Langmuir 1994,10, 4353-4356
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Chemistry and Structure of Al(OH)/OrganicPrecipitates. A Small-Angle X-ray Scattering Study. 3. Depolymerization of the All3 Polycation by Organic Ligands A. Masion,? F. Thomas,? D. Tchoubar,t J. Y. Bottero,*lt and P. Tekelys Laboratoire Environnement et Miniralurgie, Groupe de Recherche sur l'Eau et les Solides Divisis, URA 235 CNRS, ENSG, INPL, BP 40, 54501 Vandoeuvre Cedex, France, Centre de Recherche sur la Matiire Diviste, UMR 813 CRNS-Universiti, Laboratoire de Cristallographie, Universiti d'Orlians, BP 6703, 45067 Orlians Cedex, France, and Laboratoire de Mithodologie RMN, URA 406 CNRS, Universiti de Nancy I, BP 239, 54506 Vandoeuvre Cedex, France Received January 14, 1994. In Final Form: May 11, 1994@ The interaction of organic ligands, i.e., acetate, oxalate, lactate, and salicylate, with a pure A13 solution was studied by combining 27AlNMR and small-angle X-ray scattering. In all cases, the addition of organics led to the quasi complete depolymerization of the tridecamer. Variable reaction kinetics were observed according to the ligand. The proportion Of& in 2-h-aged suspensions never exceeded 2%. The mechanism of decomposition is thought to imply the depolymerization of the tridecamer into oligomers, and then into monomers.
Introduction Since its discovery,l the A l l 3 polycation, formed by partial hydrolysis of aluminum(III), has been the subject of numerous studies. Nuclear magnetic resonance (NMR) spectroscopy provided additional evidence of its e x i ~ t e n c @ - ~ and enabled the speciation of aluminum in solution.6-7 Studies of the precipitates formed during hydrolysis showed that they are formed by aggregation of A l l 3 units and that their fractal dimensionincreased with increasing hydrolysis ratio [OHy[Al].8-10 Much attention has been paid to the conditions of formation of the A l l 3 polycation, in particular the influence of initial aluminum concentration, nature of the titration agent, and mode and rate of base inje~ti0n.ll-l~The presence of organic ligands strongly modifies the hydrolysis of aluminum and leads Groupe de Recherche sur 1'Eau et les Solides Divis6s. Universit6 d'Orl6ans. Q Universit6 de Nancy I. Abstract published in Advance ACS Abstracts, July 1, 1994. (1) Johanson, G. Acta Chem. Scand. 1960, 14, 769. (2)Akitt, J. W.; Greenwood, N. N.; Lester, G. D. J. Chem. SOC.A f
@
1969,803. (3) Akitt, J. W.; Greenwood,W.; Khandelwal, B. L.; Lester, G. D. J. Chem. SOC.,Dalton Trans. 1972, 604. (4)Akitt, J. W.; Farthing, A. J. Chem. Soc., Dalton Trans. 1981, 1606. (5) Bottero, J. Y.; Cases, J. M.; Fiessinger, F.; Poirier, J. E. J.Phys. Chem. 1980.84.2933. (6) Bott&~,J.Y.;Marchal, J.P.;Poirier,J.E.;Cases,J.M.;Fiessinger, F. Bull. SOC.Chim. Fr. 1982, 11-12, 439. (7) Bertsch, P. M.; Thomas, G. W.; Barnhisel, R. I. Soil Sci. S 0 c . h . J . l986,50, 825. (8)Bottero, J. Y.; Tchoubar, D.; Cases, J. M.; Fiessinger, F. J.Phys. Chem. 1982,86,3667. (9) Bottero, J. Y.; Axelos, M.; Tchoubar, D.; Cases, J. M.; Fripiat, J. J.; Fiessinger, F. J. Colloid Interface Sci. 1987, 117, 47. (10)Axelos, M.; Tchoubar, D.; Bottero, J. Y.; Fiessinger, F. J.Phys. (Paris) 1986, 46, 1587. (11) Akitt, J. W.; Farthing, A. J. Chem. SOC.,Dalton Trans. 1981, 1617. (12) Akitt, J. W.; Farthing, A. J. Chem. SOC.,Dalton Trans. 1981, 1624. (13) Bertsch, P. M. Soil Sci. SOC.Am. J . 1987, 51, 825. (14) Parker, D. R.; Bertsch, P. M. Enuiron. Sci. Technol. 1992,26, 914. (15)Kloprogge,J. T.; Seykens, D.; Geus, J. W.; Jansen, J. H. B. J. Non-Cryst. Solids 1992, 142, 87. (16) Furrer, G.; Ludwig, C.; Schindler, P. W. J.Colloid Interface Sci. 1992, 149, 56.
