A nanopore-based strategy for sequential separation of heavy-metal

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A nanopore-based strategy for sequential separation of heavy-metal ions in water Lei Liu, and Ke Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06706 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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Environmental Science & Technology

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A nanopore-based strategy for sequential separation of

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heavy-metal ions in water

3 4

Lei Liu* and Ke Zhang

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Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical

6

Instruments, School of Mechanical Engineering, Southeast University, Nanjing

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211189, People's Republic of China

8

*

Corresponding author, email: [email protected]

9 10 11 12 13 14 15 16 17 18 19 20 21 22 1

ACS Paragon Plus Environment


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Abstract

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Developing novel methods for the removal of heavy-metal ions from wastewater

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with low costs, special selectivity and high efficiency is quite important in water

26

restoration

27

nanopore-based strategy was suggested and related segregation apparatus was built to

28

separate multiple heavy-metal ions in water by selective complexation. The results

29

indicated that the prioritization of the selecting order for the complexing agent

30

(thiacalix[4]arene-p-tetrasulfonate (TCAS)) to heavy-metal ions was Cu(II) > Cd(II) >

31

Pb(II) > Ba(II). Meanwhile, higher driven voltage corresponded to a faster separation

32

speed, while it could cause the decomposition of complexed heavy-metal ions when

33

excessed the threshold. On the other hand, pH value would affect the hydrolysis of

34

heavy-metal ions, the complexation of the calixarene to the heavy-metal ions and the

35

speed of the electroosmotic flow. In our experiments, the maximum separation

36

efficiency was achieved when the driven voltage was 1.5 V and the pH value was 5.0,

37

corresponding to the best separation rate of 94.8%, 95.2%, 92.8%, 93.6%, for Cu(II),

38

Cd(II), Pb(II) and Ba(II) respectively.

and

comprehensive

ecological

improvement.

39 40 41 42 43 44

Keywords: heavy-metal ions; separation; TCAS

45 2


In

this

work,

a

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1 Introduction

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Wastewater containing heavy-metal ions produced by industrial processes (such as

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mining, metallurgy, machinery manufacturing, chemical industry, electronics and

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instrumentation) is one of the most serious and most harmful industrial

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contaminations to water. It is difficult to eliminate heavy metals in wastewater only by

51

common methods. Generally, effective treatments to heavy-metal polluted water

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consist of two aspects: exogenous control and endogenous control. Exogenous control

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mainly deals with restricting the emissions of waste residue containing heavy metals,

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while internal control is to repair polluted water. Removal of heavy-metal ions from

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wastewater is not a simple work, while low costs, high separation selectivity and

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efficiency should also be under consideration. Among various methods such as

57

chemical precipitation, ion exchange, reverse osmosis and adsorption, 1-5 the last one

58

seems more effective and economic, which contains an efficient solid phase which

59

should consist of a stable and insoluble porous matrix with suitable active groups

60

which can interact with heavy-metal ions.6, 7

61

As an important class of ion carriers, calixarene and its derivatives bring new

62

opportunities and challenges to the separation of heavy-metal ions in water,8-14 and

63

recent reports for the applications of functionalized calixarene or its derivatives enrich

64

the research on the heavy-metal ions elimination. For example, thiacalix[4]arene

65

(TCA) and its derivatives can be incorporated into the surface of polymeric

66

membranes and can be used as selective adsorbents for some heavy-metal ions. 15-17

67

TCA can be used for the extraction of Cu(II), Cd(II), Co(II) and Cr(III) through batch 3



68

adsorption techniques.18 on the other hand, TCA possesses good efficiencies in the

69

selective extraction of metal ions. Accordingly, this compound also can be used for

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the separations of different heavy-metal ions by its selective complexation to

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heavy-metal ions.19-22

72

In this work, a “U”-type device with two separated cells was constructed and a

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nanopore-based strategy for sequential separation of heavy-metal ions was suggested,

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by which the separation of four kinds of heavy-metal ions (Cu(II), Cd(II), Pb(II) and

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Ba(II) ions) has been carried out. The basic principle of this sequential separation

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method is based on the selective complexation of TCAS to different heavy-metal ions.

