ARTICLE pubs.acs.org/IECR
Three-Liquid-Phase Extraction: A New Approach for Simultaneous Enrichment and Separation of Cr(III) and Cr(VI) Keng Xie,†,‡,§ Kun Huang,*,† Liangrong Yang,† Pinhua Yu,†,‡,§ and Huizhou Liu*,† †
Key Laboratory of Green Process and Engineering, State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing 100190, China § Graduate University of the Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: A novel and simple three-liquid-phase extraction (TLPE) approach was developed for the simultaneous removal and separation of Cr(III) and Cr(VI) from aqueous solutions. The proposed three-liquid-phase system (TLPS) consists of di(2ethylhexyl)phosphoric acid (D2EHPA), poly(ethylene glycol) (PEG) with a molecular weight of 2000, and (NH4)2SO4. The effects of various factors including the aqueous solution pH, amounts of (NH4)2SO4 and PEG, initial chromium amount, D2EHPA concentration, phase-mixing time, and diluent on the three-phase partitioning behavior of Cr(III) and Cr(VI) were evaluated. The distribution behavior of the two oxidation states of chromium was found to be highly pH-dependent. Cr(III) preferred the D2EHPA-rich top phase through a cation-exchange reaction, whereas Cr(VI) was enriched in the PEG-rich middle phase through ion-pair formation. By appropriate selection of the extraction conditions, nearly all of the Cr(III) was extracted into the D2EHPA top phase, and more than 90% of the Cr(VI) was transferred into the PEG-rich middle phase within 5 min. The present work highlights the possibility of using the TLPE approach for the extraction and separation of two different oxidation states of metal ions, such as Cr(III) and Cr(VI), in a single extraction step.
1. INTRODUCTION Chromium, a major source of environmental contamination, occurs most frequently as Cr(VI) or Cr(III) in industrial wastewaters of metallurgical processing, leather tanning, wood preservation, and electroplating.1,2 Cr(VI) is known to be highly irritating and toxic to humans and animals and is considered to cause cancer.3 Although Cr(III) is less hazardous than Cr(VI) and is even regarded as an essential trace element for the effective metabolism of glucose, lipids, and protein,4 its discharge is still regulated5 because Cr(III) can be oxidized to Cr(VI) under some circumstances,6 and safety problems arise when living creatures are exposed to high levels of Cr(III).7,8 In this case, the treatment of Cr(III) has to conform with the same stringent requirements as that of Cr(VI). Therefore, it is necessary for risk assessment to determine not only the total chromium content but also its different oxidation states and for environmental remediation to remove the chromium species. Moreover, as chromium is widely used in industry, it is also necessary for comprehensive resource utilization to recover both Cr(III) and Cr(VI).9 Liquidliquid extraction has been employed for treatment of various aqueous solutions containing chromium.1012 As an effective separation and preconcentration strategy prior to determination by atomic absorption spectroscopy (AAS), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and so on, the liquidliquid extraction improves the analytical detection limit for chromium through its high separation selectivity.1317 However, such common analyses can give only the total content of chromium rather than independent results for Cr(III) and Cr(VI). The use of liquidliquid extraction for the simultaneous precise determination of Cr(III) and Cr(VI) is unfeasible because of mutual interference from the complex matrix. r 2011 American Chemical Society
Apart from analytical determination purposes, liquidliquid extraction has also been employed for the elimination of chromium from industrial effluents and the recovery of chromium from hydrometallurgical leaching solutions.18 A literature survey covering information on chromium extraction reveals that acidic organophosphorous extractants and neutral organophosphorous extractants are often chosen for the extraction of Cr(III) and Cr(VI), respectively.1923 Owing to significant differences in their physicochemical properties, the two chromium species cannot simultaneously be extracted with one extractant. Thus, much attention has been paid to the extraction of single species. As for mixtures containing both Cr(III) and Cr(VI), liquidliquid extraction often cannot ensure total compliance with environmental regulations and economic perspective. The versatile technique employed for the removal or recovery of total chromium is based on the transformation between Cr(III) and Cr(VI) by redox reactions.2426 The introduction of oxidizing or reducing agents usually leads to instability of the system and is cost-ineffective. Thus, there has been a sustained effort to develop better, economically viable methods for the extraction and separation of the two chromium species. Three-liquid-phase extraction (TLPE), which is carried out in a three-layer liquid medium composed of an organic solvent phase and two aqueous phases containing polymer and inorganic salt, has become a promising alternative to displace traditional Received: August 15, 2011 Accepted: October 10, 2011 Revised: October 7, 2011 Published: October 10, 2011 12767
