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Humic Nanoparticles at the Oxide#Water Interface:
Interactions with Phosphate Ion Adsorption
Liping Weng, Willem H. Van Riemsdijk, and Tjisse Hiemstra
Environ. Sci. Technol., 2008, 42 (23), 8747-8752 • Publication Date (Web): 06 November 2008
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Environ. Sci. Technol. 2008, 42, 8747–8752
Humic Nanoparticles at the
Oxide-Water Interface: Interactions
with Phosphate Ion Adsorption
LIPING WENG,* WILLEM H. VAN
RIEMSDIJK, AND TJISSE HIEMSTRA
Department of Soil Quality, Wageningen University P.O. Box
47, 6700 AA, Wageningen, The Netherlands
Received June 13, 2008. Revised manuscript received
September 12, 2008. Accepted October 2, 2008.
In this work, data for the interactions between humic acid
(HA) or fulvic acid (FA) with phosphate ions at the surface of
goethite (R-FeOOH) are presented. The results show very clear
differences between HA and FA in their interactions with
phosphate at goethite surface. HA is strongly bound to goethite
but surprisingly does not strongly affect the phosphate
binding, whereas FA is less strongly bound, but these molecules
have a very large effect on the phosphate adsorption, and
vice versa. Phosphate adsorption to goethite in the presence
of adsorbed HA or FA can be well predicted with the LCD model
(ligand and charge distribution). According to the model
calculations, the nature of the interactions between HA or FA
with phosphate at goethite surface is mainly electrostatic.
The stronger effects of FA on phosphate adsorption are caused
by its spatial location which is closer to the oxide surface,
and as a consequence, the electrostatic interactions between
adsorbed FA particles and phosphate ions are much stronger
than for HA particles. This is the first time that effects of natural
organic matter (NOM) on an anion adsorption are predicted
successfully using an integrated ion-binding model for oxides and
for humics that accounts for chemical heterogeneity of
NOM.
Introduction
Natural organic matter (NOM) nanoparticles, such as humic
acid (HA) and fulvic acid (FA), bind very strongly to mineral
surfaces (1-3). These surfaces also strongly interact with
well-known oxyanions such as phosphate, arsenate, and
selenite. NOM adsorption to minerals contributes to the
storage of terrestrial carbon. Phosphate adsorbed to metal
oxide surfaces is probably the most important active pool in
the phosphorus (P) nutrient cycle in the terrestrial ecosystems. The competition between NOM and phosphate for
adsorption to minerals may strongly influence their solubility
and mobility, and consequently, their behavior in the
environment.
Several experimental studies have been carried out that
show big effects of NOM on phosphate adsorption to
minerals (4-6). However, fundamental research on the
interactions between inorganic phosphorus and NOM at
metal oxide surfaces is, up till now, not advanced enough to
allow for a quantitative description, prediction and understanding of these effects. The difficulty in the model
development arises from the chemical complexity of NOM
* Corresponding author phone: 31-317-482332; fax: 31-317-419000;
e-mail: [email protected].
10.1021/es801631d CCC: $40.75
Published on Web 11/06/2008
 2008 American Chemical Society
particles and their relatively large size compared to inorganic
ions. There is an ongoing debate on the nature of HA
molecules and their molecular mass (7, 8). For systems
containing the relatively small NOM particles (molecular
mass <1 kDalton), i.e. FA, adsorption modeling has been
done in many different ways (2, 9-11). The common aspect
of all these approaches is that the molecules are assumed to
be identical with only a few discrete reactive groups per
molecule.
Humics (both FA and HA) are mixtures of complex
molecules, showing a large degree of chemical heterogeneity. An advanced approach of treating the chemical
heterogeneity of NOM is used in the NICA (non-ideal
competitive adsorption) model (12). The ion adsorption
to NOM is described in the NICA model using a continuous
distribution of affinities of the functional groups. In case
of NOM adsorption to mineral surfaces, the NICA approach
can be combined with a surface complexation model like
the CD-MUSIC (charge distribution multi site complexation) model (13, 14) that describes the reactivity and
electrostatics of oxide surfaces. In combination, the model
iscalledtheLCDmodel(ligandandchargedistribution)(15-17).
