Shuai Xu,Qi Zhu,Shaojie Xu,Manjing Yuan,Xuliang Lin,Wenjing Lin,Yanlin Qin,*,Yuliang Li,*

1 School of Chemical Engineering and Light Industry,Guangdong University of Technology,Guangzhou 510006,China

2 Key Laboratory of Subsurface Hydrology and Ecological Effects in Arid Region,Ministry of Education,School of Water and Environment,Chang'an University,Xi'an 710064,China

3 Guangzhou Key Laboratory of Clean Transportation Energy Chemistry,Guangdong University of Technology,Guangzhou 510006,China

Keywords:Ionic liquid Aqueous two-phase systems 2-chlorophenol HPLC Extraction

ABSTRACT In this paper,the aqueous two-phase systems(ATPS)containing n-ethylpyridinium tetrafluoroborate([EPy]BF4),sodium-based salts,and water were studied and the extraction efficiency of 2-chlorophenol was measured to study the ATPS performance in extracting phenolic compounds.The binodal curves of[EPy]BF4+sodium carbonate(Na2CO3)ATPS and[EPy]BF4+sodium dihydrogen phosphate(NaH2PO4)ATPS have been determined at 308.15 K,318.15 K,and 328.15 K and atmospheric pressure.After getting good correlation with Merchuk equation,the binodal curves together with gravimetric method were used to calculate the tie-lines data.Furthermore,the reliability of tie-lines data was verified using Othmer-Tobias and Bancroft equations.Then,the salt influence and temperature influence on the phase behavior were discussed and the results show the salt-outing ability of Na2CO3 is better than NaH2PO4.With the aim of studying the ATPS performance in extracting phenolic compounds,extraction efficiency for 2-chlorophenol at different temperatures were studied and the results show that[EPy]BF4+Na2CO3 ATPS is preferred than[EPy]BF4+NaH2PO4 ATPS in applications.

1.Introduction

Phenolic compounds,which is commonly from petrochemical and coking operations,pharmaceuticals industries,petroleum refinery,coloring textiles and paper-making processes[1].Due to the weakly biodegradable,the degrade of phenolic compounds in the ecological system is limited and phenolic compounds in the natural environment tend to accumulate.The 2-chlorophenol is one of the toxic chlorinated phenols and it is water solubility and usually emit the unpleasant odor,which can cause serious threats to human health [1,2,52].Therefore,2-chlorophenol is considered as priority pollutant by many countries[3].It is essential therefore,removal of 2-chlorophenol from wastewaters prior to discharge into the environment.The aqueous two-phase systems(ATPS)extraction technology has increasingly attracted people's attention and has been successfully applied in the extraction of antibiotics before the discharge of swine husbandry effluents[4],the detection of sulfonamides in meat products [5],and the extraction of herbicides in wastewaters[6].

ATPS are formed by mixing two mutual incompatible reagents contain polymer-salt or polymer-polymer at a certain critical concentration in aqueous solutions[7,8].Compared with traditional organic solvent extractions,ATPS present efficient,economical,and served as an environmentally friendly media.Consequently,ATPS extraction technology has been widely used for extraction of proteins[9,10],drug molecules[11],heavy metal ions[12],small organic molecules[13]and cell organelles[14].In recent decades,ionic liquid(ILs)are under intense investigation and are considered a potential replacement for volatile organic solvents.ILs attracted much attention for greater application on account of that they not only present several beneficial properties such as low toxicity,negligible vapor pressure,and high ionic conductivity,but also possess unique features such as recyclability and high thermal chemical stability[15].With high performance in reactions and separations,ILs have more used in separation processes[16],which is a breakthrough in the fields of solvents.Reported by Gutowski et al.[17]for the first time,the IL+salt ATPS have been increasingly investigated as novel ATPS.Compared with ionic liquid as adjuvants or organic solvent ATPS[18,19],the IL-based ATPS is considered to be more environmentally friendly.

