Thermoseparative Regeneration of Triblock ... - ACS Publications

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Thermoseparative Regeneration of Triblock Copolymer after Aqueous Biphasic Extraction of Molybdate Species Laboni Das,† Sankar Prasad Paik,‡ and Kamalika Sen*,† †

Department of Chemistry, University of Calcutta, 92 APC Road, Kolkata 700 009, India Department of Chemistry, Sundarban Mahavidyalaya, Kakdwip 743347, India

‡

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S Supporting Information *

ABSTRACT: The present study aims at the generation of a triblock copolymer vs sodium sulfate aqueous biphasic system (ABS), its application toward extraction of molybdenum species, and finally purification and regeneration of the block copolymer. The ABS was characterized for its biphasic region by constructing the phase diagram using turbidometric titration methods. Extractions were monitored using the spectrophotometric method directly in the block copolymer medium. Complete extraction of molybdenum species was observed in this method. Interference from other closely related metal ions in the extraction process was found to be negligible excepting oxo and polyoxometallates of vanadium and tungsten. Regeneration of the polymer was done using a thermoseparation technique followed by successive treatment with ion exchange resins. ∼87% of the pure triblock copolymer could be regenerated as a result of such treatment. biocompatible reagents.2 In general, mutually incompatible aqueous polymer−polymer or polymer−salt or polymer− surfactant or salt−salt solutions are mixed at certain thermodynamic conditions (like pH, temperature, concentration etc.), to generate an ABS.3,4 Although a large number of ABSs composed of an individual unit of the block copolymers like polyethylene glycol (PEG) are reported in the literature, only a few are available involving triblock copolymers. Some of them are composed of poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) vs various sulfate salts (Li2SO4, MgSO4, Na2SO4, and ZnSO4).5 Some others are also composed of poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol) (PPG-PEG-PPG) vs sodium tartrate,6 sodium acetate, or sodium citrate.7 Design of a new ABS demands its complete characterization in terms of phase diagrams which are conveniently achieved through a cloud point determination technique or turbidometric method or node determination method. Tie lines representing the final compositions of the top and the bottom phases help to have an actual idea about the ABS.8

1. INTRODUCTION An aqueous biphasic extraction system is a complete aqueous substitute of the traditional liquid−liquid extraction technique which involves organic solvents having high volatility and toxicity.1 Aqueous biphasic systems (ABSs) deal with water as the solvent, which supports a greener future for the separation technology. ABSs also frequently deal with the use of

Figure 1. Binodal curve of ABS composed of PPG-PEG-PPG/ Na2SO4 at: black square, 293 K; red circle, 298 K; and blue triangle, 310 K. © XXXX American Chemical Society

Received: June 2, 2018 Accepted: December 14, 2018

A

DOI: 10.1021/acs.jced.8b00455 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 2. (a) Extraction pattern of molybdate at different pH of ABS. (b) Variation of aggregation number of the polymer-rich phase with extraction of molybdate at different pH.

an alumina column containing the radioactive molybdate species.22,23 In addition, molybdates are widely used as catalysts.24 Some important organic syntheses like transfer hydrodeoxygenation,25 synthesis of regiospecific dihydro pyrimidinones and pyrimido pyridazines,26 regioselective synthesis of 2-aryl-3-benzamidobenzofurans,27 etc., are catalyzed by molybdate. Extraction and regeneration studies of this particular species therefore demand emphasis in terms of both industrial and research applications. In this work we have prepared a new aqueous biphasic system composed of PPG-PEG-PPG and sodium sulfate. The phase diagram has been constructed using a turbidometric method. A ternary phase diagram was also obtained. This newly designed ABS is then used for extraction of molybdate species at different pH conditions. We have used the thermoseparation technique to recover the polymer after extraction. The methodology adopted is capable for the regeneration of the phase forming components as well as the analyte (moybdate) after the extractions.

