Role of Hydrophobic Interaction in Driving the ... - ACS Publications

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Role of Hydrophobic Interaction in Driving the Partitioning of Metal Ions in a PEG-Based Aqueous Two-Phase System Pan Sun,†,‡ Kun Huang,*,† Jieyuan Lin,†,‡ and Huizhou Liu† †

CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China

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S Supporting Information *

ABSTRACT: PEG-based aqueous two-phase systems (ATPS) for extraction and removal of metal ions from industrial wastewaters is a promising strategy to replace traditional organic-aqueous two phase systems, due to its environmentally friendly advantages and excellent separation efficiency from controllable phase structure and physicochemical properties. However, controversy still remains about the partitioning mechanism of metal ions into the PEG-rich phase in ATPS. The present contribution revealed that a specific hydrophobic interaction was a main driving force for the transferring of Cr(VI) ions into the PEG-rich phase in ATPS. The properties of aqueous media around the PEG molecules and Cr(VI) ions also played a crucial role in the partitioning of Cr(VI) ions in the PEG-base ATPS. The PEG-rich phase in the ATPS exhibited a specific affinity toward poorly hydrated HCrO4− ions, compared to the strongly hydrated CrO42− ions. The difference in the properties of aqueous media in the coexisting liquid phases was response for the partitioning of Cr(VI) ions in the PEG-base ATPS. It was confirmed by combined application of UV−vis spectroscopy and molecular dynamics simulations. The present work provides a new understanding into the microscopic mechanism of the partitioning of metal ions in the polymer-based ATPS.

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wastewaters.13 A lot of works were performed to measure the partition coefficient of different metal ions, such as Am3+, Pu4+, Th4+, Bk3+, UO22+, and Eu3+, in the PEG-based ATPS consisting of polyethylene glycol, water, and inorganic salts.14−17 However, the poor partition efficiency of some metal ions in PEG-based ATPS limited the widespread application of such a kind of green solvent system in the industry. A full understanding of the partition mechanism of metal ions in the PEG-based ATPS will be helpful for further elucidation of the physicochemical nature of ionic partition process which, in turn, will provide new insight for promoting the separation efficiency. The partition mechanism of metal ions in PEG-based ATPS was studied considerably less and was not well understood.

INTRODUCTION The salting-out induced polymer-based aqueous two-phase systems (ATPS) consist of two immiscible aqueous phases formed when a certain amount of water-soluble polymers are mixed with inorganic salts in specific concentrations.1 In 1896, Beijerinck reported the first example of such a polymer-based ATPS.2 Afterward, many efforts have been dedicated to research and application of the ATPS and others like it.3−6 Owing to the similar physic-chemical properties of the two immiscible aqueous phases in the ATPS, the interfacial tensions between them was usually ultralow.7 The ultralow interfacial tensions in the polymer-based ATPS provide a mild environment for the transport of biological molecules, such as proteins,8 nucleic acids,9 and cellular materials,10,11 across the interface and also the feasibility for separation of targets with extremely similar physic-chemical properties.12 As an economic and environmentally friendly extraction system, polymer-based ATPS exhibited a significant potential in recovery of metal ions from various complicated industrial © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

April 10, 2018 July 20, 2018 July 27, 2018 July 27, 2018 DOI: 10.1021/acs.iecr.8b01551 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

was then added into above mixed aqueous solution, with shaking thoroughly for 30 min followed by centrifugation for 10 min at a speed of 4000 rpm to obtain a stable PEG based aqueous two-phase system. After clear phase separation, the volume of each immiscible liquid layer in the ATPS was recorded. All of the above experiments were performed at room temperature (25 °C). In the present study, we chose the experimental conditions in the corresponding area labeled in Figure S1 in the Supporting Information. The concentrations of metal ions in the salt-rich bottom aqueous phase in the ATPS were analyzed by an OPTIMA 7000 DV inductively coupled plasma optical emission spectrometer (ICP-OES, PekinElmer, U.S.). The concentration of metal ions in the polymer-rich top aqueous phase in the ATPS could be calculated from the mass balance. The inherent error originating from the measurement was within ±5%. The metal extraction experiments followed by analysis of the samples were conducted three times to check the repeatability and accuracy of the measurements. The statistical errors in analysis were within ±3%. The mass fractions of metal ions in the polymer-rich phase and salt-rich phase were expressed as follows:

