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Capability of magnetic functional metal-organic nanocapsules for removal of mercury(II) ions
Accepted Manuscript Capability of Magnetic Functional Metal-organic Nanocapsules for Removal of Mercury(II) Ions
Rokhsareh Nouri, Elham Tahmasebi, Ali Morsali PII:
S0254-0584(17)30460-1
DOI:
10.1016/j.matchemphys.2017.06.018
Reference:
MAC 19758
To appear in:
Materials Chemistry and Physics
Received Date:
07 January 2017
Revised Date:
10 May 2017
Accepted Date:
07 June 2017
Please cite this article as: Rokhsareh Nouri, Elham Tahmasebi, Ali Morsali, Capability of Magnetic Functional Metal-organic Nanocapsules for Removal of Mercury(II) Ions, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.06.018
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Capability of Magnetic Functional Metal-organic Nanocapsules for Removal of Mercury(II) Ions Rokhsareh Nouria, Elham Tahmasebib, Ali Morsalia* aDepartment
of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-4838, Tehran, Islamic Republic of Iran
bDepartment
of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P.O. Box 45195-1159, Zanjan, Iran
Tel: +98-21-82883449, Fax: +98-21-8009730 E-mail: [email protected]
Abstract In this study, a magnetic infinite coordination polymer with the morphology of nanocapsule, as an efficient adsorbent for removal of Hg2+ ions has been introduced. This infinite coordination polymer was synthesized from Zn2+ ion and a ditopic organic ligand (1,3-bis(tetrazol-5-ylmethyl)benzene (btb)) and its efficiency as an adsorbent was studied in view of adsorption isotherms, kinetics and thermodynamics. The adsorption capacity of mercury(II) ions was impressed by pH value and adsorption time and the optimal adsorption conditions were pH value of 8, adsorption time of 75 min. The adsorption isotherm was analyzed by Freundlich and Langmuir equations and was fitted with Langmuir model better. Outcomes indicated that the adsorption was an endothermic process. Moreover, this adsorption process was fitted with pseudosecond order model kinetically. Finally, the magnetic properties of nanocapsules synthesized in high and low concentration of initial reagents were investigated. Studies showed that the saturation magnetization of nanocapsules synthesized in low concentration of initial reagent is higher. Keywords: Infinite coordination polymer; Functional metal-organic nanocapsule; Mercury(II) ions; Removal.
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1. Introduction Infinite coordination polymers (ICPs) are a novel fascinating group of organic-inorganic materials fabricated from metal cations interconnected by multitopic organic ligands which have been studied over the recent years [1, 2]. Attractive tunable functional ICPs networks can be designed and synthesized for many applications as exemplified by gas adsorption [3], photonics [4], drug release [5], catalysis [6] and sensing [7]. These micro and nano structures have distinctive morphology such as rod, cube and sphere which can be crystalline or amorphous [8]. Infinite coordination polymer particles are synthesized based on three approaches including fast precipitation started by addition of a solvent into the solution of metal cations and ligands, solvothermal synthesis and microemulsion route [9]. Adsorption efficiency of heavy metal ions or other pollutants by functional metal-organic nanospheres has not been investigated yet. Mercury is a persistent and bioaccumulative heavy metal that causes lots of neurotoxic effects in human body [10]. Natural processes (e.g., volcanic actions, erosion of residues which contain mercury, etc) and anthropogenic activities (e.g., mining operations, tanneries, etc) have significant roles in the presence of mercury in our planet [11]. Accordingly, it is required to develop an effective method to eliminate mercury(II) ions from water sources. Some techniques such as liquid extraction [12], ion-exchange [13], membrane separation [14], chemical precipitation [15] and adsorption [16] have been studied to eliminate mercury ions from contaminated waters over the years. But, adsorption method has attracted more attention owing to its benefits such as feasibility and economical cost. Activated carbon, zeolites and some other porous materials are common adsorbents which have been investigated for elimination of heavy metal ions from wastewaters in the recent years [17–19]. Our research group reported the synthesis of ICP nanocapsules from the metal ions (Zn2+) and a new ditopic ligand (btb: 1, 3-bis(tetrazol-5ylmethyl)benzene) Zn(btb) with spherical morphology [20]. Since these spheres have an innate capacity to automatically entrap various species present in the initial reagents mixture such as Fe3O4, they were magnetized by trapping Fe3O4 NPs during their synthesis to collect them easily by a magnet. Considering there are free nitrogen sites in the structure of these nanocapsules, we expected them to adsorb heavy metal
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cations via acid-base Lewis interactions between nitrogen sites and metal cations. Therefore, in this work, we aimed to investigate the adsorption capability of these nanospheres for removal of Hg2+ cations and adsorption isotherms, kinetics and thermodynamics of Hg2+ adsorption by Fe3O4@Zn(btb) nanocapsules. 2. Experimental
2.1. Materials and methods 1,3-phenylenediacetonitrile and mercury(II) chloride were acquired from Sigma-aldrich (Milwaulkee,WI, USA). Sodium azide, zinc bromide and other reagents for the synthesis and analysis were commercially accessible from Merck Company (Darmstadt, Germany). IR spectra were measured on a Nicolet IR100 (Madison, WI) Fourier-transform infrared (FT-IR) spectrometer. Simultaneous inductively coupled plasma optical emission spectrometry (ICP-OES) on a Varian Vista-PRO instrument (Springvale, Australia) with a radial torch connected to a concentric nebulizer and a Scott spray chamber and outfitted with a chargecoupled detector (CCD) was utilized to determine the goal element. The prepared samples were characterized by a scanning electron microscope (SEM) (Philips XL 30) with gold coating. Transmission electron microscopy (TEM) images were attained with a Hitachi H-9500 instrument. Measurements of particles sizes were performed using a Zetasizer Nano apparatus. Powder X-ray diffraction (PXRD) of compound Fe3O4@Zn(btb) was carried out using a diffractometer of Philips Company with Cu kα radiation. 2.2. Synthesis of 1,3-bis(tetrazol-5-ylmethyl) benzene ligand The (1,3-bis(tetrazol-5-ylmethyl) benzene (btb)) ligand was synthesized according to previously reported method [20]. 1,3-phenylenediacetonitrile (20 mmol), sodium azide (60 mmol), zinc bromide (20 mmol), and 40 mL of water were added to a 250 mL round-bottomed flask. The reaction mixture was stirred and heated for 48 h at 270˚C to reflux. Afterward, HCl (3 N, 30 mL) was added to the cool mixture until the aqueous layer had a pH of 1. Then 200 mL of 0.25 N NaOH was added. The mixture stirred for 30 min, until the precipitate was disappeared. The suspension was filtered. 40 mL of 3 N HCl was added to the filtrate and stirred. The precipitate was separated and washed with HCl. The product had the following data: 3
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mp 269-270 °C. NMR (d -DMSO) 1H: 7.27 (t, 1H), 7.25 (b, 1H), 7.24 (b, 1H), 7.17 (m, 3H), 4.2 (s, 4H), 13C:
173.00, 155.17, 136.50, 129.16, 127.90, 29.60. Anal. Calc. for C10H10N8: C, 49.57; H, 4.13; N, 46.28.
