Chem. Mater. 2005, 17, 5275-5281
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Heavy Metal Retention by Organoclays: Synthesis, Applications, and Retention Mechanism M. Jaber,* J. Miehe´-Brendle´, L. Michelin, and L. Delmotte Laboratoire de Mate´ riaux a` Porosite´ Controˆ le´ e, UMR-7016 CNRS, Ecole Nationale Supe´ rieure de Chimie de Mulhouse, UniVersite´ de Haute Alsace 3, rue Alfred Werner, 68093 Mulhouse CEDEX, France ReceiVed April 8, 2005. ReVised Manuscript ReceiVed June 14, 2005
Organoclays with thiol functionalities pending in the interlayer space were prepared by a single-template sol-gel synthesis. With magnesium nitrate, aluminum acetylacetonate, and mercaptopropyltrimethoxysilane involved, the reaction was established at room temperature. Characterizations were carried out using X-ray diffraction (XRD), X-ray fluorescence, IR, Raman, and solid-state 29Si and 13C NMR spectroscopies. XRD patterns exhibit broad peaks characteristics of badly organized solids. The comparison of the experimental basal distance with the theoretical one indicated that organic groups are not perpendicular to the sheets. IR spectra presented the structural bands of the framework while Raman spectroscopy was necessary to confirm the presence of the thiol function. The silicon NMR spectra showed that silanes were fully hydrolyzed but weakly condensated (presence of T1 sites). Comparison between magic-angle spinning (MAS) and cross-polarization MAS 13C NMR spectra revealed rigid and mobile chains. To evaluate the retention capacity of these materials, tests on the chelating of heavy metal cations were performed. These hybrids offer a high retention capacity for mercury and copper (∼100%). On the basis of different characterizations, a retention mechanism is proposed.
Introduction Under certain environmental conditions, heavy metals might accumulate up to a toxic concentration and cause ecological damage.1 As a consequence, aquatic ecosystem disturbance induced by heavy metal pollution causes the loss of biological diversity and increases bioaccumulation and magnification of toxicants in the food chain.2 Soils are also regarded as the ultimate sink for these pollutants.3 They are often induced to the environment through modern human activities at sites related, for example, to metal mining and metallurgical processing and waste disposal.4 Increasing importance in pollution control is actually developed, because chemicals are suspected to have negative adverse health effects. Alkaline precipitation, ion exchange columns, electrochemical removal, filtration, and membranes are currently available technologies for heavy metal removal.2 However, there is some inconvenience in the application of these methods, such as poor selectivity, production of solid residuals, outflow of dangerous ionic metals from the treatment, and organic compounds frequently inhibiting the process.2 Soluble heavy metals can be removed too, by adsorption on chemically functionalized inorganic supports.5 Among these inorganic supports, porous materials such as zeolithes, mesoporous silica, and clays have focused the attention. (1) Gumgum, B.; Unlu, E.; Tez, Z.; Gulsun, Z. Chemosphere 1994, 29, 111. (2) Eccles, H. Trends Biotechnol. 1999, 17, 462. (3) Chlopecka, A.; Bacon, J. R.; Wilson, M. J.; Kay, J. J. EnViron. Qual. 1996, 25, 69. (4) Gier, S.; Johns, W. D. Appl. Clay Sci. 2000, 16, 289. (5) Sen, T. K.; Mahajan, S. P.; Khilar, K. C. Colloids Surf., A 2002, 211, 91.