The approach can describe adsorption of FA to goethite
in FA-goethite systems over a wide range of conditions
(17). However, modeling the adsorption behavior of HA
on oxides has only become possible very recently (18).
The larger number of reactive groups per particle and the
larger physical size of HA are crucial factors when it comes
to the development of a physically realistic adsorption
model for HA adsorption (18). A large part of the HA
particles has to be positioned in the diffuse double layer
(DDL) beyond the compact part of the electric double layer
(EDL) (Figure 1) for a good description of the adsorption
data (18). This is in agreement with the size of HA molecules
(diameter about 2-7 nm) (19) when compared with an
estimate of the size of the compact part of the EDL (about
1 nm) (20). FA molecules are much smaller, having a
diameter around 1-2 nm (21), and they can come closer
to the surface and fit in the compact part of the double
layer as is shown in Figure 1.
Although the fitted particle distribution in space for HA/
FA at oxide surface might be reasonable, it is very important
to have a way to test this result. The interfacial distribution
of the charge of HA and FA molecules can be elucidated by
studying the competition of HA and FA with an anion like
phosphate at the iron oxide surface. The adsorption of
FIGURE 1. Schematic representation of phosphate, FA and HA
at goethite surface in which the extended Stern model is used
to describe the structure of the electric double layer.
VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Basic Properties, Surface Species and CD-MUSIC Model Parameters for Goethitea
specific surface area (m2/g)
site density (/nm2)
PZC
94
OH-0.5
9.3
-Fe1
-Fe3
3.45
surface species
sites
-Fe1
+0.5
-Fe1OH2
-Fe3OH+0.5
-Fe1OH...Na+0.5
-Fe3O...Na+0.5
-Fe1OH2...NO3-0.5
-Fe3OH...NO3-0.5
-(Fe1O)2PO2-2
-Fe1OPO2OH-1.5
-Fe1OOCR-0.5
OH-0.5
capacitance (F/m2)
O-0.5
2.7
charge distribution
O-0.5
∆z0
0
1
0
1
0
1
0
0
0
1
1
0
0
1
1
0.46
0.28
0.5
-Fe3
1
0
1
0
1
0
2
1
1
C1
C2
0.83
0.74
ions or ligands
-
logK
∆z1
H+
Na+
NO3
PO4
0
0
1
1
-1
-1
-1.46
-1.28
-0.5
1
1
0
0
1
1
2
2
1
0
0
1
1
0
0
0
0
0
0
0
0
0
1
1
0
0
0
0
0
0
0
0
0
1
1
0
3-
RCOO0
0
0
0
0
0
0
0
1
9.30
9.30
-0.60
-0.60
8.62
8.62
29.8
27.7
-1.0*
a
∆z0:charge attributed to 0-plane. ∆z1: charge attributed to 1-plane. R the rest of HA/FA molecule. The logK values for
protonation of goethite sites are set equal to the PZC of this goethite (14). The logK values for the ion pairs formation have
been derived by (20). The logK values and the charge distribution of adsorbed phosphate species are kept the same as
derived by (27). * This logK value is a NICA-Donnan model parameter, pertains to the reaction of an oxide surface site with
a carboxylic ligand of HA or FA adsorbed, which has been optimized in this study.
