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Liquid-liquid extraction of chlorophenols from wastewater using hydrophobic ionic liquids
Journal of Molecular Liquids 294 (2019) 111680
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Liquid-liquid extraction of chlorophenols from wastewater using hydrophobic ionic liquids R. Sulaiman a, I. Adeyemi a, S.R. Abraham b, S.W. Hasan a, I.M. AlNashef a,⁎ a b
Department of Chemical Engineering, Khalifa University, Masdar City Campus, P. O. Box 127788, Abu Dhabi, United Arab Emirates Department of Chemical Engineering, University at Buffalo (SUNY), Buffalo, NY 14260-4200, USA
a r t i c l e
i n f o
Article history: Received 9 April 2019 Received in revised form 29 August 2019 Accepted 2 September 2019 Available online 03 September 2019 Keywords: Ionic liquids Chlorophenols Wastewater Liquid-liquid extraction
a b s t r a c t In this work, liquid-liquid extraction of 3-chlorophenol, 2, 5-dichlorophenol, 2, 4, 6-trichlorophenol and pentachlorophenol from wastewater was conducted using five hydrophobic ionic liquids (ILs). Moreover, the impact of key parameters like molecular structure, pH, temperature, phase ratio and initial concentration of feed on the removal of the chlorophenols (CPs) from their aqueous solution were studied. The results showed that the molecular structure of the IL has an important impact on the removal of CPs. Generally, the evaluation of different ILs with the same CPs revealed that piperidinium and pyridinium based ILs have greater extraction efficiency than sulfonium and imidazolium based ILs. It was found that the extraction efficiency decreased with the decrease of IL:wastewater mass ratio, with a maximum extraction at a ratio of 1:1. The results showed that temperature and initial concentrations in the range of 25–45 °C and 4–1000 mg/L, respectively had no significant impact on the extraction for all studied systems. © 2019 Elsevier B.V. All rights reserved.
1. Introduction The removal of chlorophenols (CPs) from wastewater has gained significant attention due to the widespread usage and toxicity of these phenolic compounds. CPs have been used for industrial and agricultural purposes in activities ranging from pesticides/organic solvents synthesis, flame retardant and wood treatment agents. Moreover, CPs could be produced naturally in soils through a reactive interaction of inorganic chloride and organic compounds in the presence of fungi. Consequently, an annual production of 200,000 tons of CPs has been reported [1]. Most of these CPs enter into the aquatic environment in concentrations of 0–30 μg/L and sometimes reaching 1000 μg/L in contaminated surface waters and sewage [2–5]. These concentrations of CPs are significant considering their toxic effects such as carcinogenicity, endocrine disruption ability, and mutagenicity. Furthermore, CPs have relatively strong resistance to biodegradation. Hence, numerous processes have been studied for the removal of CPs from wastewater due to the cumulative implications of their environmental impacts. Amongst the top methods for the removal of CPs are biological processes, advanced oxidation process, adsorption, chemical degradation, photolysis, ion exchange, and liquid-liquid extraction [6–10]. Liquidliquid extraction provides benefits over other methods such as biological and thermal decomposition processes for the removal of chlorophenols from water. This is because biological processes utilize ⁎ Corresponding author. E-mail address: [email protected] (I.M. AlNashef).
https://doi.org/10.1016/j.molliq.2019.111680 0167-7322/© 2019 Elsevier B.V. All rights reserved.
microorganisms which are affected by a high dose of CPs. Moreover, thermal decomposition requires substantial energy due to the heating of the CPs [11,12]. However, the usage of liquid-liquid extraction often relies on solvents that are flammable, toxic and volatile. This has engendered the development of improved alternative solvents such as ionic liquids (ILs). ILs are solvents which are composed of ions and are usually liquid below 100C. These solvents could form a crucial building block of green chemistry with a careful selection of its cation and anion. They are attractive for the extraction of micro-pollutants such as polycyclic aromatic hydrocarbons (PAHs) because of their low vapor pressure, tuneability and dissolving strength for CPs. Due to these benefits, there had been a number of reported publications on the removal of CPs from wastewater with ILs [13–22]. However, majority of the research have focused on imidazolium based ILs. For instance, Bekou et al. [13] studied the removal of CPs from water with two hydrophobic ILs, 1-butyl-3methylimidazolium hexafluorophosphate, [bmim][PF6] and 1-ethyl-3methylimidazolium bis(perfluoroethylsulfonyl)imide, [emim][bmim] [Tf2N]. The extraction efficiency of the chlorinated phenols was better when [bmim][PF6] was utilized and when pH b pKa. In a similar study focused on the extraction of pentachlorophenol, Fan et al. [14] studied the use of imidazolium-based ILs with different alkali chain. It was observed from their results that increasing alkali chain length of the IL cation resulted in the increase of extraction efficiency for the same phenolic compound. Another research by Lakshmi et al. [15] reported the significance on imidazolium based ILs where they investigated the extraction of CPs with 1-butyl-3-methyl imidazolium based ILs with
R. Sulaiman et al. / Journal of Molecular Liquids 294 (2019) 111680
chloride, hexafluorophosphate and tetrafluoroborate anion, and their mixture with organic solvents. It was found that the extraction with 1-butyl-3-methylimadazolium tetrafluoroborate [bmim][BF4], which was mixed with tributyl phosphate, produced the maximum extraction efficiency. In addition, the solubility of CPs at different temperatures in bis(trifluoromethylsulfonyl)imide based hydrophobic ILs was reported by our group recently [23]. The results revealed that imidazolium and pyridinium based ILs have high capacity of dissolving CPs. The CPs showed decreasing solubility values with the increasing number of chlorine atom in their molecules. Although the investigations with imidazolium ILs have revealed their viability for the removal of CPs, there is a need to further investigate other types of ILs for CPs extraction. This evaluation is required in order to assess their capabilities, gain some insightful understanding of the process and derive resulting benefits that comes with their usage. For instance, pyridinium based ILs have been reported to show good economics [24] and less toxicity [25–27] but there are little to no studies present on their usage. Thus, in this work, we investigated the extraction efficiency of 3-Clorophenol (3-CP), 2,5-Dichlorophenol (DCP), 2,4, 6-Trichlorophenol (TCP), and Pentachlorophenol (PCP) using five bis(trifluoromethylsulfonyl)imide based ILs with different cations, namely 1-butyl-3-methylimidazolium, 1-butyl-1methylpiperidimium, diethylmethylsulfonium, 1-butyl-3methylpyridinium, 1-ethyl-3-methylpyridinium, as solvent. To the best of our knowledge, the extraction of CPs using bis (trifluoromethylsulfonyl)imide based ILs with the cation of sulfonium, piperidinium and pyridinium has not been reported before. Additionally, the effect of temperature, pH and IL:water mass ratio on extraction efficiency of selected CPs and ILs was investigated.