Grafting organic groups such as thiol or amine functions have been widely investigated in the case of mesoporous silica.6 Important soil components are layer silicates such as clay minerals or phyllosilicates.7 Smectites which belong to the 2:1 phyllosilicates family are built from layers made by the condensation of one central octahedral sheet and two tetrahedral sheets.8 Heteroionic substitutions impart to smectites a permanent negative structural charge, which is balanced by the presence of cations in the interlayer space. Owing to their important specific surface area and a high structural charge (up to 1000 mequiv per kg),9 they have interesting ion exchange properties.10 The heavy metal adsorption by clays has been studied to evaluate their use as remedial agents in contaminated waste deposits and other areas of high heavy metal concentration.4 The inconvenient of using clays for adsorption is their possibility to outflow dangerous ionic metals. To avoid the outflow process, functionalizing11 and pillaring12 have been practiced to enhance the chelating effect instead of the adsorption one. This latter process requires at least two steps: first, an ion exchange process with alkylammonium and, then, contact with organosilane groups. (6) Chong, A. S. M.; Zhao, X. S.; Kustedjo, A. T.; Qiao, S. Z. Microporous Mesoporous Mater. 2004, 72, 33. (7) Haber-Pohlmeier, S.; Pohlmeier, A. J. Colloid Interface Sci. 1997, 188, 377. (8) Brindley, G. W. Mineral. Soc. Monogr. 1980, 5, 125. (9) Caille`re, S.; Rautureau, H. M. Mine´ ralogie des argiles, tome 1: Structure et proprie´ te´ s physico-chimiques, 2nd ed.; Actualits scientifiques et agronomiques 8; Masson: Paris, France, 1982. (10) Schlegel, M. L.; Manceau, A.; Chateigner, D.; Charlet, L. J. Colloid Interface Sci. 1999, 215, 140. (11) Lagadic, I. L.; Mitchell, M. K.; Payne, B. D. EnViron. Sci. Technol. 2001, 35, 984. (12) Pinnavaia, T. J. NATO Sci. Ser., Ser. C 1986, 165, 151.
10.1021/cm050754i CCC: $30.25 © 2005 American Chemical Society Published on Web 09/16/2005
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In 1995, a one step sol-gel process was reported to obtain a hybrid inorganic-organic nickel and magnesium phyllosilicate having a talc-like structure and utilizing an organosilane as the silica source.13 This new material contains organic functionalities covalently bonded to the tetrahedral sheets via Si-C bonds. The resulting organotalc has a wellordered octahedral sheet and a slightly disordered -C-SiO3 tetrahedral sheet. In 1997, syntheses of organotalc and organopyrophyllite have been reported.14 Since 1998, various organic functionalities have been introduced into organotalc, leading to applications in the fields of heavy metal retention14-22 and clay-polymer nanocomposites.18 These hybrid lamellar materials prepared by sol-gel self-assembly methods can be used as environmental barriers, polymer fillers, and catalytic supports. Other potential applications include the improvement of optical and mechanical properties.23 We report here the synthesis and characterization of covalently linked organic-inorganic lamellar composites, containing silicon, aluminum, and magnesium. The goal was to prepare compounds having a saponite-like structure with the general formula Nax[(RSi)(4-x)AlxMg3O8+x(OH)2] presenting both ion exchange properties and organic thiol functions (known as mercaptan because of their affinity for mercury compounds). The hydrophobic/hydrophilic balance could be controlled by the variation of aluminum content.24 After describing the materials and their structural characteristics, contact with the heavy metal cations was accomplished. Different concentrations of the latter were prepared. The chelating process mechanism is proposed on the basis of experimental results. Experimental Section Reactants. Mercaptopropyltrimethoxysilane HS(CH2)3Si(OCH3)3 (Lancaster, 96 wt %), aluminum acetylacetonate [CH3COCHCOCH3]3Al (Aldrich, 99 wt %), hexahydrated magnesium nitrate (Carlo Erba, 98 wt %), mercury chloride (Prolabo, 99 wt %), copper sulfate (Merck, 99 wt %), lead nitrate (Aldrich, 99 wt %), zinc acetate (Fluka, 99 wt %), ethylenediaminetetraacetic acid (EDTA; Labosi, 99 wt %), orange xylenol (Merck, 100 wt %), ethanol (Prolabo, 99.8 wt %), sodium hydroxide (Fluka, 97 wt %), and distilled water were used. (13) Fukushima, Y.; Inagaki, S.; Kuroda, K. Prepr. Pap.sAm. Chem. Soc., DiV. Petrol. Chem. 1995, 40, 254. (14) Ukrainczyk, L.; Bellman, R. A.; Anderson, A. B. J. Phys. Chem. B 1997, 101, 531. (15) Da Fonseca, M. G.; Airoldi, C. J. Mater. Chem. 2000, 10, 1457. (16) Da Fonseca, M. G.; Airoldi, C. J. Chem. Soc., Dalton Trans. 1999, 21, 3687. (17) Da Fonseca, M. G.; Da Silva Filho, E. C.; Machado Junior, R. S. A.; Arakaki, L. N. H.; Espinola, J. G. P.; Airoldi, C. J. Solid State Chem. 2004, 177 (7), 2316. (18) Da Fonseca, M. G.; Barone, J. S.; Airoldi, C. Clays Clay Miner. 2000, 48, 638. (19) Cestari, A. R.; Vieira, E. F. S.; Bruns, R. E.; Airoldi, C. J. Colloid Interface Sci. 2000, 227, 66. (20) Da Fonseca, M. G.; Silva, C. R.; Barone, J. S.; Airoldi, C. J. Mater. Chem. 2000, 10, 789. (21) Whilton, N. T.; Burkett, S. L.; Mann, S. J. Mater. Chem. 1998, 8, 1927. (22) Silva, C. R.; Fonseca, M. G.; Barone, J. S.; Airoldi, C. Chem. Mater. 2002, 14, 175. (23) Sasai, R.; Itoh, H.; Shindachi, I.; Shichi, T.; Takagi, K. Chem. Mater. 2001, 13, 2012. (24) Jaber, M.; Miehe-Brendle, J.; Delmotte, L.; Le Dred, R. Microporous Mesoporous Mater. 2003, 65, 155.
