Article Cite This: Environ. Sci. Technol. 2018, 52, 5884−5891
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Nanopore-Based Strategy for Sequential Separation of Heavy-Metal Ions in Water Lei Liu* and Ke Zhang
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Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, School of Mechanical Engineering, Southeast University, Nanjing 211189, People’s Republic of China
ABSTRACT: Developing novel methods for the removal of heavy-metal ions from wastewater with low costs, special selectivity and high efficiency is quite important in water restoration and comprehensive ecological improvement. In this work, a nanoporebased strategy was suggested and related segregation apparatus was built to separate multiple heavy-metal ions in water by selective complexation. The results indicated that the prioritization of the selecting order for the complexing agent (thiacalix[4]arene-p-tetrasulfonate (TCAS)) to heavy-metal ions was Cu(II) > Cd(II) > Pb(II) > Ba(II). Meanwhile, higher driven voltage corresponded to a faster separation speed, while it could cause the decomposition of complexed heavy-metal ions when excessed the threshold. On the other hand, pH value would affect the hydrolysis of heavy-metal ions, the complexation of the calixarene to the heavy-metal ions and the speed of the electroosmotic flow. In our experiments, the maximum separation efficiency was achieved when the driven voltage was 1.5 V and the pH value was 5.0, corresponding to the best separation rate of 94.8%, 95.2%, 92.8%, 93.6%, for Cu(II), Cd(II), Pb(II) and Ba(II), respectively.
1. INTRODUCTION Wastewater containing heavy-metal ions produced by industrial processes (such as mining, metallurgy, machinery manufacturing, chemical industry, electronics and instrumentation) is one of the most serious and most harmful industrial contaminations to water. It is difficult to eliminate heavy metals in wastewater only by common methods. Generally, effective treatments to heavy-metal polluted water consist of two aspects: exogenous control and endogenous control. Exogenous control mainly deals with restricting the emissions of waste residue containing heavy metals, while internal control is to repair polluted water. Removal of heavy-metal ions from wastewater is not a simple work, while low costs, high separation selectivity and efficiency should also be under consideration. Among various methods such as chemical precipitation, ion exchange, reverse osmosis and adsorption,1−5 the last one seems more effective and economic, which contains an efficient solid phase which should consist of a stable and insoluble porous matrix with suitable active groups which can interact with heavy-metal ions.6,7 As an important class of ion carriers, calixarene and its derivatives bring new opportunities and challenges to the separation of heavy-metal ions in water,8−14 and recent reports for the applications of functionalized calixarene or its © 2018 American Chemical Society
derivatives enrich the research on the heavy-metal ions elimination. For example, thiacalix[4]arene (TCA) and its derivatives can be incorporated into the surface of polymeric membranes and can be used as selective adsorbents for some heavy-metal ions.15−17 TCA can be used for the extraction of Cu(II), Cd(II), Co(II) and Cr(III) through batch adsorption techniques.18 On the other hand, TCA possesses good efficiencies in the selective extraction of metal ions. Accordingly, this compound also can be used for the separations of different heavy-metal ions by its selective complexation to heavy-metal ions.19−22 In this work, a “U”-type device with two separated cells was constructed and a nanopore-based strategy for sequential separation of heavy-metal ions was suggested, by which the separation of four kinds of heavy-metal ions (Cu(II), Cd(II), Pb(II) and Ba(II) ions) has been carried out. The basic principle of this sequential separation method is based on the selective complexation of TCAS to different heavy-metal ions. Received: Revised: Accepted: Published: 5884
January 3, 2018 March 26, 2018 April 23, 2018 April 23, 2018 DOI: 10.1021/acs.est.7b06706 Environ. Sci. Technol. 2018, 52, 5884−5891
Article
Environmental Science & Technology
Figure 1. Structure of TCA (A) and TCAS (B).
make it water-soluble. 2 g of TCA and 80 mL of concentrated sulfuric acid were poured into a 100 mL beaker, and the temperature was controlled at 80 °C for 4 h.23−25 The final extracted substance was TCAS with high purity. The concentration of CuCl2, PbCl2, CdCl2 and TCAS used in the following experiments was 1.0 mol/L. Two groups of samples were prepared according to Table 1 and Table 2.
