Organic Geochemistry 30 (1999) 461±468
Formation and characterization of OH±Al±humate±
montmorillonite complexes
A. Violante*, M. Arienzo, F. Sannino, C. Colombo, A. Piccolo, L. Gianfreda
Dipartimento di Scienze Chimico-Agrarie, UniversitaÁ di Napoli `Federico II', Portici, Italy
Abstract
We studied the formation of OH±Al±humate±montmorillonite complexes as a€ected by the nature of the humic
acid and the sequence of addition of montmorillonite, humic acid and hydrolytic species of Al. Complexes were
prepared at pH 7.0 by di€erent addition sequences of montmorillonite, 3 or 6 mmol Al and 5, 10 or 20 mg of humic
acid per g of clay. d-spacings of samples ranged from 14.50 to 19.00 AÊ vs. 11.70 AÊ of montmorillonite. Interlayering
of humic acids was promoted when preformed OH±Al±humate complexes were added to montmorillonite and
partially impeded when the complexes were synthesized by neutralizing Al ions in the presence of montmorillonite.
Humic acids were not interlayered in the absence of OH±Al-species, indicating that the hydrolytic products of Al
play a vital role in the formation of organo±mineral complexes in natural environment. # 1999 Elsevier Science
Ltd. All rights reserved.
Keywords: Montmorillonite; Complexes; Aluminum; Humic acids
1. Introduction
The genesis, nature and stability of hydroxy-Al
smectite are of great interest in soil science and in surface chemistry (Sawhney, 1960; Brydon and Kodama,
1966; Rich, 1968; Harsh and Doner, 1985). HydroxyAl-interlayered expandable phyllosilicates are commonly present in acid soils. The nature and stability of
Al interlayers are in¯uenced by the amount of Al
added to the clay, type of clay mineral, OH/Al molar
ratios and presence of inorganic and organic ligands
(Barnhisel and Bertsch, 1989). In the last decade it has
been demonstrated that low molecular weight organic
ligands may interact with Al ions, retarding the crystallization process of Al(OH)3 polymorphs (Kwong and
* Corresponding author. Fax: +39-81-775-5130.
E-mail address: [email protected] (A. Violante)
Huang, 1979; Violante and Violante, 1980; Violante
and Huang, 1985; Violante and Huang, 1993; Huang
and Violante, 1986). Some authors (Violante and
Violante, 1978; Violante and Jackson, 1981; Goh and
Huang, 1984) demonstrated that bi- and tricarboxylic
organic acids (citric, oxalic, malic and tartaric acid),
usually present in the root exudates, perturb the formation of hydroxy-Al montmorillonite in acidic and
alkaline environments and stabilize Al interlayers.
Moreover, the inhibitory e€ect of Al crystallization is
due not only to low molecular weight organic ligands.
Kodama and Schnitzer (1980), Violante and Huang
(1985) and Singer and Huang (1990) demonstrated
that the presence of increasing amounts of fulvic, tannic or humic acid ®rst delays and then prevents the
crystallization of Al(OH)3 polymorphs, promoting the
formation of short-range ordered Al precipitation products.
The interactions among clay minerals, monomers
0146-6380/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 1 4 6 - 6 3 8 0 ( 9 9 ) 0 0 0 3 1 - 5
462
A. Violante et al. / Organic Geochemistry 30 (1999) 461±468
Table 1
Basal spacings of OH±Al±humate±montmorillonite (M1±M12) and Ca±humate±montmorillonite complexes (M13±M18) formed at
pH 7.0. Samples were obtained by using 5, 10 and 20 mg of lignite (LIG) or oxidized coal (COX) and 3 mmol Al per g of clay
Sample
M1
M2
M3
M4
M5
M6
M7
M8
M9
M10
M11
M12
M13
M14
M15
M16
M17
M18
Standards
Sequence of addition of componentsa
Mt/Al/OH/COX
Mt/Al/OH/COX
Mt/Al/OH/COX
Mt/Al/OH/LIG
Mt/Al/OH/LIG
Mt/Al/OH/LIG
Al/OH/COX/Mt
Al/OH/COX/Mt
Al/OH/COX/Mt
Al/OH/LIG/Mt
Al/OH/LIG/Mt
Al/OH/LIG/Mt
Mt/Ca/COXb
Mt/Ca/COX
Mt/Ca/COX
Mt/Ca/LIG
Mt/Ca/LIG
Mt/Ca/LIG
Added HA mg gÿ1 clay
5
10
20
5
10
20
5
10
20
5
10
20
5
10
20
5
10
20
Mt
Mt/Al (3 mmol
Mt/Al (6 mmol)
Mt/Ca
d001 spacing (AÊ)
258C
3008C
15.08
15.18
14.15
15.13
14.97
14.45
16.68
15.78
14.97
16.81
14.97
14.97
14.76
14.97
14.97
14.86
14.65
14.97
14.15
14.15
15.05
14.05
14.34
14.05
15.08
14.75
14.45
15.42
14.45
14.45
9.97
10.10
10.10
10.10
10.10
10.10
11.7
14.76
15.08
14.97
9.90
14.15
14.7
9.95
a
Sequence of addition of montmorillonite (Mt), Al solution (Al), NaOH (OH) and humic acid (LIG or COX). See Section 2 for
explanation.
