Article pubs.acs.org/IECR
Synthesis of Silica-Gel-Supported Sulfur-Capped PAMAM Dendrimers for Efficient Hg(II) Adsorption: Experimental and DFT Study Yuzhong Niu,* Jinyun Yang, Rongjun Qu, Yanhong Gao, Na Du, Hou Chen, Changmei Sun, and Wenxiang Wang School of Chemistry and Materials Science, Ludong University, Yantai 264025, P.R. China S Supporting Information *
ABSTRACT: A series of silica-gel-supported sulfur-capped PAMAM dendrimers (SiO 2 -G0-MITC−SiO 2 -G2.0-MITC) were synthesized and used for the adsorption of Hg(II) from aqueous solution. The optimum adsorption pH was found to be 6. Adsorption kinetics indicated that equilibrium can be approached in about 220 min and that the adsorption capacity increased with increasing generation of sulfur-capped PAMAM dendrimers. The kinetics of the adsorption process was found to be controlled by film diffusion and to follow a pseudo-secondorder model. The adsorption isotherms were fitted well by the Langmuir isotherm model, and adsorption was found to take place by a chemical mechanism. Thermodynamic analysis demonstrated that the adsorption was a spontaneous, endothermic, and randomness-increasing process. Adsorption selectivity experiments showed that SiO2-G0-MITC−SiO2-G2.0-MITC can selectively adsorb Hg(II) from binary systems containing Hg(II) with Ni(II), Cd(II), Fe(III), and Zn(II). DFT calculations revealed that G0-MITC interacts with Hg(II) through the S atom in a monocoordinated manner, whereas G1.0-MITC behaves as a pentadentate ligand to coordinate with Hg(II) through the N atom of the tertiary amine group, the O atoms of the amide groups, and the S atoms. Charge transfer from G0-MITC and G1.0-MITC to Hg(II) was found to occur during the adsorption process.
1. INTRODUCTION Water contamination by heavy-metal ions is a serious environmental problem because of the toxicity and hazardous effects to human health of such ions.1 Hg(II) is considered to be highly toxic, as it can accumulate in the human body and cause detrimental effects on the liver, kidney, digestive system, and neurological system.2−4 Hence, the selective removal of Hg(II) from aqueous media is of vital importance. The traditional methods for Hg(II) removal include precipitation, membrane filtration, coagulation, ion exchange, and adsorption.3,5 Among these approaches, adsorption is widely employed because of its high efficiency, low cost, excellent reversibility, and simplicity.6,7 Therefore, the development of novel adsorbents for effective Hg(II) removal is still a great challenge. Adsorption capacity and selectivity, which mainly depend on the type and number of organic functional groups, are the main factors that dominate the performance of any adsorbent.8 In the past decades, a wide variety of functional groups containing nitrogen, oxygen, and sulfur have been used for the removal of Hg(II). 7,9−11 Among these functional groups, poly(amidoamine) (PAMAM) dendrimers have received great attention because of their unique structures and properties © 2016 American Chemical Society
such as three-dimensional structure, availability of plenty of nitrogen and oxygen groups to capture metal ions, and ease of functionalization.7,12 However, both PAMAM dendrimers and their metal-ion complexes are generally soluble in aqueous solution, which makes it difficult to recycle the dendrimers after adsorption.13,14 To overcome this problem, PAMAM dendrimers are often immobilized on supports such as silica gel,7,15−17 chitosan,18 titania,19 and graphene oxide.20,21 Silica gel has been widely used for immobilization because of its low cost, large surface area, excellent thermal and chemical stabilities, and well-modified surface properties.7,16 According to the hard−soft acid−base (HSAB) theory, sulfur-containing ligands display a strong affinity and adsorption selectivity toward Hg(II).22,23 Thus, it can reasonably be concluded that the introduction of sulfur groups on the peripheral surface of PAMAM dendrimers should promote their absorption capacity and selectivity for Hg(II). Received: Revised: Accepted: Published: 3679
January 13, 2016 March 14, 2016 March 16, 2016 March 16, 2016 DOI: 10.1021/acs.iecr.6b00172 Ind. Eng. Chem. Res. 2016, 55, 3679−3688
Article
Industrial & Engineering Chemistry Research Scheme 1. Synthetic Routes Used for the Preparation of Silica-Gel-Supported Sulfur-Capped PAMAM Dendrimers
group with ethylenediamine. Methyl isothiocyanate (MITC) was purchased from Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan). All other reagents were of analytical grade. FTIR spectra were obtained on a Nicolet MAGNA-IR 550 (series II) spectrophotometer with a resolution of 4 cm−1. Elemental analysis was conducted with an Elementar vario EL cube. Scanning electron microscopy (SEM) was performed on a JEOL JSF5600LV microscope. Thermogravimetric analysis (TGA) was performed on a NETZSCH STA409PC instrument. Porous structure analysis was performed with an ASAP 2020 analyzer. A GBC-932 atomic adsorption spectrophotometer was employed to determine the concentration of Hg(II) in solution.
