ARTICLE pubs.acs.org/JPCC
Monitoring of the Structure of Mesoporous Silica Materials Tailored Using Different Organic Templates and Their Effect on the Adsorption of Heavy Metal Ions Munusamy Thirumavalavan, Ying-Ting Wang, Ling-Chu Lin, and Jiunn-Fwu Lee* Graduate Institute of Environmental Engineering, National Central University, Chung-Li, Taoyuan County, 320, Taiwan, Republic of China ABSTRACT: In this study, we discussed a simple hydrothermal synthesis of self-adsorbing mesoporous silica using different templates and pore size engineering under various preparation conditions. The structural properties of the synthesized mesoporous materials were tunable by varying the reaction parameters. A hexadecyltrimethylammonium bromide template resulted in the formation of meosporous materials possessing a large surface area with narrow pore size distribution. The present synthesis route has the advantage of being a single-step self-assembly approach and involves the use of a low-cost and environmentally friendly precursor. In this work, the physical adsorption of heavy metal ions was investigated. The adsorption capacities of mesoporous materials were in the order of ZC-16 > ZC-14, ZC-12 > ZC-9, and the selectivity of heavy metal ions was in the order of Pb2þ > Cu2þ > Ni2þ. A comparative study of adsorption capacity of synthesized mesoporous materials with commercially available β-zeolite was carried out and discussed.
1. INTRODUCTION The unearthing of the first mesoporous silica featuring a (hexagonal) pore-ordered system, which was obtained using a surfactant as the structure-directing agent,1 constitutes a milestone in the field of porous materials.28 The self-assembly of surfactant aggregates and mineral species can now be tailored to provide mesoporous materials with high surface area, extremely narrow pore size distribution, and perfectly adjustable pore size. This pioneering work1 has germinated huge efforts of research both in synthesis and inclusion chemistry of new mesoporous materials with various composition and structure,2 offering new opportunities for applications in various fields, including mainly heterogeneous catalysis4,5 and separation sciences6 but also some advanced applications such as confined electron-transfer chains or quantum dots, mesoporous molecular wires, hostguest encapsulation, sensors, shape-selective polymerization, optical devices, immobilization of biomolecules, and green chemistry and environment, among others.7 The still-growing development of functionalized mesoporous materials, combining in a single solid, and the structural properties of the rigid inorganic lattice with the intrinsic chemical reactivity of the organic components has led to extending widely the scope of applications of ordered mesoporous systems.810 Most of the properties of these new high-technology materials are dependent on their structural and chemical composition as well as on the dynamic properties inside the blends. Interest of ordered mesoporous organosilicas with this respect has been evidenced in several applied fields: high catalytic activity and selectivity,4,5,11 improved removal of toxic heavy metal species, radionuclides, organic solvents from aqueous effluents,1214 enhanced activity r 2011 American Chemical Society
of entrapped organometallic complexes or biomolecules,15 and thin films and sensor devices.16 To date, a wide variety of surfactants and block copolymers have been extensively studied as templates to finely tune the structure and morphology of mesoporous materials.1,17,18 However, the structure and morphology of mesoporous materials are dependent on the cooperative self-assembly of surfactant templates19,20 and inorganic precursors.21 It is well-known that many factors such as reaction temperature,22 pH,23 etc. play important roles in the cooperative self-assembly of mesopores and these key factors may interact synergistically and determine the structure of the final product. In this regard, the influence of silica source upon the resultant mesostructure directed by templates should be studied in more detail. But the detailed studies on effect of preparation conditions and surfactants have been sparse and the present task is aimed at the same. In this current study, mesoporous silica materials were obtained by simple hydrothermal methods using different organic templates (hexadecyltrimethylammonium bromide [HDTMAB], myristyltrimethylammonium bromide [MTMAB], dodecyltrimethylammonium bromide [DTMAB], nonyltrimethylammonium bromide [NTMAB]) as a portal for the combined effect of various surfactants and different preparation conditions and subsequently characterized using various techniques. Variations in the structure-directing agent and synthetic conditions have led to the formation of mesoporous materials with a large variety of pore sizes, morphologies, and structures. However, details of Received: January 2, 2011 Revised: March 21, 2011 Published: April 01, 2011 8165
