Preparation of Poly(vinylidene fluoride-co-tetrafluoroethylene)-Based

Article pubs.acs.org/IECR

Preparation of Poly(vinylidene fluoride-co-tetrafluoroethylene)Based Polymer Inclusion Membrane Using Bifunctional Ionic Liquid Extractant for Cr(VI) Transport Lin Guo,†,‡ Jianping Zhang,† Dongli Zhang,†,‡ Yinghui Liu,†,‡ Yuefeng Deng,† and Ji Chen*,† †

State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China ABSTRACT: A series of novel polymer inclusion membranes (PIMs) were prepared and applied in Cr(VI) transport. The PIMs were composed of bifunctional ionic liquid extractant (Bif-ILE) [trialkylmethylammonium][bis(2,4,4-trimethylpentyl)phosphinate] ([A336][C272]), [trialkylmethylammonium][di-2-ethylhexylphosphinate] ([A336][P204]), or [trialkylmethylammonium][di-(2-ethylhexyl)orthophosphinate] ([A336][P507]) as the carrier, 1-octyl-3-methylimidazolium hexafluorophosphate or tetrafluoroborate ([C8mim][PF6] or [BF4]) as the ionic liquid plasticizer (ILP), and poly(vinylidene fluoride-co-tetrafluoroethylene) (PVDF) as the polymer. ILP played a key role for the permeability coefficient (P). The maximum P values of [A336][C272], [A336][P204], and [A336][P507] PIMs were 1.39, 1.41, and 0.66 times higher than that without the plasticizer, respectively. The HCl concentration in the feed phase and NaOH concentration in the stripping phase had effects on the flux of Cr(VI). The maximum initial flux (Ji) was 40.08 μmol m−2 s−1 using the [A336][P204] PIM when the initial HCl concentration in the feed phase was 0.20 mol L−1 and the NaOH concentration in the stripping phase was 0.05 mol L−1. The selectivity of three PIMs followed as [A336][P204] ≈ [A336][P507] > [A336][C272]. Compared with Cyphos IL 104, the P value of Cr(VI) using [A336][C272], [A336][P204], or [A336][P507] was all higher than using Cyphos IL 104.

1. INTRODUCTION The separation technology of polymer inclusion membrane (PIM) has been widely developed since it reduces the consumption of organic solvents in extraction process and performs extraction and stripping simultaneously. PIM represents an alternative way to immobilize extractants, reduce the use of volatile organic compounds, and enhance the efficiency of extractants for metal extraction. PIMs composed of polymers, plasticizers, and ion carriers were widely used in removal of metal ions selectively.1−8 In these references, i.e., Zn(II) was separated by PIMs with calix[4]resorcinarenes derivatives1 or di(2-ethylhexyl)phosphoric acid7 as carriers. With functionalized calix[4]arene carrier, Hg(II) was selectively separated by PIMs.2 PIMs with carrier 3,7-dinonyl-naphtalene1-sulfonic acid were used for cobalt-60, strontium-90, and cesium-137 separation.3 Cu(II), Cd(II), In(III), or Pb(II) could be selectively separated by new N-6-(t-Dodecylamido)-2Pyridinecarboxylic acid, 4 Aliquat 336, 5 bis(2,4,4trimethylpentyl)phosphinic acid,6 or bis-(2-ethylhexyl) phosphoric acid8 in PIMs, respectively. A series of novel quaternary ammonium bifunctional ionic liquid extractants (Bif-ILEs) were synthesized in our laboratory.9 The extractants, i.e., [trialkylmethylammonium][bis(2,4,4-trimethylpentyl)phosphinate] ([A336][C272]), [trialkylmethylammonium][di-2-ethylhexylphosphinate] ([A336][P204]), and [trialkylmethylammonium][di-(2ethylhexyl)orthophosphinate] ([A336][P507]), which were neutral extractants, were used for rare earth ions separation.10,11 Other types of Bif-ILE such as [trialkylmethylammonium][secoctylphenoxy acetate] ([A336][CA-12]) and [trialkylmethylammonium][sec-nonylphenoxy acetate] © 2012 American Chemical Society

