Flocculation of Colloidal Silica with Hydrolyzed Aluminum: An 27Al

Jan 22, 1997 - Computer simulation of the polydispersive sol coagulation process ... Bogulsaw Pierozynski. The Canadian Journal of Chemical Engineerin...
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Langmuir 1997, 13, 147-152

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Flocculation of Colloidal Silica with Hydrolyzed Aluminum: An 27Al Solid State NMR Investigation B. S. Lartiges,*,† J. Y. Bottero,‡ L. S. Derrendinger,† B. Humbert,§ P. Tekely,| and H. Suty⊥ LEM (INPL-CNRS), Rue du Doyen Roubault, BP 40, 54501 Vandoeuvre, France, LGE/CEREGE (CNRS), BP 80, 13628 Aix-en-Provence, France, LCPE (CNRS), 405, Rue de Vandoeuvre, 54600 Villers-les-Nancy, France, Lab. de Me´ thodologie RMN, Universite´ Nancy I, BP 239, 54506 Vandoeuvre Cedex, France, and Elf-Atochem, 95 Rue Danton, BP 108, 92300 Levallois-Perret, France Received November 13, 1995. In Final Form: October 11, 1996X Flocculation of colloidal silica with aluminum fractal polymers was investigated by 27Al magic angle spinning and 29Si cross polarization magic angle spinning NMR as a function of aluminum concentration and pH. Aluminum flocculant species were prepared by dilution of a commercially available flocculant, WAC HB, the hydrolysis of which yields Al13 polymers. The results showed that destabilization with hydrolyzed aluminum has many features in common with flocculation by addition of conventional organic polymers. Interaction of aluminum polymers with silica leads to the formation of four coordinated aluminum retained at the silica surface as negatively charged aluminosilicate sites. These sites, similar to those found in clay minerals and zeolitic materials, represent potential anchors to aluminum polycations. Hence, aggregation of silica particles proceeds with either charge neutralization or bridging. Tetrahedral aluminum in contact with silica may be assimilated to polymer segments bound to the surface. Study of the effect of aluminum concentration and pH suggests that aluminum partition within silica flocs may be ascribed to a competition between structural rearrangement of individual aluminum polymers, which tend to adopt a flat conformation on the silica surface, and excluded area effects originating from neighboring flocculant species.

Introduction More than half of the drinking water used in the world comes from the treatment of a surface water.1 In conventional practice, the treatment process begins with the addition of hydrolyzing metal salts such as aluminum or iron coagulants to the raw water in order to promote the aggregation of the finely suspended matter. The aggregates formed are then usually removed by sedimentation followed by filtration through granular media, and disinfection of the clarified water finally yields the drinking water. The aggregation stage is essential to the treatment process: first it eliminates the colloidal particles which would not settle otherwise, and then it removes part of the dissolved organic compounds which are suspected to form carcinogenic by products during disinfection.2 Over one million tons of aluminum- or iron-based coagulants are used in the world every year to treat surface waters;3 however, despite this wide use, there is still disagreement over how aggregation is carried out with these chemicals, and mechanisms as different as double layer compression,4,5 neutralization of surface charge,6,7 bridging,8,9 and * Author to whom correspondence should be addressed. † LEM (INPL-CNRS). ‡ LGE/CEREGE (CNRS). § LCPE (CNRS). | Universite ´ Nancy I. ⊥ Elf-Atochem. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Degremont Memento technique de l’eau, 9th ed.; Paris, 1989. (2) Qasim, S. R.; Hasham, S. A.; Ansari, N. I. J. Environ. Eng. 1992, 118, 432. (3) Peaff, G. Chem. Eng. News 1994, November 14, 15. (4) Packham, R. F. Water Res. Assoc. 1960, 12, 3. (5) Black, A. P.; Chen, C. J. Am. Water Works Assoc. 1965, 57, 354. (6) Stumm, W.; O’Melia, C. R. J. Am. Water Works Assoc. 1968, 60, 514. (7) Dentel, S. K.; Gosset, J. M. J. Am. Water Works Assoc. 1988, 80, 187. (8) Ham, R. K.; Christman, F. R. J. Sanit. Eng. Div. 1969, 95, 6605.