Found: C, 49.12; H, 4.04; N, 46.1. 2.3. Synthesis details of Fe3O4@Zn(btb) nanospheres Fe3O4 nanoparticles (0.04 mmol) were added to a solution of 1,3-bis(tetrazol-5-ylmethyl) benzene (0.57 mmol) in DMF (4.5 ml). Afterward, the mixture was sonicated to make it homogenized and a methanolic solution (1 ml) of Zn(NO3)2·6H2O (0.57 mmol) was added to the mixture. Nanospheres that encapsulate Fe3O4 nanoparticles were formed. Fe3O4@Zn(btb) nanospheres with bigger size were prepared by the same route and same amount of Fe3O4 nanoparticles in concentration of initial reagents [Zn2+]= [btb] = 0.01 mol L-1. 2.4. Adsorption studies To analyze adsorption capacities at equilibrium and removal efficiency, the batch equilibrium method was utilized. A certain amount of adsorbent (0.01 g) was suspended in 10 mL Hg2+ aqueous solution and stirred for 80 min at RT to find out adsorption capacity. Then, magnetic nanocapsules were collected with a magnet. Finally, the concentration of Hg2+ in the supernatant was determined by ICP elemental analysis. Equations (1) and (2) were used to obtain the adsorption capacities at equilibrium (mg/g) and removal efficiency. qe = (C0 ‒ Ce) ×
V M
Hg2 + removal efficiency (%) =
(1)
(C0 ‒ Ce) C0
× 100
(2)
In this equation, C0 and Ce are the concentration of mercury ions (mg/L) at zero time and equilibrium, V is the volume of the initial solution (L) and M is the mass of Fe3O4@Zn(btb) nanospheres (g). 3. Results and discussion 4
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Our research group, reported the synthesis of tetrazolic metal-organic nanocapsules from addition of solution of Zn(NO3)2·6H2O in methanol to the DMF solution of btb previously. Investigations showed that concentration of initial reagents, affected the size of prepared ICPs. According to dynamic light scattering (DLS) studies, increasing the concentration of the initial reagents led to a decrease in the size of the Zn(btb) capsules. Fig. 1 shows (DLS) analysis of different suspensions of synthesized capsules in DMF with different concentration of initial precursors.
Fig. 1. DLS analysis of different suspensions of Zn(btb) spheres in DMF with different concentration of initial precursors (A) (96 ± 6) nm, (B) (331 ± 22) nm, (C) (667 ± 43) nm and (D) (1336 ± 87) nm for initial concentration of both metal ion and btb: 25 ×10-1 M, 5 ×10-2 M, 2.5 × 10-2 M and 1× 10-2 M respectively and the corresponding SEM images.
As mentioned, these capsules are capable of entrapping Fe3O4 nanoparticles or some other luminescent quantum dots, and fluorescent dyes as the guests distributed in the primary solution of precursors [20]. It is noteworthy to mention that there is not any chemical bonding between the guest and the surface of Fe3O4 nanoparticles. There may be some physical interactions between the guest and the infinite coordination polymer which does not seem to have a key role in the formation of Fe3O4@Zn(btb) nanospheres. According to DLS measurements, encapsulation of Fe3O4 nanoparticles does not change the size of the primary polymer. Fig. 2 displays representative reaction scheme of the formation of organic-inorganic 5
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spheres of Fe3O4@Zn(btb) in high and low concentration of initial precursors. The structure of the Fe3O4@Zn(btb) nanocapsules synthesized in low concentration of initial precursors were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 3 indicates
Fe3O4 particles are inside the metal-organic hollow sphere. In order to increase the surface area and the adsorption capacity of these magnetic nanocapsules, we synthesized these nanocapsules in high concentration of initial precursors and used them as an adsorbent in the present study.
Fig. 2. Representative reaction scheme of the formation of metal–organic hollow spheres of Fe3O4@Zn(btb) in high and low concentration of initial precursors.
Fig. 3. (a and b) HRTEM images of Zn(btb) spheres encapsulating the Fe3O4 nanoparticles prepared in the concentration of initial reagents [Zn2+]= [btb]= 2.5 × 10-2 M.
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3.1. Characterization of the Fe3O4@Zn(btb) nanocapsules before and after adsorption
Characterization of the sample before adsorption was performed by SEM. Fig. 4 shows SEM images of Fe3O4@Zn(btb) nanocapsules synthesized in high concentration of initial precursors with spherical morphology. Moreover N2 adsorption–desorption isotherms were determined at 77 K. Specific surface area of the sample was measured in accordance with the Brunaure–Emmet– Teller (BET) method which was 15 m2/g.
Fig. 4. SEM images of synthesized Fe3O4@Zn(btb) used as an adsorbent.
The magnetic properties of Fe3O4@Zn(btb) were investigated utilizing a vibrating sample magnetometer. The saturation magnetization attained for the magnetic capsules with smaller size was 2.68 emu g_1. Synthesized Fe3O4@Zn(btb) nanocapsules exhibited superparamagnetic behavior. The saturation magnetization of Fe3O4@Zn(btb) nanocapsules synthesized in low concentration of initial reagents was measured and compared with the nanocapsules synthesized in high concentration of initial reagents. From Fig. 5, the saturation magnetization of the Fe3O4@Zn(btb) nanospheres decreases from 12.38 to 2.68 emu g−1 with decrease of the ICPs sizes. Zn2+/Fe3+ molar ratio of Fe3O4@Zn(btb) synthesized in high and low concentration of initial reagents measured by ICP analysis were 2.35 and 0.44 respectively. According to Zn2+/Fe3+ molar ratio, nanocapsules synthesized in high concentration of initial reagents encapsulate lower amount 7
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of Fe3O4 nanoparticles that leads to lower saturation magnetization. The saturation magnetization (MS) of the adsorbent after removal of mercury(II) ions in concentration of 10 ppm was 2.39 emu g-1. In addition, energy-dispersive X-ray spectroscopy (EDAX) analysis (Fig. 6) of samples after adsorption signifies the presence of the Hg element, showing that the Hg2+ ions were favorably adsorbed on Fe3O4@Zn(btb) nanocapsules.
Fig. 5. a) Magnetic hysteresis loop of Fe3O4@Zn(btb) nanospheres synthesized in: I) low concentration of initial reagents II) high concentration of initial reagents III) and adsorbent after removal of mercury(II) ions from aqueous solution in concentration of 10 ppm. b) Vial including a suspension of Fe3O4@Zn(btb) nanospheres in aqueous solution of HgCl2 before and after putting a magnet close to the wall.
Fig. 6. EDX spectrum of Fe3O4@Zn(btb) after Hg2+ adsorption.