[C4mPip][Tf2N]
Table S1 presented the used chemicals and their properties. The ILs were procured from Ionic Liquid Technologies, Io-Li-Tec (Heilbronn, Germany). 3-CP, DCP, and TCP were procured from Acros, Thermo Fisher Scientific (USA). PCP, acetonitrile, and sulfuric acid were procured from Sigma-Aldrich (USA). Ultrapure water (18.2MQ.cm) used during experiment was produced by the water purification unit Suez Water (UK). Further purification was not conducted for all chemicals prior to use. Coulometric Karl Fischer Titrator, Aquamax KF (GR Scientific, UK) was used to measure the water content of pure ILs. The pH measurement of the aqueous solution was done using benchtop meter (infolab, Xylem, USA) with an uncertainty of ±0.1 pH, ±0.1 °C of temperature H2SO4 and NaOH was used to modify pH of aqueous solution having CPs. High Performance Liquid Chromatography (HPLC; Dionex Ultimate 3000 system, Shimadzu Japan) equipped with a Raptor ACR C-18 column (250 mm × 4.6 mm × 5 μm), purchased from RESTEK (USA), was utilized to quantify CPs in aqueous solution. 45% of 0.0025 M sulfuric acid solution and 55% acetonitrile based on volume was used as mobile phase to quantify PCP and TCP in water. 45% water and 55% acetonitrile based on volume was used as mobile phase to quantify 3-CP and DCP in water. The absorbance wavelength of 220 nm was chosen at for PCP. The absorbance wavelength of 285 nm was chosen for 3-CP, DCP and TCP.
[1,3emPY][Tf2N]
90 80 70 60 50 40 30 20 10 0 0
0.5
1
1.5
2
2.5
3
3.5
Fig. 1. Contact time effect on extraction efficiency of 3-CP using ILs (T = 25 °C, 250 rpm, and IL:water ratio 1:10).
in a 20 mL vial. The vial containing IL and CP aqueous solution was then mixed vigorously using an incubator shaker at 250 rpm and 25 °C for a fixed time. After mixing, 15 min of centrifugation was performed for complete phase separation. Subsequently, samples were taken from the aqueous phase and analyzed with the HPLC in order to determine CP concentrations after extraction. Sensitivity analysis of key parameters that could influence the removal of the CPs from their aqueous solution was investigated. The effect of temperature, mass phase ratio, pH, initial concentration of aqueous feed, and molecular structure was studied. The extraction efficiency (E) was calculated according to Eq. (1): E¼
1−
Cw 100% C wo
ð1Þ
where Cwo and Cw are the concentrations of chlorophenols in the water phase before and after extraction, respectively. 3. Results and discussion The 3-CP, DCP, TCP, PCP were extracted using five hydrophobic bis (trifluoromethylsulfonyl)imide based ILs from their aqueous solution. The mixing time is a crucial parameter for solvent extraction experiments because it ensured equilibrium mass transfer of the CPs to the IL phase. Consequently, the mixing time was observed at regular intervals to determine its impact on the extraction efficiency (Figs. 1– S1–4). It was found that the extraction efficiency remained practically constant when the system reached equilibrium (Tables S7–S10). These predetermined equilibrium times which were stated in Table S11 were subsequently utilized for further studies. The errors for the experiments has been conducted using duplicate measurements (n = 2) and the standard deviation was determined to be b2% for all extraction. In addition, the extraction efficiency obtained from Lakshmi et al. [15,29] was compared with those measured in this work (Table 1). The TCP extraction efficiency comparison showed that the piperidinium
2.2. Extraction procedure The extraction experiments started with the preparation of the CP aqueous solution which includes a concentration of 97 mg/L for 3-CP, DCP, and 100 mg/L for TCP. The PCP was prepared as a 10 mg/L aqueous solution due to its lower solubility in water. Thereafter, 10 g of each CP aqueous solution was added to 1 g of IL1 and the combination was placed 1 The ILs were used in the dried form as saturation has no significant impact on the extraction efficiency (Tables S2–S6)
4
Contact Time (h)
2. Materials and methods 2.1. Chemical reagents
[Et2MeS][Tf2N]
100
Extracon effeciency (%)
2
Table 1 Comparison of the extraction efficiency in this study with literature data. IL type
TCP-extraction efficiency (%)
[1,3emPY][Tf2N] [C4mPip][Tf2N] [bmim][Cl] [bmim][PF6] [bmim][BF4]
99.14 98.98 90.18 [15,29] 93.41 [15,29] 99.79 [15,29]
R. Sulaiman et al. / Journal of Molecular Liquids 294 (2019) 111680
Extraction effeciency (%)
100
3-CP
DCP
TCP
3
PCP
95 90 85 80 75 70
[bmim][Tf2N]
[C4mPip][Tf2N]
[Et2MeS][Tf2N]
[1,3bmPY][Tf2N]
[1,3emPY][Tf2N]
Ionic liquid Fig. 2. The effect of molecular structure of CPs on extraction efficiency (T = 25 °C, 250 rpm, and IL:water ratio 1:10).
and pyridinium based ILs studied has excellent extraction capacity when compared to the ones reported by Lakshmi et al. [15,29]. However, ILs with [PF6] and [BF4] produce hydrofluoric acid at a relatively fast rate compared to [Tf2N] anions. 3.1. Effect of molecular structure The impact of the molecular structure of the CPs and ILs were studied in order to gain a fundamental insight into their behavior during the extraction process. This sensitivity analysis allows for the assessment of the effect of different cations and their molecular structures on the extraction of CPs. This criterion supports the quest for optimum ILs for the extraction purposes as the IL groups differ in their extraction capacity toward different CPs. Furthermore, this effect of the classes of ILs allows for the usage of the promising properties exhibited by different ILs. The effect of molecular structure of CPs was evaluated by using four CPs (3-CP, 2, 5-DCP, TCP and PCP) with a fixed IL as the extraction media (Table S12). As depicted in Fig. 2, the extraction efficiency of CPs followed the order of PCP N TCP N DCP N 3-CP with pyridinium and sulfonium based ILs. By comparing the log Kow, (octanol/water partition) values of PCP, TCP, 2, 5-DCP and 3-CP which are 5.1, 3.7, 3.1 and 2.5, respectively, it was noted that the more hydrophobic the CPs was, the more CPs were extracted [29]. Similar observation has been reported by Bekou et al. [13] where they attributed the rising hydrophobicity to the increasing chlorine substituents in the CPs. Hence, the increased hydrophobicity of CPs with higher chlorine substituents resulted in the
[bmim][Tf2N]
[C4mPip][Tf2N]
increased hydrophobic bonding between the cation of the ILs and the CPs which enhanced the extraction process. The impact of IL structure on extraction efficiency of CPs is shown in Fig. 3. With regards to imidazolium and piperidinium based ILs, there are variations from the trend described above (PCP N TCP N DCP N 3-CP). Although the piperidinium based IL followed the general trend with the exception of DCP, imidazolium based ILs deviates significantly from this trend. The random trend observed for imidazolium based ILs could be found from some previous studies like that of Bekou et al. and Lakshmi et al. For example, in the work of Bekou et al., they reported that for the ionic liquid [emim][Beti] there was more extraction capacity for 2,4-DCP than PCP. In another study, Lakshmi et al. evaluated the extraction of chlorophenols with different solvents which includes three imidazolium based ILs. In their results, the behavior of the three imidazolium based ILs investigated was not based primarily on the number of chlorine atom and their hydrophobicity. For all the imidazolium based ILs, the extraction efficiency did not follow the general trend, rather they deviate significantly from that hydrophobic interaction mode of extraction with the CPs. Besides the reports on the behavior of imidazolium based ILs, there was a study reported by Deng et al. for the IL [3C6PC14][FeCl4] which showed that it has higher extraction of 2-CP when compared with 2,4-DCP. These reports show that the extraction of the CPs is not determined solely based on the hydrophobicity of the chlorophenols with water. The extraction mechanism is complicated by the presence of hydrophobic interaction of the CPs with the cation of the ionic liquids, the hydrogen bonding between
[Et2MeS][Tf2N]
[1,3bmPY][Tf2N]
[1,3emPY][Tf2N]
Extraction effeciency (%)
100 95 90 85 80 75 70
3-CP
DCP
TCP
PCP
Chlorophenols Fig. 3. The effect of molecular structure of ILs on extraction efficiency (T = 25 °C, 250 rpm, and IL:water ratio 1:10).