The main focus of previous works is on what is the driving force for the transport of metal ions into the PEG-rich phase in ATPS. During the past decades, electrostatic interaction mechanism was widely accepted to understand the partition behaviors of ions in PEG-based ATPS. For the partitioning of metal anions into the PEG-based ATPS, it was suggested that the protonated PEG molecules could combine with the anions through the electrostatic interaction.18,19 By contrast, it was found that the partitioning of metal cations into PEG-rich phase in ATPS was difficult. However, addition of some complexing agents could significantly improve the partitioning of metal cations. It was supposed that the metal cations could react with those complexing agents to form negatively charged species and then interacted with the protonated PEG molecules by electrostatic interaction.20,21 However, the electrostatic interaction mechanism could not interpret the difference in the partition behaviors of some other inorganic anions. For example, experimental results indicated that the partition coefficient of Cl− was higher than that of SO42−. This was inconsistent with the fact that a stronger electrostatic interaction occurred between the protonated PEG molecules and SO42−, in comparison with Cl−.1 Accordingly, a detailed study of the partition mechanism of metal ions in PEG-based ATPS was needed to give better explanation to the experimental results. To elucidate the mechanism underlying the partitioning of metal ions in PEG-based ATPS, here, as an example, we investigated the partition behaviors of Cr(VI) ions with the change of aqueous pH. The experimental results demonstrated that the partition behaviors of Cr(VI) ions were consistent with the corresponding variation in species of Cr(VI) ions and its existing form, during the increase of aqueous pH. Thus, we proposed that the pH dependence of partitioning of Cr(VI) ions in the PEG-based ATPS resulted from the conversion of the predominant existing forms of Cr(VI) ion species during the increase of aqueous pH. It has been reported that UV−vis spectra of Cr(VI) ions was sensitive to their existing forms in aqueous solutions.22 In the present work, UV−vis spectroscopy was employed to identify the Cr(VI) species extracted into the PEG-rich phase at different aqueous pH. In addition, the molecular dynamics simulations were employed to elucidate the interaction mechanism of PEG aggregates with different Cr(VI) species at the molecular level.

wM,p =

wM,s =

c M,pvp m0

× 100%

m0 − c M,pvp m0

× 100%

(1)

(2)

where M represents metal ion and wM,p and wM,s are the mass fractions of the metal ion M in the polymer-rich phase and saltrich phase of ATPS, respectively. cM,p and cM,s are the concentrations of the metal ion M in the polymer-rich phase and salt-rich phase of ATPS, respectively. vp is the volume of the polymer-rich phase in ATPS. vs is the volume of the saltrich phase in ATPS. m0 is the total amount of metal ions added into the system. UV−vis Spectroscopy Measurement. UV−vis spectra of the PEG-rich phase loaded with Cr(VI) ions were recorded within the range from 300 to 450 nm using a Hitachi U-4100 spectrometer. The optical path length of the quartz cuvette used in the present study was 1.00 cm. The inherent error originating from the measurement was within ±5%. In each experiment, UV−vis spectra of the blank PEG-rich phase without loading metal ions was used as the background. The UV−vis spectra of the samples were collected three times to check the repeatability and accuracy of the measurements. The statistical errors in analysis of the absorbance intensity and wavenumber were within ±3%. Simulation Details. MD simulations were performed using the software NAMD, version 2.10.23 The CHARMM force field24 was used for ions and PEG molecules, and the TIP3P model was employed to describe water molecules.25 The force-field parameters and geometry parameters of anions were taken from the works reported in other literature.26,27 The graphical force-field toolkit available in the VMD package28 was utilized to compute the missing parameters. All quantum-chemical calculations in the optimal of structure of ions were carried out using the Gaussian 09 suite of programs.29 The results given in Figure S2 in the Supporting Information. Other simulation details were provided in the Supporting Information.

