CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201300832
Synthesis and Application of Hydride Silica Composites for Rapid and Facile Removal of Aqueous Mercury Kseniia V. Katok,*[a] Raymond L. D. Whitby,[a] Franck Fayon,[b] Sylvie Bonnamy,[c] Sergey V. Mikhalovsky,[a, d] and Andrew B. Cundy[a] The adsorption of ionic mercury(II) from aqueous solution on functionalized hydride silicon materials was investigated. The adsorbents were prepared by modification of mesoporous silica C-120 with triethoxysilane or by converting alkoxysilane into siloxanes by reaction with acetic acid. Mercury adsorption isotherms at 20 8C are reported, and maximum mercury loadings were determined by Langmuir fitting. Adsorbents exhibited efficient and rapid removal of ionic mercury from aqueous
solution, with a maximum mercury loading of approximately 0.22 and 0.43 mmol of Hg g1 of silica C-120 and polyhedral oligomeric silsesquioxane (POSS) xerogel, respectively. Adsorption efficiency remained almost constant from pH 2.7 to 7. These inexpensive adsorbents exhibiting rapid assembly, low pH sensitivity, and high reactivity and capacity, are potential candidates as effective materials for mercury decontamination in natural waters and industrial effluents.
1. Introduction A number of heavy metal contaminants affect water supplies globally due to their industrial use, accidental release or natural abundance and migration through the environment. Mercury, one of the most toxic metals, has drawn much attention owing to its toxicity and potential impact on public health. The main anthropogenic sources contributing to mercury contamination include wastewater discharges and atmospheric deposition from mining activities, oil and coal combustion, chlor-alkali industries, cement production, municipal waste and sewage-sludge combustion, and manufacture of batteries, among others.[1, 2] Once released into the environment, mercury(II) can undergo (biologically mediated) conversion into the highly toxic methyl mercury form. Effluents containing mercury(II) or its derivatives have already caused several public health incidents, arguably the most well-known of which is the Minimata Bay disaster in 1953.[3, 4] It has been shown that the daily intake of mercury in small doses (0.25 mg) by an individual can affect the nervous system and cause neurological and
[a] Dr. K. V. Katok, Dr. R. L. D. Whitby, Prof. S. V. Mikhalovsky, Prof. A. B. Cundy Nanoscience & Nanotechnology Group Faculty of Science and Engineering, University of Brighton Brighton, BN2 4GJ (United Kingdom) E-mail: [email protected] [email protected] [b] Dr. F. Fayon CNRS, UPR3079 CEMHTI, Universit d’Orlans 45071 Orlans (France) [c] Dr. S. Bonnamy Centre de Recherche sur la Matire Divise CNRS - Universit d’Orlans 1B, rue de la Frollerie 45071 Orlans Cedex 2 (France) [d] Prof. S. V. Mikhalovsky Nazarbayev University 53, Kabanbay Batyr Ave, Astana, 010000 (Kazakhstan)
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renal disorders which are sometimes irreversible.[3, 5, 6] The World Health Organization has set the limit for mercury in drinking water at 0.001 mg L1.[7] Conventional technologies for treatment of aqueous mercury include precipitation, coagulation, reduction, membrane separation, ion exchange, and adsorption.[8, 9] However, the widespread use of these technologies remains limited due to several factors, including the need for further clarification, flocculation and/or filtration-following treatment, generation of further concentrated waste waters, and/or their high cost. Among the range of technologies available, adsorption has been widely studied because it is relatively easy to operate and cost-effective. Many adsorbents have been studied for HgII removal from aqueous solutions, including silicates, polymers, activated carbons, clays, organic matter, iron oxides, and pyrite.[10–12] Adsorbents for mercury removal generally possess sulphur-, nitrogen- or oxygen-containing functional groups as major binding sites for mercury. For example, strong interactions between mercury and various types of organic matter have been attributed to the binding of mercury with sulfhydryl-containing functional groups in these substances.[13, 14] Mesoporous silica materials functionalized with thiol and amino groups have been used previously for the removal of heavy metal ions from waste water. Thiolated SBA-15 adsorbents exhibited a high complexation affinity for Hg2 + , Cd2 + , and Pb2 + , while other metal ions (Cu2 + , Zn2 + , Cr3 + , and Ni2 + ) showed exceptional binding ability with its aminated analogue.[15] It has been previously demonstrated that silicon hydride groups SiH attached to the silica surface can effectively reduce metal ions located in the electromotive series after hydrogen to their zero-valent metallic form. In particular it has been shown that reduction of gold and silver ions from solution using silica covered with a thin film containing silicon hydride groups resulted in the immobilization of gold and ChemPhysChem 2013, 14, 4126 – 4133
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CHEMPHYSCHEM ARTICLES silver nanoparticles on the silica surface.[16, 17] Using this system, it was possible to control the nanoparticle size by altering the concentration of the metal-salt solution, time of contact, and type of silica matrix. The main drawback of the approach was that the silicon-hydride-containing film caused pore blockage in the silica support, resulting in aggregation of the metal nanoparticles produced. Despite these previously reported limitations, because mercury is present in the electromotive series after hydrogen, it is of great interest to test the capability of silica modified with SiH groups for the removal of mercury from water, as these functionalized substrates potentially provide a selective, low-cost, easily manufactured ,and scalable adsorbent. Since post-synthesis modification of silica with alkoxysilanes is limited by its specific surface area, a one-pot synthesis of polyhedral oligomeric silsesquioxane (POSS) material was carried out through self-assembly of different organosilanes. The kinetics and mechanisms of the adsorption process of mercury onto the modified silica, its pH dependence, and the potential application of hydride silica composites as adsorbents for mercury from aqueous systems were examined.
Experimental Section Synthesis of Silochrom-Based Hydride Silica and POSS Materials Cab-o-sil (fumed silica), acetic acid, sodium thiosulfate, and iodine standard solution were purchased from Fisher Scientific. Triethoxysilane (TES), tetramethyl orthosilicate (TEOS), dichlorodimethylsilane (DMDCS) and mercury nitrate from Sigma–Aldrich were used for modification and adsorption experiments. All reagents were used as received. The silochrom-based hydride silica samples were obtained using a two-step synthesis. The bulk mesoporous Silochrom C-120 silica was prepared by stirring 280 g of cab-o-sil in one litre of water until formation of a homogeneous mixture, with following gelation over 40 h.[18] The hydrogel was then cut into small pieces, dried at 150 8C for 12 h, and later calcinated at 850 8C over 6 h. The obtained C-120 silica was ground and sieved. The 0.25–0.5 mm fraction was removed for further sorption experiments. Silicon hydride groups were introduced on the surface of C-120 by reflux treatment with a solution of TES in glacial acetic acid. Approximately 3 g of silica sample was impregnated for 2 h with 15 mL of organosilane solution. TES solutions with concentrations of 0.01, 0.016, 0.034, and 0.088 mol L1 were used to obtain final molar ratios of 0.05, 0.08, 0.17, and 0.44 mmol of TES per g of silica, respectively. The obtained materials were filtered and dried at 130 8C overnight prior to use. The samples were denoted as C-120-H-x, where x indicates the molar percentage of TES in the initial mixture. The POSS materials were obtained from a one-pot synthesis. Since the alkoxide moiety strongly interacts with residual humidity, the POSS xerogels were prepared under anhydrous conditions. In order to change the concentration of functional SiH hydride groups, a variable amount of TES was added to the mixture of TEOS and acetic acid during the synthesis in the presence of DMDCS (used as a catalyst, Table 1). The solutions were left under reflux for 48 h at 80 8C (until a solid formed and aged), and the solid phase was then dried at 130 8C. The obtained xerogels do not swell and have amorphous glass-like structures. These materials were denoted as POSS-x, where x indicates the molar percentage 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Table 1. Relative amounts of TES, TEOS, acetic acid, and DMDCS used for the synthesis of POSS materials having different molar percentage of silicon hydride groups.
POSS-78 POSS-188 POSS-780
TES [mL]
TEOS [mL]
CH3COOH [mL]
DMDCS [mL]
SiH content [mmol g1]
1.0 2.0 10.0
10.77 9.6 -
6.2 6.2 6.2
0.06 0.06 0.06
0.78 1.88 7.8
of SiH groups available on the POSS surface, measured using back iodometric titration.