The objective of the present work was to synthesize silicagel-supported sulfur-capped PAMAM dendrimers and use them for the adsorption of Hg(II) by the batch method. The adsorption mechanism was investigated using density functional theory (DFT).
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Silica-gel-supported PAMAM dendrimers (SiO2-G0−SiO2-G2.0) were prepared by a divergent method.17 SiO2-G0 was prepared by the reaction of 3-aminopropyltriethoxysilane with the surface silanol groups of silica gel. Then, SiO2-G0.5−SiO2-G2.0 were obtained through the iterative reaction of Michael addition of an amino group with methyl acrylate and subsequent amidation of the ester 3680
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Industrial & Engineering Chemistry Research 2.2. Synthesis of Silica-Gel-Supported Sulfur-Capped PAMAM Dendrimers (SiO2-G0-MITC−SiO2-G2.0-MITC). SiO2-G0-MITC−SiO2-G2.0-MITC were synthesized as illustrated in Scheme 1. 2.2.1. Synthesis of SiO2-G0-MITC. SiO2-G0-MITC was synthesized by the reaction of SiO2-G0 with MITC according to a similar procedure described in ref 24. Under a nitrogen atmosphere, a suspension of 12.0 g of SiO2-G0 and 2.55 g of MITC was added to 100 mL of ethanol. The mixture was stirred at 50 °C for 18 h. Then, SiO2-G0-MITC was obtained after filtration and Soxhlet extraction with ethanol for 12 h. 2.2.2. Synthesis of SiO2-G1.0-MITC. By an approach similar to that used for SiO2-G0-MITC, 10.0 g of SiO2-G1.0 and 8.50 g of MITC were added to 150 mL of ethanol under a nitrogen atmosphere. The mixture was stirred at 50 °C for 24 h, and then the solid product of SiO2-G1.0-MITC was filtered off and extracted with ethanol for 12 h. 2.2.3. Synthesis of SiO2-G2.0-MITC. Under a nitrogen atmosphere, 8.0 g of SiO2-G2.0 and 18.13 g of MITC were added to 200 mL of ethanol. The mixture was stirred at 50 °C for 30 h, and the product was filtered off. The purification was similar to that of SiO2-G0-MITC. 2.3. Hg(II) Adsorption Properties of SiO2-G0-MITC− SiO2-G2.0-MITC. The effect of pH on the adsorption was evaluated by charging about 30 mg of adsorbent and 20 mL of 0.002 mol·L−1 Hg(II) solution with varying pH values of 1.0− 6.0 according to the method described in our previous report.25 The mixture was shaken for 12 h at 25 °C, and then the concentration of Hg(II) was determined by atomic absorption spectroscopy (AAS). The adsorption capacity was calculated as
q=
(C 0 − C )V W
Figure 1. FTIR spectra of (1) silica gel, (2) SiO2-G.0, (3) SiO2-G0MITC, (4) SiO2-G0.5, (5) SiO2-G1.0, (6) SiO2-G1.0-MITC, (7) SiO2G1.5, (8) SiO2-G2.0, and (9) SiO2-G2.0-MITC.