3.2. Effect of pH on Hg2+ adsorption
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The effect of pH on the adsorption of Hg2+ was studied in the pH range from 4 to 9. Outcomes showed that the adsorption efficiency of Hg2+ is dependent on pH of the solution by affecting their adsorption due to both the chemistry of Hg2+ ions in solution and protonation of the adsorbent donor atoms. The maximum removal of Hg2+ ions appeared in alkaline pH value of 8 (Fig. 7). At low pH values, Fe3O4@Zn(btb) nanosphere donor atoms are protonated and become positively charged, thereby decreasing electrostatic attractions between positively charged Hg2+ and positively charged adsorption sites leads to a decrease in the Hg2+ adsorption. The optimal adsorption pH is 8. The decrease in the efficiency of removal of Hg2+ ions at higher pH values may be due to the formation of hydroxide species of Hg2+ ions in the presence of high concentration of OH- ions and deformation of nanocapsules.
Fig. 7. The effect of pH on Hg2+ removal using Fe3O4@Zn(btb).
3.3. Adsorption kinetic considerations
The kinetic of adsorption process exhibits the solute uptake rate that ascertains the residence time of the adsorption process and can determine the efficiency of adsorption. The kinetic studies of mercury adsorption on Fe3O4@Zn(btb) nanospheres were accomplished by contacting 10 mg of adsorbent with 30 mg/L mercury ions solution at pH 8. Fig. 8 indicates the influence of contact time on adsorption of Hg2+ ions on the adsorbent.
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Fig. 8. Contact times for the adsorptions of Hg2+ onto Fe3O4@Zn(btb).
In the initial time, the adsorption efficiency was enhanced, and afterward declined, and attained equilibrium after 75 min. Therefore, the optimum time to gain highest adsorption capacity is 75 min. Two semi-empirical kinetic models were applied for analyzing the adsorption rate: the firstorder equation and the pseudo-second-order equation [21, 22]. Pseudo-first-order equation is shown as: ln (qe ‒ qt) = ln qe ‒ k1t
(3)
Where qt is the adsorption quantity of Hg2+ ions at any time (mg/g), qe is adsorption capacity at equilibrium (mg/g) and k1 is the rate constant of pseudo-first-order adsorption (g/mg. min). Therefore, the first order kinetic constant (k1) can be calculated by slope when the ln(qe − qt) is plotted against t. In addition, the intercept of this plot can be used to obtain ln qe. Nevertheless, the first order kinetic demonstrates poor linearity (Fig. 9). The linear model of pseudo-second-order equation can be shown by following: t 1 t = + qt kq2e qe
(4)
Another parameter is the initial adsorption rate represented as: h = kq2e (T→0)
10
(5)
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Parameters of k and h can be achieved experimentally from the slope and intercept of plot of t/qt against t. The slope (1/qe) and intercept (1/k2qe2) of plot (t/qt) against t were utilized to get the parameters of k2 and qe,
cal.
In accordance with Fig. 10 and adsorption data in Table 1, the
experimental data were fitted into pseudo-second-order equation well. 3.3. Adsorption Isotherms
To find out the relation between the quantity of metal ions adsorbed on the surface of Fe3O4@Zn(btb) nanospheres and the concentration of Hg2+ left in the solution, the adsorption isotherm was studied. For investigating the adsorption isotherm, in 10 different initial concentrations (between 10 to 200 (mg/ml)), 10 mL Hg2+ aqueous solution was poured into 10 mL erlenemeyer flask including 10 mg Fe3O4@Zn(btb) at RT and stirred for 80 min.
Fig. 9. Plot of the pseudo-first-order kinetics for the adsorptions of Hg2+ onto Fe3O4@Zn(btb).
Fig. 10. Plot of the pseudo-second-order kinetics for the adsorptions of Hg2+ onto Fe3O4@Zn(btb).
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Table 1. Kinetic adsorption parameters obtained using pseudo-first-order and pseudo-second-order models.
Pseudo-second-order
Lagergren first order
K2(g/mg.min)
h(mg/g.min)
qe, exp (mg/g)
qe, cal (mg/g)
R2
K1 (min-1)
qe, cal (mg/g)
R2
0.004
3.54
25.75
27.70
0.99
0.038
14.87
0.97
The Langmuir and Freundlich isotherm models were utilized in modeling the adsorption data [23]. The Freundlich isotherm model is applicable to explain multilayer adsorption and adsorption on heterogeneous surfaces. The Freundlich model can be expressed in the form: qe = kfC1/n e (none linear form)
(6)
1
log qe = log kf + nlog Ce (linear form) (7) In this equation, n is the constant relevant to the intensity of the adsorption and the constant kf is referred to the adsorption capacity of the adsorbent. Ce and qe are equilibrium concentration of Hg2+ ions (mg/L) and the quantity of Hg2+ ions adsorbed (mg/g), respectively. 1/n can be attained from the slope of the plot of log qe against log Ce. The intercept of this plot may be utilized to calculate Log kf. Fig. 11 shows the linear plot based on fitting the experimental adsorption data with Freundlich adsorption model.
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Fig. 11. Freundlich adsorption isotherm for the adsorptions of Hg2+ onto Fe3O4@Zn(btb).
The Langmuir equation is expressed as below: Ce
Ce 1 = + qe b Q0 Q0
(8)
In this equation, qe (mg/g) and Ce (mg/L) are the equilibrium adsorption capacity and the concentration of Hg2+ at equilibrium. Q0 and b represent the highest adsorption capacity and the equilibrium adsorption constant relating to the bonding energy of the adsorption, respectively. The highest adsorption capacity Q0 can be achieved from the reciprocal slope of a plot of Ce/qe against Ce (Fig. 12).
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Fig. 12. Adsorption curve of Hg2+ at different concentrations by using the Fe3O4@Zn(btb) nanocapsules (left), The linear regression by fitting the equilibrium adsorption data with Langmuir adsorption model (right).
The Langmuir constant b (L/mg) can be calculated from the slope/intercept of the Langmuir plot. High amount of b constant signifies more affinity of adsorbate for adsorption sites on the surface of adsorbent. Langmuir adsorption model is based on arrangement of a monolayer coating of adsorbate on the surface of the adsorbent. In the first step, amount of the solute adsorbed on the surface of the adsorbent was increased strongly with increasing the initial concentrations of Hg2+, and then gained a platform. Table 2 shows Langmuir and Freundlich isotherm parameters for Hg2+ adsorption on Fe3O4@Zn(btb) nanospheres. According to the linear regression correlation coefficients (R2), adsorption of Hg2+ ions on the surface of Fe3O4@Zn(btb) nanospheres follows the Langmuir isotherm model that approves a homogenous adsorption process and monolayer coating of Hg2+ ions on the surface of nanospheres. From Langmuir model, the highest equilibrium quantity of Hg2+ on Fe3O4@Zn(btb) nanospheres was 129.87 mg/g. The high adsorption capacity for Hg2+ ions on Fe3O4@Zn(btb) nanospheres assigns to the interaction between the nitrogen sites of tetrazolic group and mercury cations. The adsorption capacity of Fe3O4@Zn(btb) nanospheres was compared with other functionalized magnetic adsorbents reported previously for removal of Hg2+ ions in Table 3 [24-27].
