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Efficient Removal of Pb2+ from Aqueous Solution by an Ionic Covalent Organic Framework: A Molecular Simulation Study Krishna M. Gupta, Kang Zhang, and Jianwen Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00625 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Industrial & Engineering Chemistry Research

Efficient

Removal

of

Pb2+ from

Aqueous

Solution

by

an

Ionic

Covalent−Organic Framework: A Molecular Simulation Study Krishna M. Gupta, Kang Zhang and Jianwen Jiang* Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576, Singapore

Abstract: An ionic covalent−organic framework (ICOF-1 containing sp3 hybridized boron anionic centers formatted by spiroborate linkage and dimethylammonium ions) is explored as an ion exchanger for the removal of lead (Pb2+) ions from aqueous solution. From molecular simulations, the Pb2+ ions are observed to exchange with the nonframework DMA+ ions in the ICOF-1. At a concentration of 600 ppm, the Pb2+ ions are completely exchanged and reside in the ICOF-1, while the DMA+ ions are in a dynamic equilibrium with the solution. It is revealed that the exchange between Pb2+ and DMA+ is governed by the stronger attraction of Pb2+ with the negatively charged ICOF-1 framework. The radial distribution functions and mean-squared displacements further show that the exchanged Pb2+ ions are in a closer proximity to the ICOF-1 framework with a smaller mobility than DMA+ ions. The simulation study provides microscopic insight into the ion exchange process between Pb2+ and DMA+, and it suggests that the ICOF-1 might be an intriguing candidate for water purification. Keywords: ion exchange, covalent−organic framework, molecular simulation, interaction

*Email: [email protected] 1

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1. INTRODUCTION With increasing population and rapid growth in industrialization (e.g. metal plating and mining industries; paper, fertilizer, battery and pesticide industries), a large amounts of various heavy metals ions have been introduced into water.1 They are environmentally toxic and nonbiodegradable, can accumulate in living organisms and lead to dysfunction in nervous, circulatory and immune systems, thus pose a serious threat to human life even at a very low concentration.2,3 Therefore, the removal of heavy metal ions from aqueous solutions is highly important to minimize health and environmental risks. Various methods are available to remove heavy metal ions, such as precipitation, electrochemical treatment, membrane filtration, extraction, adsorption, and ion exchange.4 Among these, ion exchange is considered as the most extensively used due to its high efficiency, fast kinetics, cost effective, and efficient treatment under very dilute condition.4,5 Primarily, two types of ion exchangers (inorganic and organic) exist. The common feature is that they contain an ionic framework and nonframework cations, and the latter can exchange with metal ions in water.6 Most inorganic ion exchangers (e.g. zeolites) possess well-defined pores; however, the pore size is small and hence ion exchange is slow.7 On the other hand, organic exchangers (e.g. polymer resins) are rapid in exchange, but with relatively poor chemical and mechanical stability in ionic solutions compared against inorganic counterparts.8 Further, hybrid nanoporous materials, namely metal−organic frameworks (MOFs) have been tested as ion exchanger.9,10 Nevertheless, their stability in water remains a challenge.11,12 Consequently, there is a considerable interest towards the development of high-performance ion exchangers for the removal of heavy metal ions from aqueous solutions.

