Arsenate and phosphate adsorption on ferrihydrite nanoparticles.
Synergetic interaction with calcium ions
Juan Antelo
,
, Florencio Arce
, Sarah Fiol
a
Department of Soil Science and Agricultural Chemistry, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain
b
Department of Physical Chemistry, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 8 January 2015
Received in revised form 8 June 2015
Accepted 9 June 2015
Available online 12 June 2015
Keywords:
Ferrihydrite
Phosphate
Arsenate
Calcium
Surface complexation modelling
CD model
The geochemical behaviour of phosphate and arsenate ions in soil and aquatic systems is determined by the pres-
ence of mineral surfaces and major ions. Information about the distribution of oxyanions over the solid and solu-
tion phases is essential for understanding the transport, bioavailability and toxicity of these compounds in the
environment. Here, we studied the adsorption of both arsenate and phosphate on ferrihydrite nanoparticles in
the presence of calcium ions. The presence of calcium ions enhanced the retention of these oxyanions on ferrihy-
drite and vice versa. The arsenate
–calcium and phosphate–calcium multi-component systems were described
using a mechanistic surface complexation model. Use of this type of model enables prediction of the solution
and surface speciation, along with analysis of oxyanion mobility in relation to environmental conditions. We
were able to calibrate the charge distribution model with the macroscopic data obtained for the single-
component systems, thus obtaining surface complexation constants for later use to simulate multi-component
systems. The mutual interactions between arsenate and calcium were successfully described with these param-
eters, indicating that changes in the electrostatic forces at the solid/solution interface caused the observed
enhanced adsorption. However, adsorption in the phosphate
–calcium system was underestimated with the
parameters obtained for the single-component systems, indicating that additional mechanisms or processes
should be considered. Formation of insoluble mineral phases was ruled out, but the inclusion of a phosphate
–
calcium ternary surface complex improved the modelling predictions.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
The presence of phosphorus and arsenic in surface waters and
groundwaters is considered a major environmental problem world-
wide. Phosphorus is an essential nutrient for plant growth in soils
and often limits algal growth and eutrophication in surface waters
(
). Although phosphorus in freshwaters is not consid-
ered to be directly toxic to humans and animals, elevated concentra-
tions can adversely affect the ecosystems and ultimately produce
anoxic conditions. Usually the contamination of surface waters by
phosphorus is caused by a combination of over-fertilization and
agricultural land runoff, which may lead to concentrations above
the recommended limit proposed by USEPA, 0.1 mg/l. On the other
hand, arsenic is a very toxic element that may be present at high con-
centrations in freshwater environments due to weathering processes
or to anthropogenic sources as mining and agricultural activities
(
). The major health and ecological
issues of arsenic are related to its carcinogenicity, phytotoxicity and
biotoxicity. Usually the concentration of arsenic in natural waters is
below the guideline value proposed by the World Health Organization
for drinking water, 10
μg/l (
), although concentrations
above this value are not uncommon and its concentration can reach
up to hundreds of mg/l in systems affected by acid mine drainage
(
Nordstrom, 2002; Smedley and Kinniburgh, 2002
). The chemical be-
haviour of phosphorus and arsenic is similar and both elements tend
to form analogous species such as phosphate and arsenate; however,
the biogeochemical behaviour of these elements is different. Thus,
although phosphate can be considered as a macronutrient, arsenate is
very toxic at low concentrations.
For a better knowledge of how these two species are distributed be-
tween the solid and solution phases in natural systems, it is essential to
understand how they interact with the reactive constituents present in
these systems. Both arsenate and phosphate display a relatively strong
af
finity for iron mineral oxides (
Raven et al., 1998; Gao and Mucci,
2003; Antelo et al., 2005; Kanematsu et al., 2010; Carabante et al.,
2012
). Ferrihydrite is one of the most reactive and most common natu-
rally occurring iron oxides. Its occurrence has been reported at near-
neutral pH conditions in a variety of redox-active environments, such
as soils and sediments, or freshwater and marine settings (
). The oxidation and dissolution of Fe-bearing sulphide
Chemical Geology 410 (2015) 53
⁎ Corresponding author.
E-mail address:
(J. Antelo).
http://dx.doi.org/10.1016/j.chemgeo.2015.06.011
0009-2541/© 2015 Elsevier B.V. All rights reserved.
Contents lists available at
Chemical Geology
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h e m g e o
minerals associated with mine wastes and acid mine drainage also re-
sult in the formation of ferrihydrite, along with other secondary iron ox-
ides. Due to its abundance, large surface area and high reactivity it is
considered a key mineral for metal sequestration and plays a crucial
role in the cycling of iron and trace elements in natural systems. The rel-
evance of this iron hydroxide has been made evident from its ability to
adsorb different contaminants on its surface and to control the transport
of ions in soils and aqueous systems (
A better understanding of the adsorption of oxyanions on this iron
oxide has become increasingly important in recent years (
et al., 2010; Das et al., 2011; Wang et al., 2013; Neupane et al., 2014;
Zhu et al., 2014
). Retention of oxyanions can be affected by the presence
of other ions, which may enhance or suppress the adsorption process. A
good knowledge of the interactions and effects of co-existing elements
(e.g. calcium) in soils and aquatic systems is essential for modelling
and predicting the mobility of phosphate and arsenate. The presence
of positively charged ions such as calcium may enhance the adsorption
of negatively charged ions such as phosphate or arsenate on mineral
surfaces above pH 7 (
Rietra et al., 2001; Stachowicz et al., 2008; Arai,
). Most of the aforementioned studies
focus on goethite, which has become a model mineral surface for ad-
sorption studies. Limited studies can be found in the literature devoted
to integration of macro- and microscopic information to obtain a de-
tailed mechanistic description of the competitive and synergetic effects
found in multi-component systems containing ferrihydrite (
2003; Swedlund et al., 2003; Tiberg et al., 2013; Neupane et al., 2014
).
As stated above, information about the enhancement of arsenate and
phosphate adsorption on some iron oxides and clays due to the pres-
ence of calcium can be found in the literature, however discussion
about multi-component systems simultaneously involving environ-
mentally relevant concentrations of calcium, phosphate or arsenate
and ferrihydrite is not available to our knowledge. Thermodynamic de-
scription of the surface reactivity of ferrihydrite is crucial for developing
models that predict the fate of geochemically and environmentally
relevant species in settings where this mineral oxide is formed.
In this study, we investigated the effect of calcium on the adsorption
of arsenate and phosphate on 2-line ferrihydrite nanoparticles. We
studied the binding processes for arsenate, phosphate and calcium
separately, prior to investigating arsenate and phosphate adsorption
in the presence of calcium ions. Arsenate, phosphate and calcium con-
centrations in this study have been selected to simulate the levels that
these species can reach in polluted systems, such as waters affected by
mining activities or settings under the in
fluence of intense agricultural
activities. Furthermore, the arsenate concentrations employed enable
direct comparison with phosphate data, which have been taken for
the single-component system from
. We also
evaluated the capacity of the Charge Distribution (CD) model to pre-
dict the behaviour of these oxyanions in multi-component systems.