TABLE 2. Molar Mass and NICA Model Parameters for HA and FAa
molar mass (kDalton)
type of ligands
Qmax (mol/kg)
Nmax (mol/mol)
p
log̃KH
nH
log̃Kin
nin
b
HA
13.2
31.7
31.5
4.02
1.27
0.61
0.41
0.59
0.70
2.49
8.60
2.34
8.60
0.81
0.63
0.66
0.76
>0.81
0.31
0.68
2.40
2.39
5.88
1.86
-1.0
FA
carboxylic
phenolic
carboxylic
phenolic
-1.0
0.66
0.57
a
Qmax, Nmax: site density. p: parameter for the intrinsic heterogeneity of the ligands. K̃I: mean affinity constant. ni: ion
non-ideality parameter. b: a constant used to calculate the Donnan volume of humic substances. Subscript “in” refers to
the innersphere complex (-Fe1OOCR-0.5). For HA, the NICA parameters except log K̃in and nin have been derived by fitting
the acid-base charge curve of the HA used to the NICA-Donnan model (26). For FA, the NICA parameters except log K̃in and
nin are taken from (32). For both HA and FA, log K̃in was derived in this study and nin was assumed equal to nH.
phosphate to goethite in the absence of humics has already
been intensively studied using a whole series of techniques,
and our mechanistic understanding of this adsorption process
at the molecular scale is very good. The competition between
phosphate and HA/FA at the oxide surface can be due to site
competition as well as electrostatic effects. The calculated
potential profile depends strongly on the assumed distribution in space of the adsorbed HA/FA and the net charge of
the humics. However, it also depends on the charge
distribution of the different adsorbed phosphate species. The
interplay will determine the overall effect.
In this work, the competitive interactions between HA or
FA with phosphate ions at the surface of goethite (R-FeOOH)
were studied experimentally. The results were compared to
predictions using the LCD model. One objective of this study
is to test the applicability of the LCD model to predict ion
adsorption in ternary systems containing anions, humics
and oxides. In the modeling, the distribution in space of
adsorbed HA/FA and phosphate and other model parameters
were kept the same as those optimized using data of binary
systems (FA-goethite, HA-goethite, phosphate-goethite).
Another objective of this study is to understand the mechanisms in the humics-phosphate interactions at oxide
surfaces and the difference between HA and FA in these
interactions. A successful description of these important
interactions will be a major step forward to a realistic
modeling of surface complexation in natural systems.
Materials and Methods
Experiment. The goethite material was prepared based on
the procedure described by Hiemstra et al. (14). The specific
surface area of the goethite material was determined using
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 23, 2008
the BET-N2 adsorption method and equals 94 m2/g. The point
of zero charge (PZC) of this goethite material has been
determined as pH 9.3 (22) (Table 1). Humic acid (HA) used
in these experiments was purified from a forest soil (Tongbersven, The Netherlands) and fulvic acid (FA) from a peat
soil (Strichen Soil Association, Scotland) following the IHSS
(International Humic Substance Society) procedures (23).
The purified HA and FA material contain, respectively, 54
and 43% carbon. The average molar mass of HA and FA used
is, respectively, 13.2 and 0.68 kDalton (Table 2), which was
measured using size exclusion chromatography (24, 25).
Adsorption was measured with two batch experiments,
in which goethite suspensions (1 g/L) were prepared in a
background electrolyte solution of 10 mM (first batch) or 1
mM NaNO3 (second batch). Each batch contains five series
of treatments, i.e. phosphate-goethite, HA-goethite, FAgoethite, phosphate-HA-goethite, and phosphate-FA-goethite. In each series, one blank treatment was added that
contains no goethite. The initial phosphate concentration
was 0.15 mM, whereas it was 50 mg/L for HA and FA. The
stock solutions of phosphate and HA/FA were added to
goethite suspension simultaneously. For all series, acid or
base solution (0.1 M NaOH and HNO3) were added to the
suspension to adjust the pH to selected values in the range
of pH 3-7. During the preparation, the gastight vessels used
in the experiments were flushed with N2 gas to minimize the
influence of CO2. Thus prepared suspensions were shaken
at 20 °C for 3 days, and subsequently centrifuged at 18 000g
for 30 min. Our preliminary experiments show that pseudoequilibrium can be reached within 3 days when phosphate
and humics are added simultaneously. The phosphate
concentration in the supernatant was measured with opti-
mized molybdate blue colorimetric method using a segmented flow analyzer (Skalar). The concentration of dissolved
organic C (DOC) was measured with a TOC analyzer (Skalar)
after the supernatant solutions were acidified to pH 3-4 to
remove inorganic C. The final pH in the supernatant was
measured using a pH meter. At 1 mM NaNO3 background,
the final ionic strength in the treatments containing HA/FA
is 2 mM, due to sodium (Na) present in the stock solutions
of HA/FA (NaOH solution was used to dissolve HA/FA).