Jaber et al. A series of compounds having the following general formula Nax[(RSi)4-xAlxMg3O8+x(OH)2], where R stands for the mercaptopropyl group in the silica source and x varies from 0 (talc-like structure) to 1.20 (saponite-like structure), were prepared according to a synthetic procedure previously established.25 Typically, magnesium nitrate hexahydrate, Mg(NO3)2‚6H2O, was dissolved under stirring in ethanol and subsequently mixed with an ethanolic solution of aluminum acetylacetonate (for x ranging from 0.20 to 1.20). After the addition of the mercaptopropyltrimethoxysilane ethanolic solution, an aqueous solution of sodium hydroxide (50 cm3) was added to form a precipitate. The resulting white precipitate was kept under stirring for 24 h at room temperature. The solids were recovered by filtration, washed four times with distilled water (100 cm3), and left to dry for 48 h at room temperature under vacuum. For the chelating tests, 100 mg of as-synthesized organophyllosilicates was suspended in 50 cm3 of aqueous solutions containing a heavy metal cation (Cu2+, Cd2+, Pb2+, Hg2+) with the following concentrations: 5.0 × 10-4, 1.0 × 10-3, 5.0 × 10-3, and 5.0 × 10-2 mol dm-3. These mixtures were kept under vigorous stirring at room temperature for 24 h. After filtration, the solid and the supernatant were analyzed according to the methods described below. X-ray diffraction (XRD) patterns were recorded using a Philips PW1800 diffractometer (Cu KR radiation, λ ) 0.154 06 nm; scanning range, 1-70 (2θ); step size, 0.02°/2 s). Infrared measurements were performed using a DTGS detector and analyzed with the OPUS software. The number of scans was fixed to 100 with a resolution of 4 cm-1. The thermogravimetric curves of the assynthesized samples were performed with a TG-DSC apparatus (Setaram TG-DSC 111) under a mixture of 50% O2 and 50% N2, with a flow rate of 0.5 dm3 h-1, from 20 to 850 °C, and a heating rate of 5 °C min-1. Elemental analyses were perfomed by X-ray fluorescence (XRF) with a Magix Philips (2.4 kW). The assynthesized materials were calcined and mixed with lithium tetraborate before being fused to form the bead. Samples containing heavy metal cations were packed into pellets. The amount of metal remaining in solution was determined by complexometric titration with a standard EDTA solution. XRF analyses were performed on the solutions to confirm the previous results according to calibration curves. 29Si and 13C nuclear magnetic resonance (NMR) spectra were acquired on a Bruker MSL-300 spectrometer (B0 ) 7.04 T) with magic-angle spinning (MAS) at 59.6 and 75.5 MHz, respectively. Samples were packed in a 7-mm diameter cylindrical zirconia rotor and spun at a spinning frequency of 4 kHz. For 29Si and 13C MAS decoupling (29Si{1H} and 13C{1H}) NMR experiments (decoupling field 65.7 kHz), a π/4 pulse duration of 1.8 µs and a π/4 pulse duration of 2 µs were used respectively, with a recycle time of 60 s. 27Al MAS NMR spectra experiments were performed on a Bruker DSX-400 spectrometer at 104.2 MHz, using a spinning frequency of 25 kHz. Spectra were recorded with a recycle delay of 1 s and by using a 0.7-µs single pulse corresponding to a flip angle of π/12, to ensure selective excitation. Transmission electron microscopy (TEM) investigations were carried out on a Philips XL30. One droplet of the powder dispersed in chloroform (CHCl3) was deposited onto a carbon-coated copper grid and left to dry in air.
