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Langmuir 1997, 13, 2550-2556
NMR Evidence of Silicate and Carbonate Competition for Cations in Solution at Low Temperature: Case of Ca2+, Zn2+, Pb2+, and Al3+ S. Fraval,† J. Y. Bottero,*,‡ W. E. E. Stone,§ P. Broekaert,| F. Masin,| P. Pirotte,| and F. Mosnier⊥ CIRSEE 38, rue du Pre´ sident Wilson, 78230 Le Pecq, France, LGE-Cerege URA 132 CNRS, Universite´ Aix-Marseille III, Europole me´ diterrane´ en de l’Arbois BP 8013545 Aix en Provence, France, Section de Physico-Chimie Mine´ rale (MRAC-Tervuren), Place Croix du Sud 2/18, B1348 Louvain la Neuve, Belgium, Universite´ Libre de Bruxelles, Physique Ge´ ne´ rale, CP232, Bd. du Triomphe, B1050 Bruxelles, Belgium, and SITA, 94 Rue de Provence Paris 75009, France Received July 29, 1996. In Final Form: January 27, 1997X The characterization of solids obtained by mixing soluble silicates and carbonates with metal salts containing cations such as Zn2+, Pb2+, Ca2+, or Al3+ has been performed. X-ray diffraction, scanning electron microscopy, and high-resolution 29Si and 27Al NMR of these solids have been carried out. Pb and Ca form crystallized carbonate precipitates and amorphous silicates. Al and Zn form only amorphous silicates. The speciation of the different complexes formed is achieved by decomposition of the 29Si NMR spectra. It is then possible to explain the reaction mechanisms occurring between the soluble silicates and the cations. In no case is there any silica formed in the precipitate (no Q4 entity). The most reactive silicate species present in the samples are formed by Q3 units.
Introduction Silicates are used in essential technologies such as detergency, water treatment, mineral beneficiation, and oil recovery. These applications generally rely on the interactions occurring in solution between anionic silicate species and metal ions. The nature of these interactions and the characterization of the resulting solids have been investigated by numerous authors. Iler1 reported that metal salts react with soluble silicates to produce amorphous metal silicate precipitates formed by adsorption on gelatinous silica. Others2 suggested that Fe2+, Co2+, Zn2+, or Cu2+ ions react with sodium silicate to liberate colloidal silica that coats the metal hydroxides. Hazel et al.,3 who studied the interactions of zinc salt with silicates, suggested that the presence of silica enhances the hydrolysis of Zn2+. They conclude to the formation a complex phase Zn‚polymerized silicate in which Zn has the stoichiometry of Zn(OH)2. Mercade4 showed that the nature of the precipitates formed during these reactions depends on many parameters such as pH, reactant ratios, and concentrations. The resulting solids are then made up of a mixture of insoluble metal silicates, metal hydroxides and colloidal silica. The interactions between silica surfaces and metal ions have also been the subject of numerous studies. Falcone5 highlighted the parallel which could be drawn between the pKa values of silica gels and the existence of high polymer silicates in solution. He suggested that †
CIRSEE 38. Universite´ Aix-Marseille III. § Section de Physico-Chimie Mine ´ rale (MRAC-Tervuren). | Universite ´ Libre de Bruxelles. ⊥ SITA. X Abstract published in Advance ACS Abstracts, April 1, 1997. ‡
(1) Iler, R. K. In The chemistry of silica; Wiley-Interscience: NewYork, 1979. (2) Smolin, Y. I. In Aqueous Silicate Chemistry; Falcone, J. S., Jr., Ed.; American Chemical Society: Washington, DC, 1982. (3) Hazel, J. F.; Mc Nabb, W. M.; Machemer, P. E. J. Electrochem. Soc. 1952, 99, 301. (4) Mercade, V. Trans. Soc. Min. Eng. AIME 1981, 268, 1842. (5) Falcone, J. S. ACS Symp. Ser. 1982, 194 (9), 133.