Finally, we highlight the importance of obtaining surface parame-
ters for thermodynamic description of the adsorption behaviour of
ferrihydrite for future application of modelling parameters to
field
data.
2. Materials and methods
2.1. Reagents and materials
All chemicals were of Merck p.a. quality, and the water used in the
experiments was ultrapure (Milli-Q water-puri
fication system; resistiv-
ity 18 M
Ω cm at 25 °C) and CO
2
free. A-grade glassware was used in the
preparation of stock solutions. Plastic material was used for ferrihydrite
synthesis and adsorption experiments in order to avoid silica contami-
nation. In addition, all glassware and plastic materials were washed
with 10% HNO
3
and rinsed with ultrapure water to prevent contamina-
tion by metals.
2.2. Ferrihydrite synthesis and characterization
Two-line ferrihydrite was synthesized following the procedure pro-
posed by
Cornell and Schwertmann (1996)
. Brie
fly, a 0.1 M Fe(NO
3
)
3
solution was adjusted to pH 8.0 through dropwise addition of a 1 M
NaOH solution under continuous stirring and N
2
bubbling. The ferrihy-
drite dispersion thus obtained was aged at 20 °C for 48 h and then dia-
lyzed until the conductivity was lower than 10
μS/cm. The dispersion
was then freeze-dried to obtain a dry powder. Information found in
the literature indicates that no mineralogical alterations are produced
on mineral oxides after freeze-drying (
).
The ferrihydrite sample was previously characterized by powder X-
ray diffraction and transmission electron microscopy using a Phillips
PW1710 diffractometer and a Philips CM-12 microscope, respectively
(
). The diffractogram revealed two broad peaks,
which con
firm the presence of 2-line ferrihydrite, while microscopic
images showed that ferrihydrite particles were heavily aggregated.
The surface area, measured by N
2
adsorption with an ASAP 2000
V3.03 Micromeritics instrument, was 229 m
2
/g. This value is in agree-
ment with surface areas reported by
Cornell and Schwertmann (1996)
for different ferrihydrite samples. As the surface area values obtained
by BET measurements are of limited reliability for ferrihydrite, the
actual surface area of the ferrihydrite sample was obtained by compar-
ison of the surface charge determined by modelling calculations and the
experimental surface charge (
). The surface area
obtained by this method (350 m
2
/g) (
) is lower than
the theoretical values proposed for fresh ferrihydrite (range 600
–
750 m
2
/g) (
). The point of zero charge
(PZC) was previously determined by potentiometric titrations (8.7 ±
0.1) (
). Although the value is slightly higher than
those usually reported for freshly precipitated ferrihydrites, it is consis-
tent with other PZC values reported in the literature (
Hofmann et al., 2005; Wang et al., 2013
). Freshly precipitated and not
rigorously de-carbonated ferrihydrite samples show consistently low
PZC values (7.9
–8.1), whereas dialyzed and N
2
-purged samples after
synthesis tend towards higher values (8.6
–8.7).
2.3. Arsenate adsorption to ferrihydrite
Arsenate adsorption isotherms for ferrihydrite were obtained at
pH 4.5, 7.0 and 9.0 to cover a wide range of pH and to allow comparison
with phosphate adsorption data. For this purpose, batch experiments
were conducted with 1 g/l suspensions in 20 ml of 0.1 M KNO
3
. For
each data point, an arsenate solution (KH
2
AsO
4
) of the desired initial
concentration was prepared at the desired pH and ionic strength. The
solution was then added to the ferrihydrite suspension and the pH
was adjusted by addition of 0.1 M HNO
3
or KOH solutions. The KOH so-
lution was freshly prepared for each experiment from a non-carbonated
stock solution and maintained under N
2
atmosphere. Preliminary kinet-
ic experiments showed that a contact time of 24 h was required to
achieve equilibrium. During the equilibration period, the samples
were continuously shaken with a magnetic stirrer, and the pH was peri-
odically measured and readjusted, where necessary. Special care was
taken to prevent the presence of CO
2
, by maintaining the ferrihydrite
suspensions in an N
2
atmosphere. After 24 h, the pH was recorded and
the ferrihydrite suspensions were
filtered through 0.22 μm membrane
filters. A colorimetric method was used to determine the concentration
of arsenate in solution by UV
–visible spectroscopy (
),
and the amount adsorbed was calculated as the difference between the
total arsenate concentration and the arsenate concentration in the su-
pernatant. To ensure that no ferrihydrite particles were present in the
final filtered solution, aliquots of the filtered solution were acidified
with a 1 M solution of HNO
3
, and the iron concentration in solution
was measured after 24 h by atomic absorption spectroscopy (Perkin
Elmer 1100B). The results showed that iron concentrations were
below the detection limit (
b0.01 mg/l).
54
J. Antelo et al. / Chemical Geology 410 (2015) 53
–62
Adsorption envelope experiments were also carried out to deter-
mine the amount of arsenate adsorbed to the ferrihydrite surface as a
function of pH (within the range 4.0
–10.0 in 0.5–1.0 pH increments)
and ionic strength (0.01, 0.1 and 0.5 M in KNO
3
). Arsenate and ferrihy-
drite were mixed in 50 ml polyethylene bottles to provide a
final
arsenate concentration of 0.60 mM and a ferrihydrite concentration of
1 g/l. Separate bottles were used for each pH value studied. The pH of
the suspensions was adjusted and any change in the pH during the
experiment was corrected by adding small volumes of 0.1 M HNO
3
or
KOH. The samples were shaken for 24 h, and the pH values were record-
ed at the end of the experiment. Arsenate concentration in solution was
measured following the above-mentioned procedure.
Each experiment was carried out in duplicate (at least) to con
firm
the reproducibility (uncertainty
b2%). Polyethylene flasks were used
in the adsorption experiments in order to avoid contamination by
silicates, and the temperature was maintained at 25 ± 1 °C in all exper-
iments. The pH measurements were conducted after the combined glass
electrode (Radiometer GK-2401C) was calibrated with standard buffer
solutions.
2.4. Calcium adsorption to ferrihydrite
A similar procedure to that described above was carried out to obtain
the calcium adsorption edges on ferrihydrite. Batch experiments were
conducted at different pH values in separate vials with 1 g/l ferrihydrite
suspensions in 20 ml of 0.1 M KNO
3
. A calcium solution (0.1 M Ca(NO
3
)
2
) was added to the suspensions to yield three different concentrations
(0.5, 2 and 6 mM), and the pH of the suspensions was adjusted to the
desired pH value (within the range 4.0
–10.5 in 0.7–1.0 pH increments)
by addition of 0.1 M HNO
3
or KOH solutions. During the 24 h equilibra-
tion period, the suspensions were continuously shaken and the pH was
periodically measured and readjusted, when necessary. Special care was
taken to prevent the presence of CO
2
, by maintaining the ferrihydrite
suspensions in an N
2
atmosphere. The pH was registered after 24 h
and the suspensions were then
filtered through 0.22 μm membrane fil-
ters. The concentration of calcium in solution was measured by induc-
tively coupled plasma optical emission spectrometry (ICP-OES)
(Optima 3300 DV, Perkin Elmer). The amount of calcium adsorbed
was calculated as the difference between the concentration of calcium
initially added to the suspensions and the concentration measured in
solution at the end of the experiment.