Batch Metal Adsorption Batch experiments were carried out to investigate the effect of contact time, initial mercury concentration, and the pH of the aqueous solution on mercury loading. All batch adsorption experiments reported here were performed with 0.1 g of C-120-H silica or POSS xerogel. Samples were placed in conical flasks and 40 mL of Hg(NO3)2 of a pre-determined concentration was added to each flask. The mixture was shaken for 30 min minimum at room temperature to allow the mercury to interact with the hydride silica. The solid was filtered and dried in air, and the filtrate retained for analysis. The effect of solution pH on the adsorption of mercury(II) was investigated for two of the synthesized materials, namely C-120-H-5 and C-120-H-8. They were treated with 40 mL of a 8 mg L1 Hg(NO3)2 solution. The pH of the solution was adjusted to the required value by using 0.1 m HCl or 0.1 m NaOH. The pH values were 2, 7, 9, and 11. The mixtures were shaken for 30 min and the concentration of mercury in solution was analyzed by inductively coupled plasma mass spectroscopy (ICP-MS) techniques.
Measurements Textural properties of initial, functionalized C-120 silica and POSS materials were determined by measuring N2 adsorption–desorption at 77 K on an Autosorb-1 Sorptometer (Quantachrome, UK). The surface area was determined using the Brunauer–Emmett–Teller (BET) equation, and the pore-size distribution was obtained from the adsorption branch using the Barrett–Joyner–Halenda model with cylindrical pores; the pore volume was taken at relative pressure P P01 = 0.97. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out with a Derivatograph Q-1500D (Paulik, Paulik & Erdey, MOM, Budapest) at a heating rate of 10 K min1. FTIR spectra of C-120 and POSS materials were recorded in the 400–4000 cm1 range using a NEXUS FT-IR spectrometer. Solid-state NMR experiments were performed on a Bruker Avance I spectrometer operating at 7.0 T (Larmor frequencies of 300 and 59.6 MHz for 1H and 29Si, respectively). The quantitative 29Si magicangle spinning (MAS) spectra were recorded at a spinning frequency of 14 kHz using a 308 flip angle and a recycle delay of 30 s. 29Si1 H cross polarization[19] (CP) MAS spectra were recorded at a spinning frequency of 14 kHz with contact times of 1, 2.5, and 12.5 ms using a linear amplitude ramp[20] on the 1H channel. For both 29 Si MAS and CPMAS experiments, 1H SPINAL-64 decoupling[21] was applied during signal acquisition with a 1H nutation frequency of 64 kHz. ChemPhysChem 2013, 14, 4126 – 4133
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CHEMPHYSCHEM ARTICLES Quantification of SiH functional groups in the C-120-H and POSS samples was conducted using back iodometric titration of excess iodine with sodium thiosulfate. Mercury concentrations were measured using ICP-MS. Measurements were performed using a Varian Vista AX spectrometer after calibration with stock solutions over a 0–100 mg L1 concentration range.
2. Results and Discussion 2.1. Characterization of Silica-Based Sorbents The structures of the functionalized amorphous C-120-H silica and POSS materials were examined using high-resolution solidstate NMR. The 29Si quantitative MAS and CPMAS spectra of the initial C-120 silica sample are shown in Figure 1 a. These spectra exhibit three partly overlapping resonances located at 110, 100, and 92 ppm, which are unambiguously assigned to Q4, Q3, and Q2 sites corresponding to SiO4 tetrahedral units bearing 0, 1, and 2 OH groups, respectively.[22, 23] In the 29Si quantitative MAS spectrum, the intensity of the Q4 resonance is much larger than that of the Q3 and Q2 surface sites, as expected for a polymerized silica network. For the modified silica
www.chemphyschem.org samples, an additional 29Si resonance at about 85 ppm is clearly observed in 29Si CPMAS spectra (Figure 1 b). According to its 29Si isotropic chemical shift, this resonance is assigned to T3 silicon hydride units.[24–27] The 29Si quantitative MAS spectra of the surface-functionalized C-120-H-x samples also indicate that impregnation with the TES solution not only results in the formation of SiH groups but also slightly increases the amount of silanol moieties, as evidenced by the increased intensities of the Q3 and Q2 resonances when increasing x. The 29 Si MAS and CPMAS spectra of the POSS-78, POSS-188, and POSS-780 xerogel samples also show Q4, Q3, and Q2 peaks and the resonance at 85 ppm assigned to HSiO3/2 units. For the POSS-188 and POSS-780 samples, an additional weak-intensity peak at 77.5 ppm assigned to T2 HSiO2/2OH functional silicon hydride groups[24–27] is also observed. These spectra indicate that the POSS materials contain significantly more Q3 and Q2 silanol surface species than the modified C-120-H samples. The relative intensities of the Tn resonances for POSS-78, 188, and 780 are given in Table 2. These values are in relatively good agreement with the quantification of the SiH groups from the back iodometric titration (Table 1) assuming that some Q1
Figure 1. Experimental 29Si quantitative MAS and CPMAS spectra of a) initial C-120 silica, b) surface-modified C-120-H-18, c) POSS-78, d) POSS-188, and e) POSS-780 materials and their best fits (shown below each experimental spectrum). The 29Si CPMAS spectra were recorded with a contact time of 2.5 ms. The individual Q4, Q3, Q2, T3, and T2 resonances are shown as dotted lines.
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Table 2. Relative intensities ( 2 %) of the Qn and Tn resonances obtained from the fits of the 29Si quantitative MAS NMR spectra of C-120 and POSS materials. Sample
Q4 [%]
Q3 [%]
Q2 [%]
T3 [%]
T2 [%]
C120 C120-H-18 C120-H-44 POSS-78 POSS-188 POSS-780
83 77 73 36.5 35 11
14 18 19 51 47 6
3 3 4 7 9 64
0 2 4 5.5 7 71
0 0 0 0 2 8
groups (expected isotropic chemical shift of about 82 ppm) may also contribute to the T3 peak of the POSS-780 sample. FTIR spectroscopy was also used to evidence the presence of silicon hydride surface groups. In agreement with solid-state NMR results, the FTIR spectrum of a triethoxysilane surfacemodified silica sample (Figure 2 line 2) exhibits an intense ab-
sample the SiH stretching vibration band at 2250 cm1 disappears after contact with the mercury-salt solution (Figure 2 line 3), indicating to the (re-)oxidation of SiH groups to SiOH and subsequent reduction of the metal salt. In the case of the POSS sample, the intensity of the SiH band at 2250 cm1 decreases significantly but does not disappear, suggesting that some of the SiH groups are not oxidized (to SiOH) and do not participate in the redox reaction. N2 adsorption-desorption measurement were performed to characterize the textural properties of initial, modified C-120 silica and POSS materials. For the produced C-120 silica, the nitrogen-adsorption isotherm (Figure 3 a) shows a loop of capillary condensation hysteresis which is characteristic of a mesoporous framework. The silica framework shows a specific surface area of approximately 114 m2 g1, a total pore volume of
Figure 2. FTIR spectra of 1) initial (unmodified) silica, 2) silica with grafted SiH groups (C SiH = 0.05 mmol g1), 3) hydride silica after adsorption of mercury, 4) initial POSS-78 material, and 5) POSS-78 after interaction with a mercury-salt solution.
sorption band at 2250 cm1 which is typical of silicon hydride groups,[26, 28, 29] in addition to the vibration bands observed for the native C-120 silica (Figure 2 line 1). Similarly, the spectrum of the POSS material (Figure 2, line 4) contains the absorption bands associated to SiOH (3750 cm1) and SiH (2250 cm1) stretching vibrations and OH (1630 cm1) bending vibration. It should be noted that, for this POSS sample, weaker intensity bands at 1440, 1370, and 2980 cm1, corresponding to CH bending and stretching vibrations of non-hydrolyzed SiOEt groups, are also observed. Additional FTIR measurements were performed after immersing the samples into a mercury-salt solution to evidence the surface reaction between Hg and grafted SiH groups. For the modified silica 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3. Nitrogen adsorption–desorption isotherm at 77 K and calculated pore-size distribution for initial C-120 silica (a, b) and initial POSS-78 (c, d).