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TCAS itself is an organic compound that can form a stable complex with a variety of

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heavy-metal ions. In a mixed solution containing different heavy-metal ions, specific

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amounts of TCAS were added into the system successively according to the amount

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of each heavy-metal ion. The complex and heavy-metal ions were opposite charged in

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the solution, and the electric field force applied on heavy-metal ions also increased

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with the voltage increasing. Therefore, heavy-metal ions spent less time in

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translocating through the nanopore with higher voltage value, which was consistent

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with the experimental results in this work. Another possible reason was that quartz

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capillary was negatively charged; positive ions could bind to the inner surface of the

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quartz capillary and formed an electric double layer, which significantly changed the

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ionic distribution inside the nanopore. Increasing the velocity across the

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polycarbonate membrane, there was more ionic flux through the pore and the fast

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moving ions in the confined nanopore could shorten the separation time, which 4


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accelerated the speed of electroosmotic flow. Under the power of the electric field, the

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complex compound moves towards positive electrode, whereas other heavy-metal

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ions move through the nanopore arrays to the negative cell of “U”-type device. The

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separation experiment was carried out in this way. Here nanopore-contained film

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works as a semipermeable membrane, providing channels for ion-oriented

95

movements.

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2. Experimental methods

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2.1 Materials and Devices

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Polycarbonate membranes containing nanopore arrays (diameter: 50 nm) were

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obtained from Whatman, Inc. Copper chloride, Lead chloride, Cadmium chloride and

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Barium chloride were bought from Aladdin Industrial Corporation. TCA was

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purchased from Tokyo Chemical Industry with a purity of 98%. Concentrated sulfuric

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acid was purchased from Sinopharm chemical reagent Co. Ltd. Ultra-pure water was

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used for the preparation of all solutions and rinsing.

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2.2 Preparation of TCAS and its complexation

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TCA (Fig.1A) was converted to TCAS (Fig.1B) by sulfonating. The purpose of

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converting TCA into TCAS was to introduce sulfonic groups to TCA molecular

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structure and make it water-soluble. 2 g TCA and 80 mL of concentrated sulfuric acid

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were poured into a 100 mL beaker, and the temperature was controlled at 80oC for 4

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hours.23-25 The final extracted substance was TCAS with high purity.

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The concentration of CuCl2, PbCl2, CdCl2 and TCAS used in the following

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experiments was 1.0 mol/L. Two groups of samples were prepared according to Table 5



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1 and Table 2.

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Table 1 The proportion of heavy-metal ions and TCAS

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Number

TCAS solution /mL

CuCl2 solution/mL

CdCl2 solution/mL

PbCl2 solution/mL

Ultra-pure water / mL

S1

2

1

2

1

2

S2

2

0

2

1

3

Table 2 The proportion of Cu(II) ions solution and TCAS solution.

Number

TCAS solution /mL

CuCl2 /mL

Ultra-pure water/mL

Proportion

S3

1.0

0.5

6.5

1:0.5

S4

1.0

1.0

6.0

1:1

S5

1.0

1.5

5.5

1:1.5

S6

1.0

2.0

5.0

1:2

S7

1.0

2.5

4.5

1:2.5

2.3 Device and methods

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The concentrations of complex compounds of heavy-metal ions and TCAS were

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determined by UV-Vis absorption spectra carried on a UV-3600 spectrometer. The

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concentrations of heavy-metal ions were determined by using an atomic absorption

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spectrophotometer (AA-7000F/G). The transmembrane ionic currents were recorded

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by a Keithley 6485 multimeter (Keithley Instruments, Inc., USA). The electrode

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model used in this experiment is 213 from Leici company, which belongs to a

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platinum electrode.

