducing the SHS to its initial hydrophobic form, the oil-soluble contaminants (e.g., herbicides) or the hydrophobic complex was separated from water phase and extracted to the SHS phase, which contributed to the following rapid determination by specific detector. Because only a small amount of SHSs (200–2000 μL) is required in LPME process, the recovery of these SHSs from water becomes a key point in affecting the accuracy of this pre-concentration method. Lasarte-Aragonés et al. [142] evaluated different recovery methods of SHSs including 91

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Fig. 18. The schematics of switchable emulsions with a CO2-switchable superamphiphile (a), emulsions diluted with crude oils before CO2 bubbling (b) and emulsions diluted with crude oils after CO2 bubbling (c). Adapted from Ref. [26] with permission of the Elsevier.

as an activator. The results showed that the DBU/alcohol/AMP system reduced the viscosity to 13–42 mPa.s and increased the regeneration capacity [150]. To reduce the loss of alcohol through evaporability, Heldebrant et al. [54,150] used single-component switchable ILs for CO2 capture. However, the CO2 binding capacity of single-component switchable ILs (6–9 wt%) was lower than that of two-component switchable ILs [150]. This is because the high viscosity of single-component ILs hinders the mass transfer for CO2 capture. To further enhance the capture process, several novel single-component switchable ILs, such as alkanolguanidine, are proposed [151]. Their viscosity could be as low as 100 mPa.s when loading 5 wt% of CO2. Results showed that the CO2 capacity was about 7–12 wt%. Comprehensive reviews related to the CO2 capture from flue gas and high-pressure steams, the selection of ILs structures or the mechanisms in CO2 capture can be found elsewhere [35,152–154]. Recently, Huang et al. [155] synthesized the imide-based switchable IL with a high CO2 binding capacity (22 wt%) and excellent recycling (16 cycles). As shown in Fig. 22, the flexible [DAA]− was cyclized to constrained [Suc]− for CO2 absorption by pre-organization and cooperation method. Zhu et al. [156] synthesized two-component switchable ILs using DBU and different imidazoles, as shown in Fig. 23. The different substituent structures and substituent positions in structures affected the electric-charge distribution in imidazole ring and the reaction between ILs and CO2. The imidazole ring of ILs with phenyl ring (i.e., [DBUH+][2-PhIm−]) could perform a strong electron withdrawing effect, leading to the lowest CO2 capacity, as shown in Fig. 23. The substituent groups close to the nitrogen anion in imidazole ring could increase the steric hindrance, leading to lower reaction rates and CO2 capacities. This result extends our knowledge for selecting efficient switchable ILs in CO2 capture [156]. In addition to CO2 capture, the switchable ILs could be used in CS2, COS and SO2 capture, forming the corresponding O-alkylxanthate, Oalkylthiocarbonyl and O-alkylsulfite anions, respectively (Fig. 24) [157–159]. The capture principle of these acid gases are similar to that of CO2, while the strength of gas binding with switchable ILs is dependent on the acidity of acid gases [157]. Acid gas capture using CO2responsive polymers have been reviewed elsewhere in details

Fig. 19. The switching mechanism of DSA–TMEDA surfactant in the presence and absence of CO2. Reproduced from Ref. [127] with permission of the Springer.

3.5. Acid gas capture Gas separation is a common process in industry, such as CO2 capture, SO2 and CS2 removal. The traditional aqueous amines, such as ethanolamine, could bind with CO2 through a strong hydrogen bonding, forming bicarbonate or carbamate salts in water. However, some problems have arisen when using these aqueous amines in CO2 capture. These traditional aqueous amines generally require a large amount of water for dilution to avoid corrosion to equipment. Additionally, the low concentration of aqueous solution limits the maximum capacity of CO2 capture. The switchable ILs, as novel nitrogen-containing liquids, can be directly used as absorbents for CO2 capture, requiring no dilution by water. Because the CO2 is weakly bounded with switchable ILs to form alkylcarbonate salts, less energy is required to break down the structures of alkylcarbonate salts in the reversible process. Taking the two-component DBU/alcohol as an adsorbent, Heldebrant et al. [148] observed that the CO2 capture capacity (19 wt %) was much higher than that of using the traditional aqueous alkanolamine (7 wt%). Liu et al. [149] recently optimized the DBU/alcohol system by adding alkylol amine (2-amino-2-methyl-1-propanol, AMP) 92

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Fig. 20. The adsorption capacity of 6 types of heavy metals at pH = 5.5 (a), the desorption rate of 6 types of heavy metals after bubbling CO2 (b) and the schematic adsorption and desorption of heavy metals using POSS-PDMAEMA octopus. Adapted from Ref. [135] with permission of the Elsevier.