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Table 1. Mesoporous Materials Prepared under Various Experimental Conditions sample [M = ZC-16 (or) ZC-14 (or) ZC-12 (or) ZC-9]
calcination pH
time (h)
temp (°C)
temp (°C)
M-16 M-12H
10 10
24 12
100 100
600 600
M-48H
10
48
100
600
M-96H
10
96
100
600
M-168H
10
168
100
600
M-60
10
24
60
600
M-80
10
24
80
600
ZC-9150
10
24
150
600
M-500 M-700
10 10
24 24
100 100
500 700
ZC-9155
10
24
150
500
ZC-9157
10
24
150
700
9
24
100
600
M-P9
structure and stability of the obtained materials are highly dependent on the synthesis procedure.24,25 Therefore, numerous analytical techniques (such as X-ray diffraction, transmission and scanning electron microscopy, and adsorption) have been employed to monitor the synthesis and to characterize structures of ordered mesoporous materials. Among those methods, adsorption measurements appeared to be especially useful, providing information about the specific surface area, the pore size and uniformity, the pore volume, and the surface and textural properties of mesoporous molecular sieves.26,27 Water, the most important component of the ecosystem, and its purification with adsorbents and ion-exchangers for heavy metal ion removal is also an important topic for many scientific disciplines.28 Several different types of materials, among them resin, carbons, and clays, just to mention a few, have been studied and proposed for adsorption of heavy metal ions. Among the variety of adsorption applications, the preparation of highly effective mesoporous materials for heavy metal is clearly one of the most promising methods for environmental cleanup.29 The advantage of the present task is that these synthesized mesoporous materials can be directly used as adsorbents without any further surface modification. Investigations conducted on these materials will be discussed with an aim to gain insight into the effect of templates to confirm the structural characteristics of this new and promising mesoporous family and the effect of various parameters on their adsorption capacities.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Standard metal ion solutions were obtained commercially. All the chemicals and reagents used in this study were of Analar grade (AR grade) and used as supplied without further purification. 2.2. Preparation of Mesoporous Materials. 2.2.1. ZC-16. An aqueous solution of HDTMAB was shaken at 40 °C for 30 min at 150 rpm in a Lab-line orbit environ shaker. Sodium silicate (SiO2:Na2O = 3.27:1 g/mol, 35% 0.05 M) solution (pH = ca. 11 or 12) comprising 16.22 g/mol SiO2 (assay 27%) and 4.96 g/mol Na2O (assay 8%) was prepared by shaking aqueous sodium silicate solution at 150 rpm for 30 min at 40 °C. The HDTMAB solution was added slowly to the sodium silicate solution over a
period of 1 h under stirring and after complete addition; the reaction mixture was diluted using an appropriate amount of deionized water. Next, 1 M H2SO4 was added to maintain the pH at ca. 10. The reaction mixture was heated at 100 °C for 24 h. After cooling, the sample was washed several times with deionized water, filtered, and dried at 100 °C in an oven for 12 h. The final sample (herein referred as ZC-16) was obtained after calcination at 600 °C. 2.2.2. ZC-14, ZC-12, and ZC-9. ZC-14, ZC-12, and ZC-9 were prepared by above procedure using MTMAB, DTMAB, and NTMAB, respectively, instead of HDTMAB. 2.2.3. Preparation under Different Experimental Conditions. The experimental conditions such as temperature, time, pH, and calcination temperature were varied as given in Table 1, during the preparation of ZC-16, ZC-14, ZC-12, and ZC-9 to obtain various mesoporous materials with different pore size and surface area. 2.3. Batch Adsorption Studies. The adsorption of Cu2þ, Ni2þ, and Pb2þ was investigated in batch equilibrium experiments. Stock solutions of metal ions (1.0 g/L) were prepared using the obtained standard solutions in distilled water. The temperature was maintained at 28 °C. The experiments were performed at pH 3 in 250-mL conical flasks containing metal ion solutions (18) mg/L and the adsorbents (0.1 g), shaken for 24 h at 150 rpm in a Lab-line orbit environ shaker. Also the dosage of adsorbents, time, ionic strength, and pH were varied to study the effect of these parameters on adsorption. The ionic strength of the solution was controlled using 0.01 M CaCl2. The amount of adsorbents was varied such as 0.01, 0.02, 0.05, 0.1, 0.3, and 0.5 g with 1 ppm of metal ion solutions, and the experiment was carried out for 24 h at pH 3. To study the effect of time, the adsorption time was set to 1, 2, 4, 6, 12, 24, 48, and 72 h, and experiments were carried out by shaking 0.1 g of adsorbents with 1 ppm of metal ion solutions at pH 3. The adsorption was carried out at different pH values ranging from 2 to 8, shaking 0.1 g of adsorbents with 4 ppm of metal ion solutions. To study the role of ionic strength of the medium, the adsorption was carried out both in the presence and absence of 0.01 M CaCl2. 