To date,several IL-based ATPS have been developed including the[Bmim]BF4+salt(Na2CO3,NaCl,Na2SO3,Na2SO4,NaH2PO4,Na3PO4,NaC2H3O2,Na3C6H5O7,Na2C4H4O6,(NH4)2SO4,and (NH4)3C6H5O7)ATPS [20–23].Nevertheless,some researchers pointed out that the imidazolium-based ionic liquids possess certain toxicity[24].Furthermore,as the study found that pyridine-based ionic liquids exhibit more merits than imidazolium-based ionic liquids,for instance,lower vapor pressure,lower cost,less polluting and better stability[25].More importantly,pyridine-based ionic liquids are more and more used in extraction and separation with their good two phase-forming properties[26,27].Thus,determination of phase equilibrium data of pyridiniumbased ILs and inorganic salts ATPS has theoretical and practical significance.Since the toxicity of IL is positively related to the length of alkyl chains in cations[28],the n-ethylpyridinium([EPy]+),which possess shorter alkyl chain,was selected in this study.Notably,some researchers pointed out that the tetrafluoroborate (BF4−) anion in imidazolium-based ILs could undergo hydrolysis and form hydrofluoric acid(HF)in the presence of water and the ILs with long alkyl chains are more easily to undergo hydrolysis[29].However,for[EPy]BF4used in this work,the BF4−was not found to undergo hydrolysis in our experiment process.First,as show in Fig.S1(in the Supplementary Material),the glass centrifuge tubes(borosilicate glass)were found to no frost after one week [30].As we know,HF is the most corrosive for glass,which minor (mg·L-1) levels of HF could cause the frost in the glass and turn the glass to white[31].Second,the samples were analyzed by ion chromatography and not found fluoride response.This may result from the cation-anion interaction strength in[EPy]BF4are strong enough to prevent the hydrolysis of BF4−.

Our group had studied the phase diagrams about[EPy]BF4+Na2CO3and[EPy]BF4+NaH2PO4ATPS at 298.15 K[32].This work is the continued study of the[EPy]BF4+sodium salts ATPS,using both empirical equation and properties of ions to further studied the salt influence.The temperature influence on phase diagram is investigated to further study its effect on extraction performance.Meanwhile,2-chlorophenol was used in this work to study the ATPS extraction performance in extracting phenolic compounds.

It is therefore in this study,binodal data and tie-lines data of the[EPy]BF4+Na2CO3and[EPy]BF4+NaH2PO4ATPS were determinate at 308.15 K,318.15 K,and 328.15 K and atmospheric pressure.The appropriate methods,cloud point titration and gravimetric method,were used to obtain the binodal data and tie-lines data.The salts influence on the phase diagram was discussed using Setschenow-type equation and ions hydration Gibbs energy.The high-performance liquid chromatography(HPLC)method was used to analysis 2-chlorophenol content to study the extraction performance of [EPy]BF4+sodiumbased salts ATPS.

2.Experimental

2.1.Materials

The sources of the reagents and their purity are listed in Table 1.The salts are of analytical grade and directly used without further purification.The[EPy]BF4was put in the vacuum oven at 333.15 K and moderate vacuum (0.01 MPa) for 24 h before using.Freshly prepared deionized water(HHitech,Master Evo-S45UVF)was used for the salt solution prepared.

Table 1The sources and purities of chemicals used in the study

2.2.Experimental procedure for ATPS characterization

The cloud point titration method is used to determine binodal data,and the method has been widely used in previous works[33].Briefly,a great number of boundary data of immiscible regions can be obtained in the method.Known mass fraction of[EPy]BF4were put in a pure and dry colorimetric tube.A syringe with a small needle was used to add the salt solution dropwise and slowly until the system cloudy.When the composition of the system is close to cloud point,only one drop (about 0.0050 g)of the salt solution can make the system cloudy,which hardly miss the cloud point.The mass of[EPy]BF4,salt,and water can be easily measured and calculated.Then the water was added dropwise until the system turns to clear and the mass fraction of the components was calculated again.Repeat the procedure multiple times to obtain sufficient binodal data.The temperature was maintained by a water bath with an uncertainty of 0.05 K throughout the experiment.The analytical balance(model BS 124S,Beijing Sartorius Instrument Co.,Ltd.,China)with an uncertainty of 0.0001 g was used to measure the mass.