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Table 2. Langmuir and Freundlich parameters for adsorption of Hg2+ onto Fe3O4@Zn(btb).
Langmuir model
Freundlich model
Qm (mg/g)
b (l/mg)
R2
Kf ( mg(n−1)/nL1/ng−1)
n
R2
129.87
0.1
0.99
15.13
1.8
0.91
Table 3. The adsorption capacity of other functionalized magnetic adsorbents reported previously.
Adsorbent
Adsorption Capacity (mg/g)
Reference
humic acid coated Fe3O4
98
[24]
thiol modified Fe3O4@SiO2
79
[25]
naphthalimide functionalized Fe3O4@SiO2
32
[26]
poly(1-vinylimidazole)-grafted Fe3O4@SiO2
346
[27]
Fe3O4@Zn(btb)
129.87
Present study
3. 4. Adsorption thermodynamics
Thermodynamic investigations can reveal influence of temperature on adsorption behavior and whether adsorption process is spontaneous or not, therefore it is important to study the process in view of adsorption thermodynamics. The adsorption process was investigated at four different temperatures (288, 303, 313, 323 K) and in initial concentration of 50 ppm. The Gibbs free energy (ΔG°) was obtained based on the following equation [22, 23]. ΔG° = ΔH° ‒ TΔS° (9)
ln Kd =
ΔS° ΔH° ‒ (10) R RT
In this equation, the ΔG° is the change in Gibbs free energy in J/mol. The change in the enthalpy ( ΔH°) in J/mol and change in the entropy (ΔS°) in J/mol K are calculated from the slope and 15
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intercept of ln Kd against 1/T plots, respectively. R represents the universal gas constant (8.314 J/mol K) and T is the temperature in K. Kd indicates linear adsorption distribution coefficient explained as:
Kd =
(C0 ‒ Ce) Ce
×
V (11) M
Where V and M represent the working volume (mL) and the adsorbent mass (g), respectively. Fig. 13 shows the van’t Hoff plot that is a plot of ln Kd against 1/T for Hg2+ adsorption on Fe3O4@Zn(btb) nanospheres in initial concentration of 50 ppm.
Fig. 13. Plot of ln Kd versus 1/T for Hg2+ adsorption on Fe3O4@Zn(btb) in initial concentration of 50 ppm.
Thermodynamics parameters of Hg2+ adsorption on Fe3O4@Zn(btb) nanospheres at different temperature are shown in Table 4. The negative ΔG° values reflect the fact that Hg2+ ions adsorb on the surface of nanospheres spontaneously at 288–323 K. According to the positive values of ΔH ° and the increasing values of Kd , the adsorption of ions onto Fe3O4@Zn(btb) nanospheres is an endothermic process. Adsorption process in solid–liquid systems involves desorption of the solvent molecules from the surface of the adsorbent and afterward adsorption of the adsorbate species. Endothermicity of the adsorption process is due to replacement of more than one solvent 16
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(water) molecule by adsorbate species. In accordance with the positive ΔS° value, the adsorption process involves the liberation of solvent molecule from the adsorbent. Table 4. Thermodynamics parameters of Hg2+ adsorption on Fe3O4@Zn(btb).
C0 (ppm)
50
T (K)
ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (J/K mol)
288
-19.87
303
-22.57
313
-24.38
323
-26.1
31.15
177.17
R2
0.99
3.5. Nanocapsules stability test
Powder X-ray diffraction (PXRD) of compound Fe3O4@Zn(btb) showed that the synthesized metal-organic nanocapsules are amorphous (Fig. 14), so common ways to study these nanocapsules are SEM and IR spectroscopy [20].
Fig. 14. XRD pattern of Fe3O4@Zn(btb).
Therefore, the stability of Fe3O4@Zn(btb) nanospheres at different pH values was investigated by SEM. The SEM observations indicated that at a pH of 8, the morphology did not change but in acidic or higher alkaline pH, nanocapsules were slightly deformed. Fig. 15 (a-c) shows SEM images of Fe3O4@Zn(btb) nanospheres after adsorption process at pH value of 4, 8 and 9. In 17
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addition, the morphology of nanocapsules after adsorption at temperature of 50
○C
was
investigated by SEM and obtained images were shown in Fig. 15d.
Fig. 15. a) SEM images of Fe3O4@Zn(btb) after adsorption at pH= 4, b) pH= 9, c) pH= 8, d) 50 ○C.
According to these images, after adsorption at this temperature, nanocapsules were not decomposed and only slightly adherence of nanocapsules was seen. Moreover, the stability of these nanocapsules was investigated by FT-IR spectroscopy. As can be seen in Fig. 16, the FT-IR spectra of the Fe3O4@Zn(btb) nanospheres synthesized in low concentration of initial precursors and the adsorbent before and after adsorption process at RT temperature and pH of 8 are the same.
Fig. 16. FT-IR spectra of the adsorbent a) Fe3O4@Zn(btb) synthesized in high initial concentration of precursors [Zn2+]= [btb]= 0.1 M b) Fe3O4@Zn(btb) synthesized in low initial concentration of precursors [Zn2+]= [btb]= 0.01 M c) after adsorption of Hg2+.
4. Conclusions
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In conclusion, the magnetic properties of Fe3O4@Zn(btb) nanospheres synthesized in high and low concentration of initial reagents were studied. Investigations showed that the saturation magnetization of the Fe3O4@Zn(btb) nanospheres decreased with decrease of the ICPs sizes and Fe3O4@Zn(btb) nanospheres synthesized in low concentration of initial reagents had higher
saturation magnetization than those synthesized in high concentration of initial reagents. In addition, the capability of magnetic functional metal-organic nanocapsules for removal of mercury(II) cations as a heavy metal and its adsorption isotherms, kinetic and thermodynamics were investigated. These nanocapsules are magnetic nanoscale adsorbents which consist of nitrogen atoms that make it a good choice for mercury(II) removal. According to SEM and FT-IR studies, these nanocapsules are stable at pH of 8. The kinetic analysis indicated that the overall adsorption process followed the pseudo-second-order kinetic model. Adsorption isotherm obeyed the Langmuir model indicating that during the adsorption process a monolayer coating of the adsorbate on the adsorbent was formed. The adsorption of metal ions on the adsorbent was a function of temperature and the adsorption capacity increased with increasing the temperature of the system, confirming the endothermic character of adsorption process. Finally, in comparison with the most of functionalized magnetic adsorbents reported previously, Fe3O4@Zn(btb) nanospheres have more capacity due to their nitrogen enriched surface. Acknowledgement Support of this investigation by Tarbiat Modares University is gratefully acknowledged. References [1] M. Oh and C.A. Mirkin, Nature, 438 (2005) 651-654. [2] X. Sun, S. Dong and E. Wang, J. Am. Chem. Soc. 127 (2005) 13102-13103.
