Experimental and DFT investigation on the adsorption

Journal of Molecular Liquids 263 (2018) 390–398

Contents lists available at ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Experimental and DFT investigation on the adsorption mechanism of silica gel supported sulfur-capped PAMAM dendrimers for Ag(I) Panpan Zhang, Yuzhong Niu ⁎, Wenzhu Qiao, Zhongxin Xue, Liangjiu Bai, Hou Chen School of Chemistry and Materials Science, Ludong University, Yantai 264025, PR China

a r t i c l e

i n f o

Article history: Received 15 March 2018 Received in revised form 24 April 2018 Accepted 5 May 2018 Available online 8 May 2018 Keywords: Adsorption Sulfur-capped PAMAM dendrimer DFT Interaction, mechanism

a b s t r a c t The adsorption mechanism of silica gel supported sulfur-capped PAMAM dendrimers (SG-MITC-G0~SG-MITCG2.0) for Ag(I) from aqueous solution was investigated combined experimental and DFT methods. The effects of pH, contact time, temperature, and coexisting metal ions were investigated systematically. Results indicated that the optimum adsorption pH was 6. Adsorption kinetic showed that the adsorption equilibrium was reached at about 270 min. The kinetics adsorption process can be described by pseudo-second-order model and is controlled by film diffusion process. Adsorption isotherms indicated the adsorption capacity increased with the increase of temperature and initial Ag(I) concentration. The isotherm adsorption process can be well fitted by monolayer Langmuir model and proceeded by chemical mechanism. Thermodynamic parameters demonstrated the adsorption was spontaneous, endothermic in nature. Adsorption selectivity showed that SG-MITC-G0~ SGMITC-G2.0 exhibited excellent selectivity for Ag(I) in the presence of Zn(II), Fe (II), and Cd(II). DFT calculation demonstrated that G0-MITC tend to bind Ag(I) by sulfur atom, while G1.0-MITC interacts with Ag(I) by sulfur atom, carbonyl oxygen atom, and tertiary amine group to form bi-, tri-, tetra-, and penta-dentate complexes. Charge transfer from G0-MITC and G1.0-MITC to Ag(I) occurred during the coordination. © 2018 Elsevier B.V. All rights reserved.

1. Introduction With the rapid industrial development, a large amount of metal ions are discharged into wastewater from the fields of chemical engineering, metallurgy, electronics, etc. [1,2]. These metal ions can cause serious environmental problems due to their toxicity [3,4]. Among these metal ions, the pollution of Ag(I) has attracted considerable attention as it was widely applied in the fields of aerospace, fuel cell, and electronic industry due to its excellent ductility, electrical and thermal conductivity [5,6]. The presence of Ag(I) in aqueous media could be accumulated in aquatic organisms through the food chain and pose great harm to human and aquatic ecosystems [7]. Hence, the removal of Ag(I) from aqueous solution is of vital significance for environmental protection. The conventional approaches for the removal of Ag(I) include chemical precipitation, adsorption, ion-exchange, solvent extraction, and so on [5,7,8]. Adsorption is widely used in virtue of its simple operation, cost-effectiveness, and high efficiency [7–10]. In the past decades, adsorbents such as cellulose [2], zeolite [10], magnetic composite [9], silica gel [11], chitosan [12] have been employed to remove Ag(I). Construction of silica gel based adsorbent has attracted considerable attention due to its comparative high specific surface area, excellent thermal and mechanical stability [13–15]. The adsorption performance of these ⁎ Corresponding author. E-mail address: [email protected] (Y. Niu).

https://doi.org/10.1016/j.molliq.2018.05.023 0167-7322/© 2018 Elsevier B.V. All rights reserved.

adsorbents is mainly depends on the functional groups that covalently bonded on the surface of silica gel [13]. Among these functional groups, polyamidoamine (PAMAM) dendrimer is highly branched macromolecule with unique structure that has attracted great interests in metal ion adsorption and separation [16,17]. It contains a great deal of nitrogen and oxygen groups and can be readily functionalized to enhance the metal ion binding ability [18–20]. For example, Su [21] reported the synthesis of silica gel supported amino-terminated G4.0 PAMAM, and used for the successful preconcentration and separation of Pd(II). Gao [22] reported the synthesis of mesoporous silica immobilized EDTA functionalized PAMAM dendrimer to selective adsorption of Cr (III), Pb (II), and Zn(II) from aqueous solution. Our group also reported the synthesis of silica gel supported salicylaldehyde modified PAMAM dendrimers for effective removal of Hg(II) from aqueous solution [13]. On the basis of hard-soft acid-base (HSAB) theory, sulfur-containing functional group was proved to exhibited excellent binding ability for Ag(I) [23,24]. Therefore, it might be reasonably presume that attaching of sulfur contacting ligand to the periphery of PAMAM dendrimers would construct novel adsorbent with excellent adsorption capacity and selectivity for Ag(I). In our previous study, a series of silica gel supported sulfur-capped PAMAM dendrimers (SG-MITC-G0~SG-MITC-G2.0) were synthesized and used for the efficient removal of Hg(II) from aqueous solution [25]. The present study aimed to evaluate the feasibility to remove Ag (I) from aqueous solution by SG-MITC-G0~SG-MITC-G2.0. The effects

P. Zhang et al. / Journal of Molecular Liquids 263 (2018) 390–398

of solution pH, contact time, initial concentration of Ag(I) were investigated. The adsorption mechanism was revealed by the analysis of kinetic and isotherm adsorption data. The nature of the interaction between sulfur-capped PAMAM dendrimers and Ag(I) was proposed on the atomic level based on the density functional theory (DFT) calculations.

391

concentration of Ag(I) was determined by AAS, and the adsorption capacity was calculated according to Eq. (1):

q¼

ðC 0 −C ÞV W

ð1Þ

where q (mmol∙g−1) is the adsorption amount. C0 and C (mmol∙L−1) are the initial and equilibrium concentration of Ag(I), respectively. V (mL) is the solution volume, and W (g) is the weight of adsorbent.