b
Ca±montmorillonite was mixed with COX or LIG and pH was brought to 7.0.
and polymers of Al and humic substances has received
scant attention, in spite of the fact that the association
of montmorillonite with humic acid in the presence of
Al species has great implications for the understanding
of soil aggregation. Surprisingly, the e€ect of humic
acids on Al interlayering as well as the possible interstrati®cation of humic acid in montmorillonite has not
been studied extensively. Schnitzer and Kodama (1977)
and Schnitzer (1986) claimed that the interlayering
adsorption of fulvic and humic acids by expanding
clay minerals in the absence of hydroxy-Al species is
pH-dependent, being greatest at low pH, and no longer
occurs at pH>5.0. Recently, Mirabella et al. (1996)
found that fulvic acids can enter the interlayer of Naand Ca-saturated montmorillonite up to pH 4.5.
The aim of this work was to study the formation
and characterization of OH±Al±humate±montmorillonite complexes and interlayering of organic matter at
pH 7.0 as a€ected by the nature and concentration of
two humic acids and the sequence of addition of montmorillonite, humic acid and Al species.
2. Materials and methods
Humic acids were isolated from two di€erent raw
materials of high concentration of OC (>90%): a lignite (LIG) from North Dakota, USA, provided by
Cheetah Chemicals, Houston, TX and an oxidized coal
(COX) from Sulcis coal, Sardinia, provided by
Eniricerche SpA. These raw materials were selected
because of their availability in bulk and because of the
advanced chemical and physico-chemical characterization of the HA extracted from them (Piccolo et al.,
1992, 1996).
The <2 mm fraction of a montmorillonite from
Crook County, WY, USA was separated by sedimentation after dispersion in water and sodium or calcium
exchanged by washing three times with 1 M NaCl or
0.5 M CaCl2 solution. Excess NaCl or CaCl2 was
removed by washing in distilled water followed by dialysis until Cl-free. A total of 18 organo±mineral complexes were prepared and two basic methods of
addition of elemental components were adopted (Table
A. Violante et al. / Organic Geochemistry 30 (1999) 461±468
463
Table 2
Chemical and surface properties of organo±mineral complexes and their elemental components
Sample
d001 258C (AÊ)
Speci®c surface area (m2 gÿ1)
CEC (+) cmol kgÿ1
C1a
C4
C7
C10
Mt
M/Al 3 mmol
14.65
14.86
19.00
19.00
11.70
14.76
118
128
262
244
177
110
64
62
63
55
82
43
a
The C1, C4, C7 and C10 samples were prepared like the complexes M, but keeping the components under vigorous stirring for
3 h.
1). The M1±M6 samples were obtained by adding 0.5
M NaOH up to pH 5.0 to mixtures of Na±
montmorillonite+AlCl3 (3 or 6 mmol per g of clay);
after 1 h, humic acid (5, 10 or 20 mg COX or LIG gÿ1
clay) was added and the pH of each suspension was
brought to 7.0 by adding 0.5 M NaOH (method 1).