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Infinite coordination polymers (ICPs) for mercury(II) removal has been introduced. Adsorption isotherms, kinetics and thermodynamics were studied. The magnetic properties of ICPs were investigated.
Rokhsareh Nouri, Elham Tahmasebi, Ali Morsali PII:
S0254-0584(17)30460-1
DOI:
10.1016/j.matchemphys.2017.06.018
Reference:
MAC 19758
To appear in:
Materials Chemistry and Physics
Received Date:
07 January 2017
Revised Date:
10 May 2017
Accepted Date:
07 June 2017
Please cite this article as: Rokhsareh Nouri, Elham Tahmasebi, Ali Morsali, Capability of Magnetic Functional Metal-organic Nanocapsules for Removal of Mercury(II) Ions, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.06.018
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Capability of Magnetic Functional Metal-organic Nanocapsules for Removal of Mercury(II) Ions Rokhsareh Nouria, Elham Tahmasebib, Ali Morsalia* aDepartment
of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-4838, Tehran, Islamic Republic of Iran
bDepartment
of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P.O. Box 45195-1159, Zanjan, Iran
Tel: +98-21-82883449, Fax: +98-21-8009730 E-mail: [email protected]
Abstract In this study, a magnetic infinite coordination polymer with the morphology of nanocapsule, as an efficient adsorbent for removal of Hg2+ ions has been introduced. This infinite coordination polymer was synthesized from Zn2+ ion and a ditopic organic ligand (1,3-bis(tetrazol-5-ylmethyl)benzene (btb)) and its efficiency as an adsorbent was studied in view of adsorption isotherms, kinetics and thermodynamics. The adsorption capacity of mercury(II) ions was impressed by pH value and adsorption time and the optimal adsorption conditions were pH value of 8, adsorption time of 75 min. The adsorption isotherm was analyzed by Freundlich and Langmuir equations and was fitted with Langmuir model better. Outcomes indicated that the adsorption was an endothermic process. Moreover, this adsorption process was fitted with pseudosecond order model kinetically. Finally, the magnetic properties of nanocapsules synthesized in high and low concentration of initial reagents were investigated. Studies showed that the saturation magnetization of nanocapsules synthesized in low concentration of initial reagent is higher. Keywords: Infinite coordination polymer; Functional metal-organic nanocapsule; Mercury(II) ions; Removal.
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1. Introduction Infinite coordination polymers (ICPs) are a novel fascinating group of organic-inorganic materials fabricated from metal cations interconnected by multitopic organic ligands which have been studied over the recent years [1, 2]. Attractive tunable functional ICPs networks can be designed and synthesized for many applications as exemplified by gas adsorption [3], photonics [4], drug release [5], catalysis [6] and sensing [7]. These micro and nano structures have distinctive morphology such as rod, cube and sphere which can be crystalline or amorphous [8]. Infinite coordination polymer particles are synthesized based on three approaches including fast precipitation started by addition of a solvent into the solution of metal cations and ligands, solvothermal synthesis and microemulsion route [9]. Adsorption efficiency of heavy metal ions or other pollutants by functional metal-organic nanospheres has not been investigated yet. Mercury is a persistent and bioaccumulative heavy metal that causes lots of neurotoxic effects in human body [10]. Natural processes (e.g., volcanic actions, erosion of residues which contain mercury, etc) and anthropogenic activities (e.g., mining operations, tanneries, etc) have significant roles in the presence of mercury in our planet [11]. Accordingly, it is required to develop an effective method to eliminate mercury(II) ions from water sources. Some techniques such as liquid extraction [12], ion-exchange [13], membrane separation [14], chemical precipitation [15] and adsorption [16] have been studied to eliminate mercury ions from contaminated waters over the years. But, adsorption method has attracted more attention owing to its benefits such as feasibility and economical cost. Activated carbon, zeolites and some other porous materials are common adsorbents which have been investigated for elimination of heavy metal ions from wastewaters in the recent years [17–19]. Our research group reported the synthesis of ICP nanocapsules from the metal ions (Zn2+) and a new ditopic ligand (btb: 1, 3-bis(tetrazol-5ylmethyl)benzene) Zn(btb) with spherical morphology [20]. Since these spheres have an innate capacity to automatically entrap various species present in the initial reagents mixture such as Fe3O4, they were magnetized by trapping Fe3O4 NPs during their synthesis to collect them easily by a magnet. Considering there are free nitrogen sites in the structure of these nanocapsules, we expected them to adsorb heavy metal
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cations via acid-base Lewis interactions between nitrogen sites and metal cations. Therefore, in this work, we aimed to investigate the adsorption capability of these nanospheres for removal of Hg2+ cations and adsorption isotherms, kinetics and thermodynamics of Hg2+ adsorption by Fe3O4@Zn(btb) nanocapsules. 2. Experimental
2.1. Materials and methods 1,3-phenylenediacetonitrile and mercury(II) chloride were acquired from Sigma-aldrich (Milwaulkee,WI, USA). Sodium azide, zinc bromide and other reagents for the synthesis and analysis were commercially accessible from Merck Company (Darmstadt, Germany). IR spectra were measured on a Nicolet IR100 (Madison, WI) Fourier-transform infrared (FT-IR) spectrometer. Simultaneous inductively coupled plasma optical emission spectrometry (ICP-OES) on a Varian Vista-PRO instrument (Springvale, Australia) with a radial torch connected to a concentric nebulizer and a Scott spray chamber and outfitted with a chargecoupled detector (CCD) was utilized to determine the goal element. The prepared samples were characterized by a scanning electron microscope (SEM) (Philips XL 30) with gold coating. Transmission electron microscopy (TEM) images were attained with a Hitachi H-9500 instrument. Measurements of particles sizes were performed using a Zetasizer Nano apparatus. Powder X-ray diffraction (PXRD) of compound Fe3O4@Zn(btb) was carried out using a diffractometer of Philips Company with Cu kα radiation. 2.2. Synthesis of 1,3-bis(tetrazol-5-ylmethyl) benzene ligand The (1,3-bis(tetrazol-5-ylmethyl) benzene (btb)) ligand was synthesized according to previously reported method [20]. 1,3-phenylenediacetonitrile (20 mmol), sodium azide (60 mmol), zinc bromide (20 mmol), and 40 mL of water were added to a 250 mL round-bottomed flask. The reaction mixture was stirred and heated for 48 h at 270˚C to reflux. Afterward, HCl (3 N, 30 mL) was added to the cool mixture until the aqueous layer had a pH of 1. Then 200 mL of 0.25 N NaOH was added. The mixture stirred for 30 min, until the precipitate was disappeared. The suspension was filtered. 40 mL of 3 N HCl was added to the filtrate and stirred. The precipitate was separated and washed with HCl. The product had the following data: 3
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mp 269-270 °C. NMR (d -DMSO) 1H: 7.27 (t, 1H), 7.25 (b, 1H), 7.24 (b, 1H), 7.17 (m, 3H), 4.2 (s, 4H), 13C:
173.00, 155.17, 136.50, 129.16, 127.90, 29.60. Anal. Calc. for C10H10N8: C, 49.57; H, 4.13; N, 46.28.
