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This work studies the adsorption of Me–1-hydroxiethane-(1,1-diphosphonic acid) (HEDP) complex onto alumina in the pH range from 5.0
to 9.5. The extent of HEDP adsorption is not significatively affected by the presence of Me(II), while, HEDP has an interesting effect on Me(II)
adsorption. At high surface covering, Cu(II) adsorption is enhanced at low pH reaching a maximum of 57% at pH nearly 6, however, at pH > 6
a decrease about 20% in the amount of Cu(II) adsorbed takes place by the presence of HEDP. The model predicts a ternary surface complex
(≡AlLCu−) to justify the increase of Cu(II) adsorbed at lower pH. At the lower pH and at high Zn(II) concentration the presence of equimolar
concentration of HEDP also causes a discernible increase in the amount of Zn(II) adsorbed. At pH 5, the percentage of Zn(II) complexed with
HEDP increased from negligible to 40% as the HEDP concentration increased. However, in this case the HEDP does not have a suppressor
effect on the Zn(II) adsorption at the higher pH. Again, the presence of anionic-type complexation is here postulated to reach a good fit with the
experimental results. The effect of HEDP over Zn(II) adsorption becomes less pronounced with the excess of surface sites. Cd(II)–HEDP solution
complexes are weaker than those corresponding to Cu(II) and Zn(II), so competitive effects between surface and solution are much less significant
in comparison to Cu(II)–HEDP and Zn(II)–HEDP alumina systems. So, the effect of HEDP on the Cd adsorption at low concentration and low
pH is more stressed than in the case of Cu(II) and Zn(II). Overall, results indicate that the presence of HEDP in the aquatic systems could have a
significant impact on the mobility and distribution of Cu(II), Zn(II) and Cd(II) in the environment.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Alumina; Me(II)–phosphonate adsorption; Surface complexes
1. Introduction
Copper and zinc are both present in the environment. It is
estimated that an excess of 75,000 tons of copper is released
annually into the atmosphere of which a quarter is thought to
come from natural sources, while the rest is of anthropogenic
origin. Many soils contain copper and the copper contents can
be supplemented through human activities. Cu and Zn are es-
sential elements that can be toxic when exposures exceed phys-
iological needs [1]. Natural Cu and Zn concentration ranges in
different parts of the same environmental compartment can vary
substantially [2,3]. On the other hand, cadmium occurs natu-
rally in soils at very low concentrations. It has been reported
that the background cadmium level in soils should not exceed
0.5 mg/kg and all higher values reflect the anthropogenic im-
* Corresponding author. Fax: +54 0291 45595160.
E-mail address: mzenobi@criba.edu.ar (M.C. Zenobi).
pact on the cadmium in soils [4]. The distribution of cadmium
between the soil and the soil solution is mainly influenced by
sorption. However, the soil is not a permanent sink and sorbed
cadmium can easily be released upon changes in the compo-
sition of the soil solution [5]. The bioavailability of these ele-
ments can be drastically changed for adsorption processes on
particulate material, as soils, sediments or metal oxides and by
the presence of organic and inorganic ligands in the aquatic
media [6–8]. Both, bioavailability and sorption are strongly de-
pendent on the metal species. Attention is drawn to the fact that
for environmental risk assessment the bioavailable essential ele-
ment fraction, not its total concentration, should be considered.
The concentration and speciation of these cations in the soil
solution depends partly on the concentration of ligands in the
soil solution and the stability constants of the ligand-metal com-
plexes.
Iron and aluminum oxides are known for their adsorptive
properties. The aluminum oxides are ubiquitous in geological
environments. γ -Al2O3, the substrate chosen for this study is
Journal of Colloid and Interface S
N
Adsorption of Me–HEDP
Lidia Hein, María C.
Departamento de Química, Universidad Nacional del Su
Received 14 March 200
Available onli
Abstract
0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcis.2007.05.050
ce 314 (2007) 317–323
www.elsevier.com/locate/jcis
omplexes onto γ -Al2O3
enobi ∗, Elsa Rueda
vda. Alem 1253, (B8000CPB) Bahía Blanca, Argentina
ccepted 17 May 2007
4 May 2007
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羟基乙叉二膦酸 PO3H2-(CH3)(OH)C-PO3H2
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318 L. Hein et al. / Journal of Colloid an
an amphoteric solid with a PZNPC between pH 8 and 9 [9–11].