2. Experimental 2.1. Materials and methods Silica gel supported sulfur-capped PAMAM dendrimers (SG-MITCG0~SG-MITC-G2.0) were synthesized by divergent method according to the procedures described in our previous reported [25]. First, amino groups were introduced onto the surface of silica gel to obtained SGG0 by the reaction of silanol group with 3-aminopropyltriethoxysilane, and SiO2-G0.5 was achieved by the Michael addition of methylacrylate with the amino group of SG-G0. Then, SiO2-G1.0~SiO2-G2.0 were prepared by the iterative amidation of ester group with ethylenediamine and the subsequent Michael addition reaction of methylacrylate with amino group. Finally, SG-G0, SG-G1.0, and SG-G2.0 were functionalized with methyl isothiocyanate to obtain SG-MITC-G0, SG-MITC-G1.0, and SG-MITC-G2.0, respectively. The structure diagrams of SG-MITCG0~SG-MITC-G2.0 were shown in Scheme 1. AgNO3 was purchased from Sinopharm Chemical Reagent Co., Ltd., China. The concentration of Ag(I) was determined on a GBC-932 atomic adsorption spectrophotometer (AAS, GBC, Australia).

2.3. Adsorption kinetics The adsorption kinetics were carried out by using batch method [26]: a series of 100 mL iodine flasks were equipped with 20 mg adsorbent and 20 mL 0.002 mol∙L−1 Ag(I) solution. The mixture was shaken at 25 °C, and the concentration of Ag(I) was determined at different time intervals. 2.4. Adsorption isotherms The following procedure were used to performed adsorption isotherms: a series of 100 mL iodine flasks were charged with 20 mg adsorbent, and 20 mL Ag(I) solution with different initial concentration. The mixture was shaken for 12 h at different temperatures. Then the concentration of Ag(I) was determined and the adsorption capacity was calculated. 2.5. Adsorption selectivity

2.2. Effect of pH on the adsorption The effect of pH on the adsorption was performed by the following procedure: about 20 mg adsorbent and 20 mL of 0.002 mol∙L−1 Ag (I) solution with different pH were charged into a series of 100 mL iodine flasks. The mixture was shaken for 12 h at 25 °C. After that, the

Adsorption selectivity were conducted by the following procedure [27]: about 20 mg adsorbent was placed into the iodine flask, then 20 mL Ag(I) and coexisting metal ion solution was added. The concentration of Ag(I) and other metal ions were all 0.002 mol∙L−1 with pH = 6. The mixture was shaken for 12 h at 25 °C. After the adsorption completed,

Scheme 1. The structure diagram of SG-MITC-G0~SG-MITC-G2.0.

392


the concentration of Ag(I) and coexisting metal ions were determined and the adsorption capacity was calculated respectively. 2.6. DFT study The interaction mechanism between sulfur-capped PAMAM dendrimers and Ag(I) was calculated with Gaussian 03 program using DFT method [13]. As G2.0-MITC had similar structure with G1.0-MITC due to geometry symmetry of dendrimer, G0-MITC and G1.0-MITC were chosen as representative. Geometry of G0-MITC, G1.0-MITC, and the possible complexes formed with Ag(I) was optimized at B3LYP/ 6–31 + G(d) (LANL2DZ for Ag(I)) level [26]. Vibrational frequencies have been calculated at the same level to ensure the stationary points. Natural bond orbital (NBO) analysis carried out to reveal the nature of the interaction and the binding energy at B3LYP/6–311++G(d, p) (LANL2DZ for Ag(I)) level) [26]. 3. Results and discussion Fig. 2. Adsorption kinetic curves for Ag (I).

3.1. Effect of pH on adsorption Solution pH is a significant factor in the adsorption because it not only influences the surface active binding sites of the adsorbent but also affects the existence state of metal ion in solution. The solution pH on the adsorption of SG-MITC-G0~SG-MITC-G2.0 for Ag(I) was shown in Fig. 1. As can be observed from Fig. 1, the adsorption capacity of SG-MITC-G0~ SG-MITC-G2.0 for Ag(I) increased with the increasing of solution pH, and the optimum adsorption was reached at pH 6. At low pH, the functional groups of sulfur-capped PAMAM dendrimers were presented as protonated form which resulted in the reduction of active binding sites [25]. In addition, the exist of electrostatic repulsion between protonated functional groups and Ag(I) further prevented the contact of Ag(I) with SG-MITC-G0 ~ SG-MITC-G2.0. With the increase of pH, the protonation decreased and the number of active binding sites increased, leading to the increase of adsorption capacity. However, when the pH was higher than 6, Ag(I) can be hydrolyzed to produce hydroxide precipitation and the capacity decreased [10]. Hence, the optimum pH was 6 and the subsequent adsorption experiments were all performed under this condition. 3.2. Adsorption kinetics The adsorption kinetic curves of SG-MITC-G0~SG-MITC-G2.0 for Ag (I) was shown in Fig. 2. It is apparent that the adsorption capacity

increased rapidly during the initial stage within 100 min, and then the adsorption rate slowed down gradually until adsorption equilibrium was reached at about 360 min. This phenomenon was closely related with the concentration of Ag(I) and the number of the active binding sites. During the initial stage, the concentration of Ag(I) was higher and there were plenty of active binding sites which enable the quick capture of Ag(I) by the adsorbents. With the process of adsorption, both Ag(I) concentration and the number of active binding sites were all decreased, leading to the decrease of adsorption rate. It can also observed that the adsorption capacity for Ag(I) followed the order of SGMITC-G2.0NSG-MITC-G1.0NSG-MITC-G0, indicating the adsorption capacity increased with the increase of dendrimer generation as high generation sulfur-capped PAMAM dendrimer possess more functional groups. Pseudo-first- and pseudo-second-order models were employed to reveal the kinetic adsorption mechanism. The linear form of the two models can be expressed by Eqs. (2) and (3), respectively [26,28–30]. ln ðqe −qÞ ¼ ln qe −k1 t t 1 1 þ t ¼ q k2 q2e qe

ð2Þ ð3Þ

The fitting parameters of pseudo-first- and pseudo-second-order models were listed in Table 1. As can be seen from Table 1, The correlation coefficient (R2) is one of the important parameters to evaluate whether the experimental data is consistent with theoretical model. As can be seen from Table 1, the correlation coefficients of pseudosecond-order model were higher than those of pseudo-first-order model, implying pseudo-second-order model can better describe the kinetic adsorption. Furthermore, the calculated adsorption capacity (qe, cal) obtained from pseudo-second-order model is more close to the experimental data, further demonstrating the validity of describing adsorption kinetic process by pseudo-second-order model. In order to ensure whether film diffusion or intraparticle diffusion was the rate controlling step, Boyd film diffusion model as described by Eq. (4) was adopted to analyze the kinetic adsorption data [31]. F ¼ 1‐ Fig. 1. Effect of pH on the adsorption for Ag(I).