In spite of their positive aspects, the major issue with ABSs is that they generally use high concentration solutions of the phase-forming components. Therefore, to make it an inexpensive one and to reuse the components, recycling of the polymer is essential. There are some techniques like deproteinization, desalting by ultrafiltration, etc., to recycle the polymer phase.8 On the other hand, block copolymers show a unique property of thermoseparation which varies with their concentration.9 Earlier, recovery of the polymers after their use in ABS has been reported in the literature wherein most of the thermoseparations were observed in ethylene oxide−propylene oxide (EOPO) type random copolymers.9,10 These reports emphasize either regeneration of the copolymers from the analyte after ABS or simply the preconcentration of the polymer itself. However, a total recovery method of the copolymer itself from every possible molecule and ion after extractions is still lacking. Additionally, to the best of our knowledge no report on regeneration of triblock copolymers has been done so far. It is evident that the thermoseparation technique is a technique for regeneration of reagents in addition to its easy and convenient work up and therefore needs the special attention of separation chemists for its effective application. Extraction and separation of different metal species had been investigated using different ABSs during the past few years. Different cationic species have been extracted in the polymerrich phase with the aid of different extracting agents.11,12 Metal oxo anions like TcO42−, Cr2O72−, AsO2−, and AsO43−13,14 are, however, more easily extracted in the polymer-rich phase owing to their lower hydration energy, higher chaotropicity, and hydrogen bonding abilities as compared to the positively charged metal ions. Molybdate species were reported to have been extracted in the surfactant-rich phase of a Triton X-100/ (NH4)2SO4 system.15 Sodium molybdate and ammonium heptamolybdate salts were reported to have quantitative extractions in the PEG-rich phase of PEG/CuSO4 ABS at acidic pH.16 Molybdate salt was found to generate the ABS with PEG 2000, and the particular system was used for the separation of perrhenate from tungstate.17 Molybdates are essential for chemical as well as agricultural industries. Sodium molybdate is used as a fertilizer in the treatment of whiptail in broccoli and cauliflower.18 In physiological conditions of higher animals, molybdate mimics the actions of insulinpromoted metabolic effects.19,20 Sodium molybdate has reported effects on prevention of hypertension.21 The radioisotope of molybdenum, 99Mo, decays to give the most widely used medical radioisotope 99mTc, which is eluted from

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol) (average M.W. 2000)

Figure 3. Plot of variation of CPT of PPG-PEG-PPG solutions in the presence of different salt solutions.

(pluronic 10R5, Sigma-Aldrich), sodium sulfate anhydrous (NICE, 99% pure), sodium molybdate dehydrate (Merck, 99.0% pure), iron(III) nitrate nonahydrate purified (Merck, 98% pure), potassium thiocyanate (LOBA chemicals, 98% pure), Amberlite IR-120 (Koch-light laboratories Ltd.), B



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loading, extractions of higher molybdate concentrations were performed at optimum pH. 2.5. Determination of Aggregation Number. Aggregation number (Nagg) is the depiction of number of molecules aggregated in a micelle, once the cmc value has been reached.32 Aggregation number of a micellar solution can be determined by fluorescence quenching method.7 Determination of aggregation number generally requires a luminescent probe and a quencher.33 PPG-PEG-PPG being a block copolymer is reported to form micelles in solution with a resulting fluorescence emission upon excitation at 280 nm. This has been attributed to the availability of an electron-rich zone owing to micellar aggregations that account for the resultant fluorescence activity of such polymeric solutions.6,7 For calculating Nagg, PPG-PEG-PPG itself acts as a luminescent probe (>1.5% v/v)6 along with a 10 mM K2Cr2O7 solution which was used as a quencher.34 The quencher was gradually added to the block copolymer rich phase of the ABS to determine the aggregation number using the following equation31