4
R. Sulaiman et al. / Journal of Molecular Liquids 294 (2019) 111680 3-CP
DCP
TCP
Table 2 Temperature effect on the extraction efficiency of CPs (250 rpm, phase ratio: 10).
Extraction effeciency (%)
100 90 80 70 60 50 40 30 20 10 0
Temperature Extraction efficiency (%) (°C) 3-CP DCP IL
2
4
6
pH
8
10
12
Fig. 4. Effect of pH on extraction efficiency of 3-CP, DCP and TCP using [1,3emPY][Tf2N] (T = 25 °C, 250 rpm, and IL:water ratio 1:10).
the CPs and the anion of the ILs used and the location of the chlorine atom on the benzene ring. Therefore, the CPs behaves differently with various cations and anions of the ILs. Moreover, comparing the extraction efficiency of different ILs with the same CPs, it was observed that piperidinium and pyridinium based ILs had greater extraction than sulfonium and imidazolium based ILs, with the exception of 2,4-DCP. For 2,4-DCP, the piperidinium and pyridinium based ILs still have greater extraction than sulfonium based ILs. However, the extraction of DCP with imidazolium based ILs was close to that of piperidinium based ILs. The lower extraction efficiency of sulfonium based IL than piperidinium and pyridinium IL may suggest that it was resulted from lacking π − π stacking of aromatic rings between CPs and [Et2MeS] cation. However, when it is compared to imidazolium based ILs, there is no big difference in the extraction efficiency, thus it suggests that the main driving force of the extraction are the hydrophobic interaction between CPs and the cation of the IL and the hydrogen bonding between CPs and IL anion [14,20,28]. Fan et al. found that the extraction efficiency of phenols including pentachlorophenol increased with increasing alkali chain length of the cation in imidazolium based ILs [14]. In our work, the same behavior was observed for pyridinium based ILs. The extraction efficiency of all CPs with [1,3bmPY][Tf 2 N] was higher than [1,3emPY][Tf 2N]. It can be observed from the chemical structure, [1,3bmPY][Tf2N] has longer alkali chain than [1,3emPY][Tf2N]. Increasing alkali chain in the cation will increase the hydrophobicity of IL, which further increase the hydrophobic interaction between CPs and ILs during extraction process. This is why the extraction efficiency for a given CP is higher with [1,3bmPY][Tf2N] than with [1,3emPY][Tf2N].
3-CP
DCP
25 35 45
TCP
IL
IL
[bmim] [Tf2N]
[1,3emPY] [bmim] [Tf2N] [Tf2N]
[1,3emPY] [C4mPip] [Tf2N] [Tf2N]
[1,3emPY] [Tf2N]
84.56 84.67 83.25
87.07 86.07 87.15
94.54 94.90 92.70
94.77 89.69 91.23
94.25 93.29 92.20
94.77 90.19 93.79
3.2. Effect of pH The impact of the pH on the extraction of 3-CP, DCP from their aqueous solution into [bmim][Tf2N] and [1,3-emPY][Tf2N] and the impact of the pH on the extraction of TCP from their aqueous solution into [C4mPip][Tf2N] and [1,3-emPY][Tf2N] were evaluated for pH range 3–11. The effect of varying initial pH of the water on extraction efficiency has been studied since wastewater is present in different acidic and basic condition, and the extraction efficiency is influenced by the initial pH. The extraction of the CPs is single stage, thus, the measurement of the equilibrium pH was not conducted after the extraction. The results shown in Fig. 4 and Table S13, revealed that the CPs extraction efficiencies were higher for the acidic solutions. This increase in the extraction of the CPs in acidic solutions can be attributed to the low dissociation of the neutral CPs into ions due to their weak acidic nature. The weak acidity of the CPs is obvious from the pKa values of 9.1, 7.5 and 6.2 for 3-CP, DCP and TCP, respectively [30]. Consequently, the low dissociation of the CPs and their presence in the molecular form would allow an easy transfer into ILs. The ease of transfer can be linked to the possibility of the neutral CPs to migrate into the ILs via the two mechanisms of extraction as compared to the alkaline solution where only one route is dominant. The first extraction route involves the formation of hydrogen bond between the hydrogen atom of the neutral CPs and the anion of the ILs and the second route utilizes the hydrophobic interaction between the CPs and the cation of the IL. In the acidic solution, both routes are active. However, in the alkaline region the transfer of the CPs to the IL would occur mainly through the hydrophobic interaction alone as the dissociation of the CPs would lessen the strength of the hydrogen bond. This explains why the distribution ratio is less in the alkaline solutions.
3.3. Effect of IL:water mass ratio The extraction experiments were conducted using different IL:water mass ratio to determine the minimum quantity of IL that would be needed for efficient extraction. In this extraction process, the mass of
TCP
Extraction effeciency (%)
100 90 80 70 60
0
5 10 Mass ratio (Water: IL)
15
20
Fig. 5. Effect of phase ratio on extraction efficiency of 3-CP, DCP and TCP using [1,3emPY] [Tf2N] (T = 25 °C, 250 rpm).
Fig. 6. Effect of concentration of CP in feed phase on extraction efficiency of 3-CP and DCP using [1,3emPY][Tf2N] (T = 25 °C, 250 rpm and phase ratio: 10).