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Industrial & Engineering Chemistry Research solvent extraction because of its outstanding advantages in extended separation capacities.27 In a typical three-liquid-phase system (TLPS), each phase has its own specific selectivity to extract an individual component from the feed mixture. TLPS provides a new medium with different polarities from top to bottom, allowing the separation of components that differ only slightly in their physicochemical properties. Therefore, simultaneous extraction and three-phase separation of two or more components in three different liquid phases can be achieved by single-step extraction. The use of polymer-based TLPSs in antibiotics purification,28 herbal extract isolation,29 phenolic wastewater treatment,30,31 and multimetal separation32,33 was pioneered in our laboratory. In the present work, we extended the use of TLPSs for the selective enrichment and one-step separation of two different oxidation states of chromium ions, Cr(III) and Cr(VI). Di(2ethylhexyl) phosphoric acid (D2EHPA), which has received the most attention for the extraction of Cr(III),2123 was combined with a polymer-based aqueous two-phase system (ATPS) consisting of a phase rich in poly(ethylene glycol) (PEG) with a molecular weight 2000 and a phase rich in (NH4)2SO4 to construct a stable TLPS. It was found that Cr(III) and Cr(VI) were enriched and separated in the D2EHPA-rich top phase and the PEG-rich middle phase, respectively, in the TLPS. The effects of aqueous pH, amounts of (NH4)2SO4 and PEG added, initial chromium ion amount, D2EHPA concentration, phase-mixing time, and D2EHPA diluent in the organic phase on the partition behavior of Cr(III) and Cr(VI) were investigated in the TLPS.
2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Di(2-ethylhexyl) phosphoric acid (D2EHPA), purchased from Shanghai Laiyashi Chemical Co., Ltd., was used as the extractant without further purification. Chloroform, toluene, n-hexane, cyclohexane, nonane, kerosene, dodecane, and trialkylphosphine oxide (TRPO) were used as diluents. Poly(ethylene glycol) with average molecular weight 2000 (PEG) stock solution of 50% (w/v) was prepared. A series of (NH4)2SO4 stock solutions with different initial pH values (pHini) were prepared. Two stock solutions of mixed Cr(III) and Cr(VI) were prepared with CrCl3 3 6H2O and K2CrO4. One (Cr stock 1) contained 100 mmol 3 L1 Cr(III) and 50 mmol 3 L1 Cr(VI). The other (Cr stock 2) contained 50 mmol 3 L1 Cr(III) and 50 mmol 3 L1 Cr(VI). Solutions of 1 mol 3 L1 H2SO4 or 1.5 mol 3 L1 NaOH were used for pH adjustment as appropriate. All aqueous solutions were prepared using analytical reagent grade chemicals (Sinopharm Chemical Reagent Co., Ltd.) and deionized water. 2.2. Three-Liquid-Phase Extraction Procedure. Extraction experiments were performed in 50 mL graduated centrifugal tubes by taking fixed volumes of organic (5 mL) and aqueous (20 mL) phases. In all experiments, 30% (v/v) D2EHPA in n-hexane was used as the organic phase, whereas a mixture containing 4 g of PEG (8 mL of PEG stock solution) and 4 g of (NH4)2SO4 [8 mL of (NH4)2SO4 stock solution] was used as the aqueous phases, unless otherwise stated. After mixing of the organic and aqueous phases and pre-equilibration for a while, 1 mL of Cr stock 1 was added, except in the study of the initial chromium amount, for which Cr stock 2 was used. The system was made to volume (25 mL) with deionized water. Note that, except for the investigation of the effects of the aqueous pH, the pH of the (NH4)2SO4-rich bottom phase of the pre-equilibrated
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Table 1. Extraction of Cr(III) and Cr(VI) as a Function of pHini and pHeq in the Three-Liquid-Phase System pHini
pHeq
ECr(III),t (%)
ECr(VI),m (%)
1.0
1.4
0
72.6
2.0 3.0
2.3 2.6
0 0
80.9 86.7
4.0
2.7
0
87.9
5.0
2.8
0
88.5
6.0
2.9
0
92.5
7.0
2.9
4.6
92.3
8.0
3.1
24.5
92.6
8.2
3.2
36.1
92.5
8.4 8.5
3.4 3.5
43.6 59.4
92.5 91.6
8.6
3.8
69.5
92.4
8.7
4.1
81.5
92.6
8.8
4.4
86.5
92.5
8.9
4.8
92.1
92.4
9.0
5.0
93.4
92.2
9.1
5.3
92.8
93.2
9.2 9.3
5.8 6.3
93.2 93.7
86.2 67.8
