Journal of Colloid and Interface Science 342 (2010) 427–436
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Journal of Colloid and Interface Science
www.elsevier.com/locate/jcis
Iron-modified hydrotalcite-like materials as highly efficient phosphate sorbents
Kostas S. Triantafyllidis a,*, Efrosyni N. Peleka a, Vasilis G. Komvokis b, Paul P. Mavros a
a
b
Department of Chemistry, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece
Chemical Process Engineering Research Institute, CERTH, GR-57001 Thessaloniki, Greece
a r t i c l e
i n f o
Article history:
Received 28 May 2009
Accepted 25 October 2009
Available online 29 October 2009
Keywords:
Hydrotalcite
LDH
Iron
Phosphates
Sorption
Regeneration
a b s t r a c t
Highly efficient sorbents for phosphate removal from aqueous solutions based on the calcined forms of
Fe(III)-substituted Layered Double Hydroxides (LDH) materials have been developed in this study. Hydrotalcite-like materials with Mg/M3+ 3 (where M = Al3+, Fe3+ or combined) have been synthesized following
simple co-precipitation method and were subsequently calcined in air at 450 °C. Both as-synthesized and
calcined materials were characterized by means of X-ray Diffraction (XRD), Inductively Coupled Plasma
Atomic Emission Spectroscopy (ICP-AES), elemental (C) analysis, N2 porosimetry, Scanning Electron Microscopy (SEM). All the materials were evaluated for the sorption of phosphates by batch equilibrium sorption
experiments and kinetic measurements (effect of contact time). It was shown that chlorides or nitrates,
being the charge-balancing anions in the LDH structure, are more easily exchanged by phosphates compared to carbonates. In the Fe(III)-modified LDHs, an increase of the Fe loading led to the decrease of the
sorption efficiency. The maximum uptake of phosphates for both the Mg–Al LDH and Mg–Fe LDH samples
containing mainly carbonates as charge-balancing anions was relatively low (ca. 625 mg P/g sorbent) while
it was higher for the LDH samples containing mainly chlorides (80 mg P/g). On the other hand, the maximum sorption capacity for the calcined Mg–Al LDHs and the calcined Fe(III)-substituted sorbents were very
high, ca. 250 and 350 mg P/g, respectively. The sorption data of both the as-synthesized and calcined
LDHs was best fitted by the Freundlich model. Both the Mg–Al and Fe-substituted LDH sorbents were regenerated with mixed aqueous solution of NaCl and NaOH and were reused with a small loss of removal
efficiency.
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
The foreseeable worldwide limitation of water resources leads
to the need for water re-use, which requires treatment of water
streams to reduce the amounts of harmful components, e.g. toxic
metals, and pesticides, among others. Phosphate anions are particularly undesirable, especially in wastewaters from municipalities
and industries, since these are often responsible for eutrophication
of the stream receivers, like lakes and other confined water bodies,
as well as coastal areas, causing short- and long-term environmental and esthetic problems. According to Environmental Protection
Agency (EPA), phosphates should not exceed 0.05 mg L1 if streams
discharge into lakes or reservoirs, 0.025 mg L1 within a lake or
reservoir, and 0.1 mg L1 in streams or flowing waters not discharging into lakes or reservoirs to control algal growth [1]; in
the European Union, the effluent limits for phosphorus in wastewater treatment plants are 1–2 mg L1 of total phosphorus,
depending on the sensitiveness of the receiving water body [2].
Phosphates can be removed from aqueous streams by physical
[3], chemical [3] and biological methods [4]. Sorption is one of
* Corresponding author. Fax: +30 2310 997730.
E-mail address: [email protected] (K.S. Triantafyllidis).
0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcis.2009.10.063
the physical processes that may be suitable for phosphate ion removal. Various types of sorbents have been tested for phosphates
removal, to varying degrees of success: aluminum oxides [5,6], aluminum oxide hydroxide [7], alum sludge [8], dolomite [9], an inorganic–organic bentonite [10], goethite [11,12], akaganéite [13],
granulated ferric hydroxide [6], crystalline MnO2 [14], chemically
or thermally modified palygorskites [15], clinoptilolite [16] a synthetic zeolite [17], ion exchangers [18,19], an industrial Fe(III)/
Cr(III) hydroxide solid waste [20] as well as soil samples and industrial wastes [16], among others.
A class of materials that is being usually applied for the effective
removal of anions is the Layered Double Hydroxides (LDHs), called
also hydrotalcite-like anionic clays [21–23]. Hydrotalcite, with an
idealized unit cell formula Mg6Al2(OH)16CO34H2O, is a naturally
occurring layered material, which is isostructural to brucite
(Mg(OH)6), with octahedra of Mg2+ (6-fold coordinated to OH)
sharing edges to form infinite sheets. Partial substitution of the
divalent Mg2+ ions with the trivalent Al3+ ions forms the structure
of hydrotalcite and generates a positive charge in the hydroxyl
sheets, which is compensated by CO2
3 anions that lie in the interlayer space between two brucite-like sheets, together with randomly distributed water molecules. The mineral with the
idealized unit cell formula Mg6Fe2(OH)16CO34H2O, in which
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Mg2+ is substituted by Fe3+ instead of Al3+, is named pyroaurite; its
structure is similar to that of hydrotalcite. Thermal treatment of
the LDHs results in mixed oxides with varying structures, depending on the calcination temperature [21–24]. Heating up to 450–
500 °C transforms the Mg–Al LDH to a mixed oxide with a MgO
structure; this is accompanied with an increase in surface area
and pore volume, which can reach 250–300 m2/g (N2/BET) and
1 cm3/g (Hg intrusion), respectively. Further heating, at temperatures higher than 750 °C, produces the mixed phases of MgO oxide
and MgAl2O4 spinel structures, with a lower surface area. The formation of the mixed oxide phases at relatively low calcination
temperatures is reversible and upon rehydration and incorporation
of anions in aqueous media they are able to regain the hydrotalcite
structure.