enmeshment in a metal-hydroxide precipitate,10 have been proposed to explain the destabilization of colloidal particles with inorganic metal coagulants. Such a variety of destabilization models partly originates from a limited understanding of the aqueous chemistry of aluminum and iron; although Mattson11 demonstrated as early as 1928 that the metal hydrolysis products are the effective flocculant species, the problem of their chemical and structural nature has been a matter of controversy for years.12 Recent nuclear magnetic resonance (NMR) and small angle X-ray scattering (SAXS) investigations12-14 revealed that when added to water, an aluminum salt such as aluminum chloride dissociates, hydrolyzes, and forms dissolved polynuclear species such as Al12VI(OH)24AlIVO4(H2O)127+, known as Al13 polycations, and inorganic metal aggregates which correspond to clusters of Al13 units. Likewise, edge X-ray adsorption fine structure (EXAFS) and SAXS studies15,16 showed that the hydrolysis of iron salts follows a similar pattern: Fepolycations are produced first, and upon further neutralization, these polycations aggregate yielding fractal structures that are commonly referred to as fractal polymers.17 The identification of polymer-like flocculant species of metal salts suggests that destabilization mechanisms such (9) Bottero, J. Y.; Tchoubar, D.; Axelos, M. A. V.; Quienne, P.; Fiessinger, F. Langmuir 1990, 6, 596. (10) Packham, R. F. J. Colloid Sci. 1965, 20, 81. (11) Mattson, S. J. Phys. Chem. 1928, 32, 1532. (12) Bottero, J. Y.; Tchoubar, D.; Cases, J. M.; Fiessinger F. J. Phys. Chem. 1982, 86, 3667. (13) Bottero, J. Y.; Cases, J. M.; Fiessinger, F.; Poirier, J. E. J. Phys. Chem. 1980, 84, 2933. (14) Akitt, J. W.; Lester, L.; Khandelwal, F. H. J. Chem. Soc. 1972, 26, 609. (15) Combes, J. M.; Manceau, A.; Calas, G.; Bottero, J. Y. Geochim. Cosmochim. Acta 1989, 53, 583. (16) Tchoubar, D.; Bottero, J. Y.; Quienne, P.; Arnaud, M. Langmuir 1991, 7, 398. (17) Gray, K. A.; Yao, C.; O’Melia, C. R. J. Am. Water Works Assoc. 1995, 87, 136.

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as charge neutralization and particle bridging, which are traditionally associated with organic polymers, could be valid in the case of inorganic flocculants9. However, despite this improved understanding of hydrolysis, the exact role of those inorganic polymers in the aggregation process remains unclear; in particular, the type of association occurring between metal polymers and the surface of suspended particles requires further studies. The aim of this work is therefore to describe the nature of interaction between the colloid particles and the flocculant species. NMR spectroscopy appears well suited to investigate the particle/flocculant relationship as it provides insights into the local structural organization of a given element. As typical surface waters exhibit high silica contents (5-75 ppm)18 synthetic colloidal silica suspensions were used. They were flocculated with a commercial aluminum coagulant, and the resulting aggregates were characterized by magic angle spinning (MAS)27Al and cross polarization magic angle spinning (CP/MAS) 29Si NMR for various aluminum concentrations and pH values. EXPERIMENTAL SECTION Materials and Sample Preparation. Silica suspensions were prepared from a concentrated silica sol, Ludox HS 40, which was kindly supplied by SPCI (France). Ludox HS has already been extensively studied19-21 and consists of discrete, nonporous, almost spherical particles whose mean diameter is about 14 nm in size. Ludox HS was used as received; the stock silica suspension was diluted to 500 ppm with deionized water (MilliQ) and concentrations of 70 ppm NaCl and 336 ppm NaHCO3 were included to provide pH and ionic strength buffers similar to that of typical river water. A commercial aluminum coagulant, WAC HB (Elf-Atochem, France), was used in this study to aggregate the silica particles. WAC HB is a partially neutralized aluminum salt solution of pH 3.1 and contains about 10.4 wt % of aluminum as Al2O3, 1.8 wt % of SO42-, 8 wt % of Cl-, and 1.1 wt % of Ca2+. The principal aluminum species as detected by 27Al liquid state NMR spectroscopy in the pure coagulant solution are monomers and dimers.22 Upon dilution, Al13 polymers become the predominant species,22,23 which suggests that a small number of Al13 complexes might be initially present in the coagulant solution as small clusters.23 The flocculation experiments were conducted at room temperature (∼22 °C) in a standard 1 L glass beaker of diameter 9 cm, fitted with four Plexiglas baffles (1.2 × 15 cm). Stirring was carried out with a rectangular paddle (1.5 × 5.5 cm) located at one-third of the beaker height from the bottom. The pH of the suspension was first adjusted to 5.5 or 8.0 with diluted HCl or NaOH, and the required amount of coagulant was then added as pure solution with a microsyringe at a point just below the free surface of the suspension; the time of injection was about 3 s. Hydrolysis of the aluminum coagulant involves a slight decrease of the suspension pH, and aliquots of NaOH 0.1 M were manually added to readjust pH to its initial value within the first minute of agitation. Mixing of the coagulant solution and silica suspension follows a conventional jar test procedure: it comprises a rapid mixing period (250 rpm for 3 min, G ) 450 s-1) and a slow mixing period (60 rpm for 30 min, G ) 110 s-1), with the time of mixing measured from the time of coagulant addition. Rapid mixing provides a good dispersal of the coagulant and is commonly associated with particulate destabilization, whereas slow mixing is believed to (18) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979; p 13. (19) Ramsay, J. D. F.; Booth, B. O. J. Chem. Soc., Faraday Trans. 1 1983, 79, 173. (20) Ramsay, J. D. F.; Avery, R. G.; Benest, L. Faraday Discuss. Chem. Soc. 1983, 76, 53. (21) Axelos, M. A. V.; Tchoubar, D.; Bottero, J. Y. Langmuir 1989, 5, 1186. (22) Crozet, M. D.E.A., 1991, E.N.S.P.C.I. (France). (23) Wang, W. Z.; Hsu, P. H. Clays Clay Miner. 1994, 42, 356.