Amberlite IRA-400 chloride (Sigma-Aldrich), stannous chloride dehydrate, disodium tartrate dehydrate, sodium acetate trihydrate, sodium chloride and sodium nitrate, PAN indicator [1-(2-pyridylazo)-2-napthol] (Merck, 99% pure), cobalt acetate, ferric chloride, nickel sulfate, copper sulfate and manganese(II) chloride, sodium metavanadate, and sodium tungstate (LOBA chemicals) were used as received, and all the chemicals were of analytical grade. 2.2. Apparatus. An Agilent 8453 diode array spectrophotometer was used to record the absorption spectra. Fluorescence intensities were measured using luminescence spectrometer LS55B (PerkinElmer, USA). The solutions were mixed up using a vortexer (Remi model no-101). A Hermle microprocessor controlled universal refrigerated high speed table top centrifuge (model Z 36 K) was used for centrifugation. A Metler Toledo pH ion meter was used to adjust the pH of the solutions. A Metler Toledo ion-selective electrode was used to detect Na+ ions. The BOD incubator shaker NOVA model: SHCI 10(D) was used for maintaining the temperatures. 2.3. Construction of Phase Diagram and Tie Line. The phase diagram of the newly designed ABS composed of PPGPEG-PPG and Na2SO4 was constructed by using a simple turbidometric method at three different temperatures, 293, 298, and 310 K.8,11,28 Tie lines are drawn using the gravimetric method proposed by Merchuk et al.29,30 A ternary mixture of PPG-PEG-PPG, Na2SO4, and H2O at the biphasic region of the binodal curve was chosen and prepared gravimetrically. The mixture was shaken at the three different temperatures using a constant temperature BOD shaker for 10 min and then centrifuged for another 10 min at 5000 rpm. The top and bottom phases of the ABS were carefully separated out, and their weights were recorded to construct the tie line. 2.4. Extraction of Molybdate. The polymer-rich phase of ABS was taken to study the extraction behavior of molybdate ions spectrophotometrically. A 10 mM aqueous solution of sodium molybdate was prepared for calibration in 60% (v/v) PPG-PEG-PPG solution. An amount of 0.08 mL of the molybdate solution was added to the system containing 3 mL of 60% PPG-PEG-PPG and 3 mL of 2 M Na2SO4 solution and was shaken for 10 min followed by centrifugation at 5000 rpm for 10 min. The block copolymer-rich phase was taken for the extraction study using spectrophotometry. An amount of 0.5 mL of 1 M SnCl2 prepared in 2 M HCl medium and 0.5 mL of 100 mM aqueous solution of KSCN were added to 1 mL of a block copolymer rich phase of the ABS which gives an absorption maximum at 470 nm due to formation of the hexathiocyanato molybdate(III) complex.31 The results were compared with suitable calibration data obtained from a similar set of experiments with increasing concentrations of molybdate. The percentage of extraction was calculated using the following equation % Extraction =

[molybdate]PPG‐PEG‐PPG‐rich phase [molybdate]total

ln(I0/I ) = CqNagg /[C] − CMC

(2)

where Nagg, Cq, and [C] represent the aggregation number, concentration of quencher, and concentration of the block copolymer, respectively. “I0” symbolizes the fluorescence intensity of the block copolymer, and “I” represents the fluorescence intensity of the block copolymer after the addition of quencher. The fact that this block copolymer forms micellar aggregations is also supported by the works of earlier researchers who have also established the critical micellization temperature, cloud point, and polydispersity index (>0.6) of pluronic 10R5.35 2.6. Thermoseparative Regeneration. As mentioned earlier, a high concentration solution of block copolymer is necessary to prepare ABS. This demands development of methods for separation, purification, and recycling of the copolymer after use. The property of thermoseparation of the block copolymer may be used as a tool to recover it after extractions in ABS.9,10 To study the thermosensitive behavior of triblock copolymer, we prepared aqueous PPG-PEG-PPG solutions of different concentrations (1% to 10% v/v) and 0.1 M solutions of sulfate, chloride, nitrate, acetate, and tartrate salts of sodium. A 0.5 mL salt solution was added to 1.5 mL of PPG-PEG-PPG solutions to determine their cloud point temperatures (CPTs). After having an idea about the possible CPT, we took the top phase (polymer-rich phase) of the ABS and heated it until they reached the CPT and then slowly cooled the solutions to room temperature to recover the block copolymer at the bottom phase. To recover the polymer from contaminating ions (like Na+, SO42−, Mo complex), we collected the bottom phase containing the polymer after thermoseparation and added an equal amount of triple distilled water to it and further repeated the thermoseparation procedure for two more times. The recovered phase was collected and treated with cation exchange resin (0.1 g of resin per mL of polymer solution) and anion exchange resin (0.1 g of resin per mL of polymer solution) separately. In order to determine the concentration of recovered block copolymer after ABS extractions, a calibration plot of PPGPEG-PPG was prepared using the characteristic peak at 280 nm (Figure S1).