LDHs have been widely studied as sorbents, via ion-exchange of
the parent charge-balancing anions with other ‘‘polluting” anions,
such as arsenites [25], arsenates [26–28], chromates [29–32],
phosphates [33–38], selenites [39], selenates [40], borates [41], nitrates [42], iodates [43], perrhenates and pertechnetates [44],
molybdates [28,45] or via adsorption routes, mainly on the calcined forms of the LDH [29,35,37,40,46,47]. More specifically,
iron-substituted LDHs have been evaluated as adsorbents for the
removal of selenites [48], chromates [49], arsenates [50], boron
[41] and lead [51]. Since it has been already shown that iron-based
sorbents, such as granulated ferric hydroxide [6], goethite [11,12],
akaganéite [13,52] and others, exhibit remarkable properties for
the removal of phosphorus from aqueous solutions, it would be
of interest to combine the anion-exchange properties of LDHs with
the affinity of iron towards the anions of P. The synthesis of Fesubstituted LDH materials and of their calcined analogues has been
extensively studied previously [53–57]. Although there are few
studies available [33,58], that have shown the potential of the
iron-modified LDH materials as phosphates sorbents, more systematic studies are still required for the systematic investigation
and optimization of this sorbent/process.
In the present work, Fe(III)-modified hydrotalcite-like materials
(with varying degree of aluminum substitution by iron) have been
studied as sorbents for phosphate removal, in their as-synthesized
or calcined form. In addition, the effect of the type of exchangeable
2
anions (NO
3 , Cl , CO3 ) on the effectiveness of the LDH materials
as phosphate anion sorbents, has also been investigated. It was
shown that the calcined form of a fully Fe(III)-substituted LDH
material can be a very efficient sorbent for phosphate removal
from aqueous solutions. A thorough physicochemical characterization of both the as-synthesized, calcined and P-loaded samples enabled us to rationalize the significant differences in the sorption
capability between the as-synthesized LDH materials and the
mixed-oxides derived from their calcination.
carbonates was significantly lower (as shown in Table 1), yielding
the sample labeled ‘‘LDH2”. In addition, a third Mg–Al sample was
synthesized with low concentration of carbonates and by using
chlorides as source metal salts instead of nitrates (sample labeled
‘‘LDH3”). The LDH structure was further modified by adding varying amounts of Fe(III), which partially or totally substituted the
Al(III) in the octahedral sheets; the procedure was similar to that
for the synthesis of LDH1 sample and involved an aqueous Fe(NO3)39H2O solution, as Fe(III) source. Three samples (labeled
‘‘LDH4”, ‘‘LDH5”, ‘‘LDH6”) were prepared, having the following molar ratios Mg/Fe(III)/Al: 3/0.4/0.6, 3/0.8/0.2, 3/1/0 (Table 1), with
the last one resembling the structure of pyroaurite-like anionic
clays. In all cases, after initial mixing of all the solutions, and allowing for the precipitate to form, the solid–liquid dispersion was stirred for 2 h at room temperature and then for 18 h at hydrothermal
conditions in sealed polypropylene bottles at 65 °C (±2 °C). The
precipitates were filtered, washed with double-distilled water (until the filtrate solution was free of nitrates or chlorides) and were
dried at room temperature overnight. A small amount of each dried
LDH sample was further calcined at 450 °C in air for 4 h, in order to
convert them to their corresponding mixed oxides. The calcined
samples were named as ‘‘LDH-calc”.
2.2. Physicochemical characterization of LDH sorbents
The total metal (Mg, Al, Fe) content of the samples was determined by Atomic Emission Spectroscopy (ICP-AES) (Plasma 40,
Perkin–Elmer) after appropriate dissolution of the solid samples.
The content of carbonates was determined from C analysis using
a LECO 800 CHN Analyzer.
The X-ray powder diffraction (XRD) was utilized for the identification of the crystalline phases of the LDH samples or the respective mixed-oxide samples. XRD patterns were obtained using a
Siemens D-500 automated diffractometer (Cu Ka radiation, k
= 1.5418 Å) operating at 45 kV and 100 mA; counts were accumulated in the range of 5–75° every 0.02° (2h) at a scan speed of 1°
(2h)/min.
Specific surface area (SSA) and porosity characteristics of the
samples were determined from adsorption–desorption isotherms
of nitrogen, which were obtained at 196 °C on an Automatic Volumetric Sorption Analyzer (Autosorb-1, Quantachrome). Prior to
the determination of the adsorption isotherms, the samples were
evacuated overnight at 90 °C (for the as-synthesized samples) or
430 °C (for the calcined samples) under 1.0 103 mbar vacuum.
The relatively low outgassing temperature in the case of the assynthesized samples was applied in order not to have any destruction or re-organization of the layered structure.
The particles morphology was examined by Scanning Electron
Microscopy (SEM) images which were taken on a JEOL JSM-6300
Scanning Microscope.
2. Experimental
2.3. Sorption experiments
2.1. Synthesis of Layered Double Hydroxides (LDHs) and the respective
calcined (LDH-calc) materials
The parent Mg–Al hydrotalcite sample was synthesized based
on a well-established procedure [24]. In a typical synthesis, an
aqueous solution containing 0.3 mol of Mg(NO3)26Y2J and
0.1 mol of Al(NO3)39H2O was slowly added to an aqueous solution
containing 0.25 mol Na2CO310H2O under vigorous stirring at room
temperature, maintaining the pH between 8 and 10 by adding a
50% (w/w) NaOH aqueous solution dropwise. This procedure aimed
at the synthesis of the ‘‘standard” hydrotalcite sample (LDH1) having mainly carbonates as charge-balancing anions (see Table 1). A
variant of the ‘‘standard” hydrotalcite sample, was synthesized following the same procedure, except that the initial concentration of
The sorption of phosphates was studied by equilibrium experiments of the batch type. In a typical experiment, a 50 mL phosphate aqueous solution, prepared with potassium dihydrogen
phosphate (KH2PO4, Merck, pro analysi), was mixed with an appropriate amount of LDH sorbent (typically 50 mg) in a conical flask.
The initial P concentration of the solutions varied from 10 to
500 mg P/L while fresh working solutions were prepared daily.
The pH of the P-solution, initially 6–6.5, was not adjusted, and after
the sorbents were added its value increased to 6.5–7 (the pH of the
suspension of the LDH sorbents in water was found to be 8.4–8.7
for the as-synthesized samples and 9.5–10 for the calcined samples, irrespective of the presence of Fe(III) or not in the LDH structure). The flasks were then shaken in a water bath at room
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Table 1
Physicochemical properties of Mg–Al and Fe(III)-substituted LDH samples.
