Ind. Eng. Chem. Res. 2005, 44, 3665-3671
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Adsorption of Aqueous Mercury(II) on Propylthiol-Functionalized Mesoporous Silica Obtained by Cocondensation Jose´ Aguado,* Jesu ´ s M. Arsuaga, and Amaya Arencibia Chemical and Environmental Technology Department, School of Experimental Sciences and Technology (ESCET), Rey Juan Carlos University, c/Tulipa´ n s/n, E-28933, Mo´ stoles, Spain
Adsorption of inorganic mercury(II) from an aqueous solution on mercaptopropyl-functionalized SBA-15 mesoporous silicas has been investigated. Adsorbents were prepared by the cocondensation method using tetraethoxysilane and different quantities of (mercaptopropyl)trimethoxysilane. Pluronic P123 was employed as a structure-directing agent. Mercury adsorption isotherms at 20 °C are reported for every synthesized material. The maximum mercury loading has been obtained from Langmuir fitting. Stoichiometric ratios of 1:1 S-Hg have been observed in all cases. Adsorbents were extremely efficient in removing inorganic mercury from an aqueous solution with a maximum in mercury loading of ca. 2.9 mmol of Hg g-1 for the material with the higher organic content. The effect of the pH on mercury adsorption has been studied. The efficiency of the adsorbents has been found to remain almost constant from neutral pH up to 3 M nitric acid concentration. 1. Introduction Pollution by heavy metals is one of the major problems in wastewater treatment. In the case of mercury, which is highly toxic for both the environment and human health, there is an active research to develop effective methods to eliminate it from wastewater. Mercury is present in the environment from mining activities, urban wastes, and industrial sources such as chloroalkali processes and the manufacture of batteries, paints, and paper. Many techniques have been proposed for the treatment of wastewater containing mercury. The most employed procedures are precipitation, coagulationcoprecipitation, reverse osmosis, ionic exchange, and adsorption.1 Among them, the adsorption on activated carbon appears to be one of the most common techniques because of its simplicity of operation. In the last years, there has been an increasing interest to develop new adsorbents including resins,2 clays,3 and carbon materials obtained from nonconventional sources (biomass, agricultural products, and the like).4-8 A quite promising alternative for selective adsorption is the use of specific chelating organic groups that can be anchored to the surface of a solid support. Because mercury species show a high affinity toward sulfur, several thioorganic groups have been used for improving the adsorption efficiencies of different materials such as activated charcoal,6-8 clays,9,10 or silica.11,12 Frequently, however, not all of the functional groups that are incorporated into the material are accessible to the mercury species because such materials present small and irregular pore sizes.9 Mesostructured silica materials obtained by surfactant assembly methods present high superficial areas and large uniform pores.13-16 Therefore, they can be suitable supports for adsorbent design. For this potential application, hybrid organic-inorganic silicas have * To whom correspondence should be addressed. Tel.: 34 91 488 70 05. Fax: 34 91 488 70 68. E-mail: jose.aguado@ urjc.es.
been developed by anchoring organic moieties to the mesoporous silica surface, allowing its use in specific adsorption.17 For mercury treatment, these materials have been superficially modified by incorporating an organic monolayer that contains thiol and amine groups as specific sites of mercury anchorage.18-32 Propylthiol derivatives have been prepared by both grafting and one-step synthesis on several types of mesostructured silica. (3-Mercaptopropyl)trimethoxysilane (MPTMS) has been reported as a suitable organic source in the synthesis of thiol-functionalized mesoporous silica. Materials so prepared have been used for selective mercury adsorption18-28,32 and in acidic catalysis after oxidation from thiol to sulfonic groups.32-35 For the first purpose, Feng et al.18 and Mercier and Pinnavaia19 developed in 1997 mesostructured siliceous materials with a high selective capacity of removing mercury by grafting the silica surface of MCM-41- and HMS-type materials, respectively, with mercaptopropyl groups. Since then, the