S0743-7463(96)00743-3 CCC: $14.00
metal cations interact with silicates in solution in a manner analogous to what is observed for silica gel, that is, metal adsorption is initiated when the pH is raised to within 1-2 pH units below the pH at which the polyvalent metal hydroxide precipitates.6 Falcone et al.5 argued that the interaction of these soluble silicate species with metal ions would increase as their degree of polymerization decreased. He found, for instance, that at high SiO2/Na2O molar ratios (>2) the presence of colloidal size silicate anions having a high content of acidic surface silanol groups readily adsorbed nucleating metal ion hydroxides. This adsorption seems to be enhanced by either the presence of OH- ions or by an increased number of SiOsites on the colloidal sized silica surfaces at high pH. The presence of intermediate sized silica species capable of forming ligand complexes with metal ions was suggested but not demonstrated. To reach a better understanding of the interactions occurring between soluble silicates and metal ions in solution, it is essential first to determine the nature and structure of the soluble silicates species present and second to characterize the new solid phase formed. The different methods which can be used for the study of soluble silicate species in solution are, for instance, gel filtration chromatography, potentiometry, trimethylsilation followed by gas chromatography, and 29Si NMR. With these techniques the identity of a whole series of silicate anions of different structures has been established.7,8 In this study we mainly focus on the use of NMR spectroscopy which can probe the local environment of a particular nucleus. It can be used to study both silicate species in solution and in the solid phase. In this paper, 29Si and 27Al NMR experiments have been run on a series of samples formed by the interaction of soluble silicate species with Al, Zn, Pb, and Ca cations at different pH and Si/M molar ratios. (6) Shindler, P. W.; Kamber, H. R. Helv. Chim. 1968, 51, 1781. (7) Shimada, K.; Tarutani, T. J. Chromatogr. 1979, 168, 401. (8) Dent Glasser, L. S.; Lachowski, E. J. Chem. Soc., Dalton Trans. 1980, 394 (1), 399.
© 1997 American Chemical Society
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Langmuir, Vol. 13, No. 9, 1997 2551
Table 1. Samples and Methods Used for Characterization materials
CS CS + Ca 2+ CS + Pb 2+ CS + Zn 2+ CS + Al 3+
Si/M RMN MAS * CP/MAS * SEM * X-ray diffraction
0.5 * * *
1.1 * *
1
2.2
* *
1
2
1
2
* *
* *
* *
* *
* *
*
Materials l. Solid Sample Preparation. The reactive powder (CS, provided by Rhone-Poulenc) used in this study is formed by a mixture of sodium silicate (35%) having a SiO2/Na2O ratio around 2 and sodium carbonate (65%). The powder, in form of granules of some 2 mm in diameter, is completely soluble in water. Solutions of Ca2+, Al3+, Zn2+, and Pb2+ are prepared by dissolving chloride salts in deionized water. Initial cation concentrations are 0.5 g/L for Zn, Pb, and Al and 1 g/L for Ca. Variable quantities of CS were added to each solution in order to reach Si/M molar ratios of 0.5, 1, and 2. After a few minutes, when the solid is entirely dissolved, NaOH is added to the solutions to raise the pH up to 7 or 10. After a contact time of 1 h, the precipitates are centrifuged at 9000 rpm for 20 min. The supernatants are analyzed for Ca, Pb, Zn, and Al by inductively coupled plasma using a Jobin-Yvon 70 PLUS apparatus. The solids removed are freeze-dried for characterization. 