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In the continuous quest for new materials, a class of nanoporous materials namely covalent−organic frameworks (COFs) have drawn large attention because of their precisely ordered crystalline structure, high structural and functional tunability as well as excellent stability.13,14 They are formed by covalent linking among different building blocks of light elements, and have been investigated in many potential applications such as storage, separation, catalysis and sensing.15 For instance, a series of cationic COFs with high thermal and chemical stability were produced as anion exchangers.16 COFs stable in strong acidic and basic conditions were also explored as adsorbents for the removal of metal ions.17 Recently, an ionic COF (ICOF-1) was synthesized by the formation of spiroborate linkages, with a high surface area of 1259 m2/g, good thermal stability and excellent resistance to hydrolysis.18 The ICOF-1 consists of negatively charged framework and dimethylammonium (DMA+) ions, and is a potential ion exchanger. In this work, we report a molecular simulation study to examine the removal of lead (Pb2+) from aqueous PbCl2 solution by exchange with the DMA+ ions in the ICOF-1. With rapidly growing computational resources, molecular simulation has become an indispensable tool and played an increasingly important role in materials science and engineering. Simulation at an atomistic/molecular level can provide microscopic insight that otherwise is experimentally intractable, thus elucidate underlying physics from bottom-up. In this study, Pb2+ is chosen because it is toxic and a common contaminant during water transport through lead-bearing household pipelines. Following this introduction, the simulation models and methods are briefly described in Section 2. In Section 3, the simulation results are presented, including the density distributions of ions, the electrostatic and van der Waals energies of ions with the framework, the structural and dynamic properties of ions in the ICOF-1. Finally, the concluding remarks are summarized in Section 4.

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where εij and σij are the well depth and collision diameter, rij is the distance between atoms i and j, qi is the atomic charge of atom i, and ε0 = 8.8542 × 10-12 C2N-1m-2 is the permittivity of vacuum. The atomic charges (Table S1) of ICOF-1 were estimated by the Density-Derived Electrostatic and Chemical (DDEC) method,20 based on density functional theory (DFT) calculations using the Vienna Ab Initio Simulation Package (VASP).21 The atomic charges of DMA+ ion were calculated by DFT calculations with the Becke exchange plus the Lee-YangParr correlation functional (B3LYP) and 6-31+g(d) basis set using Gaussian 09.22 The LJ parameters (Table S2) were adopted from the universal force field (UFF).23 The bonded and nonbonded potential parameters for DMA+ were adopted from the AMBER force field.24 Moreover, Pb2+ and Cl− ions were described as charged LJ particles with potential parameters from the AMBER. Water was modeled by the TIP4P model,25 which is shown to be accurately reproduced the entropic and hydrogen bonding behavior of liquid water.26 The system was initially subjected to energy minimization using the steepest descent method, then velocities were assigned according to the Maxwell-Boltzmann distribution at 300 K. Finally, NVT molecular dynamics (MD) simulation was conducted at 300 K. The periodic boundary conditions were used in all three dimensions. While the ICOF-1 framework atoms were fixed during simulation, the nonframework DMA+ ions were free to move. A cutoff of 14 Å was used to calculate the LJ interactions, and the particle-mesh Ewald method was used to evaluate the electrostatic interactions with grid spacing of 1.2 Å and real-space cutoff of 14 Å. A time step of 2 fs was used for integration of equations of motion by the leapfrog algorithm. The simulation duration was 256 ns and the trajectory was saved every 100 ps. Further, the interaction energies (electrostatic and van der Waals) of a single Pb2+ or DMA+ ion moving from the solution to the ICOF-1 framework were calculated along the z axis. For each type of ion,

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first, 60 independent systems were constructed with the ion spacing of 2 Å at the z axis followed by energy minimization on the xy plane; then the electrostatic and van der Waals energies were calculated using a cutoff of 24 Å. To examine the structural and dynamic properties of ions in the ICOF-1 framework, the left and right compartments in the system were removed after 256 ns; then another 5 ns NVT MD simulation was conducted. All the MD simulations were performed using the GROMACS v5.0.6 package.27

3. RESULTS AND DISCUSSION First, the ion exchange process is monitored by simulation snapshots and the density distributions of ions at different times. Then, the interaction energies of DMA+ and Pb2+ ions with the framework are shown at different locations to quantify the driving force for Pb2+ ions to move from the solution into the ICOF-1. Finally, the structure and dynamics of DMA+ and Pb2+ ions in the ICOF-1 are analyzed. 3.1 Evolution of ion-exchange Figure 2a and 2b show the initial (t = 0 ns) and final (t = 256 ns) simulation snapshots. Initially, all the Pb2+ and Cl− ions are in the solution and DMA+ ions reside in the ICOF-1 framework. Upon the initiation of simulation, the Pb2+ ions start to move into the framework and simultaneously the DMA+ ions move out from the framework. A video is provided in the ESI to visualize the exchange process (water molecules are not shown for clarity). As time lapses, the number of Pb2+ ions in the ICOF-1 increases along with a decrease in the number of DMA+, implying the occurrence of ion exchange. The ions move independently rather than in pairs/clusters. Once exchanged, the Pb2+ ions prefer staying in the ICOF-1 without moving back to the solution; meanwhile, the DMA+ ions move out from the ICOF-1 and stay in the solution. It

