Solvent extraction of chromium(VI) with trialkylamine (N235) using kerosene/n-heptane as the diluent was investigated. The basic conditions for Cr(VI) extraction have been explored, and the influence of coexisting contaminants on extraction has been evaluated. The results indicated that the separation of Cr(VI) could be realized with N235, and the recovery rate could be increased by adding organophosphorus-tributyl phosphate as the modifier due to the strong bonding force of synergistic extraction. However, the inorganic phosphorus could inhibit the chemical reaction between tertiary amine and Cr(VI) ions owning to the competition of the limited adsorption sites. Besides, inorganic anions such as massive SO42− and NO3− could inhibit extraction of metal ions because of competition coordination. In addition, the separation of Cr(VI) from coexisting metal ions could be achieved with trialkylamine except Fe2+ due to its reducibility. Based on the results, the feasibility of pseudo-emulsion based hollow fiber strip dispersion used for separation and recovery of Cr(VI) has been evaluated, and the transport properties of Cr(VI) and coexistence with interfering ions have been explored.
- hexavalent chromium
- liquid membrane
- pseudo-emulsion based hollow fiber strip dispersion
- solvent extraction
Cr(VI) is toxic to organisms and difficult to biodegrade. Besides, it can accumulate inside the bodies of humans, animals and plants and spread through the food chain, existing as a threat to ecosystems (Garg et al. 2012). Heavy metal toxicity can be described as ‘murder by slow knife’ and ‘biological time bomb’. As a typical heavy metal element, Cr(VI) is widely used in the fields of leather, electroplating, metallurgy, printing, and the coating industry, and a large amount of wastewater is mostly discharged directly without appropriate recovery and treatment, which can bring about serious pollution to the environment. Thus, it is urgent to eliminate the hazards and realize the reuse of metal ions (Zhitkovich 2011).
Generally, effluent containing Cr(VI) has such characteristics as complicated element, intricate species of metal ions, organics and inorganics, where inorganic anions such as and metal ions are common species. Sulfate ions are common anions in the inorganic chemical industry, metallurgy, and acid mine drainage. The main sources of sulfate ions are chemical weathering of sulfur-bearing ore and the oxidation of sulfides and sulfur (Silva et al. 2010). Besides, tailings from coal and some metal-bearing ores are readily oxidized, resulting in acid drainage. Such acid mine drainages contain heavy metal ions, sulfate ions and acidity, and constitute one of the main challenges in the mining industry (Dong et al. 2011). Nitrogen and phosphate ions discharged into the aquatic ecosystem come from sources including agricultural fertilizers, septic tank systems, animal waste disposal, domestic sewages and industrial effluents. The inorganic anions exist as a main threat to aquifers of agricultural and urban areas (Pintar et al. 2001; Huang et al. 2008). In addition, industries of dyeing, electroplating, leather, metallurgy and mining will result in the loss of metal ions into the aquatic environment. Thus, in the fields of ion separation and recycling, it is imperative to investigate the influence of coexisting inorganic anions and metal ions.