2. Characterization of the Resulting Solids. X-ray Diffraction. X-ray diffractions are run on a Jobin-Yvon Sigma 2080 apparatus using a Cu KR (1.5406 A) source (Table 1). Scanning Electron Microscopy (SEM). Freeze-dried solids are included in a resin. The resulting samples are then slightly polished using a fine glasspaper (0.6 µm). Chemical analyses are carried out in the energy dispersive mode. The energy range used is 0-20 kV, and the number of counts are recorded during 300 s. The equipment used consisted of a Phillips SEM 515 apparatus. NMR Experiments. For the precipitates, all 29Si and 27Al NMR spectra were recorded on a MSL 300 Bruker solid-state NMR spectrometer equipped with a 7.05 T wide-bore magnet at 59.63 and 78.21 MHz, respectively. Rapid (14 or 12 kHz) sample spinning at the magic angle was used. The samples were filled into a double-bearing 4 mm cylindrical rotor. In the case of 29Si, conventional Fourier transform-magic angle spinning (FT-MAS) NMR spectra were detected with 2.5 µs radio-frequency pulses and 2 s time intervals. Typical measuring times were around 3 h. For the 27Al spectra, a 1.5 µs (i.e. corresponding to π/10) free induction decay excitation pulse was used together with a recycle time of 500 ms. In addition to direct FT-NMR spectroscopy, cross-polarization (CP) techniques were used in all cases. For 1H-29Si the spin contact time was typically 1 ms while for 1H27Al it was 300 µs. The time intervals between single-contact CP pulse sequences were 2 s for 29Si and 1s for 27Al. For 29Si the CP technique leads to a large sensitivity enhancement of the signal. All 29Si chemical shifts were referenced to tetramethylsilane. The 27Al chemical shifts were determined relative to Al(H2O)63+. Shifts toward more positive values denote low-field shifts. Liquid-state 29Si NMR were performed on a VARIAN 600 NMR spectrometer at a frequency of 115.2 MHz. The recycle time was 3 s and the excitation pulse 10 µs (π/2 pulse).
Results Cation Removal Efficiency. The amount of removed cations versus the mass of CS and Si/M molar ratios used is shown in Table 2. It is observed that for zinc and calcium the amount removed is influenced by the Si/M ratio. X-ray Diffraction. The X-ray diffraction diagrams (Figure l) of the precipitates are characteristic of amorphous materials except for Ca and Pb which also form crystallized carbonate phases. Scanning Electron Microscopy. Some chemical analyses of precipitates formed with Ca and Pb are given in Table 3. The initial Si/M ratios in the solution were 1
Figure 1. X-ray diffraction diagrams of Si/Zn ) 1 (A), Si/Pb ) 1 (B), and Si/Ca ) 0.5 (C).
for Ca and 2 for Pb. Parts a and b of Figure 2 show corresponding SEM images. NMR Results CS Solution at pH 10. The liquid-state 29Si NMR spectrum of CS dissolved in D2O at pH 10 is given in Figure 3. The different narrow lines in the range -71-96 ppm are assigned to Q0-Q3 species9-12 where Q represents a silicon atom bonded to four oxygen atoms (9) Harris, R. K.; Knight, C. T. G. ACS Symp. Ser. 1982, 194 (6), 79. (10) Kinrade, S. D.; Swadle, T. W. J. Am. Chem. Soc. 1986, 108, 7159.
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Table 2. Mass of CS Added (g/L) and Cations Removed from Solutions (%) Ca CS added % removed final pH
Pb
Zn
Al
Si/M ) 0.5
Si/M ) 1
Si/M ) 1
Si/M ) 2
Si/M ) 1
Si/M ) 2
Si/M ) 1
Si/M ) 2
3.6 87.3 10
8.6 99 10
0.8 98.6 10
1.5 96.2 10
2.3 93.2 10
5.2 58 10
5.8 99.5 7
12.9 99.6 10
Table 3. Si/M Ratios of (CS + Ca2+) and (CS + Pb2+) Precipitates CS + Ca 2+ CS + Pb 2+
particle 1
particle 2
Si/Ca ) 0.05
Si/Ca ) 0.4
particle 3 Si/Pb ) 0.4
Figure 3. Liquid-state D2O at pH ) 10.
29
Si NMR spectra of CS dissolved in
Table 4. Si Chemical Shifts of the Different Species Present in the CS Solution species monomer linear trimer Q13 dimer Q12 cyclic trimer Q23
Figure 2. Scanning electron microscopy images of (CS + Ca2+) (a, top) and (CS + Pb2+) (b, bottom) precipitates.
forming a tetrahedron and the upper index the number of other Q units attached to it. Wide bands present under the narrow lines indicate fast exchange between species. Table 4 gives tabulated values of various Si species. 29Si NMR of (CS + Cations) Precipitates. MAS Technique. Figure 4 presents solid-state 29Si NMR spectra obtained by MAS. The lines are very broad because of the magnetic dipole-dipole interactions between nuclei and the low degree of crystallinity of the samples. The CS powder exhibits a spectrum similar to the one observed in solution, i.e. three distinct regions characterized by (11) Svensson, I. L.; Sjo¨berg, S.; O ¨ hman, L. O. J. Chem. Soc., Faraday Trans. 1 1982, 82, 3635. (12) Harris, R. K.; Newman, R. H. J. Chem. Soc., Faraday Trans. 2 1977, 1204.