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CHEMPHYSCHEM ARTICLES 0.68 cm3 g1, and a mean pore diameter of approximately 25 nm, giving relatively high and accessible reactive surface area for modification and subsequent HgII adsorption. Unlike C-120 silica, nitrogen isotherm sorption of the POSS material exhibits a steep rise at low relative pressure revealing a microand meso-porous structure. For the POSS samples, the average pore diameter is of about 6 nm (four times smaller than for C120 silica) and the specific surface area is about 754 m2 g1 (six times larger than for C-120 silica). Comparison between C-120and POSS-functionalized silica will therefore allow investigation of the influence of porosity on mercury uptake. Knowledge of the thermal stability of adsorbents determines the thermal limitations on their practical application. Thermogravimetric analysis of C-120-H and POSS materials were thus performed. Total mass losses for C-120-H-5 and POSS-78 silica were 0.62 % and 3.3 %, respectively under heating to 1000 8C. Decomposition of samples occurs stepwise. The first loss of mass occurs at 100 8C (Figure 4, line 2) which corresponds to
www.chemphyschem.org ples and formulation into composites, for example, polymer processing through melting.
2.2. Mercury-Adsorption Experiments The initial (unmodified) C-120 silica showed no adsorption mercury from a solution of Hg(NO3)2 with a concentration 1.4 mg L1, at pH 4 (Figure 5, line 4). At pH ~ 3–4, only 0.1 % the silanol groups are expected to be ionized (according
of of of to
Figure 5. Adsorption kinetics of mercury onto a) C-120-H-18 hydride silica (1–3) and on initial C-120 silica (4) at pH 4, and b) onto POSS-78 (5) and POSS-188 (6) materials at pH 4.
Figure 4. DTA (1), DTG (2), and TG (3) spectra of a) C-120 silica with grafted SiH groups (C SiH = 0.05 mmol g1) and b) POSS material (C SiH = 0.78 mmol g1).
evaporation of water from both materials. Next an exothermic peak appears at 390 8C in the differential thermal gravimetry (DTG) spectra of POSS-78, which is caused by the oxidation of the SiH groups to SiOH. No peaks were observed at 390 8C in the DTG spectra of C-120-H-5 silica, which can be explained by the low amount of SiH groups. After 600 8C, the gradual decrease of the TG curve observed for both materials corresponds to condensation of SiOH groups and formation of SiOSi bonds. TGA and DTA confirm that the functionalized C-120 and POSS materials are thermally stable up to 390 8C under air atmosphere, which enables facile processing of sam 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
the Eh–pH diagram for silica[30]) and thus to be able to participate in cation exchange explaining the almost complete absence of adsorption of mercury(II) onto the unmodified silica surface. It should also be mentioned that Hg2 + forms highly stable mercuric hydroxo complexes (such as Hg2 + + 2 OH$Hg(OH)2) under weakly acidic conditions, which again may not interact with silanol groups. This, coupled with the observation that all silicon hydride groups of TES-modified C120 silica (C-120-H-x) were oxidized to silanol groups upon exposure to mercury nitrate solution (according to complete disappearance of the SiH band in the FTIR spectrum), supports that mercury adsorption onto C-120-H samples can be exclusively attributed to the presence of (specifically the oxidation of) silicon hydride groups grafted onto the silica surface. To obtain the equilibrium time of adsorption at 20 8C, a range of mercury solutions with initial concentration between 1.4–2.4 mg L1 were stirred in the presence of C-120-H silica having concentrations of silicon hydride groups of about 0.17 and 0.2 mmol g1. The mercury concentration was found to rapidly decrease within the first 3 min (Figure 5 a, line 2), after which a steady value was reached. Owing to the time reChemPhysChem 2013, 14, 4126 – 4133
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CHEMPHYSCHEM ARTICLES quired to separate the sample under filtration, measurements for shorter reaction times (< 3 min) were not possible. The extremely fast adsorption kinetics suggest that the process is not diffusion controlled, as a consequence of the open (meso)porous structure of the adsorbent. In contrast, a much slower kinetic uptake is observed for both POSS-78 and POSS-188 (Figure 5 b, line 5, 6), which can be explained by the small pore size and high hydrophobicity of these sorbents. Mercury-adsorption isotherms were obtained at 20 8C for all C-120-H-x materials (x = SiH content [mmol g1] 100) and isotherms were constructed from adsorption experiments with initial concentrations of mercury ranging from 0.15 to 190 mg L1 (Figure 6). The isotherm data were closely fitted to
Figure 6. Experimental mercury(II) adsorption on C-120-H-x silica samples containing: 1) 0.44 mmol g1, 2) 0.17 mmol g1, and 3) 0.05 mmol g1 SiH groups: a) isotherms at 20 8C; b) Langmuir linear plots.
the Langmuir equation, as demonstrated by the linear plots in Figure 6 b, where the amount of mercury A adsorbed at equilibrium (millimoles per gram of adsorbent) is plotted as a function of the equilibrium mercury concentration in the liquid phase Ceq (milligrams per liter). The maximum mercury loading Am was calculated from the linearized Langmuir equation:
Ceq 1 1 þ ¼ k l Am Am A
Ceq
where Am and kl are the characteristic parameters related to the maximum adsorption capacity and the affinity of adsorption, respectively. A high value of kl = 86.630 mL mmol1 for C120-H-17 silica (Table 2) indicates high affinity of the hydride silica adsorbent for HgII, whereas the reported kl for other functionalized ordered mesoporous silica-based sorbents is about 5.300 mL mmol1.[31] 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Table 3. Maximum mercury loading on C-120-x and POSS-x materials. Material
SiH[a] content [mmol g1]
kl[b] [mL mmol1]
Maximum mercury loading [mmol of Hg g1]
C-120-H-5 C-120-H-18 C-120-H-44 POSS-78 POSS-188
0.05 0.18 0.44 0.78 1.88
8541 43 254 86 630 – –
0.02 0.10 0.22 0.41 0.43
[a] A quantitative determination of the SiH functional groups was conducted using back iodometric titration of excess iodine with sodium thiosulfate (Table 2). [b] kl was not calculated for POSS-x samples. Maximum mercury loading of HgII on POSS materials were calculated from kinetic uptake curve data.
As summarized in Table 3, the maximum adsorption capacity of the adsorbents studied steadily increases with the rise in concentration of SiH groups. The correlation between these two parameters reveals that the silicon hydride groups on the surface are fully accessible to mercury and that the reaction proceeds with a 1:2 Hg:SiH stoichiometric ratio. As listed in Table 2, the maximum concentration of SiH groups, achieved through the post-modification of silica, was 0.44 mmol g1, which allows removal of up to 0.22 mmol Hg per gram of silica. For the POSS material, which exhibits an increased concentration of SiH groups (up to 0.78 mmol g1), the sorption capacity of mercury is two times higher (0.41 mmol g1) than for functionalized silica (0.22 mmol g1). It should be noted that further increasing the concentration of SiH groups on the POSS material (up to 1.88 mmol g1) does not result in a significant increase of mercury loading. This is probably related to both the small pore size and enhanced hydrophobicity of the POSS material. Maximum mercury loadings are compared for different Sibased adsorbents in Table 4. While the maximum mercury loading observed for POSS-78 is broadly similar to previously reported values for propylthiol-functionalized mesoporous silicas and bifunctional hybrid silica materials containing -NH2 and -SH groups, that of C-120-H silica is significantly lower (less than half that of propylthiol-functionalized mesoporous silica).[32–35] Despite the higher maximum mercury loading on the thiol-functionalized silica, the kinetics of mercury adsorption on C-120-H silica are markedly superior: complete satura-
Table 4. Analysis of mercury adsorption on different silica sorbents with grafted functional groups. Functional group
Functional group content [mmol g1] -SH -NH2 SiH
Maximum mercury loading [mmol of Hg g1]
-C3H7SH (propylthiol)[32] -SH and -NH2[33] C-120-H-44 ( SiH)[a] POSS-78 ( SiH) [a]
0.54
0.55
1.06
1.07 0.44 0.78
0.38 0.22 0.41
[a] This study.
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CHEMPHYSCHEM ARTICLES tion of SiH groups on C-120-H occurs within 3–5 min, compared to 100 min on thiol-silica, which can be explained by the different mechanisms of reaction (redox versus complex formation) and differing structural characteristics.[32] The large pores of C-120 (five times larger than propylthiol silica obtained by co-condensation) also allow for more uniform grafting of TES monomers to the silica surface, increasing the HgII accessibility to reactive sites. The chemical binding affinity of a sorbent can be expressed in terms of its distribution coefficient (kd). The higher the kd value, the more effective the sorbent material is at capturing and retaining the target species. The distribution coefficient (kd) (in mL g1) is a mass-weighted partition coefficient between the liquid supernatant phase and the solid phase: kd ¼
ðC0 Cf Þ V Cf m
where C0 and Cf are the initial and final concentrations of mercury in the solution, V is the solution volume in millilitres, and m is the mass of the sorbent in grams. The value of kd for thiol-silica was determined to be 6.234 mL g1,[32] whereas the kd for the hydride silica used in this study is about 16.000 mL g1, highlighting the improved sorption properties of the C-120-H silica.
