Article
pubs.acs.org/est
Removal of Phosphate from Eutrophic Lakes through Adsorption by
in Situ Formation of Magnesium Hydroxide from Diatomite
Fazhi Xie,†,‡,§ Fengchang Wu,†,* Guijian Liu,‡,* Yunsong Mu,† Chenglian Feng,† Huanhua Wang,†
and John P. Giesy∥
†
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences,
Beijing 100012, China
‡
CAS Key Laboratory of Crust-Mantle Materials and Environment, School of Earth and Space Sciences, University of Science and
Technology of China, Hefei 230026, China
§
Key Laboratory of Functional Molecule Design and Interface Process, School of Materials Science and Chemical Engineering, Anhui
Jianzhu University, Hefei 230022, China
∥
University of Saskatchewan, Department of Veterinary Biomedical Sciences and Toxicology Centre, 44 Campus Drive, Saskatoon,
Saskatchewan, Canada, S7N 5B3
S Supporting Information
*
ABSTRACT: Since in situ formation of Mg(OH)2 can efficiently sorb phosphate
(PO4) from low concentrations in the environment, a novel dispersed magnesium
oxide nanoflake-modified diatomite adsorbent (MOD) was developed for use in
restoration of eutrophic lakes by removal of excess PO4. Various adsorption
conditions, such as pH, temperature and contact time were investigated. Overall,
sorption capacities increased with increasing temperature and contact time, and
decreased with increasing pH. Adsorption of PO4 was well described by both the
Langmuir isotherm and pseudo second-order models. Theoretical maximum
sorption capacity of MOD for PO4 was 44.44−52.08 mg/g at experimental
conditions. Characterization of PO4 adsorbed to MOD by use of X-ray powder
diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and solid state 31P
nuclear magnetic resonance revealed that electrostatic attraction, surface
complexation and chemical conversion in situ were the major forces in adsorption
of PO4. Mg(OH)2 formed in situ had a net positive charge on the surface of the
MOD that could adsorb PO43‑ and HPO42‑ anion to form surface complex and gradually convert to Mg3(PO4)2 and MgHPO4.
Efficiency of removal of PO4 was 90% when 300 mg MOD/L was added to eutrophic lake water. Results presented here
demonstrated the potential use of the MOD for restoration of eutrophic lakes by removal of excess PO4.
■
activated alumina,16,17 ferric (hydr)oxides2 and polymer hydrogels18 for adsorption of PO4 have been investigated .
Due to greater control of external inputs of PO4 in the last few
decades, internal PO4 loading by release of PO4 from sediments
can control trophic status of the lakes.19 Among the large number
of in-lake control measures,3 adsorption of PO4 by use of
limestone, fly ash, gypsum, calcite, bentonite clay, and diatomite
have been used to control eutrophication by removal of
PO4.2,20−24 But use of these adsorbents has been restricted
because they do not have sufficient capacities for adsorption or
contain toxic compounds that can be released to the water during
treatment. Therefore, these conventional adsorbents might not
be feasible in practical treatment of wastewaters or restoration of
eutrophic lakes.2
INTRODUCTION
Phosphate (PO4) is often the limiting factor for primary
production by phytoplankton in lakes and excess PO4 can result
in hazardous algal blooms. Due to widespread development of
industry and agriculture in China point source wastewater and
nonpoint source runoff can transport PO4 to surface waters, such
as lakes and reservoirs and result in cultural eutrophication,
which affects wildlife and possibly health of humans and as a
result limit sustainable social development.1,2 To address
eutrophication, limiting PO4 can stop eutrophication from
occurring or becoming more sever, but reducing loadings alone
cannot reverse eutrophication.3Recently, removal of PO4 by use
of chemical precipitation, adsorption and biological treatment
have received attention.4,5 Among the various techniques
available to remove PO4 adsorption is simple and economical
and the nutrient-loaded sorbent can be used in agriculture as PO4
fertilizer and as a conditioner of soils.6 Various adsorbents such as
sands,7−9 fly ash,10,11 steel slag,12 red mud,13 zeolite,14,15
© 2013 American Chemical Society
Received:
Revised:
Accepted:
Published:
582
August 21, 2013
December 14, 2013
December 16, 2013
December 16, 2013
dx.doi.org/10.1021/es4037379 | Environ. Sci. Technol. 2014, 48, 582−590
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washed with deionized water until free from Cl−. The residue was
dried at 323 K for 24 h and then stored at room temperature. The
material produced was magnesium hydroxide modified diatomite
(MHD). MOD was obtained by calcination of MHD at 723 K.33
Characterization of the Adsorbent. The zero point of
charge (pHzpc) of adsorbents (raw diatomite, MHD and MOD)
were measured by using mass titrations.36 Surface morphology
was determined by use of a JEOL JSM-6700F scanning electron
microscope (SEM). X-ray diffraction analysis (XRD) to
investigate crystalline structures was conducted by use of a
Rigaku TTR-III X-ray diffraction system equipped with Cu Kα
radiation (λ = 0.15406 nm). Chemical composition of PO4
adsorbed to MOD was probed using X-ray photoelectron
spectroscopy (XPS, ESCAL AB250) with Al Kα radiation (1487
eV) at a power of 300 W under a vacuum of 1.0 × 10−5 Pa. Data
were analyzed by use of XPSpeak 41 software. N2 adsorption−
desorption analysis was performed on each sorbent by
ASAP2020 M+C system (Micromeritics). Surface areas
measured by use of the Brunauer−Emmentt−Teller (BET)
method. The solid state 31P nuclear magnetic resonance
spectrum was collected on a Bruker Avance III 400 spectrometer
operating at 161.984 MHz, by use of a Doty Scientific 4 mm H/
X/Y MAS probe.