Sample
Composition of LDH samples
d(003)
d(110)
a
c
In synthesis
mixture
In solidsc
(Å)
(Å)
(Å)
(Å)
(m2/g)
(cm3/g)
Mg3Al
Mg3Al
Mg3Al
Mg3Fe(III)0.4Al0.6
Mg3Fe(III)0.8Al0.2
Mg3Fe(III)
Mg2.63Al
Mg2.85Al
Mg2.53Al
Mg2.46Fe0.41Al0.60
Mg2.59Fe0.75Al0.20
Mg2.67Fe
2.5
0.2
0.2
2.5
2.5
2.5
0.65
0.39
0.34
0.62
0.54
0.67
7.708
7.849
7.868
7.691
7.781
7.736
1.527
1.531
1.530
1.534
1.548
1.554
3.054
3.062
3.060
3.068
3.096
3.108
23.124
23.547
23.604
23.073
23.343
23.208
83 (256)e
72
64 (281)
73
55
53 (143)
0.53 (0.86)e
0.27
0.38 (0.56)
0.39
0.50
0.58 (0.94)
Q¼
ðC init C fin ÞV
m
ð1Þ
where m (g) was the amount of sorbent used, V (L) the volume of
the solution, Cinit and Cfin the initial and final P concentrations of
the solution (mg L1) and Q (mg/g) the amount of P sorbed on the
solid. Isotherms of P uptake (Qeq) versus concentration of P at equilibrium in solution (Ceq) were plotted for every sorbent; all experiments were done in triplicate, and the data plotted are the average
of the three replications. The relative standard deviation in the analysis of phosphates was less than 4%. The effect of contact time on P
uptake (kinetic measurements) and on pH variation was also studied by conducting sorption experiments using the same batch system and solutions with initial P concentration of 50 mg P/L.
Samples were taken in regular intervals and P was determined as
in the case of the equilibrium sorption experiments.
Regeneration/re-use experiments were performed using a
mixed aqueous solution of NaCl (5 M) and NaOH (0.1 M). In a typical regeneration experiment, 0.2 g of the loaded with phosphates
LDH sorbents were mixed with 200 mL of the above solution and
stirred for 24 h at room temperature. The solid sorbents were separated by filtration and the desorbed/extracted phosphorus was
measured in the clear filtrate solution. The LDH samples were further washed with deionized water and dried at room temperature.
They were then characterized by XRD and were subjected to successive sorption/regeneration cycles in order to evaluate their
reusability. The sorption experiments were performed as above
using solutions with initial P concentration of 100 mg P/L.
3. Results and discussion
3.1. Physicochemical characteristics of Mg–Al and Fe(III)-substituted
LDH materials
3.1.1. As-synthesized (not calcined) LDH materials
Representative XRD patterns of the LDH materials are shown in
Fig. 1. The XRD patterns of all the synthesized Mg–Al and Fe-
110
113
Mg 3 Fe(III) (LDH6)
018
temperature until the solution reached equilibrium; 24 h were
found by preliminary experiments to be sufficient for the sorbent–ion mixture to reach equilibrium. The suspension was then
filtered by using a 0.45 lm membrane filter and the filtrates were
analyzed for phosphate concentration. Phosphate anions in aqueous solutions were determined by atomic absorption spectroscopy
(AAS) at 880 nm, following the ascorbic acid method [59]. Phosphate uptake by the sorbent was calculated by the following
equation:
012
015
e
006
d
Nitrate salts were used for the synthesis of all samples except for LDH3 (chloride salts).
From ICP-AES chemical analysis of the synthesized LDH samples.
Based on the results from elemental (Carbon) analysis and ICP-AES analysis (for metals Al, Fe) of the synthesized LDH samples.
From N2 sorption measurements, BET method.
Numbers in parentheses correspond to surface area and pore volume of the mixed-oxide samples (LDH-calc) derived from calcination of the respective LDH sample.
003
c
Pore volume at
P/P0 = 0.99
In solidsb
3+
3+
CO2
3 /(Al +Fe ) mole ratio
Intensity (arb. units)
a
b
Specific surface aread
In synthesis
mixturea
Mole ratio of metals
LDH1
LDH2
LDH3
LDH4
LDH5
LDH6
Lattice parameters (from XRD data)
Mg3 Fe(III)0.4 Al0.6 (LDH4)
Mg3 Al (LDH1)
5
15
25
35 45 55
2θ (degrees)
65
75
85
Fig. 1. XRD patterns of representative LDH materials: standard Mg3Al–CO2
3
hydrotalcite (LDH1) and partially (LDH4; Mg3Fe(III)0.4Al0.6–CO2
3 ) or totally
2
(LDH6; Mg3Fe(III)–CO3 ) Fe(III)-substituted LDH materials.
substituted LDH samples, exhibit the characteristic reflections of
the hydrotalcite structure and accordingly, the patterns can be indexed in a hexagonal lattice with an R3m rhombohedral space
group symmetry [21,22]. The high intensity of the main reflections,
i.e., the (0 0 3) at 11–12° 2h, the (0 0 6) at 23° 2h, and the (0 1 2)
at 34–35° 2h, reveals that the samples are highly crystalline.
The value of the crystallographic parameter a, which corresponds to the cation–cation distance in the brucite-like layer of
the LDH samples, has been calculated from the d-spacing of the
(1 1 0) reflection (a = 2d110). The value of parameter c, which is related to the thickness of the brucite-like layers and the interlayer
space, has been also calculated from the d-spacing of the (0 0 3)
reflection (c = 3d003); these values are given in Table 1. The values
of parameters a and c (3.054 and 23.124 Å, respectively) for the
LDH1 sample, which was synthesized in excess of carbonates, were
those expected for a Mg–Al hydrotalcite sample with Mg/Al 3 and
most of the charge-balancing anions being CO2
3 [21,60–62]. This is
3+
ratio determined for sample
further supported by the CO2
3 /M
LDH1 (Table 1), which is higher than the theoretical one (0.65 instead of 0.5 if we consider that each bivalent CO2
3 should compensate one cationic charge generated by each M3+ atom in the LDH
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K.S. Triantafyllidis et al. / Journal of Colloid and Interface Science 342 (2010) 427–436
structure). The higher value of the ratio could be ascribed to nonstoichiometric sorption of carbonates on the charged surface of
LDH. While the value of parameter a practically remains the same
in samples LDH2 and LDH3 since the same metals (Mg, Al) exist in
the LDH structure and in the same ratio, the value of parameter c
increases slightly due to partial replacement of carbonates in the
interlayer space by nitrates (in LDH2) and chlorides (in LDH3),
since these two anions result in higher d-spacing compared to carbonates [21,55,62]. This is again verified by the determined CO2
3 /
M3+ ratio (Table 1), which is lower than 0.5 for both the above samples. Gradual isomorphous substitution of Al3+ (with ionic radius
0.50 Å) for Fe3+ (ionic radius 0.64 Å) in the LDH structure (samples
LDH4, LDH5 and LDH6) resulted in higher a values, the increase
being more pronounced in the fully substituted sample LDH6 for
which a = 3.108, as expected for the pyroaurite structure [21].