Results and Discussion To have information about the framework, structural characterizations were performed. XRD patterns of the as(25) Jaber, M.; Miehe-Brendle, J.; Roux, M.; Dentzer, J.; Le Dred, R.; Guth, J.-L. New J. Chem. 2002, 26 (11), 1597.
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Figure 1. X-ray pattern of the as-synthesized mercaptopropylphyllosilicate (x ) 0.40).
synthesized materials (Figure 1) display the reflections characteristic of an organophyllosilicate structure: (001), (020, 110), (130, 220), (060). However, these reflections are broader than those observed for the parent phyllosilicates as previously mentioned in the literature.26 This behavior is explained by the presence of organic chains in the interlamellar space. The value of d001 varies between 1.3 and 1.7 nm and increases with the substitution ratio x. Assuming that the silicate layer thickness is equal to 0.96 nm27 and that the length of the organic group R is 0.54 nm, the d001 value should be equal to 2R + 0.96 nm (3.1 nm) considering that the organic chains are perpendicular to the sheets. The values obtained could suggest that the organic chains are bonded to adjacent lamella, and the chains are either alternatively distributed or identically tilted in the interlayer space. An additional reflection appears at 2θ ) 12° for x > 0.60 and is assigned to the (002) reflection, indicating a variation in the layers stacking. The peaks at 2θ ) 60°, associated with an interplanar distance of 0.156 nm for x ) 0 and 0.152 nm for x ) 1.20, are characteristic of the (060) reflection.28 TEM investigations were performed along a longitudinal projection direction, that is, with the electron beam perpendicular to the sheets. The micrograph presented in Figure 2 showed that the sheets appear isolated and are 20-25 nm long. No (001) distance could be estimated using this method, because there is no periodicity in the structure. The thickness of the sheet measured is about 0.30 nm, which indicates that complete exfoliation of the structure occurred in chloroform. The simplest exfoliation method could be due to a disorder that was already present in the structure and may be caused by the organic moieties. At this point, further characterizations were essential to obtain detailed structural information. The chemical formulas were determined from the XRF analysis. The content of organic matter was subsequently adjusted from thermogravimetric curves. The results presented in Table 1 show that the repartition of aluminum in both sheets does not match the theoretical formulas. Indeed, the substitution of magnesium by aluminum in the octahedral sheet confers a positive charge partially balanced by the negative charge of the tetrahedral sheet. Also, note the (26) Jaber, M.; Miehe-Brendle, J.; Le Dred, R. Chem. Lett. 2002, 9, 954. (27) Brindley, G. W. Clays Clay Miner. 1966, 14, 27. (28) Roy, D. M. Am. Mineral. 1954, 39, 957.
Figure 2. TEM micrograph of the as-synthesized mercaptopropylphyllosilicate (x ) 0.40); the arrows point to layers.
presence of magnesium in the interlayer space (shown in the formulas reported in Table 1). Incorporation of a less divalent cation (two positive charges in the case of magnesium vs one in the case of sodium) in a hydrophobic interlayer space filled with organic matter is easier to balance the charges induced by tetrahedral substitution. MAS 27Al NMR spectra (Figure 3) showed that aluminum is principally hexacoordinated (peaks in the range -10 to 10 ppm) for low values of x (0.20) and is distributed in both sheets when x is increased. These results do not correlate with previous calculations made by XRF. Because the reaction occurs in a basic medium, dissolved silica remains partially in solution. Because the silicon is deficient, defects can be present in the tetrahedral