The M7±M12 samples were prepared by mixing OH±
Al and humic acid solutions, whose pH was previously
brought to 5.0, and, subsequently, after 1 h, by adding
montmorillonite and then 0.5 M NaOH to reach pH
7.0 (method 2). Few complexes (M13±M18) were prepared by method 1 with the di€erence that montmorillonite was Ca saturated and no Al was present. Finally
four samples, hereafter called, C1, C4, C7 and C10,
were prepared as the M1, M4 M7 and M10 complexes,
except that the components were kept under vigorous
stirring for 3 h. Complexes (used as standards) were
also prepared in the absence of humic acids by neutralizing with 0.5 M NaOH mixtures of montmorillonite
and AlCl3, containing 3 or 6 mmol of Al per g of clay.
X-ray powder di€ractograms (XRD) of oriented specimens were recorded using a Rigaku Geiger¯ex D/
Max IIIC X-ray di€ractometer equipped with iron-®ltered CoKa radiation generated at 40 kV and 30 mA,
at a scanning speed of 58 2 y minÿ1. The oriented speciments were stored for 24 h at 208C in a desiccator
containing CaCl2 before XRD analysis.
Infrared (IR) spectra were recorded from 2500 to
4000 cmÿ1 with a Perkin Elmer 1720 X FTIR spectrometer using the KBr-pellet method (0.25% sample).
Speci®c surface area was determined by H2O adsorption at 20% relative humidity with ammonium acetate,
assuming that the weight of H2O required to form a
monomolecular layer on a m2 of surface is 2.78.10ÿ4 g
(Quirck, 1955).
Cation exchange capacity (CEC) was determined by
washing 100 mg of montmorillonite in 50 cm3 of a 0.4
M solution of BaCl2 in the presence of triethanolamine
bu€er solution at pH 8.0. Barium was exchanged with
magnesium (0.05 M MgSO4 solution) and magnesium
Fig. 1. X-ray di€ractograms of selected (M2, M7, M8 and M10) OH±Al±humate±montmorillonite complexes containing 3 mmol
Al gÿ1 clay and di€erent amounts of humic acid (see Table 1).
464
A. Violante et al. / Organic Geochemistry 30 (1999) 461±468
was determined by titration with 0.005 M EDTA. The
stability of the OH±Al±humate complexes (C1±C10)
was examined by the pyrophosphate treatment for the
extraction of organically bound Al.
3. Results and discussion
3.1. Nature of humic substances
Humic substances exhibited very similar surface
area, (222 (LIG) and 257 (COX) m2 gÿ1) and CEC
(33(+) cmol kgÿ1) (Table 2). However, the two isolated humic substances displayed di€erent chemical
composition and structural characteristics as reported
by Piccolo et al. (1992). Liquid-state NMR spectroscopy indicated that, though the two organic materials presented high aromatic content, the material
from oxidized coal was more aromatic (>70%) but
less acidic (10%) than the HA from lignite (56 and
14%, respectively). Moreover, LIG humic material
showed a lower average molecular weight (14 kDa)
than COX (16 kDa) (Piccolo et al., 1996).
3.2. X-ray di€raction analysis of OH±Al±humate±
montmorillonite complexes
Table 1 and Fig. 1 show the d001 spacings at 258C
and after preheating at 3008C of OH±Al±humate±
montmorillonite complexes obtained at pH 7.0 by
di€erent addition sequences of montmorillonite,
hydroxy-Al-species (3 or 6 mmol of Al/g clay) and
humic acid (LIG and COX; 5, 10 or 20 mg of humic
acid per g of montmorillonite). In Table 1 are also
reported the d-spacings of complexes containing Ca
versus OH±Al species.
The X-ray di€raction (XRD) analysis showed that
humic acid did not penetrate the interlamellar spaces
of the Ca-saturated clay (as well as those of the Nasaturated montmorillonite; data not shown). In fact,
the complexes containing Ca and di€erent quantities
of COX or LIG (M13±M18 samples; Table 1) showed
d-spacings at 258C of 14.65±14.97 AÊ, which easily collapsed to 10.16±9.97 AÊ after preheating at 3008C, similarly to the Ca-saturated montmorillonite (Table 1).
Our ®ndings are in agreement with the conclusions of
Schnitzer and Kodama (1977), who demonstrated that
interlamellar adsorption of humic and fulvic acids may
occur only at very low pH values (pH < 5.0).