2.5. Adsorption experiments in multi-component systems
Phosphate and arsenate adsorption experiments were carried out in
the presence of calcium at different pH (5.0
–10.0) and constant ionic
strength. A constant initial concentration (0.6 mM) of phosphate or ar-
senate was used at each pH, whereas different volumes of calcium solu-
tion were added to the suspensions to yield an initial concentration of
calcium between 0.3 mM and 6 mM. Additionally, adsorption experi-
ments with initial arsenate concentrations of 0.1, 0.3, 0.45 and 0.8 mM
were conducted at two calcium loadings, 0.7 and 6 mM. Appropriate
volumes of distilled water and 1 M KNO
3
solution were added to ensure
an ionic strength of 0.1 M in all experiments. The
final volume in the
suspensions for each data point was 20 ml. To minimize precipitation
of calcium phosphate or calcium arsenate, calcium was added to ferrihy-
drite suspensions (1 g/l) that had been pre-equilibrated at the desired
pH with phosphate or arsenate for 2 h. Any change in pH after the
mixing was corrected by addition of 0.1 M HNO
3
or KOH solutions.
The suspensions were equilibrated for 24 h, and the pH was periodically
monitored and readjusted, when necessary, during the experiment.
After the equilibration period, the suspensions were
filtered and the
solutions were analysed for phosphate, arsenate and calcium. The
phosphate concentration was measured spectrophotometrically by the
molybdenum blue method (
), whereas the
concentrations of arsenate and calcium were determined by the
above-mentioned methods. The concentration of adsorbed ions was cal-
culated as the difference between the initial and
final concentrations in
the solution.
2.6. CD model
Application of surface complexation models (SCMs) to describe the
adsorption behaviour of ferrihydrite may be challenging because of
the limited and contradictory information about the crystalline struc-
ture of ferrihydrite. It was recently shown that both the charging behav-
iour and ion adsorption could be modelled for ferrihydrite by using
goethite as a proxy (
Hiemstra and van Riemsdijk, 2009; Hiemstra
et al., 2009; Villalobos and Antelo, 2011; Tiberg et al., 2013
). Among
many SCMs that have been applied so far, the CD model has become
the most popular for describing the surface reactivity of crystalline
and amorphous mineral oxides (
Hiemstra and van Riemsdijk, 1996;
Hiemstra and van Riemsdijk, 2006
). This model combined with the Ex-
tended Stern concept for the description of the solid/solution interface
has been used to describe the experimental data and to provide a micro-
scopic interpretation of adsorption in the single- and multi-component
systems.
Optimization of the parameters required for describing the arsenate,
phosphate and calcium binding to the ferrihydrite surface with the CD
model was carried out with the ECOSAT (
) speciation program and the FIT program (
).
The solution species and their equilibrium constants were formulated
as summarized in Table S1 (Supplementary data). Surface complexation
parameters speci
fic for ferrihydrite and for the solid/solution interface
(such as surface area, site density, capacitance or protonation constants)
are shown in
3. Results and discussion
3.1. Arsenate adsorption
shows the arsenate adsorption isotherms in 0.1 M KNO
3
at dif-
ferent pH values and the adsorption envelopes at the different ionic
strengths for an initial arsenate concentration of 0.6 mM. The
figure
also illustrates the CD model predictions and the arsenate surface speci-
ation as a function of pH, according to the modelling calculations. No
difference was observed between adsorption on freeze-dried ferrihy-
drite or ferrihydrite stored as a wet paste (Fig. S1, Supplementary
data). Arsenate adsorption decreased gradually and continuously as
the pH of the systems increased, as expected from previous
findings re-
garding oxyanion adsorption on amorphous and crystalline iron oxides
(
Salazar-Camacho and Villalobos, 2010; Villalobos and Antelo, 2011;
). The point of zero charge of the ferrihydrite
particles is 8.7, indicating that at low pH the iron hydroxyl groups are
mainly positively charged, which will favour the interaction (via ligand
exchange) between the arsenate ions and the iron surface groups.
The effect of ionic strength is rather low at pH
b 5.0, whereas an in-
crease in the ionic strength at pH
N 5.0 produced an increase in the ad-
sorption of arsenate ions. This effect, previously observed for phosphate
binding on goethite and ferrihydrite (
Rahnemaie et al., 2007; Antelo
), is usually attributed to changes in the electrostatic poten-
tial at the solid/solution interface. An increase in the ionic strength pro-
duces a decrease in the electrostatic repulsion between the charged
surface and the arsenate ions, favouring the adsorption process at rela-
tively high pH. Ions that form inner-sphere complexes, such as arsenate
ions, are directly coordinated to surface hydroxyl groups and may not
compete, or may compete relatively weakly with electrolyte ions.
As stated above, arsenate adsorption on ferrihydrite was simulated
using the CD model, which requires molecular scale information to con-
strain the nature and the charge distribution of the surface complexes
formed at the solid/solution interface. Previous spectroscopic and mo-
lecular studies have provided very useful information about the identity
55
J. Antelo et al. / Chemical Geology 410 (2015) 53
–62
of the surface species that arsenate ions form on iron oxide surfaces.
Studies by
and by
indicated that bidentate surface complexes are thermodynami-
cally and kinetically favoured over monodentate surface complexes.
The EXAFS study conducted by
con
firmed that arse-
nate is predominantly bound to iron oxide surfaces as bidentate corner
sharing complexes, regardless of the nature of the iron oxide mineral
(goethite, ferrihydrite, lepidocrocite, or maghemite). The recent litera-
ture reports some controversy about the nature of the arsenate surface
complexes, especially after the thorough study of
.
These authors combined EXAFS information with data obtained by IR
spectroscopy, and they suggested that monodentate coordination may
be the dominant geometry for arsenate binding on iron oxides.