Found: C, 49.12; H, 4.04; N, 46.1. 2.3. Synthesis details of Fe3O4@Zn(btb) nanospheres Fe3O4 nanoparticles (0.04 mmol) were added to a solution of 1,3-bis(tetrazol-5-ylmethyl) benzene (0.57 mmol) in DMF (4.5 ml). Afterward, the mixture was sonicated to make it homogenized and a methanolic solution (1 ml) of Zn(NO3)2·6H2O (0.57 mmol) was added to the mixture. Nanospheres that encapsulate Fe3O4 nanoparticles were formed. Fe3O4@Zn(btb) nanospheres with bigger size were prepared by the same route and same amount of Fe3O4 nanoparticles in concentration of initial reagents [Zn2+]= [btb] = 0.01 mol L-1. 2.4. Adsorption studies To analyze adsorption capacities at equilibrium and removal efficiency, the batch equilibrium method was utilized. A certain amount of adsorbent (0.01 g) was suspended in 10 mL Hg2+ aqueous solution and stirred for 80 min at RT to find out adsorption capacity. Then, magnetic nanocapsules were collected with a magnet. Finally, the concentration of Hg2+ in the supernatant was determined by ICP elemental analysis. Equations (1) and (2) were used to obtain the adsorption capacities at equilibrium (mg/g) and removal efficiency. qe = (C0 ‒ Ce) ×
V M
Hg2 + removal efficiency (%) =
(1)
(C0 ‒ Ce) C0
× 100
(2)
In this equation, C0 and Ce are the concentration of mercury ions (mg/L) at zero time and equilibrium, V is the volume of the initial solution (L) and M is the mass of Fe3O4@Zn(btb) nanospheres (g). 3. Results and discussion 4
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Our research group, reported the synthesis of tetrazolic metal-organic nanocapsules from addition of solution of Zn(NO3)2·6H2O in methanol to the DMF solution of btb previously. Investigations showed that concentration of initial reagents, affected the size of prepared ICPs. According to dynamic light scattering (DLS) studies, increasing the concentration of the initial reagents led to a decrease in the size of the Zn(btb) capsules. Fig. 1 shows (DLS) analysis of different suspensions of synthesized capsules in DMF with different concentration of initial precursors.
Fig. 1. DLS analysis of different suspensions of Zn(btb) spheres in DMF with different concentration of initial precursors (A) (96 ± 6) nm, (B) (331 ± 22) nm, (C) (667 ± 43) nm and (D) (1336 ± 87) nm for initial concentration of both metal ion and btb: 25 ×10-1 M, 5 ×10-2 M, 2.5 × 10-2 M and 1× 10-2 M respectively and the corresponding SEM images.
As mentioned, these capsules are capable of entrapping Fe3O4 nanoparticles or some other luminescent quantum dots, and fluorescent dyes as the guests distributed in the primary solution of precursors [20]. It is noteworthy to mention that there is not any chemical bonding between the guest and the surface of Fe3O4 nanoparticles. There may be some physical interactions between the guest and the infinite coordination polymer which does not seem to have a key role in the formation of Fe3O4@Zn(btb) nanospheres. According to DLS measurements, encapsulation of Fe3O4 nanoparticles does not change the size of the primary polymer. Fig. 2 displays representative reaction scheme of the formation of organic-inorganic 5
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spheres of Fe3O4@Zn(btb) in high and low concentration of initial precursors. The structure of the Fe3O4@Zn(btb) nanocapsules synthesized in low concentration of initial precursors were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 3 indicates
Fe3O4 particles are inside the metal-organic hollow sphere. In order to increase the surface area and the adsorption capacity of these magnetic nanocapsules, we synthesized these nanocapsules in high concentration of initial precursors and used them as an adsorbent in the present study.
Fig. 2. Representative reaction scheme of the formation of metal–organic hollow spheres of Fe3O4@Zn(btb) in high and low concentration of initial precursors.
Fig. 3. (a and b) HRTEM images of Zn(btb) spheres encapsulating the Fe3O4 nanoparticles prepared in the concentration of initial reagents [Zn2+]= [btb]= 2.5 × 10-2 M.
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3.1. Characterization of the Fe3O4@Zn(btb) nanocapsules before and after adsorption
Characterization of the sample before adsorption was performed by SEM. Fig. 4 shows SEM images of Fe3O4@Zn(btb) nanocapsules synthesized in high concentration of initial precursors with spherical morphology. Moreover N2 adsorption–desorption isotherms were determined at 77 K. Specific surface area of the sample was measured in accordance with the Brunaure–Emmet– Teller (BET) method which was 15 m2/g.
Fig. 4. SEM images of synthesized Fe3O4@Zn(btb) used as an adsorbent.
The magnetic properties of Fe3O4@Zn(btb) were investigated utilizing a vibrating sample magnetometer. The saturation magnetization attained for the magnetic capsules with smaller size was 2.68 emu g_1. Synthesized Fe3O4@Zn(btb) nanocapsules exhibited superparamagnetic behavior. The saturation magnetization of Fe3O4@Zn(btb) nanocapsules synthesized in low concentration of initial reagents was measured and compared with the nanocapsules synthesized in high concentration of initial reagents. From Fig. 5, the saturation magnetization of the Fe3O4@Zn(btb) nanospheres decreases from 12.38 to 2.68 emu g−1 with decrease of the ICPs sizes. Zn2+/Fe3+ molar ratio of Fe3O4@Zn(btb) synthesized in high and low concentration of initial reagents measured by ICP analysis were 2.35 and 0.44 respectively. According to Zn2+/Fe3+ molar ratio, nanocapsules synthesized in high concentration of initial reagents encapsulate lower amount 7
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of Fe3O4 nanoparticles that leads to lower saturation magnetization. The saturation magnetization (MS) of the adsorbent after removal of mercury(II) ions in concentration of 10 ppm was 2.39 emu g-1. In addition, energy-dispersive X-ray spectroscopy (EDAX) analysis (Fig. 6) of samples after adsorption signifies the presence of the Hg element, showing that the Hg2+ ions were favorably adsorbed on Fe3O4@Zn(btb) nanocapsules.
Fig. 5. a) Magnetic hysteresis loop of Fe3O4@Zn(btb) nanospheres synthesized in: I) low concentration of initial reagents II) high concentration of initial reagents III) and adsorbent after removal of mercury(II) ions from aqueous solution in concentration of 10 ppm. b) Vial including a suspension of Fe3O4@Zn(btb) nanospheres in aqueous solution of HgCl2 before and after putting a magnet close to the wall.
Fig. 6. EDX spectrum of Fe3O4@Zn(btb) after Hg2+ adsorption.