CO bonds of ester groups at 1735 cm−1, whereas SiO2-G1.0 and SiO2-G2.0 exhibit the NH bending vibrations of the amide group at 1562 cm−1.17 After the peripheral amino groups had been capped with MITC, a new band appeared in the spectra of SiO2-G0-MITC−SiO2-G2.0-MITC at 1381 cm−1 that is attributed to CS stretching vibrations,26 indicating that the functionalization of SiO2-G0−SiO2-G2.0 with MITC was successful. It should be noted that, to ensure the complete functionalization of the amino groups, the amount of MITC was increased and the reaction time was prolonged for SiO2G1.0-MITC and SiO2-G2.0-MITC because of the increased amino content and steric hindrance. Elemental analysis was performed to further confirm the attachment of sulfur ligands on the surface of the silica-gelsupported PAMAM dendrimers. The sulfur contents of SiO2G0-MITC, SiO2-G1.0-MITC, and SiO2-G2.0-MITC were 2.05, 2.20, and 2.77 wt %, respectively, which demonstrated the successful introduction of sulfur ligands onto the frameworks of the adsorbents. SEM images of the adsorbents are displayed in Figure 2. It can be clearly seen that the surface morphology of the silica gel changed after functionalization. Compared with silica gel, the surfaces of SiO2-G0-MITC−SiO2-G2.0-MITC became rougher as a result of the introduction of the sulfur-capped PAMAM dendrimers. This rough surface should benefit the adsorption of metal ion; similar results were also observed for other silica-gelbased hybrid materials.7,25 The TGA results for the adsorbents are presented in Figure 3. The first weight loss below 110 °C corresponds to the dehydration of physically adsorbed water in the adsorbents. The second weight loss in the range of 110−800 °C in the profile of silica gel is attributed to the condensation of silanol groups, whereas the same weight loss appeared in the range of 110−180 °C for SiO2-G0-MITC−SiO2-G2.0-MITC.7 An obvious weight loss related to the degradation of the organic dendrimer moieties and the condensation of the residual silanol groups appeared from 180 to 800 °C. The final weight losses for SiO2-G0-MITC, SiO2-G1.0-MITC, and SiO2-G2.0-MITC are 18.13%, 24.19%, and 32.33%, respectively. Figures 4 and 5 show the nitrogen adsorption−desorption isotherms and pore size distributions, respectively, of the adsorbents. The isotherms are classified as type IV with hysteresis loops according to the IUPAC classification.27 The hysteresis loops of SiO2-G0-MITC−SiO2-G2.0-MITC were similar to that of silica gel, indicating that the porous structure of silica gel was not changed obviously during the functionalization. However, the inflection point decreased
(1)
where q is the amount adsorbed (mmol·g−1); C0 and C are the initial and equilibrium concentrations (mmol·mL−1), respectively; V is the solution volume (mL); and W is the weight of adsorbent (g). The adsorption kinetics, adsorption isotherms, and adsorption selectivity were measured by a batch method at a solution pH of 6 according to the methods described in ref 25. 2.4. DFT Study on the Adsorption Mechanism. G0MITC and G1.0-MITC were selected as representative materials for the investigation of the adsorption mechanism by DFT. Geometrical optimizations of the G0-MITC-Hg(II) and G1.0-MITC-Hg(II) complexes were carried out with the Gaussian 03 program using the B3LYP functional. The 631+G(d) basis set was used for C, H, N, and O atoms, whereas Hg(II) atoms were described by the LANL2DZ basis set.7 Natural bond orbital (NBO) analysis was used to evaluate the interactions between Hg(II) and the dendrimers according the method described in our previous report.12
3. RESULTS AND DISCUSSION 3.1. Characterization of SiO2-G0-MITC−SiO2-G2.0MITC. FTIR spectra of the adsorbents are presented in Figure 1. Silica gel shows characteristic OH and SiOSi stretching vibrations at 3430 and 1100 cm−1, respectively. The bands at 804 and 471 cm−1 are attributed to the SiO Si symmetric bending vibrations.7 For SiO2-G0, the new bands at 2924 and 2853 cm−1 related to the asymmetric and symmetric CH2 bands indicate the successful synthesis of SiO2-G0. SiO2-G0.5 and SiO2-G1.5 display the characteristic 3681
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Figure 2. SEM images of the adsorbents.