In the present study, two bidentate surface complexes (protonated
and non-protonated) were considered in the modelling calculations
conducted to describe the adsorption of arsenate on ferrihydrite. As a
simpli
fication, it was assumed that all the singly coordinated groups
present in ferrihydrite show the same reactivity against arsenate ions,
and no distinction was made between corner sharing and edge sharing
surface complexes. A second modelling scenario, which included the
formation of singly-protonated monodentate surface complexes, was
also considered, as these complexes may also occur at the ferrihydrite
surface. Neither scenario considered the formation of surface ion-pair
complexes, i.e. K
–AsO
4
, since their inclusion does not improve the
modelling predictions (
). The surface reactions
for the three surface complexes considered can be formulated as
follows:
≡2FeOH
−1=2
þ 2H
þ
þ AsO
4
−3
⇆ ≡Fe
2
O
2
−1þΔz
0
AsO
2
Δz
1
þ 2H
2
O
ð1Þ
≡2FeOH
−1=2
þ 3H
þ
þ AsO
4
−3
⇆ ≡Fe
2
O
2
−1þΔz
0
AsOOH
Δz
1
þ 2H
2
O
ð2Þ
≡FeOH
−1=2
þ 2H
þ
þ AsO
4
−3
⇆ ≡FeO
−1=2þΔz
0
AsO
2
OH
Δz
1
þ H
2
O
ð3Þ
The only parameters allowed to vary in the
fitting were the com-
plexation constants of the arsenate surface species. The complexation
constants previously obtained by
for the ad-
sorption of arsenate on goethite were used as initial estimates. The
charge distribution coef
ficients Δz
0
and
Δz
1
were also taken from the
same study. These values may not be very accurate because they were
derived from molecular orbital and density functional theory calculations
and were corrected to take into account the electrostatic dipole effect
induced by the introduction of charge at the solid/solution interface.
A relatively good
fit to the experimental results was achieved by
optimizating the complexation constants of the protonated and
non-protonated bidentate complexes (
a and
b). The sur-
face complexation parameters for arsenate are shown in
. Al-
though these results are consistent with the spectroscopic evidence
of the predominance of bidentate surface complexes, the presence
of protonated monodentate surface complexes was also considered.
Inclusion of this complex, along with the bidentate complexes, did
not signi
ficantly improve the modelling predictions (see r
2
values in
). The
fitting results showed that we were not able to distinguish
between both model scenarios, possibly due to the predominance of the
non-protonated bidentate surface complex in both model options. As
the main objective was to achieve good predictions of the experimental
data for arsenate and then to analyse the suitability of the complexation
constants in a multi-component system including arsenate and calcium,
no additional calculations or efforts were conducted to identify the
exact nature of the surface complexes occurring at the ferrihydrite
surface.
c shows the abundance of arsenate surface species as a function
of pH for an arsenate loading of 0.6 mM at 0.1 M ionic strength accord-
ing to the CD model. Under these conditions, the dominant surface
species at intermediate to high pH is the non-protonated bidentate
complex. At the lower pH values, the protonated bidentate complex
contributes less to the arsenate adsorption than the non-protonated
bidentate complex, although it makes a greater contribution at pH
b 3.
The bidentate complexes predominate when the protonated
monodentate complex is considered (Fig. S2, Supplementary
data). The inclusion of the monodentate complex accounts for ~15% of
the total arsenate adsorbed at acidic pH values and its contribution de-
creases with the increase of pH. On the other hand, for the protonated
surface complexes the ratio between monodentate and bidentate com-
plexes decreases when the arsenate loading increases. These results are
partially in agreement with the
findings of
,
who found that the contribution of the monodentate complex only
accounted for 30% of the adsorbed arsenate at the most favourable
conditions. The relative abundance of the monodentate surface com-
plex does not change with ionic strength, whereas the abundance of
bidentate complexes is ionic strength dependent (Fig. S3, Supplementa-
ry data).
Finally, a comparison can be established between arsenate and
phosphate adsorption on ferrihydrite. Considering the similar chemical
behaviour shown by both oxyanions, a similar af
finity on ferrihydrite
particles may be expected. The adsorption observed for both oxyanions
con
firms this behaviour (Fig. S4, Supplementary data). On the other
hand, the surface complexation constants calculated for the oxyanions
are comparable (
and option I in
). The slight differences
between these constants are consistent with the differences found in
the solution chemistry. According to the thermodynamic data available
for aqueous speciation of arsenate and phosphate (Table S1, Supple-
mentary data), protonation and ion-pair formation constants are
systematically higher for the phosphate species than for the arsenate
analogues. The same tendency can be observed for the surface complex-
ation constants.
Table 1
Surface species and CD model parameters for H
+
, K
+
, NO
3
−
and PO
4
−3
binding to ferrihydrite, estimated using the Extended Stern layer model and considering C
1
= 0.74 F/m
2
and C
2
=
0.93 F/m
2
.
Δz
0
,
Δz
1
, and
Δz
2
represent the change in the charge (or charge distribution) in the 0-, 1-, and 2-planes, respectively.
Surface reactions
`FeOH
`Fe
3
O
Δz
0
Δz
1
Δz
2
H
+
K
+
NO
3
−
PO
4
−3
log K
`FeOH
−1/2
1
0
0
0
0
0
0
0
0
0.00
`FeOH
2
+1/2
1
0
+1
0
0
1
0
0
0
8.70
`FeOH
−1/2
⋯K
+
1
0
0
+1
0
0
1
0
0
–1.16
`FeOH
2
+1/2
⋯NO
3
−
1
0
+1
–1
0
1
0
1
0
7.74
`Fe
3
O
−1/2
0
1
0
0
0
0
0
0
0
0.00
`Fe
3
OH
+1/2
0
1
+1
0
0
1
0
0
0
8.70
`Fe
3
O
−1/2
⋯K
+
0
1
0
+1
0
0
1
0
0
–1.16
`Fe
3
OH
+1/2
⋯NO
3
−
0
1
+1
–1
0
1
0
1
0
7.74
`Fe
2
O
2
PO
2
−2
2
0
+0.46
–1.46
0
2
0
0
1
27.78
`Fe
2
O
2
POOH
−
2
0
+0.63
–0.63
0
3
0
0
1
32.09
Surface site density of the singly and triply coordinated groups is set to N
s,1
= 6 sites/nm
−2
and N
s,3
= 1.2 sites/nm
−2
, respectively. Surface area is set to 350 m
2
/g. Values for the surface
complexation constants were taken from
.
56
J. Antelo et al. / Chemical Geology 410 (2015) 53
–62
3.2. Calcium adsorption
shows that calcium adsorption on ferrihydrite only occurs at
pH
N 8.5. An almost horizontal edge was observed for the three calcium
loadings, indicating that no adsorption takes place within the pH range
4.0
–8.5. A notable decrease in the calcium concentrations in solution
only was observed beyond pH 8.5. This pH is close to the PZC (8.7)
and as the mineral surface develops an increasingly negative charge at
pH
N PZC, attractive electrostatic interactions will favour calcium ion ad-
sorption. The adsorption of calcium is relatively weak compared with
that of heavy metals such as copper, lead, and zinc and is consistent
with the
findings reported by
. These au-
thors observed a correlation between the hydrolysis constant and the
surface complexation constant. According to this correlation, the diva-
lent cation Ca
+2
, which has a relatively low hydrolysis constant
(Table S1), is expected to adsorb weakly onto the surface of amorphous
and crystalline iron oxides. The relatively low af
finity that iron oxides
show for calcium ions was also observed by
when using goethite as adsorbent material (SSA 98 m
2
/g; PZC 9.2).