Si (ppm) -71.1 -79.3 -79.6 -81.3
species cyclic tetramer Q24 prismatic hexamer Q36 linear Q34 cubic octamer Q38
Si (ppm) -87.4 -88.8 -95.8 -96.2
three broad lines located at -76.6, -86.1, and -98 ppm (compare with Figure 3). In the presence of Ca2+ and Al3+, the center of mass of the spectra is shifted toward high fields, while for Zn2+ and Pb2+ it is shifted toward low fields. None of the samples show a line corresponding to the Q4 species around -110ppm. CP/MAS Technique. The corresponding CP/MAS spectra are given in Figure 5. The lines, narrower than those in the case of MAS, correspond to Si nuclei bonded to OH groups. 27Al NMR of (CS + Al3+) Precipitates. The MAS and CP/MAS 27Al spectra of the samples Si/Al ) 1 (pH 7) and Si/Al ) 2 (pH 10) are shown, respectively, in parts a and b of Figure 6. The former exhibits two main distinct signals at 0 and +57.2 ppm, while for the latter only one signal at +57.2 ppm is observed. A signal around 30-40 ppm seems be present in the spectra of the Si/Al ) 1 sample. Discussion Much information concerning the aqueous species present in solution can be extracted from the liquid-state 29 Si NMR spectrum of CS (see Figure 3). This information is very important as the soluble species constitute the precursors of the species found in the precipitated solids. The interpretation of the solid-state NMR spectra is, however, more difficult. In order to obtain quantitative data these spectra have been fitted with Gaussian lines using a IGOR Pro (WaveMetrics) program. The positions of the major peaks were chosen from the spectra of the Si/Zn ) 1 and CS samples. An example of decomposition is given in Figure 7 for CS. CS. Information concerning the degree of polymerization of the SiO4 tetrahedra can be obtained from the tabulated values of 29Si chemical shifts (Table 4). It is
Silicate and Carbonate Competition for Cations
Figure 4. 29Si MAS NMR spectra of CS with Ca2+, Al3+, Pb2+, or Zn2+ at different Si/M ratios.
clear from the liquid-state 29Si NMR spectrum of CS shown in Figure 3 that the degree of polymerization of the silicate species is weak. The contribution of each particular entity is given in Table 5. Considering the value of the Q2/Q1 ratio (∼2.6), it seems that the solution consists mainly of quite long linear chains (with Qi cyclic entities included) together with other single cyclic units such as trimers, tetramers and polymers. The value of the Q2/Q3 ratio (∼0.8) indicates a certain amount of branching groups. Dent Glasser and Lackowski8 showed that systems with a low value of n attached tetrahedra are those which are the most heavily protonated. Generally, the pKa value of one SiO4 tetrahedron in a chain or a ring structure decreases when the total tetrahedra number of this structure decreases.5 In our case, given the observed Q2/Ql ratio, the basicity of the terminal oxygen atoms should be low, i.e. the conjugate acid is strong. Table 6 gives, for CS in the solid state, the percentage of the different species found by MAS. When comparing these different Qn contributions to those found in the liquid state, it is observed that the Q0 species disappear mainly to the benefit of Q2 units. The Q2/Q3 ratio in the solid is 1.2, which means that the number of branching groups has decreased. It is possible that single polymers react with each other (particularly Q0 species) to form new chains. CS + Zn2+. The presence of Zn cations in contact with CS leads (see Table 6 for MAS results) to a decrease of species having a chemical shift in the -96 to -98 ppm range in favor of entities in the range -86 to -88 ppm and around -69.2 ppm. With CP/MAS, the relative quantities of species at -85 ppm in CS and CS + Zn2+ precipitates are the same while they increase in the Q0 and Q1 regions
Langmuir, Vol. 13, No. 9, 1997 2553
Figure 5. 29Si CP/MAS NMR spectra of CS with Ca2+, Al3+, Pb2+, or Zn2+ at different Si/M ratios.