3.2. Effect of pH on Hg2+ adsorption
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The effect of pH on the adsorption of Hg2+ was studied in the pH range from 4 to 9. Outcomes showed that the adsorption efficiency of Hg2+ is dependent on pH of the solution by affecting their adsorption due to both the chemistry of Hg2+ ions in solution and protonation of the adsorbent donor atoms. The maximum removal of Hg2+ ions appeared in alkaline pH value of 8 (Fig. 7). At low pH values, Fe3O4@Zn(btb) nanosphere donor atoms are protonated and become positively charged, thereby decreasing electrostatic attractions between positively charged Hg2+ and positively charged adsorption sites leads to a decrease in the Hg2+ adsorption. The optimal adsorption pH is 8. The decrease in the efficiency of removal of Hg2+ ions at higher pH values may be due to the formation of hydroxide species of Hg2+ ions in the presence of high concentration of OH- ions and deformation of nanocapsules.
Fig. 7. The effect of pH on Hg2+ removal using Fe3O4@Zn(btb).
3.3. Adsorption kinetic considerations
The kinetic of adsorption process exhibits the solute uptake rate that ascertains the residence time of the adsorption process and can determine the efficiency of adsorption. The kinetic studies of mercury adsorption on Fe3O4@Zn(btb) nanospheres were accomplished by contacting 10 mg of adsorbent with 30 mg/L mercury ions solution at pH 8. Fig. 8 indicates the influence of contact time on adsorption of Hg2+ ions on the adsorbent.
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Fig. 8. Contact times for the adsorptions of Hg2+ onto Fe3O4@Zn(btb).
In the initial time, the adsorption efficiency was enhanced, and afterward declined, and attained equilibrium after 75 min. Therefore, the optimum time to gain highest adsorption capacity is 75 min. Two semi-empirical kinetic models were applied for analyzing the adsorption rate: the firstorder equation and the pseudo-second-order equation [21, 22]. Pseudo-first-order equation is shown as: ln (qe ‒ qt) = ln qe ‒ k1t
(3)
Where qt is the adsorption quantity of Hg2+ ions at any time (mg/g), qe is adsorption capacity at equilibrium (mg/g) and k1 is the rate constant of pseudo-first-order adsorption (g/mg. min). Therefore, the first order kinetic constant (k1) can be calculated by slope when the ln(qe − qt) is plotted against t. In addition, the intercept of this plot can be used to obtain ln qe. Nevertheless, the first order kinetic demonstrates poor linearity (Fig. 9). The linear model of pseudo-second-order equation can be shown by following: t 1 t = + qt kq2e qe
(4)
Another parameter is the initial adsorption rate represented as: h = kq2e (T→0)
10
(5)
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Parameters of k and h can be achieved experimentally from the slope and intercept of plot of t/qt against t. The slope (1/qe) and intercept (1/k2qe2) of plot (t/qt) against t were utilized to get the parameters of k2 and qe,
cal.
In accordance with Fig. 10 and adsorption data in Table 1, the
experimental data were fitted into pseudo-second-order equation well. 3.3. Adsorption Isotherms
To find out the relation between the quantity of metal ions adsorbed on the surface of Fe3O4@Zn(btb) nanospheres and the concentration of Hg2+ left in the solution, the adsorption isotherm was studied. For investigating the adsorption isotherm, in 10 different initial concentrations (between 10 to 200 (mg/ml)), 10 mL Hg2+ aqueous solution was poured into 10 mL erlenemeyer flask including 10 mg Fe3O4@Zn(btb) at RT and stirred for 80 min.
Fig. 9. Plot of the pseudo-first-order kinetics for the adsorptions of Hg2+ onto Fe3O4@Zn(btb).
Fig. 10. Plot of the pseudo-second-order kinetics for the adsorptions of Hg2+ onto Fe3O4@Zn(btb).
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Table 1. Kinetic adsorption parameters obtained using pseudo-first-order and pseudo-second-order models.
Pseudo-second-order
Lagergren first order
K2(g/mg.min)
h(mg/g.min)
qe, exp (mg/g)
qe, cal (mg/g)
R2
K1 (min-1)
qe, cal (mg/g)
R2
0.004
3.54
25.75
27.70
0.99
0.038
14.87
0.97
The Langmuir and Freundlich isotherm models were utilized in modeling the adsorption data [23]. The Freundlich isotherm model is applicable to explain multilayer adsorption and adsorption on heterogeneous surfaces. The Freundlich model can be expressed in the form: qe = kfC1/n e (none linear form)
(6)
1
log qe = log kf + nlog Ce (linear form) (7) In this equation, n is the constant relevant to the intensity of the adsorption and the constant kf is referred to the adsorption capacity of the adsorbent. Ce and qe are equilibrium concentration of Hg2+ ions (mg/L) and the quantity of Hg2+ ions adsorbed (mg/g), respectively. 1/n can be attained from the slope of the plot of log qe against log Ce. The intercept of this plot may be utilized to calculate Log kf. Fig. 11 shows the linear plot based on fitting the experimental adsorption data with Freundlich adsorption model.
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Fig. 11. Freundlich adsorption isotherm for the adsorptions of Hg2+ onto Fe3O4@Zn(btb).
The Langmuir equation is expressed as below: Ce
Ce 1 = + qe b Q0 Q0
(8)
In this equation, qe (mg/g) and Ce (mg/L) are the equilibrium adsorption capacity and the concentration of Hg2+ at equilibrium. Q0 and b represent the highest adsorption capacity and the equilibrium adsorption constant relating to the bonding energy of the adsorption, respectively. The highest adsorption capacity Q0 can be achieved from the reciprocal slope of a plot of Ce/qe against Ce (Fig. 12).
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Fig. 12. Adsorption curve of Hg2+ at different concentrations by using the Fe3O4@Zn(btb) nanocapsules (left), The linear regression by fitting the equilibrium adsorption data with Langmuir adsorption model (right).
The Langmuir constant b (L/mg) can be calculated from the slope/intercept of the Langmuir plot. High amount of b constant signifies more affinity of adsorbate for adsorption sites on the surface of adsorbent. Langmuir adsorption model is based on arrangement of a monolayer coating of adsorbate on the surface of the adsorbent. In the first step, amount of the solute adsorbed on the surface of the adsorbent was increased strongly with increasing the initial concentrations of Hg2+, and then gained a platform. Table 2 shows Langmuir and Freundlich isotherm parameters for Hg2+ adsorption on Fe3O4@Zn(btb) nanospheres. According to the linear regression correlation coefficients (R2), adsorption of Hg2+ ions on the surface of Fe3O4@Zn(btb) nanospheres follows the Langmuir isotherm model that approves a homogenous adsorption process and monolayer coating of Hg2+ ions on the surface of nanospheres. From Langmuir model, the highest equilibrium quantity of Hg2+ on Fe3O4@Zn(btb) nanospheres was 129.87 mg/g. The high adsorption capacity for Hg2+ ions on Fe3O4@Zn(btb) nanospheres assigns to the interaction between the nitrogen sites of tetrazolic group and mercury cations. The adsorption capacity of Fe3O4@Zn(btb) nanospheres was compared with other functionalized magnetic adsorbents reported previously for removal of Hg2+ ions in Table 3 [24-27].