TLPS was adjust to the preferred value of pH 5.0, based on the results from the study of the effect of the aqueous pH. Then, the tubes containing the solutions were shaken in a mechanical shaker for a specified period followed by 10 min of centrifugation at a speed of 4000 rpm. After phase separation, volumes of immiscible phases could be directly read from the scale of the tube. A sample of each separated phase was carefully pipetted and used for analysis. The final equilibrium pH value (pHeq) of the (NH4)2SO4-rich bottom phase was also measured. 2.3. Instruments and Analytical Method. pH was measured using a pH211 acidometer (Hanna, Italy). The concentrations of total chromium in the middle and bottom phases of the TLPS were analyzed with an OPTIMA 7000DV inductively coupled plasma atomic emission spectrometer (Perkin-Elmer) at a wavelength of 267.716 nm. The Cr(VI) concentration in the presence of Cr(III) was determined by UVvis spectrophotometry (Perkin-Elmer Lambda Bio40) at a wavelength of 540 nm based on the complex formation between 1,5-diphenylcarbazide and Cr(VI). Each analytical value reported is the mean of three replicates. The concentrations of Cr(III) and Cr(VI) in the top organic phase were calculated based on the analytical results for these ions in the middle and bottom phases of the TLPS. Radiometrically determined distribution ratios are generally accurate to (5%; however, because of the complexity of this TLPS, a conservative accuracy of (10% is reported. The extraction percentages (E) of the two metal ions in the top and middle phases were calculated according to the equation EM, i ¼
CM, i Vi 100% CM, 0 V0
ð1Þ
where M denotes the metal ions, Cr(III) and Cr(VI), and i represents the top (t) or middle (m) phase. CM,i and Vi are the concentration of metal ion in the ith phase and the volume of the ith phase, respectively. CM,0 is the initial concentration of the metal ion in the stock solution, and V0 is the volume of stock solution added to the system. 12768
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Figure 1. Effect of pHeq on the extraction of Cr(III) and Cr(VI) in the three-liquid-phase system. The phase-mixing time was 10 min.
Fourier transform infrared (FTIR) spectra were acquired using a Bruker Vector 22 FTIR spectrometer with BaF pellets. Measurements were taken in the wavenumber range from 400 to 4000 cm1.
3. RESULTS AND DISCUSSION 3.1. Effect of Aqueous pH. To avoid the precipitation problem caused by a high initial pH, chromium was added after the D2EHPA had been pre-equilibrated with PEG/(NH4)2SO4 aqueous two phases. Extraction of chromium at different aqueous pH values was conducted in the TLPSs composed of an organic phase of 30% (v/v) D2EHPA in n-hexane and an ATPS of PEG/(NH4)2SO4. It was found that both ECr(III),m and ECr(VI),t were negligible throughout the tested pH range. The results concerning variations in ECr(III),t and ECr(VI),m with pHeq, presented in Table 1 and Figure 1, show that the D2EHPA-rich top phase and PEG-rich middle phase were very selective for Cr(III) and Cr(VI), respectively, and that the extraction was highly pH-dependent. The final equilibrium pH value of the (NH4)2SO4-rich bottom phase (pHeq) was lower than the initial pH values of the (NH4)2SO4 stock solution (pHini) because of the acidic hydrogen ions of D2EHPA released into the aqueous solution through an ion-exchange reaction. From Figure 1, at low aqueous pHeq (72%) into the PEG-rich middle phase, whereas Cr(III) completely remained in the (NH4)2SO4-rich bottom phase, indicating that Cr(III) and Cr(VI) could be separated from each other. ECr(VI),m increased with increasing pHeq, and the maximum extraction (92.5%) was observed at pHeq = 2.9, corresponding to pHini = 6.0. The maximum ECr(VI),m was maintained in the pHeq range of 2.95.3 and decreased thereafter. However, the elevated aqueous pHeq enhanced the extraction efficiency of Cr(III) into the D2EHPA-rich top phase. ECr(III),t increased dramatically from 0% at pHeq 2.9 to 93.4% at pHeq 5.0 and henceforth leveled off. pHeq around 5.0 was expected to give appreciable (>90%) values of both ECr(III),t and ECr(VI),m. Hydrolysates could be observed at pHeq values above 6.5. It is worth noting that the distribution behavior of Cr(III) and Cr(VI) is apparent by observing the color change of different phases in the TLPS. Photographs of the TLPE of Cr(III) and Cr(VI) in different pHeq ranges were accordingly included in Figure 1. The PEG-rich middle phase became yellow after
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Figure 2. FTIR spectra of (a) D2EHPA, (b) Cr(III)-loaded D2EHPArich top phase, (c) blank PEG-rich middle phase, and (d) Cr(VI)-loaded PEG-rich middle phase.