but decrease in the Q3 range (Table 7). The region -96 to -98 ppm mainly corresponds to Q3 chain branching sites and Q3 (3 Si, l OH) species. In the latter the hydroxyl group is probably (because of the high connectivity of the site) also the most acidic of the ensemble of proton-bearing sites found in the sample.13 These sites can then react with soluble Zn2+ and Zn(OH)+ ions which are those present in solution at high pH. Bonded ZnOH+ may then weaken Si-O bonds and lead to new sites and species. Because of the inherent poor resolution of these spectra, the exact identification of these new lines is difficult and somewhat speculative. The appearance of a small contribution at -69.2 ppm seems to indicate the presence of Q0 species. This chemical shift value lies in the lower midrange of what is found for monomeric SiO44- anions bound to cations of low bond strength such as Na but also in the low-field shift region of sorosilicates, i.e. dimeric silicate anions, Si2O76, bound to Na. The only reported Zn sorosilicate (hemimorphite) has a line Q12 at -78 ppm.14 The observed increase of signal in the -84 to -88 ppm range should also be considered as resulting from Zn cationic interactions. Because of the many factors which can influence the position of a 29Si line, only a tentative interpretation can be given. 29Si shifts depend on a whole host of both structural and bond energy type parameters such as bond length, bond angle, degree of polymerization, nature of second neighbors, chemical disorder, ... (for some general considerations on the problem, see e.g. Engelhardt15 and B. Sheriff16). A simplified popular approach is to use second neighbor or, more rigourosly, nearest group (13) McCormick, A. V.; Bell, A. T.; Radke, C. J. J. Phys. Chem. 1989, 93, 1741. (14) Lippmaa, E.; Ma¨gi, M.; Samoson, A.; Tarmak, M.; Engelhardt, G. J. Am. Chem. Soc. 1980, 102, 4889.
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Figure 7. 29Si MAS spectrum fitted with three Gaussian curves in the case of sample CS. Table 5. Relative Contribution of the Various Main Species Found in Liquid CS As Deduced from 29Si MAS NMR Q2
Figure 6. MAS and CP/MAS 27Al spectra for Si/Al ) 1 (pH 7) (a) and Si/Al ) 2 (pH 10) (b).
neighbor electronegativity considerations.14,17 As Zn has an electronegativity value which is smaller but close to the one of Si, its substitution (as an octahedral ion) for a proton in Q3 (3 Si, 1 OH) would shift the line toward highfield values contrary to what is observed. Because of its size, Zn could not fit into a dense rigid silica framework. It can however coordinate tetrahedrally with silicon in more open type structures such as three- or four-membered rings (as for instance in zincosilicate zeolites18), which have the effect of shifting Q3 lines downfield into the Q2 region. Zn would therefore be a strong structural reorganizer of the initial CS sample. It is important to note, however, that there is no silica particle formation. CS + Pb2+. Lead silicate and carbonate can be formed as shown very recently in polluted soils.19 X-ray diffraction patterns and SEM analysis show that 2PbCO3‚Pb(OH)2 is formed together with an amorphous phase. This last phase as shown by the 29Si NMR spectrum given on Figure 4 is some form of lead silicate. Similar to what was observed for the (CS + Zn2+) system, Tables 6 and 7 indicate that relative to the CS material the contribution of Q3 has decreased in presence of lead cations while that of Q2 has increased. The contribution at -80.5 ppm is (15) Engelhardt, G.; Michel, D. High resolution solid-state NMR of silicates and zeolites; Wiley: New York, 1987; Chapter IV. (16) Sherrif, B. L.; Grundy, H. D.; Hartman, J. S. Eur. J. Miner. 1991, 3, 751-768. (17) Janes, N.; Oldfield, E. J. Am. Chem. Soc. 1985, 107, 67-69. (18) Ro¨hrig, C.; Dierdorf, I.; Gies, H. J. Phys. Chem. Solids 1995, 56, 1369-1376. (19) Manceau, A.; Boisset, M. C.; Sarret,G.; Hazemann, J. L.; Mench, M.; Cambier, Ph.; Prost, R. Environ. Sci. Technol. 1996, 30, 15401552.