× 100 (1)

where [molybdate]PPG‑PEG‑PPG‑rich phase represents the concentration of the molybdate species in the polymer-rich phase (mM) and [molybdate]total represents the total initial concentration of the same. The percentage of extraction was studied at different pH medium. The pH was adjusted using H2SO4 or NaOH solutions. For studying the effect of metal C



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Table 1. System Compositions of the Phase Diagrams (Figure 1) at Temperatures of 293, 298, and 310 Ka final composition (%) (293 K)

system composition (%) X1 16.29 13.33 11.11 8.89 6.66 4.44

± ± ± ± ± ±

X2 0.81 0.66 0.55 0.44 0.33 0.22

8.31 12.47 15.58 18.69 21.82 24.93

± ± ± ± ± ±

Y1 0.41 0.62 0.78 0.93 1.09 1.25

8.34 7.11 6.52 5.67 4.69 3.59

± ± ± ± ± ±

final composition (%) (298 K)

Y2 0.42 0.35 0.32 0.28 0.23 0.18

4.28 6.69 9.19 11.99 15.45 20.30

± ± ± ± ± ±

Y1 0.21 0.33 0.46 0.59 0.77 1.01

7.06 6.43 5.93 5.05 4.52 3.44

± ± ± ± ± ±

final composition (%) (310 K)

Y2 0.35 0.32 0.29 0.25 0.22 0.17

3.60 6.01 8.31 10.63 14.78 19.32

± ± ± ± ± ±

Y1 0.18 0.30 0.41 0.53 0.74 0.96

6.46 5.67 5.01 4.69 3.98 3.26

± ± ± ± ± ±

Y2 0.32 0.28 0.25 0.23 0.19 0.16

3.32 5.33 7.07 9.99 13.10 18.40

± ± ± ± ± ±

0.16 0.27 0.35 0.49 0.65 0.92

X1 and X2 are the mass fraction percentages of sodium sulfate and PPG-PEG-PPG, respectively. Y1 and Y2 represent the final compositions, respectively. a

Table 2. Values of System Parameters of the Sodium Sulfate/PPG-PEG-PPG System at 293, 298, and 310 K T/K

A

B

C

R2

293 298 310

59.823 ± 12.293 46.881 ± 27.749 144.018 ± 94.788

−0.520 ± 0.109 −0.384 ± 0.329 −1.055 ± 0.381

0.002 ± 0.0002 0.004 ± 0.0010 0.004 ± 0.0010

0.997 0.988 0.990

characteristic peak at 555 nm,38 and 1 mL of 20 mM KSCN solution was added to 1 mL of the ABS top phase to detect Fe3+ ions which show a λmax at 350 nm.39 Vanadate was detected after direct addition of 10 mM solution of sodium vanadate in the block copolymer solution in increasing volumes. For detection of tungstate, the solutions were treated with 0.1 mL of 1 M SnCl2 and 0.1 mL of concentrated HCl to obtain a yellow solution.31 Extraction of molybdate in the present ABS in the presence of different ABS-forming salts has also been studied. To check the extraction of molybdate in the presence of different salts, we prepared 0.5 M aqueous solutions of sodium chloride, sodium acetate, sodium nitrate, and sodium tartrate. Amounts of 0.1 mL of salt solution and 0.08 mL of 10 mM sodium molybdate solution were added to the ABS composed of 3 mL of 60% (V/V) aqueous solution of block copolymer and 3 mL of 2 M sodium sulfate solution. Then the mixture of the solutions was vortexed and centrifuged at 5000 rpm for 10 min. The polymer-rich phase of the ABS was taken for spectrophotometric studies as mentioned earlier.