Table 4 Liquid phase microextraction of contaminants from water samples. No.

Extraction

LOD

Flame atomic absorption spectrometer Fluorescence spectrophotometer GC equipped with mass spectrometric detector Atomic absorption spectrometer Flame atomic absorption spectrometry Atomic absorption spectrometer

0.16 0.08 0.5

5.4 6.7 3.1–12.5

[129] [142] [143]

0.07 1.80 3.8 × 10−4

3.5 3.8 4.5

[145] [144] [147]

Solvent volume (μL)

Time (min)

Chelating agent

1 2 3

TEA DMCHA DMCHA

750 750 250

6 5 5

APDC / /

Cd2+ Benz[a]anthracene Triazine herbicides

4 5 6

TEA TEA TEPA/ acetonitrile

750 1000 1200–1800

3 10 2

PAN PAN PAN

Pd2+ Cu2+ Cd2+

a

The limit of detection.

b

Relative standard deviation.

Fig. 21. The microextraction of contaminants from water samples using SHSs.

93

(μg/L)

RSD

b

Detector

Name of solvent

Note:

a

Contaminant

(%)

Ref.

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Fig. 22. The pre-organization and cooperation for CO2 capture using IL. Reproduced from Ref. [155] with permission of the Wiley-VCH.

Fig. 24. Using the switchable ILs for CS2, COS and SO2 capture. Reproduced from Ref. [157] with permission of the Wiley-VCH.

emerging solvents on environment cannot be neglected. Therefore, the environmental assessments of SHSs are necessary before the large-scale applications. Some researchers predicted the environmental effects (e.g., LD50 values, skin effects, etc.) of SHSs using computing software, but other properties such as biodegradability are seldom reported. The supplement of toxic properties of SHSs could not only help to evaluate the environmental effects of SHSs, but also provide guidance for selecting the post-treatment methods. For instance, if the SHSs is biodegradable, then the biological methods can be used to remove residual solvents from aqueous solutions. Otherwise, other coupling methods (e.g., adsorption) require treating the SHSs-polluted wastewater. It is desirable to provide more comprehensive statistics and analysis to guide further studies for environmental effects of SHSs. Besides, the addition of an equal amount of water (serving as a reactant) is required to induce the switchable reaction with SHSs. The use of pure water in lab-scale is feasible. In industry, however, most of water are process water with salinity, even in high concentration. Whether the presence and accumulation of impurities in water phase influence the reversible reaction of SHSs, and how the salinity of water influences the extent of reactions are still unclear. It is necessary to obtain the exact effects of water chemistry on the efficiency of SHSs,

Fig. 23. The CO2 capture using switchable ILs with different imidazole and the possible absorption mechanism. Reproduced from Ref. [156] with permission of the American Chemistry Society.

[19,36,37].

4. Challenges and perspectives An important role of switchable-hydrophilicity solvents is regarded as an extractant in separations. Because of the high boiling point of SHSs, the volatility in atmosphere is mostly lower than that of traditional solvents. Additionally, the easy recovery properties of SHSs at moderate conditions show unique advantages in separations. However, the amount of SHSs lost in water phase and oil products cannot be neglected [17,77,89]. Especially for heavy oils, the SHSs may be entrained and wrapped into highly viscous oil phase, increasing the difficulty of using CO2 trigger to separate SHSs. Additionally, different from traditional surfactants or other additives, a relatively larger amount of solvents is required in separating system. The impacts of 94