Q0 CS aqueous solution %
Q1
8
Q3
cyclic
linear
cyclic
linear
5.9
28.5
14.2
30
13.4
34.4
44.2
Table 6. Percentages of the Different Species Obtained from Solid-State 29Si MAS NMR Data except for (CS + Al3+) Samples percentages at various chemical shifts -69.2 ppm
-76.6/-80.8 ppm
-86/-88 ppm
-96/-98 ppm
CS 12.1
47.2
40.7
CS + Zn 14.3 14.5
59.9 69.5
17.9 10
Si/Pb ) 1
CS + Pb 10.7
77.6
11.7
Si/Ca ) 0.5
CS + Ca 9
53.3
37.7
Si/Zn ) 1 Si/Zn ) 2
7.9 6
unchanged. No Q0 species are formed. If Pb was confined to a Si neighboring site without structural modifications, the corresponding Si line would probably be shifted to high-field values because the electronegativity of Pb is higher than that of Si. The opposite trend is observed. Recently Bessada et al.20 studied some high-temperature lead silicates formed of chains, rings, and cyclic units of silica tetramers. In the classical notation, these silica tetrahedra are then Q2 units with the nonbridging oxygens bonded to variable amounts of tetrahedral Pb atoms. When the 29Si chemical shifts of these compounds are plotted (20) Bessadat, C.; Massiot, D.; Coutures, J.; Douy, A.; Coutures, J. P.; Taulelle, F. J. Non-Cryst. Solids 1994, 168, 76.
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Langmuir, Vol. 13, No. 9, 1997 2555
Table 7. Percentage of the Different Species Obtained from Solid-State 29Si CP/MAS NMR Data Except for (CS + Al3+) Samples
Table 9. Percentages of the Different Species Obtained from Solid-State 29Si MAS NMR Data for (CS + Al3+) Samples
percentages at various chemical shifts -69.2 ppm
-76.6/-80.8 ppm
-86/-88 ppm
CS 12.5
55.4
32
CS + Zn 21.4 26.9
52.8 52.3
12.1 9.4
Si/Pb ) 1
CS + Pb 7.3
81.8
10.9
Si/Ca ) 0.5
CS + Ca 8.4
72.2
19.4
Si/Zn ) 1 Si/Zn ) 2
13.7 11.4
-96/-98 ppm
Table 8. Relative Amount of the Three Aluminum Species in CP/MAS and MAS 27Al Spectra for Sample Si/Al ) 1 CP/MAS MAS
+57.2 ppm
+42.6 ppm
+0 ppm
31.2 46.6
5.5a
63.3 38.3
15.1
a The simulation gives a peak centered on 31 ppm in the CP/ MAS spectra.