Solvent extraction is a promising technology for the separation of metal ions (Wang et al. 2012; Elizalde et al. 2013; Hu et al. 2013; Nikam et al. 2013; Okewole et al. 2013; Senol 2013). In the field of separation of heavy metal ions, long-chain amine extractants such as alkaline extractant-tertiary amine (Alamine 336, TOA) (Sadoun & Hassaine-Sadi 2004; El-Hussaini et al. 2012) and quaternary ammonium salt (Aliquat 336) (Galan et al. 2006; El-Hefny 2009) are considered as universal extractants in Cr(VI) ion separation. Furthermore, the organic phosphorus extractants (tributyl phosphate (TBP), Cyanex 921, Cyanex 923) (Alguacil et al. 2004; Zhang et al. 2007) and ionic liquid CYPHOS IL101 are also used to separate Cr(VI) ions (Alguacil et al. 2010b). However, currently, though a variety of studies has mainly focused on the transport property of metal ions with different extractants, the influence of coexisting contaminants on extraction also cannot be neglected. In addition, membrane-extraction processes derived from solvent extraction have attracted wide attention because of their versatility and the characteristics that they overcome problems encountered in other established separation technologies, i.e. liquid–liquid extraction. The great potential for energy conservation, low capital and operating cost, and easy scale-up makes supported liquid membranes an area deserving attention. In addition, theories of membrane materials and manufacturing methods have advanced dramatically. Thereinto, hollow fiber has been proven to be economical to construct. It has characteristics such as unique separation capabilities and ability to adapt in diversified conditions. Systems based on hollow fiber membrane contactors can make chemical plants more compact, more energy saving, cleaner, and safer by providing a lower equipment size-to-production capacity ratio and decreasing waste generation, with the correct choice of membrane material (Sirkar 1997; Kocherginsky et al. 2007; Parhi 2013). Owing to the large surface area per volume, which is typically 100 times bigger than in conventional equipment, the mass transfer rate of hollow fibers is quick and the liquid extraction in fibers is 600 times faster than in mixer settlers. Extensive studies on hollow fiber membrane based separation technology have been carried out for removal of toxic heavy metals because the rapid mass transfer rate, enhances ease of operation and scaling up the devices (Aguilar & Cortina 2007; Kislik 2010). However, the obstacle of the technology application is the stability. Recently, a new liquid membrane technique, named pseudo-emulsion based hollow fiber strip dispersion (PEHFSD) was proposed for the treatment of liquid effluent due to its stability (Alguacil et al. 2010a, b). It combines the advantages of emulsion liquid membrane and non-dispersion solvent extraction. It is noteworthy that this technique can not only separate and recycle the metal ions, but can also realize the reuse of resources (Aguilar & Cortina 2007; Kislik 2010).
Within the idea of green chemistry, the expensive and toxic extraction reagent becomes a bottleneck to utilization in the fields of separation. Therefore, the design of an efficient and economical extraction system is imperative. In addition, whether the coexisting contaminants could interfere with the extraction reaction and the action principles is the cornerstone in the exploration of technology applications. Furthermore, it is also imperative to explore whether the contaminants could react with extractants and thus disturb the transport of Cr(VI). The objective of this work is to investigate the characteristic of tertiary amine N235 in kerosene/n-heptane, and TBP as modifier on the extraction of Cr(VI). In addition, reaction mechanisms of organophosphorus TBP and inorganic phosphorus on extraction have been discussed. Besides, the influence of inorganic anions and metal ions on extraction and reaction mechanisms has been evaluated. Based on the analysis of the basic condition in the solvent extraction reaction, the derived liquid membrane technology used for separation and recovery of Cr(VI) ions has been verified, and the extraction behaviors of Cr(VI) with coexisting metal ions by the PEHFSD technique have been discussed.
MATERIALS AND METHODS
Instruments and reagents
Trialkylamine (N235), Shaoyang Institute of Chemical Industry (Hunan, China); TBP, Tianjin Hengxing Chemical Preparation Co., Ltd; Kerosene, n-heptane, analytical reagent, Tianjin Hengxing Chemical Preparation Co., Ltd. All other chemicals used were of analytical reagent grade.
H2S-H Water Bath Oscillator (Harbin Dong Lian Electronic Technology Development Co., Ltd); PHS-3C PH Meter (Shanghai INESA Corporation); 722 UV-vis Spectrophotometer (Shanghai Spectrum Instruments Co., Ltd); Optima 5300DV Plasma Atomic Emission Spectrometry (Perkin-Elmer, USA); ADVANCE III 400 MHz Nuclear Magnetic Resonance Spectrometer (Bruker Corporation, Germany).