2.7. Determination of Ions after Recovery. The regenerated polymer was tested for its residual contaminations from the possible ions using ion-selective and spectrophotometric detection methods. Sodium ion detection: Sodium ions in the aqueous phases after thermoseparation were determined using a sodium ion selective electrode in combination with a digital pH/ion meter after suitable calibration. Sulfate ion detection: For the detection of unknown sulfate in solution a suitable calibration was first prepared using a 60% (v/v) block copolymer solution as the blank. To this, 0.1 mL of 25 000 ppm Fe(NO3)3 solution was added as a standard, and aliquots of 0.01 mL of 2500 ppm of Na2SO4 solution were added. A peak at 305 nm appeared in the absorption spectrum (Figure S2). In the presence of a constant concentration of ferric ion in the solution, the absorbance of the solution increases proportionately with an increase in the concentration of sulfate ions.36 Using this calibration (shown in inset of Figure S2), sulfate concentrations in unknown solutions were determined. The analytical parameters for sulfate detection using this method are tabulated in Table S1. Molybdate ion detection: Molybdate ion was also detected spectrophotometrically after forming the hexathiocyanato molybdate(III) complex as described before using its characteristic absorption maximum at 470 nm in 60% PPGPEG-PPG medium (Figure S3). 2.8. Effect of Interfering Ions on the Extraction of Molybdate. The effect of interfering ions in aqueous biphasic extraction of molybdate ions was studied in the presence of different ions such as Co2+, Ni2+, Cu2+, Mn2+, Fe3+, VO3−, and WO42−. 10 mM aqueous solutions each of Co(CH3COO)2, NiSO4, CuSO4, MnCl2, FeCl3, NaVO3, and Na2WO4 were prepared. An amount of 0.08 mL of 10 mM salt solution of each kind was added to the ABS containing 3 mL of 60% block copolymer and 3 mL of 2 M sodium sulfate solutions of pH 3. For spectrophotometric determination of Co2+, Ni2+, and Cu2+, 0.1 mL of 20 mM methanolic solution of PAN was added to a mixture of 1 mL solution of the block copolymer rich phase and 1 mL of acetate buffer (pH = 4.8) solution which gives the characteristic absorption maxima at 577, 565, and 560 nm, respectively.11,37 To detect Mn2+, 0.1 mL of 20 mM PAN solution was added to a mixture of 1 mL of the top phase of ABS and 1 mL of borate buffer (pH = 9) which gives a

3. RESULTS AND DISCUSSION 3.1. Construction of Phase Diagram. The phase diagram of the ABS composed of PPG-PEG-PPG and sodium sulfate solutions was constructed following the turbidometric titration method at three different temperatures, 293, 298, and 310 K.11 In order to get the binodal curves, weight fractions of PPGPEG-PPG and sodium sulfate for different experimental sets were plotted [Figure 1]. The system compositions are tabulated in Table 1. The binodal curves were fitted to the Merchuk equation as follows Y = A exp[(BX 0.5) − (CX3)]

(3)

where X and Y are the mass fraction percentages of sodium sulfate and PPG-PEG-PPG block copolymer, respectively. A, B, and C are the constants obtained from the binodal curve. Experimental values of A, B, and C are given in Table 2. From Figure 1, it is clear that the biphasic region increases with increase in temperature which is in agreement with the facts observed and reported earlier.40 This observation can be explained on the basis of decreased solubility of the copolymer and increased solubility of the salt with increase in temperature. The binodal curve shifts toward the left as temperature D


X1, Y1, and Z1 indicate the mass fraction percentages of sodium sulfate in the feed, top, and bottom phases, respectively. X2, Y2, and Z2 designate the same for PPG-PEG-PPG, respectively.

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Figure 4. Thermoseparation of the block copolymer rich phase of the ABS in study where (A) represents the condition before thermoseparation, (B) represents the cloud of block copolymer at cloud point temperature, and (C) shows the situation after thermoseparation where the bottom phase is polymer rich.

Figure 5. Flowchart of the recovery process of PPG-PEG-PPG after ABS.