More recently,
conducted adsorption experi-
ments at two calcium loadings with a porous adsorbent material that
contained ~90% nanogoethite (SSA 158.1 m
2
/g; PZC 8.5). These authors
observed calcium adsorption above pH 8.0, and adsorption only reached
100% at the lower loading, 0.03 mM.
To describe the adsorption of calcium on ferrihydrite using the CD
model, as previously discussed, molecular scale information is needed.
As far as we are concerned, no spectroscopic information is available
with respect to the nature of the calcium surface complexes occurring
on iron oxides. Nevertheless, surface speciation may be inferred through
those studies that applied realistic surface complexation models, such as
the CD model or the extended triple layer model, to the adsorption of
calcium ions onto goethite (
Stachowicz et al., 2008; Hiemstra et al.,
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
2
4
6
8
10
12
pH
[A
s
O
4
]
ad
s
(
mmo
l/
l)
I = 0.5 M
I = 0.1 M
I = 0.01 M
-4.5
-4.1
-3.7
-3.3
-2.9
-8.0
-7.0
-6.0
-5.0
-4.0
-3.0
-2.0
log [AsO
4
]
sol
(mol/l)
log [
A
s
O
4
]
ad
s
(
m
o
l/l)
pH = 4.5
pH = 7.0
pH = 9.0
0
20
40
60
80
100
3
4
5
6
7
8
9
10
pH
% A
s
O
4
s
p
e
c
ie
s
a
b
c
Fe
2
O
2
AsO
2
Fe
2
O
2
AsOOH
HAsO
4
AsO
4
H
2
AsO
4
Fig. 1. Arsenate adsorption on ferrihydrite: (a) adsorption isotherms for 0.1 M KNO
3
;
(b) adsorption envelopes at initial arsenate concentration of 0.6 mM; (c) surface (dashed
lines) and solution (dotted lines) speciation for an arsenate loading of 0.6 mM according to
the CD model predictions. Solid lines represent the CD model simulations based on option
I scenario (bidentate surface complexes).
Table 2
Surface species and complexation constants estimated with the CD model for arsenate binding on ferrihydrite.
Surface species
`FeOH
`Fe
3
O
Δz
0
Δz
1
Δz
2
H
+
AsO
4
−3
log K
Option I: bidentate surface complexes
`Fe
2
O
2
AsO
2
−2
2
0
+0.47
–1.47
0
2
1
27.36 ± 0.07
`Fe
2
O
2
AsOOH
−
2
0
+0.58
–0.58
0
3
1
31.03 ± 0.45
r
2
= 0.914
Option II: bidentate and (protonated) monodentate surface complexes
`Fe
2
O
2
AsO
2
−2
2
0
+0.47
–1.47
0
2
1
27.35 ± 0.09
`Fe
2
O
2
AsOOH
−
2
0
+0.58
–0.58
0
3
1
30.81 ± 0.75
`FeOAsO
2
OH
−3/2
1
0
+0.30
–1.30
0
2
1
24.76 ± 1.02
r
2
= 0.916
Standard deviations are given for the
fitted parameters.
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
3
5
7
9
11
pH
[C
a
]
so
l
(
m
m
o
l/l)
2 mM Ca
0.5 mM Ca
6 mM Ca
Fig. 2. Effect of the pH on the equilibrium calcium concentration in solution after adsorp-
tion on ferrihydrite. Solid and dotted lines correspond to the results of the modelling
scenarios I and II, respectively.
57
J. Antelo et al. / Chemical Geology 410 (2015) 53
–62
). In the present study, two different sce-
narios were considered for the modelling of calcium adsorption using
the CD model. In the
first scenario, we considered the previous informa-
tion available for goethite, whereas some additional considerations
were assumed in the second scenario to improve the simulations for
the single-component and multi-component systems.
3.2.1. Modelling scenario I: calcium complexation constants for goethite
Different surface complexes have been postulated for the interaction
between calcium ions and synthetic and natural iron oxides (
et al., 2001; Rahnemaie et al., 2006; Stachowicz et al., 2008; Hiemstra
et al., 2010
). In these studies, the adsorption of calcium ions has been
described by combining bidentate and monodentate surface complexes
and considering the existence of both inner- and outer-sphere com-
plexes. In the present study, the adsorption of calcium on ferrihydrite
was initially explained by considering the calcium surface complexes
proposed by
for goethite. On the basis of previ-
ous EXAFS studies with strontium (
Axe et al., 1998; Sahai et al., 2000
),
these authors proposed two inner- and two outer-sphere monodentate
complexes. The same surface complexes have been used by
to describe phosphate behaviour on
field samples, although
the values of the surface complexation constants were slightly different.
In both cases, the total charge of calcium in the outer-sphere complexes
was located at the 1-plane, and no charge was assigned to the 0-plane.
Awareness should be taken on the risk of using a large number of
surface complexes in the modelling that might lead to a meaningless
description of the system. This situation can be especially relevant in
the case of species, such as calcium, that show low af
finity for the min-
eral surface. The model simulations are less sensitive to changes in the
surface complexation constants and this situation becomes more
noticeable when only one dataset is available.
The surface reactions for calcium can be formulated as follows:
≡FeOH
−1=2
þ Ca
þ2
⇆ ≡FeOH
−1=2þΔz
0
Ca
Δz
1
ð4Þ
≡FeOH
−1=2
þ H
2
O
þ Ca
þ2
⇆ ≡FeOH
−1=2þΔz
0
CaOH
Δz
1
þ H
þ
ð5Þ
≡FeOH
−1=2
þ Ca
þ2
⇆ ≡FeOH
−1=2
Ca
Δz
1
ð6Þ
≡Fe
3
O
−1=2
þ Ca
þ2
⇆ ≡Fe
3
O
−1=2
CaOH
Δz
1
þ H
þ
ð7Þ
As can be observed in
, use of the set of constants proposed by
) yielded acceptable prediction of the
behaviour of calcium ions, although the amount of calcium in solution
is underestimated above pH 9. When these model parameters were
later applied to describe phosphate and arsenate adsorption in the pres-
ence of calcium, the adsorption of both oxyanions was considerably
overestimated. The discrepancy between experimental data and model-
ling results was attributed to the inclusion of outer-sphere complexes in
the calculations. Considering the results of this modelling scenario and
the previously mentioned risk of using too many surface complexes,
an optimization process will be required to describe the single- and
multi-component systems with the same set of constants.
3.2.2. Modelling scenario II: optimization of the calcium complexation
constants
In this second scenario, an attempt was made to improve the de-
scription of calcium adsorption by optimization of the surface complex-
ation constants and the charge distribution at the solid/solution
interface. Optimization was performed by including all the calcium ad-
sorption data obtained in the present study from the single-component
systems (ferrihydrite
–Ca) and also from the multi-component systems
(ferrihydrite
–AsO
4
–Ca and ferrihydrite–PO
4
–Ca).