R. Sulaiman et al. / Journal of Molecular Liquids 294 (2019) 111680
5
the aqueous solution was increased while keeping the IL mass constant at 1 g (Fig. 5 and Table S14). The extraction efficiency decreased with the decreasing mass ratio, with a maximum extraction at IL:water ratio of 1. Extraction efficiencies N91% were attained for all studied systems for IL:water ratio of 1:5. TCP was able to sustain its extraction capability until IL:water ratio of 1:15. This could be attributed to the increase of hydrophobicity due the increase chlorine substituents which decreased its affinity toward water. However, IL:water ratio of 1:10 was utilized for further studies to allow for consistencies and uniformity for all CPs studied. The extraction efficiencies at IL:water ratio of 1:5 and 1:10 were not significantly different for 3-CP, DCP and TCP. Hence, IL:water ratio of 1:10 would reduce the quantity of the ILs used for the extraction process without significantly affecting the extraction efficiency of the CPs. 3.4. Effect of temperature The extraction experiments were done at three temperatures (25, 35, and 45 °C) and the results were shown in Table 2. In general, no substantial change in the extraction efficiency was shown from the results within the temperature range of 25–45 °C for all studied systems. Similar observation was reported by Lakshmi et al. [15] [28] on the extraction of phenols and their substituents. The ability of the extraction to be conducted within the range of 25–45 °C with no effect on the CPs removal allows for flexibility and provides the possibility to treat the wastewater at different temperatures. More importantly, there is a possibility to conduct the process at ambient temperature in order to save energy and consequently operational cost. 3.5. Effect of concentration of CPs in feed phase The effect of concentration of CP in feed phase on extraction efficiency was investigated by studying 3-CP and DCP extraction efficiency using [bmim][Tf2N] and [1,3emPY][Tf2N] as solvents. The CPs in the aqueous solution had initial concentrations ranging from 4 to 1000 mg/L. The results indicated that there was no substantial variance in the extraction efficiency within the range of concentration studied (Fig. 6 and Table S15). 3.6. Mechanism of CPs extraction with hydrophobic ILs There are two main extraction mechanisms for the CPs from the aqueous phase. The first is through the hydrogen bond interactions between the CPs and the anion of the IL. To demonstrate this pathway, we have used TMoleX and Conductor-like screening model for realistic solvents (COSMO RS) models. The sigma surfaces/profiles were obtained
Fig. 8. Sigma profile of DCP.
as depicted in Figs. 7–9. The threshold value for hydrogen bonding is σhb = ± 0.0084 eA − 2 from the sigma profile estimations. Compounds with a general peak at σ b σhb has hydrogen bond donor ability whilst those with σ N σhb has hydrogen bond acceptor ability. Based on this, it becomes clear that the CP has general peaks on the σ b σhb region, a characteristic of hydrogen bond donor capacity. However, the [Tf2N] anion exhibited a general peak in the σ N σhb region, a feature of hydrogen bond acceptor ability. Consequently, hydrogen bond interactions between the CP and [Tf2N] anion could be affirmed. The second mode of extraction is through the hydrophobic interactions between the CPs and the cation of the IL. The hydrophobic interaction is a long range attractive phenomenon where non-polar molecules interact with water in a special way. This phenomenon has been well described in various works like that of Israelachvili and Pashley [31], Feher [32], Luzar et al. [33], Pangali et al. [34], Pratt and Chandler [35] etc. Moreover, similar findings of the hydrophobic interaction between phenolic compounds and the cation of ILs have been reported by Fan et al. [14], Deng et al. [20], Lakshmi et al. [15]. 4. Conclusions The removal of 3-CP, DCP, TCP, PCP using five different hydrophobic ILs through liquid-liquid extraction was investigated. In addition,
Fig. 7. Sigma surfaces of bis(trifluoromethylsulfonyl)imide anion and DCP.
6
R. Sulaiman et al. / Journal of Molecular Liquids 294 (2019) 111680
Fig. 9. Sigma profile of [Tf2N] anion.
parametric studies based on molecular structure, pH, temperature, IL mass CP aqueous solution mass ratio and initial concentration of feed was observed. The results showed that the molecular structure of both the IL and CP had an effect on the removal of the CPs. The evaluation of different ILs with the same CPs revealed that piperidinium and pyridinium based ILs have higher extraction efficiency than sulfonium and imidazolium based ILs having the same anion. The extraction efficiency decreased with decreasing mass ratio, with a maximum extraction efficiency at 1:1 ratio. The effect of temperature and initial concentrations showed that there was no substantial difference in the extraction efficiency within the range of 25–45 °C and 4–1000 mg/L, respectively, studied for all systems. Overall, it was found that it is possible to achieve high extraction efficiency by controlling the different operational conditions during the extraction process such as temperature, contact time, and IL:CP aqueous solution mass ratio. It is also possible to select the most proper ILs based on the practical application from industry such as the temperature of the wastewater, the water quality requirement, etc. Acknowledgment This work was funded by Khalifa University of Science, Technology and Research through research grant CIRA-2018-69 and by ADEK through grant 8434000282. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.111680. References [1] T. Ge, J. Han, Y. Qi, X. Gu, L. Ma, C. Zhang, S. Naeem, D. Huang, The toxic effects of chlorophenols and associated mechanisms in fish, Aquat. Toxicol. 184 (2017) 78–93, https://doi.org/10.1016/j.aquatox.2017.01.005. [2] D.F. Grobler, J.E. Badenhorst, P.L. Kempster, PCBs, chlorinated hydrocarbon pesticides and chlorophenols in the Isipingo Estuary, Natal, Republic of South Africa, Mar. Pollut. Bull. 32 (1996) 572–575, https://doi.org/10.1016/0025-326X(96) 84578-4.