Effect of pH on Sorption of PO4. Adsorbents (0.20 mg)
(raw diatomite, MHD and MOD) and 40 mL of 50 mg/L
standard KH2PO4 solutions were added to 50 mL centrifuge
tubes with initial pH adjusted to 5.0−10.0 with dilute NaOH and
HCl, and shaken until equilibrated (12h) at a temperature of 298
K. Supernatant was filter using a 0.45 μm filter and
concentrations of PO4 in supernatant were determined using
the method of Murphy and Riley.37 The limit of quantification of
PO4 was 0.01 mg/L and the limit of detection was 1.5 μg/L.
Sorption capacity was calculated (eq 1).
Diatomite is a natural material formed from the remains of
diatoms, which grew and were historically deposited in sediments
of seas or lakes.25 It is abundant in many areas of the world and
has unique physical characteristics, such as low cost, porosity
(80−90%), small particle size, low thermal conductivity and
density26 and it has been approved as a food-grade material by
the U.S. Food and Drug Administration.2 Thus, diatomite can be
safely used to remove PO4 from eutrophic lake waters. However,
diatomite cannot be used directly as an effective adsorbent of
PO4 due to its relatively small surface area and the fact that at
neutral pH, the surface has a net negative charge. That negative
charge would repel the predominant species of PO4 at neutral pH
that are HPO42− and H2PO4−. A suitable modification of the
surface of diatomite can dramatically improve the surface
characteristics and make it more useful for adsorption of PO4
than raw diatomite. Conventional modification methods are
purification in HCl25 and calcination,25,27 which have been
applied to make treated diatomite more inert for use as a filter
support. Treatment of diatomite with NaOH and deposition of
manganese oxide on the surface have been used to make it more
effective as a scavenger of metal ions. The surface area of
modified diatomite can be as great as 80 m2/g.28 Diatomite can
be modified with lime and aluminum sulfate (Al2(SO4)3)
resulting in aluminum hydroxyl groups on the surface of
diatomite, which results in greater chemisorption efficiency for
PO4. Efficiency of removal of 80% of total PO4 was obtained by
this modified diatomite.29 A ferrihydrite-modified diatomite with
a, Brunauer−Emmentt−Teller (BET)-specific surface area of
211.1 m2/g and capacity to adsorb PO4 of 37.3 mg/g at pH 4.02
and a total efficiency for removal of PO4 of 85%.30
In situ formation of magnesium hydroxide (known as
MAGIC) is an effective method for preconcentration of
nanomolar concentrations of PO4 from fresh and sea waters
before quantification.31,32 However, reports on removal of anion
by Mg(OH)2 formed in situ are limited. Recently, it was reported
that magnesium oxide (MgO) was useful for removal of As(III),
MgO nanoflakes that are porous can form Mg(OH)2 in situ by
interactions with water, which have high affinity for As(III) anion
in aqueous environments.33
The objectives of this study were to (1) use MgO and
diatomite, both low cost and environmentally benign materials,
to prepare a novel PO4 adsorbent with high surface area and
sorption capacity; (2) characterized the mechanism of
adsorption of PO4 to modified diatomite; (3) investigate,
under laboratory conditions, the potential of modified diatomite
to adsorb PO4 for removal from eutrophic lake waters.
qe =
V (C0 − Ce)
m
(1)
Where qe is the sorption capacity at equilibrium(mg/g), C0 is the
initial concentration of PO4 (mg/L), Ce is the equilibrium
concentration of PO4 (mg/L), V is the volume of PO4 solutions,
and m is the mass of the sorbent (g).
Sorption Kinetics. A 500 mg aliquant of MOD was
equilibrated with 1000 mL of 50 mg P/L, of which the pH was
adjusted to 5.0 in a glass beaker (2000 mL) at 298 K for a fixed
period of time (15 min, 30 min, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h,
and 12 h). Approximately 5 mL aliquots were taken from the
suspension was filtered using a 0.45 μm filter and concentration
of PO4 was measured. The amount of PO4 adsorbed was
calculated (eq 1) Data were fitted by using the pseudo first-order
equation (eq 2) and the pseudo second-order equation(eq 3).38
■
MATERIALS AND METHODS
Materials. Raw diatomite was obtained through Shanghai
Kexi Chemical Co., Ltd., Shanghai, China. The raw diatomite was
composed of 86.30% SiO2, 4.91% Al2O3, 2.32% Fe2O3, 2.55%
CaO, and 0.58% MgO. All stock solutions were prepared using
reagent grade chemicals and deionized water. Stock solutions of
PO4 were prepared by dissolving KH2PO4 (Shanghai Chemical
Reagent Company, China Pharmacy Group) in deionized water.