Hollow fiber module
The hollow fiber device designed with polyvinylidenefluoride material was obtained from Tianjin Polytechnic University. The module inner and outer fiber diameters are 0.80 mm and 0.15 mm, respectively. The bore diameter of the hollow fiber is 0.16 μm and the porosity is 82–85%. The fiber length is 22 cm and the number of fibers in the module is 100. The thickness and active interfacial area of the membrane are 0.18 mm and 0.05 m2, respectively.
Solvent extraction method
Potassium dichromate is used to prepare the Cr(VI) solution of hydrochloric acid, of which pH is 1.0, and C0 [Cr(VI)] is 0.1 g L−1. The extractant used is 0.0032 mol L−1 tertiary amine N235 unless otherwise stated, kerosene or n-heptane is used as the diluent, and TBP as a modifier to investigate the recovery rate of Cr(VI) ions. Under the condition of 25°C, Vaq and Vo was mixed into the flask by 1:1, then put into a thermostatic water bath oscillator, and subjected to oscillation for 15 minutes at a speed of 145 rpm. Then, the solution was put into a separatory funnel for separation, and when the extraction equilibrium was reached, the lower solution was taken, of which the concentration of Cr(VI), Ct, was measured, while Cr(VI) concentration in the organic solution was calculated by mass balance equation C = C0 − Ct. The calculation formulae of distribution coefficients and extraction rates of Cr(VI) are shown in Equations (1) and (2). 1 2
The PEHFSD is comprised of a single membrane module for extraction and stripping. The view of PEHFSD using a single contactor in recirculation mode is shown in Figure 1. The volume of pseudo-emulsion solution is 0.4 L (0.2 L of organic solution and 0.2 L of NaOH solution). One liter of feed solution (pH = 1.0) of the desired Cr(VI) concentration was prepared by taking a suitable aliquot of the potassium dichromate stock solution. The experimental setup for the separation of the metal ions consists of a peristaltic pump and a gear pump. The organic solution wet the porous wall of the fiber because of its hydrophobic nature. The liquid membrane interface was kept at the pore by applying a higher pressure to the aqueous phase than to the pseudo-emulsion phase.
The simulated wastewater is circulated in the tube side with the peristaltic pump and the pseudo-emulsion solution is circulated in the shell side with the gear pump in counter current mode to provide organic solution to the membrane pore, and the organic solution in pseudo-emulsion was loaded on the pore of hollow fiber to form a stable membrane. The metal ions selectively transport into the liquid membrane and move into sodium hydroxide solution in pseudo-emulsion to realize ions separation. The differential pressure was always kept below the breakthrough pressure and transmembrane pressure was maintained at 0.2 × 105 Pa (0.2 bar) between aqueous solution and pseudo-emulsion solution. The stirring rate of the pseudo-emulsion solution using the mechanical stirrer was kept at 120 rpm.
Optima·5300DV Plasma Atomic Emission Spectrometry was used to analyze the metal ions concentration; Diphenylcarbazide hydrazine method was used to analyze the Cr(VI) ions concentration by 722 UV-vis Spectrophotometer; ADVANCE III 400 MHz NMR Nuclear Magnetic Resonance Spectrometer was used to characterize the structure of trialkylamine.
RESULTS AND DISCUSSION
Hydrogen nuclear magnetic resonance spectra of tertiary amine
Hydrogen nuclear magnetic resonance (H-NMR) is used to characterize the structure of tertiary amine N235, of which the nuclear magnetic resonance spectra are shown in Figure 2. From the structural analysis based on nuclear magnetic resonance spectra and tables of chemical shift, it can be seen that the chemical shift of R3N is 2.12–2.34 ppm. The chemical shift of methylene (-CH2) is 1.3 ppm and methyl (-CH3) is 0.9 ppm. Characterization of trialkylamine structure by NMR is consistent with its structural formula (N235, R3N, R = C8–C10).