The interlayering of humic acids actually occurred in
the complexes containing OH±Al species. In fact, the
OH±Al±humate±montmorillonite complexes showed dspacings at 258C, which ranged from 14.15 to 19.00 AÊ
(Tables 1 and 2, Fig. 1). These complexes slightly collapsed after preheating for 2 h at 300 and 5008C. The
presence of very broad and asymmetric XRD peaks at
Fig. 2. Basal spacings of OH±Al±humate montmorillonite
complexes containing 6 mmol Al gÿ1 clay and increasing
amounts (0±20 mg gÿ1) of lignite (LIG) or oxidized coal
(COX). The sequence of addition of the components was Mt/
Al/OH/HA.
258C and after heating at 300 or 5008C (Fig. 1) was
probably due to the continuous removal of humic acid
on heating which left OH±Al species randomly distributed in the interlamellar spaces of montmorillonite
(Buondonno et al., 1989).
The di€erent addition sequences of hydroxy-Al
species, montmorillonite and humic acid promoted the
formation of complexes with di€erent d-spacings and
surface properties, which must be attributed not only
to di€erences in the nature of OH±Al±humate complexes, but also to di€erent interlayering of OH±Al
species and humic molecules (Tables 1 and 2; Fig. 1).
In fact, in the complexes formed by neutralizing Al
ions in the presence of montmorillonite (M1±M6
samples), being OH±Al polymers previously sorbed on
the external and in the interlayers of the phyllosilicate,
interstrati®cation of humic substances was, at least in
part, prevented. As a consequence, the d-spacings of
these complexes at 258C and after preheating at 300
and 5008C were usually slightly greater or similar to
those of the complexes prepared in the absence of
humic acid (Table 1 and Fig. 1). The peaks of these
complexes, even after heating at 300 and 5008C
appeared usually relatively sharp and symmetric, indicating an uniform distribution of OH±Al and/or OH±
Al±humate species (Fig. 1). On the contrary, when suspensions of Na-saturated montmorillonite were added
to preformed OH±Al±humate complexes, containing
no more than 5 mg of humic acid per g of clay, the
M7 and M10 complexes showed particularly high dspacings (16.68±16.80 AÊ vs. 14.15±15.78 AÊ of the M1±
M6 complexes). Furthermore, when samples were prepared like M7 and M10, but keeping the components
under vigorous stirring for 3 h, the d-spacings of the
C7 and C10 Table 2, were even greater (19.00 AÊ).
These ®ndings demonstrate that relatively large OH±
Al±humate polymers may penetrate more easily into a
A. Violante et al. / Organic Geochemistry 30 (1999) 461±468
well dispersed Na-montmorillonite (Singer and Huang,
1993; Violante et al., 1996). The M7±M10 complexes
showed after heating at 5008C peaks at 10.10±10.30 AÊ,
indicating that some interlamellar spaces of montmorillonite were not ®lled by OH±Al±humate complexes.
The nature of humic acid (COX or LIG) did not
have a signi®cant in¯uence on the interlayering of
OH±Al±humate containing 3 mmol Al gÿ1 montmorillonite (Tables 1 and 2). Only the complexes containing
6 mmol Al gÿ1 clay and increasing quantities of LIG
showed d-spacings very slightly greater than those
obtained in the presence of COX (Fig. 2). However,
the e€ect of the nature of humic substances on the
interlayering of OH±Al±humate complexes deserves
closer attention.
In the presence of greater amounts of humic acid
(10 or 20 mg gÿ1 clay) the d-spacings of the complexes
signi®catively decreased (up to 2 AÊ) as appears evident
comparing M7 and M10 (d-spacings of 16.68±16.80 AÊ)
with M9 and M12 (d-spacings of 14.97 AÊ) (Table 1,
Fig. 1). A possible explanation of these ®ndings is that
the presence of larger quantities of humic acid promoted the formation of particularly large OH±Al±
humate polymers, whose interlayering into the expansible layers of montmorillonite became more and more
dicult. Furthermore, the complexes formed between
the positively charged OH±Al species and the negatively charged humic acid became less positive (or even
negative) by increasing the content of the organic matter and then less adsorbable on the external and interlamellar surfaces of the phyllosilicate. Fig. 2 shows
that organo±mineral complexes, containing the same
quantities of LIG or COX (5, 10 or 20 mg gÿ1 clay)
but 6 mmol Al gÿ1 montmorillonite, also showed a
decrease in d-spacings by increasing the content of
humic acid. Because the OH±Al±humate polymers,
richer in Al (Fig. 2) were obviously more positive in
charge than the complexes containing the same
amounts of humic acid but less Al (3 mmol gÿ1; Table
1), it seems logical that, at least under our experimental conditions, the size of the polymers, more than
their charge, played an important role in the ®lling of
the interlamellar spaces of clay.