The increase of the value of parameter c was marginal for these
three samples, since all of them contained mainly carbonates in
the interlayer space (Table 1). The a value appears to be very sensitive to the type of the M2+ and/or M3+ cations, since in a previous
study where Fe(II) substituted Mg(II) in the LDH structure, this value was decreased from the Mg–Al sample to the MgFe(II)–Al sample since the ionic radius of Fe(II) is smaller than that of Mg(II) [56].
The chemical analysis by ICP-AES showed that the determined
M2+/M3+ ratio in all the synthesized samples (Table 1) was similar
to the nominal ratio used in the synthesis mixture, indicating the
successful formation of the LDH structure in accordance with the
XRD results.
The N2 adsorption–desorption isotherms of representative
Mg–Al and Fe-substituted LDH samples are shown in Fig. 2; the
inset shows the pore size distribution determined by the Barrett–
Joyner–Halenda (BJH) analysis using the adsorption data. The
adsorption isotherms of the Mg–Al LDH samples exhibit the characteristics of the types II and IV isotherms (according to IUPAC
800
3 -1
N2 volume adsorbed (cm g / STP)
1000
Dv(log d)(cc/g) .
1200
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(B)
(E)
(A)
(D)
(E)
(C)
10
100
1000
Pore Diameter (Å)
10000
600
(D)
400
200
(C)
(B)
(A)
classification); the former type is usually associated with non-porous or macroporous materials which allow unrestricted monolayer–multilayer adsorption to occur at high P/P0 values, while
the latter type is related to mesoporous materials [63]. The isotherms of the Mg–Al samples (as can be seen in Fig. 2 for LDH1
and LDH2) exhibit clear hysteresis loops, which are characteristic
for the type IV adsorption isotherms. The shape of the hysteresis
loop changes from sample LDH-1 (Mg3Al with more carbonates)
to sample LDH2 (Mg3Al with relative less carbonates in the presence of nitrates). The hysteresis loop for LDH2 resembles type H1
loops (IUPAC classification) which are related to relatively uniform
mesopores, while the loop for LDH1 tends to adopt the shape of
type H3, which is related with slit-shaped pores or voids created
within aggregates of platy particles [63]. The isotherms (not shown
for brevity) of sample LDH3 (Mg3Al with relative less carbonates in
the presence of chlorides) show an intermediate shape between
those of LDH1 and LDH2. Similar N2 adsorption–desorption isotherms have been previously shown for Mg–Al LDH materials [56].
The adsorption isotherms of Fe-substituted LDH samples
resemble more those of type II, mainly at high Fe loadings (samples
LDH5 and LDH6), similarly to previously reported results [53]. Furthermore, the hysteresis loops observed in the isotherms of Mg–Al
LDH samples, tend to be minimized in the Fe-substituted samples
(LDH4, LDH5, LDH6).
The pore size distributions (PSD) of the LDH samples (inset in
Fig. 2) are relatively broad (compared for example to the PSD of ordered mesoporous materials of the MCM-type). The Mg–Al LDH2
sample which contains carbonates and nitrates in the intragallery
region exhibits the narrower distribution with an average pore
diameter of about 20 nm while the distribution for LDH1 (contains
mainly carbonates in the galleries) is broader and centered at
about 40 nm. The pore size distribution for LDH3 (not shown)
which contains carbonates and chlorides (instead of nitrates) is
similar to that of LDH2 exhibiting maximum at about 30 nm. The
pore size distribution for LDH4 (Fe-substituted sample with low
Fe loading; 40% substitution of Al) resembles that of LDH2, being
relatively narrow and exhibiting a maximum at about 30 nm. On
the other hand, as the Fe loading is increased in samples LDH5
(80% substitution of Al) and LDH6 (100% substitution of Al), the
distributions become broader and the average pore diameter increases significantly (about 60 nm for LDH5 and >100 nm for
LDH6).
The specific surface area (BET method) and the total pore volume for all the LDH samples are given in Table 1. It can be seen that
sample LDH1 exhibits the highest surface area (83 m2/g) among
the Mg–Al LDH samples as well as the highest total pore volume
(0.53 cc/g). The addition of Fe induces a gradual decrease in surface
area (lowest at 53 m2/g for LDH6). On the other hand, the total pore
volume remains high (except in the case of sample LDH4). No
microporosity (by V–t plot analysis) has been found in all samples.
The particle morphology of the LDH samples can be seen in the
SEM images presented in Fig. 3. The particles of both Mg–Al and
Fe-substituted LDH samples are aggregates (1–10 lm) of smaller
primary crystallites with irregular size and shape, and rough surface. However, the particles of the Mg–Al LDH1 sample appear to
be denser compared to those of the Fe-modified LDH6 sample, in
which they are formed by the aggregation of very small platy
crystallites.
0
0.0
0.2
0.4
0.6
0.8
1.0
P/Po
Fig. 2. N2 adsorption–desorption isotherms and pore size distribution based on BJH
analysis of adsorption data (inset) for representative as-synthesized LDH samples:
2
(A) LDH2 (Mg3Al–CO2
3 =NO3 ), (B) LDH1 (Mg3Al–CO3 ), (C) LDH4 (Mg3Fe(III)0.4Al0.6–
2
2
CO2
3 ), (D) LDH5 (Mg3Fe(III)0.8Al0.2–CO3 ), (E) LDH6 (Mg3Fe(III)–CO3 ); isotherms
are offset by 150 cm3/g.
3.1.2. Mixed-oxides derived from calcination of LDH materials
The as-synthesized LDH samples were calcined in air at relatively low/moderate temperature (450 °C) in order to convert them
to the respective mixed oxides (LDH-calc). Characterization data
for the mixed oxides are presented in Fig. 4 (XRD) and Fig. 5 (N2
adsorption–desorption isotherms); porosity characteristics (surface area and total pore volume) are given in Table 1. The XRD pat-
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K.S. Triantafyllidis et al. / Journal of Colloid and Interface Science 342 (2010) 427–436
2
Fig. 3. SEM images of (a and b) LDH1 (Mg3Al-CO2
3 ) and (c and d) LDH6 (Mg3Fe(III)-CO3 ).