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A “U”-type device containing a feed cell (on the left side) and a permeation cell (on

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the right side) was employed in our experiments, as showed in Fig.2. Two separated

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cells with certain solutions were linked by a piece of PC membrane containing

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nanopore arrays and they were sealed by PDMS (polydimethylsiloxane). An electric

127

field was applied to transport ions in the feed cell to the permeation cell through the

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membrane. 6


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A mixed solution containing 1.0 mL TCAS solution, 2.0 mL CuCl2 solution, 1.0 mL

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CdCl2 solution, 1.0 mL PbCl2 solution and 1.0 mL BaCl2 solution was prepared. The

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hybrid solution was diluted to 8.0 mL, followed by ultrasonic treatment for half an

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hour. Then the mixed solution was put into the feed cell of “U”-type device. 2.0 mL

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TCAS solution (0.125 mol/L) was added to the permeation cell firstly according to the

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complexation ratio of TCAS to Cd(II) ions. The separation experiment was carried out

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according to the process showed in Fig.3.

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Because of the different complexing ability, Cu(II)-TCAS complex can be first

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formed. Under the force by driven voltage, Cu(II)-TCAS complex moves towards

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positive electrode in the feed cell, while other heavy-metal ions (such as Cd(II), Pb(II)

139

and Ba(II)) move through nanopores to the permeation cell, and the added TCAS in

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permeation cell formed a complex with Cd(II) ion according to its complexing priority.

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When the first step was completed, the solution in the permeation cell was collected

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and transferred to the cleaned feed cell, and TCAS was added to the permeation cell

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secondly according to the complexation ratio of TCAS to Pb(II). The above

144

experiments were circulated until only Ba(II) ions are left in the permeation cell. All

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above experiments were carried out at room temperature. The separation time was

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controlled at 4 hours and it was measured half an hour.

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3. Results and discussion

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3.1 Selective complexation by TCAS

149

The complexation of these heavy-metal ions can be realized by the synergistic

150

coordination of the bridged sulfur atoms and the phenol hydroxyl groups in the lower 7



151

edge of TCAS molecule. According to Hard and Soft Acids and Bases (HSAB)

152

Theory, these metal ions belong to the soft acid metal ions, which are easy to combine

153

with sulfur atoms of the soft alkali. Due to lone pairs of electrons on the bridged

154

sulfur atoms and the phenol hydroxyl groups in the lower edge of TCAS molecule, it

155

can participate in the action of metal cations, making TCAS better in identifying and

156

complexing with metal ions. The specific process can be described as follows:

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heavy-metal ions enter the cavity of TCAS molecule. Heavy-metal ions provide

158

empty electron orbits, while TCAS molecule provides lone pairs of electrons. They

159

form a stable complex compound by forming coordinate bonds.

160

The characteristic UV-vis absorption peak for TCAS is located at 301 nm. The

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spectra for the complex compounds of Cd(II)-TCAS, Cu(II)-TCAS, Pb(II)-TCAS and

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Ba(II)-TCAS are showed in Fig.4A, in which the UV-vis absorption peaks are located

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at 317 nm (for Cd(II)-TCAS complex), 310 nm (for Cu(II)-TCAS complex) and 322

164

nm (for Pb(II)-TCAS complex) respectively. In a mixed solution containing Cu(II),

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Cd(II), Pb(II) and Ba(II) ions, the absorption peak attributed to Cu(II)-TCAS complex

166

appears at 310 nm (Fig.4B), which indicates TCAS has formed a complex with Cu(II)

167

ions firstly. For a mixed solution containing Cd(II), Pb(II) and Ba(II) ions, the

168

absorptive peak at 317 nm can indicate that TCAS forms a complex with Cd(II) ions

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firstly (Fig.4C). For a mixed solution containing Pb(II) and Ba(II) ions, the absorptive

170

peak at 322 nm indicates that TCAS forms a complex with Pb(II) ions firstly (Fig.4D).