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EXPERIMENTAL SECTION Materials and Chemicals. Polyethylene glycol (PEG) with average molecular weight of 2000 (denoted as PEG 2000) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. The stock metal aqueous solutions containing 50 mmol/L of Cr(VI) ions were prepared by dissolving NaCrO4· 4H2O (purity of 99%, Aladdin Industrial Corporation) into deionized water. Solutions of 1 mol/L H2SO4 and 2 mol/L NaOH were employed for pH adjustment as appropriate. The other chemicals are analytical grade reagents. All chemicals were used as received, without further purification. Partitioning of Cr(VI) in PEG-Based Aqueous TwoPhase Systems. Experimental aqueous solutions containing Cr(VI) ions were prepared by diluting its stock solutions with deionized water. A certain amount of Na2SO4 were then dissolved into above aqueous solutions, and the desired pH values of the aqueous solutions were adjusted by adding either 1 mol/L H2SO4 or 2 mol/L NaOH (measured by HANNA pH 211 digital pH meter (Italy)). A certain amount of PEG 2000 B

DOI: 10.1021/acs.iecr.8b01551 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

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RESULTS AND DISCUSSION Effect of Aqueous pH on the Partitioning of Cr(VI) in the PEG-Rich Phase. Figure 1 gives the effect of aqueous pH on the partitioning of Cr(VI) ions in the PEG-rich phase of aqueous two-phase systems.

Figure 2. Distribution of different Cr(VI) ions with the change in aqueous pH. (The concentration of Cr(VI) in the aqueous solution was 12 mg/L.)

Cr(VI) Species Extracted into the PEG-Rich Phase Investigated by UV−vis Spectroscopy. In the present study, UV−vis spectroscopy was employed to elucidate the existing form of Cr(VI) ions extracted into the PEG-rich phase. The PEG-rich phase containing different concentration of Cr(VI) ions in acid and alkaline systems was prepared, and their corresponding UV−vis spectra are depicted in panels a and b in Figure 3, respectively. As can be seen, the peaks for HCrO4− (in the acidic systems) and CrO42− (in the alkaline systems) were located at about 352 and 372 nm, respectively, which are consistent with the previous literature.22 The Cr(VI) aqueous solutions with a pH value that ranged from 2.00 to 12.46 were mixed with PEG 2000 and Na2SO4 for constructing the aqueous two-phase systems. After phase separation, the UV−vis spectra of the loaded PEG-rich phase were recorded, as shown in Figure 4a. As can be seen that the UV−vis spectra of Cr(VI) ions in the PEG-rich phase undergo significant changes with the increase of initial aqueous pH. To make its change trend more clearly, we divided it into three parts as depicted in the Figure 4b−d. HCrO4− was the main existing form of Cr(VI) within the aqueous pH range between 2 and 4. As the pH increased from 4 to 7, HCrO4− gradually converted to CrO42−. As a result, HCrO4− and CrO42− coexisted in the pH range between 4 and 7. CrO42− became the predominant form as the pH was increased to above 8. Figure 5 gives the variation of UV−vis absorbance intensity and wavenumber of Cr(VI) species in the PEG-rich phases for different initial aqueous pH. Notably, the variation in Figure 5 could also been divided into three regions. The three regions were consistent with the variation of Cr(VI) species as shown in Figure 2. Combined analysis of the pH dependence of the partitioning of Cr(VI) ions in the PEG-rich phase, we could then suppose that the partitioning of Cr(VI) in the ATPS indeed depends upon the existing form of Cr(VI) in the aqueous solutions. In detail, the affinity of the PEG-rich phase to HCrO4− was highly stronger than that to CrO42−. Molecular Dynamics Simulations for the Interaction between PEG Molecules and HCrO4− and CrO42−. In order to elucidate the specific affinity of the PEG-rich phase to HCrO4− ions than that to CrO42− ions, molecular dynamics simulations were carried out in the present study to investigate the interaction of PEG molecules with HCrO4− and CrO42− ions. The component in the simulation box is listed in Table S1 in the Supporting Information. Snapshots after different simulation times are shown in Figure 6. First, PEG molecules were placed randomly in the center of the solution box. After 2

Figure 1. Effect of aqueous pH on the partitioning of Cr(VI) ions in the PEG-rich phase of aqueous two-phase systems. (The concentration of Cr(VI) ions in the aqueous solutions was 12 mg/L. The concentration of Na2SO4 in the aqueous solutions was 1.0 mol/L. The volume of aqueous solutions was 8 mL. The added amount of PEG 2000 was 2 g.)