It is known that oxides and hydroxides of Al are important com-
ponents of soils, both as individual minerals and as coatings on
aluminosilicates clay minerals.
Phosphonates are readily adsorbed onto almost all types of
mineral surfaces [12–16]. Phosphonic ligands and their corre-
sponding phosphonates are widely employed in water treatment
for scale inhibition. These materials are also used extensively in
laundry detergents, as corrosion inhibitors, in industrial clean-
ing and in peroxy bleach stabilization. All these properties can
be found in recent reviews [17–19]. Among these compounds
1-hydroxiethane-(1,1-diphosphonic acid) (HEDP) is a biphos-
phonate extensively used in many technical applications.
This work focuses on the effect of HEDP in the uptake of
Cu(II), Zn(II) and Cd(II) by alumina as a function of pH in
stoichiometric conditions at different loading surface.
2. Material and methods
2.1. Materials
The alumina used in this study was of high purity manu-
factured by CONDEA Chemie. The specific surface area was
206 m2/g determined by the BET method. GADOR LABO-
RATORIES provided HEDP in the form of disodium salt. The
phosphorous content in the phosphate form was removed from
the original phosphonate solution previous passage through an
anion exchange resin (Amberlite IRA-400). All the solutions
were prepared using reagent-grade chemicals and bidistilled
water. Metal solutions were prepared from nitrate salts.
2.2. Adsorption experiments
In all cases the experiments were done in a sealed cylindri-
cal beaker provided with a thermostat water jacket and a total
volume of 50 mL. A stock suspension of stable hydrated alu-
mina, aged for more than a month was prepared to obtain a
solid concentration of 20.0 and 1 g L−1 of alumina aged was
used in all experiences [14,20]. A background concentration of
0.1 M NaNO3 was used to maintain a constant ionic strength.
The solutions were purged with N2 to minimize dissolved CO2.
In the first set of experiments, the solutions containing the de-
sired concentration of Cu, Zn, Cd or HEDP were equilibrated
with the alumina suspension for 2 h at 30 ± 0.2 ◦C. In the sec-
ond group of experiments, equimolar concentrations of Me(II)–
HEDP were equilibrated together prior to the addition of alu-
mina suspension. The pH of the suspension was held constant
during this period with the aid of a Metrohm automatic titrime-
ter. HNO3 or NaOH was employed to cover the range from pH
5–9.5. The samples were then filtered using Nucleopore mem-
brane (pore size 0.22 µm) prior to analysis.
Dissolved HEDP concentrations in the filtered were first
converted into orthophosphate by peroxidisulfate digestion for
◦
two hours at 100 C and then measured spectrophotometrically
as ortho-phosphate by molybdenum blue method at 880 nm.
Dissolved Cu, Zn and Cd concentrations in acidified samples
terface Science 314 (2007) 317–323
were measured using flame atomic absorption spectrophotom-
etry.
2.3. Modeling adsorption data
All experimental data were analyzed using the 2-pK con-
stant capacitance model (CCM). The concepts and assumptions
underlying the model have been already discussed in other
works [21–23]. Model calculations and optimization of equi-
librium constants for adsorption reactions were performed with
the nonlinear least squares optimization program FITEQL 3.2.
Technical details of this program were provided by Westall and
Herbelin, and Westall [24,25]. The values of these stability con-
stants were used to adjust the different experimental adsorption-
edges.
The data were modeled considering only mononuclear com-
plex formation for Me(II), HEDP and Me(II)–HEDP adsorp-
tion.
The following generalized reactions represent the adsorption
stoichiometries:
≡AlOH + L4− + (n + 1)H+
(1)�≡Al–L–H(4−n−1)−n + H2O,
(2)≡AlOH + Me2+� ≡AlOMe+ + H+,
≡AlOH + Me2+ + L4− + (n + 1)H+
(3)�≡Al–LHnMe(4−n−2−1)− + H2O,
HEDP, Me(II) and Me(II)–HEDP adsorption data were mod-
eled by defining the minimum number of surface species that
consistently yielded a good fit to all data. Equilibrium constants
for the selected surface reactions were optimized for each data
set. These selected constants were then used for all model sim-
ulations.