Amount of HA/FA adsorbed was derived from the difference
of DOC between the treatments and the blank (no goethite).
Our previous study shows that at the ionic strengths studied
there is no HA precipitation at pH above 3 (26).
Modeling. CD-MUSIC model was used to describe the
reactivity and electrostatics at goethite surface (13, 14), in
combination with the extended Stern model for the compact
part of the EDL, which has two Stern layers and two outerelectrostatic planes (1- and 2-plane) (Figure 1) (20). Two types
of surface groups on goethite are proton reactive, i.e., the
singly (-FeOH-0.5) and triply (-Fe3O-0.5) coordinated surface
groups (13). The basic CD-MUSIC parameters have been
derived previously (18) (Table 1). Phosphate can form two
types of innersphere surface species, i.e. a protonated
monodentate (-Fe1OPO2OH-1.5) and a bidentate
(-(Fe1O)2PO2-2) surface complex (27), with the charge
distributed between the surface plane (0-plane) and the
middle plane (1-plane). The model parameters for phosphate
adsorption derived by Rahnemaie et al. (27) were used
without modification to predict phosphate adsorption to
goethite (Table 1). The mean-field approximation was used
to link the charge density and the potential in the various
electrostatic planes.
Adsorption of phosphate to goethite in the presence of
adsorbed HA/FA was calculated using the LCD (ligand and
charge distribution) model (15-18, 22), which integrated the
NICA model (12) and the CD-MUSIC model (13, 14). In
addition to proton binding by both the carboxylic and
phenolic type of ligands on HA/FA, it was assumed that the
carboxylic groups (RCOO-) of adsorbed HA and FA in the
first compact layer can form innersphere complexes
(-Fe1OOCR-0.5) with the singly coordinated surface sites
(-Fe1OH2+0.5) on goethite (17, 18). The corresponding charge
is distributed between the surface (0-plane) and 1-plane
(Table 1). Both reactions (protonation, innersphere complexation) were calculated with the NICA model. It was
assumed that the NICA model parameters for adsorption of
protons to adsorbed HA/FA remain the same as for HA/FA
in the solution (Table 2). These parameters have been used
to describe the acid-base titration data of these HA/FA (26).
In our previous work of modeling FA adsorption to
goethite in the absence of phosphate with the LCD model,
the basic Stern model was used, which has only one Stern
layer and all FA charge was located in the compact part of
the EDL, at the 0-/1-plane (17). In the present work, we used
the extended Stern model for all calculations, in which the
compact part is divided into two layers (Figure 1). Model
calculations show that by adjusting the affinity constant for
the innersphere complex (-Fe1OOCR-0.5), a reasonable
description of FA adsorption data in the absence of phosphate
can be obtained for a range of spatial distributions (between
the 0-/1- and 2-plane) of adsorbed FA. However, to achieve
a good description of phosphate adsorption to goethite in
the presence of adsorbed FA, the range of spatial distribution
is narrower. The optimal value points to an even distribution,
i.e. 50% of FA ligands at the 0-/1-plane and 50% at the 2-plane.
The charge distribution of the ligands between the 0- and
1-plane depends on the amount of innersphere complex
formed (-Fe1OOCR-0.5), which results from the iterative
model calculations. With this spatial distribution, and an
adjustment of the affinity constant for the formation of the
innersphere complex (-Fe1OOCR-0.5) (logK from -2 to -1)
(Table 1), the model can describe the FA adsorption data
quite well (see the Supporting Information).