As can be seen, the extraction percentage of Cr(VI) ions in the PEG-rich phase was above 85% within the aqueous pH range from 2 to 6. However, as the aqueous pH was increased to above 6, the extraction percentage of Cr(VI) ions in the PEG-rich phase decreased abruptly to below 20%. In our previous work, the change in the concentration of hydrogen ions in the equilibrium aqueous solution was considered as a main reason for the decrease in partitioning of Cr(VI) ions in the PEG-rich phase.30 It was suggested that the ether oxygen groups in PEG molecules could be protonated by a hydrogen ion to form the positively charged PEG·H+ and then interact with the negatively charged Cr(VI) ions through electrostatic interaction. With the increase of aqueous pH, there were not enough hydrogen ions that could be provided for the protonation of PEG molecules. As a result, the partitioning of Cr(VI) ions in the PEG-rich phase would decrease abruptly during the increase of aqueous pH. However, there are still some contradictions exist in that explanation. According to the electrostatic interaction mechanism, the ether oxygen groups in the PEG molecules could also combine with other cations such as sodium ions in the aqueous solutions to form positively charged PEG·Na+. It was reasonable to suppose that Cr(VI) ions could also be extracted into the PEG-rich phase through electrostatic interaction despite the increase of aqueous pH. Obviously, the electrostatic interaction mechanism could not interpret the partitioning behaviors of Cr(VI) ions in the PEG-rich phase. Notably, the existing form of Cr(VI) ions in the aqueous solution was highly dependent on the aqueous pH.31 As depicted in Figure 2, HCrO4− was the predominant form in the aqueous pH range between 2 and 4. As the aqueous pH was increased to above 4, the mass fraction of HCrO4− decreased significantly, while the mass fraction of CrO42− increased quickly, becoming the main form when the aqueous pH was increased up to 8. The pH dependence of the partitioning of Cr(VI) might be understood from the changes in the existing form of Cr(VI) ions during the variation of aqueous pH. C

DOI: 10.1021/acs.iecr.8b01551 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 3. UV−vis spectra of the PEG-rich phase containing different concentrations of Cr(VI) in (a) acidic systems (the concentrations of HCl in the systems 1−4 were 48 mmol/L, and the concentrations of Cr(VI) in the systems 1−4 were 0.2, 0.4, 0.6, and 0.8 mg/L respectively) and (b) alkaline systems (the concentrations of NaOH in the systems 1−4 were 16 mmol/L, and the concentrations of Cr(VI) in the systems 1−4 were 0.067, 0.130, 0.200, and 0.267 mg/L respectively).

Figure 4. UV−vis spectra of the loaded PEG-rich phases for different initial aqueous pH.

ns, PEG molecules gradually assembled to form the aggregates with a loose structure. After 5 ns, the PEG aggregates with a loose structure gradually turned into a compact structure. Then those compact PEG aggregates moved to the top of the solution box after 50 ns. The evolution behaviors of PEG aggregates could be attributed to the salting-out induced effect of Na2SO4, by which water molecules would be peeled off from the PEG molecules.32 As a result, aggregation of PEG molecules was observed. Obviously, the simulation results were consistent with the formation of aqueous two phase systems in which PEG molecules assembled together to form lots of aggregates and then separated out from the salt-rich aqueous phase to form a PEG-rich phase. Figure 7a gives the distribution of HCrO4− and CrO42− around PEG aggregates. It is clear that all of the HCrO4− distributed in the PEG aggregates, while most of the CrO42−

Figure 5. Variation in absorbance intensity and wavenumber of Cr(VI) species in the PEG-rich phases for different initial aqueous pH. D

DOI: 10.1021/acs.iecr.8b01551 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 6. Snapshot of the simulation box at different simulation time. (The carbon, hydrogen, and oxygen atoms in PEG molecules are represented by the cyan, white, and red spheres, respectively. Others components in the simulation box were represented by the red−white dot line.)

Figure 7. (a) Snapshot of the simulation box at equilibrium state (CrO42− is represented by the green spheres. HCrO4− is represented by the yellow spheres. The carbon, hydrogen, and oxygen atoms in PEG molecules are represented by the cyan, white, and red spheres, respectively.) (b) The radial distribution of the chromium atoms of HCrO4− or CrO42− around all of the monomers of PEG molecules in the aggregates.