Soil humic and fulvic acid were extracted from the soil of Jiufeng
Mountain (Beijing, China), and have been characterized in
detail.34,35
Preparation of MOD. Raw diatomite (15 g) was added to
100 mL of 6 M NaOH at 353K for 2 h to partially dissolve Si. The
mixture was immediately added to 100 mL of 2.5 M MgCl2, and
stirred at room temperature for 24 h. The mixture was then aged
another 24 h and centrifuged to remove the supernatant and
ln(qe − qt ) = ln qe‐k1t
(2)
t
1
1
=
+ t
qt
qe
k 2qe 2
(3)
Where qt (mg/g) is the PO4 sorption capacity at time t, and k1
(1/min) and k2 (g/mg min) are the rate constant of pseudo firstorder adsorption and the equilibrium rate constant of pseudo
second-order adsorption, respectively.
Sorption Isotherm and Thermodynamics. Solutions of
PO4 (100 mL) over a concentration range of 4−50 mg/L were
maintained at the pH 5.0 and shaken with MOD (50 mg) at 288,
298, 308, and 318 K, respectively. After 12h of mixing, amounts
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Figure 1. Typical scanning electron micrographs for (a) raw diatomite, (b) Mg(OH)2-modified diatomite(MHD), (c) MgO-modified
diatomite(MOD).
the system, where KF is a measurement of adsorption capacity
and 1/n of adsorption intensity.
The thermodynamic parameters of the adsorption process
were determined from the experimental data obtained at 288,
298, 308, and 318 K after 12 h of mixing (eqs 6, 7, and 840):
of PO4 in the supernatant were determined. The sorption
isotherm was described using eq 4 and eq 5:39
Ce
Ce
1
=
+
qe
Q max
b*Q max
1
lgqe = lgCe + lgKF
n
(4)
ΔG 0 = −RT ln K 0
q
Kd = e
Ce
(5)
Where: b is a Langmuir constant. Qmax is the theoretical
maximum adsorption capacity (mg/g), also known as monolayer
coverage of the surface. KF and n are characteristic constants of
ln K 0 =
584
ΔSo
ΔH o
−
R
RT
(6)
(7)
(8)
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Where ΔG0, ΔH0, and ΔS0 are Gibbs free energy change(kJ/
mol), enthalpy change(kJ/mol) and entropy change(J/mol·K),
respectively. Kd (L/g) is the distribution coefficient for the
adsorption, T(K) is the absolute temperature, R(J/mol·K) is gas
constant.
Desorption Study. 1000 mL of phosphate solution (50 mg
P/L) was maintained at the pH 5.0 and shaken with MOD (500
mg) at 298 K. After 12 h of mixing, the PO4 saturated MOD was
filtered and washed gently with deionized water to remove any
unadsorbed PO4. Then the PO4 saturated MOD(50 mg) was
mixed with 100 mL of 1 mmol and 10 mmol of Ca2+, CO32‑,
HCO3−, OH−, SiO32‑ solutions and deionized water at different
pH values (5.0−10.0). After 12 h of mixing, the desorbed PO4
was determined.
Removal of PO4 from Eutrophic Lake Water. Eutrophic
lake water was collected in March 2012 from Chaohu Lake, the
fifth largest freshwater lake China, located in the lower reach of
the Yangtze floodplain. The sampling sites were on the west side
of the lake, close to the city of Hefei. Lake water was filtered using
a 0.45 μm filter, and stored in plastic bottles at 277 K. Batch
experiments were carried out in 1000 mL glass beakers using 500
mL of eutrophic lake water for measuring PO4 removal efficiency.
MOD was added to the lake water at concentrations of 0, 50, 100,
200, 300, 400, or 500 mg/L and then mixed at constant stirring
for 12 h at 298 K. Amounts of soluble reactive PO4 in the
supernatant were determined after it was filtered through a 0.45
μm membrane filter. Concentrations of total PO4 were measured
after acid-persulphate digestion.41 Amounts of Ca and Mg were
determined by use of a flame atomic absorption spectrometer
(WFX-1E2, Beijing, China). Absorption wavelengths of 422.7
and 285.2 nm were used for Ca and Mg, respectively. The limit of
quantification of Ca and Mg were 0.02, 0.006 mg/L and the limit
of detection was 5, 0.8 μg/L, respectively.
Statistical Analysis. All the experimental data were the
averages of triplicate experiments. The relative errors of the data
were less than 5%.The curve fitting and statistical analyses were
performed using SigmaPlot software (version 10.0, SPSS Inc.,
Chicago, IL, USA). The correlation coefficient (R2) obtained
from regression was used for comparing the model applicability.
Figure 2. X-ray diffraction patterns of (a) Raw diatomite, (b) MHD, (c)
MOD and (d) MOD after treated with phosphate. * for Mg(OH)2 and #
for MgO.
■
to MgO (JCPDS no. 65−0476), and no remaining Mg(OH)2
peaks could be observed (Figure 2c). This observation suggests
that Mg(OH)2 was converted to MgO completely after the
calcination at 723 K for 2 h.
Effect of pH. In the pH range of 5.0−10.0, PO4 exhibit weak
adsorption on raw diatomite (Figure 3). The sorption capacity
decreased from 4.52 mg/g to 1.09 mg/g as a function of pH.