Figure 5. Barrett−Joyner−Halenda (BJH) desorption pore size distributions of silica gel and SiO2-G0-MITC−SiO2-G2.0-MITC.
Figure 3. TGA results for the adsorbents.
Figure 4. Nitrogen adsorption−desorption isotherms of the adsorbents. 3682
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increased pH, the protonation of the functional groups decreased, and more active binding sites became available, resulting in an increase in the adsorption capacity. At higher pH, Hg(II) can be hydrolyzed to Hg(OH)+ and Hg(OH)25. Therefore, the subsequent adsorption experiments were performed at the optimum pH. 3.3. Adsorption Kinetics. The kinetics profile of SiO2-G0MITC−SiO2-G2.0-MITC for Hg(II) at different concentrations are presented in Figure 7. It is obvious that the adsorption was very rapid in the first 120 min, and then the rate gradually decreased until equilibrium was approached at about 220 min. This phenomenon is closely related to the number of active binding sites and the geometry of the sulfur-capped PAMAM dendrimers. During the primary stage, there were plenty of active binding sites, so Hg(II) could be captured quickly by the binding sites. As the adsorption proceeded, the availability of surface active binding sites decreased, and the formation of Hg(II)−dendrimer complexes on the surface of the adsorbent made it hard for Hg(II) to diffuse into the interior of the dendrimers, resulting in the decrease of the adsorption rate. Figure 7 also shows that the adsorption capacity increased with increasing dendrimer generation and with initial Hg(II) concentration, which is similar to the results obtained for salicylaldehyde-modified PAMAM dendrimers in our previous investigation.7 The positive effects of generation number and Hg(II) concentration are ascribed to the greater number of functional groups and the higher driving force for adsorption, respectively. The kinetic adsorption mechanism was elucidated by fitting the kinetic data with pseudo-first-order and pseudo-secondorder models as described by the equations7,30−32
gradually from SiO 2-G0-MITC to SiO2 -G2.0-MITC as compared with that of silica gel, which can be attributed to the blocking of some channels as a result of the formation of the dendrimer structure.5,28 As shown in Figure 5, the pore size became smaller after functionalization with increasing generation of dendrimers compared with that of silica gel. The pore diameters of SiO2-G0-MITC−SiO2-G2.0-MITC mainly fell in the range of 30−100 nm. The corresponding porous structural parameters are summarized in Table 1. It is clear that the Table 1. Parameters of the Porous Structures of the Adsorbents adsorbent
surface area (m2/g)
pore volume (cm3/g)
average pore radius (nm)
silica gel SiO2-G0-MITC SiO2-G1.0-MITC SiO2-G2.0-MITC
268.32 189.29 186.03 140.99
1.00 0.51 0.49 0.39
52.00 36.49 36.38 34.32
surface area along with the average pore volume and radius decreased gradually after functionalization as a result of the formation of the dendrimer structure. With increasing dendrimer generation, the volume of sulfur-capped PAMAM dendrimers became larger and occupied part of the pores, leading to the decrease in the surface area and pore size.7 3.2. Effect of pH on Adsorption. Solution pH is a critical parameter in adsorption because it atrongly affects the existence of surface active binding sites and the metal-ion chemistry.7,29 As shown in Figure 6, the adsorption capacities of SiO2-G0-
ln(qe − q) = ln qe − k1t
(2)
1 1 t = + t q qe k 2qe 2 −1
(3) −1
−1
where k1 (min ) and k2 (g·mmol ·min ) are the rate constants of the pseudo-first- and pseudo-second-order models, respectively. qe is the amount adsorbed at equilibrium, whereas q is the amount adsorbed at time t (mmol·g−1). The kinetic parameters obtained from the fitting results are presented in Table 2. It is clear that the correlation coefficients (R2) of the pseudo-second-order model are higher than those of the pseudo-first-order model, implying that the kinetics of the Hg(II) adsorption process can be better described by the pseudo-second-order model. Moreover, the equilibrium adsorption capacities (qe,cal) calculated with the pseudosecond-order model are generally closer to the experimental values (qe,exp), further indicating that the pseudo-second-order
Figure 6. Effect of pH on the adsorption of Hg(II).