mercury adsorption capacity on mercaptopropyl-functionalized HMS,20 MCM-41,21,22 and SBA-1523 obtained by grafting of the preformed mesoporous silica has been reported. These new adsorbents present a significant capacity of mercury adsorption (up to 3 mmol g-1) because they contain high amounts of functional groups that are accessible to the mercuric species because of the uniform open-framework mesoporosity of the materials. Unfortunately, it is necessary to employ a large excess of thio-organic precursor in the grafting procedure. Moreover, to achieve the highest sulfur incorporation, a previous treatment of the calcined mesoporous silica is required. So, the material presenting the highest mercury capacity (FMMS) was obtained by repeated hydrolysis and sililation processes.18 More recently, adsorption of mercury on mesostructured silica MCM-41,32 HMS,24,25 and MSU26,27 obtained by the cocondensation method has been studied. MPTMS was used as an organic precursor in all cases. This method has gained importance because of the simplicity of the procedure, the smaller consumption of the organic precursor, and the smaller synthesis time. The resulting
10.1021/ie0487585 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/06/2005
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materials exhibit a high content and good dispersion of functional groups, although for some of them a constriction of pores into the micropore range has been observed. This constriction can avoid the total access of mercury species to the active sorption sites.24,25,32 For instance, thiol-MCM-41 obtained by the cocondensation procedure contains a high sulfur content, but mercury adsorption loading (2.1 mmol of Hg g-1) is only about half of the sulfur content of the microporous material. A similar behavior has been observed for the one-step synthesis of the thiol-HMS material. Conversely, Brown et al. developed a thiol-functionalized MSU material with a high sulfur content that preserved the mesostructured channel network, so allowing full access of mercury to every binding site.26 As is well established, mesoporous silica SBA-15, prepared by using a poly(alkylene oxide) triblock copolymer as the structure-directing agent, has the largest pore size found for siliceous mesostructured materials.16 In addition, its thicker silica walls are expected to impart significant thermal and mechanic stabilities. Therefore, it could be the ideal support for metal adsorbent design. However, to the best of our knowledge, there are no studies that describe the use of thiolfunctionalized SBA-15-type silica materials synthesized by the cocondensation procedure as the mercury adsorbent. In this work, we report a study on the mercury adsorption capacity of thiol-functionalized mesoporous SBA-15 silica obtained by cocondensation using different proportions of MPTMS as the organic precursor. Our aim is to investigate the adsorption of aqueous mercuric chloride on synthesized materials at room temperature. Also, the effect of the pH conditions on adsorption loading has been studied because the wastewater pH is one of the common variables that affect the adsorption efficiency. 2. Materials and Methods 2.1. Chemicals. Surfactant Pluronic P123, triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO20PPO70PEO20) was used as the structure-directing agent. Tetraethoxysilane [TEOS; (CH3CH2O)4Si] and (3-mercaptopropyl)trimethoxysilane [MPTMS; (CH3O)3Si(CH2)3SH] have been employed as silica and organic precursors, respectively. All reagents were purchased from Aldrich and used without further purification. Mercuric chloride from Fluka was used in adsorption experiments. 2.2. Adsorbent Synthesis. Propylthiol-functionalized mesoporous silicas SBA-15 were synthesized following the previously reported procedure developed by Margolese et al. with minor changes.35 A total of 4 g of Pluronic P123 was dissolved at room temperature in 125 g of 1.9 M HCl. After the solution was heated to 40 °C, 0.037 mol of TEOS was added, and the resulting solution was stirred for 45 min before the addition of MPTMS. Different quantities of the organosilane precursor were used to obtain molar ratios of MPTMS/ (MPTMS + TEOS) of 0.05, 0.1, 0.15, 0.20, and 0.30 in the initial mixture. The reactant mixture was stirred for 20 h and finally aged for 24 h at 100 °C. Silica products were recovered by filtration and then dried. The template was extracted with ethanol under reflux for 24 h, filtered, and dried at 100 °C for 2 h. We have denoted these materials as SBA15SH-x, where x indi-