Figure 8. Radial density profiles of all of the components in (a) the whole simulation box and (b) the partly enlarged range of the radial density profiles within the PEG aggregates. (1 and 3 in Figure 8b represent the peaks in the density profile of Na+ in the inner of PEG aggregates. 2 and 4 in Figure 8b represent the peaks in the density profile of HCrO4− in the inner of PEG aggregates.)

was excluded from the PEG aggregates and distributed in the aqueous solutions. In addition, similar distribution behaviors of HCrO4− and CrO42− were also observed in the periodic simulation boxes as given in Figure S3. Furthermore, the interaction between PEG aggregates and Cr(VI) ions was studied by analysis of the radial distribution function of Cr(VI)

ions around PEG aggregates. Figure 7b gives the radial distribution function of the chromium atoms of HCrO4− or CrO42− around all of the monomers of PEG molecules in the aggregates. As can be seen, a significant peak occurred in the radial distribution function of the chromium atoms of HCrO4− E

DOI: 10.1021/acs.iecr.8b01551 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

interaction, as shown in Figure 9b. As can be seen in Figure 9b, PEG molecules actually interacted with HCrO4− through Na+ acting as a bridge. In detail, Na+ was surrounded by the ether oxygen groups in PEG molecules to form the positively charged PEG·Na+ and then interacted with HCrO4−. It seems that the electrostatic interaction between PEG·Na+ and HCrO4− might be the predominant driving force for the partition of HCrO4− in the PEG aggregates. However, compared with HCrO4−, CrO42− also bears a negative charge, even more negative. Its electrostatic interaction with PEG·Na+ would be stronger than that for HCrO4−. Obviously, the interaction mechanism between PEG molecules and HCrO4− could not be simply considered as a kind of electrostatic interaction. In fact, it has been demonstrated that the partitioning of various solutes in the polymer based ATPS were driven by different properties of water in the coexisting liquid phases.34,35 In the present system, the partitioning behaviors of different Cr(VI) ions species in the PEG-rich phase might be originating from the different properties of water in each of the coexisting liquid phases. Generally, the radial distribution function of water molecules around the target ions or molecules could be employed to estimate the affinity of water molecules to the target ions or molecules.33 Herein, we analyzed the radius distribution function of water around Na+, CrO42−, SO42−, HCrO4−, and PEG molecules, respectively. The result is depicted in Figure 10. Na+ and SO42− ions are the main components in the salt-rich bottom aqueous phase. As can be seen in Figure 10, the interactions of Na+ and SO42− ions with water molecules were strong enough in the salt-rich bottom aqueous phase. Therefore, there might occur a strong hydrogen-bonding network in the aqueous media in the salt-rich bottom aqueous phase. Comparatively speaking, such a kind of the aqueous media in the salt-rich bottom aqueous phase could be considered as a hydrophilic environment. On the other hand, the result suggested that the interactions of CO42− ions with water molecules were also strong enough. It indicated that CrO42− ions could be regarded as a hydrophilic species. Therefore, CrO42− ions preferred to remain in the salt-rich bottom aqueous phase. The properties of the aqueous media in the PEG-rich phase were quite different from that in the salt-rich bottom aqueous phase. The main components in the PEG-rich phase are the hydrated PEG molecules. The radius distribution function of water around PEG molecules suggested that the interaction of PEG molecules with water molecules were comparatively weak. Therefore, the aqueous media in the PEG-rich phase could be regarded as a hydrophobic environment. Besides, the radius distribution function of water around HCrO4− indicated that HCrO4− ions could be regarded as a hydrophobic species. Therefore, HCrO4− ions preferred to transport into the PEGrich phase. The main driving force for the transport of HCrO4− ions into the PEG-rich phase might be attributed to a hydrophobic interaction. The difference in the hydration properties of CrO42− and HCrO4− could be understood from the Hofmeister series.36 According to the Hofmeister series, the hydration properties of ions was determined by their polarizability, whereas the polarizability of ions was mainly dependent on their charges and radii. Compared with the double charged CrO42−, the single charged HCrO4− exhibited a higher polarizability. Thus,