2.3. Mercury-Adsorption Mechanism, Influence of pH on Adsorption Capacity and Competitive Adsorption X-ray photoelectron spectroscopy (XPS) analysis was performed on C-120-H-5 samples before and after contact with the mercury-salt solution to characterize the oxidation state of the absorbed mercury. As shown in Figure 7, the obtained spectra allow the presence of elemental mercury at the C-121-
www.chemphyschem.org The sorption properties of TES-modified C-120 silica are related to the properties of SiH bonds at the silica surface. According to their relative electronegativities, silicon is more positive than hydrogen, and therefore, the hydrogen atom of the silicon hydride unit bears a negative charge and can be eliminated as a hydride ion (H).[17, 36] Hydration of hydride silica occurs due to the nucleophilic attack OH ions at the silicon atom with subsequent evolution of hydrogen (as a hydride ion reacts with a proton of water). SiH þ H2 O ! SiOH þ H þ Hþ
ð1Þ
Assuming that the redox potential of the metal species at ambient pH is lower than the redox potential of H + , H will recombine with the metal ion. Under such conditions, mercury removal using hydride silica might be described as oxidation of silicon hydride groups and reduction of HgII to HgI, followed by adsorption of HgI onto the adsorbent surface, according to the following reaction scheme: ð SiHÞ þ 2 Hg2þ þ H2 O ! ð SiOÞHg2 2þ þ 2 Hþ
ð2Þ
The pH of the solution is an important factor that can influence the mercury adsorption efficiency, as it is relevant to both the aqueous metal speciation and the stability of the silicon hydride groups. The adsorption efficiency of both the C-120-H5 and the C-120-H-8 silica samples was thus investigated over a broad pH range. It is worth noting that the adsorption capacities for both materials are nearly identical: the highest efficiency is observed at pH 3 and decreases with increasing pH (Figure 8). Only a slight reduction in removal efficiency oc-
Figure 8. Plot of removal efficiency of C-120-H-x silica containing 1) 0.05 mmol g1 and 2) 0.08 mmol g1 of SiH groups as a function of the pH of the Hg2 + solution.
Figure 7. XPS spectra of C-120-H-5 silica before (1) and after (2) sorption of HgII.
H-5 surface to be discounted, as the 4f7/2 binding-energy peak characteristic of Hg0 (99.9 eV) is not observed. However, the use of silica as a support substrate presents difficulties in assessing the presence or absence of HgI and HgII, for which the 4f5/2 and 4f7/2 binding-energy peaks strongly overlap with the intense broad Si 2p binding-energy peak of silica centred around 102.5 eV. Therefore, these XPS spectra allow exclusion of the presence of Hg0 at the surface, and they are consistent with the formation of either HgI or HgII surface complexes. 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
curred at pH 7, although the removal efficiency at higher pH (> 8) declined significantly, because splitting of the silicon hydride bond occurs in alkaline media with subsequent formation of a silanol group (Reaction 1). This suggests that sorbent regeneration should be possible at high pH, although studies of mercury desorption/sorbent regeneration are complicated by readsorption of Hg onto residual silicon hydride groups and facile mercury hydroxide precipitation. Finally, the competitive adsorption of metal cations in aqueous solution onto C-120-5 silica was investigated for a Hg2 + (aq)/Cu2 + (aq) mixture with initial concentrations of 0.8 and 8 mg L1. The selectivity of the process towards mercury was ChemPhysChem 2013, 14, 4126 – 4133
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Figure 9. Competitive adsorption of HgII and CuII ions on 0.1 g of C-120-5 in a 40 mL solution containing a mixture of 0.8 mg L1 Hg(NO3)2 and 8 mg L1 Cu(NO3)2.
determined by the redox properties of SiH groups and other heavy metals. In this competitive adsorption study, despite a tenfold excess of CuII over HgII, mercury is effectively removed from the aqueous solution (Figure 9). It should be noted that the mercury adsorption process is almost unaffected by the presence of copper and that no metallic copper was detected in the system.
4. Conclusions The effectiveness of ionic mercury removal from an aqueous solution by adsorption onto silica and POSS xerogels with grafted silicon hydride groups occurs mainly by a redox mechanism: oxidation of SiH groups and reduction of HgII ions. The competitive adsorption of a mixture of copper and mercury ions showed specificity of the adsorbent toward mercury removal. The structure of the silica framework (including the pore size) and the concentration of SiH groups are recognized as key factors that determine the total metal-adsorption capacity. The larger silica pore channels provide low flow resistance to solutes passing through the matrix and may control access of Hg2 + ions to the adsorbent’s binding sites. Mercury adsorption onto hydride silica composites is a fast and efficient process, allowing the loading of up to 0.22 mmol of Hg per g of C-120-H-44 and 0.44 mmol per g of POSS-188 adsorbent with a stoichiometric molar ratio 1:2 between Hg and SiH groups on the silica surface. Their near-constant adsorption efficiency under mildly alkaline to acidic conditions and their inexpensive and facile assembly indicates the potential usefulness of hydride silica composites as effective materials for mercury decontamination in natural waters and industrial effluents.
Acknowledgements
www.chemphyschem.org [1] E. G. Pacyna, J. M. Pacyna, Water Air Soil Pollut. 2002, 137, 149 – 165. [2] N. Pirrone, G. J. Keeler, J. O. Nriagu, Atmos. Environ. 1996, 30, 2981 – 2987. [3] C. D. Carrington, B. Montwill, P. M. Bolger, Regul. Toxicol. Pharmacol. 2004, 40, 272 – 280. [4] L. Valentino, M. V. Torregrossa, L. J. Saliba, Water Sci. Technol. 1995, 32, 41 – 47. [5] S. M. Ullrich, T. W. Tanton, S. A. Abdrashitova, Crit. Rev. Environ. Sci. Technol. 2001, 31, 241 – 293. [6] W. F. Fitzgerald, C. H. Lamborg in 9.04—Geochemistry of Mercury in the Environment (Eds.: D. H. Heinrich, K. T. Karl), Pergamon, Oxford 2007, pp. 1 – 47. [7] W. H. Organization, Guidelines for Drinking Water Quality: Recommendations, World Health Organization, 2004. [8] T. A. Kurniawan, G. Y. S. Chan, W.-H. Lo, S. Babel, Chem. Eng. J. 2006, 118, 83 – 98. [9] Y. K. Henneberry, T. E. C. Kraus, J. A. Fleck, D. P. Krabbenhoft, P. M. Bachand, W. R. Horwath, Sci. Total Environ. 2011, 409, 631 – 637. [10] A. Walcarius, L. Mercier, J. Mater. Chem. 2010, 20, 4478 – 4511. [11] J. Alauzun, A. Mehdi, C. Reye, R. J. P. Corriu, Chem. Commun. 2006, 347 – 349. [12] J. Wang, B. Deng, H. Chen, X. Wang, J. Zheng, Environ. Sci. Technol. 2009, 43, 5223 – 5228. [13] D. Hesterberg, J. W. Chou, K. J. Hutchison, D. E. Sayers, Environ. Sci. Technol. 2001, 35, 2741 – 2745. [14] M. Ravichandran, Chemosphere 2004, 55, 319 – 331. [15] A. M. Liu, K. Hidajat, S. Kawi, D. Y. Zhao, Chem. Commun. 2000, 1145 – 1146. [16] K. V. Katok, V. A. Tertykh, V. V. Yanishpolskii, Synthesis and application of metal-containing silicas, Springer, Heidelberg, 2008. [17] V. A. Tertykh, K. V. Katok, V. V. Yanishpolskii, Russ. J. Phys. Chem. 2008, 82, 1438 – 1441. [18] A. V. Kiselev, Experimental methods in adsorption and chromatography, Moscow University Publisher, 1973. [19] A. Pines, M. G. Gibby, J. S. Waugh, J. Chem. Phys. 1973, 59, 569 – 590. [20] G. Metz, X. L. Wu, S. O. Smith, J. Magn. Reson. Ser. A 1994, 110, 219 – 227. [21] B. M. Fung, A. K. Khitrin, K. Ermolaev, J. Magn. Reson. 2000, 142, 97 – 101. [22] G. E. Maciel, D. W. Sindorf, J. Am. Chem. Soc. 1980, 102, 7606 – 7607. [23] D. W. Sindorf, G. E. Maciel, J. Am. Chem. Soc. 1983, 105, 1487 – 1493. [24] W. E. Rudzinski, T. L. Montgomery, J. S. Frye, B. L. Hawkins, G. E. Maciel, J. Catal. 1986, 98, 444 – 456. [25] A. Mehdi, P. H. Mutin, J. Mater. Chem. 2006, 16, 1606 – 1607. [26] C. H. Chu, E. Jonsson, M. Auvinen, J. J. Pesek, J. E. Sandoval, Anal. Chem. 1993, 65, 808 – 816. [27] B. A. Ashu-Arrah, J. D. Glennon, K. Albert, J. Chromatogr. A 2012, 1236, 42 – 50. [28] B. Lynch, J. D. Glennon, C. Troltzsch, U. Menyes, M. Pursch, K. Albert, Anal. Chem. 1997, 69, 1756 – 1762. [29] J. E. Sandoval, J. J. Pesek, Anal. Chem. 1989, 61, 2067 – 2075. [30] L. A. Belyakova, O. M. Shvets, D. Y. e. Lyashenko, Inorg. Chim. Acta 2009, 362, 2222 – 2230. [31] M. Puanngam, F. Unob, J. Hazard. Mater. 2008, 154, 578 – 587. [32] J. Aguado, J. M. Arsuaga, A. Arencibia, Ind. Eng. Chem. Res. 2005, 44, 3665 – 3671. [33] B. Lee, Y. Kim, H. Lee, J. Yi, Microporous Mesoporous Mater. 2001, 50, 77 – 90. [34] J. Brown, R. Richer, L. Mercier, Microporous Mesoporous Mater. 2000, 37, 41 – 48. [35] M. H. Lim, C. F. Blanford, A. Stein, Chem. Mater. 1998, 10, 467 – 470. [36] M. G. Voronkov, V. B. Pukhnarevich, Izv. Akad. Nauk SSSR Ser. Khim. 1982, 5, 1056.