cates the molar percentage of MPTMS in the initial synthesis mixture. 2.3. Adsorbent Characterization. Textural properties were determined by measuring the N2 adsorptiondesorption isotherm at 77 K on a Micromeritics Tristar 3000 sorptometer. The surface area was determined by using the Brunauer-Emmett-Teller (BET) equation, and the pore size distribution was obtained from the adsorption branch by means of the Barrett-JoynerHalenda model with a cylindrical geometry of the pores; the pore volume was taken at P/P0 ) 0.97. Low-angle X-ray diffraction (XRD) patterns of the samples were obtained on a powder PW3040/00 X’Pert MPD/MRD diffractometer using the Cu KR line. Transmission electron micrographs (TEMs) were acquired on a Philips Tecnai 20 electronic microscope working at 200 kV. Scanning electron micrographs (SEMs) were obtained with a XL30 ESEM microscope. Covalent bonding of propylthiol to the silica structure was evidenced by 29Si NMR recorded on an Infinity Plus Varian multinuclear spectrometer with a 400-MHz resonance frequency. The thiol groups’ qualitative presence was determined by infrared spectra with a Fourier transform infrared (FTIR) Infinity Series spectrometer in the 400-4000-cm-1 range. A quantitative determination of the thiol functional group content was done by elemental microanalysis. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out with a TA Instruments SDT 2960 apparatus at a heating rate of 5 °C min-1. 2.4. Mercury Adsorption Experiments. Different sets of batch experiments have been carried out to investigate the effect of mercury loading on the contact time, the initial mercury concentration, and the pH of the aqueous solution. Single runs were carried out by stirring 25 mg of functionalized silica in 45 mL of aqueous HgCl2 solutions at 20 °C. Mixtures were filtered with a syringe filter of 0.22 µm, and the final solutions were collected. The mercury concentrations in both the initial and final solutions were determined by inductively coupled plasma atomic emission spectroscopy. Measurements were performed in a Varian Vista AX spectrometer after calibration with stock solutions in the range of concentrations of 0-10 ppm. Two emission mercury lines (194 and 253 nm) were used according to the standard EPA method for mercury analysis.36 The mercury adsorption capacity was determined by the difference between the initial and final mercury concentrations in the solution. In addition, some experiments have been carried out with larger quantities of adsorbent, namely, 100 mg in 175 mL of a 650 ppm Hg(II) aqueous solution. The mercury-loaded samples were recovered by filtration and analyzed in order to determine the quantity of adsorbed mercury. Chemical analysis was carried out on a Philips X-ray fluorescence spectrometer, MagiX model. 2.5. Modeling of Mercury Speciation in an Aqueous Solution. To study the influence of the mercury chemical state in an aqueous solution on the adsorption process, speciation diagrams have been calculated as a function of the pH using the chemical equilibrium modeling software MINEQL+.37 Stability constants of mercury complexes used in these calculations are shown in Table 1.
Ind. Eng. Chem. Res., Vol. 44, No. 10, 2005 3667 Table 1. Stability Constants of Mercury Species Used by the Modeling Program MINEQL+ reaction
log K
Hg2+ + Cl- T HgCl+ Hg2+ + 2Cl- T HgCl2 Hg2+ + 3Cl- T HgCl3Hg2+ + 4Cl- T HgCl42Hg2+ + Cl- + OH- T HgClOH Hg2+ + OH- T HgOH+ Hg2+ + 2OH- T Hg(OH)2 Hg2+ + 3OH- T Hg(OH)3+ Hg2+ + NO3- T HgNO3+ Hg2+ + 2NO3- T Hg(NO3)2
6.72 13.23 14.2 15.3 10.44 10.97 22.36 21.46 0.77 1.00
3. Results and Discussion 3.1. Adsorbent Characterization. The structure of the silica framework and the pore size are recognized as key factors determining the total mercury adsorption capacity.20,24-26,32 So, the porous structure of the synthesized samples has been studied by low-angle XRD. XRD patterns are plotted in Figure 1. SBA15SH-5 and
Figure 2. (a) TEMs in the direction of the pore axis and the perpendicular direction (inner graph). (b) SEM of thiol-modified mesoporous SBA15SH-10.
Figure 1. Powder low-angle XRD patterns of the thiol-modified SBA-15 materials: (a) SBA15SH-5; (b) SBA15SH-10; (c) SBA15SH15; (d) SBA15SH-20; (e) SBA15SH-30.