([A336][CA-100]) were used for the Co(II) and Ni(II) separation 12 and rare earth ions separation. 13 Also [trialkylmethylammonium][salicylate] was used for the selective separation of Fe(III) from a Fe(III), Zn(II), Ni(II), Co(II), Mn(II), and Cr(III) solution.14 The advantages of Bif-ILEs used in extractions are low acidity for extraction, good interfacial phenomena, and avoidance of the saponification of wastewater from the application of acidic extractants. Cr(VI), one of the most toxic heavy metal elements, is a pollutant in groundwater and soil and a carcinogen to animals and humans.15 Cr(VI) and other chromium compounds are widely used in many areas, i.e., textiles, electroplating, leather tanning, metallurgy, pigments, and stainless steel production. In order to remove and separate Cr(VI), many methods, i.e., liquid−liquid extraction,16,17 adsorption,18−23 and membrane separation24−29 are used in the laboratory and in industry. Aliquat 336,30 tertiary amines,26,27,31 and trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos IL 104)32 were used as PIMs carriers for Cr(VI) transport in the references. Imidazolium ILs were used as ionic liquid plasticizers (ILPs) in PIMs in our previous study. Compared with other traditional molecular plasticizers (MPs), ILPs could be used instead of MPs, since ILPs in PIMs also enhanced the flux of Cr(VI) transport.32 In this study, Bif-ILEs [A336][C272], [A336][P204], and [A336][P507] were found to be Received: Revised: Accepted: Published: 2714

August 16, 2011 December 30, 2011 January 9, 2012 January 9, 2012 dx.doi.org/10.1021/ie201824s | Ind. Eng.Chem. Res. 2012, 51, 2714−2722

Industrial & Engineering Chemistry Research

Article

Figure 1. Chemical structures of the Bif-ILEs and ILPs used in this study.

maintained for 6 h and washed several times to remove solvent. Then the membrane was dried at room temperature for 24 h. The dry membrane was cut to a suitable size for transport experiments. 2.3. Membrane Characterization. The morphology of the samples was observed using field-emission scanning electron microscopy (FESEM, XL30, Philips). Atomic force microscopy (AFM) (tapping mode) was used to investigate the surface morphology of the membranes with a Nanoscope IIIa (Veeco). The water contact angles were measured by a KRüSS DSA10-MK2 (KRüSS GmbH, Germany) drop shape analysis system at ambient temperature. The average contact angle value was obtained by measuring the same sample at five different positions. 2.4. Liquid−Liquid Extraction. Batch experiments of liquid−liquid extraction of Cr(VI) were explored to investigate the extraction mechanism of Bif-ILE and determine the appropriate chemical conditions for membrane transport. A 1.0 mL volume of Bif-ILE diluted with toluene and 5.0 mL of aqueous phase solutions were combined using a vibrating mixer for 60 min in equilibrium tubes at 25 ± 1 °C. The pH value was obtained by adding the required amount of HCl or NaOH. The concentration of Cr(VI) in the liquid−liquid extraction and transport experiment was measured at 540 nm based on the reaction of diphenylcarbazide and Cr(VI)19 using a UV−vis spectrophotometer. The extraction efficiency (E) and the distribution ratio (D) were obtained by the following equations:

the extractants for Cr(VI), and they were used as carrier with ILPs in PIMs for Cr(VI) transport.

2. EXPERIMENTAL SECTION 2.1. Material. PVDF (poly(vinylidene fluoride-co-tetrafluoroethylene)) commercial product of F2.4 was used as the membrane material, provided by Professor Yonglie Wu from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. The stock solution of Cr(VI) was prepared by dissolving K2Cr2O7 (primary standard grade, Tianjin Benchmark Chemical Reagent Co., Ltd., China) in deionized water. The other chemicals were analytical grade reagents (Beijing Beihua Fine Chemicals Co., Ltd., China). The Bif-ILEs [A336][C272], [A336][P204] and [A336][P507] were synthesized in our laboratory according to a published method.9 The imidazolium ILs used in this experiment were 1-octyl-3-methylimidazolium tetrafluoroborate ([C8mim][BF4]), 1-octyl-3-methylimidazolium hexafluorophosphate ([C 8 mim][PF 6 ]), and 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C8mim][NTf2]). They were synthesized as previously reported.33 The chemical structures of these ILs were shown in Figure 1. 2.2. Membrane Preparation. Composite PVDF flat membranes were prepared by the phase inversion method, and other type of membranes were prepared by a similar method in the literature.34,35 PVDF (0.5 g) was dissolved into the N,N-dimethylformamide (DMF, 5.0 mL), and ILPs [C8 mim][BF 4], [C8 mim][PF6 ], or [C8 mim][NTf2] was added. Then carrier [A336][C272], [A336][P204], or [A336][P507] was added. The mixture was stirred for 5 min at room temperature and kept at 40 °C for 72 h. Subsequently the casting solution was stirred for 30 min. Membranes were cast using a 500 μm casting knife onto a glass plate at room temperature. The nascent membrane was evaporated at room temperature for 20 s, then immersed in deionized water

