Environ. Sci. Technol. 1993, 27, 2511-2516

Aluminum( I I I ) Speciation with Hydroxy Carboxylic Acids. 27AI NMR Study

Fablen Thomas,'vt Armand Maslon,t Jean Yves Bottero,? James Rouiller,* Fr6d6ric Montlgny,s and Francine Gen6vrlere

Laboratoire Environnement et Minhralurgie, URA 235 CNRS, BP 40, 54501 Vandoeuvre Cedex, France, Centre de Phdologie Biologique, UPR 6831 du CNRS associhe A I'Universit6 Nancy I, BP 5, 54501 Vandoeuvre Cedex, France, and Laboratoire de Mbthodologie RMN, Universith Nancy I, BP 239, 54506 Vandoeuvre Cedex, France

Aluminum(II1) hydrolysis and precipitation in the pres- ence of lactic acid and salicylic acid have been studied by liquid-state 27Al NMR. The main aluminum species appearing for pH values up to 6.5 have been identified and quantified by decomposition of the NMR spectra: unreacted and hydrolyzed aluminum species, complexed monomers, and the All3 tridecamer. Lactic acid forms various types of aluminum complexes according to the ligand/metal (L/M) ratio. At low lactate content (L/M I l), 1:l monodentate and 1:l chelate are formed and the formation and aggregation of the All3 tridecamer takes place. As soon as 2:l and 3:l multiligand complexes are formed (L/M > l ) , lactate strongly occupies the aluminum bonding sites and the hydrolysis is hindered. Salicylic acid forms a 1:l bidentate complex and the formation of All3 is hindered for L/M > 0.5. Colloids were detected at pH above 4.5. In the presence of lactate, they are made up of A113 linked by ligand molecules. In the presence of salicylate, the colloidal phase is different and of undefined nature.

Introduction

Aluminum may be released from soil minerals into interstitial water, due to naturally occurring local acidity or acid precipitations. Its high reactivity leads to hydro- lytic reactions as well as to strong complexation with naturally occurring organic acids. The relative distribution of hydrolyzed or complexed A1 species is the determining factor for its toxicity and mobility in the environment. The speciation of aqueous A1 was classically quantified by (i) chromatographic separation and categorization methods such as the timed ferron reaction (1, 2) and (ii) compu- tational methods derived from thermodynamic equilib- rium constants (3, 4 ) . In laboratory experiments, 27Al NMR provided direct information on the local structure of the aluminum polyhedron. The chemical nature of the aluminum species occurring on hydrolysis [hexaaquo monomer Al(H20)s3+, hydrolyzed monomers, dimer Alz- (OH)z(HzO)84+, and tridecamer All3 (A104A112(OH)24- (H20)1z7+)1 has thus been ascertained (5-8). 27Al NMR and ferron assay showed that high amounts of A113 can be formed in various neutralizing conditions and at concen- trations as low as lo4 M (9, 10). A recent observation of the tridecamer in an organic soil (11) supports the environmental relevance of the laboratory results.

Few 27Al NMR studies on A1 speciation in the presence of organic ligands have been reported. NMR was used for studying aluminum complexation with some organic acids, such as acetate (12, 131, oxalate (4 ) , lactate, citrate, and

* Corresponding author. t Laboratoire Environnement et MinBralurgie. * Centre de PBdologie Biologique.

Laboratoire de MBthodologie RMN.

0013-936X/93/0927-2511$04.00/0 0 1993 American Chemical Society

EDTA (14), or a number of acids (15, 16). These works presented qualitative results in terms of complexation mechanisms and stoichiometry, but no quantitative data were available from NMR.

The aim of the present work was to study in a quantitative way the hydrolysis of aluminum in the presence of organic acids. For this purpose, liquid-state 27Al NMR was used as the most powerful technique for identifying and quantifying the aluminum species formed at various pH values and for various initial ligand/metal (L/M) ratios. The quantitation of the formed aluminum species was obtained from NMR by decomposing the spectra into elemental resonances using a novel least- squares method (17). The chosen pH range corresponds to natural soil conditions (pH 2-7). In a previous paper the hydrolysis and precipitation of aluminum in the presence of acetic acid and oxalic acid were described (18). Mondentate and bidentate complexes represented the two model situations encountered with natural organics. Strong multiligand chelates formed with oxalate were shown to decrease and even inhibit the tridecamer formation; weak monodentate complexes formed with acetate displayed less inhibiting effect. The present paper addresses the hydroxy carboxylic acids, the most common among the aquatic humic acids, represented here by lactic acid and salicylic acid.