Recently, De Cristofaro et al. (1998) demonstrated
that proteic molecules may be easily intercalated in the
interlayers of montmorillonite even at pH values
slightly higher than those of their isoelectric points
(i.e.p.), where the surfaces of proteins present a net but
low negative charge. These authors demonstrated that
urease (MW=480 kDa), catalase (MW=238 kDa) and
albumin (MW=60 kDa) formed protein±montmorillonite complexes at a pH values of 1.5 pH unit higher
than their respective i.e.p. The complexes showed
broad peaks centered respectively at 18.5, 25.3 and
32.5 AÊ, indicating that the smaller proteic molecules
penetrated the interlayers of montmorillonite more
465
easily than the bigger ones. These authors also demonstrated that relatively small proteins, like pepsin
(MW=34 kDa; i.e.p.=2.5), did not intercalate the
interlayers of montmorillonite at a pH (pH 7) much
higher than that of their i.e.p. Many evidence seem to
demonstrate that large polymers or organo±mineral
complexes with a net positive, neutral or, even, light
negative charge may be intercalated into the interlamellar spaces of expandable phyllosilicates (Schnitzer
and Kodama, 1977; De Cristofaro et al., 1998).
However, when polymers or organo±mineral complexes are particularly large or present a too high negative charge their intercalation is dicult or impossible.
Clearly, humic acids in the absence of OH±Al species
may be intercalated into the interlayers of montmorillonite only at very low pH values (pH < 5), where they
show a relatively low negative charge density. On the
contrary, humic acids complexed with monomers and
polymers of Al (or with Fe species) may present at certain pH values and humic acid/Al ratios a positive
charge and may be intercalated into the interlamellar
spaces of montmorillonite in neutral (Tables 1 and 2;
Figs. 1 and 2) or even in alkaline environments. These
®ndings are of paramount importance in pedology.
3.3. Chemical and surface properties of selected organo±
mineral complexes
Table 2 shows the d-spacings, the speci®c surface
area and the CEC of four selected complexes (C1, C4,
C7 and C10) which were prepared as the M1±M12
complexes (Table 1), but keeping the components
under vigorous stirring for 3 h. As discussed before
these samples showed basal spacings slightly di€erent
from those of the M1, M4, M7 and M10 complexes,
but strengthen the ®nding that interlayering of humic
acid is promoted when preformed OH±Al±humate are
added to montmorillonite.
The C7 and C10 complexes showed a surface area
(244±262 m2 gÿ1) much greater than that of the C1
and C4 samples (118±128 m2 gÿ1). Probably, the surface area of each complex was in¯uenced by the nature
of hydroxy-Al±humate polymers and by their distribution on the surfaces and interlayer space of montmorillonite and, after drying by the formation,
stability and strength of aggregates formed between
the particles (Buondonno et al., 1989). Probably, the
particles of the C7 and C10 samples were much more
aggregated than those of the C1 and C4 complexes.
The CECs (55±64 (+) cmol kgÿ1) of the organo±mineral complexes were lower than the CEC of montmorillonite (82 (+) cmol kgÿ1), but higher than that of the
OH±Al clay complex (43 (+) cmol kgÿ1) (Table 2).
The decrease of CEC with respect to montmorillonite
must be attributed to the interlayering of OH±Al±
humate ions, whereas the increase of CEC with respect
466
A. Violante et al. / Organic Geochemistry 30 (1999) 461±468
interlamellar and external surfaces of montmorillonite.
Evidently, the Na2P2O7 treatment, which removes
organically bound Al, solubilized more Al from the
former than the latter complexes.
3.5. FT-IR analysis of OH±Al±humate±montmorillonite
complexes
Fig. 3. X-ray di€ractograms of hydroxy-Al±montmorillonite
complexes (C1, C4, C7, C10; Table 2) after pyrophosphate
treatment.
to OH±Al±montmorillonite complex was due to the
dissociation of some functional groups of humic acid
complexed with hydroxy-Al. The CEC of the organo±
mineral complexes increased by increasing the content
of humic acid (data not shown).