Mg(OH)2 deposited on diatomite resulted in grater sorption
capacities of 9.10−4.77 mg/g in the pH range of 5.0−10.0 . After
calcining, the surface area and sorption capability of adsorbent
became greater. Sorption capacity can reach 45.72 mg/g with an
initial pH of 5.0, which is 42-fold greater than that of raw
diatomite. This sorption behavior usually can be explained in
terms of pHZPC of the adsorbent and PO4 speciation occurring in
solution.42 The PO4 in aqueous solution exists mainly as H2PO4−
and HPO42‑ in the pH range of 5.0−10.0 (SI Figure S2).At pH<
pHzpc, the surface of the adsorbent was positively charged,
which is favorable for adsorption of the PO4 anion. Weaker
sorption of PO4 by raw diatomite can be attributed to lower
surface area and the negative charge on its surface under
experimental conditions. Greater soption of PO4 by MOD and
MHD can be attributed to the positive charge on its surface in the
pH range of 5.0−10.0. The specific surface area of MOD larger
than that of MHD is likely more responsible for the enhanced
sorption of PO42. Meanwhile, the tendency for sorption of PO4
RESULTS AND DISCUSSION
Characterization of Modified Diatomite. Physical
parameters of adsorbents (Raw diatomite, MHD and MOD),
such as the BET surface areas, total pore volume, average pore
diameter and magnesium content, are summarized in Supporting
Information (SI) Table S1. Surface morphology of raw diatomite
in the present study was disk-like and many pores can be seen on
its surface (Figure 1a). The SEM micrograph of MOD reveals
that the original geometry of the pores is destroyed by
pretreatment with NaOH and deposition of magnesium (Figure
1b and 1c). After being calcined at 723 K for 2 h, MgO nanoflakes
were formed on the surface of the diatomite (Figure 1c). The
surface area and the total pore volume of raw diatomite were
improved significantly. The BET surface area of MOD was 72.53
m2/g, which is 113-fold greater than that of raw diatomite. The
pHzpc of raw diatomite, MHD and MOD were experimentally
found to be at pH 4.5, 10.9 and 9.6, respectively (SI Figure S1).
Two diffraction bands were centered at 22.0° and 36.2° (2θ
degree) of the XRD, which is the characteristic peak for
amorphous SiO2 (JCPDS 29−0085) (Figure 2a). Four new
diffraction peaks were observed at 18.8°, 38.2°, 51.0° and 58.8° in
MHD (Figure 2b), which belong to Mg(OH)2 (JCPDS no. 07−
0239). For MOD, the diffraction peaks at 42.9° and 62.3° belong
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adsorption on MOD is more negative at higher temperature,
which demonstrates that the spontaneity of the adsorption
process increases with increasing the temperature (SI Table S4
and Figure S9). The positive value of entropy change (ΔS°)
implies an increase in the disorderness of the solid-solution
system,46 which is probably because of structural changes take
place in adsorbate and adsorbent during the adsorption. A
positive value of the standard enthalpy change (ΔH°) indicates
that the adsorption process is endothermic.47
Effects of Coexisting Substance. Many coexisting anions,
cations and dissolved organic matter such as Na + ,K + ,
Ca2+,Mg2+,Cl−, NO3−, SiO32−, CO32−‑,SO42‑, fulvic acid, and
humic acid in environmental waters can interfere with the
adsorption of PO4 on MOD through competitive adsorption. For
this purpose, 100 mL of the test solutions containing 5 mg/L of
PO4 and various amounts (1.0 and 10.0 mM for ions; 10, 30, and
50 mg C/L for dissolved organic matter) of other coexisting
substance were equilibrated with MOD (50 mg) at pH 5.0. The
negligible effects of cations on adsorption PO4 were demonstrated by the experimental data in Figure 4. Cl− and NO3− had
Figure 3. Effect of pH on the retention of PO4 by MOD, MHD and raw
diatomite experimental conditions: Vtotal, 40 mL;adsorbent, 20 mg; T,
298 K.
to MOD to be the same as sorption to MHD can be explained by
competition adsorption between PO4 anion and OH− with at
greater pH.
Sorption Kinetics. The time taken for PO4 removal using
MOD is of considerable importance for its possible application
for remediation of eutrophic lakes. Sorption of PO4 can reach
91% of maximum within 3 h, and then changed slowly until to
reach sorption equilibrium (SI Figure S3). The process of
sorption consisted of two distinct steps: an initial rapid
adsorption which a rapid diffusion of ions from the solution to
external adsorbent surfaces, followed by a slower adsorption by
diffusion of ions into pores of inner adsorbent surfaces of
Donnan spaces.43,44 Under the experimental conditions of
continuous stirring, it is unlikely that the external diffusion could
control the adsorption process. Chemical reaction might take
place on the surfaces of MOD. The obtained data were fitted by
using the pseudo first-order equation and the pseudo secondorder equation (SI Figure S4, S5, and Table S2). Correlation
coefficients of the pseudo second-order kinetic model (R2 =
0.998) was relatively greater than that of the pseudo first-order
kinetic model (R2 = 0.961). The adsorption system obeys
pseudo, second-order kinetics for the entire adsorption period
and thus supports the assumption that the adsorption is probably
via surface reactions until the surface functional sites are fully
occupied, thereafter PO4 diffuse into the MOD for further
complexation interactions.45
Sorption Isotherms. Sorption capacity of MOD for PO4
increased with increasing of temperature and initial PO4
concentrations and gradually to reach equilibrium (SI Figure
S6). Langmuir and Freundlich constants were determined using
the adsorption data at different temperature. The adsorption
isotherm data fit the Langmuir adsorption model well (Langmuir
model: R2 >0.997, Freundlich model: R2 <0.946) (SI Figure S7,
S8 and Table S3), indicating the adsorption of PO4 on the surface
of MOD was taking place in a monolayer fashion. According to
the Langmuir equation, the theoretical maximum sorption
capacity of MOD for P was 44.44−52.08 mg/g at experimental
conditions.