E(%) =

D=

2715

Co − Ce × 100 Co

Co − Ce ⎛ Va ⎞ ⎜ ⎟ Ce ⎝ Vo ⎠

(1)

(2)

dx.doi.org/10.1021/ie201824s | Ind. Eng.Chem. Res. 2012, 51, 2714−2722


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where Co (mg L−1) and Ce (mg L−1) are the initial and final concentrations of Cr(VI) in the aqueous phase. Va (mL) is the volume of the aqueous phase and Vo (mL) is the volume of the organic phase. 2.5. Transport Experiments. All Cr(VI) transport experiments were performed at 25 ± 0.5 °C in a two compartments membrane cell of 90 cm3 each. The PIM was placed on the circular window separating the two compartments. The effective membrane contact area was 7.07 cm2. The feed and stripping solutions were placed in each compartment of the cell, and both the compartments were stirred at 600 rpm by stirrers. The pH value of feed solution was adjusted by adding the required amount of HCl. NaOH was chosen as the stripping solution in this work. The permeability coefficient (P, μm s−1) of Cr(VI) in the membrane was calculated by

⎛c⎞ ln⎜ ⎟ = − kt ⎝ ci ⎠

(3)

where ci (mg L−1) and c (mg L−1) are the concentration of the metal ions in the feed phase at the initial time and a selected time, respectively, k is the rate constant (min−1), and t is the selected time of transport (min). In order to obtain k values, the plots of ln(c/ci) versus time were prepared. The relationship of ln(c/ci) versus time is linear, and the determined coefficients (r2) were ≥0.98. Then P was calculated with following equation:

P=−

⎛V ⎞ ⎜ ⎟k ⎝ A⎠

Figure 2. SEM images of the surface of PIMs. (a) PVDF membrane, (b−d) PVDF membrane containing 0.4 mmol g−1 carrier and 3.0 mmol g−1 [C8mim][BF4] (carriers in parts b−d are [A336][C272], [A336][P204], and [A336][P507], respectively).

37.7 nm, and 23.7 nm, respectively. The Ra value was larger than the others when [A336][P204] was used as the carrier. The surface unevenness would enlarge the effective contact area of the membrane. The investigation about the contact angle of the above membrane was shown in Table 1. The value of PVDF membrane was 90. 43 ± 0.69°. When the carrier and ILP were added in the membrane, the contact angle was higher. Therefore, the hydrophobicity of the membrane surface followed as b > d > c > a. With comparison of b, c, and d, both the hydrophobicity of the carriers and the surface morphology of the membranes have effects on the contact angles. In the hydrophobic region, the contact angle increased with the increasing of the surface roughness.37 Contact angles at rough hydrophobic surfaces are higher since the wrinkle could arrest the liquid.38 Sun et al.9 reported the hydrophobic property of carriers (Table 1). The hydrophobic property of [A336][C272] was larger than the other carriers, and then the contact angle of b was the largest. Compared with c and d, the hydrophobic property of [A336][P507] was higher than [A336][P204], and the Ra value of c was larger than d. The contact angle values of c and d were similar because of the two factors. Therefore, the contact angle values of PIMs were consistent with the hydrophobicity of carriers and Ra values of the membranes. 3.2. Effect of Carrier on the Cr(VI) Transport. 3.2.1. Extraction Mechanism of Bif-ILEs. The effect of the initial pH value on extraction of Cr(VI) by [A336][C272], [A336][P204], or [A336][P507] was shown in Figure 4. With the increasing of the pH value from 0 to 12, the extraction efficiency of Cr(VI) decreased. The optimum pH value range of Cr(VI) extraction for [A336][C272] was 0 to 2 and for [A336][P204] or [A336][P507] was 0 to 1. The dominant species of Cr(VI) in aqueous solution were H2CrO4 when the pH value was below 2.39,40 E decreased, since H2CrO4 in