2.4. Analytical Procedure. The adsorption of metal ions from aqueous solutions was studied. After the desired incubation period for each batch, the aqueous phases were separated from the materials, and the concentration of metal ions was measured using a Varian AA-400 atomic absorption spectrophotometer (AAS). The amount of metal adsorbed was calculated using the following equation qe ¼ ½ðC0 CÞV =M where qe is the amount of metal ion adsorbed per unit amount of the adsorbent (mg/g); C0 and C are the concentrations of the metal ion in the initial solution (mg/L) and after adsorption respectively; V is the volume of the adsorption medium (L); M is the amount of the adsorbent (g). The value of qmax was calculated from qe using the Langmuir and Freundlich linear isotherms and the corresponding values were obtained using a linear method. 2.5. Desorption and Reuse of the Adsorbents. The reusability of the adsorbents for metal ion adsorption was determined. Measurements were repeated after 4 consecutive adsorptiondesorption cycles using the same adsorbent and the maximum deviation was found to be (4%. The desorption of metal ions was measured in a 10 mM HCl:HNO3 solution. The adsorbents with metal ions were placed in the desorption 8166
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Table 2. Surface Area and Pore Size of Different Mesoporous Materials BET surface area (m2/g)
average pore diameter (nm)
ZC-16
1568.72
3.07
ZC-14 ZC-12
1156.32 957.47
3.44 2.43
sample
Figure 1. Chemical structure of surfactants (templates).
ZC-9
779.83
4.51
medium and stirred at 150 rpm for 60 min at 25 °C. The final metal ion concentration in the aqueous phase was determined by using an AAS as described above. The desorption ratio was calculated from the amount of metal ion adsorbed by the adsorbents and the final metal ion concentration in the desorption medium using the following equation
ZC-1612H
860.85
3.44
ZC-1648H
891.27
3.45
ZC-1696H
925.18
3.41
ZC-16168H
776.88
3.15
ZC-1660
865.54
2.87
ZC-1680 ZC-9150
1005.69 1195.77
3.10 1.68
desorption ratioð%Þ
ZC-9105
556.33
2.28
¼ ðamount of metal ions desorbed to the elution mediumÞ 100=ðamount of metal ions adsorbed onto adsorbentsÞ
ZC-9107
691.82
2.32
ZC-9157
14.97
11.40
ZC-16-P9
656.73
6.38
β-zeolite
402.27
2.66
2.6. Characterization Techniques. The specific surface area and average pore diameter [BrunauerEmmettTeller (BET) surface and pore volume] were measured using a Quantochrome NOVA 1000. The adsorbents were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD patterns were obtained at room temperature using a Mac Science M3X Model 1030 instrument equipped with a Cu KR X-ray source (40 kV and 20 mA). SEM was performed using a HITACHI-S-800, field emission scanning electron microscope. The surface composition was calculated through elemental analysis using energy dispersive spectroscopy (EDS) combined with SEM. Fourier transform infrared (FT-IR) spectra were recorded using a Neclit 6700 model spectrometer. Thermogravimetric analyses (TGA) were performed with a TA Instrument, Perkin-Elmer TGA 7. A LM-595R rotary incubator (YIH DER Company) was used in the experiments requiring incubation. The pH was determined using a Jenco 6171 microcomputerbased bench pH meter.
3. RESULTS AND DISCUSSION To study the effect of templates, we used different organic templates, and the chemical structures of surfactants are shown in Figure 1. The experimental conditions such as hydrothermal temperature, time, pH, and calcination temperature were varied as shown in Table 1. The techniques FT-IR spectroscopy, BET, SEM, TGA, and XRD were used to characterize the resulting mesoporous materials. We obtained various mesoporous materials with different surface area and pore size under the various experimental conditions using different templates. In addition, in this study we also compared the activities of these mesoporous materials and their adsorption behavior toward various metal ions such as Cu2þ, Ni2þ, and Pb2þ. 3.1. FT-IR Spectroscopy, BET, SEM, TEM, TGA, and XRD. The surface compositions of the mesoporous (as-synthesized) materials were calculated using EDS elemental analysis. The Si:C ratio for ZC-9 is 1.25 (Si = 55.69% and C = 44.31%) and for ZC16 is 0.96 (Si = 49.20% and C = 50.80%). The Si:C ratio for ZC-9 is greater when compared to ZC-16 due to lower carbon chain length in ZC-9. Thus, the difference in Si:C ratio confirms the effect of organic templates (containing different carbon chain
Figure 2. XRD patterns of ZC-16 before and after Cu2þ adsorption.