∞ 6 X 1 exp ‐n2 Bt π2 n¼1 n2

ð4Þ


393

Table 1 The parameters of pseudo-first- and pseudo-second-order models for Ag(I). Adsorbents

SG-MITC-G0 SG-MITC-G1.0 SG-MITC-G2.0

qe, exp (mmol⋅g−1)

0.89 0.93 1.11

Pseudo-first-order kinetics

Pseudo-second-order kinetics

k1 (min−1)

qe,cal (mmol⋅g−1)

R21

k2 (g⋅mmol−1⋅min −1)

qe,cal (mmol⋅g−1)

R22

0.0127 0.0128 0.0127

0.37 0.32 0.46

0.9721 0.9873 0.9749

0.0745 0.1008 0.0687

0.92 0.92 1.13

0.9990 0.9993 0.9967

Table 2 Linear equations of Bt-t and the corresponding parameters. Adsorbents

Linear equation

Intercept error

R2

SG-MITC-G0 SG-MITC-G1.0 SG-MITC-G2.0

Bt = 0.01208 t + 0.4337 Bt = 0.01301 t + 0.5763 Bt = 0.01283 t + 0.4623

0.0650 0.0467 0.0440

0.9718 0.9873 0.9883

diffusion was the rate controlling step [32]. The fitting results were presented in Table 2. As can be seen from Table 2, the fitting curves exhibited excellent linearity and do not pass through the origin, indicating the film diffusion is rate controlling step.

3.3. Adsorption isotherms where n is an integer defines the infinite series solution; B is time constant; F is the fractional attainment of equilibrium at time t, which can be obtained by Eq. (5): F¼

qt qe

ð5Þ

where qt and qe are the adsorption capacity at time t and equilibrium (mmol∙g−1). The values of Bt can be obtained by the corresponding F value [32]. The linear fitting of Bt vs t can be used to determine the rate controlling step. The rate controlling step was conformed to be intraparticle diffusion if the fitting curve pass through the origin, otherwise, the film

Adsorption isotherm is used to describe the interactive behavior between adsorbate and adsorbent. It can provide vital information for optimizing the application of the adsorbent. The adsorption isotherm of Ag (I) were shown in Fig. 3. As can be seen from Fig. 3, the adsorption capacity of SG-MITC-G0~SG-MITC-G2.0 for Ag(I) increased with the increase of temperature and initial concentration of Ag(I). The reason for the phenomena can be attributed to the endothermic nature of the adsorption and the greater driving force by higher concentration gradient pressure [25]. In order to well understand the isotherm adsorption mechanism, Langmuir and Freundich models were used to fit the isotherm adsorption data. Langmuir model describe the adsorption occurs on a uniform surface by monolayer adsorption without any interaction between the

Fig. 3. Isotherm adsorption curves for Ag(I).

394


Table 3 The parameters of Langmuir and Freundlich models for Ag(I). Adsorbents

T(°C)

SG-MITC-G0

Langmuir model

15 25 35 15 25 35 15 25 35

SG-MITC-G1.0

SG-MITC-G2.0

Freundlich model

q(mmol∙g−1)

KL(ml∙mmol−1)

R2L

KF (mmol∙g−1)

n

R2F

1.01 1.16 1.24 1.06 1.24 1.49 1.20 1.30 1.39

776.1 992.2 1044.8 2170.4 2620.9 3523.1 1000.8 1450.6 1832.1

0.9976 0.9952 0.9965 0.9964 0.9953 0.9957 0.9957 0.9943 0.9944

5.78 4.91 5.34 2.27 2.39 2.52 5.20 4.04 3.77

2.81 3.24 3.24 6.20 7.16 8.89 3.22 4.20 4.81

0.9837 0.9905 0.9911 0.9920 0.9732 0.9534 0.9937 0.9918 0.9880

adsorbed ions. The linear from of Langmuir model can be expressed by Eq. (6) [13,26,33]. Ce Ce 1 ¼ þ qe qm qm K L

ð6Þ

where Ce (mmol∙mL−1) and qe (mmol∙g−1) are the equilibrium concentration and equilibrium adsorption capacity, respectively. qm (mmol∙g−1) is maximum adsorption capacity, and KL(mL∙mmol−1) is the Langmuir constant. Freundlich model is an empirical regularity and assumes the adsorption occurs on inhomogeneous surface by multilayer adsorption. The linear form of Freundlich equation is described by Eq. (7) [13,26]. ln qe ¼ ln K F þ

ln C e n

ð7Þ

where KF(mL∙mmol−1) is Freundlish constant, and n is integer. The fitting results of Langmuir and Freundlich models were shown in Table 3. It is clear that the correlation coefficients of Langmuir model were higher than Freundlish model, indicating the adsorption for Ag(I) can be well described by the Langmuir model with monolayer adsorption behavior. The maximum adsorption capacity (qm) for SGMITC-G0~ SG-MITC-G2.0 at 25 °C were 1.16, 1.24, and 1.30 mmol∙g−1, respectively. The comparison of qm with other alternative adsorbents Table 4 Comparison of the maximum adsorption capacities (qm) with alternative adsorbents. Adsorbents

qm (mmol∙g−1)

References

SG-MITC-G0 SG-MITC-G1.0 SG-MITC-G2.0 Ag+-imprinted biosorbent Nanocelluloses Functionalized Nano-TiO2 Klebsiella sp. 3S1 Clinoptilolite Chitosan