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with metal ions makes these materials suitable as adsorbents for removing metal ions from aqueous solutions. However, in practical use, the impurities in wastewater may occupy the adsorption sites and behave competitive adsorptions with metal ions. It is necessary to simulate real industrial wastewater samples in lab-scale. The desorption efficiency and recyclability of adsorbents also require consideration. Similar problems occur in membrane filtration process. After several recycling use, membrane flux may decrease as impurities or contaminants plug the pores of membrane materials. The simple membrane generation method of using CO2 trigger may lose efficacy. Actually, the environmentally stimulus-responsive materials have received significant attention in recent years. Research on CO2-responsive materials in separation fields is still in its early stage showing potential advantages and disadvantages, when comparing with other triggers (temperature, light, pH, magnetism, etc.). The pH-responsive materials have been widely studied in developing switchable emulsions for demulsification [163–165]. These materials can response to complex systems quickly when adding particular acid or base. However, the addition of acid and base leads to salt accumulation in system, limiting the multiple cycles [163]. Light-responsive materials are composed of light-responsive nanoparticles, which can adjust different types of emulsions (e.g., oil-in-water emulsions and water-in-oil emulsions) for demulsification [166–168]. However, due to the relatively poor light transmittance of emulsions especially for the black heavy oils, the lightresponsive speed is limited. Meanwhile, in some examples, light trigger may degrade the long-chain molecules into small molecules, destroying the re-emulsification process [168]. Strictly speaking, the CO2-responsive materials are types of pH-responsive materials. However, differently, they could switch between emulsification and demulsification easily without the addition of acid or base. Bubbling gas assisted with heating allows for faster demulsification. However, the adjustable pH range of CO2 trigger is limited. With regard to oil-water mixture separation, temperature-responsive and light-responsive wettability materials have been extensively studied [169–172]. They could switch between super-hydrophobicity and super-hydrophilicity with fast response and simple operation. In this field, research on CO2-responsive materials primarily focus on the super-wettability membrane materials and more in-depth studies are needed. In the future, the synthesis of multi-responsive materials, such as coupling with CO2-responsive materials with lightresponsive materials, may take advantage of each materials and overcome the disadvantages, achieving rapid response and efficient separation. However, up to now, there is little information related to the quantitative comparison between CO2-responsive materials and other environmentally stimulus-responsive materials, especially in a life-time cycle. In addition to the specific characteristics of materials above, some common problems require consideration for improving and enhancing the synthesis and applications of CO2-responsive materials. Firstly, because the separating ability of materials is dependent on their properties and structures, researchers could make more efforts at the beginning of designing materials. The current designing principles primarily focus on how to synthesize the qualified CO2-responsive materials with suitable switchable properties. Further studies require developing the CO2-responsive materials with multi-targeted functions, considering the influence of structures on separating efficiency and environmental effect. The computing science, using the quantitative structure-activity relationship (QSAR), is helpful in designing SHSs by saving time and cost [41,173]. Secondly, from a practical point of view, during the industrial applications of chemicals, mass and heat transfer processes should be carefully tested, which play important roles in successful reactions, conversions and process enhancement. For example, the extending research in reaction kinetics and fluid dynamics may help the protonation and deprotonation process of CO2-responsive materials to be more efficient and more controllable. The amount of SHSs lost in water phase may decrease, solving secondary pollution.