5
15
25
35
45
2θ (degrees)
55
65
75
Fig. 4. XRD patterns of the mixed-oxides derived from calcination at 450 °C in air of
2
the samples LDH1 (Mg3Al-CO2
3 ) and LDH6 (Mg3Fe(III)-CO3 ).
terns of both the Mg(Al)O (derived from LDH1) and Mg(Fe)O (derived from LDH6) correspond to that of the MgO phase (periclase)
[24,53,57]. The shape of the N2 adsorption–desorption isotherms
(Fig. 5) for both mixed oxides is similar to that observed for the
corresponding parent samples (Fig. 2). However, the surface area
and total pore volume have been significantly increased in all
cases, as can be seen in Table 1 for the calcined samples, in accordance with previously reported results [53]. No microporosity has
been determined in the mixed oxides (similar to the parent samples), indicating that the observed increase of surface area and pore
(B)
0.8
0.6
0.4
0.2
0.0
10
3
Mg(Al)O mixed oxide
from calcination of LDH1
(A)
1.0
Dv(log d) (cc/g)
800
-1
N2 volume adsorbed (cm g / STP)
Intensity (arb. units)
.
1000
.
1.2
Mg(Fe)O mixed oxide from
calcination of LDH6
600
100
1000
Pore Diameter (Å)
10000
(B)
400
200
(A)
0
0.0
0.2
0.4
0.6
0.8
1.0
P/Po
Fig. 5. N2 adsorption–desorption isotherms and pore size distribution based on BJH
analysis of adsorption data (inset) for representative mixed-oxides derived from
calcination of the LDH samples: (A) from LDH1 (Mg3Al–CO2
3 ) and (B) from LDH6
3
(Mg3Fe(III)–CO2
3 ); isotherms for (B) are offset by 400 cm /g.
volume is mainly attributed to the increased mesoporosity in the
calcined LDH samples. This is also corroborated by the presence
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K.S. Triantafyllidis et al. / Journal of Colloid and Interface Science 342 (2010) 427–436
of the relatively small peak in the pore size distribution curve of
both mixed oxides shown in Fig. 5, which exhibit maxima in the
range of 2.4–3.5 nm. No significant changes in the morphology of
the particles could be identified after calcination, at least at the
micrometer scale, as evidenced from SEM images of the mixed oxides (not shown for brevity), compared to the parent LDH samples.
3.2. Removal of phosphates with the LDH sorbents
3.2.1. Sorption by as-synthesized LDH samples
There has been considerable interest in the use of LDHs to remove negatively charged species from surface water and wastewater. LDHs can take up anion species from solution by three different
mechanisms: surface adsorption, interlayer anion-exchange and
re-construction of a calcined LDH precursor by the ‘‘memory
effect” [64,65]. The anion-exchange process of the as-synthesized
(not calcined) LDHs is mainly influenced by the charge-balancing
anions in the interlayer and the layer charge density. Fig. 6a presents the effect of Mg–Al LDHs charge-balancing ions on phosphates sorption ability. The maximum phosphate uptake of the
‘‘standard” Mg–Al LDH (LDH1) having mainly carbonates in the
interlayer was found to be 15 mg P/g sorbent, while the sorption
affinity was increased, when the carbonate ions were replaced partially by nitrates or chlorides. The Mg–Al LDH sample (LDH3),
which contains both carbonates and chlorides, exhibited a higher
sorption capacity for lower P initial concentrations compared to
the carbonate/nitrate containing sample (LDH2). However, the
maximum uptake for these two sorbents was similar (about
28 mg P/g). The observed relatively low sorption capacity for all
the as-synthesized LDH samples can be attributed to the high affinity of the Mg–Al hydrotalcite structures for carbonate anions [21].
On the other hand, when a Mg–Al LDH sample containing mainly
chlorides as charge-balancing anions was tested, the sorption
capacity was increased to 80 mg P/g sorbent. This LDH sample
was prepared by calcination of the parent Mg–Al LDH sample
which contained mainly carbonates, consequent loading with
phosphates and regeneration with alkaline solution of NaCl, as described in the experimental section and in the results of Section
3.2.4 and Table 3. The superior performance of the Mg–Al LDHs
containing chlorides as charge-balancing anions for the removal
of phosphates has also been previously recognized [27,34].
a
With regard to the iron-substituted LDHs (samples LDH4, LDH5
and LDH6, having mainly carbonates as charge-balancing anions)
in which Al3+ was partially or totally substituted by Fe3+, we can
observe a gradual decrease in phosphates removal efficiency by
increasing the iron loading (Fig. 6b). The performance of LDH4, in
which only a small portion (40%) of Al3+ was substituted by Fe3+,
is similar to that of the parent LDH1, while both LDH5 (80% substitution of Al3+ by Fe3+) and LDH6 (total substitution of Al3+ by Fe3+)
samples exhibited a lower sorption ability. The structural (XRD),
porosity (N2 sorption) and morphological (SEM) characteristics of
the Mg–Al and Fe-substituted LDH samples, as described above,
do not differ to such a significant extent, so as to support the observed lower P sorption ability of the latter type of sorbents. Therefore, it may be postulated that the replacement of Al3+ by Fe3+ in
the LDH structure strengthens the bond between the doublehydroxide layer and carbonates by increasing the positive surface
charge, leading to an increased affinity for carbonates over other
anions.
3.2.2. Sorption by mixed-oxides derived from calcination of LDH
The mixed oxides (LDH-calc) that derive from the calcination of
the LDH solids exhibit a much higher surface area (as is also evidenced from the samples of our work, see Table 1) and they usually
show superior sorption properties compared to the parent LDH
samples [35,40]. The sorption results for the mixed-oxides derived
from the calcination of the LDH samples are shown in Fig. 7. It can
be seen that for both the Mg–Al and Fe-modified LDHs, the sorption efficiency of their calcined analogues (mixed oxides) is much
higher than that of the corresponding parent LDH samples.
The maximum phosphate uptake by the calcined Mg–Al LDH
sample that contained mainly carbonates (LDH1), for the range of
phosphate concentration tested, was found 220 mg/g, 15-fold
higher than the corresponding as-synthesized sample (Fig. 7a).