Initially, only the complexation constants for the inner-sphere com-
plexes were optimized. Although the description of calcium adsorption
was improved relative to the previous modelling scenario, adsorption in
multi-component systems was still overestimated. We tried to correct
the excess charge in the 1-plane by transferring part of the charge to
the surface plane. When strong hydrogen bonds are formed between
the surface sites and the outer-sphere complex, a small charge attribu-
tion to the surface plane is feasible (
Rietra et al., 2001; Rahnemaie
). In our case, a small transfer of charge from the 1-plane to
the 0-plane (
Δz
0
= +0.2 and
Δz
1
= +1.8) was not suf
ficient to predict
the mutual effects of calcium and arsenate/phosphate in the multi-
component systems.
The best
fits were obtained when the outer-sphere complexes were
excluded from the model. Analysis of the distribution of the calcium sur-
face species reveals that the outer-sphere complexes were not relevant
and the hydrolysed (inner-sphere) form is dominant under the condi-
tions studied (Fig. S5, Supplementary data). Outer-sphere complexation
cannot be completely ruled out, although its inclusion did not improve
the modelling simulations. The contribution of these complexes is
slightly signi
ficant in the pH range 8.5–9.5, where the calcium adsorbed
is lower than 10%. The outer-sphere complexes would predominate
over the other surface species at pH below 8.5, where adsorption barely
reaches 1%. Although most of the modelling work on calcium adsorption
conducted so far considers outer-sphere complexes,
have also discussed the possibility of describing calcium adsorp-
tion to goethite by considering only inner-sphere complexes. Recalcula-
tion of the goethite
–Ca and goethite–PO
4
–Ca systems studied by
using their model parameters, without the
inclusion of the outer-sphere complexes, yielded an equally good de-
scription of their experimental results (Figs. S6 and S7, Supplementary
data).
Optimization of the complexation constants (
) for the inner-
sphere surface species was required along with relocation of the excess
charge in the 1-plane by not considering the formation of the outer-
sphere complexes. Application of the parameters obtained improved
the prediction of the calcium adsorption in the single-component
Table 3
Surface species and complexation constants estimated with the CD model for calcium binding on ferrihydrite.
Surface species
`FeOH
`Fe
3
O
Δz
0
Δz
1
Δz
2
H
+
Ca
+2
log K
Scenario I: Two inner- and two outer-sphere surface complexes
`FeOHCa
+3/2
1
0
+0.31
+1.69
0
0
1
3.23 ± 0.05
`FeOHCaOH
+1/2
1
0
+0.31
+0.69
0
–1
1
–6.42 ± 0.08
`FeOH
−1/2
⋯Ca
+2
1
0
0
+2
0
0
1
1.80 ± 0.95
`Fe
3
O
−1/2
⋯Ca
+2
0
1
0
+2
0
0
1
1.80 ± 0.95
r
2
= 0.992
Scenario II: Two inner-sphere surface complexes
`FeOHCa
+3/2
1
0
+0.31
+1.69
0
0
1
–0.89 ± 0.35
`FeOHCaOH
+1/2
1
0
+0.31
+0.69
0
–1
1
–7.20 ± 0.15
r
2
= 0.998
a
Values taken from
b
Standard deviations are given for the
fitted parameters.
58
J. Antelo et al. / Chemical Geology 410 (2015) 53
–62
system (
). This modelling scenario also yields adequate prediction
of calcium adsorption in the multi-component systems, which will be
discussed in the following section along with the effect that calcium
ions have on oxyanion adsorption. The lower calcium complexation
constants obtained for ferrihydrite, compared with those proposed by
for goethite, are consistent with the differences
observed for the arsenate or phosphate constants in both iron oxides.
3.4. Arsenate and calcium adsorption in multi-component systems
The arsenate adsorption results for the multi-component systems
are shown in
for a 0.6 mM arsenate loading, along with the predic-
tions of the CD model for the different cases studied. Results obtained
for two other arsenate loadings, 0.1 and 0.3 mM, are shown in Fig. S8
(Supplementary data). For purposes of comparison, the arsenate ad-
sorption level in the absence of calcium is also shown. The presence of
calcium ions enhanced the adsorption of arsenate ions on ferrihydrite
at relatively high pH values (
a). At pH
b 8.0 no influence of calcium
was observed, due to the weak adsorption of this cation. The degree to
which the adsorption is enhanced depends on the concentration of cal-
cium in the system, i.e. at pH ~10.2 a concentration of 0.3 mM slightly
affects the arsenate adsorption, whereas a concentration of 6 mM in-
creased the arsenate adsorption by up to 30%. Adsorbed calcium ions
are positively charged and will promote the adsorption of the negatively
charged forms of the arsenate ions through changes in the electrostatic
potential. As the calcium loading on ferrihydrite increases, the repulsive
electrostatic interactions between the negatively charged surface (at
relatively high pH values) and the arsenate ions decrease.
The effect of the arsenate loading has been evaluated separately at
constant calcium concentration and constant pH (
b). As expected,
increasing the As/Ca ratio at constant calcium loading produced a higher
adsorption of arsenate on ferrihydrite. The observed results for the ad-
sorption of arsenate are sensitive to the calcium loading in the system.
The ferrihydrite surface is close to saturation when the calcium loading
is relatively low and the maximum adsorption capacity was ~0.45 mmol
AsO
4
/l, whereas it is evident that arsenate did not reach saturation
when the calcium loading is relatively high and the maximum adsorp-
tion capacity of the ferrihydrite increased (
N0.55 mmol AsO
4
/l). The
good performance of the CD model, which is able to predict the en-
hancement of arsenate adsorption on ferrihydrite in the presence of cal-
cium, is discussed below.
a and b shows the equilibrium concentration of calcium ions in
the presence of arsenate and ferrihydrite. Comparison of the results in
the absence and in the presence of arsenate, analysed at two different
calcium loadings (2 and 6 mM), indicated that calcium adsorption is en-
hanced at pH
N 7.5 when arsenate is present in the system. Initially, in
the absence of arsenate, calcium adsorption only took place at
pH
N 8.5. Binding of phosphate or arsenate ions on iron mineral surfaces
strongly decreases the surface charge and shifts the isoelectric point to
lower pH values (
). Therefore, adsorption of arsenate
ions onto the ferrihydrite nanoparticles stimulated changes in the elec-
trostatic forces at the solid/solution interface and favoured the adsorp-
tion of the positively charged calcium ions.
The mutual effect of arsenate and calcium ions was simulated using
the CD model, with the complexation parameters previously obtained
in the single-component experiments (
). As stated
pH
[A
s
O
4
]
ad
s
(
m
m
o
l/l)
0.2
0.3
0.4
0.5
0.6
0.7
5
6
7
8
9
10
11
no Ca
0.3 mM Ca
0.7 mM Ca
1 mM Ca
2 mM Ca
6 mM Ca
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.0
0.3
0.6
0.9
1.2
As/Ca
[A
s
O
4
]
ad
s
(
mmo
l/
l)
[Ca] = 6 mM
[Ca] = 0.7 mM
a
b
Fig. 3. Arsenate adsorption experiments in the ferrihydrite
–AsO
4
–Ca multi-component
system. a) Effect of calcium in the arsenate adsorption to ferrihydrite; b) effect of ar-
senate loading at two
fixed calcium concentrations. Lines correspond to the CD model
predictions.