Materials and Methods

Liquid-state z7Al NMR spectra were obtained as pre- viously described (18) on aBruker XWP 200 spectrometer a t 52.1 MHz and 25 "C using a 2H lock with D2O at a level of 20% in the external reference. Typical experimental parameters included 10-ps pulses a t 6 2 , recycle delays of 500 ms, 1024 or 4096 transients (according to the total A1 concentration), 8000-Hz sweep width, and 2-Hz line broadening. The free induction decays (FID) were col- lected in 8K of data memory. Samples were placed in a 10-mm standard tube; inside the tube a well-centered capillary contained an Al(0Hd- solution (5 X 10-2 or 2 X 10-1 M according to the total A1 concentration of the samples) used as external reference.

Unhydrolyzed aluminum solutions at pH 3.3 were first analyzed by 27A1 NMR in order to get accurate information about the most probable complexes and their stoichiometry in the absence of hydrolysis. Stock solutions were prepared with deionized, 0.22-pm-filtered water: 5 X 10-l M AlC13, 6H20, and 5 X 10-1 M sodium lactate or sodium salicylate acidified with 5 X 10-1 M HCl. Volumes of acidified ligand were added to the aluminum solution so that the aluminum concentration was 10-1 M and the L/M (total ligand concentration/total metal concentration) ratio was 0.1,0.2, 0.5,1,2, or 3 for lactic acid and 0.2,0.5,1, or 2 for salicylic acid.

Partially hydrolyzed mixtures were prepared at various pH values. The total aluminum concentration was 2 X

Environ. Sci. Technoi., Vol. 27, No. 12, 1993 2511

8 d I1

60 LO 20 0 - 2 0 - 4 0 PPm

Flgure 1. 27AI NMR spectra of aluminum chloride (0.1 M) with lactic acid (a) 0.01, (b) 0.02, (c) 0.05, (d) 0.1, (e) 0.2, and (f) 0.3 M, at pH 3.3 f 0.1,

M, and the L/M ratios were 0.5,1, and 2 for lactic acid and 0.5 and 1 for salicylic acid. Titration was run on well- stirred solutions by slowly adding lo-' M NaOH from a CO2-free stock solution until the required pH was reached. The use of 2 X M aluminum and lo-' M NaOH minimized the effect of local oversaturation of titrant. The pH values of the samples were the following: (initial pH of the unhydrolyzed mixtures, imposed pH of the partially hydrolyzed solutions) 3.3,4.0,4.5,5.0,5.5,6.0 and 6.5. The samples were analyzed by NMR within 2 h after prepa- ration.

The concentrations of the formed aluminum species were obtained from the NMR spectra. For this purpose a novel method was developed to decompose the spectra. Cal- culated free induction decays (FIDs) were fitted to the experimental signal by means of a nonlinear least-squares method. The calculated FIDs were obtained from a set of four adjustable parameters (line width, frequency, amplitude, and phase) for each presumable resonance (1 7). The results were expressed as percentages of the total initial aluminum. The experimental uncertainity was always lower than 10%.

Results and Discussion

Lactate-Aluminum Complexes in Unhydrolyzed Solution. Liquid-state 27Al NMR spectra of lactate- aluminum complexes at pH 3.3 and for L/M ratios ranging from 0.1 to 3 are shown in Figure 1. The spectra consisted of a sharp line (Av = 8 Hz) a t 0 ppm, corresponding to A ~ ( H z O ) ~ ~ + , and broad lines at lower field, corresponding to the lactate-aluminum complexes. When L/M was increased from 0.1 to 1, the hexaaquoaluminum monomer peak decreased in intensity (not shown on Figure 1) and the broad peak downfield increased in a reciprocal fashion and was shifted from 6 to 9 ppm. For L/M up to 2, this peak was strongly broadened by a second component at 15 ppm. For L/M = 3, the 15-ppm peak was the major resonance and was broadened by a 24-ppm shoulder. The fact that the peaks from hydrated aluminum and lactate- complexed aluminum vary in a reverse way indicates that in these experimental conditions, slow exchange between both species occurs, with respect to the 27Al chemical shift time scale (14). The changes in the 27Al spectra may be explained by a gradual displacement of H20 by lactate molecules according to the L/M ratio.