layer, hence, weakening the framework. Therefore, completing the tetrahedral layer composition with aluminum may be erroneous, and this latter can essentially be present in the octahedral layer. This suggested explanation fits well with the TEM observations. The infrared spectra of the original matrix display a sequence of bands in agreement with the presence of organic moieties bonded to the inorganic network with absorptions at 2900 and 1180 cm-1 associated with ν(C-H) and ν(SiC), respectively. A very weak signal is observed at 2560 cm-1 and is attributed to the -SH functions related to the carbon chain. Characteristic framework vibration modes corresponding to the presence of Si-O-Si (1019-1130 cm-1), Si-O-Al (550 cm-1), and Si-O-Mg (400 cm-1) are also present, confirming the formation of the clay matrix. As the signal corresponding to thiol functions is almost absent in IR measurements, a Raman experiment was required to distinguish a very sharp and intense signal (Figure 4). Complementary information about the organic chain was obtained by 13C NMR spectroscopy. All the carbons of the organic chain are identified on the MAS-DEC (proton
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Table 1. Chemical Formula per Half Unit Cell of the As-Synthesized Mercaptopropylphyllosilicate relative mass of elements (g/100 g dried at 60 °C) substitution ratio
Na
Mg
Al
Si
chemical formula per half unit cell
0 0.20 0.40 0.60 0.80 1.00 1.20
0 0 0.11 0 0.58 0.82 0.17
9.29 7.09 7.45 18.50 6.71 13.30 13.30
0 0.60 1.23 4.94 2.41 12.70 7.97
10.80 9.45 10.20 27.90 8.05 6.64 12.40
(Mg)3[(RSi)4] O8,0(OH)2,1MgO Mg0,1(Mg)3[(RSi)3,7Al0,3]O8,1(OH)2 Na0,1(Mg2,9Al0,1)[(RSi)3,6Al0,4]O8,2(OH)2 Mg0,1(Mg2,6Al0,4)[(RSi)3,7Al0,3]O8,3(OH)2 Na0,2Mg0,2[(Mg2,8Al0,2)(RSi)3,2Al0,8]O8,4(OH)2 Na0,2Mg0,5(Mg2,8Al0,2)[(RSi)2,7Al1,3]O8,7(OH)2 Na0,1Mg0,5(Mg2,7Al0,3)[(RSi)2,6Al1,4]O8,7(OH)2
decoupling experiment) NMR spectra (Figure 5): the peak at 13 ppm is assigned to the C3 of the mercaptopropyl chain, the second one at 29 ppm corresponds to the C1 and C2, and the third one at 44 ppm is attributed to the C4 of the methoxy group.15 This latter indicates that the hydrolysis of the mercaptopropyltrimethoxysilane is not complete. The presence of sodium and magnesium cations in the interlamellar space, to balance the negative charges created by the silicon to aluminum substitution, could be correlated: multiplying the environments of the carbon chain led to an enlargement of the peaks observed with the increasing of the substitution ratio (x). Comparison of the MAS-DEC and
CP-MAS spectra exhibits an enhancement of the peaks assigned to the C2 and C4 (Figure 6). The mercaptopropyl chain is, therefore, present into different states: one rigid and one mobile. Mobile groups are probably located in the
Figure 5. 13C MAS-DEC NMR spectra of the as-synthesized mercaptopropylphyllosilicate with variable substitution ratios.
Figure 3. 27Al MAS NMR spectra of the as-synthesized mercaptopropylphyllosilicate with variable substitution ratios.
Figure 4. Raman spectrum of the as-synthesized mercaptopropylphyllosilicate (x ) 0.40).
Figure 6. Comparison of 13C MAS-DEC and CP-MAS NMR spectra of the as-synthesized mercaptopropylphyllosilicate (x ) 0.40).
HeaVy Metal Retention by Organoclays
Figure 7. 29Si MAS-DEC NMR of the as-synthesized materials with variable substitution ratios.