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
6
7
8
9
10
11
pH
[C
a
]
so
l
(
m
m
o
l/l)
0.6 mM AsO
4
pH
[C
a
]
so
l
(
m
m
o
l/l)
0.3 mM Ca
0.7 mM Ca
1 mM Ca
0.0
0.2
0.4
0.6
0.8
1.0
1.2
6
7
8
9
10
11
a
b
2 mM Ca
6 mM Ca
Fig. 4. Calcium behaviour in the ferrihydrite
–AsO
4
–Ca multi-component system. a) Equi-
librium calcium concentrations in solution in the presence (open symbols) and absence
(closed symbols) of arsenate; b) Effect of the pH on the equilibrium calcium concen-
tration (lower loadings) in the presence of arsenate. Lines correspond to the CD
model predictions.
59
J. Antelo et al. / Chemical Geology 410 (2015) 53
–62
above (
), consideration of outer-sphere complexes for calci-
um, along with inner-sphere complexes, introduced an excess of posi-
tive charge on the 1-plane and led to overestimation of the adsorption
of arsenate in the multi-component systems. The optimized complexa-
tion parameters obtained for calcium in modelling scenario II, which
excluded the formation of outer-sphere complexes, led to successful
predictions of both the arsenate and calcium adsorption in the multi-
component systems. In order to consider in the modelling calculations
the possible formation of insoluble arsenic-calcium mineral phases,
solubility products, recently compiled by
, for
different mineral phases (haidingerite, CaHAsO
4
·H
2
O, log K
sp
=
–4.79;
tricalcium-arsenate, Ca
3
(AsO
4
)
2
·3H
2
O, log K
sp
=
–21.14; johnbaumite,
Ca
5
(AsO
4
)
3
OH, log K
sp
=
–40.12) were included in these simulations.
Saturation indices were calculated for these mineral phases, and the re-
sults indicated that under the conditions analysed the multi-component
systems were undersaturated and therefore no precipitation was
expected. Only at pH
N 9 and for the highest Ca/AsO
4
ratio the effect of
precipitation was slightly signi
ficant, although its contribution to the ar-
senate retention is lower than 10%. The adequate modelling predictions
showed that the electrostatic changes produced in the solid/solution
interface, when both ions were present, had a major role in the mutual
effect produced between arsenate and calcium. Nevertheless, the possi-
ble formation of ternary complexes cannot be ruled out. Finally, model-
ling simulations of the multi-component systems were also conducted
with the generalized two-layer model, using the complexation parame-
ters proposed by
. Although the simulations
predicted an arsenate adsorption enhancement in the presence of calci-
um ions, the amount of arsenate adsorbed was not accurately predicted
for the entire range of calcium loadings (Fig. S9, Supplementary data).
3.5. Phosphate and calcium adsorption in multi-component systems
The mutual effect produced by phosphate and calcium on their ad-
sorption is shown in
. Qualitatively, this effect is similar to that
discussed above for the arsenate
–calcium multi-component system,
i.e. it is a synergetic interaction. The presence of calcium enhanced phos-
phate adsorption on ferrihydrite nanoparticles and vice versa, although
the effect of calcium seems to be greater for phosphate than for arse-
nate. This result is consistent with the solution chemistry of both
oxyanions, since slightly higher Ca ion-pair formation constants have
been reported for phosphate (Table S1, Supplementary data). At pH
~ 9.3, when the concentration of calcium present in the system is
1 mM, adsorption of phosphate and arsenate reached 83% and 74%, re-
spectively, whereas at pH ~ 10.2 the corresponding levels were 88%
and 66%. On the other hand, the concentration of calcium in solution
was slightly lower in the presence of phosphate in comparison with
the system containing arsenate (Fig. S10 in the Supplementary data).
Initially, the enhanced adsorption was explained by the changes pro-
duced in the electrostatic forces at the solid/solution interface following
the adsorption of phosphate and calcium ions. However, in contrast to
the successful modelling prediction of the arsenate
–calcium interac-
tions, the CD model underestimated the adsorption of phosphate in
the presence of calcium when the constants obtained in the single-
component systems were used (Fig. S11 in the Supplementary data).
Phosphate adsorption was only adequately described for the lower
calcium loading, indicating that changes in the electrostatic forces
were not the only mechanism responsible for the adsorption en-
hancement at intermediate to high calcium loadings. Precipitation
of insoluble mineral phases or the formation of ternary complexes
at high pH may also cause the observed effects. Saturation indices
were calculated for the most insoluble phosphate
–calcium mineral
(hydroxyapatite, Ca
5
(PO
4
)
3
OH, log K
sp
=
–58.2) and the results in-
dicated that precipitation may occur under the conditions analysed.
However, if precipitation was included in the calculations no fur-
ther improvement in the prediction of the experimental results
was achieved. On the contrary, a considerable reduction in the
amount of phosphate immobilized (adsorbed and precipitated) in
the system was predicted at the higher Ca/PO
4
ratios (Fig. S12 in
the Supplementary data). Considering the discrepancy that arises
from the assumption of the formation of phosphate
–calcium pre-
cipitates, and taking into account the experimental conditions
(phosphate pre-equilibration period reduces the presence of phos-
phate in solution before calcium addition) and the kinetics of the
hydroxyapatite formation, which requires the formation of the
more soluble tricalcium-phosphate (Ca
3
(PO
4
)
2
, log K
sp
=
–28.9)
as a precursor (
), the precipitation of these solid
phases was eventually ruled out. For this reason, ternary complexes
were considered in the modelling calculations. In the absence of
0.6 mM PO
4
0.0
0.2
0.4
0.6
0.8
1.0
1.2
6
7
8
9
10
11
pH
[C
a
]
so
l
(m
m
o
l/
l)
0.3 mM Ca
0.7 mM Ca
1 mM Ca
0.2
0.3
0.4
0.5
0.6
0.7
5
6
7
8
9
10
11
pH
[P
O
4
]
ad
s
(m
m
o
l/
l)
no Ca
0.3 mM Ca
0.7 mM Ca
1 mM Ca
2 mM Ca
6 mM Ca
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
6
7
8
9
10
11
pH
[C
a
]
so
l
(m
m
o
l/
l)
a
b
c
2 mM Ca
6 mM Ca
Fig. 5. Adsorption experiments in the ferrihydrite
–PO
4
–Ca multi-component system.
a) Effect of calcium in the phosphate adsorption to ferrihydrite; b) equilibrium calcium
concentrations in solution in the presence (open symbols) and absence (closed symbols)
of phosphate; c) effect of the pH on the equilibrium calcium concentration (lower
loadings) in the presence of phosphate. Lines correspond to the CD model predictions.