(4)

where V is the volume of the aqueous feed phase (mL) and A is the area of the effective membrane (cm2). The initial flux (Ji, μmol m−2 s−1) was determined by

Ji = Pci

(5)

Both transport and batch experiments were conducted in duplicate under the same conditions, and the relative error between duplicates was less than 10%. The results were reported as mean values. The concentrations of the mixture of metal ions (i.e., Fe(III), Co(II), Cu(II), Zn(II), and Cr(VI)) were determined by inductively coupled plasma optical emission spectrometers (ICP-OES, Thermo iCAP 6000).

3. RESULTS AND DISCUSSION 3.1. Membrane Characterization. The morphology of the PVDF membrane and PIMs was shown in Figure 2. When some compounds such as LiCl, LiClO4·3H2O, and [C8mim][BF4] were added in PVDF, the membrane structure was changed.32,36 Compared with Figure 2a, the surface morphology and pore size of membranes in Figure 2b−d were changed when different carriers and [C8mim][BF4] were added. At the same time, there were no essential changes in the porous structure of the PVDF, and the pores did not vanish. AFM images of PIMs in three-dimensional format of 5.0 μm × 5.0 μm were shown in Figure 3. Compared with other PIMs, the surface of the PVDF membrane without other ILs was smooth (Figure3a). When [C8mim][BF4] and carriers were added in membranes, the wrinkle of PIMs became larger in Figure 3b−d. It was attributed to the surface condition of PIMs. Also the similar phenomenon was found in ref 32. As shown in Table 1, the roughness (Ra) values of the PVDF membrane and various PIMs from Figure 3a−d were 6.61 nm, 25.1 nm, 2716



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Figure 3. AFM images of the surface of PIMs. (a) PVDF membrane, (b−d) PVDF membrane containing 0.4 mmol g−1 carrier and 3.0 mmol g−1 [C8mim][BF4] (carriers in parts b−d are [A336][C272], [A336][P204], and [A336][P507], respectively).

Also, the phenomena were similar to Cr(VI) extraction using Cyphos IL 104 as the extractant in our previous work.32 To explore the extraction mechanism by [A336][C272], [A336][P204], and [A336][P507] in toluene, respectively, the conventional slope analysis method was adopted. As shown in Figure 5, the plot of log D versus log [A336][C272] gave a straight line with a slope of 2.00. Similarly, the plot of log D

Table 1. Roughness and Contact Angle of PIMs and the Hydrophobic Property of Bif-ILEs

a b c d

carrier

roughness (Ra) (nm)

[A336][C272] [A336][P204] [A336][P507]

6.61 25.1 37.7 23.7

contact angle (deg) 90.43 127.89 91.42 92.43

± ± ± ±

0.69 1.55 0.70 0.69

water content (ppm) 378.4 967.2 576.0

Figure 5. Slope analysis of Cr(VI) extraction as a function of the concentration of Bif-ILE in toluene (100 mg L−1 Cr(VI), 1.0 mol L−1 HCl).

Figure 4. Effect of initial pH value on the extraction of 100 mg L−1 Cr(VI) by 0.009 mol L−1 Bif-ILE in toluene.

versus log [A336][P204] gave a straight line with a slope of 1.99 and the plot of log D versus log [A336][P507] gave a straight line with a slope of 2.01. It was shown that the stoichiometry of [A336][C272], [A336][P204], or [A336][P507] with Cr(VI) was 2:1. The extraction mechanism by three Bif-ILEs was similar, although the anion of each extractant was different.

aqueous solution decreased with the pH value increasing. Hence, [A336][C272], [A336][P204], and [A336][P507] were effective extractants for the removal of H2CrO4. The E decreased to 0 with a pH value larger than 6 for [A336][C272] and [A336][P507]. On the other hand, the extraction efficiency decreased to 0 when the pH value was larger than 5 for [A336][P204]. The variation trends of E using [A336][C272], [A336][P204], or [A336][P507] as the extractant were similar. 2717



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3.2.2. Effect of [A336][C272] Concentration on Cr(VI) Transport. The flux of Cr(VI) was low when the concentration of [A336][C272] was low. When the amount of [A336][C272] was higher than 0.15 mmol g−1, the flux of Cr(VI) increased smoothly. With the increasing of [A336][C272], the P enhanced and reached a plateau at the concentration of 0.4 mmol g−1 (Figure 6). Compared with Aliquat 336, the carriers

Figure 7. The P of Cr(VI) versus different carriers. Feed phase: 100 mgl L−1 Cr(VI), 0.10 mol L−1 HCl. Stripping phase: 0.05 mol L−1 NaOH. PIM: 0.4 mmol g−1 carrier.