The tie lines were determined by the gravimetric method proposed by Merchuk et al.[34],and tie-lines data for a large amount of ATPS were successfully obtained using gravimetric method[35–37].The total compositions were selected in the biphasic regions of the phase diagram to determine the tie lines data.Mixing[EPy]BF4,salt,and water at the selected compositions and total mass of the systems are approximately 10 g,then vortex mixer was used to mix the system thoroughly and stand for 24 h to achieve complete phase separation.The syringe with a small needle was used to take the liquid of top and bottom phases,and then weight the top phase and bottom phase by analytical balance.The lever arm rule was used to calculate the compositions of the top and bottom phases using the following equations:

In the equations,w1,and w2denote mass fraction of[EPy]BF4and salt,respectively;The superscript m,b,and t denote mixture,bottom phase,and top phase,respectively;a,b,and c are parameters from fitting to binodal curves;and α denotes the ratio between the topphase mass and the total mass.Based on the tie-line data,the following Eqs.(4)and(5)were used to calculate the TLL and S:

where TLL and S are tie-line length and slope of tie line,respectively.

2.3.2-chlorophenol extraction and quantification

The 2-chlorophenol extraction process is similar to abovementioned tie-lines preparation.The salt,water,and [EPy]BF4were added in the centrifuge tubes according to the selected composition.The 2-chlorophenol was added to the tubes and the concentration was controlled at 0.200 g·L−1.The vortex mixer was used to mix the system thoroughly and then stand for 24 h.The 2-chlorophenol concentrations in the top and bottom phase were determined using high-performance liquid chromatography(HPLC,Thermo Fisher Scientific U-3000,Germany)equipped with a Hypersil Gold C18 column(3.0 μm particle size,3 mm×150 mm,Thermo Fisher Scientific)and a UV detection.The mobile phase consisted of methanol(A)and water(B).A constant system with the condition of 0–15 min,60%A and 40%B was used.The flow rate of the mobile phase was 1.0 ml·min−1.The injection volume was 5 μl.The column temperature was set at 30°C and the detection wavelength was set at 275 nm.

Fig.1.Binodal curves and tie lines of[EPy]BF4(1)+Na2CO3(2)+H2O(3)ATPS at 308.15 K,318.15 K and 328.15 K.

3.Results and Discussion

3.1.Binodal curves and correlation

The experimental binodal data of the[EPy]BF4+Na2CO3/NaH2PO4+H2O ATPS at 308.15,318.15 and 328.15 K and atmospheric pressure are given in Table S1 (in the Supplementary Material).Figs.1 and 2 show the binodal curves of[EPy]BF4+Na2CO3/NaH2PO4+H2O ATPS.For calculation convenience,the binodal curves can be transformed to mathematical formulas by fitting the binodal curves with the empirical equation in general.Besides the Merchuk equation[Eq.(1)],the following two equations were also used to fit the binodal curves:

Table 3Values of the parameters of Eq.(6)for the IL+Salt+Water ATPS at different temperatures

Table 4Values of the parameters of Eq.(7)for IL+Salt+Water ATPS at different temperatures

Fig.2.Binodal curves and tie lines of[EPy]BF4(1)+NaH2PO4(2)+H2O(3)ATPS at 308.15 K,318.15 K and 328.15 K.

Table 2Values of the parameters of Eq.(1)for IL+Salt+Water ATPS at different temperatures

where a1,a2,b1,b2,a,b,c,and d are fitting parameters.The following equation can be used to calculate the standard deviations(sd)values:

where the superscript cal and exp.denote calculated data and experimental data;N represents the number of data.It was found that binodal data for many IL-based ATPS could fit these equations very well[38,39].Tables 2–4 give the fitting parameters,sd values,and correlation coefficient (R2).It is obvious that all the sd values are lower than 0.0057,which show the satisfactory fitting results of Eqs.(1),(6),and(7).Compared the R2values of the three equations,it can be found that the R2values of Eqs.(6)and(7)are larger than Eq.(1)and both are larger than 0.999.This is mainly because Eqs.(6)and(7)possess more fitting parameters than Eq.(1).However,for more convenient calculations,thethree-parameter equation,which the fitting results are good enough,was still used to calculate the tie-lines data.