Table I. Stability Constants of Aluminum Species, Aluminum Organic Complexes, Lactic Acid, and Salicylic Acida

compd log K compd log K

5.02 AlL+ 2.38 8.71 AlL2- 4.56

10.4 AlLa" 6.66

A 1 u m i n u m Lactate-Aluminum Al(OH)Z+ Al(OH)2+ A1 (OH) 3O

A113 97.6 Salicylate-Aluminum A12(0H)2 6.27 AIL+ 12.9

Lactic Acid AlLZ- 23.2 LH- 13.0 AIL& 29.8 LH2 16.8

Salicylic Acid LH- 13.4 LH2 16.3

a Taken from refs 19, 20, and 29.

0 1 2 3 LIM

Flgure 2. Broadening of the 27Ai NMR resonances of the lactate- aluminum complexes, shifted from 6 to 24 ppm.

An attribution of the downfield peaks has been proposed by Karlik et al. (141, except for the 6-ppm peak (Figure 11, not described by these authors, who studied solutions at L/M 2 1. This resonance is close to that of the aluminum dimer (4 ppm), but at this pH this species can be considered to be negligible (13, 18). However, a t higher pH, the presence of the dimer cannot be totally excluded, since its stability constant is of the order of magnitude of the 2:l and the 3:l lactate-aluminum complexes (Table I). On the other hand, a 1:l monodentate complex is probable, since at pH 3.3 the acid group of lactic acid is only partly dissociated. Since this 6-ppm resonance was always detected as a component of the broad lactate-aluminum complex peak up to L/M = 3, it was presumably assigned to the monodentate mononuclear complex (LM2+). The 9-ppm peak at L/M = 1 could be attributed to the 1:l lactate-aluminum bidentate (LM+), in agreement with Karlik et al. (14). Similarly, the 1:l oxalate-aluminum bidentate gives a 6-ppm peak (18); the slight difference in the chemical shift may be due to different magnetic shielding by the ligand. The 15-ppm shift appearing at L/M = 2 (Figure 1) may be due to a 2:l lactate-aluminum complex (L2M-). A similar shift between the 1:l and the 2: 1 complex was observed with oxalate-aluminum mixtures at L/M = 2 under the same experimental conditions (18). Following this reasoning, the 24-ppm shoulder (Figure 1) at L/M = 3 may presumably by attributed to the emerging 3:l lactate-aluminum complex (L3M3-). The linewidth of the resonance corresponding to -the complexed species linearly increased from 220 to 1850 Hz according to the L/M ratio (Figure 2). The slope, 574 Hz or 11 ppm per L/M unit, represents the gradual addition of new com- plexes when more ligand was added. Thus there must be

2512 Environ. Scl. Technol., Vol. 27, No. 12, 1993

a 8ol 9

100 Is b

I I I I I I I

PPm Flgure 3. *'AI NMR spectra of aluminum chloride (0.02 M) with (a) 0.01 M lactic acid and (b) 0.01 M salicylic acid, at pH 4.5. These spectra are the result of the convolution of decomposed FIDs (77). The peak at 80 ppm corresponds to the AI(OH),- external reference.

ao 60 40 20 o

fast exchange between the 1:1, the 2:1, and the 3:l complexes. This hypothesis is supported by the fact that the formation constants of these complexes are close to each other (Table I).

A very interesting point is the contrast between the low complexation constants between lactate and aluminum (Table I) and the high chemical shifts and complex peak areas recorded by NMR. Indeed, computation of the complexes' concentrations using MINEQL (19) yielded values lower than 10-'1 M in all cases, whereas the NMR spectra yielded total complex concentrations ranging between 2.5 X 10-3 and 9.8 X M when L/M varied from 0.1 to 3. The chemical shifts and NMR peak areas were similar to those observed with oxalate (181, the aluminum-organic complexation constants of which are 11.1 and 15.1, respectively, for the 1:l and 2:l complexes (20). Thus the lactate-aluminum complexation constants or the methods by which they were determined warrant reexamination in light of the above NMR results.

Lactate-Aluminum Complexes in Partially Hy- drolyzed Solutions. 27Al NMR spectra were recorded at various pH values ranging between 2 and 6.5 on lactic acid-aluminum mixtures a t L/M = 0.5, 1, and 3. The total aluminum concentration, 2 X M, was suitable for recording valuable NMR spectra and required mod- erately concentrated NaOH (10-1 M) for titration. How- ever, the chemical shifts of these NMR spectra were identical to those recorded with 10-l M aluminum. The same complexes could then be taken into account in the following part of the study. The 63-ppm resonance from the tetrahedrally coordinated aluminum of Al13 was also detected at pH 4.5 for L/M = 0.5 and 1 (Figure 3a).