3. Results and discussion
Table 1 gives the surface complexation reactions and the
values of the optimized equilibrium constants used for model-
ing the experimental data. We found that these values are very
similar to those employed to simulate HEDP adsorption onto
boehmite [26].
Table 1
Surface complexation reaction used in adsorption modeling employing the ca-
pacitance model (I = 0.1 M)
Surface complexation reactions logK
≡Al–OH + L4− + H+ = ≡Al–L3− + H2O 12.40
≡Al–OH + L4− + 2H+ = ≡Al–LH2− + H2O 19.80
≡Al–OH + L4− + 3H+ = ≡Al–LH−2 + H2O 25.90
≡Al–OH + L4− + 4H+ = ≡Al–LH3 + H2O 29.88
≡Al–OH + Cu++ = ≡Al–OCu+ + H+ −0.13
≡Al–OH + Cu++ + L4− + H+ = ≡Al–LCu− + H2O 25.40
≡Al–OH + Cu++ + L4− + 2H+ = ≡Al–LHCu + H2O 30.80
≡Al–OH + Zn++ = ≡Al–OZn+ + H+ −1.50
≡Al–OH + Zn++ + L4− + H+ = ≡Al–LZn− + H2O 25.50
≡Al–OH + Cd++ = ≡Al–OCd+ + H+ −3.34
≡Al–OH + Cd++ + L4− + H+ = ≡Al–LCd− + H2O 25.40
≡Al–OH + Cd++ + L4− + 2H+ = ≡Al–LHCd + H2O 30.50
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Fig. 1. Adsorption of HEDP and Cu(II) onto alumina as a function of pH. Aged
(a, b) 5 × 10−4 M; (c, d) 1 × 10−4 M; 5 × 10−5 M.
L. Hein et al. / Journal of Colloid and Interface Science 314 (2007) 317–323 319
Values for alumina surface-site density, surface acidity con-
stants and total specific capacitance values can be seen in Ta-
ble 2.
Stability constants for solution speciation reactions used in
adsorption model were reported in earlier works [26,27].
Table 2
Equilibrium model describing the acid/base properties of alumina surfaces
logK
≡Al–OH + H+ = ≡Al–OH+2 7.17≡Al–OH = ≡Al–O− + H+ −8.87
Total specific capacitance 1.40 F/m2
Site density 1.17 sites/nm2
BET surface area 206 m2/g
3.1. Cu(II)–HEDP adsorption
Figs. 1a, 1c and 1e show the adsorption of HEDP together
with Cu(II) as function of pH at three different total concentra-
tions of HEDP and Cu(II). To assist in the interpretation of these
results, adsorption edges of solely HEDP is also displayed.
The extent of HEDP adsorption is not practically affected by
the presence of Cu(II) in equimolar concentrations. The model
predicts less HEDP adsorption in the presence of Cu(II) than
that observed experimentally (Fig. 1a). That is, a sufficiently
high concentration of Cu(II) in solution can, to a certain ex-
tent, successfully compete with the surface for complexation
with HEDP. At lower Cu(II)–HEDP concentrations the model
predicts that competition between the free Cu(II) and the free
HEDP for adsorption sites is not important.
alumina 1 g/L, I = 0.1 M NaNO3. Total concentration HEDP and Cu(II):
d In
Fig. 2. Adsorption of HEDP and Zn(II) onto alumina as a function of pH. Aged alumina 1 g/L, I = 0.1 M NaNO3. Total concentration HEDP and Zn(II): (a, b)
5 × 10−4 M; (c, d) 1 × 10−4 M; 5 × 10−5 M.