In modeling the HA adsorption to goethite in the absence
of phosphate using the LCD approach, the distribution of
adsorbed HA in space is an adjustable parameter, which is
a function of the ionic strength of the solution (18). The
fraction of HA attributed to the 0-/1-plane has been optimized
previously (18). The fraction found is, respectively, 5 and 1%
for the 10 and 2 mM ionic strength. The fraction of HA at the
2-plane was kept constant at 25%. The rest of HA has been
located in the DDL, which is 70 and 74% for, respectively, 10
and 2 mM ionic strength. In the present work, the affinity
constant for the formation of the innersphere complex
(-Fe1OOCR-0.5) was assumed to be the same for HA and FA
(logK ) -1) (Table 1). Changing this constant from -2.0,
which was used in the previous work (18), to present value
(-1.0) did not influence much the predicted HA adsorption
for HA-goethite systems (see the Supporting Information)
because only a small fraction of HA was located at the 0-/
1-plane and allowed to form the innersphere complex.
The NICA model calculates fractions of reactive ligands
on adsorbed HA/FA particles that are protonated or complexed with surface sites. Together with the amount of HA/
FA adsorbed to goethite, the total charge carried by adsorbed
HA/FA can be derived. The charge balance and electrostatic
potential at each electrostatic plane at the goethite surface
was calculated with the CD-MUSIC model by taking into
account the charge contribution from adsorbed HA/FA.
Adsorption of phosphate to goethite in the presence of
adsorbed HA/FA was calculated with the CD-MUSIC model
using the same parameters for phosphate adsorption to
goethite without HA/FA (27) (Table 1). Although the spatial
distribution of FA was optimized in the present work using
FA/phosphate competition data, the results for HA/phosphate competition calculations are a pure prediction. In the
present study, the measured amount of HA/FA adsorbed
was used as an input to the model, to simplify the calculations
and discussions. The model calculations were carried out
using the computer program ORCHESTRA (28).
Results and Discussion
Adsorption of Phosphate. More than 90% of the added
phosphate (1.55 µmol/m2) remained adsorbed at changing
conditions, but a very large variation in the equilibrium
concentration of phosphate in solution was observed (Figure
2). The data in the figure show that the addition of FA led
to a very large increase in the equilibrium concentration of
phosphate. The presence of FA increased the phosphate
concentration by a factor of up to 100 at low pH. It illustrates
that FA is in strong competition with phosphate at the surface
of goethite. HA molecules have also competition with
adsorbed phosphate, but the increase of the phosphate
concentration is much smaller. The data show that the effect
of the ionic strength is rather limited in contrast to the effect
of a change in pH.
Adsorption of HA and FA. The competitive effect between
HA or FA with phosphate not only changed phosphate
concentrations in solution, it also affected the binding of HA
and FA molecules. The stronger competition between FA
and phosphate than between HA and phosphate found in
Figure 2 for phosphate is mutual. The presence of phosphate
led to a large decrease of the amount of FA adsorbed over
the entire pH range (Figure 3b). This is very different for the
systems with HA. First of all, HA was more strongly bound
in the absence of phosphate, and it remained also strongly
bound in particular in the lower pH range when phosphate
was added (Figure 3a). It has been shown previously that
this strong binding of HA is related to the larger number of
ligands per molecule and its corresponding charge (16, 18).
VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Phosphate concentration in solution in the absence and presence of HA or FA in systems of 1 g/L goethite and 10 mM
NaNO3 (closed symbols and thick lines) or 1 mM NaNO3 (open symbols and thin lines) with initially 0.15 mM P, 50 mg/L HA or FA. a:
Results for HA. b: Results for FA. Symbols: experimental data. Lines: model calculations. Grey lines: model calculations by assuming
an even distribution of adsorbed HA between the two compact layers (see text).