Lartiges et al.

Figure 1. 27Al MAS NMR spectra of WAC HB hydrolyzed at pH 8 with and without silica particles: [Al] ) 3.0 × 10-4 mol/L. optimize floc growth by limiting the breakdown of aggregates.24 The mean velocity gradient G is defined as G ) (P/Vµ)1/2, where P, V, and µ represent, the power dissipated in stirring the flocculating suspension, the volume of suspension, and its dynamic viscosity.25 G was calculated by measuring the torque exerted on the stirrer drive shaft.26 At the end of slow mixing, the suspension was allowed to settle for 24 h; the sediment was then freeze-dried and used for NMR experiments. A 24 h period was required to obtain a sufficient amount of sediment at low coagulant dosages. As flocculant species are metastable and have been reported to evolve toward aluminum hydroxide upon aging,27,28 a sample of freshly flocculated silica was quickly frozen in liquid nitrogen and then freeze-dried. The 27Al MAS NMR spectrum of this sample was virtually identical to the one obtained after 24 h of settling, indicating that this delay does not alter the nature of the species present. NMR Spectroscopy. 27Al MAS and 29Si CP/MAS NMR spectra were acquired on a Bruker MSL 300 spectrometer operating at 27Al and 29Si frequencies of 78.2 and 59.6 MHz, respectively. A zirconia rotor was used with rotation speed of 3 kHz. For 27Al, 1 µs excitation pulses were used in conjunction with recycle delays of 1 s to enable quantitative estimates of aluminum species.29 For cross-polarization experiments, the contact time for the transfer of magnetization between protons and 29Si was 2 ms. Chemical shifts of the 27Al and 29Si resonances were reported with respect to 1 M Al(H2O)63+ solution and tetramethylsilane (TMS) signals, respectively, both of which taken as 0.0 ppm.

Results Figure 1 shows the Al MAS NMR spectra of a 3.0 × 10-4 mol/L aluminum concentration of WAC HB hydrolyzed at pH 8 with and without silica particles. In the absence of silica, the 27Al MAS NMR spectrum of hydrolyzed WAC HB exhibits two resonances: a strong 6 ppm signal that falls in the six coordinate chemical shift region of aluminum and has been assigned to the octahedral aluminum of Al13 polycations in fresh amorphous Al13 gels,28,30 and a 63.5 ppm signal which corresponds to the central tetrahedral aluminum of the Keggin Al13 structure.13 This suggests that at pH 8.0, hydrolysis of the coagulant solution primarily yields Al13 polymers. Previous characterization23 using kinetics of Al-ferron color development has shown that a freshly diluted WAC HB 27