TCAS itself is an organic compound that can form a stable complex with a variety of heavy-metal ions. In a mixed solution containing different heavy-metal ions, specific amounts of TCAS were added into the system successively according to the amount of each heavy-metal ion. The complex and heavy-metal ions were opposite charged in the solution, and the electric field force applied on heavy-metal ions also increased with the voltage increasing. Therefore, heavy-metal ions spent less time in translocating through the nanopore with higher voltage value, which was consistent with the experimental results in this work. Another possible reason was that quartz capillary was negatively charged; positive ions could bind to the inner surface of the quartz capillary and formed an electric double layer, which significantly changed the ionic distribution inside the nanopore. Increasing the velocity across the polycarbonate membrane, there was more ionic flux through the pore and the fast moving ions in the confined nanopore could shorten the separation time, which accelerated the speed of electroosmotic flow. Under the power of the electric field, the complex compound moves toward positive electrode, whereas other heavy-metal ions move through the nanopore arrays to the negative cell of “U”-type device. The separation experiment was carried out in this way. Here nanopore-contained film works as a semipermeable membrane, providing channels for ionoriented movements.
Table 1. Proportion of Heavy-Metal Ions and TCAS Number
TCAS solution/ mL
CuCl2 solution/ mL
CdCl2 solution/ mL
PbCl2 solution/ mL
Ultrapure water/mL
S1 S2
2 2
1 0
2 2
1 1
2 3
Table 2. Proportion of Cu(II) ions solution and TCAS solution Number
TCAS solution/ mL
CuCl2 /mL
Ultrapure water/ mL
Proportion
S3 S4 S5 S6 S7
1.0 1.0 1.0 1.0 1.0
0.5 1.0 1.5 2.0 2.5
6.5 6.0 5.5 5.0 4.5
1:0.5 1:1 1:1.5 1:2 1:2.5
2.3. Device and Methods. The concentrations of complex compounds of heavy-metal ions and TCAS were determined by UV−vis absorption spectra carried on a UV-3600 spectrometer. The concentrations of heavy-metal ions were determined by using an atomic absorption spectrophotometer (AA-7000F/G). The transmembrane ionic currents were recorded by a Keithley 6485 multimeter (Keithley Instruments, Inc., USA). The electrode model used in this experiment is 213 from Leici company, which belongs to a platinum electrode. A “U”-type device containing a feed cell (on the left side) and a permeation cell (on the right side) was employed in our experiments, as shown in Figure 2. Two separated cells with certain solutions were linked by a piece of PC membrane containing nanopore arrays and they were sealed by PDMS (polydimethylsiloxane). An electric field was applied to
2. EXPERIMENTAL METHODS 2.1. Materials and Devices. Polycarbonate membranes containing nanopore arrays (diameter: 50 nm) were obtained from Whatman, Inc. Copper chloride, lead chloride, cadmium chloride and barium chloride were bought from Aladdin Industrial Corporation. TCA was purchased from Tokyo Chemical Industry with a purity of 98%. Concentrated sulfuric acid was purchased from Sinopharm chemical reagent Co. Ltd. Ultrapure water was used for the preparation of all solutions and rinsing. 2.2. Preparation of TCAS and its Complexation. TCA (Figure 1A) was converted to TCAS (Figure 1B) by sulfonating. The purpose of converting TCA into TCAS was to introduce sulfonic groups to TCA molecular structure and 5885
DOI: 10.1021/acs.est.7b06706 Environ. Sci. Technol. 2018, 52, 5884−5891
Article
Environmental Science & Technology
Figure 2. “U”-Type separation device containing a feed cell and a permeation cell, a Keithley 6485 multimeter was used to detect the transmembrane ionic current through nanopores. (A) The exact dimensions of the electrochemical cells (B); the SEM scan imaging of the polycarbonate nanopore arrays (C).
Because of the different complexing ability, Cu(II)−TCAS complex can be first formed. Under the force by driven voltage, the Cu(II)−TCAS complex moves toward positive electrode in the feed cell, while other heavy-metal ions (such as Cd(II), Pb(II) and Ba(II)) move through nanopores to the permeation cell, and the added TCAS in permeation cell formed a complex with Cd(II) ion according to its complexing priority. When the first step was completed, the solution in the permeation cell was collected and transferred to the cleaned feed cell, and TCAS was added to the permeation cell second according to the complexation ratio of TCAS to Pb(II). The above experiments were circulated until only Ba(II) ions are left in the permeation cell. All above experiments were carried out at room temperature. The separation time was controlled at 4 h and it was measured half an hour.
transport ions in the feed cell to the permeation cell through the membrane. A mixed solution containing 1.0 mL of TCAS solution, 2.0 mL of CuCl2 solution, 1.0 mL of CdCl2 solution, 1.0 mL of PbCl2 solution and 1.0 mL of BaCl2 solution was prepared. The hybrid solution was diluted to 8.0 mL, followed by ultrasonic treatment for half an hour. Then the mixed solution was put into the feed cell of “U”-type device. 2.0 mL of TCAS solution (0.125 mol/L) was added to the permeation cell first according to the complexation ratio of TCAS to Cd(II) ions. The separation experiment was carried out according to the process showed in Figure 3.