MITC−SiO2-G2.0-MITC were found to depend strongly on solution pH, and the optimum pH is 6. At low pH, the bulk of the active binding sites such as NH and CS were protonated, leading to a decrease of active binding sites.25 Moreover, the positively charged surface blocks the contact of Hg(II) with the adsorbent through electrostatic repulsion. With
Figure 7. Adsorption kinetics at different concentrations: (a) 0.002 and (b) 0.004 mol·L−1. 3683
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Industrial & Engineering Chemistry Research Table 2. Kinetic Parameters for Hg(II) Adsorption pseudo-first-order kinetics adsorbent SiO2-G0 -MITC SiO2-G1.0 -MITC SiO2-G2.0 -MITC
pseudo-second-order kinetics
C (mol·L−1)
qe,exp (mmol·g−1)
qe, cal (mmol·g−1)
k1 (min−1)
R12
qe,cal (mmol·g−1)
k2 (mmol·g−1·min−1)
R2 2
0.002 0.004 0.002 0.004 0.002 0.004
0.87 0.96 0.91 1.04 1.01 1.15
0.71 1.40 1.36 1.28 0.86 1.31
0.0191 0.0187 0.0232 0.0155 0.0148 0.0197
0.9893 0.9895 0.9722 0.9890 0.9957 0.9696
1.05 1.12 1.00 1.26 1.14 1.30
0.0180 0.0271 0.0323 0.0342 0.0367 0.0539
0.9976 0.9950 0.9974 0.9937 0.9966 0.9975
Figure 8. Adsorption isotherms of Hg(II).
the practical application of the adsorbents.7 The adsorption isotherms of Hg(II) are depicted in Figure 8. It is clear that the amount adsorbed increased with increasing Hg(II) concentration and temperature. The reasons for this phenomena can be attributed to the increased driving force arising from a high concentration gradient and the endothermic nature of the adsorption.25 To reveal the adsorption mechanism, the Langmuir and Freundlich models represented by eqs 6 and 7 were employed to analyze the isotherm data35,36
model is more suitable for describing the kinetics of the adsorption of Hg(II). The Boyd film diffusion model was employed to determine whether film or intraparticle diffusion was the rate-controlling steps of the adsorption, as described in the equation33 F=1−
6 π2
∞
∑ n=1
1 exp( −n2Bt ) 2 n
(4)
where n is an integer that defines the infinite-series solution and B is a time constant. F is the fractional attainment of equilibrium at time t:
F=
Ce C 1 = e + qe qm qmKL
qt qe
(5)
ln qe = ln KF +
The values of Bt were derived from the corresponding values of F given by Reichenberg.34 Plots of Bt versus t were used to determine whether film diffusion or intraparticle diffusion was the rate-controlling step. If the plots show good linearity and pass through the origin, the adsorption is characterized by intraparticle diffusion; otherwise, film diffusion is the ratecontrolling step.7,34 The fitting results presented in Table S1 indicate that the plots of Bt versus time exhibited excellent linearity without passing through the origin, implying that film diffusion dominates the kinetics of the adsorption process. 3.4. Adsorption Isotherms. Adsorption isotherms that describe the interactive behavior between adsorbate and adsorbents could provide valuable information for optimizing
ln Ce n
(6)
(7)
where qe and qm are the equilibrium and maximum adsorption amounts (mmol·g−1), respectively; Ce (mmol·mL−1) is the equilibrium concentration of Hg(II); KL (mL·mmol−1) is the Langmuir constant; and KF (mmol·g−1) and n are the Freundlich constant and exponent related to the adsorption intensity, respectively. The Langmuir model assumes that adsorption occurs by monolayer adsorption at homogeneous sites, whereas the Freundlich model suggests the uptake of metal ions by multilayer adsorption on a heterogeneous surface.37 The fitting results of the Langmuir and Freundlich models are reported in Table 3. It can be seen that the adsorption isotherms can be 3684