Figure 6. Extraction percentage of molybdate and possible interfering ions using ABS at pH 3.

increases. This is because lower salt concentrations are required at higher temperatures to reach the critical points as the polymers become more hydrophobic, resulting in salting out effect at lower concentrations.

a

STL

−2.185 ± −0.109 −2.289 ± −0.114 −3.407 ± −0.170

TLL Z2 Z1 Y2

19.191 ± 0.959 18.319 ± 0.915 12.338 ± 0.616

Y1 X2 X1

16.679 ± 0.833 11.119 ± 0.555 5.559 ± 0.277

7.799 ± 0.389 15.598 ± 0.779 23.397 ± 1.169

0.0910 ± 0.00455 0.0912 ± 0.00456 0.0911 ± 0.004555

58.917 ± 2.945 39.398 ± 1.969 42.223 ± 2.111

bottom phase (%) top phase (%) feed composition (%)

Table 3. Tie-Line Compositions of the ABSs at 298 Ka

0.000012 ± 0.0000006 0.00015 ± 0.0000075 0.0053 ± 0.000265

45.902 ± 2.295 45.546 ± 2.277 43.494 ± 2.174


E



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Na2SO4 ABS was studied at different pH (Figure 2a). In acidic medium molybdate extraction efficiency of block copolymer is higher than that in the basic conditions. Maximum extraction was found at pH 2 and 3 (100%). Extraction decreases to 86% at pH 1. It has also been observed that lesser extractions are observed upon increasing the pH beyond 3. At a higher pH (6 and above), the extraction decreases and becomes almost negligible. The reason behind this pH-dependent extraction may be explained on the basis of the fact that molybdate species are polymerized in acidic medium to H2Mo7O244− and HMo7O245− which have higher hydrophobicity due to low charge density of the species.15 Therefore, the extraction of the species in the polymer-rich phase becomes feasible. This result is further reflected in the aggregation number of the polymerrich phase as explained below. On increasing the metal loading from 0.13 mM to 5 mM the extraction percentage remained unaltered. 3.3. Aggregation Number. The fluorescence property of PPG-PEG-PPG was explored to obtain the aggregation number of the polymer-rich phase after extraction of the molybdate species at different pH conditions.41,32,7 A solution of K2Cr2O7 was used for the quenching experiment. Figure S4 shows the fluorescence quenching property of block copolymer. The fluorescence property of the linear chain polymers has been reported extensively in the literature, which was validated using several instrumental and experimental procedures including dynamic light scattering, transmission electron microscopy, confocal microscopy, concentration, and molecular weight based fluorescence spectroscopic methods.6,7 The organized self-assembly or aggregations with a hydrophilic core and a similar outside, similar to a vesicle, in the polymer solutions offer an electron-rich zone for the generation of fluorescence. Fluorescence spectroscopy served as an important and reliable experimental method for investigating the environment, prevailing within organized assemblies over a nanodimensional length scale. The micellar aggregations were reported to be 20 ± 5 μm in 30% v/v block copolymer solution.7 Upon increasing the pH after neutral point, molybdenum exists as MoO42− species which has a tendency to salt out the polymer owing to its lower hydrophobicity and higher charge density.15 Hence the extraction falls drastically. At pH 2 and 3 a slight protonation of the −OH moieties of block copolymer becomes responsible for attracting the negatively charged molybdate ions. At even lower pH, protonation becomes higher, resulting in mutual hindrance in extraction procedure, and both extraction percent and aggregation number decrease to some extent. The results of the extraction pattern tally with that of the aggregation number (Figure 2b) which explains the variation of extraction of molybdate ions and aggregation numbers of block copolymer at different pH. According to Figure 2b, the aggregation number (Nagg) is highest at pH 2 and 3 where the extraction of molybdate is highest. The Nagg values follow the exact trend for the extent of extraction at all other pH conditions. 3.4. Recovery of Block Copolymer. After the extraction of the analyte, the main aim of the work is to recover the polymer used in ABS. The block copolymer was heated at different conditions to ensure its thermoseparation and subsequent regeneration from the ABS. Thermoseparation: Recovery of block copolymer is essential, keeping in view the environmental and economic factors. It has

Table 4. Charge Density Values of Different Metal Species Extracted species

charge densitya(|Z|/M)

H2Mo7O244− HMo7O245− MoO42− Co2+ Ni2+ Mn2+ Fe3+ Cu2+ VO2+ H2VO42− WO42− HWO4−

0.0037 0.0046 0.0125 0.0339 0.0340 0.0364 0.0537 0.0314 0.0149 0.0117 0.0093 0.00461

|Z| = absolute value charge of the ions; M = molar mass of ions.

a

Figure 7. Extraction of molybdate in block copolymer vs sodium sulfate ABS in the presence of other ABS-forming salts at pH 3.