3. RESULTS AND DISCUSSION 3.1. Selective Complexation by TCAS. The complexation of these heavy-metal ions can be realized by the synergistic coordination of the bridged sulfur atoms and the phenol hydroxyl groups in the lower edge of TCAS molecule. According to Hard and Soft Acids and Bases (HSAB) Theory, these metal ions belong to the soft acid metal ions, which are easy to combine with sulfur atoms of the soft alkali. Because of lone pairs of electrons on the bridged sulfur atoms and the phenol hydroxyl groups in the lower edge of TCAS molecule, it can participate in the action of metal cations, making TCAS better in identifying and complexing with metal ions. The specific process can be described as follows: heavy-metal ions enter the cavity of TCAS molecule. Heavy-metal ions provide empty electron orbits, while TCAS molecule provides lone pairs of electrons. They form a stable complex compound by forming coordinate bonds. The characteristic UV−vis absorption peak for TCAS is located at 301 nm. The spectra for the complex compounds of Cd(II)−TCAS, Cu(II)−TCAS, Pb(II)−TCAS and Ba(II)− TCAS are shown in Figure 4A, in which the UV−vis absorption peaks are located at 317 nm (for Cd(II)−TCAS complex), 310
Figure 3. Schematic diagram of sequential separation. 5886
DOI: 10.1021/acs.est.7b06706 Environ. Sci. Technol. 2018, 52, 5884−5891
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Environmental Science & Technology
Figure 4. UV−vis absorption spectrum of TCAS (A); UV−vis absorbance spectra of TCAS to four kinds of heavy-metal ions respectively (B); UV− vis absorbance spectra of TCAS to hybrid solutions of multiple heavy-metal ions (C and D).
Figure 5. Absorbance spectra of Cu(II)−TCAS complex (A), Pb(II)−TCAS complex (B) and Cd(II)−TCAS complex (C) under different ion to TCAS molar ratio.
nm (for Cu(II)−TCAS complex) and 322 nm (for Pb(II)− TCAS complex) respectively. In a mixed solution containing Cu(II), Cd(II), Pb(II) and Ba(II) ions, the absorption peak attributed to Cu(II)−TCAS complex appears at 310 nm (Figure 4B), which indicates TCAS has formed a complex with Cu(II) ions first. For a mixed solution containing Cd(II), Pb(II) and Ba(II) ions, the absorptive peak at 317 nm can indicate that TCAS forms a complex with Cd(II) ions first (Figure 4C). For a mixed solution containing Pb(II) and Ba(II) ions, the absorptive peak at 322 nm indicates that TCAS forms a complex with Pb(II) ions first (Figure 4D). Therefore, the priority of TCAS complexation to heavy-metal ions was determined as Cu(II) > Cd(II) > Pb(II) > Ba(II). (Ba(II) ion cannot be complexed by TCAS.) The complexing priority of TCAS with different heavy-metal ions can be explained as follows: the arrangement of extranuclear electrons abides by Pauli Exclusion Principle, the principle of the lowest energy and Hund’s rule. Electrons must
take up the lowest energy track as far as possible. According to the nuclear electronic arrangement of Cu(II), Cd(II) and Pb(II) ion, the energy of the empty electron orbit of Cu(II) ion is the lowest, therefore, the lone pairs of electrons on the TCAS will first occupy the electron orbit of the Cu(II) ion. In this way, the complex is formed first with Cu(II) ion. The energy of the empty electron orbit of Cd(II) ion is the second one, while the energy of the empty electron orbit of Pb(II) ion is the highest one. TCAS finally forms a stable complex with the Pb(II) ion, which explains the complexing priority of TCAS to the three heavy-metal ions in the mixed solution. In addition, Ba(II) ion cannot form a complex with TCAS. According to our experiments, the complexation between Cu(II) ions and TCAS tended to be saturated and stable after 1.5 h. Cu(II) ions will settle down in alkaline, so in the following experiments the pH value was controlled from 3.0 to 7.0. When the molar ratio of TCAS and CuCl2 is 1:2, the absorbance of the complex compound reaches the maximum 5887
DOI: 10.1021/acs.est.7b06706 Environ. Sci. Technol. 2018, 52, 5884−5891
Article
Environmental Science & Technology
Figure 6. Concentration of complex compounds and Ba(II) ions in each step under different voltages. Change of concentration of copper complex compound in the first step feed cell (A); change of concentration of cadmium complex compound in the first step permeation cell (B); change of concentration of lead complex compound in second step permeation cell (C); change of the concentration of barium ions in the last step permeation cell (D).