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Industrial & Engineering Chemistry Research Table 3. Fitting Parameters of the Langmuir and Freundlich Models Langmuir adsorbent SiO2-G0 -MITC
SiO2-G1.0 -MITC
SiO2-G2.0 -MITC
qthe (mmol·g−1)
KL (mL·mmol−1)
RL2
KF (mmol·g−1)
n
RF 2
15 25 35 15 25 35 15 25 35
1.44 1.53 1.54 1.44 1.54 1.74 1.51 1.68 1.89
4827.22 7142.86 8878.82 2173.46 1888.49 2792.81 1920.95 2577.31 2055.25
0.9994 0.9995 0.9996 0.9981 0.9965 0.9999 0.9980 0.9990 0.9984
3.92 3.79 3.61 4.77 5.51 6.98 5.80 6.52 8.15
5.21 5.87 6.29 4.11 3.84 3.69 3.67 3.75 3.44
0.9169 0.9260 0.9353 0.9034 0.9058 0.8563 0.8795 0.8576 0.9089
better described by the Langmuir model, as it provides higher correlation coefficients (R2) than the Freundlich model. In addition, the adsorption capacities calculated with the Langmuir equation (qthe) are closer to the experimental results, which indicates that the adsorption process proceeded by monolayer adsorption. To determine whether the adsorption was physical or chemical,7,38 we used the Dubinin−Radushkevich (D−R) isotherm model, given by ln qe = ln qm − βε 2
Table 4. Thermodynamic Parameters at Different Temperatures adsorbent SiO2-G0 -MITC
SiO2-G1.0 -MITC
(8)
SiO2-G2.0 -MITC
where β (mol2·J−2) is the activity coefficient and ε is Polanyi potential, which can be obtained as
⎛ 1⎞ ε = RT ln⎜1 + ⎟ Ce ⎠ ⎝
1 2β
(10)
The nature of the adsorption can be determined from the E value. If the E value falls within the range of 8−16 kJ·mol−1, the adsorption proceeds chemically. If it falls below 8 kJ·mol−1, the adsorption is dominated by physical processes. The fitting parameters of the D−R model are presented in Table S2. As can be seen from Table S2, the E values are all lie in the range of 8−16 kJ·mol−1, indicating that the adsorption process proceeds by a chemical mechanism.25 The thermodynamic parameters ΔG, ΔH, and ΔS were calculated as38 ΔG = −RT ln KL ln KL =
ΔS ΔH − R RT
T (°C)
ΔG (kJ mol−1)
ΔH (kJ mol−1)
ΔS (J mol−1 K−1)
15 25 35 15 25 35 15 25 35
−2.36 −5.01 −7.65 −3.61 −4.97 −6.34 −3.53 −4.98 −6.42
73.88
264.72
35.78
136.76
38.08
144.49
Hg(II) on the investigated adsorbents proceeded spontaneously and was promoted by higher temperatures. 3.5. Adsorption Selectivity. Adsorption selectivity is one of the most important parameters for the separation of metal ions from aqueous solutions. The adsorption selectivities of SiO2-G0-MITC−SiO2-G2.0-MITC for Hg(II) were evaluated by choosing a series of binary metal-ion systems such as Hg(II)Pb(II), Hg(II)Ni(II), Hg(II)Cd(II), Hg(II) Fe(III), and Hg(II)Zn(II). The results presented in Table S3 show that SiO2-G0-MITC−SiO2-G2.0-MITC exhibited markedly higher adsorption capacities for Hg(II) than the coexisting metal ions. In particular, they exhibited 100% selectivity for Hg(II) in the presence of Ni(II), Cd(II), Fe(III), and Zn(II), suggesting that SiO2-G0-MITC−SiO2-G2.0-MITC have excellent selectivities for Hg(II). This phenomenon can be reasonably interpreted by HSAB theory as a sulfur-containing ligand exhibiting a remarkable affinity and adsorption selectivity toward Hg(II) over other metal ions.22,23 3.6. DFT Study on the Adsorption Mechanism. The optimized geometries of G0-MITC-Hg(II) and G1-MITCHg(II) complexes are shown in Figure 9. G0-MITC coordinates with Hg(II) through a S atom in a monocoordinated mode. The bond