FIGURE 3. Fraction of HA and FA adsorbed to goethite in the absence and presence of phosphate in systems of 1 g/L goethite and 10
mM NaNO3 (closed symbols) or 1 mM NaNO3 (open symbols) with initially 0.15 mM P, 50 mg/L HA or FA.
Despite the high amount of HA that was bound, a much
smaller effect was found in terms of competition with
phosphate as follows from Figure 2. It indicates that HA has
only a moderate effect on phosphate adsorption. In other
words, HA is strongly bound but surprisingly does not very
strongly affect the phosphate binding, while FA is less strongly
bound, but these molecules have a very large effect on the
phosphate adsorption.
Mechanisms of Interaction. The LCD model predicted
the effects of adsorbed HA on phosphate concentration rather
well (lines in Figure 2). Calculations show that the low fraction
of the HA that is located by the model in the compact layer
is the key factor for the correct prediction of the effect of HA
on phosphate adsorption. If all HA ligands are located in the
compact part of the EDL as is done for FA, the model would
strongly overestimate the effect of HA on phosphate adsorption. As shown by the gray lines in Figure 2-a, using the same
spatial distribution as assumed for FA, i.e., an even distribution between the two Stern layers, the model would largely
over predict phosphate concentration in solution in the
presence of HA.
For phosphate adsorption in the presence of FA, the
description of the data is equally good (lines in Figure 2). An
equal distribution of FA over both compact layers is essential
for the description of the interaction with phosphate. Locating
all ligands of FA in the first compact layer (0-/1-plane) leads
to overestimation of the competition for phosphate adsorption especially at low pH (results not shown). It is important
to note that the charge distribution of the adsorbed humics
is a strong function of pH and phosphate concentration
because these factors affect the electrostatic potentials at
goethite surfaces for a given distribution of the reactive ligands
in space. The resulting charge distribution of HA/FA and the
electrostatic potentials are output of the model calculations.
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The above suggests that the main reason for the much
stronger competition effects of FA in comparison with HA
for phosphate adsorption to goethite is the difference in
the spatial distribution of these types of molecules in the
goethite-water interface. According to the model, the
nature of the interaction of HA or FA with phosphate is
mainly electrostatic. Site binding effects are of secondary
importance. The HA and FA have large differences in the
number of reactive groups per particle (mol/mol), which
are related to the difference in particle size and therefore
the molecular mass (Table 2). The smallest species is the
phosphate ion (Figure 1). Its charge is divided between
the surface plane (0-plane) and the 1-plane in the CD model
(29). The potential at the surface (0-plane) is mainly
determined by the concentration of the potential determining ion, i.e., by the pH. The electrostatic potential at
the 0-plane does not change significantly at a given pH
upon adsorption of other ions, in contrast to the potential
at the 1-plane. The potential at the 1-plane will decrease
as a function of adsorption of phosphate, HA and FA. The
strongest effects of the adsorption of humics on the
potential at the 1-plane will occur when all the added
negative charge is located at the 1-plane itself. In the modeling, 50% of the FA ligands has been located at the 0-/1plane, whereas for HA it is only 1-5%. Although the charge
of the humic acid beyond the 1-plane also affects the
potential at the 1-plane, the major effect is caused by the
charge that is directly present at the 1-plane. The much
lower number of ligands of HA at the 1-plane will thus
produce much less negative charge at the 1-plane compared to FA, which results in a less strong interaction with
the phosphate ions that are located there. In other words,
the difference in the degree of overlap of the location of
charge of HA or FA with the charge of phosphate is the
FIGURE 4. Schematic representation of goethite surface (about 100 nm2) covered with phosphate and HA (a), or phosphate and FA
(b).