μm s−1 for 0.9 mmol g−1 ILP in [A336][P204] PIM, and 6.15 μm s−1 for 0.9 mmol g−1 ILP in [A336][P507] PIM, respectively. When the amount of [C8mim][BF4] was higher than each corresponding value for [A336][C272], [A336][P204], or [A336][P507] PIM, the flux of Cr(VI) increased sharply with the increasing of ILP. The maximum P were 17.48 μm s−1 for 2.9 mmol g−1 ILP in [A336][C272] PIM, 18.35 μm s−1 for 1.7 mmol g−1 ILP in [A336][P204] PIM, and 15.45 μm s−1 for 3.0 mmol g−1 ILP in [A336][P507] PIM, respectively. After the P value reached a maximum, the flux of Cr(VI) decreased with the increasing of ILP in [A336][C272], [A336][P204], or [A336][P507] PIM. The excess ILP would dilute the carrier, so the P was decreased. The variation trends of P were similar when PIM was used [A336][C272], [A336][P204], or [A336][P507] as the carrier. The P of Cr(VI) decreased when a small amount of plasticizer was added. A small amount of [C8mim][BF4] may affect the carrier ([A336][C272], [A336][P204], or [A336][P507]) with continuous distribution in the membrane then inducing the P value decrease.32 This result was not observed in CTA PIMs with MPs.42,43 With the amount of plasticizer [C8mim][BF4] increasing, carrier ([A336][C272], [A336][P204], or [A336][P507]) was dispersed in liquid microdomains by the plasticizer. When the amount of plasticizer was enough to coalesce, the liquid microdomains created continuous pathways and connected between the two interfaces of membranes.42 The continuous pathways had benefits for Cr(VI) transport in PIMs, since the maximum P value of [A336][C272] PIM was 1.39 times higher than that of PIM without plasticizer (Figure 7). Similarly, the maximum P values of [A336][P204] and [A336][P507] PIMs were 1.41 and 0.66 times higher than that of PIMs without plasticizer, respectively. Hence the diffusion of Bif-ILE carrier in [C8mim][BF4] was faster than in the polymer matrix. Compared with [A336][C272], [A336][P204], and [A336][P507] as carriers for Cr(VI) transport, [A336][P204] was the better choice than the others, since [A336][P204] PIM consumed less plasticizer to the maximum P value and brought a higher flux of Cr(VI) than the other carriers. [A336][C272] and Cyphos IL 104 had the same anion and different cations. The variation trends of P using Cyphos IL 10432 as the carrier in our previous work were similar to that using [A336][C272] as the carrier (Figure 7). The flux of Cr(VI) using Cyphos IL 104 was less than using [A336][C272]. Therefore, quaternary ammonium salt cation was more effective for Cr(VI) transport than quaternary phosphonium salt. As shown in Figure 7, the P

Figure 6. Effect of [A336][C272] concentration in PIM on Cr(VI) transport. Feed phase: 100 mg L−1 Cr(VI), 0.10 mol L−1 HCl. Stripping phase: 0.05 mol L−1 NaOH. PIM: 2.3 mmol g−1 [C8mim][BF4].