SBA15SH-10 XRD diffractograms show an intense reflection (100) and one additional weak peak at higher angle (110), denoting high hexagonal mesoscopic order. Samples containing higher MPTMS ratios provide a single XRD signal with a significant intensity decrease and a peak broadening. Both effects are attributed to the higher contrast between silica and organic groups and the loss of mesostructured order of the material with increasing organic content. Interplanar distances d100 and unit cell parameters a0 are reported in Table 2. TEMs exhibit a typical hexagonal SBA-15 arrangement of mesopores (Figure 2a). SEM images of materials
(Figure 2b) evidence morphology of ropelike domains with a uniform particle size of around 1 µm. Nitrogen adsorption-desorption experimental isotherms at 77 K as well as the calculated pore size distributions of SBA15SH-10, SBA15SH-15, and SBA15SH-30 samples are plotted in Figure 3. Characteristic mesoporous type IV IUPAC isotherms have been found. As seen in the figure, a change in the shape of isotherms and a broadening of the pore size distribution are observed when thiol functionalization increases. Pore diameter Dp, pore volume Vp, and superficial area BET SBET are also found to decrease significantly with increasing sulfur content. Conversely, silica wall thickness ep extracted from XRD and nitrogen sorption parameters steadily increases in the same order (Table 2). Nevertheless, materials preserve its mesoporous nature in all cases, showing a suitable porosity to act as mercury adsorbents. To verify the incorporation of propylthiol groups in the structure, 29Si NMR spectra of the samples have been carried out. Figure 4 shows the resonances due to siloxane and organosiloxane groups. Qn signals at -112, -103, and -94 ppm are assigned to silicon in siloxane environments such as Q4 for (SiO)4Si, Q3 for (SiO)3SiOH, and Q2 for (SiO)2Si(OH)2; Tm signals at -67 and -58 ppm, which are attributed to silicon covalently bonded to propylthiol groups (SiO)3Si-(CH2)3SH, T 3, and (SiO)2OHSi-(CH2)3SH, T 2, respectively, are also detected. Analysis of the spectra has been carried out by splitting
Table 2. Physicochemical Properties of Thiol-Modified Mesoporous SBA-15 Materials and Maximum Mercury Loading structural properties
sulfur content (mmol of S g-1)
maximum mercury loading Q0 (mmol of Hg g-1)
0.54 1.19 1.45 2.04 2.91
0.55 1.21 1.45 2.00 2.88
textural properties
material
d100 (Å)
a0 (Å)
ep (Å)
DBJH (nm) p
SBA15SH-5 SBA15SH-10 SBA15SH-15 SBA15SH-20 SBA15SH-30
103 95 93 87 90
119 109 107 101 104
49 46 51 53 67
7.0 6.3 5.6 4.8 3.7
SBET
(m2 934 885 873 611 640
g-1)
Vp
(cm3
g-1)
1.04 0.92 0.83 0.49 0.44
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Figure 3. Nitrogen adsorption-desorption isotherms at 77 K and calculated pore size distributions (inner graph) for (a) SBA15SH10, (b) SBA15SH-15, and (c) SBA15SH-30.
Figure 5. FTIR spectrum of the material SBA15SH-10 and an enlarged view of the spectral region near 2570 cm-1.
Figure 6. Final mercury(II) concentration after adsorption on SBA15SH-10 as a function of the stirring time.
Figure 4. 29Si NMR spectra for synthesized materials: (a) SBA15SH-5; (b) SBA15SH-10; (c) SBA15SH-15; (d) SBA15SH-20; (e) SBA15SH-30. RI reads for the intensity ratio I(T m)/[I(Qn) + I(T m)] calculated from spectral profile analysis.
the profiles into Gaussian bands. Intensity ratios, RI ) I(T m)/[I(Qn) + I(T m)], obtained from this analysis quantify the degree of organic incorporation as the ratio of silicon-bonded to organic groups with respect to the total silicon in the sample. As labeled in Figure 4, the measured ratios are in good agreement with the molar proportion MPTMS/(MPTMS + TEOS) employed in the initial mixture of the synthesis process. These results confirm high yields of organic incorporation to the silica structure by covalent bonding. Qualitative evidence of the thiol groups’ presence in the materials has been verified by FTIR spectra that show a very weak vibrational band assigned to the S-H stretching vibration at 2570 cm-1 (Figure 5). Quantitative determination of thiol groups was carried out by elemental chemical analysis, and the results are summarized in Table 2. The sulfur content of mmol of S (g of adsorbent)-1 increases from 0.54 (SBA15SH-5) to 2.91 (SBA15SH-30).