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Mohanty, Liquid-liquid equilibrium (LLE) data for ternary mixtures of [C4DMIM]-[PF6]+[PCP]+[water] and [C4DMIM][PF6]+[PA]+[water] at T=298.15K and p=1atm, Fluid Phase Equilib. 381 (2014) 12–19, https://doi. org/10.1016/j.fluid.2014.08.004. [17] S. Apfalter, R. Krska, T. Linsinger, A. Oberhauser, W. Kandler, M. Grasserbauer, Interlaboratory comparison study for the determination of halogenated hydrocarbons in water, Fresenius J. Anal. Chem. 364 (1999) 660–665, https://doi.org/10. 1007/s002160051409. [18] O.G. Sas, I. Domínguez, B. González, Á. Domínguez, Liquid-liquid extraction of phenolic compounds from water using ionic liquids: literature review and new experimental data using [C2mim]FSI, J. Environ. Manag. 228 (2018) 475–482, https://doi. org/10.1016/j.jenvman.2018.09.042. [19] O.G. Sas, I. Domínguez, Á. Domínguez, B. González, Using bis (trifluoromethylsulfonyl)imide based ionic liquids to extract phenolic compounds, J. Chem. Thermodyn. 131 (2019) 159–167, https://doi.org/10.1016/j.jct.2018.11. 002. [20] N. Deng, M. Li, L. Zhao, C. Lu, S.L. De Rooy, I.M. Warner, Highly efficient extraction of phenolic compounds by use of magnetic room temperature ionic liquids for environmental remediation, J. Hazard. Mater. 192 (2011) 1350–1357, https://doi.org/ 10.1016/j.jhazmat.2011.06.053. [21] F. Huang, P. Berton, C. Lu, N. Siraj, C. Wang, P.K.S. Magut, I.M. Warner, Surfactantbased Ionic Liquids for Extraction of Phenolic Compounds Combined With Rapid Quantification Using Capillary Electrophoresis, 2014 2463–2469, https://doi.org/ 10.1002/elps.201300589. [22] K.S. Khachatryan, Æ.S.V. Smirnova, A.A. Formanovsky, Æ.I.V. Pletnev, Solvent Extraction and Extraction – Voltammetric Determination of Phenols Using Room Temperature Ionic Liquid, 2005 464–470, https://doi.org/10.1007/s00216-004-2872-y. [23] R. Sulaiman, M.K. Hadj-Kali, S.W. Hasan, S. Mulyono, I.M. AlNashef, Investigating the solubility of chlorophenols in hydrophobic ionic liquids, J. Chem. Thermodyn. 135 (2019) 97–106, https://doi.org/10.1016/j.jct.2019.03.026. [24] E.S. Sashina, D.A. Kashirskii, Pyridinium-based ionic liquids — application for cellulose processing, Ion. Liq. - Curr. State Art. (2015)https://doi.org/10.5772/ 59286. [25] C.W. Cho, Y.C. Jeon, T.P.T. Pham, K. Vijayaraghavan, Y.S. Yun, The ecotoxicity of ionic liquids and traditional organic solvents on microalga Selenastrum capricornutum, Ecotoxicol. Environ. Saf. 71 (2008) 166–171, https://doi.org/10.1016/j.ecoenv. 2007.07.001. [26] D.J. Couling, R.J. Bernot, K.M. Docherty, J.N.K. Dixon, E.J. Maginn, Assessing the factors responsible for ionic liquid toxicity to aquatic organisms via quantitative structure-property relationship modeling, Green Chem. 8 (2006) 82–90, https:// doi.org/10.1039/b511333d. [27] K.M. Docherty, C.F. Kulpa, Toxicity and antimicrobial activity of imidazolium and pyridinium ionic liquids, Green Chem. 7 (2005) 185–189, https://doi.org/10.1039/ b419172b. [28] A.B. Lakshmi, S. Sindhu, S. Venkatesan, Performance of Ionic Liquid as Bulk Liquid Membrane for Chlorophenol Removal, vol. 5, 2013 1129–1137.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Liquid-liquid extraction of chlorophenols from wastewater using hydrophobic ionic liquids R. Sulaiman a, I. Adeyemi a, S.R. Abraham b, S.W. Hasan a, I.M. AlNashef a,⁎ a b
Department of Chemical Engineering, Khalifa University, Masdar City Campus, P. O. Box 127788, Abu Dhabi, United Arab Emirates Department of Chemical Engineering, University at Buffalo (SUNY), Buffalo, NY 14260-4200, USA
a r t i c l e
i n f o
Article history: Received 9 April 2019 Received in revised form 29 August 2019 Accepted 2 September 2019 Available online 03 September 2019 Keywords: Ionic liquids Chlorophenols Wastewater Liquid-liquid extraction
a b s t r a c t In this work, liquid-liquid extraction of 3-chlorophenol, 2, 5-dichlorophenol, 2, 4, 6-trichlorophenol and pentachlorophenol from wastewater was conducted using five hydrophobic ionic liquids (ILs). Moreover, the impact of key parameters like molecular structure, pH, temperature, phase ratio and initial concentration of feed on the removal of the chlorophenols (CPs) from their aqueous solution were studied. The results showed that the molecular structure of the IL has an important impact on the removal of CPs. Generally, the evaluation of different ILs with the same CPs revealed that piperidinium and pyridinium based ILs have greater extraction efficiency than sulfonium and imidazolium based ILs. It was found that the extraction efficiency decreased with the decrease of IL:wastewater mass ratio, with a maximum extraction at a ratio of 1:1. The results showed that temperature and initial concentrations in the range of 25–45 °C and 4–1000 mg/L, respectively had no significant impact on the extraction for all studied systems. © 2019 Elsevier B.V. All rights reserved.
1. Introduction The removal of chlorophenols (CPs) from wastewater has gained significant attention due to the widespread usage and toxicity of these phenolic compounds. CPs have been used for industrial and agricultural purposes in activities ranging from pesticides/organic solvents synthesis, flame retardant and wood treatment agents. Moreover, CPs could be produced naturally in soils through a reactive interaction of inorganic chloride and organic compounds in the presence of fungi. Consequently, an annual production of 200,000 tons of CPs has been reported [1]. Most of these CPs enter into the aquatic environment in concentrations of 0–30 μg/L and sometimes reaching 1000 μg/L in contaminated surface waters and sewage [2–5]. These concentrations of CPs are significant considering their toxic effects such as carcinogenicity, endocrine disruption ability, and mutagenicity. Furthermore, CPs have relatively strong resistance to biodegradation. Hence, numerous processes have been studied for the removal of CPs from wastewater due to the cumulative implications of their environmental impacts. Amongst the top methods for the removal of CPs are biological processes, advanced oxidation process, adsorption, chemical degradation, photolysis, ion exchange, and liquid-liquid extraction [6–10]. Liquidliquid extraction provides benefits over other methods such as biological and thermal decomposition processes for the removal of chlorophenols from water. This is because biological processes utilize ⁎ Corresponding author. E-mail address: [email protected] (I.M. AlNashef).
https://doi.org/10.1016/j.molliq.2019.111680 0167-7322/© 2019 Elsevier B.V. All rights reserved.
microorganisms which are affected by a high dose of CPs. Moreover, thermal decomposition requires substantial energy due to the heating of the CPs [11,12]. However, the usage of liquid-liquid extraction often relies on solvents that are flammable, toxic and volatile. This has engendered the development of improved alternative solvents such as ionic liquids (ILs). ILs are solvents which are composed of ions and are usually liquid below 100C. These solvents could form a crucial building block of green chemistry with a careful selection of its cation and anion. They are attractive for the extraction of micro-pollutants such as polycyclic aromatic hydrocarbons (PAHs) because of their low vapor pressure, tuneability and dissolving strength for CPs. Due to these benefits, there had been a number of reported publications on the removal of CPs from wastewater with ILs [13–22]. However, majority of the research have focused on imidazolium based ILs. For instance, Bekou et al. [13] studied the removal of CPs from water with two hydrophobic ILs, 1-butyl-3methylimidazolium hexafluorophosphate, [bmim][PF6] and 1-ethyl-3methylimidazolium bis(perfluoroethylsulfonyl)imide, [emim][bmim] [Tf2N]. The extraction efficiency of the chlorinated phenols was better when [bmim][PF6] was utilized and when pH b pKa. In a similar study focused on the extraction of pentachlorophenol, Fan et al. [14] studied the use of imidazolium-based ILs with different alkali chain. It was observed from their results that increasing alkali chain length of the IL cation resulted in the increase of extraction efficiency for the same phenolic compound. Another research by Lakshmi et al. [15] reported the significance on imidazolium based ILs where they investigated the extraction of CPs with 1-butyl-3-methyl imidazolium based ILs with
R. Sulaiman et al. / Journal of Molecular Liquids 294 (2019) 111680
chloride, hexafluorophosphate and tetrafluoroborate anion, and their mixture with organic solvents. It was found that the extraction with 1-butyl-3-methylimadazolium tetrafluoroborate [bmim][BF4], which was mixed with tributyl phosphate, produced the maximum extraction efficiency. In addition, the solubility of CPs at different temperatures in bis(trifluoromethylsulfonyl)imide based hydrophobic ILs was reported by our group recently [23]. The results revealed that imidazolium and pyridinium based ILs have high capacity of dissolving CPs. The CPs showed decreasing solubility values with the increasing number of chlorine atom in their molecules. Although the investigations with imidazolium ILs have revealed their viability for the removal of CPs, there is a need to further investigate other types of ILs for CPs extraction. This evaluation is required in order to assess their capabilities, gain some insightful understanding of the process and derive resulting benefits that comes with their usage. For instance, pyridinium based ILs have been reported to show good economics [24] and less toxicity [25–27] but there are little to no studies present on their usage. Thus, in this work, we investigated the extraction efficiency of 3-Clorophenol (3-CP), 2,5-Dichlorophenol (DCP), 2,4, 6-Trichlorophenol (TCP), and Pentachlorophenol (PCP) using five bis(trifluoromethylsulfonyl)imide based ILs with different cations, namely 1-butyl-3-methylimidazolium, 1-butyl-1methylpiperidimium, diethylmethylsulfonium, 1-butyl-3methylpyridinium, 1-ethyl-3-methylpyridinium, as solvent. To the best of our knowledge, the extraction of CPs using bis (trifluoromethylsulfonyl)imide based ILs with the cation of sulfonium, piperidinium and pyridinium has not been reported before. Additionally, the effect of temperature, pH and IL:water mass ratio on extraction efficiency of selected CPs and ILs was investigated.