especially at the large-scale application. The switchable surfactants are generally applied in soil remediation to achieve oil-solid separation and in destabilization of emulsion to achieve oil-water separation. In soil remediation, the switchable properties of switchable surfactants provide them more advantages than conventional surfactants in the recovery of surfactants. However, an inevitable problem is the adsorption of surfactants on ionic sites of soil, which could lead to the pollution of soil and loss of surfactants. Ceschia et al. [27] calculated that the content of residual switchable surfactants on sand was only about 0.45%, lower than expected. But subsequent studies related to the influence of soil type and oil compositions remain rare. Actually, the adsorption of SHSs is highly dependent on soil type. The different oil compositions (containing metal ions and fine particles) may also result in the synergistic or competitive effects at the oil-solid and oil-water interfaces. Certainly, the switchable surfactants with different chain lengths and functional groups lead to a content variation of residual surfactants on soil, but the reported studies related to the structures of switchable surfactants are still limited. In recent years, the destabilization of emulsion is a main application of switchable surfactants and switchable water additives, showing potentials in oil pipeline transportation, oil removal, etc. Especially for heavy oil emulsions, the electrostatic interaction between alkaline compounds (e.g., surfactants, water additives) and acid active components (e.g., naphthenic acid) is widely studied in stabilizing and destabilizing oil-water emulsions [21,117,160,161]. More in-depth mechanisms and interface behaviors require further investigation. It is worth noting that some of switchable surfactants are oil-soluble after destabilization of emulsions by CO2 trigger, which lead to extra postprocessing for separating surfactants from oil products [26,123,125]. Therefore, a water-soluble surfactant after bubbling CO2 is desirable in practical use because the water phase containing switchable surfactants could be further applied in establishing switchable emulsions. The switchable ILs, as alternatives for traditional aqueous amines, are promising materials in acid gas capture. However, when considering their practical applications, things become much more complex. Industrial waste gas usually contains a certain amount of impurities (e.g., HCl, NH3, ash particles, etc.). Whether switchable ILs could still efficiently absorb the low-concentration acid gas require careful consideration. Moreover, the highly reactive ILs may interact with impurities, changing their structures and switchable properties. In addition, the source of waste gas is crucial in gas capture study. In industry, the flue gas, with a higher temperature than the room temperature, is directly injected into absorption liquids. The relatively high absorption temperature may lead to the evaporation of alcohol components of switchable ILs, destroying their switchable properties. The switchable ILs, as special solvents, could be directly used in biooil and solid separations, but their high synthesis costs and limited polarity range may impede their applications, especially compared with traditional or novel solvents. However, except for the role of solvents, researchers could turn to exploit other roles of switchable ILs, such as catalysts and process aids, in oil-solid separation. For example, the catalysis of DBU/ethanol has been revealed in soybean extraction [79]. Solvents assisted with traditional ILs (serving as process aids) extraction have demonstrated the ability of ILs for decreasing the fine sands from oil phase [162]. Using switchable ILs as substitutes combined with solvents may not only improve the quality of oil products, but also simplify the separating step and decrease the recovery cost of ILs. If it works, the switchable ILs would widen their applications in oil-solid separation. In terms of CO2-responsive polymers, their applications are more extensive than other CO2-responsive materials. They can be used as membranes for filtration, as demulsifiers for adjusting emulsions and as absorbents for removing impurities from water. Although the synthesis process of CO2-responsive polymers is relatively complex, the designability and diversity of structures could make this type of materials more promising in the future. The chelation of polymer components 95