versus the number of Pb atoms located in the second coordination sphere, a decrease is observed (around 10 ppm toward low-field values). In our samples, the important decrease of the Q3 sites confirms their high reactivity toward metal cations, leading to more open Q2 configurations. CS + Ca2+. SEM analysis and the X-ray diagram show for this sample two phases: a CaCO3 precipitate and an amorphous calcium silicate. As shown in Table 6, the latter is characterized by three species. Following many authors21-25 these contributions can indeed be attributed to Ql (-76.6 ppm), Q2 (-86 ppm), and Q3 (-96 ppm) species. Apparently no new contribution is found in the final product. Relative to CS, the total distribution of species in the MAS spectra does not change drastically. This agrees with results obtained by Fernandez.22 Calcium probably reacts with the OH groups of the Q3 (3 Si, 1 OH) species which leaves the chemical shift of the corresponding entity in approximately the same Q3 frequency region. Grutzeck23 or Brough24 found a weak contribution for Q3 when the Ca/Si ratio was large. In our case the Ca/Si ratio is about 2.5 (Table 3) with a Q3 contribution of about 40%. When calcium is added, the Q2/Q1 ratio increases from 3.9 to 5.9, indicating an increase of chain lengths. CS + Al3+. 27Al NMR. As seen in Figure 6, the 27Al spectra of the two examined samples are quite different. For Si/Al ) 1 (pH ) 7), the spectrum consists of three different lines at 0, +42.6, and +57.2 ppm. The relative amounts of these three different species are given in Table 8. The large contribution in the CP/MAS spectrum for the peak located around 0 ppm indicates that these octahedral aluminum are bonded to OH groups. This signal is probably due to the presence of a residual gibbsitic phase Al(OH)3.26 (21) Rassem, R.; Zanni-Theveneau, H.; Shneid, I.; Regourd, M. J. Phys. Chim. 1989, 86, 1253-1264. (22) Fernandez, L.; Zanni, H.; Couty, R.; Barret, P.; Bertrandie, D. J. Chem. Phys., 1989, 89, 453. (23) Grutzeck, M.; Benesi, A.; Fanning, B. J. Am. Ceram. Soc. 1989, 72 (4), 665. (24) Brough, A. R.; Bobson, C. M. J. Am. Ceram. Soc. 1994, 77 (2), 593-596. (25) Cong, X.; Kirkpatrick, R. J. Cem. Concr. Res. 1993, 23, 1065. (26) De Jong, B. H. W. S.; Schramm, C. M.; Parziale, V. E. Geochim. Cosmochim. Acta 1983, 47, 1223.
Si/Al ) 1, pH 7 Si/Al ) 2, pH 10
-86 ppm
-93 ppm
-98 ppm
45.1 51.4
14.8 20.8
40.1 27.8
According to many authors,26-31 the line at +57 ppm can be attributed to tetrahedral aluminum bonded to 3 or 4 Si nuclei. If all of this Al was bonded to four Si well within a dense framework, the corresponding CP/MAS 27Al NMR would be close to zero which is not the case here (Figure 6a). This is probably due to the open structure of these compounds wherein part of the Al is surrounded by three or less Si and in close contact with OH groups. For this material there also seems to be a line at +42.6 ppm. This contribution has been attributed for model compounds such as andalousite to pentacoordinated Al.32 It has also been suggested to be present in pyrophyllite after thermal transformations33 or kaolinite after dehydroxylation,34,35 but the question is still debatable. For the sample Si/Al ) 2 (pH ) 10), only one tetrahedral line is observed at +57 ppm, which is quite different from the value of +80 ppm for Al(OH)4- found in solution at pH 10 (Al is then located in an isolated nonstrained tetrahedron). Again CP/MAS tells us that these Al are located quite close to OH groups. 29 Si NMR. The presence of aluminum leads to the formation of a new species located at -93 ppm and to the disappearance of the line at -76.6 ppm corresponding to Q1 species (Table 9). For the Si/Al ) 1 sample, the contribution at -86 and -98 ppm remain constant relative to the CS product. The 27Al NMR data showed that at pH 7 aluminum is tetrahedrically bonded to two or three silicon atoms. This means therefore that in presence of Al, a partial condensation of the initial Q1 takes place together with the formation of Si-O-Al bonds. Both these effects involve a shift toward high-field values, from -76.6 to -93 ppm.36-38 For the Si/Al ) 2 sample and relative to the CS product, the contribution at -98 ppm decreases while that at -86 ppm increases. At pH 10, all of the Al present is four-coordinated. Here again, the disappearance of the Q1 species will lead toward a high-field shift, while aluminum substitution for Si in the initial Q3 species would lead to a shift toward low-field values (∼7 ppm) and a decrease of the -93 ppm line. Conclusion This work clearly demonstrates that even in the presence of carbonates at normal pressure and temper(27) Mu¨ller, D.; Gessner, M.; Berhens, H. J.; Scheler, G. Chem. Phys. Lett. 1981, 79, 59. (28) Thomas, J. M.; Klinowski, J. Adv. Catal. 