FIGURE 5. Fraction of the bidentate phosphate species
(-(Fe1O)2PO2-2) over the total amount of phosphate adsorbed to
goethite in the absence or presence of HA or FA calculated
with the LCD model. Systems contain 1 g/L goethite and 10 mM
NaNO3 (thick lines) or 1 mM NaNO3 (thin lines) with initially
0.15 mM P, 50 mg/L HA or FA.
major reason for the difference in interaction of HA and
FA molecules with phosphate (Figure 1).
The difference between HA and FA can also be visualized
in the following way. Without HA or FA, the amount of
phosphate adsorbed to goethite for the conditions of the
experiments is on average 90 phosphate ions per 100 nm2
in the pH range studied. When HA or FA is present, there is
25-50 mg/m2 HA and 10-35 mg/m2 FA adsorbed. Using the
average molecular mass of HA and FA (Table 2), the amount
of HA and FA adsorbed is equivalent to, respectively, 1-2 HA
nanoparticles and 11-26 FA nanoparticles per 100 nm2
(Figure 4). The larger effect of the FA on phosphate adsorption
is thus due to a combination of a much larger number of
particles adsorbed (although the amount in terms of mass
is lower) and a shorter distance to the surface.
Phosphate Surface Speciation. Adsorption of HA/FA to
goethite not only influences the amount of phosphate
adsorbed, but also its speciation. Model calculations show
that the presence of adsorbed HA/FA led to a shift from the
bidentate (-(Fe1O)2PO2-2) to monodentate (-Fe1OPO2OH-1.5)
adsorbed phosphate species especially at low pH (Figure 5).
At pH around 3, the percentage of bidentate species decreased
from about 60% to about 50% in the presence of HA and in
the presence of FA from about 60% to about 30%. In principle,
such a change in speciation is experimentally accessible. The
shift from the bidentate to monodentate phosphate species
is mainly due to the electrostatic potential at the 1-plane,
which is more negative when HA or FA are present. The
reduction of available surface sites on goethite due to the
formation of innersphere complex with adsorbed HA/FA also
favors the formation of monodentate over bidentate phosphate species, but model calculation shows that it is of much
less importance compared to the effects of electrostatic
potential. This change in potential at the 1-plane is unfavorable for formation of both types of phosphate species.
However, because the monodentate species contributes less
negative charge, i.e., -1.28 v.u. (valence units) to the 1-plane
than the bidentate species (-1.46 v.u.) (Table 1), the reduction
in the amount of the monodentate species is less than that
of the bidentate. This leads to the relative shift toward
monodentate species when HA or FA are present. This shift
is stronger in the presence of FA than HA because of the
closer location of FA to the surface and the larger number
of adsorbed FA molecules, which led to a stronger reduction
of the electrostatic potential at the 1-plane.
Implications and Relevance. The strong interactions
between HA/FA and phosphate that we have studied in this
work have practical relevance. In nature, effects of natural
organic matter (NOM) on solution concentration of phosphate can be stronger than observed in this study, due to a
possible larger solid-solution ratio in natural samples than
used in this experiment. For a proper application of the
advanced knowledge in ion adsorption to mineral oxides in
natural systems, the interaction with humics should be
considered explicitly because it will have large effects on the
adsorption of many ions. The knowledge on phosphate-NOM
interaction at mineral surfaces may contribute to the
improvement of our insights in the role of NOM in influencing
the availability of inorganic phosphate for organisms in both
terrestrial and aquatic ecosystems. NOM may also influence
behavior of other anions such as arsenate and selenite in the
environment. Competition between arsenic (As) with NOM
for sorption may contribute to the mobility of this toxic
element as found in Bangladesh (30, 31). The model used in
this work is, as far as we are aware, the first attempt to describe
or predict this interaction quantitatively using a mechanistic
approach. The fact that it can predict the major effects that
are observed experimentally is very encouraging.
Acknowledgments
This research was partly funded by the EU project, FUNMIG
(516514, F16W-2004). We thank Gerlinde Vink for helping
with the experiment.
VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
8751
Supporting Information Available
Two additional figures. This material is available free of charge
via the Internet at http://pubs.acs.org.
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