interlayer space whereas the rigid one must surely be in a confined space. Further characterizations were essential before concluding the placement of these rigid organic chains. Indeed, taking into account the available surface of the interlayer space (45 × 10-2 nm2, a × b) and the surface that a mercaptopropyl chain (28 × 10-2 nm2 calculated by assimilating the organic matter to a cylinder; l × L, l, and L were determined taking into account the C-H, Si-C and S-H bounds) occupies, the interlayer space could contain 1.5 organic chains per unit cell. However, the number of organic chains was higher (Table 1). The defects found in the tetrahedral layer already mentioned above render the penetration of the organic chain possible into the hexagonal cavities of the adjacent sheet in addition to those present in the interlayer space. This could be a probable site for the rigid carbon chain. MAS-DEC 29Si NMR spectra exhibit three bands: the first one between -70 and -58 ppm corresponds to the T3 sites [(RSi(OM)3] (where M stands for Si or Al), the second one
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between -57 and -50 ppm is attributed to T2 sites [RSi(OM)2(OH)], and the last one between -45 and -35 ppm is assigned to the T1 sites [RSi(OM)(OH)2] (Figure 7). The distribution of aluminum in both tetrahedral and octahedral layers induces an enlargement of the bands as the value of x increases. A shift is observed from -70 to -60 ppm and is probably correlated to the electonegative difference of silicon and aluminum. No signal was detected in the range -102 to -90 ppm indicating that, as expected, all the silicon is bonded to mercaptopropyl groups. The different analysis and characterization results can be summerized as follows: 1. The broad reflections on the XRD patterns suggest a disorder in the structure. 2. TEM observations indicate an exfoliation of the sheets in chloroform. 3. XRF analysis indicates probable defects in the tetrahedral layer. 4. An incomplete polycondensation of silicic species occurs because the T2 and T1 sites are observable by 29Si NMR spectroscopy. 5. The SiO4 tetrahedra are not all bonded to the magnesium or aluminum present in the octahedral layer. To evaluate the capacity of these materials for water and soil depollution, the solids obtained were suspended in heavy metal cation solutions. Indirect titration of the filtrate was then performed. The inorganic cations were chosen for their pollutant strengths. The influence of both concentration and x values was investigated (Figure 8). A total of 100% of mercury was retained even for the highest concentration (5.0 × 10-2 M). In the case of copper and cadmium solutions, the retention was almost complete (∼90%). Lead cations were less retained than others, and their amount in the solids decreased (∼40%) when the solution concentration increased (5.0 × 10-2 M). The retention ratio decreased when the aluminum amount increased (Figure 8b). Indeed, the lower the thiol function concentration, the lower the chelating ratio. Many factors must be taken into consideration: the ionic radius of the cation, its hydration state, and its complex stability with the thiol function. Mercury and copper are smaller than cadmium and lead, which facilitates their penetration into
Figure 8. Retention ratio as a function of the cation concentration (x ) 0.20; a) and the substitution ratios (C ) 1.0 × 10-3 M; b).
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Figure 9. XRD patterns of the hybrids after contact with the heavy metal cations. Table 2. Molar Ratio of the Hybrids after Their Contact with the Heavy Metal Cations product
aqueous solution
molar ratios obtained by XRF
concentration (M2+ mol L-1)
a
x
Si/Mg
Si/Al
M2+/Si
0
0
1.1
a
a
Cu/5 × 10-3 Cu/5 × 10-2 Cu/5 × 10-1
0 0 0
2.0 116 88
a a a
0.4 1.5 1.9
Cd/5 × 10-3
0
2.1
a
0.7
Pb/5 × 10-3 Pb/5 × 10-2
0 0
2.6 98
a a
0.2 1.0
Cu/5 × 10-3 Cu/5 × 10-2
0.20 0.20
4 87
3.0 10.2
0.8 3.1
Hg/5 × 10-3 Hg/5 × 10-2
0.20 0.20
3.0 12.5
11.1 12.5
0.5 1.6
Cd/5 × 10-3 Cd/5 × 10-2
0.20 0.20
1.8 20.0
12.5 10.0
0.01 0.62
Pb/5 × 10-3 Pb/5 × 10-2 Cu/5 × 10-3 Hg/5 × 10-3 Cd/5 × 10-3 Pb/5 × 10-3
0.20 0.20 0.60 0.60 0.60 0.60
3.3 100 2.3 3.0 2.4 3.4
12.5 4.2 3.0 4.2 4.3 3.6
0.6 0.6 0.8 0.5 0.1 0.4
No available data.