171

Therefore, the priority of TCAS complexation to heavy-metal ions was determined as

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Cu(II) > Cd(II) > Pb(II) > Ba(II). (Ba(II) ion cannot be complexed by TCAS.) 8


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The complexing priority of TCAS with different heavy-metal ions can be explained

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as followings: the arrangement of extra-nuclear electrons abides by Pauli Exclusion

175

Principle, the principle of the lowest energy and Hund's rule. Electrons must take up

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the lowest energy track as far as possible. According to the nuclear electronic

177

arrangement of Cu(II), Cd(II), and Pb(II) ion, the energy of the empty electron orbit of

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Cu(II) ion is the lowest, therefore, the lone pairs of electrons on the TCAS will first

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occupy the electron orbit of the Cu(II) ion. In this way, the complex is formed firstly

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with Cu(II) ion. The energy of the empty electron orbit of Cd(II) ion is the second one,

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while the energy of the empty electron orbit of Pb(II) ion is the highest one. TCAS

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finally forms a stable complex with the Pb(II) ion, which explains the complexing

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priority of TCAS to the three heavy-metal ions in the mixed solution. In addition,

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Ba(II) ion cannot form a complex with TCAS.

185

According to our experiments, the complexation between Cu(II) ions and TCAS

186

tended to be saturated and stable after 1.5 hours. Cu(II) ions will settle down in

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alkaline, so in the following experiments the pH value was controlled from 3.0 to 7.0.

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When the molar ratio of TCAS and CuCl2 is 1:2, the absorbance of the complex

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compound reaches the maximum value (Fig.5A). This result shows that one TCAS

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molecule is capable of complexing with two Cu(II) ions. The complexation ratio of

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TCAS to Pb(II) and Cd(II) ions is 1: 1(Fig.5B and Fig.5C). The maximum values of

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absorbance of the copper complex compound, the lead complex compound and the

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cadmium complex compound are 0.132, 0.136 and 0.142 respectively.

194

3.2 Effects of driven voltage 9



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0.5 V, 1.0 V, 1.5 V, and 2.0 V are employed as the driven voltage in the sequential

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separation experiments. The concentration changes of Cu(II)-TCAS complex in the

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feed cell in the first step at different voltage values are showed in Fig.6A. When the

198

voltage is 1.5 V and below 1.5 V, the concentrations of Cu(II)-TCAS complex varies

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slightly, which shows it possesses good stability. When the voltage is increased to 2.0

200

V, the concentration of Cu(II)-TCAS complex is reduced obviously, which indicates

201

that some of them have been decomposed. The concentration changes of Cd(II)-TCAS

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complex in the permeation cell is presented in Fig.6B. When the voltage is 0.5 V, the

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changements of Cd(II)-TCAS complex concentration are raised slowly, and it tends to

204

be stable after 3.5 hours. Obviously, the force applied on heavy-metal ions is rather

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smaller when the voltage is lower, which will influence the migration velocity and the

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separation speed of the heavy-metal ions. After 3 hours of separation, the

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concentration is no longer obviously increased.

208

According to Fig.6B, the system can reach a balanced situation after three hours

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under the driven voltage of 1.0 V. Similarly, the time for the system reaching balance

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is 2.5 hours (Fig.6C) and 2.0 hours (Fig.6D) under the driven voltage of 1.5 V and 2.0

211

V respectively. In addition, the concentration has a downward trend as the time goes

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on under the driven voltage of 2.0 V, mainly because high voltage can result in a

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breakdown of the complex compound in the feed cell. In the second step, the

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concentration changing tendency of Pb(II)-TCAS complex is in accordance with that

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of Cd(II)-TCAS complex (Fig.6C). On the other hand, Fig.6D reveals the

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concentration changing tendency of Ba(II) ion in permeation cell after the last step, 10


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which is determined by an atomic absorption spectrophotometer. In general, higher

218

driving voltage is beneficial to increase the separation speed, while it also could cause

219

partial decomposition of heavy-metal ion complexes when it excessed the threshold.

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According to our experiments, the best voltage for the sequential separation is 1.5 V.

221

3.3 Effects of pH value

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The pH value has an important influence on the formation of the complex

223

compound. On the one hand, hydrogen ion in acid solution can suppress the

224

hydroxylation of Cd(II), Cu(II) and Pb(II) ions, which leads to generate more

225

effective heavy-metal ions in the sample solution. On the other hand, lower pH value

226

can affect the complexation of TCAS to heavy-metal ions. Both metal ions and

227

hydrogen ions are positively charged, and hydrogen ions enter the cavity of TCAS,

228

getting the empty cavity and suppressing the complexation of heavy-metal ions. By

229

the couple influence of the hydrogen ions, the absorbance of complex compounds

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reaches a maximum value when the pH value is 5.0 (Fig.7A, Fig.7B and Fig.7C).