1.16 1.24 1.30 1.85 1.26 1.19 1.06 0.40 0.31

This study This study This study [5] [2] [34] [7] [35] [36]

were listed in Table 4. As can be seen from Table 4, SG-MITC-G0~SGMITC-G2.0 exhibited relative higher adsorption capacity than the most of the alternative adsorbents. The results indicated that SG-MITC-G0 ~ SG-MITC-G2.0 can be used as promising adsorbents for the removal of Ag(I) form aqueous solution. In order to further estimate whether the adsorption process is physical or chemical, Dubinin−Radushkevich (D − R) model was employed to analyze the equilibrium data. The D − R equation can be described by Eq. (8) [13,37]. ln qe ¼ ln qm −βε2

ð8Þ

whereβ(mol2⋅J−2) is the activity coefficient of average free energy, and ε is Polanyi potential described as Eq. (9). 1 ε ¼ RT ln 1 þ Ce

ð9Þ

Then, the values of average free energy (E, kJ⋅mol−1) can be calculated by Eq. (10). 1 E ¼ pffiffiffiffiffiffi 2β

ð10Þ

The adsorption mechanism can be distinguished according to the E value. If the E value lies between 8 and 16 kJ⋅mol−1, the adsorption proceeds chemically. Otherwise, if the value below 8 kJ⋅mol−1, the adsorption belong to physical adsorption. The fitting results of D-R model were shown in Table 5. It is clearly that the E values were all higher than 8 kJ⋅mol−1, indicated the adsorption process was chemical in nature. The thermodynamics parameters of the adsorption, such as Gibbs free energy changes (ΔG), enthalpy (ΔH), and entropy (ΔS) were calculated by the following thermodynamic equations [38,39]: ln K L ¼

ΔS ΔH − R RT

ð11Þ

ΔG ¼ ΔH−TΔS

ð12Þ

Table 5 The fitting parameters of D-R model for Ag(I). Adsorbent

T (°C)

Linear equation

qm (mmol·g−1)

k (mol2·J−2)

E (kJ·mol−1)

R2

SG-MITC-G0

15 25 35 15 25 35 15 25 35

y = −2.65·10−9x + 0.76 y = −3.97·10−9x + 0.55 y = −3.81·10−9x + 0.65 y = −2.19·10−9x + 0.32 y = −2.38·10−9x + 0.56 y = −2.29·10−9x + 0.77 y = −4.20·10−9x + 0.69 y = −3.01·10−9x + 0.63 y = −2.47·10−9x + 0.67

2.14 1.73 1.92 1.38 1.74 2.16 1.98 1.87 1.96

2.65·10−9 3.97·10−9 3.81·10−9 2.19·10−9 2.38·10−9 2.29·10−9 4.20·10−9 3.01·10−9 2.47·10−9

13.76 11.22 11.47 15.11 17.15 20.85 10.91 12.89 14.23

0.9968 0.9969 0.9702 0.9813 0.9921 0.9778 0.9838 0.9716 0.9410

SG-MITC-G1.0

SG-MITC-G2.0

P. Zhang et al. / Journal of Molecular Liquids 263 (2018) 390–398 Table 6 Thermodynamics parameters for the adsorption of Ag(I). Adsorbent

T(°C)

△G(kJ mol−1)

△H(kJ mol−1)

△S(J mol−1 K−1)

SG-MITC-G0

15 25 35 15 25 35 15 25 35

−27.01 −27.96 −28.89 −36.16 −37.42 −38.67 −38.77 −40.12 −41.47

11.01

93.82

17.82

125.57

22.19

134.65

SG-MITC-G1.0

SG-MITC-G2.0

where KL is the Langmuir constant (mL⋅mmol−1). R is the gas constant (8.314 J⋅mol−1⋅K−1), and T is a temperature (K). The calculated thermodynamics parameters were presented in Table 6. As can be seen from Table 6, ΔG values are all negative and the values decreased with the increasing temperature. This result indicated that the adsorption of Ag(I) is spontaneous and favors high temperature. The values of ΔH and ΔS are all positive, indicating that the adsorption process is an endothermic and randomness increased process. The increase of randomness mainly attributed to the release of hydration water molecules during the adsorption. Before adsorption, Ag (I) exist in the solvation form with water molecules surrounded. Then, these water molecules would be partially released during the adsorption due to the coordination of SG-MITC-G0~SG-MITC-G2.0 with Ag (I) [26]. Therefore, the entropy change was positive. 3.4. Adsorption selectivity The adsorption selectivity of SG-MITC-G0~SG-MITC-G2.0 were measured by selecting a series of mixed metal ions system as representatives,

Table 7 Adsorption selectivity for Ag(I). Adsorbent

Systems

Metal ions

q (mmol/g)

Selective coefficienta

SG-MITC-G0

Ag(I)-Ni(II)

Ag(I) Ni(II) Ag(I) Pb(II) Ag(I) Zn(II) Ag(I) Fe(II) Ag(I) Cd(II) Ag(I) Ni(II) Ag(I) Pb(II) Ag(I) Zn(II) Ag(I) Fe(II) Ag(I) Cd(II) Ag(I) Ni(II) Ag(I) Pb(II) Ag(I) Zn(II) Ag(I) Fe(II) Ag(I) Cd(II)

0.83 0.17 0.93 0.08 1.01 0 1.01 0 1.01 0 0.97 0.15 0.95 0.07 1.12 0 1.12 0 1.12 0 0.93 0.06 0.95 0.20 1.32 0 1.32 0 1.32 0

4.88

Ag(I)-Pb(II) Ag(I)-Zn(II) Ag(I)-Fe(II) Ag(I)-Cd(II) SG-MITC-G1.0

Ag(I)-Ni(II) Ag(I)-Pb(II) Ag(I)-Zn(II) Ag(I)-Fe(II) Ag(I)-Cd(II)

SG-MITC-G2.0

Ag(I)-Ni(II) Ag(I)-Pb(II) Ag(I)-Zn(II) Ag(I)-Fe(II) Ag(I)-Cd(II)

11.63 ∞ ∞ ∞ 6.47 13.57 ∞ ∞ ∞ 13.3 4.11 ∞ ∞ ∞

a The selective coefficient were the ratio of adsorption capacities of metal ions in binary mixture.