Figure 7. Absorbance spectra of Cu(II)−TCAS complex (A), Pb(II)−TCAS complex (B) and Cd(II)−TCAS complex (C) under different pH value.
Cd(II)−TCAS complex concentration are raised slowly, and it tends to be stable after 3.5 h. Obviously, the force applied on heavy-metal ions is rather smaller when the voltage is lower, which will influence the migration velocity and the separation speed of the heavy-metal ions. After 3 h of separation, the concentration is no longer obviously increased. According to Figure 6B, the system can reach a balanced situation after 3 h under the driven voltage of 1.0 V. Similarly, the time for the system reaching balance is 2.5 h (Figure 6C) and 2.0 h (Figure 6D) under the driven voltage of 1.5 and 2.0 V respectively. In addition, the concentration has a downward trend as the time goes on under the driven voltage of 2.0 V, mainly because high voltage can result in a breakdown of the complex compound in the feed cell. In the second step, the concentration changing tendency of Pb(II)−TCAS complex is in accordance with that of Cd(II)−TCAS complex (Figure 6C). On the other hand, Figure 6D reveals the concentration changing tendency of Ba(II) ion in permeation cell after the last step, which is determined by an atomic absorption spectropho-
value (Figure 5A). This result shows that one TCAS molecule is capable of complexing with two Cu(II) ions. The complexation ratio of TCAS to Pb(II) and Cd(II) ions is 1:1 (Figure 5B,C). The maximum values of absorbance of the copper complex compound, the lead complex compound and the cadmium complex compound are 0.132, 0.136 and 0.142, respectively. 3.2. Effects of Driven Voltage. 0.5, 1.0, 1.5 and 2.0 V are employed as the driven voltage in the sequential separation experiments. The concentration changes of Cu(II)−TCAS complex in the feed cell in the first step at different voltage values are shown in Figure 6A. When the voltage is 1.5 V and below 1.5 V, the concentrations of Cu(II)−TCAS complex varies slightly, which shows it possesses good stability. When the voltage is increased to 2.0 V, the concentration of Cu(II)− TCAS complex is reduced obviously, which indicates that some of them have been decomposed. The concentration changes of Cd(II)−TCAS complex in the permeation cell is presented in Figure 6B. When the voltage is 0.5 V, the changements of 5888
DOI: 10.1021/acs.est.7b06706 Environ. Sci. Technol. 2018, 52, 5884−5891
Article
Environmental Science & Technology
Figure 8. Influence of pH value on separation efficiency, the driven-voltage is 1.5 V, the separation time based on the voltage is 2.5 h. Separation efficiency of Cu(II) ions in the first step (A); separation efficiency of Cd(II) ions in the second step (B); separation efficiency of Pb(II) and Ba(II) ions in the last step (C and D).
increases with pH value increasing. The reason is that the degree of dissociation of the SiO− group in the inner surface of the quartz capillary increases with pH value increasing. The velocity of electroosmotic flow increases due to the change of zeta potential. The electroosmotic flow is very small when pH is less than 3.0, while it increases rapidly with pH value increasing from 3.0 to 8.0, and it slow down when the pH value is higher than 8.0. 3.4. Applicability for Real Sample Analysis. A real sample was taken from a river in the city. The solid impurities contained in the water were purified by filtration. It was found that the sample contained Cu(II) and Cd(II) ions, and their concentration were 3.7 × 10−5 mol/L and 1.3 × 10−6 mol/L, respectively. 2 mL of untreated sample was employed for separation. 1 mL of TCAS solution (3.7 × 10−5 mol/L) was added into the sample. Then ultrasonic treatment and separation were carried out. After separation, the concentration of Cu(II) ion in the feed cell was 3.08 × 10−5 mol/L, and the concentration of Cd(II) ion in the permeation cell was 1.07 × 10−6 mol/L. The separation rate of Cu(II) and Cd(II) ions were 83.4% and 82.7%, respectively. In this way, the separation of two heavy-metal ions was completed, which showed that the method could be used for the real applications in heavy-metal ions separation. In addition, the heavy-metal ion complex near the electrode was dissolved in the solution after the separation. Heavy-metal ion complex can be dissociated into heavy-metal ion and TCAS by adding a certain amount of nitric acid or hydrochloric acid.29−31 Some common methods for recovery of heavy-metal ions can be used for subsequent treatment.