length) on surface composition of mesoporous materials. The Si: C ratio confirms that the surfactant remained throughout the synthesis.30 The experimental data show that varying the synthetic conditions affected the specific surface area and pore size distribution as shown in Table 2. The XRD patterns of ZC-16, ZC-14, ZC-12, and ZC-9 exhibit peaks (2θ) at 2.14, 3.70, 4.20, and 5.60° as shown in Figure 2. These peaks are indicative of typical silicate mesoporus materials (MCM-41), and it is identified as the thermally stable porous medium having a hexagonal lattice structure with a unidimensional pore distribution.30,31 After the adsorption of metal ions (Figure 2), the d(100) value of the mesopores decreased slightly, and the intensity of all diffraction peaks decreased (but not significantly), which might be caused by scattering of the X-rays by metal particles rather than degradation of the pore structure. This shows that the adsorption of metal ions did not alter the structure of the mesopores and that the structure of the mesopores possesses a high thermal stability. The mesoporous materials synthesized under various experimental 8167
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Figure 4. TEM of (a) ZC-16; (b) ZC-9157.
Figure 3. SEM of (a) ZC-16; (b) ZC-14; (c) ZC-9.
conditions were characterized using the FT-IR spectroscopy before and after removing the organic templates. The peaks at 800 and 1082 cm1 are due to symmetric and asymmetric ν(SiOSi); the peak at 963 cm1 is due to terminal ν(SiOH); the peak at 2800 cm1 is due to ν(CH3); the peak at 2920 cm1 is due to ν(CH2), and the peak at 3480 cm1 is due to ν(OH) and hydrogen-bonded ν(SiOH). The disappearance of the sharp peaks at 2800 and 2920 cm1 in the FT-IR spectra of the meosoporus materials after removing the surfactants confirms the complete removal of the templates. The SEM morphological studies of the mesoporous materials confirm the presence of holes (pores) on the surface. The detailed study of SEM reports that the different experimental conditions affected the structural properties30 of the mesoporous materials as shown in Figure 3. Upon comparison of SEM images as shown in Figure 3, we observed that the hole size of ZC-9 is higher with respect to others. The comparison of TEM images of ZC-16 and ZC-9157, as shown in Figure 4, led to the observation that the hole size of ZC-9 157 is larger relative to that of ZC-16. But at the same time, we clearly observed that the surface area of ZC-16
is significantly larger relative to that of others. So, it was concluded that ZC-16 could be the most effective adsorbent due to its high surface area. TGA of the as-synthesized samples (without calcination) and calcined mesopores were conducted from room temperature (RT) to 850 °C. Figure 5 shows the TG curve of the as synthesized sample using NTMAB and ZC-9 before and after Cu2þ adsorption. The weight loss curves of the as-synthesized materials are dominated by the relatively large loss (4045 wt %) due to the decomposition of surfactants at 300600 °C (Figure 5a). A loss of 25 wt % below 120 °C is due to the loss of residual water. Gradual matrix decomposition from 300 to 600 °C accounts for the completion of decomposition of surfactants at 600 °C. Hence, the as-synthesized samples were calcined at 600 °C to get the final mesopores referred as ZC-16, ZC-14, ZC-12, and ZC-9. The weight loss curves of the calcined mesopores (Figure 5b) show no evidence of any residual surfactant. A similar profile of the TG curves over the entire temperature range (from room temperature to 850 °C) was observed for all the measured samples before and after the adsorption of metal ions (Figure 5b). This confirms that no changes in thermal stability of the samples occurred during adsorption. 3.2. N2 Adsorption Isotherms. The obtained nitrogen adsorption isotherms as shown in Figure 6 obviously convey that all the samples exhibited type IV curves.32 The hysteresis was not obvious initially (it may occur only in the high relative pressure range. At low relative pressure (P/P0 = 0.3) the adsorbed volume 8168
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Figure 5. Thermogravimetric weight loss curve of (a) as-synthesized sample (without calcination) using NTMAB; (b) ZC-9 before and after Cu2þ adsorption.