395

such as Ag(I)-Ni(II), Ag(I)-Pb(II), Ag(I)-Zn(II), Ag(I)-Fe(II), and Ag(I)-Cd (II). The selective adsorption results were compiled in Table 7. It is obvious that the adsorption capacity for Ag(I) was higher than the selected metal ions. Particularly, SG-MITC-G0~SG-MITC-G2.0 exhibited 100% selectivity for Ag(I). in the system of Ag(I)-Zn(II), Ag(I)-Fe(II), and Ag(I)-Cd(II). The results indicated that the SG-MITC-G0~SG-MITC-G2.0 exhibited good adsorption selectivity, and can be potentially used for the removal and selective recycle of Ag(I) from aqueous solution. This phenomenon can be reasonably interpreted by HSAB theory as sulfur-containing functional group exhibits remarkable affinity and adsorption selectivity toward Ag(I) than other metal ions [23,24]. 3.5. DFT calculations DFT method is a value tool that can provide better understanding to the nature of the interaction during the adsorption [40,41]. The optimized geometries of the complexes are illustrated in Fig.4, and the corresponding geometry parameters are listed in Table 8. It is clearly that G0-MITC intends to coordinate with Ag(I) through sulfur atom by monodentate coordination with the binding energy of −570.96 kcal/ mol. The bond distance and NAO bond order of S\\Ag is 2.45 Å and 0.32. For G1.0-MITC, it interacts with Ag(I) to form bi-, tri-, tetra-, and penta-dentate complexes during the adsorption process as shown by G1.0-MITC- Ag(I)-1~G1.0-MITC-Ag(I)-4. For G1.0-MITC-Ag(I)-1, G1.0MITC acts as bidentate ligand to chelate with Ag(I) by peripheral sulfur atoms. The S\\Ag bond length and bond order for G1.0-MITC-Ag(I)-1 are 2.46 Å and 0.29, which is similar to that of G0-MITC-Ag(I). For G1.0-MITC-Ag(I)-2, G1.0-MITC tends to coordinate with Ag(I) by two carbonyl oxygen atoms and tertiary amine to form tri-coordinated chelate. The N\\Ag bond length is 2.54 Å, while the bond lengths of O\\Ag are 2.23 and 2.24 Å, respectively. The bond order of N\\Ag and O\\Ag are 0.09 and 0.14, which indicated that the interaction between carbonyl oxygen atom and Ag(I) is stronger than that of tertiary amine group in the complex of G1.0-MITC- Ag(I)-2. G1.0-MITC-Ag(I)-3 was formed by the chelation of Ag(I) with one sulfur atoms, two carbonyl oxygen atoms, and nitrogen atom of tertiary amine. The O\\Ag bond lengths are 2.41 and 3.23 Å, the bond length gap is 0.82 Å, which suggest that the binding ability of oxygen atoms for Ag(I) is different in G1.0MITC-Ag(I)-3. The bond orders of the O\\Ag are 0.09 and 0.02, further demonstrates the difference in the binding ability of oxygen atom. The bond lengths of N\\Ag and S\\Ag are 2.36 Å and 2.50 Å, which are similar to the corresponding bonds in the complexes of G1.0-MITC-Ag(I)-2 and G1.0-MITC-Ag(I)-1. The bond order of N\\Ag is larger than that of G1.0-MITC-Ag(I)-2 by 0.06, indicating tertiary amine plays more important part when G1.0-MITC behaves as tetradentate ligand. With respect to G1.0-MITC- Ag(I)-4, G1.0-MITC tends to bind Ag(I) by two sulfur atoms, two carbonyl oxygen atoms, and tertiary amine nitrogen atom to form penta-dentate complexes. Similar to that of G1.0-MITC-Ag(I)2, the bond length and bond order of N\\Ag is 2.56 Å and 0.10. The bond length is longer than that of G1.0-MITC-Ag(I)-3 by 0.20 Å and the bond order is reduced by 0.05. The bond lengths of S\\Ag are 2.60 Å and 2.63 Å, while those of O\\Ag bonds are 2.64 and 2.79 Å. The binding energy (absolute value) of G1.0-MITC-Ag(I)-1~G1.0-MITC-Ag(I)-4 followed the order of G1.0-MITC-Ag(I)-1 N G1.0-MITC-Ag(I)-3 N G1.0MITC-Ag(I)-4 N G1.0-MITC-Ag(I)-2. The binding energy difference between G1.0-MITC-Ag(I)-1, G1.0-MITC-Ag(I)-3, and G1.0-MITC-Ag(I)-4 was very small, suggesting G1.0-MITC inclined to complex with Ag (I) in bi-, tetra-, and penta- coordinated manner. Natural population analysis that describes the charge distribution of the complex is an effective method to evaluate the charge transfer during the coordination [42]. The NBO partial charge of Ag(I) in G0-MITCAg(I) is 0.72, while the electron configuration of Ag(I) is 5s0.30 4d9.96 6p0.01, indicated charge transfer from ligand to Ag(I) during the coordination. The charge mainly transferred from G0-MITC to the 5 s orbital of Ag(I), only a small part transferred to the 6p orbital. Similar to G0-MITC-

396


Fig. 4. Optimized geometries for the complexes of G0-MITC and G1.0-MITC with Ag(I).

Ag(I), the NBO partial charges of Ag(I) in the complexes of G1.0-MITCAg(I)-1~G1.0-MITC-Ag(I)-4 are all lower than 1, further demonstrating charge transfer occurred from ligand to Ag(I). The electron configurations

of Ag(I) for G1.0-MITC-Ag(I)-1~G1.0-MITC-Ag(I)-4 also indicated the charge mainly transferred to the 5 s orbital of Ag(I). As can be seen from Table 8, the dipole moment value of G0-MITC changed from 5.91

Table 8 The calculated parameters of the complexes for G0-MITC and G1.0-MITC. Complexes

Binding energy (kcal/mol)

G0-MITC-Ag(I) G1.0-MITC-Ag(I)-1 G1.0-MITC-Ag(I)-2 G1.0-MITC-Ag(I)-3 G1.0-MITC-Ag(I)-4

−570.96 −620.86 −592.47 −616.43 −614.18

a b

NBO partial charge Ligand

Ag(I)

0.28 0.42 0.15 0.25 0.27

0.72 0.58 0.85 0.75 0.73

The ground-state electron configurations of free Ag(I) is 4d10. The dipole moment for G0-MITC and G1-MITC are 5.91 and 9.41, respectively.

