Science in China Series B: Chemistry
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Preparation of polymer-supported hydrated ferric
oxide based on Donnan membrane effect and its application for arsenic removal
ZHANG QingJian1, PAN BingCai1†, CHEN XinQing1, ZHANG WeiMing1, PAN BingJun,
ZHANG QuanXing1, L. LV2 & X. S. ZHAO2
1
2
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093,
China;
Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge, Singapore 119260,
Singapore
In the present study a novel technique was proposed to prepare a polymer-supported hydrated ferric
oxide (D201-HFO) based on Donnan membrane effect by using a strongly basic anion exchanger D201
as the host material and FeCl3-HCl-NaCl solution as the reaction environment. D201-HFO was found to
exhibit higher capacity for arsenic removal than a commercial sorbent Purolite ArsenX. Furthermore, it
presents favorable adsorption selectivity for arsenic removal from aqueous solution, as well as satisfactory kinetics. Fixed-bed column experiments showed that arsenic sorption on D201-HFO could result in concentration of this toxic metalloid element below 10 μg/L, which was the new maximum concentration limit set recently by the European Commission and imposed by the US EPA and China. Also,
the spent D201-HFO is amenable to efficient regeneration by NaOH-NaCl solution.
hydrated ferrous oxide, anion exchanger, Donnan membrane effect, arsenic, sorption
1 Introduction
Water pollution by arsenic has been of a serious concern
particularly in Southeast Asia, North America, and Taiwan because of its negative impact to human health that
ranges from acute lethality to chronic and carcinogenic
effects[1,2]. Recent research demonstrated that arsenic
threat posed to human health had been underestimated to
some extent[3]. Therefore, the World Health Organization has recommended a maximum contaminant level
(MCL) for arsenic in drinking water of 10 μg/L. In 1998,
the European Commission adopted the same value as the
MCL and all European countries were required to comply with this limit since 2003. Recently, the same limit
has been adopted by the United States, and the date by
which water treatment systems must comply with the
new standard is January 2006.
Arsenic is present in natural waters mainly in its in-
organic oxyanionic forms, including arsenate and arsenite. Various treatment technologies have been developed to remove arsenic from water, including flocculation-precipitation, membrane separation, and adsorp－
tion[4 6]. In the past decades adsorption has attracted
increasing interest, and adsorption on hydrated Fe(III)
oxide (HFO) has been generally considered as a poten－
tial way for its high capacity for arsenic removal[7 10].
As a nonporous material, the freshly precipitated amorphous HFO particles were found to vary between 20 and
100 nm, which is unusable for fix beds or any flowthrough systems because of excessive pressure drops
Received November 7, 2006; accepted April 29, 2007
doi: 10.1007/s11426-007-0117-6
†
Corresponding author (email: [email protected])
Partially supported by the National Natural Science Foundation of China (Grant No.
20504012), the Natural Science Foundation of Jiangsu Province (Grant No.
BK2006129) and the Scientific Research Foundation of Graduate School of Nanjing
University (Grant No. 2006CL11)
Sci China Ser B-Chem | Apr. 2008 | vol. 51 | no. 4 | 379-385
and poor mechanical strength. To overcome the foregoing problems, HFO was always loaded on different porous materials, such as granular activated carbon[11],
cellulose[12], alginate beads[13], sand[14], polymeric adsorbent[15,16], and so on. However, little is known about
the role of surface chemistry of host materials in arsenic
removal. In 2005, Sengupta and coworkers developed a
novel hybrid sorbent based on Donnan membrane effect
for arsenic removal[17]. It was achieved by using strongly
basic anion exchangers as supporting material, where
the targeted anions would be pre-concentrated as a result
of the electrostatic interaction between the immobilized
positively charged groups on the exchanger matrix and
the targeted anions. However, Fe3+ as a traditional HFO
precursor cannot directly enter into the pores of a
strongly basic exchanger due to the charge expulsion. A
proprietary technique was proposed by Sengupta to successfully load HFO onto a strongly basic exchanger and
obtain a specific sorbent ArsenX for arsenic removal
from contaminated water[18,19].