3.4. XRD analysis of complexes after pyrophosphate
treatment
Fig. 3 shows the X-ray di€raction patterns of the
C1, C4, C7 and C10 organo±mineral complexes Table
2 after pyrophosphate treatment. The peaks appeared
very broad and asymmetric, showing maxima centered
at 013.80±14.80 AÊ with shoulders of much lower dspacings, indicating that large amounts of OH±Al±
humate complexes were solubilized. The decrease of
the basal spacings was more evident for C7 and C10
than C1 and C4 complexes. Indeed, as discussed
before, in the C7 and C10 samples humic acids were
mainly complexed with Al, because the OH±Al±
humate complexes were preformed and then added to
montmorillonite. Vice versa, in the C1 and C4 complexes humic acids were mainly adsorbed on the previously formed OH±Al species, which coated the
In the Fig. 4 are reported the FT-IR absorption
spectra in the 2500±4000 cmÿ1 range of montmorillonite, OH±Al clay sample and C1 and C7 complexes
before and after heating at 3008C. The C4 and C10
complexes showed FT-IR spectra (not shown) practically similar to those of C1 and C7 complexes, respectively.
The strong and sharp absorbance at 03630 cmÿ1 of
all the samples is attributed to OH stretching of montmorillonite. The band at 03450 cmÿ1 was stronger in
the samples containing humic acid due to presence of
OH group in the humic substance. This band was
stronger in the C7 and C10 than in the C1 and C4
samples, probably due to the di€erent nature and distribution of OH±Al±humate complexes on the surfaces
of clay. The band at 03450 cmÿ1 strongly decreased in
the C1 and C4 samples after heating at 3008C and
appeared smaller than the band at 03630 cmÿ1; vice
versa this band appeared stronger than the band at
03630 cmÿ1 in the C7 and C10 samples after preheating at 3008C. A possible explanation of this behavior
is that humic substances being more intercalated in the
interlayers of montmorillonite in C7 and C10 were
also more stable to heating decomposition.
4. Conclusions
This study has shown that preformation of hydroxyAl±humate polymers added to montmorillonite
increased the extent of interlayering of humic acid into
montmorillonite surfaces at pH 7.0. This phenomenon
was even more evident at lower level of addition of
humic acid due to more favorable dimensions of
organo±mineral complexes. Vigorous stirring during
complex formation facilitated penetration of OH±Al±
humate polymers into the interlayers of montmorillonite. OH±Al±humate complexes formed by di€erent
addition sequences of components showed di€erent
physicochemical and chemical properties.
Acknowledgements
This work received the support of the Environment
Research Program of the European Community, contract 94-EV5V-0470. Contribution number 156 of the
Dipartimento di Scienze Chimico-Agrarie (DISCA).
A. Violante et al. / Organic Geochemistry 30 (1999) 461±468
467
Fig. 4. Infrared spectra in the 2500 to 4000 cmÿ1 range of Na-saturated montmorillonite, Al(OH)x±montmorillonite complex, containing 3 mmol Al gÿ1 clay and C1 and C7 complexes (Table 2).
References
Barnhisel, R.I., Bertsch, P.M., 1989. Chlorites and hydroxy
interlayered vermiculite and smectite. In: Dixon, J., Weed,
S.B. (Eds.), Minerals in Soil Environments, 2nd edn. Soil
Science Society of America, Madison, WI, pp. 729±788.
Brydon, J.E., Kodama, H., 1966. The nature of aluminum hy-
droxide±montmorillonite
complexes.
American
Mineralogist 51, 875±889.
Buondonno, A., Felleca, D., Violante, A., 1989. Properties of
organo mineral complexes formed by di€erent addition
sequences of hydroxy-Al, montmorillonite and tannic acid.
Clays & Clay Minerals 37, 235±242.
De Cristofaro, A., Colombo, C., Gianfreda, L., Violante, A.,
468
A. Violante et al. / Organic Geochemistry 30 (1999) 461±468
1998. E€ect of pH, exchange cations and hydrolytic
species of aluminum and iron on formation and properties
of montmorillonite±protein complexes. Proceedings of
ISMOM, 3±6 September, Nancy, France, in press.