Table 5Values of the parameters of Eq.(9)for IL+salt+water ATPS at different temperatures

Table 6Values of the parameters of Eq.(10)for IL+salt+water ATPS at different temperatures

3.2.Tie-line data and correlation

The tie lines,values of TLL,and S of ATPS composed of[EPy]BF4+Na2CO3/NaH2PO4+H2O at 308.15 K,318.15 K and 328.15 K and atmospheric pressure were provided in Table S2(in the Supplementary Material)and shown in Figs.1 and 2.We found that the values of S are almost equal to each other at the same temperature,which implies the tie lines are almost parallel to each other at the same temperature.This phenomenon can be observed in Figs.1 and 2.

In this work,the Othmer-Tobias[40]and Bancroft[41]equations were used to ascertain the reliability of tie-lines data:

where k1,n,k2,and r denote the fitting parameters.Eqs.(9)and(10)have been widely used to correlate the tie lines for IL-based ATPS[42–44].Tables 5 and 6 give the k1,n,k2,r,sd,and R2values.The lower sd values in the tables indicate the tie lines data fit the Eqs.(9)and(10)well,and ascertain the tie lines data are reliable.

Table 7Values of the Parameters of Eq.(11)for IL+Salt+Water ATPS at Different Temperatures

3.3.Influence of temperature on binodal curves and tie lines

The influence of temperature on the[EPy]BF4+Na2CO3ATPS and[EPy]BF4+NaH2PO4ATPS at 308.15 K,318.15 K and 328.15 K areillustrated in Fig.S2(in the Supplementary Material).In order to compare the temperature dependence of the studied systems,the published binodal curve of the[EPy]BF4+Na2CO3and[EPy]BF4+NaH2PO4ATPS at 298.15 K[32]is also shown in Fig.S2.In the phase diagram,the immiscible regions are above the binodal curve,and the monophasic is below the curve.It is obvious from the figure that the temperature has no remarkable influence on the binodal curve with the studied temperature range.This phenomenon can also be found in some PEG+salt ATPS[45].

In order to investigate the influence of temperature on the tie lines,the tie lines of the studied ATPS at 308.15 K,318.15 K,and 328.15 K were presented in Fig.S3(in the Supplementary Material).For both[EPy]BF4+Na2CO3/NaH2PO4ATPS,the bottom phase is the salt-rich phase,and the top phase is the IL-rich phase.The two-side points of tie lines are the composition of the bottom phase and top phase,respectively.From Table S2 and Fig.S3,the slope of the tie lines almost no change with the variety of temperature,which indicates the [EPy]BF4+Na2CO3/NaH2PO4ATPS are insensitive to temperature.

3.4.Salting-out ability of salts

To study the salting-out ability of sodium salt,the Setschenow-type equation [46] was used to fit the tie-lines data for the [EPy]BF4+Na2CO3/NaH2PO4ATPS:

In this equation,parameters c1and c2denote the molality of the IL and salt;β is a fitting parameter;and ksis a parameter related to the salting-out ability.Table 7 gives the ks,β,and sd values of the studied ATPS.It is obvious that the ksvalues of Na2CO3systems were higher than the NaH2PO4systems,which indicate the Na2CO3have a better salt-outing ability than NaH2PO4.