The basic conditions for Cr(VI) extraction
Effect of the concentration of trialkylamine
The influence of N235 concentration on extraction of Cr(VI) with kerosene/n-heptane as diluent has been explored and the results are shown in Figure 3. Results indicated that the recovery rate of metal ions improved with the increase of extractant concentration. Extraction equilibrium could be reached with 0.0048 mol L−1 N235 diluent in kerosene or n-heptane and the distribution coefficients are more than 100. However, it has been reported that the extractive property of quaternary ammonium salt Aliquat 336 is superior to tertiary amine Alamine 336 (El-Hefny 2009). To verify whether this conclusion is correct between Aliquat 336 and N235, the effect of Aliquat 336 on Cr(VI) extraction has been evaluated. Results indicated that the distribution coefficients with 0.01 mol L−1 Aliquat 336 diluent in kerosene or n-heptane are 9.45 and 11.17, respectively. The recovery rates of Cr(VI) are approximately 92%. Based on the above conclusions, trialkylamine is used as an extractant in this work since the solubility and distribution coefficient are better than Aliquat 336 which is widely used internationally as an extractant for Cr(VI). Therefore, choosing N235 as the extractant to extract Cr(VI) ions can not only save consumption, but also is economical.
Effect of pH
Kerosene/n-heptane is used as diluent and 0.0032 mol L−1 tertiary amine N235 as extractant to explore the recovery rate of 0.1 g L−1 Cr(VI) under different pH ranging from 0.5 to 6. The extraction rate decreased with the increase of pH value. The reason for this is that Cr(VI) ions exist as anions and in acidic condition (pH = 1.0), while tertiary amine N235 can be combined with H+ to form R3NH+, and the association reaction could occur between ammonium salt and Cr(VI) ions (Bachmann et al. 2010; El-Hussaini et al. 2012). The corresponding chemical reactions are shown in Equations (3) and (4). With the increase of pH value, the concentration of H+ declines, and thus the concentration of the R3NHCl that can react with hexavalent chromium anions to achieve separation decreases. Results in Figure 4 show that the optimum pH is 1.0. 3 4where R3N(org), HX, R3NHX(org) and Bn−(aq) denote the tertiary amine N235, inorganic acids, ammonium salt in the organic solution, and metal anions, respectively.
Effect of TBP
Kerosene/n-heptane is used as diluent and tertiary amine N235 as extractant to explore the influence of TBP on the extraction of Cr(VI). The results are illustrated in Figure 5. TBP can improve the extraction rate due to the behavior of the synergistic reaction and the reaction occurring due to the strong donating ability of phosphoryloxy in neutral phosphonic. Besides, the oxygen functional groups of TBP could combine with amine molecules and break the polymer of alkyl amines, and increase the solubility of alkyl amines in diluents. Observations have found that association complexes H2Cr2O7·nTBP or H2CrO4·nTBP could be formed with neutral phosphonic TBP as the extractant (Ouejhani et al. 2003; Kumbasar 2010). Thus, the chemical reactions of synergistic extraction are shown in Equations (5) and (6). However, when the TBP concentration is higher than 0.145 mol L−1 with kerosene as diluent, the homogenization time of organic solvent becomes longer. Therefore, 0.145 mol L−1 TBP can not only improve recovery rate and save resource consumption, but also slightly improve the homogenization rate of solution. 5 6
Extraction behavior of Cr(VI) and coexistence with interfering ions by solvent extraction method
Extraction behavior of Cr(VI) with coexisting phosphate ions
The effect of inorganic phosphorus on extraction has been evaluated. The anion exchange reaction between tertiary amine and Cr(VI) ions preferentially occurred with the concentration of phosphate ion lower than 0.06 mol L−1. The extraction rate of Cr(VI) ions declines sharply when concentration is between 0.08 and 0.1 mol L−1 (Figure 6). This phenomenon is attributed to the competition of the limited adsorption sites on the surface of tertiary amine between the massive ion and Cr(VI) ion due to the affinity between N235 and phosphate ions. In addition, the competition of adsorption sites of slightly decreased due to the strong bonding force of synergistic extraction in the N235-TBP system.