extraction of Cr(VI), whereas the D2EHPA-rich top phase became violet-blue-gray after extraction of Cr(III) and then green at pHeq above 5.5. The extraction behavior can be understood from the solution chemistry of Cr(III) and Cr(VI). It is known that, in the pH range of 18, the possible Cr(III) species are Cr3+, Cr(OH)2+, Cr(OH)2+, Cr(OH)3, Cr(OH)4, etc.34 In acidic aqueous solutions with pH < 2.9, Cr(III) occurs mainly in the form of Cr3+, which is difficult to extract with D2EHPA because Cr(III) is strongly hydrated ([Cr(H2O)6]3+) in aqueous solutions and displacement of the coordinated water molecules by the ligand is difficult. The experimentally observed steep increase in ECr(III),t can be explained by rapid changes in the non-hydroxide and hydroxide forms of trivalent chromium ions. Cr(OH)2+ exists at pH g 2.9 and dominates between pH 3.5 and 5.5. In addition to Cr(OH)2+, Cr(OH)2+ also exists. Insoluble Cr(OH)3 species and anionic Cr(OH)4 form at higher pH values. Only those chromium(III) hydroxide cationic forms would exchange with H+ ions (or substituted NH4+) of D2EHPA. As DEHPA is dissolved as a dimer in nonpolar solvents,7,35 the extraction reaction of Cr(III) by D2EHPA can be described as CrðOHÞ2þ ðbottomÞ þ 2ðHRÞ2 ðtopÞ S CrðOHÞðHR 2 Þ2 ðtopÞ
þ 2Hþ ðbottomÞ
ð2Þ
where HR represents the D2EHPA and top and bottom denote the top and bottom phases, respectively, of the TLPS. Comparison of the FTIR spectra of pure D2EHPA (Figure 2a) with that of Cr(III)-loaded D2EHPA-rich top phase (Figure 2b) shows that the overlapping stretching vibrations of POC and POH shifted from 1036 cm1 in pure D2EHPA to 1070 cm1 in the Cr(III)-loaded D2EHPA-rich top phase, whereas the PdO stretching vibration shifted from 1226 to 1193 cm1 and the OH deformation vibration at 1684 cm1 shifted to 1630 cm1, indicating the involvement of the phosphoryl group and liberation of hydrogen during Cr(III) extraction. Moreover, the stretching region of NH that peaks at 3215 cm1 appeared, confirming the presence of ammonium cations in the D2EHPA-rich top phase. It is clear that D2EHPA favors the extraction of metal cations. Cr(VI) exists in anionic forms such as dichromate (Cr2O72), hydrogen chromate (HCrO4), and chromate (CrO42) in aqueous solutions34 and therefore was not extracted by D2EHPA. The fraction of any particular Cr(VI) species is dependent on the Cr(VI) 12769
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Figure 4. Effect of (NH4)2SO4 amount on the extraction of Cr(III) and Cr(VI) and volume of the middle phase in the three-liquid-phase system at a pre-equilibrated pH of 5.0. The phase-mixing time was 10 min.
Figure 3. Possible scheme for Cr(III) and Cr(VI) extraction mechanism in the three-liquid-phase system.