The salting-out ability of salts can also be quantitatively described by checking the ions hydration Gibbs energy(ΔGhyd),which have successfully used to describe the polymer+salt ATPS[47,48].In our study,the common cation(Na+)was shared by the two inorganic salts,while the different anions involve different values of ΔGhyd,and the absolute values are in the following order:(ΔGhyd=−465 kJ·mol−1).These values were taken from Marcus[49].The higher absolute values of the ΔGhydrepresent hydration of anion happened easier and trend to hydrate more water molecules.Therefore,the water molecules available for IL to hydrate are reduced,and the system is easier to separate to IL-rich phase and saltrich phase.For the salt,it is described as better salting-out ability.In other words,Na2CO3is easier to obtain water from ionic liquid than NaH2PO4.

3.5.Application to 2-chlorophenol extraction

The concentration of 2-chlorophenol in both two phases were determined using HPLC method.As an example,a liquid chromatogram is provided in Fig.S4(in the Supplementary Material).Then,the partition coefficient(K)can be calculated using the following equation:

where Ctand Cbare concentration of 2-chlorophenol in top and bottom phase respectively.For the same ATPS at a constant temperature,the logarithm of K and TLL are in accordance with the following equation[38]:

where λ is a constant value that related to ATPS composition.It is therefore in this study,the same total composition was prepared at each temperature to study the 2-chlorophenol extraction.The extraction efficiency(E)can also be calculated:

where RVis the volume ratio of top phase and bottom phase.The selected total composition,TLL,K,and E are given in Table 8,from which we found the extraction efficiency for the[EPy]BF4+Na2CO3/NaH2PO4ATPS are higher than 98%.From the table,extraction capacities are satisfactory for both studied systems,but[EPy]BF4+NaH2PO4ATPS requires more ionic liquid to achieve the same extraction efficiency,i.e.,the addition of NaCO3tend to reduce the use of ionic liquid,which reduced the cost of extraction process.Compared with the ATPS for extracting phenolic compounds in the literature,the extraction efficiency of [EPy]BF4-based ATPS are very close to the [BPy]OTF-based ATPS [33],[Bmim]BF4-based ATPS [50],[Pmim][NTf2]-based ATPS,[Bmim][NTf2]-based ATPS,and[Hmim][NTf2]-based ATPS[51],while higher than PEG-based ATPS[19].This phenomenon is mainly because of the ionic liquids have better solubility for phenolic compounds.

Table 8Partition coefficient and extraction efficiency of 2-chlorophenol for IL(1)+salt(2)+water(3)ATPS at different temperatures and atmospheric pressure

The flow chart of 2-chlorophenol wastewater treatment is presented in Fig.S5(in the Supplementary Material).In summary,this work studied the salting-out ability of two sodium salts and the application of[EPy]BF4-based ATPS in 2-chlorophenol extraction.Although the extraction condition in real phenolic compound wastewater is different(e.g.temperature,pH values,presence of undefined compounds,etc.),the data provided in this work are significant to further application.

4.Conclusions

This work provided a pyridine-based ionic liquid ATPS and its application in 2-chlorophenol extraction.Binodal data of and tie lines data for the[EPy]BF4+sodium salts ATPS were obtained at 308.15 K,318.15 K,and 328.15 K and atmospheric pressure.The satisfactory fitting result of the empirical equations with binodal curve provides a mathematical formula for the tie lines determination using gravimetric method.The correlation of Bancroft and Othmer-Tobias equations present the good reliability of tie-lines data.The temperature has no apparent effect onthe[EPy]BF4+Na2CO3/NaH2PO4ATPS.The Setschenow-type equation indicate the salting-out ability are in the order of the Na2CO3>NaH2PO4,and this order is exactly consistent with the ΔGhydvalues of the anions.Bothe the[EPy]BF4+Na2CO3/NaH2PO4ATPS perform well in 2-chlorophenol extraction,while NaCO3is more preferred because of the less use of ionic liquid.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was sponsored by the Natural Science Foundation for Distinguished Young Scholars of Guangdong Provence(2019B151502038),the National Natural Science Foundation of China (21706038,21808042,21808039),the Fundamental Research Funds for the Central Universities(300102299202),and the National Training Projects of the University Students' Innovation and Entrepreneurship program(201910710125),the Scientific Innovation Practice Project of Postgraduates of Chang'an University(300103703058,300103703016).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.07.024.