(24) Montgomery, J. M. Water Treatment, Principles and Design; John Wiley & Sons: New York, 1985. (25) Camp, T. R.; Stein, P. C. J. Boston Soc. Civil Eng. 1943, 30, 219. (26) Laine, J. M. D.E.A., 1987, Universite Paris XII. (27) Bottero, J. Y.; Axelos, M. A. V.; Tchoubar, D.; Cases, J. M.; Fripiat, J. J.; Fiessinger, F. J. Colloid Interface Sci. 1987, 117, 47. (28) Bradley, S. M.; Kydd, R. A.; Howe, R. F. J. Colloid Interface Sci. 1993, 159, 405. (29) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites, John Wiley & Sons: New York 1987. (30) Stone, W. E. E.; El Shafei, G., Sanz, J.; Selim, S. A. J. Phys. Chem. 1993, 97, 10127.

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Figure 2. 29Si CP/MAS NMR spectra of freeze-dried Ludox HS and flocculated silica ([Al] ) 3.0 × 10-4 mol/L, pH 8).

Figure 3. 27Al MAS NMR spectra of colloidal silica flocculated at pH 8 and pH 5.5 with [Al] ) 3.0 × 10-4 mol/L.

solution of pH 4.4 already contains about 75% of slowreacting OH complexes, which can be assumed to be Al13 polymers on the basis of work of Bertsch31 and Bottero13 on aluminum chloride hydrolysis. It can therefore be hypothesized that Al13 polymers are the main flocculant species in solution over the pH range investigated in this study. The 27Al MAS NMR spectrum of the silica aggregates also shows two resonances located at 6 and 55 ppm downfield from Al(H2O)63+. The 6 ppm signal is similar to the octahedral line observed above; it is therefore attributed to the six-coordinate aluminums in Al13 polymers although the 63.5 ppm resonance which would confirm such an assignment cannot be resolved with the MAS rate used here. The 55 ppm signal occurs in the tetrahedral region of the 27Al NMR chemical shift scale; it corresponds to a AlO4 unit with silica as nearest neighbors and is typically observed in amorphous aluminosilicates32,33 and aluminasilica gels.34 X-ray fluorescence analysis of the original Ludox HS sample reveals the presence of a small amount of Al as impurity (0.03 g/100 g of SiO2) which also yields a 55 ppm resonance when examined by 27Al MAS NMR spectroscopy.30,35 It could then be assumed that the 27Al MAS NMR spectrum of the silica aggregates is a simple combination of the signals obtained for Ludox HS and WAC HB hydrolyzed in the absence of silica. However, the quantity of Al added as coagulant is approximately 50 times more than the amount of aluminum initially present in the silica particles of the suspension, while the relative intensities of the 6 and 55 ppm signals indicate a smaller quantity of octahedrally coordinated aluminum and hence fewer flocculant species. This implies that most of the tetrahedral aluminums detected have been formed during the destabilization of colloidal silica with the aluminum polymers; the 55 ppm resonance can then be assigned to aluminum originating from the flocculant species in contact with the surface of silica, whereas the 6 ppm signal is representative of unaffected Al13 polymers. The CP/MAS 29Si NMR results obtained for the same silica aggregates (pH 8.0, [Al] ) 3.0 × 10-4 mol/L) and for a sample of freeze-dried Ludox HS are shown in Figure 2. Both spectra exhibit two broad signals with peak chemical shifts of -112 and -101 ppm relative to TMS, which are normally attributed to Si in Q4 (Si(OSi)4) framework silica and Q3 (Si(OSi)3OH) single silanol