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SHSs, switchable surfactants, CO2-responsive polymers and switchable water additives. These materials have different switchable properties, such as hydrophilicity-hydrophobicity conversion, switching “on” or “off” surface-activity and switchable change in ionic strength, which behave dual roles in separation fields. It is feasible to apply these materials in a wide range of separations including oil-solid separation, oilwater separation, wastewater treatment and acid gas capture, which provide a new pathway for the utilization of CO2. Great progress has been made on these types of materials. However, research on CO2-responsive materials is still in its early stages. There are still some gaps or challenges for designing CO2-responsive materials and applying in industry. Future research should not only emphasize the design of CO2responsive materials with low environmental hazards and efficient separating capabilities, but also pay more attention on the engineering considerations and push forward these materials into industrial applications. Acknowledgements This work was financially supported by the Municipal Natural Science Foundation of Tianjin (Grant18JCQNJC06500) and National Natural Science Foundation of China (NSFC, No.21506155, No.41471258). References [1] E.M. Shahid, Y. Jamal, Production of biodiesel: a technical review, Renew. Sustain. Energy Rev. 15 (9) (2011) 4732–4745. [2] Z.K. Feng, W.J. Niu, J.Z. Zhou, C.T. Cheng, H. Qin, Parallel multi-objective genetic algorithm for short-term economic environmental hydrothermal scheduling, Energies 10 (2) (2017) 163–184. [3] Y. He, X. Yang, Y. Pang, H. Tian, R. Wu, A regulatory policy to promote renewable energy consumption in China: review and future evolutionary path, Renew. Energy 89 (2016) 695–705. [4] L. He, F. Lin, X. Li, H. Sui, Z. Xu, Interfacial sciences in unconventional petroleum production: from fundamentals to applications, Chem. Soc. Rev. 44 (15) (2015) 5446–5494. [5] J.S. Kim, Production, separation and applications of phenolic-rich bio-oil–a review, Bioresour. Technol. 178 (2015) 90–98. [6] S.P. Jeevan Kumar, G. Vijay Kumar, A. Dash, P. Scholz, R. Banerjee, Sustainable green solvents and techniques for lipid extraction from microalgae: a review, Algal Res. 21 (2017) 138–147. [7] B. Xu, P. Li, C. Chan, Application of phase change materials for thermal energy storage in concentrated solar thermal power plants: a review to recent developments, Appl. Energy 160 (2015) 286–307. [8] H. Sui, P. An, X. Li, S. Cong, L. He, Removal and recovery of o- xylene by silica gel using vacuum swing adsorption, Chem. Eng. J. 316 (2017) 232–242. [9] P.Q. Fu, K. Kawamura, C.M. Pavuluri, T. Swaminathan, J. Chen, Molecular characterization of urban organic aerosol in tropical India: contributions of primary emissions and secondary photooxidation, Atmos. Chem. Phys. 10 (6) (2010) 2663–2689. [10] Q.C. Le, R.M. Andrew, J.G. Canadell, S. Sitch, J.I. Korsbakken, G.P. Peters, A.C. Manning, T.A. Boden, P.P. Tans, R.A. Houghton, Global carbon budget 2016, Earth Syst. Sci. Data 7 (2) (2016) 521–610. [11] E.S. Sanz-Pérez, C.R. Murdock, S.A. Didas, C.W. Jones, Direct capture of CO2 from ambient air, Chem. Rev. 116 (19) (2016) 11840–11876. [12] X. Li, J. Li, H. Sui, L. He, X. Cao, Y. Li, Evaluation and determination of soil remediation schemes using a modified AHP model and its application in a contaminated coking plant, J. Hazard. Mater. 353 (2018) 300–311. [13] L. Ritchie, C. Ferguson, S. Saini, A novel sensor for monitoring leakage of petroleum and other liquid hydrocarbons into soil environments, J. Environ. Monitor. 2 (2) (2000) 193–196. [14] W. Liu, L.I. Xiaosen, Y. Liu, Z. Zhang, The application status in remediation of petroleum contaminated soil, Oilfield Chem. 32 (2) (2015) 307–312. [15] B. Petrie, R. Barden, B. Kasprzyk-Hordern, A review on emerging contaminants in wastewaters and the environment: current knowledge, understudied areas and recommendations for future monitoring, Water Res. 72 (2015) 3–27. [16] X. Li, Y. He, H. Sui, L. He, One-step fabrication of dual responsive lignin coated Fe3O4 nanoparticles for efficient removal of cationic and anionic dyes, Nanomaterials 8 (3) (2018) 162–177. [17] X. Li, Z. Yang, H. Sui, A. Jain, L. He, A hybrid process for oil-solid separation by a novel multifunctional switchable solvent, Fuel 221 (2018) 303–310. [18] J. Zhang, D.W. Agar, X. Zhang, F. Geuzebroek, CO2 absorption in biphasic solvents with enhanced low temperature solvent regeneration, Energy Proced. 4 (2011) 67–74. [19] A. Darabi, P.G. Jessop, M.F. Cunningham, CO2-responsive polymeric materials: synthesis, self-assembly, and functional applications, Chem. Soc. Rev. 45 (2016) 4391–4436.

Fig. 25. The future potential development of CO2-respmsive materials.

Thirdly, the basic property of CO2-responsive materials should be supplemented to promote the industrial amplification. The basic property could be firstly established based on experiments and simulations. Subsequently, the transfer process could be analyzed according to basic theoretical models with rational modifications. For example, bubbling CO2 into SHS-water system may be a more complex gas-liquid-liquid mass transfer process, compared with the classical Lewis and Whitman two-film theory. After a careful analysis, the operational conditions and reactors could be designed and optimized. Finally, an entire industrial process often involves not only a single unit operation, but also the system combination, heat network optimization, material recycling in an environmentally friendly way, as well as the economic considerations. For example, how to collect CO2 after reactions and how to achieve a cyclic utilization of CO2 in an entire system require careful consideration. The comparison of energetic and capital costs of CO2-responsive materials with traditional materials will be important to identify the unique separating advantages of new materials However, the detailed energetic calculations and evaluations of CO2-responsive materials are rarely reported. In the future, to facilitate the industrialization, more efforts should be made on unlocking the engineering problems (Fig. 25).

5. Conclusions CO2, a relatively green, cheap and eco-friendly gas trigger, has been widely studied for synthesizing CO2-responsive materials including 96

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