increased linearly upon increasing the pressure. This region corresponds to a monolayermultilayer adsorption on the pore walls. By increasing the value of P/P0 from 0.25 and 0.5, we observed a sharp increase in the adsorbed volume, which we attribute to capillary condensation. At this stage, the changes in nitrogen adsorption can be used as a measure of uniform pore size distribution. If greater the adsorption the larger will be the uniform size distribution and the rate of nitrogen adsorption increases in the following order as ZC-16 > ZC-14 > ZC-12 > ZC-9 indicating the increase in the order of pore size distribution. The curve became relatively flat below P/P0 = 0.3, and the surface area was measured by BET based on nitrogen adsorption desorption. This BET measurements showed that ZC-16 possess a large surface area among others and the order of increase in surface area of the mesopores is ZC-16 > ZC-14 > ZC-12 > ZC-9. At higher relative pressures multilayer adsorption occurred on the external surface, resulting in a gradual increase in the adsorbed volume. At a value of P/P0 of 1, saturation was reached indicating that all of the mesopores were filled with the condensed adsorbents. The isotherm for the mesoporous materials
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exhibits a sharp inflection characteristic of capillary condensation within a narrow pore size distribution, where the value of P/P0 of the inflection point is related to the average diameter of the pores.33 These N2 adsorption data confirmed the uniformity of the pore size distribution of the samples. In the case of ZC-16 and ZC-14, the observed hysteresis was narrow and less obvious, whereas both ZC-12 and ZC-9 showed obviously widespread hysteresis loop. These nitrogen adsorption curves clearly reveal that the different organic templates affect the pore size and its distribution. Thus narrow pore size distribution is uniform and accumulated in the case of ZC-16 and ZC-14, whereas in the case of ZC-12 and ZC-9, the pore size distribution is smooth and less accumulated. This study vividly reveals that the organic templates containing higher carbon chain resulted in mesopores with a larger surface area whereas the organic templates containing lower carbon chain resulted in mesopores with a lower surface area but contrarily relatively larger pore size. Thus, it could be concluded that HDTMAB is the most effective surfactant among all the templates used in this study. 3.3. Variation of ζ Potential with pH of the System. To study the stabilities and the textural properties of the medium synthesized using different templates, ζ potentials of the medium were measured at different pH values as shown in Figure 7. An insight into the variation of potential with pH indicates that the negatively charged ζ potential for ZC-9 is higher relative to that of ZC-14 and ZC-16. The ζ potential indicates the degree of repulsion between adjacent, similarly charged particles in dispersion. For molecules and particles that are small enough, a high ζ potential will confer stability, that is, the solution or dispersion will resist aggregation. When the potential is low, attraction exceeds repulsion and the dispersion will break and flocculate. This conveys that the accumulation of pore size distribution is more in the case of ZC-16 and ZC-14, whereas in the case of ZC9 the pore size distribution is smooth. Hence, HDTMAB is the effective surfactant among the templates, as ZC-16 possesses narrow pore size distribution. This clearly reveals that the templates containing lower carbon chain resulted in the formation of dispersed particles whereas the templates containing higher carbon chain led to the formation of aggregated (accumulated) particles. So, we can say that the carbon chain length of the templates played a vital role on morphology. It is noticed that the ζ potential vs pH curve is positive (less negative) at low pH and lower (more negative) at high pH. It can be seen that if pH is 610, the stability of the system is sufficient. However, if the pH of the system is below 6 or above 10, the system may be unstable. Hence, the stability of the dispersion decreased at high pH values and synthesis of mesoporous materials at high pH value is not feasible (preferable) in this study. 3.4. Effect of Experimental Conditions. In this study, we varied the experimental conditions such as heating time, temperature, pH, and calcination temperature using different templates. The surface area decreased when the reaction time was prolonged. When the time was 24 h, it led to an increase in the pore size distribution but beyond 24 h surface area of the porous medium decreased significantly. It suggests that extension of the reaction time is not beneficial to enhance the textural properties of the materials.34 From the experimental results, we found that increasing the hydrothermal temperature favored the formation of porous materials having large surface areas and pore sizes, which is contrary to the works reported earlier34 where the pore size decreased. This increase in temperature corresponds to an 8169
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Figure 6. N2 adsorption isotherms of (a) ZC-16, (b) ZC-14, (c) ZC-12, (d) ZC-9.
