Langmuir 1994,10, 4349-4352 4349

Chemistry and Structure of Al(OH)/Organic Precipitates. A Small-Angle X-ray Scattering Study. 2. Speciation and

Structure of the Aggregates A. Masion,? J. Y. Bottero,*>? F. Thomas,? and D. Tchoubart

Laboratoire Environnement et Midralurgie, Groupe de Recherche sur l%au et les Solides Divisks, URA 235 CNRS, ENSG, INPL, BP 40, 54501 Vandoeuvre Cedex, France, and Centre de Recherche de la Matikre Diviske, UMR 813 CRNS-Universitt, Laboratoire de

Cristalographie, Universitk d'orlkans, BP 6703, 45067 Orlians Cedex, France

Received January 14, 1994. In Final Form: June 19, 1994@

The fitting of experimental SAXS curves of Allorganics suspensions by synthetic curves allowed the determination of the Al speciation within the precipitates. The simulations showed that the subunits of the precipitates are mainly uncondensed monomers, which partly originate from the depolymerization of &. The precipitates are built by dense aggregation of uncondensed monomers (log Z(Q)-log Q slope > -2.3), formation of dense-Al(0H)Al- colloids, and loose linear aggregates of oligomers.

Introduction Organic acids are known to modify the reactions

occurring during hydrolysis of aluminum salts. They act as competitors against hydroxyls for the bonding sites of the aluminum atoms. The outcomes of this phenomenon are (i) formation of precipitates with high turbidity at lowered precipitation pH,l (ii) low amounts of Al l3 in solution and in the precipitate^,'-^ (iii) poorly polymerized a lumin~m, l ,~ and finally, (iv) polymorphic, amorphous hydroxides.6,6 However, the nature of the components and the mechanisms ofgrowth of these precipitates remain unclear.

The present work aimed at investigating these mecha- nisms from the study of the final structure of the precipitates. For this purpose, small-angle X-ray scat- tering (SAXS) was used as a powerful method for the study of these disordered systems. Scattering curves were obtained from Al(0H) precipitates in the presence of organic acids with various complexing power (acetic, lactic, oxalic, and salicylic acids). The structure of the precipi- tates was described at two organization levels. The local connectivity of the scattering subunits was described by models inferred from previous liquid- and solid-state 27Al NMR ~tudies , l -~ consisting of few, poorly condensed aluminum species: monomers, oligomers, and Al13. From these models, scattering curves were calculated in the range -0.8 < log Q < -0.2 (part 1). The long-range structure of the precipitates was described by the analysis of the scatterinng curves at log Q < -0.8.

Esperimental Section Sample Preparation. Aluminum chloride-organic acids

mixtures were hydrolyzed by adding slowly NaOH until the

* To whom correspondence should be addressed at Laboratoire de Geosciences de l'Environnement URA CNRS 132, Univ. Aix- Marseille I11 Case 431, 13397 Marseille Cedex 20, France.

t Laboratoire Environnement et MinBralurgie. * Universit-6 d'Orl6ans. @ Abstract published inAdvance ACSAbstracts, August 15,1994. (1) Masion, A.; Thomas, F.; Bottero, J. Y.; Tchoubar, D.; Tekely, P.

J . Non-Cryt. Solids, in press. (2) Thomas, F.; Masion, A.; Bottero, J. Y.; Rouiller, J.; GenBvrier, F.;

Boudot, D. Enuiron. Sci. Technol. 1991,25, 1553. ( 3 ) Thomas, F.; Masion, A.; Bottero, J. Y.; Rouiller, J.; Montigny, F.;

GenBvrier, F. Environ. Sci. Technol. 1993,27, 2511. (4)Thomas, F.; Masion, A,; Bottero, J. Y.; Tekely, P. In NMR

Spectroscopy in Environmental Science and Technology; Minear, R. A., Nanny, M. A., Eds.; Lewis: Chelsea, MI, 1993.

(5) Violante, A.; Violante, P. Clays Clays Miner. 1980,28, 425. ( 6 ) Violante, A.; Huang, P. M. Clays Clays Miner. 1985, 35, 181.

Table 1. Characteristic Parameters of the Sample Preparation. Total Aluminum Concentration: 0.1 M organic ligand L/M pH of Drecipitation

~~

acetate 0.50, 1.00 6.5 oxalate 0.25, 1.00 6.0 lactate 0.50, 1.00 7.0 salicylate 0.25,0.50 6.0

maximum turbidity of the suspensions was reached. The total aluminum concentration was 0.1 M. The organic acids were acetic, lactic, oxalic, and salicylic acids. The experimental conditions (ligandmetal ratios (L/M) and final pH) were detailed in part 1 and are listed in Table 1.