This research has been supported by a Marie Curie International Incoming Fellowship FP7-PIIF-GA-2009-255635-HYREM within the 7th European Community Framework Programme. Keywords: adsorption · hydride silica · mercury · rapid uptake · water remediation
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: September 8, 2013 Published online on November 20, 2013
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Synthesis and Application of Hydride Silica Composites for Rapid and Facile Removal of Aqueous Mercury Kseniia V. Katok,*[a] Raymond L. D. Whitby,[a] Franck Fayon,[b] Sylvie Bonnamy,[c] Sergey V. Mikhalovsky,[a, d] and Andrew B. Cundy[a] The adsorption of ionic mercury(II) from aqueous solution on functionalized hydride silicon materials was investigated. The adsorbents were prepared by modification of mesoporous silica C-120 with triethoxysilane or by converting alkoxysilane into siloxanes by reaction with acetic acid. Mercury adsorption isotherms at 20 8C are reported, and maximum mercury loadings were determined by Langmuir fitting. Adsorbents exhibited efficient and rapid removal of ionic mercury from aqueous
solution, with a maximum mercury loading of approximately 0.22 and 0.43 mmol of Hg g1 of silica C-120 and polyhedral oligomeric silsesquioxane (POSS) xerogel, respectively. Adsorption efficiency remained almost constant from pH 2.7 to 7. These inexpensive adsorbents exhibiting rapid assembly, low pH sensitivity, and high reactivity and capacity, are potential candidates as effective materials for mercury decontamination in natural waters and industrial effluents.
1. Introduction A number of heavy metal contaminants affect water supplies globally due to their industrial use, accidental release or natural abundance and migration through the environment. Mercury, one of the most toxic metals, has drawn much attention owing to its toxicity and potential impact on public health. The main anthropogenic sources contributing to mercury contamination include wastewater discharges and atmospheric deposition from mining activities, oil and coal combustion, chlor-alkali industries, cement production, municipal waste and sewage-sludge combustion, and manufacture of batteries, among others.[1, 2] Once released into the environment, mercury(II) can undergo (biologically mediated) conversion into the highly toxic methyl mercury form. Effluents containing mercury(II) or its derivatives have already caused several public health incidents, arguably the most well-known of which is the Minimata Bay disaster in 1953.[3, 4] It has been shown that the daily intake of mercury in small doses (0.25 mg) by an individual can affect the nervous system and cause neurological and
[a] Dr. K. V. Katok, Dr. R. L. D. Whitby, Prof. S. V. Mikhalovsky, Prof. A. B. Cundy Nanoscience & Nanotechnology Group Faculty of Science and Engineering, University of Brighton Brighton, BN2 4GJ (United Kingdom) E-mail: [email protected] [email protected] [b] Dr. F. Fayon CNRS, UPR3079 CEMHTI, Universit d’Orlans 45071 Orlans (France) [c] Dr. S. Bonnamy Centre de Recherche sur la Matire Divise CNRS - Universit d’Orlans 1B, rue de la Frollerie 45071 Orlans Cedex 2 (France) [d] Prof. S. V. Mikhalovsky Nazarbayev University 53, Kabanbay Batyr Ave, Astana, 010000 (Kazakhstan)
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
renal disorders which are sometimes irreversible.[3, 5, 6] The World Health Organization has set the limit for mercury in drinking water at 0.001 mg L1.[7] Conventional technologies for treatment of aqueous mercury include precipitation, coagulation, reduction, membrane separation, ion exchange, and adsorption.[8, 9] However, the widespread use of these technologies remains limited due to several factors, including the need for further clarification, flocculation and/or filtration-following treatment, generation of further concentrated waste waters, and/or their high cost. Among the range of technologies available, adsorption has been widely studied because it is relatively easy to operate and cost-effective. Many adsorbents have been studied for HgII removal from aqueous solutions, including silicates, polymers, activated carbons, clays, organic matter, iron oxides, and pyrite.[10–12] Adsorbents for mercury removal generally possess sulphur-, nitrogen- or oxygen-containing functional groups as major binding sites for mercury. For example, strong interactions between mercury and various types of organic matter have been attributed to the binding of mercury with sulfhydryl-containing functional groups in these substances.[13, 14] Mesoporous silica materials functionalized with thiol and amino groups have been used previously for the removal of heavy metal ions from waste water. Thiolated SBA-15 adsorbents exhibited a high complexation affinity for Hg2 + , Cd2 + , and Pb2 + , while other metal ions (Cu2 + , Zn2 + , Cr3 + , and Ni2 + ) showed exceptional binding ability with its aminated analogue.[15] It has been previously demonstrated that silicon hydride groups SiH attached to the silica surface can effectively reduce metal ions located in the electromotive series after hydrogen to their zero-valent metallic form. In particular it has been shown that reduction of gold and silver ions from solution using silica covered with a thin film containing silicon hydride groups resulted in the immobilization of gold and ChemPhysChem 2013, 14, 4126 – 4133
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CHEMPHYSCHEM ARTICLES silver nanoparticles on the silica surface.[16, 17] Using this system, it was possible to control the nanoparticle size by altering the concentration of the metal-salt solution, time of contact, and type of silica matrix. The main drawback of the approach was that the silicon-hydride-containing film caused pore blockage in the silica support, resulting in aggregation of the metal nanoparticles produced. Despite these previously reported limitations, because mercury is present in the electromotive series after hydrogen, it is of great interest to test the capability of silica modified with SiH groups for the removal of mercury from water, as these functionalized substrates potentially provide a selective, low-cost, easily manufactured ,and scalable adsorbent. Since post-synthesis modification of silica with alkoxysilanes is limited by its specific surface area, a one-pot synthesis of polyhedral oligomeric silsesquioxane (POSS) material was carried out through self-assembly of different organosilanes. The kinetics and mechanisms of the adsorption process of mercury onto the modified silica, its pH dependence, and the potential application of hydride silica composites as adsorbents for mercury from aqueous systems were examined.