The behavior of the calcined LDH2 sample (parent LDH contained
both carbonates and nitrates) is similar to that of LDH1. On the
other hand, calcination of the Mg–Al LDH sample which contained
both carbonates and chlorides (LDH3) resulted in a lower sorption
capacity compared to the other two calcined LDHs. With regard to
the Fe-substituted samples, the reverse trend is observed for the
sorption efficiency of the calcined LDHs compared to the as-synthesized materials. Increasing the loading of Fe3+ in the parent
LDH4, LDH5 and LDH6 samples resulted in monotonic increase of
b 100
100
LDH as - synthesized
LDH1[Mg 3Al-CO 3 ]
LDH4 [[Mg3Fe(III)0.4 Al0.6-CO3]
LDH5 [Mg 3Fe(III)0.8 Al0.2-CO3 ]
Qeq [mg P/g]
Qeq [mg P/g]
LDH6 [Mg 3Fe(III)-CO3]
10
10
LDH3 [Mg3Al-Cl / CO3 ]
LDH2 [Mg3Al-NO3 / CO3]
LDH as - synthesized
LDH1 [Mg3Al-CO3]
1
0.001 0.01
0.1
1
10
C eq [mg P/L]
100 1000
10
4
1
0.001 0.01
0.1
1
10
100
1000
4
10
C eq [mg P/L]
Fig. 6. Sorption of phosphates by various as-synthesized (not calcined) Mg–Al and Fe(III)-substituted LDH materials: Effect of (a) the type of anion and (b) the degree of Fe
substitution (equilibrium batch experiments, measurements taken at 24 h).
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K.S. Triantafyllidis et al. / Journal of Colloid and Interface Science 342 (2010) 427–436
a 1000
b 1000
LDH 1
LDH1 calcined
LDH2 calcined
LDH3 calcined
Qeq [mg P/g]
Qeq [mg P /g]
100
10
1
0.001 0.01
LDH4 calcined
LDH5 calcined
LDH6 calcined
LDH1
LDH1 calcined
100
10
0.1
1
10
100
1000
4
10
1
0.001 0.01
0.1
1
10
100
1000
104
Ceq [mg P/L]
Ceq [mg P/L]
Fig. 7. Sorption of phosphates by various calcined Mg–Al and Fe(III)-substituted LDH materials: Effect of (a) the type of anion and (b) the degree of Fe substitution of the
parent LDH samples (equilibrium batch experiments; measurements taken at 24 h).
3.2.3. Sorption models and mechanistic aspects
In order to better understand the mechanism of P removal by
the LDH sorbents, it is important to identify the nature (type) of
phosphate anions present in the aqueous solution to be treated.
Phosphates exist in different ionic states such as monovalent
2
3
H2 PO
4 , divalent HPO4 and trivalent PO4 ions depending on the
pH of the solution. The dissociation equilibrium of H3PO4 can be
written as follows [34]:
2
2
ture of H2 PO
4 and HPO4 , and at pH 9–10 as HPO4 anions. Thus,
the initial anions (monovalent nitrates or chlorides and bivalent
carbonates) can be exchanged by either the monovalent or bivalent
phosphates available in solution, depending on its pH.
Classical sorption models have been extensively used to describe the equilibrium established between the pollutant ions on
sorbent and their concentration in solution, at a constant temperature. The Freundlich equation (Eq. (2)), is the most important
multi-site sorption isotherm for heterogeneous surfaces.
Q eq ¼ K F C 1=n
eq
where Qeq is the quantity of solute sorbed per unit weight of solid
sorbent, Ceq is the concentration of solute in the solution at equilibrium and KF and 1/n are constants related to the sorption capacity
and the sorption intensity respectively (1/n < 1). Langmuir equation
(Eq. (3)), is the most important model for monolayer sorption.
Q eq ¼
Q max K L C eq
1 þ K L C eq
ð3Þ
where KL is an energy term which varies as a function of surface
coverage strictly due to variations in the heat of sorption and Qmax
is the maximum loading capacity.
þ
þ
H3 PO4 $ H2 PO4 $ Hþ $ HPO2
$ PO3
4 þ 2H
4 þ 3H
100
% removal of phosphates
where pK1 = 2.15, pK2 = 7.20 and pK3 = 12.33. In the present study
the pH was not adjusted or kept constant by the use of any buffer
solutions. As mentioned in the experimental section, the pH of
the phosphate aqueous solutions was 6–6.5 and after addition of
the LDH sorbents it was raised instantly to 6.5–7. However, as it
can be seen in Fig. 8, the pH changed with time and reached gradually within the first 2 h a value of 9.5 for the as-synthesized LDHs
and a value of 10 for the calcined LDHs, after which it remained constant. These pH values are similar to those found for solid–water
suspensions (see experimental) for the as-synthesized and calcined
LDHs, respectively. This indicates that the uptake of phosphates by
the sorbents and their removal from the aqueous phase is practically completed within the first 2 h of the process, and this is confirmed by the kinetic experiment curves shown also in Fig. 8, for
both the parent (as-synthesized) and the calcined LDH2 sample.
From speciation calculations, it was found that at pH 6.5 the phosphates exist in solution mainly as H2 PO
4 , at pH 7.5 as a 50:50 mix-
ð2Þ
14
80
12
60
10
40
pH
the sorption capacity of their calcined analogues (Fig. 7b). The calcined sample LDH5 (with 80% substitution of Al3+ with Fe3+)
showed similar sorption behavior with that of the calcined LDH1
sample. A significantly higher maximum phosphate uptake
(300 mg P/g) was however observed with the calcined LDH6 (total
substitution of Al3+ with Fe3+). Das et al. [33], showed that the calcined Mg–Al LDH material was slightly more efficient for phosphates removal compared to calcined Mg–Fe LDH. A maximum
loading capacity of 44 mg/g was observed in that work with the
calcined Mg–Al LDH sample, which is significantly lower compared
to the maximum sorption capacity measured in the present study
with the calcined Mg–Fe LDH sample (300 mg P/g). The relatively
lower adsorption dose (0.4 g/L) and/or initial phosphate concentration (50 mg L1) used in the work of Das et al. [33] could be the
reason for this discrepancy.
8
20
LDH 2 calcined
LDH 2
0
0
30
60
6
90 120 150 180 210 240
contact time (min)
Fig. 8. Effect of contact time on solution pH variation and% removal of phosphates
by as-synthesized (LDH2; Mg3Al–CO2
3 =NO3 ) and corresponding calcined LDH
sample (Cinit = 50 mg L1) (open symbols: phosphate removal, closed symbols: pH).
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K.S. Triantafyllidis et al. / Journal of Colloid and Interface Science 342 (2010) 427–436
a
b
16
14
LDH 1c
LDH 5c
LDH 6c
300
12
250
10
Qeq [mg P /g]
Qeq [mg P /g]
350
8
6
4
2
LDH 1
LDH 5
LDH 6
solid line: Freundlich model
dash line: Langmuir model
0
0
50
100
150 200
Ceq [mg P/L]
200
150
100
50
solid line: Freundlich model
dash line: Langmuir model
0
250
300
0
50
100
150 200 250
Ceq [mg P/L]
300
Fig. 9. Sorption isotherms of phosphates for three representative LDH sorbents: (a) for as-synthesized, and (b) for the calcined samples. The Freundlich model fit is indicated
by the solid lines and the Lamgmuir model fit by the dashed lines.