SAXS DataRecordingandTreatment. SAXS curves were obtained over a wide Q range by combining the data recorded on two experimental settings: the D24 line ofthe LURE synchrotron (Universitk de Paris Sud) and a laboratory bench (UniversitC d'Orl6ans). Background correction, smoothing, normalization, and connection of the curves recorded on both benches were carried out by following the procedure described in part 1.

Results Two structural levels were taken into account for the

analysis of the SAXS data, since all the curves displayed two characteristic features (Figure 1): The outermost part of the curves (log Q > -0.8) corresponds to the scattering by the subunits and describes the local organization. This part was simulated by the numerical procedure described in part 1. The innermost part (log Q < -0.8) corresponds to the scattering by aggregates. The slope of the curves characterizes the long-range organization of the precipi- tates. '

Al Speciation from SAXS Curves in the -0.8 < log Q < -0.2 Range. The scattering a t log Q > -0.4 was fitted by calculated curves from spheres whose radii ( r ) simulated the aluminum species: Al monomers (r = 1.88 A), Al dimers and trimers (r = 2.97 A), and 11113 (r = 7 A). In the range -0.8 < log Q < -0.4, the scattering results from the local connectivity of the subunits. Since the log I-log Q slope was invariably close to -1 in this range, unidimensional connection was hypothetized for the calculation of the scattering in this range.

Figure 2 shows the outermost part of the experimental scattering curves and the best-fitting calculated curves obtained with the models. All the scattering curves were simulated by models built with four components based on the three subunits described above: uncondensed mono- mers, unidimensional three- or four-membered oligomer chains, uncondensed Al13, and three-membered chains of

0743-746319412410-4349$04.50/0 0 1994 American Chemical Society

4350 Langmuir, Vol. 10, No. 11, 1994 Masion et al.

SALICYLATE , I--

ACETATE I --.

I

\>\ UM=1.00 0 - -0,5 -1 .

-7-4 -1,5 UM-0.25

-2 -2 -1,5 -1 -0,5 0

log (Q) -2 -1,5 -1 -0,5 0

log (Q) Figure 1. Corrected and connected SAXS curves: organic ligandAl mixtures.

SALICYLATE ,

i -0,;

ACETATE 0 -0,2 -04 1 -0,6 \,

OXALATE -

-0,4 -0.6 P-.

-1 -0,8 -0,6 -0,4 -0,2 0 log (Q)

-2 7

-1 -0,8 -0,6 -0,4 -0,2 0 log (Q)

Figure 2. Simulation of SAXS curves of organics/Al mixtures. Bold lines, experimental; dashed lines, calculated.

A113 (Table 2). The most striking point is the huge majority of monomeric aluminum (70-97%) in all the precipitates. Oligomeric or tridecameric aluminum are only minor species. In the cases ofacetate and lactate, the composition of the models and the proportions yielded by the fit were not affected by the L/M ratio (Table 2), as expected from the similarity of the scattering curves (Figure 1). The models consisted of uncondensed monomers and unidi- mensional chains of AlI3 (lactate) or oligomers (acetate). Marked differences appeared between the L/M ratios with oxalate-AI and salicylate-AI precipitates (Table 2): the

proportion of monomers in oxalate-Al precipitates de- creased from 94 to 70% when L/M increased; in the presence of salicylate, All3 was associated in three- membered chains a t L/M = 0.25 and was uncondensed at higher L/M.

Structure of the Precipitates (log Q < -0.8 Range). The intensity scattered by an aggregate is

I(&> = I,&Q) SCQ)

where Io corresponds to the scattering of the subunits and

AKOH) / Organics Precipitates. Part 2 Langmuir, Vol. 10, No. 11, 1994 4351

Table 2. Speciation of Aluminum within the Precipitates. Proportions (%) of AI Involved in the Species lacate acetate oxalate Salicylate

0.50" 1.00" 0.50" 1.00" 0.25" 1.00" 0.25" 0.50" uncondensed monomers 96 97 87 90 94 70 92 90 chains of 3-4 oligomers 13 10 6 30 uncondensed A113 10 chains of 3 A113 4 3 8 " L/M value.

Table 3. Values of the Slopes of log I(&)-log Q plots acetate lactate oxalate salicylate

0.50" 1.00" 0.50" 1.00" 0.25" 1.00" 0.25" 0.50" log Qmax -1.3 -1.3 -1.3 -1.3 -0.8 -1.3 -1.3 -1.3 slope values 2.59 2.77 2.60 2.42 2.30 2.92 2.47 2.79

" L/M value. * Log Qmax is the higher log Q limit of the linear part of the scattering curves.