interlamellar space filled with organic matter and in the octahedral sheet. After contact with heavy metal cations, the hybrids were characterized by XRD. None of the prepared samples did contain peaks corresponding to mercaptopropylphyllosilicate. However, a band between 20 and 30° in 2θ° appeared that corresponds to amorphous silica. Many peaks present can be assigned to the crystallization of copper, cadmium, and lead into basic salts when mercury cations Hg2+ are reduced into Hg22+ and form calomel Hg2Cl2 (Figure 9). The molar ratio of Si/Mg, Si/Al, and M2+/Si increased considerably when the concentration of the heavy metal cations increased in the solution (Table 2). The diminution of Si, Mg, and Al concentrations in the lattice indicated the destruction of the structure which was already observed by XRD. Calculations of the thiol function amount and heavy metal cations (millimoles per gram of organoclay) present in the lattice were completed to determine if a chelating mechanism could be applied. Taking into account that there is 5.8 mmol -SH/g of organoclay, 10 mmol Pb/g of
organoclay, and 24 mmol Cu2+/ and Hg2+/g of organoclay, we could not just acknowledge a simple chelating process. The infrared spectrum of the resulting matrix showed a sequence of bands in agreement with the presence of organic moieties at 2900 and 1180 cm-1 associated with ν(-C-H) and ν(-Si-C), respectively. A very weak signal was observed at 2560 cm-1 and was attributed to the -SH functions related to the carbon chain. Bands corresponding to the presence of a phyllosilicate structure were absent. The 13C CP-MAS NMR spectra of solids containing copper, cadmium, and lead exhibited bands between 13 and 42 ppm assigned to the different carbons of the organic chain. The spectrum of the solid retaining mercury showed a shielding of the band attributed to the C1-C2 (Figure 10). This is probably due to the oxidation of two mercaptopropyl chains by a mercury cation (Hg2+)29,30 according to the reaction shown below: 2(-C3H6SH)(l) + 2(Hg2+ + 2Cl-)(l) f -C3H6-S-S-C3H6-(l)+ Hg2Cl2(s) + 2(H+ + Cl-)(l) (1) Contact between the heavy metal cation solution and the organophyllosilicate seemed to destroy the structure and led to the formation of basic salts with copper, cadmium, and lead cations. In the case of mercury cations, Hg2+ was reduced into Hg22+ by the mercaptan functions present in the organic chain bounded to silicon. No oxydo-reduction reaction occurred for other cations. The retention process of the heavy metal cation solution by a mercaptopropylphyllosilicate could be explained as follows: The chelating of the heavy metal cations by the mercaptopropylphyllosilicate induce the destruction of the structure based on the organic groups. This decreases the acid character of the aqueous solution according to the reaction below: Na[(HSC3H7Si)3Al]Mg3O8,5(OH)2(s) + 4(Cu2+ + SO42-)(s) + 4H2O(l) f 3HSC3H6SiO1,5(l) + Al(OH)3(s) + 3(Mg2+ + SO42-)(s) + Cu4(SO4)(OH)6(s) + (Na+ + OH-)(l) (2) The decrease of the acid character was exemplified by copper cations. The same results were obtained with cadmium and lead. Indeed, the mercaptopropylphyllosilicate was badly organized, which was demonstrated by all the characterizations. As explained above, mercaptopropyl groups were located either in the interlayer space or in the hexagonal cavities constituted by the silica tetrahedral. During the contact with heavy metal cations, hexacoordinated cations such as copper and cadmium could substitute magnesium and aluminum in the octahedral sheets. The chelating of the hexacoordinated cations by thiol functions induced the distortion of both (29) Encyclopedia of industrial chemistry; VCH: New York, 1995; Vol. A26, p 767. (30) Coffey, S. Rodd’s Chemistry of Carbon Compounds, Aliphatic compounds, 2nd ed.; Elsevier: Amsterdam, 1994; Vol. I, part B, p 74.
HeaVy Metal Retention by Organoclays
Figure 10.
13C
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CP-MAS NMR spectra of a mercaptopropylphyllosilicate (x ) 0.20) after contact with mercury.
tetrahedron and octahedron and, thus, the destruction of the structure. The ratio of all elements constituting the sheet in the solution increased strongly. Conclusion Organophyllosilicates with thiol functions have been prepared to chelate heavy metal cations. A one-step synthesis at room temperature for 24 h involved magnesium nitrate, aluminum acetylacetonate, and mercaptopropyltrimethoxysilane. These hybrids consisting of octahedral sheet surrounded with two tetrahedral sheets seem to have a high disorder in their structure as was confirmed by XRD and TEM characterizations. The T1 sites observed by 29Si NMR spectra indicate an incomplete condensation of silanol groups. This results in the presence of defects in the framework inducing a very unstable structure. On the basis of calculations done on the available surface of the interlayer space and the surface occupied by a mercapropyl chain, organic groups cannot be located only between the layers. Therefore,
pending groups can penetrate the adjacent layer and be positioned in the hexagonal cavities constituted by the tetrahedral sheets. The contact of these materials with heavy metal cation solutions at different concentrations showed that they have a high retention capacity. However, XRD patterns of the resulting solids exhibit the formation of basic salts and calomel. Because there are mercaptopropyl functions in the hexagonal cavities, migration of hexacoordinated cations in the octahedral sheet was accelerated. The chelating of these cations induced the destruction of the structure. Stabilization of the latter via the preparation of hybrid organosilanes and silanes is under progress. Acknowledgment. We would like to acknowledge Dr. O. Larlus, Dr. F. Gaslain, and Prof. J.-L. Guth for the fruitful discussions. CM050754I