concentration and pH. Under the condition that the pH is between 1 and 6 and the concentration of Cr(VI) is less than 0.02 mol 3 L1, Cr(VI) predominantly exists as HCrO4.36 Moreover, CrO42 increases with increaing pH value and becomes the main form at pH values above 7. The probable equilibrium reactions are as follows HCrO4 þ Hþ S H2 CrO4
ð3Þ
HCrO4 S CrO4 2 þ Hþ
ð4Þ
HCrO4 is assumed to be readily attracted to PEG. The interaction between PEG and hydrogen chromate can be understood from Figure 2. The CO stretching vibration of PEG (1096 cm1) shifted to 1082 cm1 (Figure 2c) in the blank PEG-rich middle phase and to 1074 cm1 (Figure 2d) in the Cr(VI)-loaded PEGrich middle phase, whereas the CH bending vibration of PEG (1461 cm1) shifted 20 cm1 lower in frequency in the Cr(VI)loaded PEG-rich middle phase. Ion-pair formation can be considered as one of the important mechanisms in this extraction process.37 The vacant O sites of PEG are positively charged, which leads to an electrostatic interaction between the PEG and the HCrO4 anion. PEG is proton-dissociable and easy to form PEGH+ cation in an acid medium.38 Subsequently, the PEGH+ cation can associate with the HCrO4 anion in the solution and forms a neutral ion-pair complex of [HCrO4 3 PEGH+], which can be extracted into the PEG-rich middle phase. The extraction process can be represented by the equations Hþ ðbottomÞ þ PEGðmiddleÞ S PEGHþ ðmiddleÞ
ð5Þ
HCrO4 2 ðbottomÞ þ 2PEGHþ ðmiddleÞ S HCrO4 3 PEGHþ ðmiddleÞ
ð6Þ
A possible scheme for the Cr(III) and Cr(VI) extraction mechanism in the TLPS of D2EHPA/PEG/(NH4)2SO4 is illustrated in Figure 3.
Figure 5. Effect of PEG amount on the extraction of Cr(III) and Cr(VI) and volume of the middle phase in the three-liquid-phase system at a preequilibrated pH of 5.0. The phase-mixing time was 10 min.
3.2. Effect of (NH4)2SO4. (NH4)2SO4 was used as a phaseforming substance because it has an excellent salting-out ability, good solubility, small temperature coefficient, low price, and no side effects.39 The effect of the amount of (NH4)2SO4 on the partitioning of Cr(III) and Cr(VI) in the TLPS under a fixed preequilibrium at pH 5.0 is shown in Figure 4. It was found that an increase in the (NH4)2SO4 amount had little impact on ECr(III),t but resulted in an increase in ECr(VI),m. This was attributed to the progressive increase of hydrophobility of the middle phase. With an increase in the amount of (NH4)2SO4 added, the salting-out of PEG molecules resulted in more water being removedfrom the middle phase, and therefore a decrease in the volume of the middle phase (Vm), as shown in Figure 4. Vm decreased from 13.7 to 6.5 mL as the (NH4)2SO4 amount increased from 2 to 5 g. The phase separation between PEG and water tends to be thorough, thus, the middle phase became more hydrophobic and its affinity to Cr(VI) species is believed to be enhanced while the entrainment of Cr(III) species was reduced. ECr(III),m decreased from about 7% to 0 when the (NH4)2SO4 amount varied from 2 to 4 g and reached a plateau beyond. Obviously, increasing the amount 12770
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Figure 6. Effect of initial chromium amount on the extraction of Cr(III) and Cr(VI) in the three-liquid-phase system at a pre-equilibrated pH of 5.0. The phase-mixing time was 10 min.
Figure 7. Effect of phase-mixing time on the extraction of Cr(III) and Cr(VI) in the three-liquid-phase system at a pre-equilibrated pH of 5.0.
of (NH4)2SO4 to some extent benefited the extraction and separation of Cr(III) and Cr(VI) in the upper two phases of the TLPS. 3.3. Effect of PEG Amount. The PEG amount was varied from 3 to 5 g, and its effects on the extraction percentages of Cr(III) and Cr(VI) are shown in Figure 5. It was found that the extraction percentage of chromium remained constant even though the volume of the middle phase increased as the PEG amount increased. ECr(VI),m was expected to increase with an increase in PEG amount as more binding sites of PEG molecular chains were available for Cr(VI) species. It can be concluded from the unexpected experimental results that the loading capacity of Cr(VI) in the PEG-rich phase is large and a small amount of PEG (90%) of Cr(III) and Cr(VI) were achieved by using 40% (v/v) D2EHPA in aliphatic hydrocarbon diluents, 20% (w/v) PEG and 20% (w/v) (NH4)2SO4 in two lower aqueous phases, at equilibrium pH about 5.0. The new method is characterized to be simple, highly selective and kinetically attractive. The separation of Cr(III) and Cr(VI) in the top and middle phases of TLPS provides a potential possibility to detect two different oxidation states of chromium species and a prior step for their corresponding determination by AAS, ICP-AES, and so on. Another advantage arising from the use of this method is that both Cr(III) and Cr(VI) can be removed or recovered from aqueous solutions in a single extraction procedure, reducing the risk of releasing toxic metals into the environment and increasing the profit from recycling the total chromium as much as possible. Generally, many transition metals and nonmetal elements, such as iron
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