[A336][C272], [A336][P204], and [A336][P507] had the same quaternary ammonium cation and the different anions. The anion of Aliquat 336 was Cl−, and the anions of [A336][C272], [A336][P204], and [A336][P507] were phosphinate anions. Aliquat 336 in PIMs reacted as an anionexchanger forming an ion-pair with Cr(VI).41 On the other hand, [A336][C272], [A336][P204], and [A336][P507] were neutral extractants and they formed extracted complex 2[A336][C272]·H2CrO4, 2[A336][P204]·H2CrO4, and 2[A336][P507]·H2CrO4 with Cr(VI) in PIMs. The proposed structure of 2[A336][C272]·H2CrO4 was in Scheme 1, and the Scheme 1. Proposed Structure of 2[A336][C272]·H2CrO4

structure of 2[A336][P204]·H 2 CrO 4 or 2[A336][P507]·H2CrO4 was similar to 2[A336][C272]·H2CrO4. There were hydrogen bonds between PO in carriers and H in H2CrO4. The mechanism was similar to carrier Cyphos IL 104 for Cr(VI) transport.32 This reference also drew the conclusion that P of Cr(VI) was low, when PIMs used Aliquat 336 as the carrier and used ILPs. The low P was probably attributed to the anion exchange mechanism of Cr(VI) ions extracted by Aliquat 336.16 3.2.3. Effect of Different Carrier and Amount of ILP on Cr(VI) Transport. In order to compare the effect of [A336][C272], [A336][P204] and [A336][P507], as the carrier on Cr(VI) transport, the three carriers concentration were all 0.4 mmol g−1, and the amount of ILP [C8mim][BF4] in membrane was increased. As shown in Figure 7, with the amount of [C8mim][BF4] in membrane increased, the P of Cr(VI) changed greatly. The minimum P of Cr(VI) transport were 4.88 μm s−1 for 0.3 mmol g−1 ILP in [A336][C272] PIM, 3.17 2718



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values of Cr(VI) using [A336][C272], [A336][P204], and [A336][P507] were all higher than that using Cyphos IL 104. 3.3. Effect of Anions of ILPs on the Cr(VI) Transport. The effect of different ILPs on the Cr(VI) transport in [A336][C272] PIM was investigated, and the variation of P with [C8mim][PF6 ] and [C 8mim][NTf2] as ILPs was illustrated in Figure 8. The trend of P using [C8mim][PF6]

concentration at this pH value was too low. [A336][C272], [A336][P204], and [A336][P507] were effective extractants for removal of H2CrO4 (Figure 4). Therefore, Ji was low. When the initial concentration of HCl reached to 0.10 mol L−1, the primary ion fraction of Cr(VI) was H2CrO4, then the flux of Cr(VI) increased sharply. When the initial concentration of HCl was enhanced from 0.10 to 0.20 mol L−1, Ji increased smoothly. The maximum Ji values were 35.27 μmol m−2 s−1 using [A336][C272] PIM, 40.08 μmol m−2 s−1 using [A336][P204] PIM, and 32.36 μmol m−2 s−1 using [A336][P507] PIM, respectively (Figure 9). The initial concentration of NaOH used as a stripping solution had an effect on Cr(VI) transport. As shown in Figure

Figure 8. The P of Cr(VI) versus different ILPs. Feed phase: 100 mgl L−1 Cr(VI), 0.10 mol L−1 HCl. Stripping phase: 0.05 mol L−1 NaOH. PIM: 0.4 mmol g−1 [A336][C272].

as ILP was similar with [C8mim][BF4]. The maximum P was 18.18 μm s−1 for 2.5 mmol g−1 [C8mim][PF6] in PIM. However, [C8mim][NTf2] could not enhance the P. With the increasing of amount of [C8mim][NTf2], P decreased. When the amount of [C8mim][NTf2] was 3.1 mmol g−1, the P almost reduced to 0. The state of [A336][C272] was a waxy solid.9 The viscosity values of [C8mim][PF6], [C8mim][BF4], and [C8mim][NTf2] were 601.0 cP, 291.0 cP, and 95.0 cP, respectively.44 The viscosity of [C8mim][NTf2] was much lower than the other ILPs. [A336][C272] may not disperse well in [C8mim][NTf2]. When membrane was cast, probably [A336][C272] and [C8mim][NTf2] would be separated quickly. 3.4. Effect of HCl Concentration in Feed Phase and NaOH Concentration in Stripping Phase on Cr(VI) Transport. As shown in Figure 9, the initial concentration of

Figure 10. Effect of NaOH concentration on Cr(VI) transport. Feed phase: 100 mg L−1 Cr(VI), 0.10 mol L−1 HCl., PIM a: 0.4 mmol g−1 [A336][C272], 2.9 mmol g−1 [C8mim][BF4]. PIM b: 0.4 mmol g−1 [A336][P204], 1.7 mmol g−1 [C8mim][BF4]. PIM c: 0.4 mmol g−1 [A336][P507], 3.0 mmol g−1 [C8mim][BF4].