groups.36 In addition, the CP/MAS 29Si spectrum of the parent silica contains a minor resonance located around -90 ppm and assigned to Q2 (Si(OSi)2(OH)2) geminate silanol groups,36 while that of silica aggregates reveals what appears to be a noisy low-intensity resonance with a peak chemical shift of -86 ppm and a line width of 10 ppm. A -86 ppm signal would be consistent with chemical shifts observed for both framework and amorphous aluminosilicates.33,37 In fact, the assignment of the latter resonance to a specific structural environment is not straightforward: 29Si NMR studies of solid aluminosilicates have shown that the replacement of a silicate unit attached to a resonating 29Si by an aluminate tetrahedron increases the 29Si frequency by about 5 ppm.38,39 As a 29Si chemical shift also strongly depends on the degree of silicon polymerization, different structural environments, e.g. Q4(4Al) (Si(OAl)4) and Q2(1Al) (Si(OSi)2(OAl)(OH)), may yield resonances of similar chemical shifts.40 On the basis of results reported in the literature,40 the -86 ppm signal could be assigned to Q2(1Al) sites. Therefore, CP/MAS 29Si NMR results also suggest that aluminum atoms of the flocculant species have been incorporated as AlIV-O-Si units in the silica lattice during aggregation. Comparison of the 27Al NMR spectra of silica aggregates obtained at pH 5.5 and 8.0 with the same 3.0 × 10-4 mol/L aluminum concentration (Figure 3) reveals that the results obtained at pH 5.5 do not differ substantially from what was observed at pH 8.0: both four and six coordinate signals are again present, although the peak chemical shifts of the octahedral and tetrahedral lines have been slightly shifted to 4 and 57 ppm, respectively. It can be inferred that a similar “alumination” of the silica surface occurs at pH 5.5. However, the intensity of the 4 ppm resonance is more intense at pH 5.5, indicating that the extent of aluminum incorporation is less important at acidic pH. Figure 4 presents some of the 27Al MAS NMR spectra obtained for colloidal silica suspensions flocculated at pH 5.5 with different aluminum concentrations. All spectra were normalized to give equal height of the predominant signal. As coagulant dosage is increased, the relative intensity of the 4 ppm signal increases; octahedral and tetrahedral resonances have then almost the same importance at high aluminum concentration. Increasing

(31) Bertstch, P. M.; Layton, W. J.; Barnhisel, R. I. Soil Sci. Soc. Am. J. 1986, 50, 1449. (32) Ildefonse, Ph.; Kirkpatrick, R. J.; Montez, B.; Calas, G.; Flanck, A. M.; Lagarde, P. Clays Clay Miner. 1994, 42, 276. (33) MacKenzie, K. J. D.; Bowden, M. E.; Meinhold, R. H. Clays Clay Miner. 1991, 39, 337. (34) Couty, R.; Taulelle, F.; Zanni-Theveneau, H. C. R. Acad. Sci. Paris 1987, serie II, 165. (35) Lartiges, B. S. PhD, 1994, I.N.P.L.

(36) Ligner, et al. Adv. Colloid Interface Sci. 1990, 33, 91. (37) Smith, K. A.; Kirkpatrick, R. J.; Oldfield, E.; Henderson, D. M. Am. Mineral. 1983, 68, 1206. (38) Lipmaa, E.; Ma¨gi, M.; Samoson, A.; Tarmak, M.; Engelhardt, G. J. Am. Chem. Soc. 1981, 103, 4992. (39) Lipmaa, E.; Samoson, A.; Ma¨gi, M. J. Am. Chem. Soc. 1986, 108, 1730. (40) Fyfe, C. A. Solid State NMR for Chemists; C. F. C. Press, 1983.

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Figure 6. Change in AlIV/AlVI ratio with aluminum concentration at pH 5.5 and pH 8.

Figure 4. Typical 27Al MAS NMR spectra of silica flocculated at pH 5.5: 1, [Al] ) 1.2 × 10-4 mol/L; 2, [Al] ) 1.9 × 10-4 mol/L, 3, [Al] ) 3.0 × 10-4 mol/lL, 4, [Al] ) 4.3 × 10-4 mol/L.

aluminum incorporated in the silica framework changes with aluminum concentration. To assess the fraction of polymeric aluminum unaffected by the presence of silica, the height ratio of the 55-58 to 4-6 ppm signals (noted AlIV/AlVI hereafter) was reported as a function of coagulant dosage for the two pH values studied (Figure 6). The influence of the 63.5 ppm resonance on this ratio can be neglected since it is present in a 1:12 ratio with the 4-6 ppm resonance and AlIV/AlVI estimates were all greater than 1 in the range of aluminum dosage investigated. Figure 6 shows an exponential decay of AlIV/AlVI with aluminum concentration. This result is confirmed by the log-log diagram shown in the inset which gives a straight line of slope equal to -0.96 at both pH 5.5 and 8.0. Therefore, it can be concluded that during silica aggregation with WAC HB, aluminum partition between aluminosilicate sites and Al13 polymers follows the relation AlIV/ AlVI ∝ 1/(Total Al). Discussion

Figure 5. Peak chemical shift of 27Al tetrahedral and octahedral resonances as a function of aluminum concentration.