Extraction behavior of Cr(VI) with coexisting sulfate ions and nitrate ions
To evaluate the role of inorganic anions on extraction of metal ions, an experiment has been designed. The results are illustrated in Figure 7. Results indicated that the recovery rate of Cr(VI) ions with N235 declines as the concentrations of and increase. The reason for this is that massive inorganic anions such as and could inhibit extraction of metal ions with N235 mainly by competing adsorption sites, and lead to the decrease of metal recovery rates. Chemical reactions of inorganic anions with trialkylamine are shown in Equations (7) and (8). It is also indicated from Figure 7 that the recovery rate declined to 60% from 95% with N235 as the extractant, whereas in the N235-TBP system, the recovery rate decreased to 80% due to the strong bonding force of synergistic extractants and or which could slightly inhibit the reaction of inorganic anions with extractant. Besides, when the recovery rate decreases with the increase of concentration, its relationship with ions concentration is a linear correlation (Figure 7(b)). 7 8
Extraction behavior of Cr(VI) with coexisting metal ions
To investigate the effect of the coexisting metal ions on extraction, studies have been designed and the distribution coefficient has been calculated. The extraction properties of N235 on Cr(VI) in synthetic effluents are illustrated in Table 1. It is indicated that the ions separation could be achieved with the individual concentration of coexisting metal ions below or equal to 0.1 g L−1, and the recovery rate is more than 96% with 0.0048 mol L−1 N235 diluent in kerosene/heptane. The reason for this is that metal ions Cd2+, Ni2+, Cu2+, Zn2+, Fe3+, Mn2+ and Pb2+exist as cations at pH 1.0, and cannot react with the tertiary amine chemically, while Cr(VI) ions exist as and , which can lead to anion exchange reaction with ammonium salt to achieve separation. Therefore, the separation could be achieved due to the specificity of trialkylamine for Cr(VI) ions. However, the recovery rate could be declined when Cr(VI) coexists with Fe2+, and its relationship with Fe2+ ions concentration is a linear correlation (Figure 8). The reason for this is that Fe2+can reduce Cr(VI) to Cr(III), and thus result in a decrease in the recovery rate of Cr(VI). The chemical reaction is shown in Equations (9) and (10). In addition, TBP can promote the reduction rate of Fe2+ on Cr(VI). Its cause may be that the synergistic extraction of Fe3+ with N235-TBP occurred at the pH value of 1.0, and thus resulted in the increase in reduction rate of hexavalent chromium due to the decrease of Fe3+ion concentration in Equations (9) and (10). 9 10
Evaluation of the feasibility of extraction behavior of Cr(VI) and coexistence with interfering ions by PEHFSD technology
Evaluation of the feasibility of Cr(VI) extraction by PEHFSD
To verify the feasibility of extraction of Cr(VI) with tertiary amine N235, the PEHFSD for separation and recovery of Cr(VI) ions has been evaluated. This work was carried out using feed phases of 0.01 g L−1 Cr(VI) in 0.1 mol L−1 HCl, whereas the pseudo-emulsion phase was composed of an organic solution of 1% (0.02 mol L−1) N235 in kerosene or n-heptane with TBP as the modifier and 1.0 mol L−1 sodium hydroxide or 10 g L−1 hydrazine sulfate as the stripping solution. The results are displayed in Figure 9. It is indicated from Figure 9 that 94% of the metal ions in simulated wastewater transport into the liquid membrane, and metal transport is not significantly influenced by the stripping phase composition. Cr(VI) stripping from metal-loaded N235 organic solutions with hydrazine sulfate solutions can be related to the reduction, and the recovery rate of Cr(III) is about 78.6%. However, the recovery rate of Cr(VI) is more than 90% with 1 M NaOH as the strippant in different organic solutions and the regenerated solvent could be reused in succeeding separation and recovery of Cr(VI). Thus, NaOH is suitable as the re-extraction solution. In addition, the turbid liquid could be formed with kerosene as the diluent and the difficulties in delamination could result in difficulties for regeneration of the carrier. Besides, the difficulties in delamination with 0.145 mol L−1 TBP as the modifier are also obvious. Based on the above discussion, the optimum reaction conditions are that 1.0 mol L−1 NaOH is used as the strippant and tertiary amine N235 diluent in n-heptane as the organic solution in the pseudo-emulsion phase. The pseudo-emulsion could separate rapidly when mixing in the tank is stopped. The regenerated solution is exhibited in Figure 10. The results indicated that the organic solution could be reused and closed circulation could be realized. In conclusion, it is reasonable to use tertiary amine N235 diluent in n-heptane as the extraction solution for the separation and recovery of Cr(VI) with the PEHFSD technique. This technology is a promising alternative to conventional methods.