TGA and DTA confirm that SBA15SH materials are thermally stable up to 270 °C in air. From this temperature, decomposition of the residual template and mercaptopropyl moiety removal occur. 3.2. Mercury Adsorption Experiments. Aqueous inorganic mercury solutions were prepared by dissolving solid HgCl2 in pure water. Because mercuric chloride is mostly a molecular compound, only a slight hydrolysis takes place without the formation of insoluble species. Mercury adsorption on a SBA-15 silica sample was negligible, so mercury adsorption loading of the modified SBA-15 samples should be exclusively attributed to the presence of thiol groups anchored to the silica mesostructure. Determination of the Equilibrium Time. To obtain the equilibrium time of adsorption at 20 °C, a collection of mercury solutions with the same initial concentration was stirred in the presence of the adsorbent. Each solution was stopped and analyzed at different selected times ranging from 10 min to 24 h. This routine was repeated for all of the SBA15SH-x adsorbents and three initial mercury concentrations. Figure 6 shows the final concentration of mercury versus the contact time for SBA15SH-10 with initial concentrations of 60, 150, and 196 ppm aqueous Hg(II). The mercury concentration was found to quickly decrease up to 100 min; since then, a steady value was reached. Similar equilibrium times were obtained for the other materials synthesized in this work. Therefore, a stirring time of 150 min was used as the standard for all of the subsequent adsorption experiments. Adsorption Isotherms. Mercury adsorption isotherms have been obtained at 20 °C for all of the
Ind. Eng. Chem. Res., Vol. 44, No. 10, 2005 3669 Table 3. Comparative Analysis of Mercury Adsorption on Different Propylthiol Functionalized Mesoporous Silicas Synthesized by the Co-condensation Method
adsorbent
pore size (nm)
sulfur content (mmol of S g-1)
maximum mercury loading (mmol of Hg g-1)
Hg-S molar ratio
MP-MCM-4132 M1-HMS25 MP(2)-MSU-226 SBA15SH-30a
1.4 1.7 2.8 3.7
4.7 2.38 2.3 2.91
2.1 1.51 2.3 2.88
0.45 0.63 1.0 0.99
a
Figure 7. Experimental mercury(II) adsorption on SBA15SH-x: (a) isotherms at 20 °C; (b) Langmuir linear plots.
SBA15SH-x materials. Isotherms were constructed from many single experimental runs with the initial mercury concentration ranging from 30 to 600 mg L-1. Figure 7a displays the isotherms corresponding to SBA15SH5, -10, -20, and -30 materials, where the amount of mercury adsorbed at equilibrium, qe (millimoles per gram of adsorbent), is plotted as a function of the equilibrium mercury concentration in the liquid phase, Ce (milligrams per liter). The maximum adsorption mercury loading Q0 was calculated from the linearized Langmuir equation:
Ce/qe ) 1/Q0b + Ce/Q0
(1)
where Q0 and b are the characteristic Langmuir parameters related to the maximum adsorption capacity and the intensity of adsorption, respectively. Langmuir linear plots are shown in Figure 7b. As summarized in Table 2, the maximum adsorption capacity of the different adsorbents steadily increases with their sulfur content. The agreement between these two parameters reveals that the materials provide a full accessibility of mercury species toward all of the thiol groups with a 1:1 Hg-S stoichiometric bonding.
This work.
Comparison with Previous Studies. Table 3 shows a comparative analysis of mercury adsorption on different propylthiol-functionalized mesoporous silicas synthesized by the cocondensation method. A very high sulfur content is observed in the cocondensation silica thiol-functionalized MCM-41 synthesized by Lim et al. (MP-MCM-41).32 However, the mercury adsorption process was not fully efficient because of the small pore size of these materials (1.4 nm). Likewise, so is the case for HMS materials modified with propylthiol by direct synthesis as obtained by Lee et al.25 Conversely, Brown et al. have obtained efficient mercury adsorbents by incorporating MPTMS in MSU by the one-step procedure, because the mesoporous structure of the adsorbent is preserved with pore sizes of around 2.8 nm. These materials remove as much mercury(II) as sulfur contained (2.3 mmol g-1).26 SBA15SH-x materials reported here exhibit a great adsorption loading. In fact, the SBA15SH-30 material exhibits the highest mercury adsorption capacity of all of the (mercaptopropyl)thiol-functionalized mesostructured silicas obtained by the cocondensation procedure. This capacity exceeds the high loading of the MP(2)MSU-2 material and is similar to that reported for the FMMS material obtained by the coating method.18 So, a simple cocondensation method applied to obtain thiol derivatives of mesoporous SBA-15 is presented as an ideal method to design good mercury adsorbents. The broad mesoporous channel structure of these new adsorbents takes into account their high efficiency of mercury removal. Mercury Adsorption Mechanism. The initial and final pHs of aqueous solutions have been measured for all of the mercury adsorption equilibrium experiments. Mercury solutions directly prepared from HgCl2 and pure water exhibit a pH of around 4.5 because of the weak hydrolysis previously mentioned. The final pH of the equilibrium solutions has been found to be significantly less than the initial one. This pH decrease, which depends on the mercury loading on the adsorbent, agrees with the expected amount of protons released from the thiol groups by 1:1 exchange with mercury(II) species according to
(SiO)xSi-(CH2)3S-H + Hg-R f (SiO)xSi-(CH2)3S-Hg-R′ + H+ To characterize the chemical nature of the mercury(II) species adsorbed on the SBA15SH-x materials, some equilibrium experiments have been carried out by stirring 100 mg of each adsorbent in 175 mL of a 650 ppm Hg(II) aqueous solution. Solid mercury-loaded samples were recovered by filtration, dried at 100 °C for 1 h, and then analyzed by X-ray fluorescence. Molar contents of sulfur, mercury, and chlorine were found to
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Figure 8. Aqueous mercury speciation. Initial concentration of HgCl2: (a) 450 ppm; (b) 50 ppm.