10, with the increasing of the NaOH initial concentration, the Ji of Cr(VI) increased. When the concentration of NaOH was higher than 0.05 mol L−1, the flux of Cr(VI) was increased slowly. Also, Ji values of three types of membranes all reached a maximum when the initial concentration of NaOH was 0.20 mol L−1. The maximum Ji values were 35.63 μmol m−2 s−1 using [A336][C272] PIM, 36.92 μmol m−2 s−1 using [A336][P204] PIM, and 34.33 μmol m−2 s−1 using [A336][P507] PIM, respectively. In the feed phase, H2CrO4 combined with carrier in PIMs and went into membrane phase. In the membrane phase, extracted complex 2[A336][C272]·H 2 CrO 4 , 2[A336][P204]·H2CrO4 or 2[A336][P507]·H2CrO4 was diffused from feed phase to stripping phase. When the extracted complex was in contact with the stripping phase, the complex was decomposed in the base environment. As shown in liquid− liquid extraction (Figure 4), the extraction efficiency of Cr(VI) was 0 and 2[A336][C272]·H2CrO4, 2[A336][P204]·H2CrO4, or 2[A336][P507]·H2CrO4 could not form, when the pH value was larger than 6. OH− in the stripping phase may combine with the H+ in H2CrO4, and CrO42‑ went into the stripping phase. Then the carrier was reused and transported Cr(VI) from the feed phase to the stripping phase repeatedly in one experiment. 3.5. Reusability of PIMs. The repeated transport experiments were performed, where both the aqueous in the feed and stripping phases were renewed every time while the membrane was not changed. As can be seen in Figure 11, with the number of runs increasing, P of Cr(VI) decreased from 19.11 μm s−1 to 11.20 μm s−1 after six cycles. P was decreased to 76% after five

Figure 9. Effect of HCl concentration on Cr(VI) transport. Feed phase: 100 mg L−1 Cr(VI). Stripping phase: 0.05 mol L−1 NaOH. PIM a: 0.4 mmol g−1 [A336][C272], 2.9 mmol g−1 [C8mim][BF4]. PIM b: 0.4 mmol g−1 [A336][P204], 1.7 mmol g−1 [C8mim][BF4]. PIM c: 0.4 mmol g−1 [A336][P507], 3.0 mmol g−1 [C8mim][BF4].

HCl was from 0 to 0.20 mol L−1. Without HCl added in the feed phase, the pH value was 4.6. With this condition, the primary ion fraction of Cr(VI) was HCrO4−,40 and the H2CrO4 2719



Article

the stripping condition for Fe(III), Co(II), Cu(II), or Zn(II). With the conditions, the extraction process of Fe(III), Co(II), Cu(II), or Zn(II) by the carrier was difficult. Then it was difficult to transport these cations through the PIMs. 3.7. Effect of Stirring Speed on Cr(VI) Permeation. The hydrodynamic condition also had an effect on the P value of Cr(VI) transport. In order to investigate the influence of mass transfer resistance, the stirring speed in the feed phase was changed and the stirring speed in the stripping phase was kept constant at 600 min−1. As shown in Table 3, when the stirring Table 3. Effect of Stirring Speed on Cr(VI) Permeationa Figure 11. Repeated runs of Cr(VI) transport across PIM. Feed phase: 100 mg L−1 Cr(VI), 0.10 mol L−1 HCl. Stripping phase: 0.15 mol L−1 NaOH. PIM: 0.4 mmol g−1 [A336][P204], 1.7 mmol g−1 [C8mim][BF4].

cycles and to 59% after six cycles. From the first to the fifth run, only 24% of the P value was lost, but from the fifth to the sixth run, 17% of the P value was lost. The reusability of this PIM was less than Cyphos IL 104 PIM, which made the P value decreased to 69% after nine cycles.32 The drop of the P value was probably attributed to the partial decomposition of BF4− from the PIM and the influence of Cr(VI) oxidation to the membrane material during the Cr(VI) transport experiment. 3.6. Effect of Other Ionic Species on Cr(VI) Transport. In order to investigate the selectivity of these membranes for Cr(VI) and other metal ions, the effects of Fe(III), Co(II), Cu(II), and Zn(II) on the Cr(VI) transport were explored. As shown in Table 2, the final amount of each metal ion in the