Figure 7. Plot of ζ potential vs pH.
increase in solvating power during the synthesis. This is due to the fact that at higher temperatures the reaction is accelerated and the uniform homogeneity occurs quickly due to the increase in solvating power. But the capillary condensation step shifted slightly in the direction of lower relative pressures as the reaction temperature was increased from 100 to 150 °C in the case of ZC9, which reflected the decrease in the pore width and pore volume of the materials.34 On the basis of the resulting pore size distributions, we found that the effect of calcination was insignificant. When we increased the calcination temperature from 500 to 600 °C, the surface area increased but pore size decreased. When the calcination temperature was further increased to 700 from 600 °C, the surface area decreased (but comparatively
larger than that of 500 °C), whereas the pore size remained approximately unchanged (but comparatively lower than that of 500 °C), suggesting that calcination at higher temperatures may lead to structural damage. It is noteworthy to discuss that in the case of ZC-9 calcination synthesized at 100 °C the hysteresis is observed more obviously relative to that of others due to different textural properties of ZC-9. But in the case of ZC-9 synthesized at 150 °C, the shape of the hysteresis loop started to change upon calcination. This observation suggests that both higher hydrothermal and calcination temperatures definitely cause for structural damage to large extent due to the particles transformation creating perturbation in the distribution of pores. But at the same time in the case of ZC-9, an interesting controversial observation was noticed that the increase in hydrothermal temperature from 100 to 150, increased the surface area significantly, but significantly reduced the pore size, relatively the lowest pore size among all the mesopores in this study. When ZC-9 synthesized at 150 °C was subjected for higher calcination, say for example from 500 to 700 °C, mesopores with relatively the largest pore size and the lowest surface area (among all the mesopores) was obtained. We suspected that high pH (pH 11 or 12) would not favor the formation of porous materials because the use of strong base to adjust the pH would lead to structural disintegration of the alkyl silicates. N2 adsorption isotherms of the porous materials obtained at pH 9 and 10 were compared. Even though mesoporous materials synthesized at pH 10 had larger surface area and a greater pore size distribution than that of pH 9, the pore size was significantly reduced in the case of pH 10. From Table 2, among the reaction temperature, time, calcination, and pH under investigation, the optima were found to be 100 °C, 24 h, 600 °C, and pH 10 respectively. Among all the 8170
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Table 3. Adsorption of Heavy Metal Ions by Mesoporous Materials and Langmuir Isotherm Parameter adsorption of metal R2
ions (qm, mg/g) Pb2þ
Cu2þ
Ni2þ
Pb2þ
Cu2þ
Ni2þ
ZC-16
9.85
3.85
2.58
0.9922
0.9907
0.9949
ZC-14 ZC-9
7.83 6.61
3.46 2.67
2.27 2.23
0.9972 0.9991
0.9816 0.9971
0.9946 0.9933
sample
Figure 8. Adsorption of heavy metal ions (Cu2þ and Pb2þ) by ZC-16, ZC-14, and ZC-9.
surfactants under investigation, HDTMAB provided the mesopore (ZC-16) with relative a large surface area and pore size. All these observations suggesting that the present synthesis strategy permits a way to fine-tune the structural properties by varying the synthetic conditions. Most importantly, the present synthesis route provides an easy way to accelerate the entire synthesis by reducing the time, cost, complication and functionalization. 3.5. Adsorption Isotherms. By using the mesoporous materials, the isothermal adsorption experiments were carried out for Pb2þ, Cu2þ, and Ni2þ ions at 28 °C, respectively, and the adsorption isotherms are presented in Figure 8. For this purpose, various concentrations (18) mg/L of heavy metals were used in solutions and were mixed with 0.1 g of the adsorbent and the adsorption behavior was studied after 24 h. To access the adsorption effect of adsorbents, the Langmuir and Freundlich adsorption models were used in this study. The obtained adsorption data of metal ions and Langmuir parameters for all the adsorbents are given in Table 3. For the Freundlich isotherm model, the calculated R2 values of ZC-16, ZC-14, and ZC-9 for Pb2þ adsorption were 0.8321, 0.8155, and 0.7946, respectively. For the Langmuir isotherm model, the correlation coefficients (R2) were greater (Table 3) than that of Freundlich model, indicating that the Langmuir model could be applied to these systems. The results suggest that the adsorption process involved a physical adsorption with the adsorption capacity
ZC-1612H
5.34
2.21
1.24
0.9926
0.9908
0.9946
ZC-1648H
5.45
2.31
1.89
0.9907
0.9978
0.9933
ZC-1696H
5.62
2.51
2.13
0.9908
0.9944