Experimental Section Synthesis of Silochrom-Based Hydride Silica and POSS Materials Cab-o-sil (fumed silica), acetic acid, sodium thiosulfate, and iodine standard solution were purchased from Fisher Scientific. Triethoxysilane (TES), tetramethyl orthosilicate (TEOS), dichlorodimethylsilane (DMDCS) and mercury nitrate from Sigma–Aldrich were used for modification and adsorption experiments. All reagents were used as received. The silochrom-based hydride silica samples were obtained using a two-step synthesis. The bulk mesoporous Silochrom C-120 silica was prepared by stirring 280 g of cab-o-sil in one litre of water until formation of a homogeneous mixture, with following gelation over 40 h.[18] The hydrogel was then cut into small pieces, dried at 150 8C for 12 h, and later calcinated at 850 8C over 6 h. The obtained C-120 silica was ground and sieved. The 0.25–0.5 mm fraction was removed for further sorption experiments. Silicon hydride groups were introduced on the surface of C-120 by reflux treatment with a solution of TES in glacial acetic acid. Approximately 3 g of silica sample was impregnated for 2 h with 15 mL of organosilane solution. TES solutions with concentrations of 0.01, 0.016, 0.034, and 0.088 mol L1 were used to obtain final molar ratios of 0.05, 0.08, 0.17, and 0.44 mmol of TES per g of silica, respectively. The obtained materials were filtered and dried at 130 8C overnight prior to use. The samples were denoted as C-120-H-x, where x indicates the molar percentage of TES in the initial mixture. The POSS materials were obtained from a one-pot synthesis. Since the alkoxide moiety strongly interacts with residual humidity, the POSS xerogels were prepared under anhydrous conditions. In order to change the concentration of functional SiH hydride groups, a variable amount of TES was added to the mixture of TEOS and acetic acid during the synthesis in the presence of DMDCS (used as a catalyst, Table 1). The solutions were left under reflux for 48 h at 80 8C (until a solid formed and aged), and the solid phase was then dried at 130 8C. The obtained xerogels do not swell and have amorphous glass-like structures. These materials were denoted as POSS-x, where x indicates the molar percentage 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Table 1. Relative amounts of TES, TEOS, acetic acid, and DMDCS used for the synthesis of POSS materials having different molar percentage of silicon hydride groups.
POSS-78 POSS-188 POSS-780
TES [mL]
TEOS [mL]
CH3COOH [mL]
DMDCS [mL]
SiH content [mmol g1]
1.0 2.0 10.0
10.77 9.6 -
6.2 6.2 6.2
0.06 0.06 0.06
0.78 1.88 7.8
of SiH groups available on the POSS surface, measured using back iodometric titration.
Batch Metal Adsorption Batch experiments were carried out to investigate the effect of contact time, initial mercury concentration, and the pH of the aqueous solution on mercury loading. All batch adsorption experiments reported here were performed with 0.1 g of C-120-H silica or POSS xerogel. Samples were placed in conical flasks and 40 mL of Hg(NO3)2 of a pre-determined concentration was added to each flask. The mixture was shaken for 30 min minimum at room temperature to allow the mercury to interact with the hydride silica. The solid was filtered and dried in air, and the filtrate retained for analysis. The effect of solution pH on the adsorption of mercury(II) was investigated for two of the synthesized materials, namely C-120-H-5 and C-120-H-8. They were treated with 40 mL of a 8 mg L1 Hg(NO3)2 solution. The pH of the solution was adjusted to the required value by using 0.1 m HCl or 0.1 m NaOH. The pH values were 2, 7, 9, and 11. The mixtures were shaken for 30 min and the concentration of mercury in solution was analyzed by inductively coupled plasma mass spectroscopy (ICP-MS) techniques.
Measurements Textural properties of initial, functionalized C-120 silica and POSS materials were determined by measuring N2 adsorption–desorption at 77 K on an Autosorb-1 Sorptometer (Quantachrome, UK). The surface area was determined using the Brunauer–Emmett–Teller (BET) equation, and the pore-size distribution was obtained from the adsorption branch using the Barrett–Joyner–Halenda model with cylindrical pores; the pore volume was taken at relative pressure P P01 = 0.97. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out with a Derivatograph Q-1500D (Paulik, Paulik & Erdey, MOM, Budapest) at a heating rate of 10 K min1. FTIR spectra of C-120 and POSS materials were recorded in the 400–4000 cm1 range using a NEXUS FT-IR spectrometer. Solid-state NMR experiments were performed on a Bruker Avance I spectrometer operating at 7.0 T (Larmor frequencies of 300 and 59.6 MHz for 1H and 29Si, respectively). The quantitative 29Si magicangle spinning (MAS) spectra were recorded at a spinning frequency of 14 kHz using a 308 flip angle and a recycle delay of 30 s. 29Si1 H cross polarization[19] (CP) MAS spectra were recorded at a spinning frequency of 14 kHz with contact times of 1, 2.5, and 12.5 ms using a linear amplitude ramp[20] on the 1H channel. For both 29 Si MAS and CPMAS experiments, 1H SPINAL-64 decoupling[21] was applied during signal acquisition with a 1H nutation frequency of 64 kHz. ChemPhysChem 2013, 14, 4126 – 4133
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CHEMPHYSCHEM ARTICLES Quantification of SiH functional groups in the C-120-H and POSS samples was conducted using back iodometric titration of excess iodine with sodium thiosulfate. Mercury concentrations were measured using ICP-MS. Measurements were performed using a Varian Vista AX spectrometer after calibration with stock solutions over a 0–100 mg L1 concentration range.
2. Results and Discussion 2.1. Characterization of Silica-Based Sorbents The structures of the functionalized amorphous C-120-H silica and POSS materials were examined using high-resolution solidstate NMR. The 29Si quantitative MAS and CPMAS spectra of the initial C-120 silica sample are shown in Figure 1 a. These spectra exhibit three partly overlapping resonances located at 110, 100, and 92 ppm, which are unambiguously assigned to Q4, Q3, and Q2 sites corresponding to SiO4 tetrahedral units bearing 0, 1, and 2 OH groups, respectively.[22, 23] In the 29Si quantitative MAS spectrum, the intensity of the Q4 resonance is much larger than that of the Q3 and Q2 surface sites, as expected for a polymerized silica network. For the modified silica
www.chemphyschem.org samples, an additional 29Si resonance at about 85 ppm is clearly observed in 29Si CPMAS spectra (Figure 1 b). According to its 29Si isotropic chemical shift, this resonance is assigned to T3 silicon hydride units.[24–27] The 29Si quantitative MAS spectra of the surface-functionalized C-120-H-x samples also indicate that impregnation with the TES solution not only results in the formation of SiH groups but also slightly increases the amount of silanol moieties, as evidenced by the increased intensities of the Q3 and Q2 resonances when increasing x. The 29 Si MAS and CPMAS spectra of the POSS-78, POSS-188, and POSS-780 xerogel samples also show Q4, Q3, and Q2 peaks and the resonance at 85 ppm assigned to HSiO3/2 units. For the POSS-188 and POSS-780 samples, an additional weak-intensity peak at 77.5 ppm assigned to T2 HSiO2/2OH functional silicon hydride groups[24–27] is also observed. These spectra indicate that the POSS materials contain significantly more Q3 and Q2 silanol surface species than the modified C-120-H samples. The relative intensities of the Tn resonances for POSS-78, 188, and 780 are given in Table 2. These values are in relatively good agreement with the quantification of the SiH groups from the back iodometric titration (Table 1) assuming that some Q1
Figure 1. Experimental 29Si quantitative MAS and CPMAS spectra of a) initial C-120 silica, b) surface-modified C-120-H-18, c) POSS-78, d) POSS-188, and e) POSS-780 materials and their best fits (shown below each experimental spectrum). The 29Si CPMAS spectra were recorded with a contact time of 2.5 ms. The individual Q4, Q3, Q2, T3, and T2 resonances are shown as dotted lines.