Goh, T.B., Huang, P.M., 1984. Formation of hydroxy Al±
montmorillonite complexes as in¯uenced by citric acid.
Canadian Journal of Soil Science 64, 411±421.
Harsh, J.B., Doner, H.E., 1985. The nature and stability of
aluminum hydroxide precipitated on Wyoming montmorillonite. Geoderma 36, 45±56.
Huang, P.M., Violante, A., 1986. In¯uence of organic acids
on crystallization and surface properties of precipitation
products of aluminum. In: Huang, P.M., Schnitzer, M.
(Eds.), Interaction of Soil Minerals with Natural Organic
and Microbes, vol. 6. Soil Science Society of America,
Madison WI, pp. 159±221 Special Publication 17.
Kodama, H., Schnitzer, M., 1980. E€ect of fulvic acid on the
crystallization of aluminum hydroxydes. Geoderma 24,
195±205.
Kwong, N.F.K.F., Huang, P.M., 1979. The relative in¯uence
of low molecular weight complexing organic acids on the
hydrolysis and precipitation of aluminum. Soil Science
128, 337±342.
Mirabella, A., Piccolo, A., Pietramellara, G., 1996.
Intercalation between a well characterized andisol fulvic
acid and montmorillonite. Fresenius Enviromental Bulletin
5, 430±435.
Piccolo, A., Nardi, S., Concheri, G., 1992. Structural characteristics of humic substances as related to nitrate uptake
and growth regulation in plant systems. Soil Biology &
Biochemistry 24, 373±380.
Piccolo, A., Celano, G., Conte, P., 1996. Adsorption of glyphosate by humic substances. Journal of Agricultural and
Food Chemistry 44, 2422±2446.
Quirck, J.P., 1955. Signi®cance of surface areas calculated
from water vapour sorption isotherms by use of B.E.T.
equation. Soil Science 80, 423±430.
Rich, C.I., 1968. Hydroxy interlayers in expansible layer silicates. Clays & Clay Minerals 16, 15±30.
Sawhney, B.L., 1960. Aluminum interlayers in clay minerals,
montmorillonite and vermiculite. Nature 187, 261±262.
Schnitzer, M., 1986. Binding of humic substances by soil mineral colloids. In: Huang, P.M., Schnitzer, M. (Eds.),
Interaction of Soil Minerals with Natural Organic and
Microbes, vol. 4. Soil Science Society of America,
Madison, WI, pp. 78±101 Special Publication 17.
Schnitzer, M., Kodama, H., 1977. Reactions of minerals with
soil humic substances. In: Dixon, J.B., Weed, S.B. (Eds.),
Minerals in Soil Environments, vol. 21, pp. 741±770.
Singer, A., Huang, P.M., 1990. The e€ect of humic acid on
the crystallization of precipitation products of aluminum.
Clays & Clay Minerals 38, 47±52.
Singer, A., Huang, P.M., 1993. Humic acid e€ect on aluminum interlayering in montmorillonite. Soil Science Society
of America Journal 57, 271±279.
Violante, A., Violante, P., 1978. In¯uence of carboxylic acids on
the stability of chlorite-like complexes and on the crystallization of Al(OH)3 polymorphs. Agrochimica 22, 335±343.
Violante, A., Violante, P., 1980. In¯uence of pH, concentration and chelating power of organic anions on the synthesis of aluminum hydroxides and oxyhydroxides. Clays
& Clay Minerals 28, 425±434.
Violante, A., Jackson, M.L., 1981. Clay in¯uence on the crystallization of Al(OH)3 polymorphs in the presence of
citrate, sulfate or chloride. Geoderma 25, 199±224.
Violante, A., Huang, P.M., 1985. In¯uence of inorganic and
organic ligands on precipitation products of aluminum.
Clays & Clay Minerals 33, 181±192.
Violante, A., Huang, P.M., 1993. Formation mechanisms of
aluminum hydroxide polymorphs. Clays & Clay Minerals
41, 590±597.
Violante, A., De Cristofaro, A., Rao, M.A., Gianfreda, L.,
1996. Physicochemical properties of protein±smectite and
protein±Al(OH)x±smectite complexes. Clay Minerals 30,
325±336.