distance between S and Hg(II) is 2.55 Å. G1.0-MITC acts as a pentadentate ligand, interacting with Hg(II) through the N atoms of the tertiary amine group, the O atoms of the amide groups, and the S atoms. The HgN bond length is 2.58 Å. The HgO bond lengths are 2.48 and 2.49 Å, whereas the HgS bond lengths are 2.66 and 2.68 Å. Therefore, the chelating mechanism of sulfur-capped PAMAM dendrimer can be proposed as shown in Scheme 2. The adsorption selectivity was also clarified based on DFT calculation using G1.0-MITC and Pb(II) as representative species. The optimized geometry of the G1-MITC-Pb(II) complex is shown in Figure S1. Compared with the G1-MITC-
(9)
The mean free energy (E, kJ·mol−1) can be derived from the equation E=
Freundlich
T (K)
(11)
(12)
where KL is the Langmuir constant, R is the gas constant, and T is the temperature (K). The calculated results are listed in Table 4. The values of ΔG are all negative, suggesting that the adsorption of Hg(II) is a spontaneous. Moreover, the values of ΔG became more negative with increasing temperature, indicating that the adsorption is feasible at high temperature, which is consistent with the isotherm adsorption results, as the amount adsorbed increased at higher temperature for the same concentration. The values of ΔH and ΔS are all positive, suggesting that the adsorption is an endothermic and randomness-increasing process.25 Therefore, the adsorption of 3685
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predominantly localized on the Hg(II) ion, S, and the neighbor N atoms. Similarly to that of G0-MITC, the HOMO of G1.0MITC is characterized by strong localization on S and the neighboring N atoms, while the LUMO is mainly localize on the parts of the C atoms. For G1.0-MITC-Hg(II), the HOMO is almost spread on S and the neighbor N atoms, whereas the LUMO of G1.0-MITC-Hg(II) is dominantly localized on Hg(II) and the S, N, and O atoms that participate the coordination. The variations of the HOMOs and LUMOs before and after coordination indicate that charge transfer occurs from G0-MITC and G1.0-MITC to Hg(II) during the formation of the complexes. To further explore the nature of the charge transfer during coordination, natural population analysis was employed to evaluate the charge distributions on the complexes.39,40 The NBO partial charges on the complex fragments, the electron configurations of Hg(II), and dipole moments of the complexes are summarized in Table 5. It is clear that the net NBO charges on Hg(II) in the complexes were all less than 2, whereas the net charges on G0-MITC and G1.0-MITC became positive after coordination. These facts indicate that charge transfer occurred from G0-MITC and G1.0-MITC to Hg(II) during the formation of the complexes. The electron configuration of Hg(II) change from 5d10 to 6s0.765d9.996p0.017p0.01 and 6s0.565d9.99 6p0.027p0.01 in the G0-MITC-Hg(II) and G1.0MITC-Hg(II) complexes, suggesting that the charge transfer mainly occurs from G0-MITC and G1.0-MITC to the empty 6s orbital of Hg(II). It can also be observed that after, the formation of the complexes, the dipole moment of G0-MITC changed from 5.91 to 4.26 D, whereas this value for G1.0MITC changed from 9.41 to 3.76 D. These facts further indicate that charge transfer occurs as a result of the interactions between the ligand and Hg(II), leading to changes in the electron density distribution.41
Figure 9. Optimized geometries of G0-MITC-Hg(II) and G1.0MITC-Hg(II) complexes with atom numbering.