SCQ) is the interference function, which describes the arrangement of the particles within the aggregate.7 In the case of fractal aggregates, SCQ) scales as Q-Q, where Df is the fractal dimension of the object.8 In this study, the function IO is the normalized computed scattering intensity of the subunits. Thus the fractal dimension is easily obtained by calculating the slopes of log(I(Q)/ZoCQ)) vs log(&) plots. In all cases the log(I(Q)/Io(Q)) vs log(&) slope values follow the variation of the connectivity of the aggregates. Large values ofthe slope correspond to dense aggregates and low values to loose aggregates.

For all samples, the slope was calculated for log Q < -1.3. A crossover separated this linear part and the Q domain from which the speciation was determined (log Q > -0.81, except for oxalate at L/M = 0.25, where the linearity was observed from log Q = -0.8 (Figure 1). In all cases, the slope values were very high and spread between 2.30 and 2.92 (Table 3). The slope values corresponding to acetate-AI and lactate-Al precipitates were little sensitive to the L/M ratio, whereas they were increased with increasing L/M in the presence of oxalate and salicylate.

Discussion

Aluminum Speciation in the Precipitates Accord- ing to the Ligand. Lactate. The two methods used for the speciation of aluminum gave different AI13 proportions. On one hand, liquid-state NMR with freshly prepared solutions indicated 50% Al13; solid-state NMR of freeze- dried fresh precipitates also indicated high413 a m ~ u n t s . ~ ~ ~ On the other hand, the SAXS curves, which required 20 h recording time (10 x 2 h) on the laboratory bench, had to be fitted with models comprising only 3-4% 4 1 3 . This strong decrease in AI13 may originate from its depolym- erization by the ligand into smaller units during the counting time. Moreover, the 10 x 2 h recordings were identical. This means that the speciation and the final structure of the aggregates were worked out in the first 2 h and did not change afterward.

The mechanism of formation ofthe solid phase probably comprised several steps: precipitation of hydrolyzed complexes and aggregation of AI13 poly~ations,~ followed by the depolymerization of the tridecamer by the ligand. This step was studied more thoroughly and is reported in part 3 of this work.

Acetate. It has been shown previously that, with acetate, the formation of AI13 is hindered by the trapping of the precursors by the ligand, thus forming oligomeric com-

(7) Porod, G. In Small Angle X-ray Scattering; Glatter, O. , Kratky,

(8) Viscek, T. Fractal Growth Phenomena; World Scientific: London, O., Eds.; Academic Press: New York, 1982.

1989.

plexes which precipitate.l The fits provided models consisting of monomers and oligomers for both L/M ratios (Table 2). This confirms that the polymerization of Al into Al l3 is interrupted at the stage of the oligomers. The fit showed that the precipitates contained no Al13. This drastic decrease of the tridecamer proportion in the 2 h following the formation of the solid phase was already observed in the case of lactate-Al precipitates. This suggests that acetate has a depolymerizing power on Al l3 comparable to that of lactate. Thus the formation of acetate-AI precipitates may follow the same stages as in the case of lactate, but with a much greater involvement of the oligomers.

Salicylate. The proportion of solubleAll3 in the presence of salicylate is of the same order as that with As in the two preceding cases, the proportion of Al13 determined by SAXS was lower than that in fresh solutions or freeze-dried precipitates (Table 2). However, this decrease was less drastic than with lactate or acetate. Two hypotheses can be formulated: the depolymerizing power of salicylate is weaker than that observed in the two previous cases, or its reaction kinetics are slower. Again, the 10 SAXS recordings were identical, which means that the majority of structural modifications were completed within the 2 h following the formation of the aggregate. Since the low energy of the X-ray source used did not allow shorter time scales, we cannot clearly privilege one of the two hypotheses. The high proportion of uncondensed monomers probably results from the hydrolysis of the 1:l complexes and the depolymerization of All&

Oxalate. Liquid-state NMR and potentiometric studies have revealed the existence of stable bidentated mono- meric and polynuclear oxalate-AI c o m p l e x e ~ . ~ * ~ J ~ The high affinity of oxalate for the monomeric and oligomeric Al species results in inhibition of AI13 formation.1*2 The speciation obtained by SAXS curve fitting agreed with this result: the precipitates contained only uncondensed monomers and oligomeric AI species.

Structure of the Precipitates. The simulations of the scattering curves at the largest log Q values range revealed that uncondensed monomers appear as the main subunits in the precipitates. Oligomers or All3 are in the minority and form locally small linear aggregates. Except for oxalate at L/M = 0.25, a more or less extended crossover joins the scattering by the subunits and the scattering by the fractal aggregates (Figure 1). It may originate from

(9) Sjoberg, S.; Ohman, L. 0. J . Chem. Soc., Dalton Trans. 1986,

(10) Nordstrom, D. IL; May, H. M. In TheEnuironmental Chemistry 2665.

ofAluminum; Sposito, G., Ed.; CRC Press: Boca Raton, FL, 1989.