PIM b

PIM c

Fe(III) Co(II) Cu(II) Zn(II) Cr(VI)

1.55 0.18 0.63 9.96 93.41

1.47 0 0.11 0.64 95.08

1.42 0.01 0.29 0.92 96.07

300 450 600 800 1000

10.48 14.67 15.45 15.64 15.82

Feed phase: 100 mg L−1 Cr(VI), 0.10 mol L−1 HCl. Stripping phase: 0.05 mol L−1 NaOH. PIM: 0.4 mmol g−1 [A336][P507], 3.0 mmol g−1 [C8mim][BF4].

speed in the feed phase was higher than 600 min−1, the P value was almost constant. Then the thickness of the aqueous diffusion layer and the aqueous resistance to mass transfer was minimized.

4. CONCLUSIONS In this work, Bif-ILEs [A336][C272], [A336][P204], and [A336][P507] were found to be effective extractants for the removal of Cr(VI) when the pH value was 0−2. Bif-ILEs carriers and ILPs used in PIMs could enhance the flux of Cr(VI) transport. The P of Cr(VI) was influenced by the type and the amount of carrier and ILPs. A membrane with [A336][P204] and [C8mim][BF4] showed the higher P value and the better selectivity for Cr(VI) transport.

recovery (%) PIM a

P (μm s−1)

a

Table 2. Effect of Other Metal Ions on Recovery of Cr(VI)a ion in feed phase

speed (min−1)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-431-8526-2646. E-mail: [email protected].

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a

Feed phase: mixture of 5 metal ions; each metal ion concentration, 0.001 mol L−1; 0.15 mol L−1 HCl. Stripping phase: 0.15 mol L−1 NaOH. PIM a: 0.4 mmol g−1 [A336][C272], 2.9 mmol g−1 [C8mim][BF4]. PIM b: 0.4 mmol g−1 [A336][P204], 1.7 mmol g−1 [C8mim][BF4]. PIM c: 0.4 mmol g−1 [A336][P507], 3.0 mmol g−1 [C8mim][BF4].

ACKNOWLEDGMENTS The project was sponsored by SRF for ROCS, Ministry of Education of China, the Natural Science Foundation of China (Grant No. 51174184), and the National Basic Research Program of China (Grant 2012CBA01202). We thank Professor Yonglie Wu for helpful discussions.

feed or stripping phase was determined by ICP-OES at the end of 10 h. When more than 95% Cr(VI) was transported through the PIM, less than 1.0% Zn(III), Co(II), and Cu(II) and less than 1.5% Fe(II) went through the PIM used [A336][P204] or [A336][P507] as the carrier, respectively. On the other hand, less than 2.0% Fe(III), Co(II), and Cu(II) and less than 10% Zn(II) transported with 93.41% Cr(VI) through the PIM used [A336][C272] as the carrier. The selectivity of three PIMs followed as PIM b ≈ PIM c > PIM a. The pH value had an effect on extraction metal ions by [A336][C272], [A336][P204], or [A336][P507]. The optimum pH value of Eu(III) cations extraction using [A336][C272], [A336][P204], or [A336][P507] was 6.02, and stripping occurred since the HNO3 concentration was 0.01 mol L−1.10 Therefore, when the initial pH value of the feed phase was ≤1.0 in this study, it was

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NOMENCLATURE E = extraction efficiency (%) D = distribution ratio Co = the initial concentrations (mol L−1) of metal ions in the aqueous phase Ce = the final concentrations (mol L−1) of metal ions in the aqueous phase Va = the volume (mL) of the aqueous phase Vo = the volume (mL) of the organic phase ci = the concentration (mg L−1) of metal ions in the feed phase at time 0 c = the concentration (mg L−1) of metal ions in the feed phase at the selected time dx.doi.org/10.1021/ie201824s | Ind. Eng.Chem. Res. 2012, 51, 2714−2722


Article

k = the rate constant (min−1) t = the selected time of transport (min) P = permeability coefficient (μm s−1) V = the volume (mL) of the aqueous feed phase A = membrane effective area (cm2) Ji = initial flux (μmol m−2 s−1) Ra = the roughness of the membrane

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