to much lower or n o A l l 3 formation.16-21 In p a r t 222of this work, we showed from simulations of small-angle X-ray scattering (SAXS)curves that the organics not onlyhinder the formation of the tridecamer, but also provoke its depolymerization. The aim ofthe present paper is to study more extensively the depolymerization effect of organic ligands on pure A l l 3 solutions using 27AlNMR and SAXS. Experimental Section Solutions of A113 were prepared by hydrolysis of AlCl3 by 1N NaOH up to a hydrolysis ratio R ([OH]RAl]) equal to 2.2. The Al concentration was 0.1 M. The pH ofthe A113 solution was 4.3. Organic ligands (acetate, oxalate, lactate, salicylate) were mixed with the tridecamer at various [ligandMmetal] (L/M)ratios as 0.5 M sodium salt solutions, whose pH was adjusted to 4.3, except for salicylate where the pH was adjusted to 5.0 in order to avoid the precipitation of salicylic acid. Turbidity of the samples was measured by means of a Hach Ratio/XR turbidimeter calibrated in standard nephelometric turbidity units (N"). Liquid-state 27A1 NMR spectra were obtained on a Bruker MSL 300 spectrometer at 78.2 MHz. Typical experimental pulses, 500-ms recycle delays, parameters used were n/2 10;~s 8000-Hz sweep width, and 2-Hz line broadening. Samples were placed in 10-mm standard tubes, inside which a well-centered capillary containing a 0.5 M NaAI(OH)4 solution was used as the reference for peak area calibration. The areas of the peaks were approximated using a nonlinear least-squares method.23 Synthetic free induction decays were fitted to the experimental signal. Solid-state 27A1NMR spectra of freeze-dried precipitates were recorded under magic angle spinning (MAS) conditions. The apparatus, frequency, and pulse sequence were identical to those used for liquid-state experiments. The spinning frequency was (17) Furrer, G.; Trusch, B.; Muller, C. Geochim. Cosmochim. Acta 1992,56,3831. (18) Thomas,F.;Bottero,J.Y.;Masion,A.; Genbvrier, F. Chem. Geol. 1990, 84 (1/4), 227. (19)Thomas, F.; Masion, A.; Bottero, J. Y.; Rouiller, J.; Genbvrier, F.; Boudot, D. Enuiron. Sci. Technol. 1991,25, 1553. (20) Thomas, F.; Masion, A.; Bottero, J. Y.; Rouiller, J.; Montigny, F.; GenBvrier, F. Enuiron. Sci. Technol. 1993,27, 2511. (21) Masion, A.; Thomas, F.; Bottero, J. Y.;Tchoubar,D.; Tekely, P. J.Non-Cryst. Solids, in press. (22) Masion, A.; Bottero, J. Y.;Thomas, F.; Tchoubar, D. submitted for publication to Langmuir. (23) Montigny, F.; Brondeau, J.; Canet, D. Chem. Phys. Lett. 1990, 170, 175.
0743-746319412410-4353$04.50/00 1994 American Chemical Society
Masion et al.
4354 Langmuir, Vol. 10, No. 11, 1994 SALlCYLATE
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Figure 2. Evolution with time of the proportion of A13 calculated from fitted liquid-state 27Al NMR spectra (circles) and the line width of the 63 ppm line (open squares) for the salicylate-All3 system at L/M = 0.50.