keep a 1:1:1 ratio in all cases. These results evidence that each mercury atom anchored to the sulfur of the thiol group is also bonded to a chlorine atom as (SiO)xSi-(CH2)3S-Hg-Cl. 3.3. Influence of the pH on the Maximum Adsorption Capacity. One of the main factors that can modify the mercury adsorption efficiency is the solution pH. The proton concentration is relevant in both aqueous metal speciation and the stability of thiol groups. Because multiple inorganic mercury species could be present in an aqueous solution depending on pH and chloride concentration, aqueous mercury(II) speciation calculations have been restricted to the range from pH 0 to 8.0 and an aqueous HgCl2 concentration of up to 450 ppm. Speciation diagrams calculated from stability constants are plotted in Figure 8 as molar percentages of the different mercury species versus solution pH for two initial mercury concentrations (450 and 50 mg L-1). It was not feasible to extend the experimental adsorption study to values higher than pH 7 because insoluble mercury oxide was expected to precipitate above pH 7 for the higher mercury concentration. The effect of the solution pH on the adsorption of mercury(II) has been investigated for one of the synthesized materials, namely, SBA15SH-10, chosen as the representative of the overall behavior. Isotherms have been obtained at selected pH values for initial mercury concentrations ranging from 50 to 450 ppm. Isotherms at initial pH 7, pH 5.5, and pH 4.5 (pure water) are compared in Figure 9a. Experiments with initial pH 5.5-7.0 have been performed in the presence of NaOH. As observed, there is no noticeable difference in the mercury(II) adsorption capacity of SBA15SH-10 with increasing pH in this range. Because aqueous mercury species HgCl2, HgClOH, and Hg(OH)2 exist in the solution for the range from pH 4.5 to 7.0 (Figure 8), our experiments reveal that this material adsorbs efficiently all of the inorganic mercuric species in the solution up to pH 7. Experiments at low pH have been carried out in the presence of HNO3. Mercury adsorption at values of pH 3 and 0.75 as well as at 1 and 3 M acid concentrations has been investigated. A decrease in the pH from 4.5 to 0.75 does not affect the maximum mercury adsorption.
Figure 9. pH dependence of mercury(II) adsorption on SBA15SH10: (a) isotherms at pH 7, 5.5, and 4.5; (b) isotherms at pH 4.5, 3.0, and 0.75 and 1 and 3 M HNO3 concentrations.