231

Under such a pH value, the efficiency of separation will reach the maximum value, as

232

showed in Fig.7. Here the separation efficiency is defined by the ratio of the

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concentration of heavy-metal ions separated by each step to their initial concentration.

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According to the results in Fig.8, the maximum separation efficiency for Cu(II),

235

Cd(II), Pb(II) and Ba(II) can be achieved under the pH value of 5.0. On one hand,

236

acid conditions can inhibit the hydrolysis of Cu(II), Cd(II), Pb(II) ions; on the other

237

hand, lower pH value can affect the complexation of TCAS to heavy-metal ions.

238

When the pH value is equal to 5.0, the most metal ions pass through nanopores and 11



239

combine with TCAS in the permeation cell. Of course, Ba(II) ion is not easy to

240

hydrolyze and are relatively less affected by the pH value. In addition, lower pH value

241

accelerates the electroosmotic flow, which can help to reach separation equilibrium

242

faster.26-28 For the quartz capillary, the velocity of electroosmotic flow increases with

243

pH value increasing. The reason is that the degree of dissociation of the SiO- group in

244

the inner surface of the quartz capillary increases with pH value increasing. The

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velocity of electroosmotic flow increases due to the change of zeta potential. The

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electroosmotic flow is very small when pH is less than 3.0, while it increases rapidly

247

with pH value increasing from 3.0 to 8.0, and it slow down when the pH value is

248

higher than 8.0.

249

3.4 The applicability for real sample analysis

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A real sample was taken from a river in the city. The solid impurities contained in

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the water were purified by filtration. It was found that the sample contained Cu(II)

252

and Cd(II) ions, and their concentration were 3.7×10-5 mol/L and 1.3×10-6 mol/L

253

respectively. 2 mL of untreated sample was employed for separation. 1 mL of TCAS

254

solution (3.7×10-5 mol/L) was added into the sample. Then ultrasonic treatment and

255

separation were carried out. After separation, the concentration of Cu(II) ion in the

256

feed cell was 3.08×10-5 mol/L, and the concentration of Cd(II) ion in the permeation

257

cell was 1.07×10-6 mol/L. The separation rate of Cu(II) and Cd(II) ions were 83.4%

258

and 82.7% respectively. In this way, the separation of two heavy-metal ions was

259

completed, which showed that the method could be used for the real applications in

260

heavy-metal ions separation. 12


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In addition, the heavy-metal ion complex near the electrode was dissolved in the

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solution after the separation. Heavy-metal ion complex can be dissociated into

263

heavy-metal ion and TCAS by adding a certain amount of nitric acid or hydrochloric

264

acid.29-31 Some common methods for recovery of heavy-metal ions can be used for

265

subsequent treatment.

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4 Conclusion

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In summary, a nanopore-based strategy was suggested and related segregation

268

apparatus was built for the sequential separation of multiple heavy-metal ions in water.

269

The basis of these sequential separations was the different complexation sensitivity of

270

heavy-metal ions to specific complexing agent. The efficiency of the sequential

271

separations related to two factors, such as driven voltage and pH value. According to

272

the experimental results, when the driven voltage was 1.5 V and the pH value was 5.0,

273

the best separation efficiency was achieved, as 94.8%, 95.2%, 92.8%, 93.6%, for

274

Cu(II), Cd(II), Pb(II) and Ba(II) respectively.