Table 2
Equilibrium parameters of the Freundlich and Langmuir models for the as-synthesized and calcined LDH sorbents.
Sorbent
LDH1
LDH2
LDH3
LDH4
LDH5
LDH6
LDH1-calc
LDH2-calc
LDH3-calc
LDH4-calc
LDH5-calc
LDH6-calc
Freundlich model
Langmuir model
R2
KF
n
R2
Qmax
KL
0.97
0.96
0.90
0.94
0.94
0.97
0.92
0.93
0.74
0.93
0.84
0.96
5.69
6.13
14.64
6.07
2.78
1.41
19.95
7.96
10.98
17.14
16.68
28.60
5.47
3.64
8.56
5.86
4.20
2.93
2.36
1.84
3.67
2.87
2.39
2.25
0.63
0.71
0.61
0.62
0.60
0.82
0.89
0.89
0.88
0.88
0.83
0.94
13.12
23.78
23.29
12.19
10.10
9.69
247
215
45.9
117
134
356
0.365
0.102
1.219
0.520
0.064
0.032
0.016
0.009
0.065
0.032
0.041
0.020
mechanism involves the rehydration of mixed metal oxides and
concurrent intercalation of oxyanions into the interlayer to reconstruct the LDH structure. The XRD patterns (Fig. 10) of the P-loaded
calcined LDHs verified the re-construction of the Layered Double
Hydroxide structure of the materials during sorption of the phosphates from the aqueous solutions. The basal spacing of the Ploaded LDH1 sample (after calcination and phosphates sorption)
was 7.79 Å which is close to that observed for the parent as-synthesized LDH1 sample (Table 1). Similar basal spacing values for
Mg–Al LDH with phosphates as charge-balancing anions have also
been previously reported by Frost et al. [37]. However, they also
showed that the basal spacing depends on the pH of the solution
thus affecting the type of phosphates anion present and the degree
of their hydration when attached to the LDH layer. In
another
0
work, a maximum increase of the basal spacing by 0.7 Å
A occurred
when the chlorides of the parent Mg–Al LDH sorbent were completely exchanged with phosphates [67].
LDH6 after calcination, adsorption of
phos phates & re generation with NaCl
LDH6 after calcination &
adsorption of phosphates
Intensity (arb. units).
The experimental phosphorus-uptake isotherms for all the assynthesized and calcined LDH sorbents have been fitted to the
Langmuir and Freundlich equations. The isotherms for representative LDH sorbents are shown in Fig. 9. It is obvious that the Langmuir model cannot predict satisfactory the sorption properties of
the as-synthesized LDH sorbents (Fig. 9a). On the other hand, the
Freundlich model provides a very good fit of the sorption data
points for all the as-synthesized LDH sorbents, as is confirmed by
the relative high values of the correlation coefficients shown in
Table 2 (each data series was fitted to the Freundlich model by
employing non-linear regression analysis). Similarly, the sorption
data for the calcined LDH sorbents was smoothly fitted by the Freundlich model (solid lines in Fig. 9b and data in Table 2). However,
in this case, the Langmuir model provided also a reasonably good
fit (dashed lines in Fig. 9b and respective correlation coefficients
in Table 2), at least for the Ceq range of the present study.
The maximum sorption capacity estimated from the Langmuir
isotherms for the calcined Mg–Al LDH was 250 mg P/g while
the respective value for the calcined Fe-substituted LDH6 (in which
Al3+ has been completely replaced by Fe3+) was 350 mg P/g. This
latter value is the highest maximum sorption capacity reported so
far, at least to our knowledge [65,66].
The significant difference in the sorptive behavior among the
calcined and the uncalcined LDHs can mainly be attributed to the
different mechanisms of the anions sorption. For the uncalcined
material, the sorption process is primarily due to the ion-exchange
of the interlayer anions. For the calcined material, the sorption
LDH1 after calcination, adsorption of
phos phates & re generation with NaCl
LDH1 after calcination &
adsorption of phosphates
5
15
25
35
45
2θ (degrees)
55
65
2
Fig. 10. XRD patterns of LDH1 (Mg3Al-CO2
3 ) and LDH6 (Mg3Fe(III)-CO3 ) after
calcination at 450 °C followed by sorption of phosphate ions and regeneration with
NaCl/NaOH aqueous solution.
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K.S. Triantafyllidis et al. / Journal of Colloid and Interface Science 342 (2010) 427–436
In order to further elucidate the mechanism of phosphates sorption by the LDH sorbents we estimated the mole ratio between adsorbed P and M3+ of the LDH materials. If it is assumed that each
positive charge, generated by one M3+ atom in the LDH structure,
is potentially able to participate in the sorption/anion-exchange
process that occurs with the as-synthesized LDHs, then the above
ratio should have a value of 0.5 if the total P adsorbed was considered as bivalent HPO2
4 . However, the values of this ratio that we
estimated for all the as-synthesized sorbents which contained significant amounts of carbonates (Table 1) were between 0.03 and
0.08, indicating that only a small portion of the potentially
exchangeable sites have been utilized, mainly due to the high affinity of the LDH structure for the parent carbonates, as it is explained
above (Section 3.2.1). This could be also the reason why the Langmuir model does not fit the sorption data of the as-synthesized
LDHs, although one would expect that sorption on the specific exchange sites on the surface of LDH layers would successfully satisfy
the requirements and assumptions of that model. On the other
hand, the above ratio for the Mg–Al LDH sample that contained
mainly chlorides was estimated to 0.25, indicating the enhanced
mobility of the chloride anions compared to carbonates. Significantly higher values of the above ratio (between 0.2 and 1.1) were
however estimated for the calcined forms of the LDH sorbents. This
is an additional indication that the re-construction process of the
LDH utilizes a great deal of the available ‘‘pollutant” anions. The
values of the above ratio which are higher than 0.5 could further
suggest that sorption of phosphates occur also on sites that are
not associated with the exchangeable charge-balancing anions of
the LDH structure, via different type of interactions.