4352 Langmuir, Vol. 10, No. 11, 1994

a size distribution of the particles or from the succession of several growth regimes of the aggregates.'l

Since at pH 6-7 (OWN close to 3) the charge of the uncondensed monomers is low, the condensation through OH bonds is highly privileged, except if all the condensa- tion sites are occupied by organic ligands. The calculations showed that, despite the high pH, the aggregates es- sentially consist of uncondensed monomers. Since the L/M ratios never exceeded 1, not all bonding sites can be occupied by the ligand. Therefore, it is necessary to consider that, besides aggregates of uncondensed mono- mers, the precipitates also contain dense -Al(OH)Al- colloids.

Dense aggregation of uncondensed monomers can only take place by charge screening by the ligands. Bridging induces a supplementary densification of the aggregates. Indeed, with bridging ligands (salicylate and oxalate), the slope values increased with increasing L/M and the precipitates formed in the presence of these ligands had the highest slope values. On the contrary, the structure ofthe aggregates formed with nonbridging ligands (acetate and lactate) is only slightly affected by the L/M ratio.

Former studies on the A13+ hydrolysis are consistent in showing that the origin of the dense Al(0H)Al colloids is the dissolution of Al l3 with time. At pH values cor- responding to OWAl =- 2.5, the formation of dense plateletlike particles through A113 dissolution starts after a few minutes.12J3 Thus the precipitates consist of A l l 3 aggregates (Df = 1.8) and dense Al(OH)3 colloids. The Al(OH)3 colloids form aggregates with a fractal dimension close to 1.9.12J4

When organic ligands were added to the aluminum salt as in the current work, this evolution of aluminum sols and precipitates is similar. The A l l 3 content is higher in fresh suspension^^^^ than after several hours of aging. So the decomposition of Al l3 partly results in the formation of organically complexed oligomers or monomers and dense Al(0H)Al colloids. Then the crossover on the SAXS curves between very dense Al-organics aggregates and the subunits corresponds to aggregates of dense Al(0H)Al colloids.

At the highest log Q values the scattering curves are simulated by small linear aggregates of oligomers or N13. The investigated system seems to be similar to that described by Kratky,' which consists of the scattering by close-packed aggregates associated with loose ones (cel- lulose micelles). The close-packed aggregates are detected

Masion et al.

at the lowest angles and the loose ones at the largest angles. The Porod behavior is observable only at the largest angles and yields the specific inner surface of the loose aggregates. In the present case, the Porod law is not observable even if large slope values close to -3 are detected. This is probably due to a situation in which dense (large slope value) and loose aggregates coexist.

Here the question is whether such a structure is formed at the onset of hydrolysis or whether it is inherited from the evolution of the precipitates with time. Such linear aggregates are known to be present in aluminum hy- droxide sols at low pH (beginning of the hydrolysis)12J4 as well as in ferric s o l ~ . l ~ - ~ ~ They correspond to the first steps of the aggregation process, which include the sticking on specific sites of the subunits.18 In the current case, these small linear aggregates may result from the ag- gregation of oligomeric Al species originating from the dissolution of M13. The spacing of 1 A between the members of a chain indirectly reveals the presence of organics between them.

Conclusion The fitting of SAXS curves of Allorganics aggregates

provided new information on the precipitation mecha- nisms. The precipitates formed by hydrolysis of All organics mixtures contain mainly uncondensed monomers and a minority of oligomeric and tridecameric Al species. They result from precipitation of the complexes present in solution and from the depolymerization of Al13. The precipitates are formed by dense aggregates of uncon- densed monomers linked by organic ligands and by dense Al(0H)Al colloids built by condensation of Al octahedra through OH bridging. The most dense aggregates are those built by uncondensed monomers. They form ag- gregates with nonuniversal slope values of the log(Z(Q)/ lo(&)) vs log(&) plots larger than 2.3. Aggregates ofAl(0H) colloids contribute to the scattering in the crossover region of the scatterinng curves between subunits and dense aggregates. The oligomers or tridecamers form locally small linear aggregates during the chemical evolution of the precipitates with time. This evolution is associated with the dissolution of Al13, condensation of Al-OH-Al colloids, and aggregation through organic complexation.

Acknowledgment. The authors thankDr. P. Vachette for SAXS facilities at LURE (Universit6 de Paris Sud) and the program Dynamique et Bilan de la Terre DBT CNRS-INSU 9211 no. 709 for financial support.

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