tometer. In general, higher driving voltage is beneficial to increase the separation speed, while it also could cause partial decomposition of heavy-metal ion complexes when it excessed the threshold. According to our experiments, the best voltage for the sequential separation is 1.5 V. 3.3. Effects of pH Value. The pH value has an important influence on the formation of the complex compound. On the one hand, hydrogen ion in acid solution can suppress the hydroxylation of Cd(II), Cu(II) and Pb(II) ions, which leads to generate more effective heavy-metal ions in the sample solution. On the other hand, lower pH value can affect the complexation of TCAS to heavy-metal ions. Both metal ions and hydrogen ions are positively charged, and hydrogen ions enter the cavity of TCAS, getting the empty cavity and suppressing the complexation of heavy-metal ions. By the couple influence of the hydrogen ions, the absorbance of complex compounds reaches a maximum value when the pH value is 5.0 (Figure 7A,B,C). Under such a pH value, the efficiency of separation will reach the maximum value, as shown in Figure 7. Here the separation efficiency is defined by the ratio of the concentration of heavy-metal ions separated by each step to their initial concentration. According to the results in Figure 8, the maximum separation efficiency for Cu(II), Cd(II), Pb(II) and Ba(II) can be achieved under the pH value of 5.0. On one hand, acid conditions can inhibit the hydrolysis of Cu(II), Cd(II), Pb(II) ions; on the other hand, lower pH value can affect the complexation of TCAS to heavy-metal ions. When the pH value is equal to 5.0, the most metal ions pass through nanopores and combine with TCAS in the permeation cell. Of course, Ba(II) ion is not easy to hydrolyze and are relatively less affected by the pH value. In addition, lower pH value accelerates the electroosmotic flow, which can help to reach separation equilibrium faster.26−28 For the quartz capillary, the velocity of electroosmotic flow
4. CONCLUSION In summary, a nanopore-based strategy was suggested and related segregation apparatus was built for the sequential 5889
DOI: 10.1021/acs.est.7b06706 Environ. Sci. Technol. 2018, 52, 5884−5891
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Environmental Science & Technology
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separation of multiple heavy-metal ions in water. The basis of these sequential separations was the different complexation sensitivity of heavy-metal ions to specific complexing agent. The efficiency of the sequential separations related to two factors, such as driven voltage and pH value. According to the experimental results, when the driven voltage was 1.5 V and the pH value was 5.0, the best separation efficiency was achieved, as 94.8%, 95.2%, 92.8%, 93.6%, for Cu(II), Cd(II), Pb(II) and Ba(II), respectively.
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AUTHOR INFORMATION
Corresponding Author
*L. Liu. Email:
[email protected]. ORCID
Lei Liu: 0000-0001-6202-9966 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the Natural Science Funds for Distinguished Young Scholar of Jiangsu Province (BK20170023), the National Natural Science Foundation of China (51675360, 51675502, 51775105, 51775001, 51775530, 51775051), the Fundamental Research Funds for the Central Universities (3202006301, 3202006403), the Qing Lan Project of Jiangsu Province, the International Foundation for Science, Stockholm, Sweden, the Organization for the Prohibition of Chemical Weapons, The Hague, Netherlands, through a grant to Lei Liu (F/4736-2), the grants from Top 6 High-Level Talents Program of Jiangsu Province (2017-GDZB-006, Class A), the Natural Science Foundation of Jiangsu Province (BK20150505), the Tribo1ogy Science Fund of State Key Laboratory of Tribology (SKLTKF15A11), Open Research Fund of State Key Laboratory of High Performance Complex Manufacturing, Central South University (Kfkt2016-11), Open Research Fund of State Key Laboratory of Fire Science (HZ2017-KF05) and Open Research Fund of State Key Laboratory of solid lubrication (LSL-1607).
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DOI: 10.1021/acs.est.7b06706 Environ. Sci. Technol. 2018, 52, 5884−5891
Article
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DOI: 10.1021/acs.est.7b06706 Environ. Sci. Technol. 2018, 52, 5884−5891