To draw the tie lines, we prepared different sets of ABSs with different compositions (% weight fractions) of PPG-PEGPPG and sodium sulfate at 298 K. The tie line length (TLL) and slope of the tie line length (STL) were determined by liver arm rule using the relationship between the top phase composition and the overall compositions. The following equations with four unknown variables were solved using MATLAB to determine the TLL and STL values. 3 [Block]Block = A exp[(B[Salt]0.5 Block ) − (C[Salt]Block )]

(4)

0.5 3 [Block]Salt = A exp[(B[Salt]Salt ) − (C[Salt]Salt )]

(5)

i [Salt]M yz 1 − α zzz − [Salt]Block = jjjj [Block]Salt α k α {

i [Salt]M zy 1 − α zzz − [Salt]Block = jjjj [Salt]Salt α k α {

(6) (7)

TLL = [([Salt]Block − [Salt]Salt )2 + ([Block]Block − [Block]Salt )2 ]1/2

(8) ([Block]Block − [Block]Salt ) STL = ([Salt]Salt − [Salt]Block )

(9)

Here the subscripts “Block”, “Salt”, and “M” refer to the PPGPEG-PPG-rich phase, salt-rich phase, and the mixture, respectively. [Block] and [Salt] designate the weight fraction of PPG-PEG-PPG block copolymer and sodium sulfate, respectively. The compositions of block copolymer and Na2SO4 in the top phase and bottom phase and values of TLL and STL are given in Table 3. The TLL and STL values for each set are found to be in agreement with each other. 3.2. Extraction of Molybdate. The extraction pattern of molybdate in the polymer-rich phase of PPG-PEG-PPG/ F



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Table 5. Table of Comparison Showing the Result of Molybdenum Extraction in the Earlier Reports with the Present Work ABS CuSO4/ PEG#4000

TX-100/ (NH4)2SO4 PEG1000/ Na2SO4 PEG2000/ Na2SO4 PEG4000/ Na2SO4 PPG-PEGPPG/Na2SO4

analyte (a) Na2MoO4 (b) (NH4)6Mo7O24 Na2MoO4 Na2MoO4

Na2MoO4

optimum extraction conditions 93.07% at pH = 2.5 and T = 338.15 K 92.66% at pH = 2.5 and T = 318.15 K 97% at pH = 3 and T = 303.15 K 94.87% at pH = 2 and T = 313.15 K 96.17% at pH = 2 and T = 313.15 K 97.24% at pH = 2 and T = 313.15 K 100% at pH = 2, 3 and T = 298 K

interference

ref

__

16

Fe(III) (D = 0.240), Co(II) (D = 0.225), Ni(II) (D = 0.194), Cu(II) (D = 0.153), Zn(II) (D = 0.267) D = distribution coefficient Al (E = 0.067%), Na (E = 0.95%), Ca (E = 4.56%) and Si (E = 7.66%)

15 44

Al (E = 0.086%), Na (E = 0.82%), Ca (E = 4.69%) and Si (E = 7.34%) Al (E = 0.068%), Na (E = 0.68%), Ca (E = 6.30%) and Si (E = 6.63%) E = percentage of extraction Co2+ (E = 2.97%), Mn2+ (E = 1.51%), Ni2+ (E = 1.5%), Cu2+ (E = 2.4%) and Fe3+ this (E = 4%) work

ppm of SO42− ions (coming from both Na2SO4 and H2SO4 which was used to adjust pH 3) was reduced to SO4 2 − = C4 H4O6 2 − > HPO4 2 − > CrO4 2 − > CH3COO− > HCO3− > Cl− > NO3− > ClO3−