the coagulant dosage also gradually shifts the signal due to four-coordinate aluminum toward higher resonance frequencies: a peak chemical shift of 58 ppm is reached at an aluminum concentration of 4.3 × 10-4 mol/L while the octahedral line remains centered around 4 ppm (Figure 5). The observed shift can be due to the contribution of the 63.5 ppm signal overwhelmed by the 55 ppm line: as the importance of the 63.5 ppm resonance is directly proportional to the size of the 4 ppm signal, it should also increase with aluminum concentration, thus displacing the peak chemical shift of the 55 ppm resonance. In consequence, the shift of the 55 ppm line from 55 to 58 ppm is consistent with the presence of increasing amounts of Al13 polymers within the silica aggregates. The modification of the relative intensities of tetrahedral and octahedral resonances means that the fraction of

Nature and Charge of Aluminosilicate Sites. Interaction of aluminum with silica has already received a lot of attention in the literature. Typically, the presence of a small amount of aluminum on the surface of silica particles affects properties such as sol stability toward gelation,41 electrophoretic mobility,42 solubility in water,43,44 catalytic activity,45 or citotoxic and fibrosing action.46 In most cases, the mechanism underlying the above observations has been suggested to be the formation of aluminosilicate anions on the surface of silica particles, aluminum changing its coordination from 6-fold to 4-fold upon contact with the silica tetrahedral structure and giving then rise to a negative charge.47 Direct evidence of the formation of such tetrahedrally coordinated aluminum has been provided recently by NMR examination of aluminated silica gels30,48 and Si/Al hydrosols.49 In these studies, 27Al MAS NMR spectra of aluminum incorporated in silica compounds exhibit a 53(41) Iler, R. K. J. Colloid Interface Sci. 1976, 55, 25. (42) Allen, L. H.; Matijevic, E. J. Colloid Interface Sci. 1969, 31, 287. (43) Iler, R. K. J. Colloid Interface Sci. 1973, 43, 399. (44) Browne, B. A.; Driscoll, C. T. Science 1992, 256, 1667. (45) Milliken, T. H., Jr.; Mills, G. A.; Oblad, A. G. Discuss. Faraday. Soc. 1950, 8, 279. (46) Quinot, E.; Cavelier, C.; Merceron, M. O. Biomedecine 1979, 30, 155. (47) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: New York, 1980. (48) Yokoyama, T.; Takahashi, Y.; Tarutani, T. J. Colloid Interface Sci. 1991, 141, 559. (49) Fitzgerald, J. J.; Murali, C.; Nebo, C. O.; Fuerstenau, M. C. J. Colloid Interface Sci. 1992, 151, 299.

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Figure 7. Identification of potential Si/Al reaction sites on silica surface modeled with polyhedra containing 14 Si atoms.

56.5 ppm resonance located in the chemical shift region of tetrahedral aluminum. Stone et al.30 further indicate that cross-polarization between protons and aluminum decreases the intensity of the AlIV signal relative to the MAS spectrum; this suggests that four-coordinated aluminums are not appreciably coupled to neighboring protons and therefore that AlIV-O-H linkages are not present. Several authors45,50,51 have also noted that at low alumina contents, washed and dried aluminated silica gels retain 1 equivalent sodium per incorporated Al; this sodium ion can be readily exchanged for other cations which means that aluminum and sodium are on the available surface of the solid. It follows that the 55 ppm resonance observed in this work is representative of negatively charged aluminosilicate sites similar to those found in clay minerals and zeolitic materials.52,53 Mechanism of Formation of Aluminosilicate Sites. It is generally agreed that the aluminosilicate sites described above are formed by condensation of the silanol groups present on the silica surface with the hydroxyl groups of the hydrolyzed aluminum ions.45,50,53 Investigation of the hydrolysis behavior of aluminum in the presence of an excess silica gel,50 and examination of the reaction products by infrared spectroscopy54 and 29Si CP/MAS NMR30 have provided evidence to support this mechanism without indicating the stoichiometry of the reaction. Study of the reverse system, i.e., reaction of silicic acid with the surface of aluminum hydroxide, reveals that at pH 9.2, 1 mol of H3O+ is released for every 3 mol of Si(OH)4 reacted.52 However, other data suggest that at lower pH values, reaction of silicic acid with hydroxy-Al ions is not accompanied by a decrease in pH.54 Considering Si/Al hydrosols synthesis, Fitzgerald et al.49 propose that the reaction between silicate species and Al(H2O)63+ proceeds as a two-step reaction involving the formation of an aluminate ion prior to its incorporation in silicate complexes. Models of silica growth reveal however that the presence of aluminate ions is not required to account for the change in coordination number of aluminum. According to Humbert et al.,55 the condensation of polyhedra containing 14 silica atoms results in the creation of tetrahedral vacancies limited by 4 silanol groups. From Figure 7, it is clear that these sites are (50) Tamele, M. W. Discuss Faraday Soc. 1950, 8, 270. (51) De Kimpe, C.; Gastuche, M. C.; Brindley, G. W. Am. Mineral. 1961, 46, 1370. (52) Hingston, F. J.; Raupach, M. Aust. J. Soil. Res. 1967, 5, 295. (53) Cloos, P.; Leonard, A. J.; Moreau, J. P.; Herbillon, A.; Fripiat, J. J. Clays Clay Miner. 1969, 17, 279. (54) Luciuk, G. M.; Huang, P. M. Soil Sci. Soc. Am. Proc. 1974, 38, 235. (55) Humbert, B.; Burneau, A.; Gallas, J. P.; Lavalley, J. C. J. nonCryst. Solids 1992, 143, 75.