275

Acknowledgements

276

This work is financially supported by the Natural Science Funds for Distinguished

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Young Scholar of Jiangsu Province (BK20170023), the National Natural Science

278

Foundation of China (51675360, 51675502, 51775105, 51775001, 51775530,

279

51775051), the Fundamental Research Funds for the Central Universities

280

(3202006301, 3202006403), the Qing Lan Project of Jiangsu Province ， the

281

International Foundation for Science, Stockholm, Sweden, the Organization for the

282

Prohibition of Chemical Weapons, The Hague, Netherlands, through a grant to Lei Liu 13



283

(F/4736-2), the grants from Top 6 High-Level Talents Program of Jiangsu

284

Province(2017-GDZB-006, Class A),the Natural Science Foundation of Jiangsu

285

Province (BK20150505), the Tribo1ogy Science Fund of State Key Laboratory of

286

Tribology (SKLTKF15A11), Open Research Fund of State Key Laboratory of High

287

Performance Complex Manufacturing, Central South University (Kfkt2016-11), Open

288

Research Fund of State Key Laboratory of Fire Science (HZ2017-KF05) and Open

289

Research Fund of State Key Laboratory of solid lubrication (LSL-1607).

290

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structures and photophysical properties. Dalton T. 2015, 44 (17), 7991-8000.

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399 400

Figure captions:

401

Figure 1. The structure of TCA (A) and TCAS (B).

402

Figure 2. “U”-Type separation device containing a feed cell and a permeation cell, a

403

Keithley 6485 multimeter was used to detect the transmembrane ionic current through

404

nanopores. (A); The exact dimensions of the electrochemical cells (B); The SEM scan

405

imaging of the polycarbonate nanopore arrays (C).

406

Figure 3. The schematic diagram of sequential separation.

407

Figure 4. UV-Vis absorption spectrum of TCAS(A); UV-Vis absorbance spectra of

408

TCAS to four kinds of heavy-metal ions respectively (B); UV-Vis absorbance spectra

409

of TCAS to hybrid solutions of multiple heavy-metal ions (C and D).

410 411

Figure 5. The absorbance spectra of Cu(II)-TCAS complex (A), Pb(II)-TCAS complex (B), Cd(II)-TCAS complex (C) under different ion to TCAS molar ratio.

412

Figure 6. The concentration of complex compounds and Ba(II) ions in each step

413

under different voltages; Change of concentration of copper complex compound in the

414

first step feed cell(A); Change of concentration of cadmium complex compound in the 19



415

first step permeation cell(B); Change of concentration of lead complex compound in

416

second step permeation cell(C); Change of the concentration of barium ions in the last

417

step permeation cell (D).

418 419

Figure 7. The absorbance spectra of Cu(II)-TCAS complex (A), Pb(II)-TCAS complex (B), Cd(II)-TCAS complex (C) under different pH value.

420

Figure 8. Influence of pH value on separation efficiency, the driven-voltage is 1.5 V,

421

the separation time based on the voltage is 2.5 hours. Separation efficiency of Cu(II)

422

ions in the first step(A); Separation efficiency of Cd(II) ions in the second step(B);

423

Separation efficiency of Pb(II) and Ba(II) ions in the last step (C and D).

20


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TOC 281x134mm (150 x 150 DPI)



Fig.1 The structure of TCA (A) and TCAS (B). 217x106mm (149 x 149 DPI)


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Fig.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). 191x136mm (150 x 150 DPI)



Fig.3 The schematic diagram of sequential separation. 285x265mm (72 x 72 DPI)


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Fig.4 UV-Vis absorption spectrum of TCAS(A); UV-Vis absorbance spectra of TCAS to four kinds of heavymetal ions respectively (B); UV-Vis absorbance spectra of TCAS to hybrid solutions of multiple heavy-metal ions (C and D). 141x107mm (220 x 220 DPI)



Fig.5 The absorbance spectra of Cu(II)-TCAS complex (A), Pb(II)-TCAS complex (B), Cd(II)-TCAS complex (C) under different ion to TCAS mole ratio. 304x89mm (96 x 96 DPI)


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Fig.6 The 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). 239x199mm (300 x 300 DPI)



Fig.7 The absorbance spectra of Cu(II)-TCAS complex (A), Pb(II)-TCAS complex (B), Cd(II)-TCAS complex (C) under different pH value. 301x87mm (96 x 96 DPI)


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Fig.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 hours. 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). 289x202mm (300 x 300 DPI)


A nanopore-based strategy for sequential separation of heavy-metal

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