Sorption Thermodynamics. Thermodynamic parameters
(ΔGo, ΔSo, and ΔHo) of the adsorption process were calculated
using the obtained experiment data at several temperatures.
Gibbs free energy changes (ΔG°) during sorption of PO4
Figure 4. Effect of coexisting ions, humic acid and fulvic acid on PO4
removal experimental conditions: Vtotal, 100 mL;adsorbent, 50 mg;
initial pH 5.0.
little negative effect when the concentration of coexisting ions
was increased from 1.0 to 10 mM. Fulvic acid and humic acid also
show a slight negative effect on PO4 adsorption. When the
concentration of coexisting dissolved organic matter were
increased from 10 to 50 mg C/L, the sorption efficiency
decreasing from 94.4% and 99.2% to 86.9% and 92.6% for humic
acid and fulvic acid, respectively. The presence of CO32‑, SO42‑
and SiO32‑ showed a significantly negative influence on
adsorption of PO4 with sorption efficiency decreasing from
90.6%, 91.8%, and 89.6% to 78.5%, 81.4%, and 75.3% for CO32‑,
SO42−, and SiO32‑, respectively. The main reason was that the
increased concentration of dissolved organic matter and anions
could completion or blocks the surface sorption sites and
reduced the positive charge density on the surface of adsorbent,
which decrease the adsorption of the negative charged PO4
anions through electrostatic force of attraction. Although some of
the coexisting anions have negative effects, the experimental
results showed that larger than 75% of PO4 ions can be removal
even in a solution with a large excess amount of coexisting ions.
The results imply that MOD has high sorption selectivity toward
PO4 anion and has high potential to removal PO4 from real
eutrophic lake water.
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Mechanism of Sorption. In order to clarify the mechanism
of sorption of PO4 on MOD, the structure changes of MOD
before and after the saturation sorption of PO4 was examined
with XRD, and the chemical status of phosphate species after its
sorption on MOD was examined by high-resolution XPS scans
and solid state 31P NMR.
The XRD pattern of MOD after sorption in water solution
with initial concentration PO4 of 50 mg/L was given in Figure
2(d). After 12h of mixing, peaks attributed to MgO disappeared.
Two new diffraction peaks at the 2θ values of 18.8° and 38.2°
could be clearly distinguished (Figure 2(d)). According to the
intensity and quantity of the diffraction peaks, it can be
speculated that MgO on the surface of diatomite was gradually
converted in situ into Mg(OH)2.33
The high resolution-spectrum of P 2p was divided into two
peaks, 132.9 and 133.8 eV, which corresponded to PO43−and
HPO42−, respectively (Figure 5(a)).48 This result demonstrated
The chemical shift of physical adsorbed PO4 and the noncrystal
structure of Mg3(PO4)2·8H2O and MgHPO4·3H2O appeared at
6.4 ppm,50 0.5 ppm and −2.4 ppm,51,52 respectively. The peak
present here attributed to the mixture of physical adsorbed PO4,
the noncrystal structure of Mg3(PO4)2·8H2O and MgHPO4·
3H2O. The solid state 31P NMR results showed great accordance
with the results of the XPS analysis and imply that new bond
formation dominated the sorption process.
From the above analysis, the removal of PO4 by MOD is not a
simple adsorption process. The whole removal process involves
the in situ formation of Mg(OH)2 from MgO on the surface of
diatomite; the adsorption of PO4 anion onto in situ formed
positive charged Mg(OH)2 on the surface of diatomite through
electrostatic attraction to form surface complex; and the reaction
between surface Mg(OH)2 and sorbed PO4 to conversion
Mg3(PO4)2 and MgHPO4.
MOD with high specific surface area were added into the PO4
solution, MgO nanoflakes on the surface of diatomite became
hydrated and much more fresh active adsorption site for
enhanced PO4 removal was in situ formed (Mg(OH)2) due to
their reaction with water. The in situ formed surface hydroxide
maybe protonated in acid media and can deprotonated in alkaline
solutions. Therefore, the density of positive charged surface
groups of MOD change at different pH values. Under the
experimental conditions, H2PO4− and HPO42‑ can be formed and
the adsorption process is rapid on the surface of MOD. With
increasing pH, the number of negatively charged hydroxide
groups increases, and the negatively charged adsorbent surface
sites do not favor the adsorption of PO4 anion due to the
electrostatic repulsion. The pH dependent adsorption indicates
that the adsorption is dominated by surface complexation,53
which also showed in the sorption kinetic study. At the same
time, surface complexed PO4 species onto MOD could in situ
reaction with Mg(OH)2 and to conversion the noncrystal
structure compound of Mg3(PO4)2 and MgHPO4, which will be
beneficial to the continuous formation of Mg(OH)2. With the
increase of contact time, highly dispersed MgO nanoflakes on the
surface of diatomite could be in situ converted to Mg(OH)2
completely,33 and the reaction between Mg(OH)2 and PO4
could remove excess PO4 from water continuously. The
proposed sorption mechanism of MOD for PO4 from aqueous
solution is given (eqs 9-11).