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should be noted that during the entire simulation, none of the Cl− ion enters the ICOF-1. This is attributed to the negative framework of the ICOF-1, which exerts repulsive interaction with the Cl−. It is worthwhile to note that some Cl− ions were found to enter a MOF10 and charged porous media28 during exchange. The complete exchange of Pb2+ ions was observed at 252 ns and there was no quantitative change by further extending the simulation by 4 ns, implying the occurrence of equilibrium state. Figure 2c shows the xy plane of the simulation system at the final stage. From the snapshot, it is clear that there are two preferred locations for the exchanged Pb2+ ions, one is near the boron anionic center (black dotted circle) formed by spiroborate linkage, which is similar to the locations of DMA+ ions in the ICOF-1, and the other is near the C2 atom (red dotted circle).

(a) t = 0 ns (yz plane) 245 Å

245 Å

60.59 Å

(c) t = 256 ns (xy plane) (b) t = 256 ns (yz plane)

Figure 2. (a) Initial and (b-c) final simulation snapshots. Color code for ions: Pb2+, green; Cl-, red; DMA+, yellow. The dashed lines indicate the solution/ICOF-1 interfaces. For clarity, water molecules are not shown in (a-c); DMA+ ions in ICOF-1 are not shown in (b) and (c).

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Figure 3 shows the numbers of Pb2+ and DMA+ ions in the ICOF-1 versus simulation time. As discussed above, all the Cl− ions are in the solution during entire simulation, thus their plot is not shown. Consistent with Figure 2, along with simulation time, the number of Pb2+ ions increases, while the number of DMA+ ions decreases in the ICOF-1. The initial rate of exchange is fast as half of the Pb2+ ions are exchanged within the first 50 ns. This is attributed to the fast movement of the interfacial DMA+ ions into the solution, thus producing favorable interaction for the Pb2+ ions with the anionic framework. Once the interfacial DMA+ ions are exchanged, the rate is slowed down as nearly 200 ns is needed to exchange the rest half of the Pb2+ ions. After approximately 252 ns, the numbers of both Pb2+ and DMA+ ions in the ICOF-1 are nearly constant. While all the Pb2+ ions reside in the ICOF-1, the DMA+ ions in the ICOF-1 are in dynamic equilibrium with their counterparts in the solution. 40

700

30

650

25 20

600

15 10

550

Number of DMA+

35

Number of Pb2+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5 0

500

0

50

100

150

200

250

t (ns)

Figure 3. Numbers of Pb2+ and DMA+ ions in ICOF-1 versus simulation time.

To quantify the exchange dynamics, a pseudo-second order (PSO) rate equation is proposed

dqt = k2 (qe − qt )2 dt

(2)

where qt is the amount of Pb2+ in the ICOF-1 at time t, k2 rate constant, qe is the amount at equilibrium. By integration equation (2), we have 8


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qt =

t

(3)

 1 t  +   2  k2 qe qe 

Equation (3) is used to fit the simulated Pb2+ ~ t profile in Figure 3 (based on mg/g of ICOF-1). As shown in Figure S2, the PSO model can fit the simulation result well. The values of qe and k2 are 12.68 mg/g and 0.0012 g/(mg⋅ns), respectively. 0.04

ρN (nm-3)

(a) Pb2+

0 ns 25 ns 256 ns

0.03

0.02

0.01

0.00 0

5

10

15

20

25

30

35

40

45

50

55

z (nm) 8

(b) DMA+

0 ns 25 ns 256 ns

6

ρN (nm-3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

2

0 0

5

10

15

20

25

30

35

40

45

50

55

z (nm)