60
J. Antelo et al. / Chemical Geology 410 (2015) 53
–62
spectroscopic data, a monodentate ternary surface complex,
`FeOPO
3
Ca,
was proposed. This approach was used as a modelling exercise to assess
the effect that the formation of ternary complexes had on the adsorption.
The surface reaction and stoichiometry for the proposed ternary surface
complex were as follows:
≡FeOH
−1=2
þ H
þ
þ PO
4
−3
þ Ca
þ2
⇆ ≡FeO
−1=2þΔz
0
PO
3
Ca
Δz
1
þ H
2
O
ð8Þ
The charge distribution over the 0- and 1-plane was calculated by
combination of the actual values of the monodentate non-protonated
phosphate surface complex (FeOPO
3
;
Δz
0
= +0.22,
Δz
1
=
–2.22) and
the inner-sphere monodentate calcium surface complex (FeOHCa;
Δz
0
= + 0.31,
Δz
1
= + 1.69). Since the complexation constants for
the single-component systems had already been obtained, the only pa-
rameter
fitted in this modelling exercise was the complexation constant
of the ternary surface complex.
The inclusion of the ternary complex in the modelling calculations
yielded adequate prediction of the phosphate adsorption at the dif-
ferent calcium loadings. The optimized complexation constant was
log K
FeOPO3Ca
= + 20.5. Description of the calcium adsorption did
not improve considerably, considering that rather good modelling
simulation was already obtained without the inclusion of the ter-
nary surface complex. The surface speciation for the phosphate
–
calcium multi-component system is shown in Fig. S13 and Fig.
S14 (Supplementary data). The ternary complex is the dominant
phosphate surface species at intermediate to high pH (
N8.0) for a
calcium loading of 1 mM or higher. On the other hand, its contribu-
tion to the calcium surface speciation is relevant at intermediate pH
values (7.0
–9.0), while the dominant surface species at high pH
(
N9.0) is the hydrolysed monodentate complex. Although the
modelling simulations yielded good prediction of the experimental
data, this does not imply the existence of the ternary surface com-
plex proposed. Another likely mechanism for the enhancement of
phosphate adsorption is surface precipitation, which cannot be en-
tirely excluded as the formation of insoluble phases in the solution
was. Concentration of the ionic species near the ferrihydrite surface
might be larger than concentrations in the bulk solution. This situation
might lead to conditions where surface precipitation is occurring. Sur-
face precipitation may also account for the differences observed be-
tween the phosphate and arsenate in the multi-component systems.
Solubility products for the phosphate
–calcium minerals are usually
lower than those for the analogous arsenate
–calcium minerals. There-
fore, in the surface region, oversaturation with respect to the solid-
phases considered may be achieved with lower concentrations in the
case of phosphate compared with arsenate. Nevertheless, both surface
precipitation and ternary surface complexation involved multilayer
complex formation and can be considered as similar mechanisms. Fu-
ture spectroscopic studies should be carried out to clarify the real nature
and structure of ternary complexes or surface precipitates of phosphate
and calcium at relatively high pH.
4. Conclusions
The results of the present study demonstrate that the presence of a
major ion such as calcium enhances the adsorption of arsenate and
phosphate on ferrihydrite nanoparticles. Retention of these oxyanions
on mineral surfaces is important for controlling their transport and bio-
availability in soil and aquatic systems. Improved knowledge of multi-
component systems is critical to enable prediction of the mobility of nu-
trients and contaminants in natural systems. On the basis of the results
of the experimental and modelling studies carried out, the following
conclusions were reached:
• Qualitatively, the adsorption of arsenate on ferrihydrite showed simi-
lar trends to those found for different ionic species (phosphate,
arsenate, chromate
…) on mineral surfaces with relatively high PZC.
Adsorption decreased with increasing pH, due to the decrease in posi-
tively charged surface sites at the solid/solution interface. The effect of
ionic strength was only signi
ficant at intermediate and high pH, indicat-
ing the formation of inner-sphere surface complexes. The arsenate ad-
sorption behaviour was successfully described with the CD model
under the different conditions studied. The arsenate surface complexes
were selected according to the spectroscopic and molecular data report-
ed in the literature. The model predictions indicated that the dominant
surface species were bidentate complexes, although the presence of
monodentate surface species could not be ruled out.
• Adsorption of calcium onto the ferrihydrite surface is relatively weak,
showing similar behaviour to that observed for the adsorption of calci-
um and magnesium on goethite. Retention of calcium ions was only
relevant at pH
N 8.5, which was the expected result taking into account
the relatively low hydrolysis constant for calcium ions and the relatively
high PZC found for the ferrihydrite nanoparticles. The CD model ade-
quately describes the calcium adsorption considering the same type of
surface complexes previously proposed for goethite. However, the pres-
ence of outer-sphere complexes produced an excess charge in the elec-
trostatic 1-plane, and then the adsorption of oxyanions in the multi-
component systems was overestimated. Exclusion of these complexes
not only improved the predictions in the multi-component systems,
but also yielded an equally good prediction of the calcium adsorption
in the single-component systems.
• The presence of calcium ions enhanced the adsorption of both arsenate
and phosphate on ferrihydrite. The degree to which calcium ions en-
hanced the oxyanion adsorption depended on the pH, calcium loading
and the nature of the oxyanion. A similar synergetic interaction was
observed when the adsorption of calcium was studied in the presence
of these oxyanions. The enhanced adsorption may be explained by
changes in the electrostatic forces at the solid/solution interface,
although the presence or the formation of ternary surface complexes
cannot be ruled out.
• The mutual interaction between arsenate and calcium was successfully
described using the CD model, with the surface complexation constants
previously derived in the single-component systems. On the other
hand, the phosphate
–calcium interactions were only successfully
described at the lower calcium loading. At higher calcium loadings,
the formation of ternary complexes or surface precipitates may also
be involved in enhancing the adsorption of phosphate ions.
Acknowledgements
This work was supported through the Ministerio de Educación y
Ciencia under research project CTM2011-24985 and by the Xunta
Galicia under research project EM2013/040. The authors are grateful
to Pilar Bermejo and Paloma Herbello of the Department of Analytical
Chemistry, Nutrition and Bromatology of the University of Santiago de
Compostela (USC) for the ICP-OES measurements. The authors also
thank María Santiso of the Department of Soil Science and Agricultural
Chemistry for assistance in the AAS measurements. The associate editor,
Carla Koretsky, and four anonymous reviewers are gratefully acknowl-
edged for their feedback and constructive comments, which have great-
ly contributed to improve this manuscript.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
doi.org/10.1016/j.chemgeo.2015.06.011
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Document Outline
- Arsenate and phosphate adsorption on ferrihydrite nanoparticles. Synergetic interaction with calcium ions
- 1. Introduction
- 2. Materials and methods
- 3. Results and discussion
- 4. Conclusions
- Acknowledgements
- Appendix A. Supplementary data
- References
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