In the above series the salting-in property of the anions increases as we move toward the right; as a consequence, cloud-point temperature of the polymer also increases in the presence of such ions. This trend is also reflected in the nature of Figure 3 where the biphasic region is maximum for the sulfate and tartrate salts and minimum for the nitrate salt. The other ions in the intermediate region follow a similar trend. After aqueous biphasic extractions, the polymer-rich phase is carefully separated out from the ABS for thermoseparation studies. Figure 4 shows a digital image of the thermoseparation of PPG-PEG-PPG block copolymer and indicates that the first cloud appears upon heating at 45 °C, and the actual thermoseparation can be realized upon heating at 75 °C. After thermoseparation, the PPG-PEG-PPG concentrates at the bottom of the container, leaving the excess components, viz., water, ions of the salt-rich phase, and analyte ions in the supernatant. The bottom phase is now richer in its polymer content which is carefully separated out and taken for absorption spectral studies. By using this thermoseparation technique we could recover up to 87% of the block copolymer. The lowest temperature at which this (87%) recovery is achieved is observed to be 75 °C. Any further increase in temperature does not help to enhance the recovery percent. Further purification of the block copolymer from different ions: After performing repeated thermoseparations of the block copolymer rich phase of the ABS, we tried to purify the polymer from different ions. Cation and anion exchange resins were used for the removal of the ions which entered the polymer-rich phase due to equilibration with the salt-rich phase and extraction of the molybdate species. An amount of 206 ppm of Na+ ions which were initially present in the block copolymer phase was reduced to a level of 4 ppm, and 2880 G



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4. CONCLUSION We have constructed a new ABS composed of PPG-PEG-PPG and Na2SO4 at 25 °C. The newly prepared ABS is capable to extract molybdenum species up to 100% at the optimum physical and chemical conditions as previously described. The present extraction method compares well with other literature reports in having higher extraction percent and analogous interference from associated metal ions, oxo, and polyoxometallates. A table of comparison showing the observation of the present work and that of the others is presented in Table 5. We applied the thermoseparation technique for the regeneration of the block copolymer which helps to revive 87% of the pure copolymer. This eco-friendly water-based ABS and the thermoseparative regeneration of the polymer direct toward a greener technology in the field of extraction chemistry.

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ABBREVIATIONS PPG-PEG-PPG, poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol); PEG-PPG-PEG, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol); CPT, cloud point temperature; ABS, aqueous biphasic system; TLL, tie line length; STL, slope of tie line; PAN, [1-(2-pyridylazo)-2-napthol]

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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.jced.8b00455. Absorption spectrum of PPG-PEG-PPG; spectrum showing maximum absorption for sulfate ion determination (λmax = 305 nm) in 60% PPG-PEG-PPG medium; different analytical parameters for the detection of the SO 4 2− ; spectrum showing wavelength of maximum absorption of the molybdate−thiocyanate complex (λmax = 470 nm) in 60% PPG-PEG-PPG medium; different analytical parameters for the detection of the molybdate using SCN−; fluorescence spectra of the block copolymer showing decrease in intensity with the addition of Cr(VI); spectrum showing wavelength of maximum absorption of the Co(II)−PAN complex (λmax = 577 nm) in 60% PPG-PEG-PPG medium; spectrum showing wavelength of maximum absorption of the Ni(II)−PAN complex (λmax = 565 nm) in 60% PPGPEG-PPG medium; spectrum showing wavelength of maximum absorption of the Cu(II)−PAN complex (λmax = 560 nm) in 60% PPG-PEG-PPG medium; spectrum showing wavelength of maximum absorption of the Mn(II)−PAN complex (λmax = 555 nm) in 60% PPGPEG-PPG medium; spectrum showing wavelength of maximum absorption of the Fe(III)−SCN complex (λmax = 350 nm) in 60% PPG-PEG-PPG medium; spectrum showing wavelength of maximum absorption of VO3− solution (λmax = 344 nm) in 60% PPG-PEGPPG medium; spectrum showing wavelength of maximum absorption of SnCl2-treated WO42− solution (λmax = 340 nm) in 60% PPG-PEG-PPG medium; different analytical parameters for the detection of the interfering metal ions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kamalika Sen: 0000-0003-1005-5147 Funding

Authors acknowledge UGC CAS V and UGC-Ref. No.: 259/ (CSIR-UGC NET JUNE 2017) for funding. Notes

The authors declare no competing financial interest. H



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