around all of the monomers of PEG molecules in the aggregates, suggesting an enrichment of HCrO4− around the PEG molecules. On the contrary, no peak was observed in the radial distribution function of the chromium atoms of CrO42− around all of the monomers of PEG molecules in the aggregates, which indicated that CrO42− was away from PEG molecules. The simulation results further demonstrated the specific affinity of PEG aggregates toward HCrO4− compared with CrO42−. To understand this specific affinity, it is necessary to investigate the interaction mechanism between PEG molecules and HCrO4−. Herein, the radial density profiles of all of the components around the center of PEG aggregates were analyzed, as presented in Figure 8. As can be seen in Figure 8a, almost all HCrO4− was distributed in the inner of the PEG aggregates, some Na+ was enriched in the PEG aggregates, while the majority of SO42− and CrO42− was distributed in the aqueous solution outside around the PEG aggregates. Figure 8b depicts the partly enlarged range of the radial density profiles within the PEG aggregates. As shown in Figure 8b, four peaks could be observed in the radial density profiles of ions in the inner of PEG aggregates. The first and third peaks located at 3.55 and 8.35 Å, respectively, represent an enrichment of Na+ in the inner of PEG aggregates, whereas the second and fourth peaks located at 5.85 and 12.15 Å, respectively, represent the enrichment of HCrO4− in the inner of PEG aggregates. Notably, the distribution of Na+ and HCrO4− in the inner of PEG aggregates exhibited a special oscillation distribution. Moreover, the peak of Na+ was surrounded by that of HCrO4−. We supposed that such an oscillation phenomenon might be attributed to the occurrence of a special interaction between PEG molecules and HCrO4− ions, in which Na+ ions act as a bridge to connect them. Owing to such a specific interaction, a kind of special layer by layer self-assembled structure of HCrO4− layer and Na+ layer was formed within the inner of PEG aggregates. The number of the layers within the inner of PEG aggregates was decided by the size of PEG aggregates. To visualize the distribution of Na+ and HCrO4− in the PEG aggregates, snapshot in the inner of PEG aggregates at last simulations frame is presented in Figure 9a. As shown in Figure 9a, some combinations exist between PEG molecules, HCrO4−, and Na+. Furthermore, we select a pair of those interaction to clarify the mechanism of

Figure 9. (a) Snapshots in the inner of PEG aggregates and (b) snapshots of the complex formed between PEG molecules and HCrO4− ions. (The carbon, hydrogen, and oxygen atoms in PEG molecules are represented by the cyan, white and red spheres, respectively. HCrO4− is represented by the yellow spheres. Na+ is represented by the blue spheres.) F

DOI: 10.1021/acs.iecr.8b01551 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 10. Radial distribution function of water molecules around Na+, CrO42−, SO42−, HCrO4−, and PEG molecules.

the hydration of CrO42− was stronger than that of HCrO4−. During the increase of aqueous pH, Cr(VI) ions converted gradually from the poorly hydrated HCrO4− to the strongly hydrated CrO42−. Thus, the partitioning of Cr(VI) ions in the PEG-rich phase of PEG-based ATPS decreased gradually with the increase of aqueous pH. Hydrophobic Interaction Induced the Increase in the Partitioning of Cr(VI) in the PEG-Rich Phase. To further verify the interfacial hydrophobic interaction mechanism underlying the partitioning of Cr(VI) ions in the PEG-based ATPS, it is essential to investigate the effect of hydrophobicity of the PEG-rich phase in the ATPS on the partitioning of Cr(VI) ions. It is well-known that SO42− is a strongly hydrated anion, and it could peel off water molecules from the hydration shell of the PEG molecule chains, thus resulting in an increase in the incompatibility and the difference in chemical potential between the electrolyte solutions and PEG molecules.37 In addition, these differences would increase with the increase of SO42− concentration. As a result, spontaneous phase separation of the PEG-rich phase from the salt-aqueous phase was observed, when the concentration of SO42− increased to a certain value. Meanwhile, the separating of the PEG-rich phase from the salt-aqueous phase reduced the entropy of systems, which was also in favor of the stability of systems. It was demonstrated that the concentration of the salting-out agent has significant influence on the composition of aqueous two phase systems, such as the concentration of PEG molecules and water molecules in the PEG-rich phase and salt-rich phase, respectively.37 Thus, the change in concentration of Na2SO4 might have great influence on the hydrophobicity of the PEGrich phase. Herein, the hydrophobicity of the PEG-rich phase was determined by fluorescence spectroscopy using pyrene as a probe. The results are depicted in Figure 11. As can be seen in Figure 11, an increase in the intensity of the fluorescence-emission peaks of pyrene was observed, suggesting a more hydrophobic environment occurred around pyrene molecules, upon increasing the concentration of Na2SO4.38,39 Besides, previous work demonstrated that content of water in the PEG-rich phase would decrease with the increase of Na2SO4 concentration.40 Therefore, it can be concluded that the hydrophobicity of the PEG-rich phase would increase due to the increase of the concentration of Na2SO4. Figure 12 gives the effect of Na2SO4 concentration on the extraction percentage of Cr(VI) in the PEG-rich phase. The significant increase in the extraction percentage of Cr(VI) ions in the PEG-rich phase was observed during the increase of