DS ≡ Mg = O ⇒ DS ≡ Mg‐O‐OH 2+
(9)
DS ≡ Mg‐O‐OH 2+ + HPO4 2 ‐ ⇒ DS ≡ MgHPO4
(10)
DS ≡ Mg‐O‐OH 2+ + PO4 3 ‐ ⇒ DS ≡ Mg 2(PO4 )3
(11)
(DS = surface of diatomite)
Phosphate Desorption. To assess stability of adsorbed PO4
on the surface of MOD, the effect of pH and some chemical water
components such as Ca2+, CO32−, HCO3−, OH−, and SiO32‑ on
desorption PO4 from MOD saturated with PO4 was studied
using batch experimental method. The PO4 desorbability was
defined as the ratio of the desorbed PO4 to the total adsorbed
PO4 by the adsorbent. The desorbability of PO4 depends on the
pH of the extraction solutions(5.0−10.0) and chemical water
components were shown in Figure.6 and Table 1. The
desorbabilities of PO4 in the different pH solutions were 3.35−
7.67%. Effect of Ca2+, OH− and SiO32‑ on PO4 desorption was
negligible (<11%) for all the concentrations of these chemical
water components studied. Alkalinity had significant effect on
Figure 5. XPS spectra of MOD after treated with PO4. (a) P 2p and (b)
Mg 2p.
that the surface of MOD was composed of PO43− and HPO42−.
To confirm this conclusion deconvolution of the XPS spectra of
Mg 2p, (Figure 5 (b)) demonstrated the presence of two
compounds. The peak at 52.6 eV probably corresponds to
Mg3(PO4)2 and the peak at 51.8 eV to MgHPO4.49
A broad band in the solid state 31P NMR spectrum of MOD
after adsorption of PO4 appeared at 1.54 ppm (SI Figure S10).
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Article
Comparison with Other Adsorbents for Removal of
PO4. Increasing numbers of adsorbents have been used in recent
years to remove PO4 from environmental waters. The PO4
sorption capacity is usually chosen as the criterion for adsorbent
selection. For there are not standard procedures for batch
sorption experiments, direct comparison with other adsorbent
materials is difficult due to the different applied experimental
conditions.54 To illustrate the potential in the use of MOD for
removal of PO4 from waters, a comparative evaluation of the
SBET, pHzpc adsorption capacities and experimental conditions
of various PO4 adsorbents were provided (Table 2). Under the
similar experimental conditions, the PO4 sorption capacity of
MOD is slight greater than that of ferrihydrite-modified
diatomite (FHMD).2 Some of adsorbent such as biochar,55 Fe
loaded skin split waste56 and alunite57 have relative high capacity
for PO4, but the initial P concentrations were also higher.
Meanwhile, the ratio of material to solution in present study is
0.5g/L, which much less than that of reported adsorbents in
Table 2. The results indicate that the removal of PO4 by the
MOD was efficient and performance of MOD is better than most
of other adsorbents for which data is available. Another
advantage of the present adsorbent is its cost-effective and the
fact that it leaves no toxic residues or released toxic ions to waters
which cause a second pollution like some other adsorbents.
Removal of PO4 form Eutrophic Chaohu Lake Water.
To test the ability of MOD to remove PO4 from real eutrophic
waters, MOD was added to water from Chaohu Lake as a
representative eutrophic lake (SI Table S5). The pH value of the
water was 8.12, the concentration of total phosphorus was 0.158
mg/L. Lake water parameters such as TP, SRP, pH, Ca, and Mg
were determined after using different dose of MOD (Figure 7
and SI Figures S11 and S12). The efficiency of removal of total
phosphorus (TP) was proportional to the amount of MOD
applied. Efficiencies of removal of TP and SRP were 87.2% and
75% when the concentration of MOD was 250 mg/L. The
concentration of TP in the treated lake water samples was 19.9
μg/L, which is comparable with the amounts of alum and
ferrihydrite-modified diatomite applied to Lake Sønderby,
Denmark 58and Jackfish Lake,Canada.30When the concentration
of MOD was 300 mg/L, efficiency of removal of TP was 90% and
left a residual concentration of TP of less than 15.0 μg/L, which
falls within the oligotrophic range(3.0−17.7 μg/L).59 The pH
value of lake water was slight increased from 8.12 to 8.67 due to
Figure 6. Effect of pH on PO4 desorption experimental conditions:
Vtotal, 100 mL; PO4 saturated adsorbent, 50 mg.
Table 1. Effect of Chemical Water Composition on
Desorbability of PO4 Saturated MOD
desorbability(%)
parameters
1 mmol
10 mmol
2+
0.13 ± 0.05
34.69 ± 1.56
12.75 ± 0.21
3.80 ± 0.15
6.68 ± 0.10
0
38.57 ± 1.94
18.61 ± 0.46
9.29 ± 0.09
11.01 ± 0.34
Ca
CO32‑
HCO3−
OH−
SiO32‑
desorption of PO4 from the surface of MOD. Desorbabilities of
PO4 increased from 34.69% and 12.75% to 38.57% and 18.61%
with increasing the concentration of CO32− and HCO3− from 1.0
to 10 mM, respectively. Low desorbability at different pH and in
present of Ca2+, OH−, SiO32‑ indicates that a small fraction of
PO4 is weakly adsorbed or complexed on the surface of MOD.