Electron configuration of Ag(I)a

Dipole moment (Debye)b

5s0.304d9.966p0.01 5s0.504d9.906p0.01 5s0.214d9.936p0.01 5s0.314d9.926p0.02 5s0.294d9.955p0.016p0.01

3.76 3.38 6.88 7.44 3.12


397

Fig. 5. The calculated FTIR spectra G0-MITC~G1.0-MITC before and after Ag(I) adsorption.

Debye to 3.76 Debye, while the dipole moment of G1.0-MITC decreased from 9.41 Debye to 3.38, 6.88, 7.44, and 3.12 for G1.0-MITC-Ag(I)-1~ G1.0-MITC-Ag(I)-4, respectively. The results further revealed the existence of charge transfer between ligand and Ag(I), which changed the distribution of the electron density. Second-order perturbation theory analysis of the Fock matrix in NBO basis was performed to deepen the understanding of the interaction mechanism [26]. The value of stabilization energy E(2) is an important parameter to evaluate the interaction between ligand (donor) and metal ion (acceptor). For G0-MITC-Ag(I), the binding of G0-MITC for Ag(I) dominated by the σ donation of sulfur atom to the 5 s empty orbital of Ag(I) with the E(2) values of LP(S) → LP*(Ag) and LP(S) → BD* (S\\Ag) by 0.99 and 1.59 kcal/mol, respectively. Similar phenomena can also observed in the complex of G1.0-MITC-Ag(I)-1, the E(2) values of LP(S) → LP*(Ag) and LP(S) → BD*(S\\Ag) were 4.21 and 45.32 kcal/ mol, which were higher than G0-MITC-Ag(I) by 3.22 and 43.76 kcal/ mol. The result indicated that the binding ability of sulfur atoms of G1.0-MITC was higher than that of G0-MITC, which consistent with the experiment result as the adsorption capacity of SG-MITC-G1.0 was higher than SG-MITC-G0. For G1.0-MITC-Ag(I)-2, the E(2) value of LP (N) → LP*(Ag) is 12.04 kcal/mol, while those for LP(O) → LP*(Ag) were 13.49 and 19.83 kcal/mol. The facts demonstrated the binding ability of carbonyl oxygen group was superior to tertiary amine group, which agreed with the bond order analysis. The interaction for G1.0MITC-Ag(I)-3 was dominated by LP(N) → LP*(Ag), LP(N) → LP*(Ag), and LP(S) → LP*(Ag). The E(2) value of LP(N) → LP*(Ag) and LP(S) → LP*(Ag) were 23.30 and 31.40 kcal/mol, whereas those of LP(O) → LP*(Ag) were 6.94 and 0.59 kcal/mol. This suggested that tertiary amine and sulfur group were the main contributors when coordinate with Ag(I) by G1.0-MITC-Ag(I)-3 mode. As respect to G1.0-MITC-Ag (I)-4, the E(2) value of LP(N) → LP*(Ag) was 10.71 kcal/mol, while those of LP(S) → LP*(Ag) were 40.14 and 24.45 kcal/mol. However, the E(2) values of LP(O) → LP*(Ag) were merely 3.81 and 3.20 kcal/ mol, which indicated sulfur and nitrogen atoms dominated the interaction in G1.0-MITC-Ag(I)-4. The FTIR spectra based on the calculation were also employed to elucidate the interaction between G0-MITC~G1.0-MITC and Ag(I). G0MITC-Ag(I), G1.0-MITC-Ag(I)-1, and G1.0-MITC-Ag(I)-3 were selected as representative. As is shown in Fig. 5, the spectrum of G0-MITC

exhibited the characteristic C_S stretching vibration at 1382 cm−1, which is consistent with the experimental results [25]. After adsorption, the absorption bands of C_S shifted to 1392 cm−1, indicating strong interaction between C_S groups and Ag(I). In the spectrum of G1.0-MITC, C_S stretching vibration located at 1387 cm−1, while the characteristic C_O bonds appeared at 1730 cm−1. Similar to G0-MITC, the C_S absorption bond shifted to 1397 cm−1, while the C_O absorption bonds moved to 1735 cm−1. In addition, the C_O peak was diminished as compared to G1.0-MITC. These changes indicated the participation of both oxygen and sulfur atoms in the coordination with Ag(I). 4. Conclusion The adsorption property of silica gel supported sulfur-capped PAMAM dendrimers (SG-MITC-G0~SG-MITC-G2.0) for Ag(I) was investigated combined experimental and DFT methods. The effect of pH, adsorption kinetic, adsorption isotherm, and adsorption selectivity were investigated. The adsorption optimum pH was found to be 6. Adsorption kinetics can be described by pseudo-second-order model and film diffusion process was the rate controlling step. The adsorption isotherms can be well suited with the Langmuir model and occurred by monolayer adsorption behavior with chemical mechanism. Thermodynamic parameters demonstrated the adsorption was spontaneous, endothermic in nature. Adsorption selectivity showed that SG-MITC-G0~SG-MITC-G2.0 exhibited excellent selectivity for Ag(I) in the presence of Zn(II), Fe(II), and Cd(II). DFT calculation demonstrated that G0-MITC tended to bind Ag(I) by sulfur atom, while G1.0-MITC interacted with Ag(I) by sulfur atom, carbonyl oxygen atom, and tertiary amine group to form bi-, tri-, tetra-, and pentadentate complexes. Charge transfer from G0-MITC and G1.0-MITC to Ag (I) occurred during the coordination. Acknowledgements The authors are grateful for the financial support by the National Natural Science Foundation of China (Nos. 21307053, 21501087), Natural Science Foundation of Shandong Province (No. ZR2018MB039), A Project of Shandong Province Higher Educational Science and Technology Program (No. J15LD01), Science and Technology Research Program of Yantai (No. 2017ZH060), Innovation Foundation for Students of

398


Ludong University (Nos. ldu16w159, ldu16w160), and Chengxin Innovation Scholarship of Ludong University.