Figure 11. Effect of Na2SO4 concentration on the emission spectra of the PEG-rich phase (The aqueous pH was 3.00. The volume of the aqueous solutions was 8 mL. The added amount of PEG 2000 was 2 g. The concentrations of Na2SO4 were 0.50, 0.75, 1.00, 1.25, 1.50, and 1.75 mol/L, respectively. The concentration of pyrene in the PEGrich phase was 1.25 μmol/L.)

Figure 12. Effect of Na2SO4 concentration on the extraction percentage of Cr(VI) in the PEG-rich phase of ATPS (The concentration of Cr(VI) ions in the aqueous solutions was 12 mg/ L. The aqueous pH was 3.00. The volume of aqueous solutions was 8 mL. The added amount of PEG 2000 was 2 g. The concentrations of Na2SO4 were 0.50, 0.75, 1.00, 1.25, 1.50, and 1.75 mol/L, respectively.)

Na2SO4 concentration, suggesting that the increase of hydrophobicity of the PEG-rich phase facilitated the partitioning of Cr(VI) ions in the ATPS. The experimental results further verified that the hydrophobic interaction was a main driving force for the transport of Cr(VI) ions into the PEG-rich phase of ATPS. G

DOI: 10.1021/acs.iecr.8b01551 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

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CONCLUSIONS In present work, we demonstrated that the hydrophobic interaction played a crucial role in determining the partitioning behaviors of Cr (VI) ions in PEG-based ATPS. The pH dependence of the partitioning of Cr (VI) ions into the PEGrich phase in the ATPS was attributed to the conversion of the predominant existing forms of Cr(VI) ions in the aqueous solutions from HCrO4− to CrO42− with the increase of aqueous pH values. UV−vis spectroscopy experiments illustrated that the PEG-rich phase in the ATPS exhibited a specific affinity toward the poorly hydrated HCrO4− ions compared to the strongly hydrated CrO42− ions. Molecular dynamics simulations further revealed that the aqueous media in the salt-rich bottom phase of ATPS could be considered as a hydrophilic environment. Therefore, the strongly hydrated CrO42− ions prefer to remain in the salt-rich bottom aqueous phase of ATPS. In contrast, the aqueous media in the PEG-rich phase of ATPS exhibited a hydrophobic environment, which had a strong interaction with poorly hydrated HCrO4− ions through hydrophobic interaction. Thus, the hydrophobic interaction was suggested as a main driving force for the transport of Cr(VI) ions into the PEG-rich phase in ATPS. The increase in the hydrophobicity of the PEG-rich phase enhanced the transport of Cr(VI) ions into the PEG-rich phase and that further demonstrated the reasonability of the suggested mechanism about hydrophobic interaction. The present work provides novel insight into the driving force for the transport of Cr(VI) ions in the PEG-rich phase of ATPS, which broadens our understanding into the nature of ionic partitioning in the ATPS. The hydration properties of ions and their interface propensity were first proposed to be a key factor in determining the partitioning of ions in the ATPS. It is of benefit for further understanding the partition mechanism of other ions or biomolecules in the ATPS and developing new ecological methods to remove other heavy metal ions from industrial wastewater.

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and the Key Project of Chinese National Programs for Fundamental Research and Development (973 Program No. 2013CB632602). We thank the Supercomputing Centre of Chinese Academy of Sciences for allowing us to use the ScGrid for theoretical calculations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b01551. Simulation details; details of the simulation box used for MD simulation; the optimal structures of CrO42− and HCrO4−; variation of the volume of Na2SO4-rich phase and PEG-rich phase in the concentration of Na2SO4; and snapshot of the periodic simulation boxes at the equilibrium state (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-10-82544910. Fax: 86-1062554264. ORCID

Pan Sun: 0000-0002-6128-8656 Kun Huang: 0000-0002-6933-6480 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51574213 and 51074150) H

DOI: 10.1021/acs.iecr.8b01551 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

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