At higher concentrations of nitric acid, a slight but perceptible effect is observed (Figure 9b). The maximum mercury adsorption capacity decreases from 1.2 mmol of Hg g-1 in pure water (pH 4.5) to 1.1 and 1.0 mmol of Hg g-1 for acid concentrations of 1 and 3 M, respectively. The reduction in the adsorption capacity is probably due to the fact that a higher concentration of protons in the solution stabilizes the thiol groups in the functionalized adsorbent, hindering the replacement of hydrogen by mercury species. Nevertheless, for SBA15SH-x materials, the pH appears not to play a crucial role in the mercury removal capacity as frequently occurs for other adsorbent materials such as activated carbon, clays, or silica gel.4-6,10,11 4. Conclusions The effectiveness of inorganic mercury removal from an aqueous solution by adsorption on thiol-modified mesoporous SBA-15 prepared via cocondensation has been demonstrated. Propylthiol-functionalized silica SBA15SH-x materials can be obtained by this simple method with variable and high organic contents without loss of suitable mesoporosity. Mercury adsorption is a fast and efficient process, allowing loading of up to 3
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mmol of Hg (g of adsorbent)-1. Langmuir isotherms have been fitted to experimental mercury adsorption data. This work shows that the pH of the solution in the 0.75-7 range does not affect the effective mercury uptake, which is independent of the predominant inorganic mercury species. For lower pH values, a slight decrease in mercury adsorption was observed. Acknowledgment This work has been supported by the regional government of Madrid under the “Grupos Estrate´gicos” (07M/0050/1998) research project. Literature Cited (1) USEPA. Aqueous mercury treatment; Capsule Report EPA 625-R-97-004; U.S. EPA: Washington, DC, 1997. (2) Chiarlie, S.; Ratto, M.; Rovatti, M. Mercury removal from water by ion exchange resins adsorption. Water Res. 2000, 34, 2971. (3) Sarkar, D.; Essington, M. E.; Misra, K. C. Adsorption of mercury(II) by Kaolinite. Solid Sci. Soc. Am. J. 2000, 64, 1968. (4) Ranganathan, K. Adsorption of Hg(II) ions from aqueous chloride solutions using powdered activated carbons. Carbon 2003, 41, 1087. (5) Budinova, T.; Savova, D.; Petrov, N.; Razvigorova, M.; Minkova, V.; Ciliz, N.; Apak, E.; Ekinci, E. Mercury adsorption by different modifications of furfural adsorbent. Ind. Eng. Chem. Res. 2003, 42, 2223. (6) Carrot, P. J. M.; Ribeiro Carrot, M. L.; Nabais, J. M. V. Influence of surface ionization on the adsorption of aqueous mercury chlorocomplexes by activated carbons. Carbon 1998, 36, 11. (7) Mohan, D.; Gupta, V. K.; Srivastava, S. K.; Chander, S. Kinetics of mercury adsorption from wastewater using activated carbon derived from fertilizer waste. Colloids Surf. A 2001, 177, 169. (8) Krishnan, K. A.; Anirudhan, T. S. Removal of mercury(II) from aqueous solutions and chlor-alkali industry effluent by steam activated and sulphurised activated carbons prepared from bagasse pith: kinetics and equilibrium studies. J. Hazard. Mater. 2002, B92, 161. (9) Mercier, L.; Pinnavaia, J. T. A functionalized porous clay heterostructure for heavy metal ion (Hg2+) trapping. Microporous Mesoporous Mater. 1998, 20, 101. (10) Manohar, D. M.; Krishnan, K. A.; Anirudhan, T. S. Removal of mercury(II) from aqueous solutions and chlor-alkali industry wastewater using 2-mercaptobenzimidazole-clay. Water Res. 2002, 36, 1609. (11) Venkatesan, K. A.; Srinivasan, T. G.; Vasudeva Rao, P. R. Removal of complexed mercury from aqueous solutions using dithiocarbamate grafted on silica gel. Sep. Sci. Technol. 2002, 37, 1417. (12) Knam, H. N.; Gomez-Salazar, S.; Tavlarides, L. Mercury(II) adsorption from wastewaters using a thiol functional adsorbent. Ind. Eng. Chem. Res. 2003, 42, 1955. (13) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered mesoporous molecular sieves tailored using different synthesis conditions. Nature 1992, 359, 710. (14) Tanev, P. T.; Pinnavaia, T. J. A neutral templating route to mesoporous molecular sieves. Science 1995, 267, 865. (15) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Templating of mesoporous molecular sieves by nonionic polyethylene oxide surfactants. Science 1995, 269, 1242. (16) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock copolymers synthesis of mesoporous silica with periodic 50 to 300 angstrom pores. Science 1998, 279, 548. (17) Stein, A.; Melde, B. J.; Schroden, R. C. Hybrid inorganicorganic mesoporous silicates-nanoscopic reactors coming of age. Adv. Mater. 2000, 12, 1403.
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Received for review December 22, 2004 Revised manuscript received March 2, 2005 Accepted March 4, 2005 IE0487585