[C4mPip][Tf2N]
Table S1 presented the used chemicals and their properties. The ILs were procured from Ionic Liquid Technologies, Io-Li-Tec (Heilbronn, Germany). 3-CP, DCP, and TCP were procured from Acros, Thermo Fisher Scientific (USA). PCP, acetonitrile, and sulfuric acid were procured from Sigma-Aldrich (USA). Ultrapure water (18.2MQ.cm) used during experiment was produced by the water purification unit Suez Water (UK). Further purification was not conducted for all chemicals prior to use. Coulometric Karl Fischer Titrator, Aquamax KF (GR Scientific, UK) was used to measure the water content of pure ILs. The pH measurement of the aqueous solution was done using benchtop meter (infolab, Xylem, USA) with an uncertainty of ±0.1 pH, ±0.1 °C of temperature H2SO4 and NaOH was used to modify pH of aqueous solution having CPs. High Performance Liquid Chromatography (HPLC; Dionex Ultimate 3000 system, Shimadzu Japan) equipped with a Raptor ACR C-18 column (250 mm × 4.6 mm × 5 μm), purchased from RESTEK (USA), was utilized to quantify CPs in aqueous solution. 45% of 0.0025 M sulfuric acid solution and 55% acetonitrile based on volume was used as mobile phase to quantify PCP and TCP in water. 45% water and 55% acetonitrile based on volume was used as mobile phase to quantify 3-CP and DCP in water. The absorbance wavelength of 220 nm was chosen at for PCP. The absorbance wavelength of 285 nm was chosen for 3-CP, DCP and TCP.
[1,3emPY][Tf2N]
90 80 70 60 50 40 30 20 10 0 0
0.5
1
1.5
2
2.5
3
3.5
Fig. 1. Contact time effect on extraction efficiency of 3-CP using ILs (T = 25 °C, 250 rpm, and IL:water ratio 1:10).
in a 20 mL vial. The vial containing IL and CP aqueous solution was then mixed vigorously using an incubator shaker at 250 rpm and 25 °C for a fixed time. After mixing, 15 min of centrifugation was performed for complete phase separation. Subsequently, samples were taken from the aqueous phase and analyzed with the HPLC in order to determine CP concentrations after extraction. Sensitivity analysis of key parameters that could influence the removal of the CPs from their aqueous solution was investigated. The effect of temperature, mass phase ratio, pH, initial concentration of aqueous feed, and molecular structure was studied. The extraction efficiency (E) was calculated according to Eq. (1): E¼
1−
Cw 100% C wo
ð1Þ
where Cwo and Cw are the concentrations of chlorophenols in the water phase before and after extraction, respectively. 3. Results and discussion The 3-CP, DCP, TCP, PCP were extracted using five hydrophobic bis (trifluoromethylsulfonyl)imide based ILs from their aqueous solution. The mixing time is a crucial parameter for solvent extraction experiments because it ensured equilibrium mass transfer of the CPs to the IL phase. Consequently, the mixing time was observed at regular intervals to determine its impact on the extraction efficiency (Figs. 1– S1–4). It was found that the extraction efficiency remained practically constant when the system reached equilibrium (Tables S7–S10). These predetermined equilibrium times which were stated in Table S11 were subsequently utilized for further studies. The errors for the experiments has been conducted using duplicate measurements (n = 2) and the standard deviation was determined to be b2% for all extraction. In addition, the extraction efficiency obtained from Lakshmi et al. [15,29] was compared with those measured in this work (Table 1). The TCP extraction efficiency comparison showed that the piperidinium
2.2. Extraction procedure The extraction experiments started with the preparation of the CP aqueous solution which includes a concentration of 97 mg/L for 3-CP, DCP, and 100 mg/L for TCP. The PCP was prepared as a 10 mg/L aqueous solution due to its lower solubility in water. Thereafter, 10 g of each CP aqueous solution was added to 1 g of IL1 and the combination was placed 1 The ILs were used in the dried form as saturation has no significant impact on the extraction efficiency (Tables S2–S6)
4
Contact Time (h)
2. Materials and methods 2.1. Chemical reagents
[Et2MeS][Tf2N]
100
Extracon effeciency (%)
2
Table 1 Comparison of the extraction efficiency in this study with literature data. IL type
TCP-extraction efficiency (%)
[1,3emPY][Tf2N] [C4mPip][Tf2N] [bmim][Cl] [bmim][PF6] [bmim][BF4]
99.14 98.98 90.18 [15,29] 93.41 [15,29] 99.79 [15,29]
R. Sulaiman et al. / Journal of Molecular Liquids 294 (2019) 111680
Extraction effeciency (%)
100
3-CP
DCP
TCP
3
PCP
95 90 85 80 75 70
[bmim][Tf2N]
[C4mPip][Tf2N]
[Et2MeS][Tf2N]
[1,3bmPY][Tf2N]
[1,3emPY][Tf2N]
Ionic liquid Fig. 2. The effect of molecular structure of CPs on extraction efficiency (T = 25 °C, 250 rpm, and IL:water ratio 1:10).