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Table 2. Relative intensities ( 2 %) of the Qn and Tn resonances obtained from the fits of the 29Si quantitative MAS NMR spectra of C-120 and POSS materials. Sample
Q4 [%]
Q3 [%]
Q2 [%]
T3 [%]
T2 [%]
C120 C120-H-18 C120-H-44 POSS-78 POSS-188 POSS-780
83 77 73 36.5 35 11
14 18 19 51 47 6
3 3 4 7 9 64
0 2 4 5.5 7 71
0 0 0 0 2 8
groups (expected isotropic chemical shift of about 82 ppm) may also contribute to the T3 peak of the POSS-780 sample. FTIR spectroscopy was also used to evidence the presence of silicon hydride surface groups. In agreement with solid-state NMR results, the FTIR spectrum of a triethoxysilane surfacemodified silica sample (Figure 2 line 2) exhibits an intense ab-
sample the SiH stretching vibration band at 2250 cm1 disappears after contact with the mercury-salt solution (Figure 2 line 3), indicating to the (re-)oxidation of SiH groups to SiOH and subsequent reduction of the metal salt. In the case of the POSS sample, the intensity of the SiH band at 2250 cm1 decreases significantly but does not disappear, suggesting that some of the SiH groups are not oxidized (to SiOH) and do not participate in the redox reaction. N2 adsorption-desorption measurement were performed to characterize the textural properties of initial, modified C-120 silica and POSS materials. For the produced C-120 silica, the nitrogen-adsorption isotherm (Figure 3 a) shows a loop of capillary condensation hysteresis which is characteristic of a mesoporous framework. The silica framework shows a specific surface area of approximately 114 m2 g1, a total pore volume of
Figure 2. FTIR spectra of 1) initial (unmodified) silica, 2) silica with grafted SiH groups (C SiH = 0.05 mmol g1), 3) hydride silica after adsorption of mercury, 4) initial POSS-78 material, and 5) POSS-78 after interaction with a mercury-salt solution.
sorption band at 2250 cm1 which is typical of silicon hydride groups,[26, 28, 29] in addition to the vibration bands observed for the native C-120 silica (Figure 2 line 1). Similarly, the spectrum of the POSS material (Figure 2, line 4) contains the absorption bands associated to SiOH (3750 cm1) and SiH (2250 cm1) stretching vibrations and OH (1630 cm1) bending vibration. It should be noted that, for this POSS sample, weaker intensity bands at 1440, 1370, and 2980 cm1, corresponding to CH bending and stretching vibrations of non-hydrolyzed SiOEt groups, are also observed. Additional FTIR measurements were performed after immersing the samples into a mercury-salt solution to evidence the surface reaction between Hg and grafted SiH groups. For the modified silica 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3. Nitrogen adsorption–desorption isotherm at 77 K and calculated pore-size distribution for initial C-120 silica (a, b) and initial POSS-78 (c, d).
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CHEMPHYSCHEM ARTICLES 0.68 cm3 g1, and a mean pore diameter of approximately 25 nm, giving relatively high and accessible reactive surface area for modification and subsequent HgII adsorption. Unlike C-120 silica, nitrogen isotherm sorption of the POSS material exhibits a steep rise at low relative pressure revealing a microand meso-porous structure. For the POSS samples, the average pore diameter is of about 6 nm (four times smaller than for C120 silica) and the specific surface area is about 754 m2 g1 (six times larger than for C-120 silica). Comparison between C-120and POSS-functionalized silica will therefore allow investigation of the influence of porosity on mercury uptake. Knowledge of the thermal stability of adsorbents determines the thermal limitations on their practical application. Thermogravimetric analysis of C-120-H and POSS materials were thus performed. Total mass losses for C-120-H-5 and POSS-78 silica were 0.62 % and 3.3 %, respectively under heating to 1000 8C. Decomposition of samples occurs stepwise. The first loss of mass occurs at 100 8C (Figure 4, line 2) which corresponds to
www.chemphyschem.org ples and formulation into composites, for example, polymer processing through melting.
2.2. Mercury-Adsorption Experiments The initial (unmodified) C-120 silica showed no adsorption mercury from a solution of Hg(NO3)2 with a concentration 1.4 mg L1, at pH 4 (Figure 5, line 4). At pH ~ 3–4, only 0.1 % the silanol groups are expected to be ionized (according
of of of to
Figure 5. Adsorption kinetics of mercury onto a) C-120-H-18 hydride silica (1–3) and on initial C-120 silica (4) at pH 4, and b) onto POSS-78 (5) and POSS-188 (6) materials at pH 4.
Figure 4. DTA (1), DTG (2), and TG (3) spectra of a) C-120 silica with grafted SiH groups (C SiH = 0.05 mmol g1) and b) POSS material (C SiH = 0.78 mmol g1).
evaporation of water from both materials. Next an exothermic peak appears at 390 8C in the differential thermal gravimetry (DTG) spectra of POSS-78, which is caused by the oxidation of the SiH groups to SiOH. No peaks were observed at 390 8C in the DTG spectra of C-120-H-5 silica, which can be explained by the low amount of SiH groups. After 600 8C, the gradual decrease of the TG curve observed for both materials corresponds to condensation of SiOH groups and formation of SiOSi bonds. TGA and DTA confirm that the functionalized C-120 and POSS materials are thermally stable up to 390 8C under air atmosphere, which enables facile processing of sam 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
the Eh–pH diagram for silica[30]) and thus to be able to participate in cation exchange explaining the almost complete absence of adsorption of mercury(II) onto the unmodified silica surface. It should also be mentioned that Hg2 + forms highly stable mercuric hydroxo complexes (such as Hg2 + + 2 OH$Hg(OH)2) under weakly acidic conditions, which again may not interact with silanol groups. This, coupled with the observation that all silicon hydride groups of TES-modified C120 silica (C-120-H-x) were oxidized to silanol groups upon exposure to mercury nitrate solution (according to complete disappearance of the SiH band in the FTIR spectrum), supports that mercury adsorption onto C-120-H samples can be exclusively attributed to the presence of (specifically the oxidation of) silicon hydride groups grafted onto the silica surface. To obtain the equilibrium time of adsorption at 20 8C, a range of mercury solutions with initial concentration between 1.4–2.4 mg L1 were stirred in the presence of C-120-H silica having concentrations of silicon hydride groups of about 0.17 and 0.2 mmol g1. The mercury concentration was found to rapidly decrease within the first 3 min (Figure 5 a, line 2), after which a steady value was reached. Owing to the time reChemPhysChem 2013, 14, 4126 – 4133
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CHEMPHYSCHEM ARTICLES quired to separate the sample under filtration, measurements for shorter reaction times (< 3 min) were not possible. The extremely fast adsorption kinetics suggest that the process is not diffusion controlled, as a consequence of the open (meso)porous structure of the adsorbent. In contrast, a much slower kinetic uptake is observed for both POSS-78 and POSS-188 (Figure 5 b, line 5, 6), which can be explained by the small pore size and high hydrophobicity of these sorbents. Mercury-adsorption isotherms were obtained at 20 8C for all C-120-H-x materials (x = SiH content [mmol g1] 100) and isotherms were constructed from adsorption experiments with initial concentrations of mercury ranging from 0.15 to 190 mg L1 (Figure 6). The isotherm data were closely fitted to
Figure 6. Experimental mercury(II) adsorption on C-120-H-x silica samples containing: 1) 0.44 mmol g1, 2) 0.17 mmol g1, and 3) 0.05 mmol g1 SiH groups: a) isotherms at 20 8C; b) Langmuir linear plots.
the Langmuir equation, as demonstrated by the linear plots in Figure 6 b, where the amount of mercury A adsorbed at equilibrium (millimoles per gram of adsorbent) is plotted as a function of the equilibrium mercury concentration in the liquid phase Ceq (milligrams per liter). The maximum mercury loading Am was calculated from the linearized Langmuir equation:
Ceq 1 1 þ ¼ k l Am Am A
Ceq
where Am and kl are the characteristic parameters related to the maximum adsorption capacity and the affinity of adsorption, respectively. A high value of kl = 86.630 mL mmol1 for C120-H-17 silica (Table 2) indicates high affinity of the hydride silica adsorbent for HgII, whereas the reported kl for other functionalized ordered mesoporous silica-based sorbents is about 5.300 mL mmol1.[31] 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Table 3. Maximum mercury loading on C-120-x and POSS-x materials. Material
SiH[a] content [mmol g1]
kl[b] [mL mmol1]
Maximum mercury loading [mmol of Hg g1]
C-120-H-5 C-120-H-18 C-120-H-44 POSS-78 POSS-188
0.05 0.18 0.44 0.78 1.88
8541 43 254 86 630 – –
0.02 0.10 0.22 0.41 0.43
[a] A quantitative determination of the SiH functional groups was conducted using back iodometric titration of excess iodine with sodium thiosulfate (Table 2). [b] kl was not calculated for POSS-x samples. Maximum mercury loading of HgII on POSS materials were calculated from kinetic uptake curve data.