Hg(II) complex, the PbN bond length is equal to the Hg N bond length, and the PbO bond lengths are nearly the same as the corresponding HgO bond lengths. However, the PbS bonds are longer than the HgS bonds by 0.24 and 0.47 Å, indicating the S atom exhibits a higher binding ability toward Hg(II) than toward Pb(II).The binding energies calculated based on the optimized geometries are 309.97 and 308.83 kcal/mol for G1-MITC-Hg(II) and G1-MITC-Pb(II), respectively, furthering indicating that G1-MITC has a higher binding ability for Hg(II). According to frontier orbital theory, the highest occupied molecular orbital (HOMO) and the lowest unoccupied orbital (LUMO) play important roles in the formation of complexes.12 The contour plots of the HOMOs and LUMOs of G0-MITC, G1.0-MITC, and their complexes with Hg(II) are presented in Figure 10. The HOMO of G0 is localized mainly on S and the neighboring N atoms, whereas the LUMO is localized mainly on the C atoms. However, the HOMO of G0-MITC-Hg(II) is nearly spread over the entire complex except the S atom and Hg(II) ion, whereas the LUMO of G0-MITC-Hg(II) is
4. CONCLUSIONS A series of silica-gel-supported sulfur-capped PAMAM dendrimers (SiO2-G0-MITC−SiO2-G2.0-MITC) were synthesized and used for Hg(II) adsorption. The optimum adsorption
Scheme 2. Proposed chelating mechanism of SiO2-G0-MITC−SiO2-G2.0-MITC with Hg(II)
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Industrial & Engineering Chemistry Research
Figure 10. Contour plots of the HOMOs and LUMOs of G0-MITC, G1.0-MITC, and their complexes with Hg(II).
Table 5. NBO Partial Charges on the Fragments, Electron Configurations of Hg(II), and Dipole Moments of the Complexes NBO partial charges
a
complex
G0-MITC
Hg(II)
Hg(II) electron configurationa
dipole momentb (D)
G0-MITC-Hg(II) G1.0-MITC-Hg(II)
0.77 0.58
1.23 1.42
6s0.765d9.996p0.017p0.01 6s0.565d9.996p0.027p0.01
4.26 3.76
Ground-state electron configuration of free Hg(II) is 5d10. bDipole moments of G0-MITC and G1-MITC are 5.91 and 9.41 D, respectively.
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pH was found to be 6. Adsorption kinetics indicated that the adsorption capacity increased with increasing dendrimer generation and Hg(II) concentration. The kinetics of adsorption was found to be controlled by the film diffusion process and can be described by a pseudo-second-order model. Adsorption isotherms were fitted well with the monolayer Langmuir isotherm model, and adsorption was found to take place by a chemical mechanism. Thermodynamic parameters demonstrated the adsorption was a spontaneous, endothermic, and randomness-increasing process. SiO2-G0-MITC−SiO2G2.0-MITC exhibited excellent adsorption selectivities for Hg(II) in the presence of Ni(II), Cd(II), Fe(III), and Zn(II). DFT calculations revealed that G0-MITC interacts with Hg(II) through a S atom in a monocoordinated manner, whereas G1.0MITC behaves as a pentadentate ligand to coordinate with Hg(II) through the N atom of the tertiary amine group, the O atoms of the amide groups, and the S atoms. Charge transfer was found to occur from G0-MITC and G1.0-MITC to Hg(II) during the formation of the complexes.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 535 6672176. Fax: +86 535 6696281. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial support by the National Natural Science Foundation of China (Nos. 21307053, 51373074, 51302127), a Project of Shandong Province Higher Educational Science and Technology Program (No. J15LD01), Promotive Research Fund for Excellent Young and Middle-Aged Scientists of Shandong Province (Nos. BS2013CL044, BS2014CL040), China Postdoctoral Science Foundation Funded Project (No. 2013M541911), Innovation Foundation for Students of Ludong University (Nos. ld15l020, ld15l021).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00172. Boyd film diffusion parameters (Table S1), D−R isotherm model parameters (Table S2), adsorption selectivity (Table S3), and optimized geometry of the G1.0-MITC- Pb(II) complex (Figure S1) (PDF) 3687
DOI: 10.1021/acs.iecr.6b00172 Ind. Eng. Chem. Res. 2016, 55, 3679−3688
Article
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