Figure 4. Density profiles of (a) Pb2+ and (b) DMA+ at t = 0, 25 and 256 ns. The dashed lines indicate the solution/ICOF-1 interfaces. The distributions of Pb2+ and DMA+ ions at different times are depicted by their density profiles in Figure 4. The density profiles were calculated by dividing the simulation box into 100 identical slices along the z axis, and the number density (per nm3) in each slice was estimated. As

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10

(a)

Ecoul (kJ/mol)

0 -10 -20 -30

DMA+ Pb2+

-40 -50 0

2

4

6

8

10

12

14

10

12

14

10

12

14

z (nm) 1

(b)

EvdW (kJ/mol)

0 -1 -2

DMA+ Pb2+

-3 -4 0

2

4

6

8

z (nm) 10

(c) 0

Etotal (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-10 -20 -30

DMA+ 2+ Pb

-40 -50 0

2

4

6

8

z (nm)

Figure 5. (a) Electrostatic energy (Ecoul) (b) van der Waals energy (EvdW) (c) total energy (Etotal) between a DMA+ or Pb2+ ion and ICOF-1. The dashed lines indicate the solution/ICOF-1 interfaces.

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can be seen, all the DMA+ ions are initially in the ICOF-1, whereas the Pb2+ ions are in the solution. At t = 25 ns, i.e. after initiation of ion exchange, the Pb2+ ions move from the solution into the ICOF-1; thus, the density of Pb2+ increases in the ICOF-1 but decreases simultaneously in the solution. The opposite trend is observed for the DMA+ ions with density in the solution increases from zero, particular at the solution/ICOF-1 interfaces. The density profiles vary with time until dynamic equilibrium is reached at approximately 252 ns. Thereafter, all the Pb2+ ions reside in the ICOF-1 with pronounced peaks. Figure 5 depicts the electrostatic (Ecoul), van der Waals (EvdW) and total energy (Etotal) between a single Pb2+ or DMA+ ion with the ICOF-1 along the z axis. All the energy terms are zero far away from the ICOF-1, but decrease (more negative) upon moving towards the ICOF-1 due to the attractive interaction exerted by the negatively charged ICOF-1 framework. At the interface, the attraction is strong and the energy terms decrease sharply, particularly for Ecoul and finally reach a plateau in the ICOF-1 interior. The Pb2+ ion possesses a larger Ecoul (−37.8 kJ/mol) than the DMA+ ion (−22.8 kJ/mol), revealing that the Pb2+ interacts more strongly with the ICOF-1 framework. The EvdW for each ion share a similar shape, but the numerical value is much lower than Ecoul. In other words, the total energy is dominated by the electrostatic interaction. In contract to Ecoul, the EvdW for Pb2+ is lower (−1.3 kJ/mol) than for DMA+ (−1.7 kJ/mol) in the ICOF-1 interior. Obviously, with more atoms in DMA+, EvdW is higher. By combining the two energy terms, Etotal follows the similar trend. More precisely, the Etotal for Pb2+ and DMA+ ions are −39.2 and −24.5 kJ/mol, respectively. Therefore, Pb2+ is more strongly attracted by the ICOF-1 compared with DMA+. This is the driving force for the exchange between Pb2+ and DMA+, also the reason that Pb2+ cannot move back to the solution once exchanged.