The spectra were decomposed using the method de- scribed by Montigny et al. (17) in order to calculate the proportions of the previously hypothetized aluminum species: hydrated aluminum, 1:l monodentate (6 ppm), 1:l bidentate (9 ppm), 2:l biligand (15 ppm) lactate- aluminum complexes, and the tridecamer (63 ppm).

The distribution of the soluble aluminum species for L/M = 0.5 (Figure 4a) was similar to that obtained with

8ot \ Alc6 A Alc9 1 Alc15 1

4 0 t w PH

PH Flgure 4. Speciation of aluminum (0.02 M) in the presence of lactate, calculated from the fitted *'AI NMR spectra: (a) 0.01, (b) 0.02 and (c) 0.06 M lactate. Alm, monomer; Alc6, 6-ppm complex; AlcQ, 9-ppm complex; Alc15, 15-ppm complex; A113, tridecamer.

acetate (18). At the lowest pH value (3.51, the aluminum was mainly in the noncomplexed monomeric form. Its concentration decreased to the benefit of the 1:l mono- dentate lactate aluminum complex (6 ppm) when titrant was added more complex was formed as dissociation of the lactic acid occurred. At pH above 4.2, tridecamers were formed to a maximum proportion of 60% at the expense of both the monomers and the complexes.

For L/M = 1 (Figure 4b), the soluble aluminum distribution remained similar to the preceding, except that the 1:l bidentate lactate-aluminum complex (9 ppm) and the 2:l lactate-aluminum complex (15 ppm) appeared successively at pH values up to 5.5. The formation of the tridecamer was somewhat hindered by the presence of these chelates but resulted in about 50% of the total aluminum at pH 6 being AlI3.

For L/M = 3 (Figure 4c), the 1:l monodentate lactate- aluminum complex (6 ppm) was the minor species that appeared. Mainly 1:l and 2:l bidentates (respectively 9

Envlron. Scl. Technol., Vol. 27, No. 12, 1993 2519

1 LACTATE 1

0 UM=3 I "'1 SALICYLATE

B

0 8

A

A

e

2 3 4 5 PH

Flgure 5. Broadening of the 27AI NMR 0-ppm line from the hydrated aluminum, according to pH and various amounts of lactate and salicylate.

L, 60 LO 20 0 - 2 0 - L O

PPm

Figure 6. 27AI NMR spectra of aluminum chloride (0.1 M) with salicylic acid, (a) 0.02, (b) 0.05, (c) 0.1, and (d) 0.2 M, at pH 3.3 f 0.1.

and 15 ppm) were formed. No tridecamers were formed. Here all the aluminum was strongly complexed to lactate ligands, and the nucleation and aggregation were impos- sible.

The 0-ppm line contains information on the symmetry of the monomeric A1 octahedra: its linewidth increased regularly with the pH for the three L/M ratios (Figure 51, confirming that primary hydrolysis of the noncomplexed monomers occurs without perturbation by the organic ligands (18).

Salicylate Aluminum Complexes in Unhydrolyzed Solution. Liquid-state 27Al NMR spectra of salicylate- aluminum complexes at pH 3.3 and for L/M ratios ranging from 0.1 to 2 are shown in Figure 6. A broad line was observed at 3 ppm downfield of the 0-ppm line from the hydrated aluminum. This line may be ascribed to a salicylate-aluminum complex. In this case, the formation of the aluminum dimer can be excluded because of the high stability constants of the salicylate-aluminum com- plexes (Table I). The 3-ppm line increased in intensity according to L/M, and reciprocally the 0-ppm line de- creased, indicating slow exchange between hydratd and complexed aluminum. Although a unique 1:l bidentate (LM+) stoichiometry has been described for this complex (21), the highly stable 2:l complex (L2M-) may also be formed (Table I).

The 3-ppm chemical shift (Figure 6) was surprisingly low for such strong complexes, since it could be supposed to occur a t 6 to 9 ppm, according to the results with oxalate (18) and lactate (Figure 1). In the present case, the T

a

EO - A A l l 3

1 60

%

40

6 0 2 3 4 5

PH

100

80

60

O h t A \ i

/ \ 2 0 t / \ \

n v

2 3 4 5 6

PH

Flgure 7. Speciation of aluminum (0.02 M) In the presence of salicylate, calculated from the fltted 27AI NMR spectra: (a) 0.01 and (b) 0.02 M salicylate. Alm, monomer; Alc3, 3-ppm complex; A113, tridecamer.

electrons of the benzene ring create a variable secondary induction which results in a local magnetic field inho- mogeneity. Here the complexed aluminum atoms are placed in such a way that this induction is added to the magnetic field of the apparatus, and a shift upfield of unknown magnitude is added to the resonance of the complex (22). This situation has already been described for a number of organic ligands such as phthalate and inorganic ligands such as phosphate (23,24) and sulfate (25). The dissymetry in the 3-0-ppm resonance (Figure 6) toward high field, which increased according to L/M, could result from the presence of a second salicylate molecule when the 2:l complex is formed.