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Table 2. Langmuir and Freundlich parameters for adsorption of Hg2+ onto Fe3O4@Zn(btb).
Langmuir model
Freundlich model
Qm (mg/g)
b (l/mg)
R2
Kf ( mg(n−1)/nL1/ng−1)
n
R2
129.87
0.1
0.99
15.13
1.8
0.91
Table 3. The adsorption capacity of other functionalized magnetic adsorbents reported previously.
Adsorbent
Adsorption Capacity (mg/g)
Reference
humic acid coated Fe3O4
98
[24]
thiol modified Fe3O4@SiO2
79
[25]
naphthalimide functionalized Fe3O4@SiO2
32
[26]
poly(1-vinylimidazole)-grafted Fe3O4@SiO2
346
[27]
Fe3O4@Zn(btb)
129.87
Present study
3. 4. Adsorption thermodynamics
Thermodynamic investigations can reveal influence of temperature on adsorption behavior and whether adsorption process is spontaneous or not, therefore it is important to study the process in view of adsorption thermodynamics. The adsorption process was investigated at four different temperatures (288, 303, 313, 323 K) and in initial concentration of 50 ppm. The Gibbs free energy (ΔG°) was obtained based on the following equation [22, 23]. ΔG° = ΔH° ‒ TΔS° (9)
ln Kd =
ΔS° ΔH° ‒ (10) R RT
In this equation, the ΔG° is the change in Gibbs free energy in J/mol. The change in the enthalpy ( ΔH°) in J/mol and change in the entropy (ΔS°) in J/mol K are calculated from the slope and 15
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intercept of ln Kd against 1/T plots, respectively. R represents the universal gas constant (8.314 J/mol K) and T is the temperature in K. Kd indicates linear adsorption distribution coefficient explained as:
Kd =
(C0 ‒ Ce) Ce
×
V (11) M
Where V and M represent the working volume (mL) and the adsorbent mass (g), respectively. Fig. 13 shows the van’t Hoff plot that is a plot of ln Kd against 1/T for Hg2+ adsorption on Fe3O4@Zn(btb) nanospheres in initial concentration of 50 ppm.
Fig. 13. Plot of ln Kd versus 1/T for Hg2+ adsorption on Fe3O4@Zn(btb) in initial concentration of 50 ppm.
Thermodynamics parameters of Hg2+ adsorption on Fe3O4@Zn(btb) nanospheres at different temperature are shown in Table 4. The negative ΔG° values reflect the fact that Hg2+ ions adsorb on the surface of nanospheres spontaneously at 288–323 K. According to the positive values of ΔH ° and the increasing values of Kd , the adsorption of ions onto Fe3O4@Zn(btb) nanospheres is an endothermic process. Adsorption process in solid–liquid systems involves desorption of the solvent molecules from the surface of the adsorbent and afterward adsorption of the adsorbate species. Endothermicity of the adsorption process is due to replacement of more than one solvent 16
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(water) molecule by adsorbate species. In accordance with the positive ΔS° value, the adsorption process involves the liberation of solvent molecule from the adsorbent. Table 4. Thermodynamics parameters of Hg2+ adsorption on Fe3O4@Zn(btb).
C0 (ppm)
50
T (K)
ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (J/K mol)
288
-19.87
303
-22.57
313
-24.38
323
-26.1
31.15
177.17
R2
0.99
3.5. Nanocapsules stability test
Powder X-ray diffraction (PXRD) of compound Fe3O4@Zn(btb) showed that the synthesized metal-organic nanocapsules are amorphous (Fig. 14), so common ways to study these nanocapsules are SEM and IR spectroscopy [20].
Fig. 14. XRD pattern of Fe3O4@Zn(btb).
Therefore, the stability of Fe3O4@Zn(btb) nanospheres at different pH values was investigated by SEM. The SEM observations indicated that at a pH of 8, the morphology did not change but in acidic or higher alkaline pH, nanocapsules were slightly deformed. Fig. 15 (a-c) shows SEM images of Fe3O4@Zn(btb) nanospheres after adsorption process at pH value of 4, 8 and 9. In 17
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addition, the morphology of nanocapsules after adsorption at temperature of 50
○C
was
investigated by SEM and obtained images were shown in Fig. 15d.
Fig. 15. a) SEM images of Fe3O4@Zn(btb) after adsorption at pH= 4, b) pH= 9, c) pH= 8, d) 50 ○C.
According to these images, after adsorption at this temperature, nanocapsules were not decomposed and only slightly adherence of nanocapsules was seen. Moreover, the stability of these nanocapsules was investigated by FT-IR spectroscopy. As can be seen in Fig. 16, the FT-IR spectra of the Fe3O4@Zn(btb) nanospheres synthesized in low concentration of initial precursors and the adsorbent before and after adsorption process at RT temperature and pH of 8 are the same.
Fig. 16. FT-IR spectra of the adsorbent a) Fe3O4@Zn(btb) synthesized in high initial concentration of precursors [Zn2+]= [btb]= 0.1 M b) Fe3O4@Zn(btb) synthesized in low initial concentration of precursors [Zn2+]= [btb]= 0.01 M c) after adsorption of Hg2+.
4. Conclusions
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In conclusion, the magnetic properties of Fe3O4@Zn(btb) nanospheres synthesized in high and low concentration of initial reagents were studied. Investigations showed that the saturation magnetization of the Fe3O4@Zn(btb) nanospheres decreased with decrease of the ICPs sizes and Fe3O4@Zn(btb) nanospheres synthesized in low concentration of initial reagents had higher
saturation magnetization than those synthesized in high concentration of initial reagents. In addition, the capability of magnetic functional metal-organic nanocapsules for removal of mercury(II) cations as a heavy metal and its adsorption isotherms, kinetic and thermodynamics were investigated. These nanocapsules are magnetic nanoscale adsorbents which consist of nitrogen atoms that make it a good choice for mercury(II) removal. According to SEM and FT-IR studies, these nanocapsules are stable at pH of 8. The kinetic analysis indicated that the overall adsorption process followed the pseudo-second-order kinetic model. Adsorption isotherm obeyed the Langmuir model indicating that during the adsorption process a monolayer coating of the adsorbate on the adsorbent was formed. The adsorption of metal ions on the adsorbent was a function of temperature and the adsorption capacity increased with increasing the temperature of the system, confirming the endothermic character of adsorption process. Finally, in comparison with the most of functionalized magnetic adsorbents reported previously, Fe3O4@Zn(btb) nanospheres have more capacity due to their nitrogen enriched surface. Acknowledgement Support of this investigation by Tarbiat Modares University is gratefully acknowledged. References [1] M. Oh and C.A. Mirkin, Nature, 438 (2005) 651-654. [2] X. Sun, S. Dong and E. Wang, J. Am. Chem. Soc. 127 (2005) 13102-13103.
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Infinite coordination polymers (ICPs) for mercury(II) removal has been introduced. Adsorption isotherms, kinetics and thermodynamics were studied. The magnetic properties of ICPs were investigated.