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LIM
Figure 1. Proportion Of&3 calculatedfrom fitted liquid-state 2 7 A l NMR spectra (circles)and the line width of the 63 ppm line (opensquares)for acetate-Alla (a)and lactate-All3 (b) systems. 3 kHz in order to prevent superposition of spinning side bands with isotropic resonance signals. Precise quantitative analysis of solid-state spectra was not possible, mainly because of the presence of spinning side bands. SAXScurves were recorded on the laboratory bench (University of OrlBans,France)with the experimentalparameters described in parts 1 2 4 and 2.22
Results Turbidity. According to the organic ligand used, two situations were observed. The mixtures with acetate and lactate, for LJM ratios of 0.5,1.0, and 2.0, remained clear. The turbidity of the solutions was 0.2 and 0.3 NTU for acetate- and lactate-Alu mixtures, respectively, for all three LIM ratios. When oxalate or salicylate was mixed with the A l l 3 solution, precipitation occurred. The phenomenon was immediate with oxalate a t IJM = 0.25 (turbidity of the suspension 197 NTU), whereas with salicylate a t LJM = 0.50, the turbidity first remained low (1-2 NTU) for 30 min after mixing, and then increased quickly to reach 1050 NTU after 60 min. Liquid-State NMR. A reference spectrum was recorded with a n All3 solution at R = 2.2. Previous studies had shown that, at this hydrolysis ratio, the A13 po!ycations are isolated species representing nearly the totality , ~ Jwas ' confirmed in the current of the Al p r e ~ e n t . ~ , ~This study from a quantitative decomposition ofthe NMR spectrum: 93% of the total aluminum is involved in the tridecamer. The spectra of ligand-All3 mixtures were recorded immediately after mixing, and their aquisition took 10 min. For all samples, only one peak at 63 ppm, the characteristic resonance of the central tetrahedron of Al13, is present. No octahedrally coordinated aluminum was detected in solution. With acetate, the proportion of detected A l l 3 decreased lineary with increasing L/M (Figure la). At L/M = 2.0, more than half of the initially present A l l 3 is removed from solution. The 63 ppm resonance broadens from 19 Hz a t LJM = 0.50 to 44 Hz a t L/M = 2.0. With lactate, the proportion of soluble A l l 3 decreased more markedly than for acetate. When LJM was increased (24) Masion, A.; Tchoubar, D.; Bottero, J. Y.; Thomas, F.; VilliBras, F. submitted for publication to Langmuir.
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Figure 3. 27AlMAS NMR spectra of (a) Al13, (b) salicylateat L/M = 0.50, and (c) oxalate-All3 at L/M = 0.25 (ssb = spinning side bands).
All3
from 0.5 to 2, the proportion of detected A l l 3 decreased from 65% to 20%(Figure lb). The broadening of the 63 ppm line was larger (120 Hz a t L/M = 2.0) than in the case of acetate-All3 mixtures. In the salicylate-Al13 mixture a t L/M = 0.50, precipitation occurred slowly, allowing the monitoring of the proportion of soluble A l l 3 with time. The first spectrum, obtained 10 min after mixing, showed a decrease by 35% of the A l l 3 proportion (Figure 2). This decrease continued with a slower rate for the following 30 min. At this time, half of the A l l 3 had disappeared. Between 40 and 50 min, the precipitation phenomenon led to a total loss of NMR signal. The line width of the A l l 3 peak remained constant at 18 Hz with time. In the case of the oxalate-A113 mixture a t L/M = 0.25, no liquid-state NMR signal was detected. Solid-stateNMR. The reference A l l 3 precipitate was prepared following Johanson's procedure.' The samples were freeze-dried precipitates obtained from oxalate- and salicylate-Al13 mixtures a t L/M = 0.25 and 0.50, respectively. Since acetate and lactate did not induce precipitation, these two ligands were not studied by solid-state NMR. Compared to the reference A l l 3 precipitate, the spectra showed a strong decrease of the characteristic 63 ppm line (Figure 3). The low chemical shift (2.8 and 5.0 ppm for salicylate and oxalate, respectively) of the octahedrally coordinated Al is a n indication that, within