1985, 33, 199. (29) Mueller, D.; Hoebbel, D.; Gessner, W. Chem. Phys. Lett. 1981, 84, 25. (30) Fitzgerald, J. J.; Murali, C.; Nebo, C. O.; Fuerstenau, M. C. J. Colloid Interface Sci. 1992, 151, 299. (31) Stone, W. E. E.; El Shafei, G. M. S.; Sanz, J.; Selim, S. A. J. Phys. Chem. 1993, 97, 10127-10132. (32) Lippmaa, E.; Ma¨gi, M.; Samoson, A.; Engelhardt, G.; Grummer, A. R J. Am. Chem. Soc. 1986, 108, 1730-1735. (33) Sanchez-Soto, P.; Sobrados, I.; Sanz, J.; Perez Rodriguez, J. I. J. Am. Ceram. Soc. 1993, 76 (12), 3024-3028. (34) Watanabe, T.; Shimizu, H.; Nagasawai, K.; Masuda, A.; Saito, H. Clay Miner. 1987, 22, 37-48. (35) Lambert, J. F.; Millman, W. S.; Fripiat, J. J. J. Am. Chem. Soc. 1989, 111, 3517. (36) Lippmaa, E.; Samoson, A.; Ma¨gi, M. J. Am. Chem. Soc. 1981, 103, 4992. (37) Kirkpatrick, R. J.; Kinsey, R. J.; Smith, K. A.; Hendersen, D. M.; Oldfield, E. Am. Mineral. 1985, 70, 106. (38) Kinsey, R. A.; Kirkpatrick, R. J.; Hower, J.; Smith, K. A.; Oldfield, E. Am. Mineral. 1985, 70, 537.
2556 Langmuir, Vol. 13, No. 9, 1997
ature conditions, interactions occur between soluble silicates and metal cations. The use of 29Si and 27Al NMR helps in extracting information about the speciation of the different species present in the solids. Comparison between the species present in the initial material, CS, and the newly formed solids highlights the different interactions which take place according to the type of cation present in solution. Zinc cations react with Q3 species to form entities with signals around -86 ppm. This reaction leads to the formation of a monomer or dimer bond to Na. Lead cations react also with Q3 units, involving structural modification of the initial structure. These new entities are in the Q2 range. Simultaneously a basic lead carbonate (2PbCO3‚Pb(OH)2) is formed. The SEM images and the corresponding analyses show that both phases precipitate at the same time. Calcium cations react first with carbonate anions to form a CaCO3 precipitate. The presence of calcium leads to an increase in the size of the chain lengths within the formed calcium silicate. Three kinds of coordination have been found for aluminum: (i) a tetrahedral aluminum surrounded by two, three, or four Si nuclei and OH goups; (ii) a signal located at +40 ppm that perhaps originates from pentacoordinated aluminum, and (iii) an octahedral aluminum located around 0 ppm and resulting from residual Al(OH)3. The final product obtained after the reaction of aluminum on the CS material depends on the pH and the
Fraval et al.
aluminum coordination number. At pH 7 and Si/Al ) 1, condensation and Si-O-Al bond creation are the dominant reactions. In the second case, at pH ) 10 and Si/Al ) 2, there is a Si-O-Al bond formation and substitution of Si by Al(4) in the Q3 species. The nucleophile oxygen atom in the Q3 species present in the initial CS material is much more reactive than the other sites. The presence of this reactive site in addition to the variability in electronegativity of the cation lead to a reorganization of the initial structure and formation of preferential linkages in the solid. Finally, it is very important to note that in the present case no silica colloids were formed. Acknowledgment. This study was supported by SITA and the CIRSEE (Lyonnaise des Eaux). Within the frameworkof the EUREKA-INCIPRO program contract Rhone-Poulenc is also acknowledged for providing samples of silicate-carbonates and for helpful discussions. This research was also supported by the FRSM (Fonds de la Recherche Scientifique Me´dicale-Loterie Nationale) cre´dit no. 9.5478.90 and by the Communaute´ Franc¸ aise de Belgique cre´dit A. R. C. convention no. 91/96-149. This work forms part of a joint exchange research program within the COST action D5/0003/94. LA960743R