0.9961
ZC-16168H
5.57
2.11
1.91
0.0883
0.9914
0.9905
ZC-1660
5.99
3.17
1.97
0.9946
0.9971
0.9975
ZC-1680
6.12
3.32
2.06
0.9626
0.9484
0.9942
ZC-16-P9 ZC-9105
8.16 5.45
2.94 2.05
2.18 2.08
0.9738 0.9991
0.9884 0.9971
0.9949 0.9933
ZC-9107
6.08
2.45
2.17
0.9881
0.9484
0.9787
ZC-9157
6.21
2.53
2.19
0.9657
0.9371
0.9905
β-zeolite
4.35
0.9842
depending upon the surface area and pore size of the adsorbents. One possible explanation for physical adsorption occurring at lower concentration may be the release of weakly bonded ions into the solution making it easy for metal ions to be removed through ion exchange.35 It is known that the structure of the negatively charged porous medium such as (AlO4)5 surface is uneven and therefore need to rely on the surface of the positive charge of cations such as Naþ, Hþ, and Ca2þ, which are located outside the porous structure, making porous adsorption media also has cation exchange capacity. Such ion exchange capacity influences the removal of heavy metal ions to certain degree in physical adsorption. It can be seen from Figure 8 that ZC-16, displaying a faster adsorption rate and the adsorption ability of mesoporous materials for metal ions is in the order: ZC-16 > ZC-14, ZC-12 > ZC-9. This is mainly due to the reason that ZC-16 particles have excellent swelling property in water resulting in the smaller diffusion resistance of metal ions, so that metal ions are easy to diffuse within the particles, and a faster adsorption kinetics behavior of ZC-16 is displayed. Adsorption by each of the adsorbents increased upon increasing the initial concentration of the metal ions in solution. In all cases, when the initial concentration exceeded (34) mg/L, the adsorption capacity remained almost constant presumably because, at that initial concentration, the active adsorption sites of the adsorbents were almost gradually filled by metal ions and hence the adsorption activity was limited.36 Thus, when the initial concentration of metal ions was increased further, no additional adsorption occurred because of the unavailability of adsorption sites suggesting that an optimal concentration of metal ions existed for effective adsorption. After the adsorption of metal ions, the BET surface area, pore width, and pore volume of the mesopores decreased in all cases due to an increase in sample density, which was caused by filling of metal ions into the adsorbent pore system. For example, the surface area of ZC-16 decreased from 1568.72 to 1006.12 m2/g and the pore volume decreased from 3.07 to 2.98 nm after Cu2þ adsorption. Among different experimental conditions of all mesoporous materials, ZC-16, ZC-14, ZC-12, 8171
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Figure 9. Effect of (a) adsorption time, (b) adsorbent dosage, (c) ionic strength, (d) pH on metal ions adsorption.
and ZC-9 obtained at 100 °C for 24 h at pH 10 after calcination at 600 °C showed enhanced adsorption capacity. These experiments vividly reveal that the choice of preparation conditions altered not only the surface morphology of the adsorbents but also the extent of their adsorption of metal ions. 3.6. Adsorption Properties of Mesopores toward Various Heavy Metal Ions. On the basis of the experimental results, it is displayed that the adsorption capacities of metal ions are in the order Pb2þ > Cu2þ > Ni2þ. This could be attributed to the relatively larger size and larger ionic radius of Pb despite Pb2þ, Cu2þ, and Ni2þ having almost similar higher electronegativity.37,38 So Pb2þ ions are easily diffused within the mesoporous layers and hence higher adsorption was displayed. Notably, the selectivity for Pb2þ was greater than that for Cu2þ and Ni2þ, suggesting that the size of the pores played a vital role in the adsorption process. The saturated amount of Pb2þ by ZC-16 is 9.85 mg/g; Cu2þ is 3.85 mg/g, and Ni2þ is 2.58 mg/g. Our experimental observations suggest that the size and concentration of metal ions and the surface area and pore size of the adsorbents are all important key factors that greatly affect the adsorption phenomenon. This study reports that the adsorption capacity of the reported mesoporous materials is higher relative to that of commercially available β-zeolite as shown in Table 3. 3.7. Effects of Different Parameters on Adsorption. In this section, effect of various parameters such as adsorption time, adsorbent dosage, pH, and ionic strength of the solution was examined and discussed. It can be seen from the experimental results that the adsorption efficiency was gradually increased