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Figure 8. Schematic illustration of the destabilization mechanism of colloidal silica with aluminum polymers: 1, formation of aluminosilicate sites; 2, charge neutralization; 3, bridging of silica particles. Triangles and filled circles represent aluminosilicate sites and flocculant species, respectively.

structurally suited to receive aluminum and may be identified as potential reaction sites. The work reported herein is not directed toward a better understanding of the Si/Al interaction mechanism; as hydrolysis of aluminum polycations continues during the course of aggregation, the pH adjustments carried out cannot even be used to infer the stoichiometry of the reaction. It is nethertheless reasonable to assume that the aluminosilicate sites originate from the depolymerization of some Al13 polycations contacted with silanol groups of specific surface sites of silica. Destabilization of Silica by Polymerized Aluminum. Flocculation of colloidal silica with polymeric aluminum should begin with the formation of negatively charged aluminosilicate sites. Kinetics of the associated reaction is not known but is likely to be the ratedetermining step of aggregation as light scattering study of flocs structure reveals an adhesion-limited clustercluster aggregation regime.35,56 The second stage of destabilization involves the charge compensation of the newly created aluminosilicate sites. Al13 polymers are expected to remain in the vicinity of the silica surface after reaction and represent therefore the principal cations available for charge balance. Aggregation of silica particles with Al13 polymers can then proceed with either charge neutralization or bridging. Preservation of stoichiometric charge balance implies that Al polycations participate to the charge neutralization of several aluminosilicate sites located on either one or two particles. In consequence, the aggregation model described above and illustrated schematically in Figure 8, resembles the pillaring of smectite clays.57 This intercalation of flocculant species between silica particles may then explain why the decomposition of Al13 polymers is largely slower than what is observed usually in solution.27,28 It is also interesting to note that a similar reaction schemessilica core with hydrolyzed aluminum coating attached on aluminosilicate anionsshas been proposed to account for the evolution of catalytic properties of aluminated silica gel with alumina content53 and to explain the kinetics of silicic acid polymerization in the presence of increasing amounts of aluminum.48 Influence of pH. Interpretation of the effect of pH on aluminum partition within silica flocs is not straightforward. An increase in pH promotes aluminum hydroly(56) Lartiges, B. S.; Derrendinger, L. S.; Bottero, J. Y.; Tchoubar, D.; Suty, H. In preparation. (57) Bergaya, F. Materiaux Argileux. Structure, Proprie´ te´ s et Applications; Decarreau, A., Ed.; Societe Franc¸ aise de Mineralogie et de Cristallographie 1990; p 513.