and pyridinium based ILs studied has excellent extraction capacity when compared to the ones reported by Lakshmi et al. [15,29]. However, ILs with [PF6] and [BF4] produce hydrofluoric acid at a relatively fast rate compared to [Tf2N] anions. 3.1. Effect of molecular structure The impact of the molecular structure of the CPs and ILs were studied in order to gain a fundamental insight into their behavior during the extraction process. This sensitivity analysis allows for the assessment of the effect of different cations and their molecular structures on the extraction of CPs. This criterion supports the quest for optimum ILs for the extraction purposes as the IL groups differ in their extraction capacity toward different CPs. Furthermore, this effect of the classes of ILs allows for the usage of the promising properties exhibited by different ILs. The effect of molecular structure of CPs was evaluated by using four CPs (3-CP, 2, 5-DCP, TCP and PCP) with a fixed IL as the extraction media (Table S12). As depicted in Fig. 2, the extraction efficiency of CPs followed the order of PCP N TCP N DCP N 3-CP with pyridinium and sulfonium based ILs. By comparing the log Kow, (octanol/water partition) values of PCP, TCP, 2, 5-DCP and 3-CP which are 5.1, 3.7, 3.1 and 2.5, respectively, it was noted that the more hydrophobic the CPs was, the more CPs were extracted [29]. Similar observation has been reported by Bekou et al. [13] where they attributed the rising hydrophobicity to the increasing chlorine substituents in the CPs. Hence, the increased hydrophobicity of CPs with higher chlorine substituents resulted in the
[bmim][Tf2N]
[C4mPip][Tf2N]
increased hydrophobic bonding between the cation of the ILs and the CPs which enhanced the extraction process. The impact of IL structure on extraction efficiency of CPs is shown in Fig. 3. With regards to imidazolium and piperidinium based ILs, there are variations from the trend described above (PCP N TCP N DCP N 3-CP). Although the piperidinium based IL followed the general trend with the exception of DCP, imidazolium based ILs deviates significantly from this trend. The random trend observed for imidazolium based ILs could be found from some previous studies like that of Bekou et al. and Lakshmi et al. For example, in the work of Bekou et al., they reported that for the ionic liquid [emim][Beti] there was more extraction capacity for 2,4-DCP than PCP. In another study, Lakshmi et al. evaluated the extraction of chlorophenols with different solvents which includes three imidazolium based ILs. In their results, the behavior of the three imidazolium based ILs investigated was not based primarily on the number of chlorine atom and their hydrophobicity. For all the imidazolium based ILs, the extraction efficiency did not follow the general trend, rather they deviate significantly from that hydrophobic interaction mode of extraction with the CPs. Besides the reports on the behavior of imidazolium based ILs, there was a study reported by Deng et al. for the IL [3C6PC14][FeCl4] which showed that it has higher extraction of 2-CP when compared with 2,4-DCP. These reports show that the extraction of the CPs is not determined solely based on the hydrophobicity of the chlorophenols with water. The extraction mechanism is complicated by the presence of hydrophobic interaction of the CPs with the cation of the ionic liquids, the hydrogen bonding between
[Et2MeS][Tf2N]
[1,3bmPY][Tf2N]
[1,3emPY][Tf2N]
Extraction effeciency (%)
100 95 90 85 80 75 70
3-CP
DCP
TCP
PCP
Chlorophenols Fig. 3. The effect of molecular structure of ILs on extraction efficiency (T = 25 °C, 250 rpm, and IL:water ratio 1:10).
4
R. Sulaiman et al. / Journal of Molecular Liquids 294 (2019) 111680 3-CP
DCP
TCP
Table 2 Temperature effect on the extraction efficiency of CPs (250 rpm, phase ratio: 10).
Extraction effeciency (%)
100 90 80 70 60 50 40 30 20 10 0
Temperature Extraction efficiency (%) (°C) 3-CP DCP IL
2
4
6
pH
8
10
12
Fig. 4. Effect of pH on extraction efficiency of 3-CP, DCP and TCP using [1,3emPY][Tf2N] (T = 25 °C, 250 rpm, and IL:water ratio 1:10).
the CPs and the anion of the ILs used and the location of the chlorine atom on the benzene ring. Therefore, the CPs behaves differently with various cations and anions of the ILs. Moreover, comparing the extraction efficiency of different ILs with the same CPs, it was observed that piperidinium and pyridinium based ILs had greater extraction than sulfonium and imidazolium based ILs, with the exception of 2,4-DCP. For 2,4-DCP, the piperidinium and pyridinium based ILs still have greater extraction than sulfonium based ILs. However, the extraction of DCP with imidazolium based ILs was close to that of piperidinium based ILs. The lower extraction efficiency of sulfonium based IL than piperidinium and pyridinium IL may suggest that it was resulted from lacking π − π stacking of aromatic rings between CPs and [Et2MeS] cation. However, when it is compared to imidazolium based ILs, there is no big difference in the extraction efficiency, thus it suggests that the main driving force of the extraction are the hydrophobic interaction between CPs and the cation of the IL and the hydrogen bonding between CPs and IL anion [14,20,28]. Fan et al. found that the extraction efficiency of phenols including pentachlorophenol increased with increasing alkali chain length of the cation in imidazolium based ILs [14]. In our work, the same behavior was observed for pyridinium based ILs. The extraction efficiency of all CPs with [1,3bmPY][Tf 2 N] was higher than [1,3emPY][Tf 2N]. It can be observed from the chemical structure, [1,3bmPY][Tf2N] has longer alkali chain than [1,3emPY][Tf2N]. Increasing alkali chain in the cation will increase the hydrophobicity of IL, which further increase the hydrophobic interaction between CPs and ILs during extraction process. This is why the extraction efficiency for a given CP is higher with [1,3bmPY][Tf2N] than with [1,3emPY][Tf2N].
3-CP
DCP
25 35 45
TCP
IL
IL
[bmim] [Tf2N]
[1,3emPY] [bmim] [Tf2N] [Tf2N]
[1,3emPY] [C4mPip] [Tf2N] [Tf2N]
[1,3emPY] [Tf2N]
84.56 84.67 83.25
87.07 86.07 87.15
94.54 94.90 92.70
94.77 89.69 91.23
94.25 93.29 92.20
94.77 90.19 93.79
3.2. Effect of pH The impact of the pH on the extraction of 3-CP, DCP from their aqueous solution into [bmim][Tf2N] and [1,3-emPY][Tf2N] and the impact of the pH on the extraction of TCP from their aqueous solution into [C4mPip][Tf2N] and [1,3-emPY][Tf2N] were evaluated for pH range 3–11. The effect of varying initial pH of the water on extraction efficiency has been studied since wastewater is present in different acidic and basic condition, and the extraction efficiency is influenced by the initial pH. The extraction of the CPs is single stage, thus, the measurement of the equilibrium pH was not conducted after the extraction. The results shown in Fig. 4 and Table S13, revealed that the CPs extraction efficiencies were higher for the acidic solutions. This increase in the extraction of the CPs in acidic solutions can be attributed to the low dissociation of the neutral CPs into ions due to their weak acidic nature. The weak acidity of the CPs is obvious from the pKa values of 9.1, 7.5 and 6.2 for 3-CP, DCP and TCP, respectively [30]. Consequently, the low dissociation of the CPs and their presence in the molecular form would allow an easy transfer into ILs. The ease of transfer can be linked to the possibility of the neutral CPs to migrate into the ILs via the two mechanisms of extraction as compared to the alkaline solution where only one route is dominant. The first extraction route involves the formation of hydrogen bond between the hydrogen atom of the neutral CPs and the anion of the ILs and the second route utilizes the hydrophobic interaction between the CPs and the cation of the IL. In the acidic solution, both routes are active. However, in the alkaline region the transfer of the CPs to the IL would occur mainly through the hydrophobic interaction alone as the dissociation of the CPs would lessen the strength of the hydrogen bond. This explains why the distribution ratio is less in the alkaline solutions.