Extraction behavior of Cr(VI) with coexisting phosphate ions
The influence of inorganic phosphorus on extraction has been investigated with the PEHFSD technique. The results are illustrated in Figure 11. Results also indicated that the extraction rate decreases with the increase of concentration. But a critical value exists. The recovery rate of Cr(VI) metal ions declines sharply when phosphate ion concentration reaches the critical value. The reason for this is that massive compete for the limited adsorption sites on the surface of trialkylamine due to the bonding force between N235 and phosphate ions. Besides, massive would also inhibit the diffusion of Cr(VI) ions into the surface of the liquid film and then result in the decrease of metal permeation.
Extraction behavior of Cr(VI) with coexisting sulfate and nitrate ions
The influence of inorganic anions on Cr(VI) permeation was evaluated; the module was designed with polyvinylidene fluoride hollow fiber. Because can react with R3NH+Cl− in acidic solution, it is imperative to discuss the role of coexisting inorganic anions such as . The results are illustrated in Figure 12. It is also indicated that the metal permeability decreases with the increase of anions due to the competition for adsorption sites between inorganic anions and Cr(VI). Besides, the trends were basically identical to the data obtained from solvent extraction.
Extraction behavior of Cr(VI) with coexisting metal ions
The influence of coexisting metal ions on Cr(VI) ions transport was investigated with PEHFSD. The results are illustrated in Figure 13. Observations of solvent extraction indicated that Cd2+, Ni2+, Cu2+, Zn2+, Fe3+, Mn2+ and Pb2+ in 0.1 mol L−1 HCl exist as cations, and could not disturb the anion exchange reaction. Figure 13 shows that the recovery rate of Cr(VI) slightly decreases with the increase of the concentration of coexisting metal ions. The reason for this may be that massive metal ions would inhibit the reaction of Cr(VI) anions with trialkylamine due to the obstruction of ion collisions on the surface of the liquid membrane. But the disturbance of coexisting metal cations on Cr(VI) transport could almost be ignored after 2 hours of operation of the module.
Extraction behavior of Cr(VI) and coexistence with contaminants by trialkylamine using kerosene/n-heptane as the diluent and TBP as the modifier was investigated. According to the results, 0.145 mol L−1 TBP can not only improve extraction rate, but can also not disturb the homogenization rate of the organic solution. The competition coordination of inorganic anions such as , and with Cr(VI) ions resulted in the decrease of the extraction rate. Besides, N235 has specificity for Cr(VI) extraction in acid solution, and is able to achieve Cr(VI) separation with its coexisting metal cations (Cd2+, Ni2+, Cu2+, Zn2+, Fe3+, Mn2+, Pb2+) except for Fe2+ ions. In addition, it is proved that the PEHFSD can realize the separation of Cr(VI) by trialkylamine using n-heptane as the diluent. Extraction behavior of Cr(VI) and coexistence with interfering ions was basically identical to the data obtained from solvent extraction. This technology is a promising alternative to conventional methods.
This study was funded by the Commonweal Project of the Environmental Protection Agency through grant number 201209048.
- First received 17 November 2014.
- Accepted in revised form 6 March 2015.
- © IWA Publishing 2015