3.2.4. Regeneration/re-use of LDH sorbents
The possibility of regeneration and re-use of sorbents is one of
the desirable features of all such materials. However, it is usually
difficult and rather costly part of sorption processes, accounting
for over 70% of the total operating and maintenance cost of a sorption system [65]. A successful desorption/regeneration process
should in principle restore the initial properties of the sorbent,
while sorbates may be recovered for proper disposal or for re-use
if there is such a market demand. Using various alkaline salt solutions, oxyanion-loaded LDHs have been successfully regenerated.
The desorption of phosphates from the LDHs may be achieved
using mixed aqueous solutions of NaCl and NaOH [35,36].
In order to investigate the regeneration/reusability of the LDH
sorbents of the present study, representative samples were subjected to repeated sorption/desorption cycles, as described in the
experimental section. The calcined LDH1 and LDH6 samples were
loaded with phosphates and were then subjected to regeneration
by treating with the alkaline NaCl solution. The desorption efficiency shown in Table 3 was 70% and 75%, respectively, for the
two LDH sorbents. The regenerated sorbents were characterized
by XRD (Fig. 10). Both samples exhibited the characteristics peaks
of the LDH structure; however, the regenerated samples were not
as highly crystalline as their as-synthesized analogues, suggesting
a partial deformation of the Layered Double Hydroxide structure.
Nevertheless, when the regenerated LDHs were subjected to a
subsequent sorption step, they exhibited a phosphate uptake significantly higher compared to the uptake provided by their as-synthesized analogues. In Table 3 it can be seen that the regenerated
LDH1 (Mg–Al LDH sample) and LDH6 (Mg–Fe LDH sample) sorbed
75 and 82 mg P/g solid which is significantly higher compared to
the sorption capacity of the same LDHs in their parent, as-synthesized form in which they contained mainly carbonates as chargebalancing anions (Fig. 6 and discussion in Section 3.2.1). This
increase in sorption ability is attributed to the presence mainly
of chlorides in the interlayer space after the first regeneration step
with concentrated NaCl solution.
Table 3
Regeneration and re-use of LDH sorbents.
Samplea
Desorption
efficiency1b (%)
P uptakec (after
the 1st sorption/
regeneration
cycle) (mg/g)
Desorption
efficiency2d (%)
P uptakec (after
the 2nd sorption/
regeneration
cycle) (mg/g)
LDH1 (R)
LDH6 (R)
70
75
75.2
82.0
72
74
59.6
66.9
a
The samples shown in the table have undergone the following successive
treatments: (1) calcination of parent, as-synthesized samples, (2) equilibrium
sorption experiments for 24 h using solutions with initial P concentration of
100 mg L1, (3) regeneration with NaCl/NaOH aqueous solution for 24 h (desorption
efficiency-1 in 2nd column), (4) repeat of step 2 (3rd column), (5) repeat of step 3
(4th column), and (6) repeat of step 2 (5th column).
b
It is estimated based on the amount of desorbed phosphates from the loaded
calcined LDH samples.
c
These values can be compared with the P uptake by the as-synthesized LDHs
since regeneration with NaCl/NaOH provided re-constructed LDH structures
(Fig. 10).
d
It is estimated based on the amount of desorbed phosphates from the loaded
regenerated LDH samples after the 1st sorption/regeneration cycle.
A second regeneration step with the alkaline NaCl solution provided the same desorption efficiency (Table 3), while a subsequent
sorption step led to a somehow lower phosphate uptake (80% of
initial uptake). Since desorption efficiency with the specific alkaline
NaCl solution seems to be stable between successive sorption/
regeneration cycles, the observed decrease in sorption ability after
the second regeneration step could be attributed to partial dissolution of the LDH sorbent. We performed stability studies of representative LDH samples and it was found (not shown) that after contact
with an aqueous solution of pH 7 for 24 h, a 10% of the solid LDH was
dissolved, while for pH 8 and higher the loss was less than 1%. The
above results indicate that the development of a stable sorption
system based on LDH materials for potential practical applications
requires optimization of the regeneration conditions, such as type
of eluting agent(s) and pH conditions.
4. Conclusions
Highly crystalline Fe(III)-substituted (with partial or complete
replacement of Al3+ with Fe3+) hydrotalcite-like materials (LDHs)
have been synthesized via simple co-precipitation procedures,
usually applied for the synthesis of LDHs. The as-synthesized
LDH sorbents, having chlorides or nitrates as charge-balancing anions in addition to carbonates, were more efficient for the removal
of phosphates compared to those that possessed mainly carbonates
in the interlayer space due to the high affinity of these solids for
carbonates. Replacement of Al(III) by Fe(III) in the LDH samples
containing carbonates led to a small decrease in sorption efficiency, indicating a stronger bonding of carbonates due to the presence of Fe(III). When the anions were mainly chlorides the LDH
sorbents exhibited significantly higher phosphates’ uptake (as high
as 80 mg P/g sorbent) compared to the samples containing mainly
carbonates. The calcined forms of the LDHs were more effective
sorbents; a maximum sorption capacity (estimated by the Langmuir fit of the sorption data) of 250 mg P/g was observed for
the calcined Mg–Al LDHs and 350 mg P/g for the calcined
Fe(III)-substituted sorbent. This latter value is the highest one reported so far, at least to our knowledge, for the various types of
phosphates sorbents that are being studied. The sorption data for
both the as-synthesized and calcined LDHs was best fitted by the
Freundlich model, while the Langmuir model exhibited also an
acceptable fit of the sorption data for the calcined LDHs and it
was used in order to estimate the maximum sorption capacities
for these sorbents. It was also shown that only a relatively small
portion of the positively charged sites (exchangeable anions) were
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K.S. Triantafyllidis et al. / Journal of Colloid and Interface Science 342 (2010) 427–436
utilized in the as-synthesized LDH sorbents during removal of
phosphates via ion-exchange pathways. On the other hand, the significant higher amount of phosphates sorbed on the calcined LDHs
(30-fold higher removal efficiency compared to the as-synthesized
LDHs) can be attributed to both the re-construction of the layered
structure which utilized a great deal of phosphates as well as to the
adsorption on sites different from the interlayer anion-exchange
sites, which can exist on the external surface or on defects of the
LDH crystals. Both the Mg–Al and Fe-substituted LDH sorbents
were successfully regenerated with mixed aqueous solution of
NaCl and NaOH and were reused with a small loss of sorption
efficiency.
Acknowledgments
The partial support from the European Union and the Greek
General Secretariat for Research and Technology via programme
EPAN is gratefully acknowledged. The authors would like also to
thank Prof. N. Lazaridis for helpful discussions on sorption process
by LDHs.
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