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3.2 Structure and Dynamics of Ions in ICOF-1 The structure of ions in the ICOF-1 is characterized by radial distribution functions g(r)

gij ( r ) =

N ij ( r, r + ∆r )V

(4)

4π r 2 ∆r N i N j

where r is the distance between atoms i and j, Nij (r, r + ∆r ) is the number of atom j around i within a shell from r to r + ∆r, V is the volume of framework, Ni and Nj are the numbers of atoms i and j, respectively. Figure 6 shows the g(r) for Pb2+ and DMA+ ions around the framework atoms of the ICOF-1. Particularly, the most favorable framework atoms O1 and C2 atoms (see Figure 2c) were selected to calculate g(r) for Pb2+ and the O1 and C3 atoms were selected for DMA+. Clearly, the Pb2+ has pronounced peaks around all the framework atoms; nevertheless, the DMA+ shows broader and lower peaks. As discussed above, this is attributed to the stronger interaction of Pb2+ with the ICOF-1 than DMA+. 4

4

(a)

(b)

2+

Pb - O1 of ICOF-1 Pb2+ - C2 of ICOF-1

+

HD of DMA - O1 of HA of DMA+ - O1 of + HA of DMA - C3 of + HD of DMA - C3 of

3

g(r)

3

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

1

ICOF-1 ICOF-1 ICOF-1 ICOF-1

2

1

0

0 0

1

2

3

4

0

r (nm)

1

2

3

4

r (nm)

Figure 6. Radial distribution functions for (a) Pb2+ ion around the atoms O1 and C2 of ICOF-1 (b) the HD and HA atoms of DMA+ around the atoms O1 and C3 of ICOF-1. In addition, the dynamics of Pb2+ and DMA+ ions in the ICOF-1 are quantified by meansquared displacement (MSD)

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MSD (t ) =

1 N

N

∑ 〈| r (t ) − r (0) | 〉 2

i

(5)

i

i =1

where N is the number of glucose and ri(t) is the position of the ith glucose at time t. To improve statistical accuracy, the multiple time-origin method was used to estimate the MSD. As shown in Figure 7, the MSD of Pb2+ is small, which denotes the strong interaction of Pb2+ with the framework and thus leading to a low mobility of Pb2+. In contrast, DMA+ exhibits a larger MSD because of the relatively weaker interactions of DMA+ with the framework. Overall, the stronger interaction of Pb2+ with the ICOF-1 drives the exchange between Pb2+ and DMA+. It should be noted that the MDS ~ t curves in a logarithm scale (Figure S3) have a slope less than one. This implies the motion of the ions in the ICOF-1 is not normal diffusion within the simulation time scale. 6 5

MSD (nm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

DMA+ 3 2

Pb2+

1 0 0

1

2

3

4

5

t (ns)

Figure 7. Mean-squared displacements of DMA+ and Pb2+ ions in ICOF-1. 4. CONCLUSIONS From molecular simulations, a thermally and chemically stable ionic COF (ICOF-1) has been investigated for the removal of Pb2+ ions from PbCl2 aqueous solution. Energy analysis demonstrates that the interaction energies of Pb2+ and DMA+ with the ICOF-1 framework are

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−39.2 and −24.5 kJ/mol, respectively. Therefore, Pb2+ is more strongly attracted by the ICOF-1 compared with DMA+, which is the driving force for the exchange between Pb2+ and DMA+. At 600 ppm, all the Pb2+ ions are completely exchanged, while none of the Cl− ion is exchanged due to repulsion by the ICOF-1. In the first 50 ns, the exchange rate is fast, but slowed down at a later stage. Once exchanged, the Pb2+ ions reside in the ICOF-1 and cannot move back to the solution; nevertheless, there is a dynamic equilibrium for the [DMA+] ions in the framework and the solution. Due to the stronger interaction with the framework, Pb2+ ions are less mobile than DMA+ ions. This study provides clear bottom-up understanding for the observed ion exchange process, which is useful for the development of new porous materials towards the efficient removal of heavy metal ions from aqueous solution.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Atomic types and charges; Lennard-Jones parameters of ICOF-1, [DMA+], Pb2+ and Cl−; amount of Pb2+ ions in the ICOF-1 from simulation and model fitting; mean-squared displacements in a logarithm scale.

Acknowledgements We gratefully thank the A*star of Singapore (R-279-000-431-305) and the National University of Singapore (R-279-000-474-112) for financial support.

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A molecular simulation study is reported for Pb2+ exchange with DMA+ in an ionic covalent−organic framework.

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A Molecular Simulation Study - ACS Publications

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