The results from desorption experiments show that PO4 sorbed
on MOD is mostly irreversible and that the bond between the
MOD and adsorbed PO4 is strong. It is relatively difficult to
desorb PO4 from MOD. These results imply that MOD can be a
potential adsorbent of PO4 for some eutrophic lake water
treatment.
Table 2. PO4 Adsorption by Different Adsorbentsa
sorbent
SBET (m2/
g)
pHzpc
adsorption capacity
(mg/g)
reaction condition
reference
MOD
Fe−Mn binary oxide
FHMD
ACF-LaOH
milled furnace slag
biochar
ZnCl2-activated carbon
Fe-EDA-SAMMS
La-loaded orange waste
Fe loaded skin split waste
Al pillared bentonite
iron impregnated coir pith
alunite
72.53
309
211.1
NA
0.75
336
NA
169
NA
5.23
200
155.2
148
9.6
6.6
10
8.0
NA
NA
NA
NA
NA
NA
NA
5.8
NA
45.7
36
37.3
16.1
43.1
133
4.2
43.3
14
68
5.05
40.55
106.6
pH5.0,T = 298K,adsorbent=0.5g/L,C 0 = 50 mg/L
pH5.6, T = 298 ± 1K, adsorbent=0.2g/L, C 0 = 5 mg/L
pH4.0, T = room temperature, adsorbent=0.5g/L, C0 = 40 mg/L
pH NA,T = room temperature, adsorbent=2.5g/L,C0 = 30 mg/L
pH7.0−7.2,T = 293K, adsorbent=10g/L,C0 = 500 mg/L
pH7.0,T = 295 ± 0.5K, adsorbent=2g/L,C0 = 640 mg/L
pH4.0,T = 308K, adsorbent=6g/L,C0 = 40 mg/L
pH5.0,T = room temperature, adsorbent=0.1g/L, C 0 = 18.5 mg/L
pH7.5,T = 303K,adsorbent=1.7g/L,C 0 = 40 mg/L
pH7.0,T = 303,adsorbent=1g/L,C0 = 93 mg/L
pH3.0,T = NA,adsorbent=4g/L,C0 = 20 mg/L
pH3.0,T = 303K,adsorbent=2g/L,C0 = 100 mg/L
pH5.0,T = 298K,adsorbent=10g/L,C0 = 200 mg/L
present study
1
2
60
61
55
62
63
64
56
65
66
57
a
C0,initial P concentration NA, Date not available.
588
dx.doi.org/10.1021/es4037379 | Environ. Sci. Technol. 2014, 48, 582−590
Environmental Science & Technology
Article
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Figure 7. Effects of MOD dose on TP, SRP removal efficiency in the lake
water sample.
in situ formation of Mg(OH)2. Magnesium ion increased from
14.84 to 35.16 mg/L while calcium ion decreased from 178.15 to
157.88 mg/L in the eutrophic lake water when concentrations of
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waters.
■
ASSOCIATED CONTENT
■
AUTHOR INFORMATION
S Supporting Information
*
Twelve figures and five tables showing the results of BET data for
raw diatomite, MHD and MOD, pHzpc, the data of linear
regression, the parameters of sorption kinetic, sorption isotherm
and thermodynamics, solid state 31P NMR spectroscopy of
MOD after treated with PO4, characteristics of lake water, effects
of MOD dose on Ca, Mg concentration and pH in the lake water
sample. This material is available free of charge via the Internet at
http://pubs.acs.org.
Corresponding Author
*Phone: +86-10-84915312; fax: +86-10-84931804, e-mail:
wufengchang@vip.skleg.cn, lgj@ustc.edu.cn.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the National Basic Research
Program of China (No.2008CB418200), the National Natural
Science Foundation of China (No.21107001) and the China
Postdoctoral Science Foundation (2013M541842).
■
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Supporting information for
Removal of Phosphate from Eutrophic Lakes through Adsorption by in situ
Formation of Magnesium Hydroxide from Diatomite
Fazhi Xiea,b,c, Fengchang Wua＊, Guijian Liub＊, Yunsong Mua, Chenglian Fenga, Huanhua Wanga and
John P.Giesyd
a
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy
of Environmental Sciences, Beijing 100012, China
b
CAS Key Laboratory of Crust-Mantle Materials and Environment, School of Earth and Space
Sciences, University of Science and Technology of China, Hefei 230026, China
c
Key Laboratory of Functional Molecule Design and Interface Process, School of Materials Science
and Chemical Engineering, Anhui Jianzhu University, Hefei 230022, China
d
University of Saskatchewan, Department of Veterinary Biomedical Sciences and Toxicology
Centre, 44 Campus Drive, Saskatoon, SK, Canada, S7N 5B3
*
Corresponding authors: Tel: +86-10-84915312; Fax: +86-10-84931804,
E-mail addresses: wufengchang@vip.skleg.cn, lgj@ustc.edu.cn.
Number SI pages: 18
Number the figures: 12
Number the tables: 5
S1
12
MOD
MHD
Raw Diatomite
Final pH
10
8
6
4
2
0
5
10
15
20
25
30
Solid Fraction（ %,w/w)
Fig. S1 Determination of the point of zero charge of MOD, MHD and Raw Diatomite.