when the adsorption time increased from 12 h. Figure 9a gives the adsorption kinetics curve from which it can be seen that in 24 h the adsorption can reach equilibrium state, beyond which, the adsorption remained almost steady. By varying the adsorbent dosage to study its effect on adsorption as shown in Figure 9b, we observed that the adsorption gradually increased with the adsorbent dosage up to 0.1 g where the steady state of adsorption reached. The studies on controlling the ionic strength of the medium using 0.01 M CaCl2 revealed that the presence of CaCl2 definitely accelerated the adsorption of heavy metal ions as shown in Figure 9c. This is due to the fact that increase in ionic strength makes the diffusion of ions faster bestowing a faster adsorption rate. By variation of the pH value of the medium, the isothermal adsorption experiments at different pH values were carried out, respectively, and Figure 9d represents the adsorption isotherms for Pb2þ ion at different pH values. The following facts can be found that the adsorption capacity increased with increasing pH value;39 as pH = 5, the adsorption capacity gets up to a maximum; over pH 5, the adsorption capacity remained steady with the increase of pH value. The above facts reflect the interaction mechanism between mesopores and metal ions, and it can be explained as follows; At lower pH, the medium is highly protonized, displaying the cationic character, whereas at higher pH, the hydroxyl groups will dissociate40 (pKa of OH groups), exhibiting the anionic character. As pH is lower, the metal ions will lose the adsorption ability due to protonation, leading to the low adsorption ability of mesoporus materials for metal ions. Along with the increase of pH value, the protonation degree of 8172
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The Journal of Physical Chemistry C medium will be weakened, and the adsorption ability of these mesopores toward metal ions will be strengthened. However, the heavy metal ions are easy to be hydrolyzed. As pH > 5, the hydrolysis of heavy metal ions has become obvious, and the surfaces of mesopores will be covered with the hydrolysis product. This will badly affect the adsorption property of the solid adsorbent41 and lead to the decline of the adsorption capacity. It was believed that the solubility of metal ions is also one of the major parameters in metal ion adsorption. The state of the metal ions could also decide the adsorption behavior of metal ions. In the case of copper, the main species at different pH values are possibly CuOHþ, Cu(OH)2, Cu(OH)3, and Cu2þ, and in the case of lead, the main species at different pH values are possibly PbOHþ, Pb(OH)2, Pb(OH)3, Pb(NO3)2, Pb(NO3)þ, and Pb2þ. At optimum pH value, 85% of removal efficiency was observed. In a word, for the strong adsorption action of mesoporous materials toward heavy metal ions, the driving force comes from the cooperation of electrostatic interaction and adsorption capacity of OH group. 3.8. Desorption and Reuse. Desorption of the adsorbed metal ions from the adsorbents was also studied. The desorption efficiency of the adsorbed metal ions ranged from (8590)%. The reusability of the adsorbents was measured by repeating the metal ion adsorptiondesorption cycle 4 times using the same adsorbents. One possible cause of reduction in the adsorption capacity of the adsorbents could be the adverse effects of the desorbing agents on the binding sites. The regeneration of the adsorbents shows that the adsorptiondesorption process is a reversible process and that the adsorbents can be used repeatedly for the removal of metal ions from aqueous solution.
4. CONCLUSION By using different organic templates, the hydrothermal synthesis of self-adsorbing mesoporous materials was reported to examine the effects of templates and preparation conditions. On the whole, the template and preparation conditions play a more decisive role in the formation of ordered mesopore structures. ZC-16 (using HDTMAB) mesopores have a large surface area and hence showed high adsorption capacity for heavy metal ions. The adsorption of heavy metal ions is a physical adsorption process. The mesoporous material synthesized at 100 °C for 24 h at pH 10 was the most effective adsorbent, and this synthesis was therefore considered to feature the optimal preparation conditions. The effect of various parameters on adsorption was explored. The adsorption ability of mesoporous materials for metal ions is in the order ZC-16 > ZC-14, ZC-12 > ZC-9, and the selectivity for heavy metal ions is in the order Pb2þ > Cu2þ > Ni2þ. The degree of adsorption depended on surface area and pore size of the adsorbents and the size and concentration of metal ions in solution. The result of this study indicated that the adsorbents used in this present study are better than that of commercially available β-zeolite and also cost-wise more economic. This study suggests that a cost-effective and viable technology for the elimination of heavy metal ions from aqueous solution can be developed using simple hydrothermal methods. ’ AUTHOR INFORMATION Corresponding Author
*E-mail: jfl
[email protected]. Phone: þ886-3-422715134658. Fax: þ886-3-4226742.
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