Chemistry and Structure of AKOH) I Organic Precipitates
Table 1. Speciation of Aluminum within the AIS-Organic Precipitates: Proportions (%) of Al(II1) Involved in the Species ligand acetate lactate oxalate salicvlate L/M 0.50 0.50 0.25 0.50 uncondensed Al(II1) 98 98.5 99.3 99
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Langmuir, Vol. 10, No. 11, 1994 4355
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Figure 4. Experimental and simulated SAXScurves obtained with Aha-organic suspensions: (a) A13, R = 2.5; (b) lactate, L/M = 0.50; (c) acetate, L/M = 0.50; (d) salicylate, L/M = 0.50; (e)oxalate, L/M = 0.25; (bold lines) experimental data; (dotted lines) calculated data. the precipitate, aluminum is poorly polymerized, since polymerized Al species usually exhibit chemical shifts around 10 ppm.9s21*25 Small-AngleX-ray Scattering. The organic ligandAll3 mixtures were studied by SAXS in order to examine the nature of the subunits found in the aggregates; the and 222was followed. data treatment described in parts lZ4 Data aquisition took 20 h, divided into 10 records of 2 h. Samples with L/M ratios of 0.25 for oxalate and 0.50 for the three other ligands were studied. A suspension of aggregated All3 was used as the reference. Its hydrolysis ratio R was 2.5 in order to form All3 aggregates in which C1- and OH- ions constitute the briding ligands. The shape of the curves, in a log-log plot of the scattered intensity I vs the wave vector modulus Q, corresponding to the organic-All3 mixtures was very different from that obtained with the A13 suspension (Figure 4). The pure A l l 3 suspension has a slope of 1.75, in agreement with results reported earlier,10*26 whereas the other curves are nearly flat. Since the slope of the curves is a n indication of the fractal dimension of the precipitate, the low slope indicates that, in the case of the mixtures, no branched structures were formed a t this scale. Thus, the simulation of the scattering curves of Ails-organic suspensions was carried out using the numerical procedure described in part l.24In each case, the 10 records of 2 h were identical. The best fits were obtained with models combining uncondensed monomers and 2-4-membered chains Offill3 (Figure 4). In all cases, the proportion of aluminum involved in AI13 did not exceed 2%) the quasi totality of Al being monomeric (Table 1). These results are similar to those obtained with partially hydrolyzed organic-AI mixtures, as seen in part 2.22
Discussion Although acetate and lactate did not lead to any measurable precipitation (0.2 and 0.3 in turbidity, respectively), it is observed that a n increase in the mixtures of the proportion of these two nonbridging ligands leads to a decrease 0fAl13 detected in solution (Figure 1).Since
2 3
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no octahedrally coordinated aluminum was found, it can be inferred that no soluble aluminum complexes are formed with these ligands through a depolymerization of Al13. That part of the aluminum removed from solution can be assumed to be involved in very small colloids or polynuclear Al species (hypothetically dimers and trimers), undetectable by liquid-state NMR because of the quadrupolar nature of the 27Alnucleus. Since only the central tetrahedron of A l l 3 is detected by liquid-state NMR, the observed concurrent broadening of the 63 ppm line is probably not due to a loss of symmetry of the tridecamer, but to a n increase of the size of the observed species, i.e., the formation of small A l l 3 clusters.27 Aggregation 0fAl13 is likely to occur through charge screening by acetate or lactate, as pointed out in earlier hydrolysis experim e n t ~ . To ~ ~obtain , ~ ~satisfactory fits of the SAXS curves (Figure 4, Table 1))it was necessary to include a very small proportion of these clusters in our models. With the bridging ligands oxalate and salicylate, dramatic precipitation occurred. Solid-state NMR clearly shows that these fresh precipitates only contain small amounts 0fAl13. Oxalate