As summarized in Table 3, the maximum adsorption capacity of the adsorbents studied steadily increases with the rise in concentration of SiH groups. The correlation between these two parameters reveals that the silicon hydride groups on the surface are fully accessible to mercury and that the reaction proceeds with a 1:2 Hg:SiH stoichiometric ratio. As listed in Table 2, the maximum concentration of SiH groups, achieved through the post-modification of silica, was 0.44 mmol g1, which allows removal of up to 0.22 mmol Hg per gram of silica. For the POSS material, which exhibits an increased concentration of SiH groups (up to 0.78 mmol g1), the sorption capacity of mercury is two times higher (0.41 mmol g1) than for functionalized silica (0.22 mmol g1). It should be noted that further increasing the concentration of SiH groups on the POSS material (up to 1.88 mmol g1) does not result in a significant increase of mercury loading. This is probably related to both the small pore size and enhanced hydrophobicity of the POSS material. Maximum mercury loadings are compared for different Sibased adsorbents in Table 4. While the maximum mercury loading observed for POSS-78 is broadly similar to previously reported values for propylthiol-functionalized mesoporous silicas and bifunctional hybrid silica materials containing -NH2 and -SH groups, that of C-120-H silica is significantly lower (less than half that of propylthiol-functionalized mesoporous silica).[32–35] Despite the higher maximum mercury loading on the thiol-functionalized silica, the kinetics of mercury adsorption on C-120-H silica are markedly superior: complete satura-
Table 4. Analysis of mercury adsorption on different silica sorbents with grafted functional groups. Functional group
Functional group content [mmol g1] -SH -NH2 SiH
Maximum mercury loading [mmol of Hg g1]
-C3H7SH (propylthiol)[32] -SH and -NH2[33] C-120-H-44 ( SiH)[a] POSS-78 ( SiH) [a]
0.54
0.55
1.06
1.07 0.44 0.78
0.38 0.22 0.41
[a] This study.
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CHEMPHYSCHEM ARTICLES tion of SiH groups on C-120-H occurs within 3–5 min, compared to 100 min on thiol-silica, which can be explained by the different mechanisms of reaction (redox versus complex formation) and differing structural characteristics.[32] The large pores of C-120 (five times larger than propylthiol silica obtained by co-condensation) also allow for more uniform grafting of TES monomers to the silica surface, increasing the HgII accessibility to reactive sites. The chemical binding affinity of a sorbent can be expressed in terms of its distribution coefficient (kd). The higher the kd value, the more effective the sorbent material is at capturing and retaining the target species. The distribution coefficient (kd) (in mL g1) is a mass-weighted partition coefficient between the liquid supernatant phase and the solid phase: kd ¼
ðC0 Cf Þ V Cf m
where C0 and Cf are the initial and final concentrations of mercury in the solution, V is the solution volume in millilitres, and m is the mass of the sorbent in grams. The value of kd for thiol-silica was determined to be 6.234 mL g1,[32] whereas the kd for the hydride silica used in this study is about 16.000 mL g1, highlighting the improved sorption properties of the C-120-H silica.
2.3. Mercury-Adsorption Mechanism, Influence of pH on Adsorption Capacity and Competitive Adsorption X-ray photoelectron spectroscopy (XPS) analysis was performed on C-120-H-5 samples before and after contact with the mercury-salt solution to characterize the oxidation state of the absorbed mercury. As shown in Figure 7, the obtained spectra allow the presence of elemental mercury at the C-121-
www.chemphyschem.org The sorption properties of TES-modified C-120 silica are related to the properties of SiH bonds at the silica surface. According to their relative electronegativities, silicon is more positive than hydrogen, and therefore, the hydrogen atom of the silicon hydride unit bears a negative charge and can be eliminated as a hydride ion (H).[17, 36] Hydration of hydride silica occurs due to the nucleophilic attack OH ions at the silicon atom with subsequent evolution of hydrogen (as a hydride ion reacts with a proton of water). SiH þ H2 O ! SiOH þ H þ Hþ
ð1Þ
Assuming that the redox potential of the metal species at ambient pH is lower than the redox potential of H + , H will recombine with the metal ion. Under such conditions, mercury removal using hydride silica might be described as oxidation of silicon hydride groups and reduction of HgII to HgI, followed by adsorption of HgI onto the adsorbent surface, according to the following reaction scheme: ð SiHÞ þ 2 Hg2þ þ H2 O ! ð SiOÞHg2 2þ þ 2 Hþ
ð2Þ
The pH of the solution is an important factor that can influence the mercury adsorption efficiency, as it is relevant to both the aqueous metal speciation and the stability of the silicon hydride groups. The adsorption efficiency of both the C-120-H5 and the C-120-H-8 silica samples was thus investigated over a broad pH range. It is worth noting that the adsorption capacities for both materials are nearly identical: the highest efficiency is observed at pH 3 and decreases with increasing pH (Figure 8). Only a slight reduction in removal efficiency oc-
Figure 8. Plot of removal efficiency of C-120-H-x silica containing 1) 0.05 mmol g1 and 2) 0.08 mmol g1 of SiH groups as a function of the pH of the Hg2 + solution.
Figure 7. XPS spectra of C-120-H-5 silica before (1) and after (2) sorption of HgII.
H-5 surface to be discounted, as the 4f7/2 binding-energy peak characteristic of Hg0 (99.9 eV) is not observed. However, the use of silica as a support substrate presents difficulties in assessing the presence or absence of HgI and HgII, for which the 4f5/2 and 4f7/2 binding-energy peaks strongly overlap with the intense broad Si 2p binding-energy peak of silica centred around 102.5 eV. Therefore, these XPS spectra allow exclusion of the presence of Hg0 at the surface, and they are consistent with the formation of either HgI or HgII surface complexes. 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
curred at pH 7, although the removal efficiency at higher pH (> 8) declined significantly, because splitting of the silicon hydride bond occurs in alkaline media with subsequent formation of a silanol group (Reaction 1). This suggests that sorbent regeneration should be possible at high pH, although studies of mercury desorption/sorbent regeneration are complicated by readsorption of Hg onto residual silicon hydride groups and facile mercury hydroxide precipitation. Finally, the competitive adsorption of metal cations in aqueous solution onto C-120-5 silica was investigated for a Hg2 + (aq)/Cu2 + (aq) mixture with initial concentrations of 0.8 and 8 mg L1. The selectivity of the process towards mercury was ChemPhysChem 2013, 14, 4126 – 4133
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Figure 9. Competitive adsorption of HgII and CuII ions on 0.1 g of C-120-5 in a 40 mL solution containing a mixture of 0.8 mg L1 Hg(NO3)2 and 8 mg L1 Cu(NO3)2.
determined by the redox properties of SiH groups and other heavy metals. In this competitive adsorption study, despite a tenfold excess of CuII over HgII, mercury is effectively removed from the aqueous solution (Figure 9). It should be noted that the mercury adsorption process is almost unaffected by the presence of copper and that no metallic copper was detected in the system.
4. Conclusions The effectiveness of ionic mercury removal from an aqueous solution by adsorption onto silica and POSS xerogels with grafted silicon hydride groups occurs mainly by a redox mechanism: oxidation of SiH groups and reduction of HgII ions. The competitive adsorption of a mixture of copper and mercury ions showed specificity of the adsorbent toward mercury removal. The structure of the silica framework (including the pore size) and the concentration of SiH groups are recognized as key factors that determine the total metal-adsorption capacity. The larger silica pore channels provide low flow resistance to solutes passing through the matrix and may control access of Hg2 + ions to the adsorbent’s binding sites. Mercury adsorption onto hydride silica composites is a fast and efficient process, allowing the loading of up to 0.22 mmol of Hg per g of C-120-H-44 and 0.44 mmol per g of POSS-188 adsorbent with a stoichiometric molar ratio 1:2 between Hg and SiH groups on the silica surface. Their near-constant adsorption efficiency under mildly alkaline to acidic conditions and their inexpensive and facile assembly indicates the potential usefulness of hydride silica composites as effective materials for mercury decontamination in natural waters and industrial effluents.
Acknowledgements
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This research has been supported by a Marie Curie International Incoming Fellowship FP7-PIIF-GA-2009-255635-HYREM within the 7th European Community Framework Programme. Keywords: adsorption · hydride silica · mercury · rapid uptake · water remediation
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: September 8, 2013 Published online on November 20, 2013
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