Figs. 1b, 1d and 1f show adsorption of Cu(II) alone and
Cu(II) together with HEDP. HEDP has an interesting effect on
Cu(II) adsorption. At high surface covering Cu(II) adsorption
is enhanced at low pH, but the presence of HEDP suppresses
the amount of Cu(II) adsorbed at pH > 6 compared to that ad-
sorbed when only Cu(II) is present. The predominance of the
anionic CuL− species in solution with the increase of the pH
limits the amount of Cu(II) adsorbed at the surface at higher
pH, whereas, in the absence of HEDP the Cu(II) should be
adsorbed (or precipitated). The model predicts a ternary sur-
face complex (≡AlLCu−) to justify the increase of the Cu(II)
adsorbed at lower pH. In a previous work we found a simi-
tion of Cu(II) on goethite at low pH and a suppressor effect at
pH > 5. Alí et al. found a similar trend in the adsorption of
Cu(II) onto goethite by the presence of simple organic acids at
the higher organic acids concentrations [29]. At lower concen-
trations, the Cu(II) adsorption is less affected by the presence
of HEDP, so the adsorption edges of Cu adsorbed when Cu and
HEDP are together exhibit similar profiles to those where Cu
is alone (Figs. 1d and 1f). So, the results obtained at low con-
centrations are in agreement with those reported by Nowack
and Stone [30]. In those conditions the invocation of ternary
surface complexes was not necessary. The model predicts that
at 5 × 10−4 M Cu(II)–HEDP, practically the total amount of
320 L. Hein et al. / Journal of Colloid an
lar effect in the Cu adsorption onto boehmite by the presence
of this phosphonate [26]. Sheals et al. [28] also reported that
the presence of glyphosate produces an increase in the adsorp-
terface Science 314 (2007) 317–323
Cu(II) adsorbed at low pH is from anionic-type ternary sur-
face complexes. However, at lower concentrations, 1 × 10−4
and 5 × 10−5 M Cu(II)–HEDP, the adsorption model predicts
d In
). (c
u(I
an important decrease in the ternary surface complexes being
the free HEDP and Cu(II) the species preferentially absorbed
on the surface.
3.2. Zn(II)–HEDP adsorption
Figs. 2a, 2c and 2e displays the adsorption of HEDP on alu-
mina as a function of pH without Zn and in the presence of
equimolar concentrations of Zn. In the absence of HEDP, Zn
adsorbs or precipitates from solution at pH� 8, however, when
HEDP is present the Zn either is adsorbed or remains in solu-
tion as Zn–HEDP. Fig. 2a shows that at intermediate pH and
high surface covering the HEDP adsorption is lightly favored
by the presence of Zn, while a pronounced increment in the Zn
adsorption is observed by the presence of HEDP (Fig. 2b). In
both cases a good fit with the experimental results could be ob-
tained. Stability constant values for Zn–HEDP and Cu–HEDP
are responsible for the different behavior presented by Zn and
Cu adsorption in the presence of HEDP. Fig. 3 shows the so-
lution speciation of HEDP in the presence of Cu(II) and Zn(II)
solution and the corresponding surface species at 5 × 10−4 M
Me–HEDP. It can be clearly seen that the ternary surface com-
plexes formation is more important for Zn(II) than for Cu(II).
Since, HEDP is a more effective chelating agent for Cu(II) than
for Zn(II) the competitive effect is higher than for the former.
On the other hand, at the lower pH and at high Zn(II) con-
centration the presence of equimolar concentration of HEDP
causes a discernible increase in the amount of Zn(II) adsorbed
(Fig. 2b). However, HEDP has not a suppressor effect on the
Zn(II) adsorption at the higher pH. The presence of citrate also
produces an increase in the Zn adsorption in soils being this
effect more marked in acid soils [31]. Again, the presence of
ternary surface complexes is here necessary to reach a good fit
with the experimental results. Furthermore, the model predicts
very well the maximum observed at intermediate pH through
the ligand-like complexes. The effect of HEDP over Zn(II) ad-
sorption becomes less pronounced with the excess of surface
sites (Figs. 2d and 2f). Hence it can be concluded, from both
the model and the experimental results, in excess of surfaces
sites the adsorption of free Zn(II) will be favored as compared
with that of the Zn(II) complex.
3.3. Cd(II)–HEDP adsorption
Cd(II)–HEDP solution complexes are weaker than those cor-
responding to Cu(II) and Zn(II), so competitive effects between
the surface and the solution are much less significant in com-
parison to Cu(II)–HEDP and Zn(II)–HEDP alumina systems.