exhibits a strong depolymerizing power toward Al13. Precipitation and depolymerization take place within seconds, whereas with salicylate, even with higher L/M ratios, the process takes a t least 30 min. This slower kinetics, clearly shown by the low decrease of the proportion of soluble A l l 3 between 10 and 40 min (Figure 2)) may originate from steric effects due to the benzene ring.21 The broadening of the 63 ppm line from 10 to 18 Hz shows that, in this case, A l l 3 does not form clusters. The SAXS results are unequivocal: as shown by the marked differences from the reference scattering curve (Figure 4)) 2 h after mixing, the precipitated phases are not formed by aggregated Al13. The difference of proportions of A l l 3 between the fresh samples observed by NMR and the aged ones studied by SAXS shows that the initial All3-organic acid complexes evolve with time. This evolution leads to formation of uncondensed monomers bound by organic ligands. This process is not accomplished through dissolution of the precipitates. The transformation is achieved in the solid phase. A similar solid-state reorganization has been previously described for the growth of bayerite from A13 gels.g The final high proportion of monomeric aluminum is in agreement with results reported p r e v i o ~ s l y . ~ ~ ~ ~ ~ The mechanism of depolymerization of A l l 3 cannot be clearly determined by the current results. However, two possible pathways can be proposed: (1)If the decomposition proceeds by extraction monomers from the A13 polycation, they must aggregate immediately, since no monomeric aluminum was detected in solution. (2) If larger units (dimers, trimers) are detached, they must, as shown by the SAXS simulations, be depolymerized into monomers by the organic ligands. This latter hypothesis can be supported by a thorough observation of the chemical evolution of the acetate- and (27)Canet, D.La RMN, Concepts et Mkthodes; Inter Editions: Paris,
(25)Kirkpatrick, R.J.;Smith,K A.; Schramm, S.; Turner, G.; WangHong Yang Annu. Rev. Earth Planet Sci. 1986,13,29. (26)Axelos, M.; Tchoubar, D.; Jullien, R. J.Phys. (Paris) 1986,47, 1843.
1991.
(28)Akitt, J. W.; Farthing, A. J. Chem. Soc., Dalton Trans. 1981, 1609. (29)Wehrli, B.;Wieland, E.; Furrer, G. Aquat. Sci. 1990,52, 3.
4356 Langmuir, Vol. 10,No.11, 1994
lactate-All3 solutions, in which no precipitation occurred. The liquid-state NMR data show that the A l l 3 concentration decreases, but no soluble aluminum monomers are detected. Thus, it can be assumed that the first step of the depolymerization of All3 results in the formation of small organically complexed oligomers (dimers and trimers), which are undetectable by liquid-state NMR under the present experimental conditions.
Conclusion Liquid- and solid-state 27AlNMR and SAXS results clearly show that organic ligands added to A l l 3 solutions induce the depolymerization of the tridecamer. The undetected part of A l l 3 in solution precipitates with structural changes. Solid-state NMR and SAXS curve simulations indicate that the precipitates consist essentially of uncondensed monomers and only very low amounts of 4 1 3 . The mechanism of decomposition of the
Masion et al. tridecamer probably comprises several steps: depolymerization into oligomers and then into monomers. This confirms the results presented in part 2,22where the low A l l 3 content of aged precipitates obtained by hydrolysis of Al-organic mixtures compared to its initial proportion in fresh material was found to originate from the depolymerization of A l l 3 during the experiment. The current work shows that, when available, the organic ligands lead to a nearly complete decomposition of the tridecamer and probably of the oligomers. Thus, they have to be considered not only as strong inhibitors of the A l l 3 formation but also as strong depolymerization agents.
Acknowledgment. The authors wish to thank Dr. P. Vachette for use of the SAXSfacilities at LURE (Universit4 de Paris Sud) and the program Dynamique et Bilan de la Terre DBT CNRS-INSU9W1 for financial support.