[21]

References [1] L. Ma, Q. Wang, S.M. Islam, Y. Liu, S. Ma, M.G. Kanatzidis, Highly selective and efficient removal of heavy metals by layered double hydroxide intercalated with the MoS2− 4 ion, J. Am. Chem. Soc. 138 (2016) 2858–2866. [2] P. Liu, P.F. Borrell, M. Božič, V. Kokol, K. Oksman, A.P. Mathew, Nanocelluloses and their phosphorylated derivatives for selective adsorption of Ag+, Cu2+ and Fe3+ from industrial effluents, J. Hazard. Mater. 294 (2015) 177–185. [3] J.Z.Y. Tan, N.M. Nursam, F. Xia, M.A. Sani, W. Li, X. Wang, R.A. Caruso, Highperformance coral reef-like carbon nitrides: synthesis and application in photocatalysis and heavy metal ion adsorption, ACS Appl. Mater. Interfaces 9 (2017) 4540–4547. [4] G. Zong, H. Chen, R. Qu, C. Wang, N. Ji, Synthesis of polyacrylonitrile-grafted crosslinked N-chlorosulfonamidated polystyrene via surface-initiated ARGET ATRP, and use of the resin in mercury removal after modification, J. Hazard. Mater. 186 (2011) 614–621. [5] H. Huo, H. Su, T. Tan, Adsorption of Ag+ by a surface molecular-imprinted biosorbent, Chem. Eng. J. 150 (2009) 139–144. [6] R. Das, S. Giri, A.L.K. Abia, B. Dhonge, A. Maity, Removal of noble metal ions (Ag+) by mercapto group-containing polypyrrole matrix and reusability of its waste material in environmental applications, ACS Sustain. Chem. Eng. 5 (2017) 2711–2724. [7] A.J. Muñoz, F. Espínola, E. Ruiz, Biosorption of Ag(I) from aqueous solutions by Klebsiella sp. 3S1, J. Hazard. Mater. 329 (2017) 166–177. [8] X. Liu, H. Chen, C. Wang, R. Qu, C. Ji, C. Sun, Y. Zhang, Synthesis of porous acrylonitrile/methyl acrylate copolymer beads by suspended emulsion polymerization and their adsorption properties after amidoximation, J. Hazard. Mater. 175 (2010) 1014–1021. [9] E. Ghasemi, A. Heydari, M. Sillanpaa, Superparamagnetic Fe3O4@EDTA nanoparticles as an efficient adsorbent for simultaneous removal of Ag(I), Hg(II), Mn(II), Zn(II), Pb (II) and Cd(II) from water and soil environmental samples, Microchem. J. 131 (2017) 51–56. [10] T. Wajima, Synthesis of zeolitic material from green tuff stone cake and its adsorption properties of silver (I) from aqueous solution, Microporous Mesoporous Mater. 233 (2016) 154–162. [11] M.Z. Hong, X. Wang, W.J. You, Z.Y. Zhuang, Y. Yu, Adsorbents based on crown ether functionalized composite mesoporous silica for selective extraction of trace silver, Chem. Eng. J. 313 (2017) 1278–1287. [12] M. Zhang, R. Helleur, Y. Zhang, Ion-imprinted chitosan gel beads for selective adsorption of Ag+ from aqueous solutions, Carbohydr. Polym. 130 (2015) 206–212. [13] Y. Niu, R. Qu, H. Chen, L. Mu, X. Liu, T. Wang, Y. Zhang, C. Sun, Synthesis of silica gel supported salicylaldehyde modified PAMAM dendrimers for the effective removal of Hg(II) from aqueous solution, J. Hazard. Mater. 278 (2014) 267–278. [14] S. Radi, Y. Toubi, M. El-Massaoudi, M. Bacquet, S. Degoutin, Y.N. Mabkhot, Efficient extraction of heavy metals from aqueous solution by novel hybrid material based on silica particles bearing new Schiff base receptor, J. Mol. Liq. 223 (2016) 112–118. [15] Q.P. Wu, R.R. You, Q.F. Lv, Y.L. Xu, W.J. You, Y. Yu, Efficient simultaneous removal of Cu(II) and Cr2O2− 7 from aqueous solution by a renewable amphoteric functionalized mesoporous silica, Chem. Eng. J. 281 (2015) 491–501. [16] D.A. Tomalia, A.M. Naylor, W.A. Goddard, Starburst dendrimers: molecular-level control of size, shape, surface chemistry, topology, and flexibility from atoms to macroscopic matter, Angew. Chem. Int. Ed. 29 (1990) 138–175. [17] D.A. Tomalia, H. Baker, J. Dewald, M. Hall, G. Kallos, S. Martin, J. Roeck, J. Ryder, P. Smith, A new class of polymers: starburst-dendritic macromolecules, Polym. J. 17 (1985) 117–132. [18] A. Maleki, B. Hayati, F. Najafi, F. Gharibi, S.W. Joo, Heavy metal adsorption from industrial wastewater by PAMAM/TiO2 nanohybrid: preparation, characterization and adsorption studies, J. Mol. Liq. 224 (2016) 95–104. [19] Q. Zhang, N. Wang, L. Zhao, T. Xu, Y. Cheng, Polyamidoamine dendronized hollow fiber membranes in the recovery of heavy metal ions, ACS Appl. Mater. Interfaces 5 (2013) 1907–1912. [20] M.R. Kotte, A.T. Kuvarega, M. Cho, B.B. Mamba, M.S. Diallo, Mixed matrix PVDF membranes with in situ synthesized PAMAM dendrimer-like particles: a new

[22]

[23] [24]

[25]

[26]

[27]

[28] [29] [30]

[31] [32] [33]