Experimental conditions : ionic strength, 0.01mol/L KNO3; T, 298 K.
S2
1.2
1.0
Percent（ × 100%）
0.8
H3PO4
H2PO4
HPO4
0.6
3-
PO4
0.4
0.2
0.0
0
2
4
6
8
10
12
pH
Fig. S2 PO4 speciation as a function of pH1
S3
14
-
2-
50
45
40
qt（ mg/g）
35
30
25
20
15
10
0
2
4
6
8
10
12
Time(h)
Fig.S3 Sorption kinetics of PO4 on MOD
Experimental conditions : Vtotal, 1000mL;adsorbent, 500mg; T, 298 K.
S4
14
6
y = -0.9748x + 3.9136
2
R = 0.961
4
2
ln(qe-qt)
0
-2
-4
-6
-8
-10
0
2
4
6
8
10
12
14
Time(h)
Fig.S4 The linear regression of pseudo first-order kinetic model
S5
0.30
y = 0.0206x + 0.0114
R2 = 0.998
0.25
t/qt
0.20
0.15
0.10
0.05
0.00
0
2
4
6
8
10
12
Time(h)
Fig.S5 The linear regression of pseudo second-order kinetic model
S6
14
60
288 K
298 K
308 K
318 K
50
qe（ mg/L）
40
30
20
10
0
0
5
10
15
20
25
30
35
Ce（ mg/L）
Fig. S6 Equilibrium isotherms of PO4 on MOD, Experimental conditions : Vtotal, 100mL;adsorbent,
50mg; initial pH 5.0.
S7
0.8
c e/q e
0.6
288 K
298 K
308 K
318 K
0.4
0.2
0.0
0
5
10
15
20
25
30
35
ce(mg/L)
Fig.S7 Experimental data for PO4 absorption fitted to Langmuir isotherm
S8
2.0
288 K
298 K
308 K
318 K
1.8
logqe
1.6
1.4
1.2
1.0
0.8
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
logce
Fig.S8 Experimental data for PO4 absorption fitted to Freundlich isotherm
S9
4.5
4.4
lnK0
4.3
4.2
4.1
4.0
3.10
3.15
3.20
3.25
3.30
3.35
3.40
3.45
3.50
1000/T
Fig.S9 Plot of lnK0 versus 1000/T for estimation of thermodynamic parameters
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250
200
150
100
50
0
ppm
-50
-100
-150
-200
-250
Fig.10 Solid state 31P NMR spectroscopy of MOD after treated with PO4
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Ca and Mg concentration in lake water(mg/L)
200
Mg
Ca
180
160
40
20
0
0
100
200
300
400
500
600
Dose of MOD (mg/L)
Fig. S11 Effects of MOD dose on Ca and Mg concentration in the lake water sample
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9.0
pH
8.5
8.0
7.5
7.0
0
100
200
300
400
500
600
Dose of MOD (mg/L)
Fig. S12 Effects of MOD dose on pH in the lake water sample
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Table S1 Physical parameters for raw diatomite, MHD and MOD
Sample
SBET
(m2/g)
Total pore volume
(cm3/g)
Average pore
diameter (nm)
Magnesium
content (%)
Raw
Diatomite
0.64
0.00079
49.50
0.35
MHD
49.81
0.19
15.66
24.86
MOD
72.53
0.26
14.41
27.49
SBET,Brunauer–Emmentt–Teller (BET)-specific surface area
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Table S2 Kinetic parameters for the adsorption of PO4 onto MOD
Pseudo first-order kinetic model
T
298 K
qe,exp(mg/g)
45.72
Pseudo second-order kinetic model
k1(h-1)
qe,1(mg/g)
R2
k2(g/mg·h)
qe,2(mg/g)
R2
0.97
50.08
0.961
0.037
48.54
0.998
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Table S3 The parameters for the Langmuir and Freundlich fitting of PO4 adsorption on MOD
Correlation
parameters
T=288 K
T=298 K
T=308 K
T=318 K
qm(mg/g)
44.44
47.39
51.02
52.08
b（L/mg）
0.69
0.65
0.63
0.81
R2
0.9992
0.9982
0.9972
0.9986
1-1/n 1/n
L /g） 16.28
K（mg
F
17.13
18.16
20.27
n
2.95
2.91
2.82
2.99
R2
0.9374
0.9394
0.9458
0.9427
Langmuir
Freundlich
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Table S4 Thermodynamic parameters for the adsorption of PO4 on MOD
T(K)
lnKo
△Go(kJ/mol)
288
4.08
-9.77
298
4.15
-10.29
308
4.24
-10.86
318
4.42
-11.69
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△So(J/mol·K)
△Ho (kJ/mol)
62.85
8.40
Table S5 Characteristics of water sampled from Chaohu Lake
Parameter
Value
Parameter
Value
Na (mg/L)
203
pH
8.12
K (mg/L)
25
Ca (mg/L)
178
SRP(µg/L)
91
Mg (mg/L)
14.84
TP(µg/L)
156
TN (mg/L)
2.58
Reference
1.
Karageorgiou, K.; Paschalis, M.; Anastassakis, G. N., Removal of phosphate species from solution by
adsorption onto calcite used as natural adsorbent. J. Hazard. Mater. 2006, 139, 447-452.
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