3.3. Effect of IL:water mass ratio The extraction experiments were conducted using different IL:water mass ratio to determine the minimum quantity of IL that would be needed for efficient extraction. In this extraction process, the mass of
TCP
Extraction effeciency (%)
100 90 80 70 60
0
5 10 Mass ratio (Water: IL)
15
20
Fig. 5. Effect of phase ratio on extraction efficiency of 3-CP, DCP and TCP using [1,3emPY] [Tf2N] (T = 25 °C, 250 rpm).
Fig. 6. Effect of concentration of CP in feed phase on extraction efficiency of 3-CP and DCP using [1,3emPY][Tf2N] (T = 25 °C, 250 rpm and phase ratio: 10).
R. Sulaiman et al. / Journal of Molecular Liquids 294 (2019) 111680
5
the aqueous solution was increased while keeping the IL mass constant at 1 g (Fig. 5 and Table S14). The extraction efficiency decreased with the decreasing mass ratio, with a maximum extraction at IL:water ratio of 1. Extraction efficiencies N91% were attained for all studied systems for IL:water ratio of 1:5. TCP was able to sustain its extraction capability until IL:water ratio of 1:15. This could be attributed to the increase of hydrophobicity due the increase chlorine substituents which decreased its affinity toward water. However, IL:water ratio of 1:10 was utilized for further studies to allow for consistencies and uniformity for all CPs studied. The extraction efficiencies at IL:water ratio of 1:5 and 1:10 were not significantly different for 3-CP, DCP and TCP. Hence, IL:water ratio of 1:10 would reduce the quantity of the ILs used for the extraction process without significantly affecting the extraction efficiency of the CPs. 3.4. Effect of temperature The extraction experiments were done at three temperatures (25, 35, and 45 °C) and the results were shown in Table 2. In general, no substantial change in the extraction efficiency was shown from the results within the temperature range of 25–45 °C for all studied systems. Similar observation was reported by Lakshmi et al. [15] [28] on the extraction of phenols and their substituents. The ability of the extraction to be conducted within the range of 25–45 °C with no effect on the CPs removal allows for flexibility and provides the possibility to treat the wastewater at different temperatures. More importantly, there is a possibility to conduct the process at ambient temperature in order to save energy and consequently operational cost. 3.5. Effect of concentration of CPs in feed phase The effect of concentration of CP in feed phase on extraction efficiency was investigated by studying 3-CP and DCP extraction efficiency using [bmim][Tf2N] and [1,3emPY][Tf2N] as solvents. The CPs in the aqueous solution had initial concentrations ranging from 4 to 1000 mg/L. The results indicated that there was no substantial variance in the extraction efficiency within the range of concentration studied (Fig. 6 and Table S15). 3.6. Mechanism of CPs extraction with hydrophobic ILs There are two main extraction mechanisms for the CPs from the aqueous phase. The first is through the hydrogen bond interactions between the CPs and the anion of the IL. To demonstrate this pathway, we have used TMoleX and Conductor-like screening model for realistic solvents (COSMO RS) models. The sigma surfaces/profiles were obtained
Fig. 8. Sigma profile of DCP.
as depicted in Figs. 7–9. The threshold value for hydrogen bonding is σhb = ± 0.0084 eA − 2 from the sigma profile estimations. Compounds with a general peak at σ b σhb has hydrogen bond donor ability whilst those with σ N σhb has hydrogen bond acceptor ability. Based on this, it becomes clear that the CP has general peaks on the σ b σhb region, a characteristic of hydrogen bond donor capacity. However, the [Tf2N] anion exhibited a general peak in the σ N σhb region, a feature of hydrogen bond acceptor ability. Consequently, hydrogen bond interactions between the CP and [Tf2N] anion could be affirmed. The second mode of extraction is through the hydrophobic interactions between the CPs and the cation of the IL. The hydrophobic interaction is a long range attractive phenomenon where non-polar molecules interact with water in a special way. This phenomenon has been well described in various works like that of Israelachvili and Pashley [31], Feher [32], Luzar et al. [33], Pangali et al. [34], Pratt and Chandler [35] etc. Moreover, similar findings of the hydrophobic interaction between phenolic compounds and the cation of ILs have been reported by Fan et al. [14], Deng et al. [20], Lakshmi et al. [15]. 4. Conclusions The removal of 3-CP, DCP, TCP, PCP using five different hydrophobic ILs through liquid-liquid extraction was investigated. In addition,
Fig. 7. Sigma surfaces of bis(trifluoromethylsulfonyl)imide anion and DCP.
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R. Sulaiman et al. / Journal of Molecular Liquids 294 (2019) 111680
Fig. 9. Sigma profile of [Tf2N] anion.
parametric studies based on molecular structure, pH, temperature, IL mass CP aqueous solution mass ratio and initial concentration of feed was observed. The results showed that the molecular structure of both the IL and CP had an effect on the removal of the CPs. The evaluation of different ILs with the same CPs revealed that piperidinium and pyridinium based ILs have higher extraction efficiency than sulfonium and imidazolium based ILs having the same anion. The extraction efficiency decreased with decreasing mass ratio, with a maximum extraction efficiency at 1:1 ratio. The effect of temperature and initial concentrations showed that there was no substantial difference in the extraction efficiency within the range of 25–45 °C and 4–1000 mg/L, respectively, studied for all systems. Overall, it was found that it is possible to achieve high extraction efficiency by controlling the different operational conditions during the extraction process such as temperature, contact time, and IL:CP aqueous solution mass ratio. It is also possible to select the most proper ILs based on the practical application from industry such as the temperature of the wastewater, the water quality requirement, etc. Acknowledgment This work was funded by Khalifa University of Science, Technology and Research through research grant CIRA-2018-69 and by ADEK through grant 8434000282. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.111680. References [1] T. Ge, J. Han, Y. Qi, X. Gu, L. Ma, C. Zhang, S. Naeem, D. Huang, The toxic effects of chlorophenols and associated mechanisms in fish, Aquat. Toxicol. 184 (2017) 78–93, https://doi.org/10.1016/j.aquatox.2017.01.005. [2] D.F. Grobler, J.E. Badenhorst, P.L. Kempster, PCBs, chlorinated hydrocarbon pesticides and chlorophenols in the Isipingo Estuary, Natal, Republic of South Africa, Mar. Pollut. Bull. 32 (1996) 572–575, https://doi.org/10.1016/0025-326X(96) 84578-4.
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