[34]

[35] [36]

[37]

[38]

[39]

[40] [41]

[42]

class of sorbents for Cu(II) recovery from aqueous solutions by ultrafiltration, Environ. Sci. Technol. 49 (2015) 9431–9442. X.Z. Wu, P. Liu, Q.S. Pu, Q.Y. Sun, Z.X. Su, Preparation of dendrimer-like polyamidoamine immobilized silica gel and its application to online preconcentration and separation palladium prior to FAAS determination, Talanta 62 (2004) 918–923. Y. Jiang, Q. Gao, H. Yu, Y. Chen, F. Deng, Intensively competitive adsorption for heavy metal ions by PAMAM-SBA-15 and EDTA-PAMAM-SBA-15 inorganic–organic hybrid materials, Microporous Mesoporous Mater. 103 (2007) 316–324. D.R. Alfonso, A.V. Cugini, D.S. Sholl, Density functional theory studies of sulfur binding on Pd, Cu and Ag and their alloys, Surf. Sci. 546 (2003) 12–26. R. Randazzo, A. Di Mauro, A. D'Urso, G.C. Messina, G. Compagnini, V. Villari, N. Micali, R. Purrello, M.E. Fragalà, Hierarchical effect behind the supramolecular chirality of silver(I)–cysteine coordination polymers, J. Phys. Chem. B 119 (2015) 4898–4904. Y. Niu, J. Yang, R. Qu, Y. Gao, N. Du, H. Chen, C. Sun, W. Wang, Synthesis of silica-gelsupported sulfur-capped PAMAM dendrimers for efficient Hg(II) adsorption: experimental and DFT study, Ind. Eng. Chem. Res. 55 (2016) 3679–3688. X. Song, Y. Niu, P. Zhang, C. Zhang, Z. Zhang, Y. Zhu, R. Qu, Removal of Co(II) from fuel ethanol by silica-gel supported PAMAM dendrimers: combined experimental and theoretical study, Fuel 199 (2017) 91–101. Y. Niu, R. Qu, C. Sun, C. Wang, H. Chen, C. Ji, Y. Zhang, X. Shao, F. Bu, Adsorption of Pb (II) from aqueous solution by silica-gel supported hyperbranched polyamidoamine dendrimers, J. Hazard. Mater. 244–245 (2013) 276–286. Y.S. Ho, Second-order kinetic model for the sorption of cadmium onto tree fern: a comparison of linear and non-linear methods, Water Res. 40 (2006) 119–125. Y.S. Ho, G. McKay, Kinetic models for the sorption of dye from aqueous solution by wood, Process Saf. Environ. Prot. 76 (1998) 183–191. J. Zhao, Y. Niu, B. Ren, H. Chen, S. Zhang, J. Jin, Y. Zhang, Synthesis of Schiff base functionalized superparamagnetic Fe3O4 composites for effective removal of Pb(II) and Cd(II) from aqueous solution, Chem. Eng. J. 347 (2018) 574–584. G.E. Boyd, A.W. Adamson, L.S. Myers, The exchange adsorption of ions from aqueous solutions by organic zeolites. II. kinetics1, J. Am. Chem. Soc. 69 (1947) 2836–2848. D. Reichenberg, Properties of ion-exchange resins in relation to their structure. III. Kinetics of exchange, J. Am. Chem. Soc. 75 (1953) 589–597. X. Song, Y. Niu, Z. Qiu, Z. Zhang, Y. Zhou, J. Zhao, H. Chen, Adsorption of Hg(II) and Ag (I) from fuel ethanol by silica gel supported sulfur-containing PAMAM dendrimers: kinetics, equilibrium and thermodynamics, Fuel 206 (2017) 80–88. N. Pourreza, S. Rastegarzadeh, A. Larki, Nano-TiO2 modified with 2mercaptobenzimidazole as an efficient adsorbent for removal of Ag(I) from aqueous solutions, J. Ind. Eng. Chem. 20 (2014) 127–132. M. Akgül, A. Karabakan, O. Acar, Y. Yürüm, Removal of silver (I) from aqueous solutions with clinoptilolite, Microporous Mesoporous Mater. 94 (2006) 99–104. Y. Yi, Y. Wang, H. Liu, Preparation of new crosslinked chitosan with crown ether and their adsorption for silver ion for antibacterial activities, Carbohydr. Polym. 53 (2003) 425–430. L.L. Zhang, D. He, J.M. Chen, Y. Liu, Biodegradation of 2-chloroaniline, 3chloroaniline, and 4-chloroaniline by a novel strain Delftia tsuruhatensis H1, J. Hazard. Mater. 179 (2010) 875–882. S. Yuan, J. Gu, Y. Zheng, W. Jiang, B. Liang, S.O. Pehkonen, Purification of phenolcontaminated water by adsorption with quaternized poly(dimethylaminopropyl methacrylamide)-grafted PVBC microspheres, J. Mater. Chem. A 3 (2015) 4620–4636. X. Li, W.C. Cao, Y.G. Liu, G.M. Zeng, W. Zeng, L. Qin, T.T. Li, Property variation of magnetic mesoporous carbon modified by aminated hollow magnetic nanospheres: synthesis, characterization, and sorption, ACS Sustain. Chem. Eng. 5 (2017) 179–188. T. Wyttenbach, D.F. Liu, M.T. Bowers, Interactions of the hormone oxytocin with divalent metal ions, J. Am. Chem. Soc. 130 (2008) 5993–6000. A.K. Singha Deb, V. Dwivedi, K. Dasgupta, S. Musharaf Ali, K.T. Shenoy, Novel amidoamine functionalized multi-walled carbon nanotubes for removal of mercury(II) ions from wastewater: combined experimental and density functional theoretical approach, Chem. Eng. J. 313 (2017) 899–911. J.R. DeBackere, H.P.A. Mercier, G.J. Schrobilgen, Noble-gas difluoride complexes of mercury(II): the syntheses and structures of Hg(OTeF5)2·1.5NgF2 (Ng = Xe, Kr) and Hg(OTeF5)2, J. Am. Chem. Soc. 136 (2014) 3888–3903.