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sis and formation of large size aluminum polymers12,13 but affects also charge and surface properties of both silica particles21,58 and flocculant species.59 According to the proposed aggregation model, tetrahedral and octahedral resonances of 27Al NMR spectra represent the respective amounts of aluminum in contact with the silica surface and in flocculant species. Results of Figure 3 indicate therefore that the same aluminum dosage yields a lower fraction of aluminum in polymer bridges at pH 8 than at pH 5.5. This observation, consistent with a higher affinity of aluminum for silica at basic pH, is in accordance with the slightly better alumination reported by Stone et al.30 for silica particles contacted with aluminum solutions of increasing hydrolysis ratio. The formation on precipitated silicas of a surface gel layer at high pH21,58 suggests also that an enhanced Si/Al reactivity may be likely responsible for the higher number of aluminosilicate sites found at pH 8. However, an alternative explanation may be provided by taking into account the intrinsic dynamic nature of aggregation and a difference in the charge of flocculant species with pH. If we assume that the destabilization stages discussed above do not occur in sequence but simultaneously, it is clear that the deposit of an initial aluminum polymer influences the subsequent transfer of other flocculant species to the silica surface. Hence, at pH 5.5, highly charged aluminum polycations59 tend to repel each other and thus limit aluminum incorporation, whereas at pH 8 the almost neutral flocculant species59 permit a higher surface coverage of silica particles by giving access to more reaction sites. Influence of Aluminum Concentration. This dynamic approach may be extended to polymer sorption to account for the influence of flocculant dosage on aluminosilicate site formation at constant pH. Indeed, the decrease of AlIV/AlVI ratio with aluminum concentration (cf. Figure 6), compares favorably with curves giving the fraction p of organic polymer segments bound to a surface (trains) as a function of adsorbed amount.60,61 In that case, the evolution of p with polymer concentration indicates a gradual change in polymer conformation from a rather flat structure to a coiled configuration with loops and tails extending in solution. As shown in studies investigating the effect of rate of polymer supply on adsorbed polymer amount,62,63 this phenomenon arises from a competition between structural reorganization of (58) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. J. Colloid Interface Sci. 1994, 165, 367. (59) Furrer, G.; Ludwig, C.; Schindler, P. W. J. Colloid Interface Sci. 1992, 149, 56. (60) Bottero, J. Y.; Bruant, M.; Cases, J. M. J. Colloid Interface Sci. 1988, 124, 515. (61) Cohen-Stuart, M. A.; Fleer, G. J.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1982, 90, 321.

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individual macromolecules which tend to adopt a flat conformation on the sorbent surface and excluded area effects originating from other adsorbed polymer chains which tend to limit the extent of flattening. Applied to aluminum fractal polymers attached to the silica surface, this kinetic aspect of polymer sorption suggests that at low flocculant concentration, aluminum polymers rearrange to attain a nearly flat configuration which results in the creation of a high number of aluminosilicate sites, whereas at high aluminum dosage, the presence of neighboring flocculant species restricts this reorganization and then minimizes the fraction of aluminum in contact with silica. It is interesting to note that structural reorganization of aluminum polymers is certainly subject to additional constraints as the formation of negative aluminosilicate sites should verify both Loewenstein’s rule64 (two AlIV cannot be neighbors to the same oxygen atom) and charge balance. Concluding Remarks 27

Al MAS and 29Si CP/MAS NMR results indicate that colloidal silica destabilization with hydrolyzed aluminum has many features in common with flocculation by addition of organic polymers. In particular, the aluminosilicates sites formed during the interaction of aluminum polymers with the silica surface may be assimilated to a bound fraction of polymer segments as they represent potential anchors to the flocculant species. Finally, it has become common in the literature65,66 to describe aggregation with aluminum salts with concentration-versus-pH diagrams, on which flocculation regions correponding to different destabilization mechanisms are recognized. Typically, it is proposed that charge neutralization occurs at low aluminum dosage and acid pH, whereas high flocculant concentration and basic pH lead to sweep flocculation. The present study suggests that such an approach is not correct for colloidal silica, as the same destabilization mechanism may be derived from NMR results at pH 5.5 and pH 8. Acknowledgment. Support provided by Elf-Atochem is gratefully acknowledged. B.S.L. wishes to express his sincere thanks to Professor Mark Wiesner and Dr. Srinivas Veerapaneni for helpful discussions. LA951029X (62) Elaissari, A.; Pefferkorn, E. J. Colloid Interface Sci. 1991, 143, 85. (63) Pefferkorn, E.; Elaissari, A. J. Colloid Interface Sci. 1990, 138, 187. (64) Loewenstein, W. Am. Mineral. 1954, 39, 92. (65) Amirtharajah, A.; Mills, K. M. J. Am. Water Works Assoc. 1982, 74, 210. (66) Dentel, S. K. Environ. Sci. Technol. 1988, 22, 825.