Fig. 4 effectively shows a significant effect of Cd(II) adsorp-
tion by the presence of equimolar concentration of HEDP. This
increase in the Cd adsorption at low pH can be justified as-
L. Hein et al. / Journal of Colloid an
Fig. 3. (a, b) Speciation of HEDP in solution in the presence of Cu(II) and Zn(II
the fraction of HEDP found in respective solution and surface species. [HEDP–C
At low surface covering, however, the presence of equimolar
concentrations of Zn had no discernible effect on HEDP ad-
sorption.
terface Science 314 (2007) 317–323 321
, d) Surface speciation of HEDP in the presence of Cu(II) and Zn(II). FHEDP is
I)] 5 × 10−4 M.
suming the metal adsorption by the anionic type surface ternary
complexes. Other authors [32,33] also invoked ternary surfaces
complexes to describe the effects of organic ligands on Cd(II)
d In
ged
• A possible interference in the removal of Me(II) by the
presence of phosphonate could occur when the logK for
Me(II)–HEDP complex formation in solution is high. Sig-
nificant Cu(II)–HEDP complexation in solution explains
the decrease in Cu(II) adsorption at high pH and high
HEDP concentrations.
• The Me(II)–HEDP concentration and the available surface
sites ratio is an important factor in the removing of metal
ions from solution. This effect explains the differences in
the adsorption edges between boehmite and alumina at the
lower Me(II)–HEDP concentrations.
• The extension of HEDP can significantly modify the mo-
bility of metals in an aqueous environment. Under acidic
conditions, an enhancement in metal adsorption is noted,
lowering the metal mobility from that in the absence of this
diphosphonate. However, at higher HEDP concentrations
[7] L. Charlet, P.W. Schindler, L. Spadini, G. Furrer, M. Zysset, Aquat. Sci. 55
(1993) 291.
[8] W. Rudzin´ski, W. Piasecki, W. Janusz, G. Panas, R. Charmas, Adsorp-
tion 7 (2001) 327.
[9] C.-P. Huang, W. Stumm, J. Colloid Interface Sci. 43 (1973) 409.
[10] H. Hohl, W. Stumm, J. Colloid Interface Sci. 55 (1976) 281.
[11] S.-F. Cheah, G.E. Brown Jr., G.A. Parks, J. Colloid Interface Sci. 208
(1998) 110.
[12] E. Morillo, T. Undabeytia, C. Maqueda, Environ. Sci. Technol. 31 (1997)
3588.
[13] B.C. Barja, M.I. Tejedor-Tejedor, M.A. Anderson, Langmuir 15 (1999)
2316.
[14] E. Laiti, L.O. Öhman, J. Nordin, S. Sjöberg, J. Colloid Interface Sci. 175
(1995) 230.
[15] E. Laiti, L. Öhman, E. Laiti, L. Öhman, J. Colloid Interface Sci. 183 (1996)
441.
[16] B. Nowack, A.T. Stone, J. Colloid Interface Sci. 214 (1999) 20.
[17] A.T. Stone, M.A. Knight, B. Nowack, in: R.L. Lipnick, R.P. Mason, M.L.
Phillips, C.U. Pittman Jr. (Eds.), Chemicals in the Environment: Fate,
322 L. Hein et al. / Journal of Colloid an
Fig. 4. Adsorption of HEDP and Cd(II) onto alumina as a function of pH. A
5 × 10−5 M.
interactions with aluminum oxides. At pH 7 an increase of
about 40% in the Cd removing is observed by the presence
of phosphonate, while Cd does not influence on the phospho-
nate adsorption within experimental error. Venema et al. [34],
applying the charge distribution multisite complexation model
(CD-MUSIC) reported the positive influence of phosphate on
Cd adsorption. In agreement with our results for phosphonate,
phosphate is not affected by de presence of Cd.
4. Summary
• The CC model is able to describe data sets of both simul-
taneous and single adsorption of Cu(II), Zn(II), Cd(II) and
HEDP with the same parameters.
• The model predicts the formation of ternary surface com-
p
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