The concentrations of complexed aluminum, from the decomposition of NMR spectra, respectively 3.84 X 4.81 X 6.24 X and 9.06 X M for L/M = 0.2, 0.5,1, and 2, were in fair agreement with the concentrations computed using the MINEQL program, respectively 1.94 X 10-2, 4.87 X 9.34 X and 9.96 X M.

Salicylate-Aluminum Complexes in Partially Hy- drolyzed Solutions. The 27Al NMR spectra, recorded at various pH values, were decomposed (17) and yielded noncomplexed monomeric aluminum, salicylate-alumi- num complex, and tridecamer, the 63-ppm resonance of which appeared at pH 4.5 for L/M = 0.5 (Figure 3b).

The evolution of the aluminum species on hydrolysis in the presence of salicylate at a L/M ratio of 0.5 (Figure 7a) was similar to that obtained under the same conditions in the presence of bidentate-forming oxalate (18): the tridecamer was formed at the expense of the monomeric aluminum and not of the complex. The tridecamer content was only 30% at pH 5.5. For L/M = 1 (Figure 7b), almost all the aluminum was in the complexed form at pH 3.5

2514 Envlron. Sci. Technol., Vol. 27, No. 12, 1993

150 0 L A C 0 5

A LAC1 ;El S A L 0 5

y/m i; 100 3 f 3 ‘i

50 zi

M 0

A LAC L/M=l 0 LAC L/M=3

SAL L/M=O5 A SALL/M=l

/” / I

/ L , * . .I. . * . . - . .

3 4 5 6 7

PH

Figure 9. pH dependence of the proportion of “solid” aluminum (AIS), Calculated from the difference between the total Initial aluminum and the sum of the species detected by 27AI NMR.

clusters aggregated by lactate ligands (Figures 4a,b and 8), the precipitate may also be composed of Ah3. When the formation of A113 is partially hindered by salicylate a t L/M = 1, the precipitate must be of different nature. This aspect is now under investigation.

Environmental Consequences. Monomeric and poly- nuclear aluminum have been shown to be phytotoxic species (10,26,27). However,somestudiesinthepresence of organic matter showed that complexation of aluminum considerably reduces its phytotoxicity (28). The conse- quences of aluminum interactions with various complexing organic acids on the formation of hydrolytic products obtained at near neutral pH are then of high environmental interest. The foregoing results, as well as previously published results (18), show the detoxifying power of organic acids toward aluminum, by entrapping 50-80% of the toxic aluminum species, monomers and A113, within soluble complexes and organo-mineral precipitates a t moderately acidic pH. Monomers are involved within organic complexes, the strength of which are related to the complexing capacity of the acid. Complexing acids strongly occupy the aluminum bonding sites, causing inhibition of nucleation and aggregation of A113. Moreover, organic acids cause the formation of a nonsoluble phase by aggregation of All3 through charge neutralization or, when the latter is not formed, by precipitation of a different organo-mineral phase.

In the present study, the analyzed solutions are more concentrated in aluminum than natural waters. However, it has been established that A113 is formed at total aluminum concentrations ranging between 10-1 and 106 M (29,301. The general trends concerning the evolution of organic acid-aluminum mixtures a t various pH values may also be valid at lower concentrations, a t least qualitatively. At very low aluminum concentration, pre- cipitation can be retarded because of the low number of All3 or other aluminum units. However, microenviron- ments such as soil interstitial pores, organic or organo- mineral surfaces, plant roots, or animal tissues, given as examples of high local pH (291, may provide local conditions for the concentration of the precolating solu- tions. Therefore, the phenomena occurring in these local conditions may be correctly described by experiments at high concentrations.

Acknowledgments

This work was supported by the Programme Dynamique et Bilans de la Terre 91-02 INSU-CNRS (Article 568).

Envlron. Scl. Technol., Vol. 27, No. 12, 1993 2515

The authors wish to thank Dr. J. P. Boudot (Centre de Pbdologie Biologique) for help in computing aluminum speciation using MINEQL.

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Received for review March 3, 1993. Accepted June 13, 1993.'

Abstract published in Advance ACS Abstracts, August 15,1993.

2518 Envlron. Scl. Technol., Vol. 27, NO. 12, 1993