In the present study, we aimed at developing a more
feasible and simple process to prepare a similar hybrid
sorbent to enhance arsenic removal from water[20]. Arsenic removal by the new sorbent named D201-HFO was
evaluated by batch and fixed-bed column tests. Results
indicated that the new sorbent D201-HFO exhibited an
excellent sorption performance for arsenic removal in
terms of high capacity and improved selectivity.
2 Materials and methods
2.1 Materials
All chemicals were of reagent grade, and solutions were
prepared by double-distilled water. Sodium arsenate
(Na2HAsO4·7H2O, 99%) and sodium arsenite (NaAsO2,
99%) were obtained from Sigma. ArsenX samples were
kindly provided by Purolite China. The strongly basic
exchanger D201 was provided kindly by Hangzhou
Zhenguang Resin Co., China. Prior to use, it was rinsed
with alcohol in a glass column and dried under vacuum
at 45℃ for 48 h.
2.2 Sorbent preparation
Taken into account the fact that tetrachloroferrate anion
−
(FeCl4 ) is readily formed in a ferric chloride (FeCl3)
solution in the presence of an excess amount of hydrochloric acid or chloride, and it is a large and poorly hy−
drated anion[21], FeCl4 is preferably loaded onto a
380
strongly basic exchanger from aqueous solution even in
the presence of chloride at a high level[22]. Moreover, as
−
the precursor of HFO, FeCl4 is decomposed in dilute
NaOH solution or neutral solution. Naturally occurring,
−
FeCl4 loaded onto the anion exchanger D201 is anticipated to precipitate by alkaline solution and subsequently transform into HFO by thermal treatment. Consequently, we obtained hybrid sorbent D201-HFO. Detailed process for D201-HFO preparation can be referred
to a Chinese patent[20] and HFO is formed on the inner
surface of D201 beads as the following reactions
FeCl3 + Cl− → FeCl−4 (a)
+
−
R Cl (s) +
R
+
FeCl−4 (a)
FeCl−4 (s) +
→R
+
FeCl−4 (s) +
(1)
−
Cl (a)
(2)
−
OH (a)
→ R + OH − (s) + Fe(OH)3 (s) + Cl− (a)
(3)
50－60℃
Fe(OH)3 (s) ⎯⎯⎯→
FeOOH(s)
12 h
+ amorphous HFO particles
(4)
XRD pattern of D201-HFO was obtained with a
ESCLAB-2 diffractometer using Mg Kα radiation. Nitrogen adsorption isotherms of sorbent samples were
measured by using a Micromeritics ASAP 2010 system.
The samples were outgassed for 10 h at 300℃ before
the measurements. Pore size distribution of sorbent
samples were calculated using the Barrett-Joyner-Halenda model. SEM analysis was conducted using a
LEO-1530 VP electron microscope, and TEM analysis
was performed using Hitachi Model H-800 transmission
electron microscope using an accelerating voltage of 200
kV with a tungsten filament.
2.3 Batch sorption experiments
Batch sorption runs were carried out in several 250 mL
glass flasks. To start the experiment, the known amount
of sorbent particles was introduced to arsenic solution
with the desired arsenic concentration. The flasks were
then transferred to a G25 model incubator shaker with
thermostat (New Brunswick Scientific Co. Inc.) and
shaken under 200 r/min for 24 h at a desired temperature
to ensure that the sorption process reached equilibrium.
Hydrochloride acid or sodium hydroxide was used to
adjust the solution pH throughout the experiment. The
arsenic uptake on the resin particles is calculated by conducting a mass balance on the solute before and after the
test as
qe = V (C0 − Ce ) / W .
(5)
ZHANG QingJian et al. Sci China Ser B-Chem | Apr. 2008 | vol. 51 | no. 4 | 379-385
Sorption kinetic experiment was performed at 298 K and
the sorbent and solution were determined as 0.500 g and
500 mL, respectively. A 0.5 mL solution at various time
intervals was sampled from the flasks to determine the
sorption kinetics[23].
2.4 Fixed-bed column tests
Fixed-bed column experiments were carried out with a
glass column (5.6 mm in diameter) equipped with a water bath to maintain a constant temperature. A Lange580 pump (China) was used to ensure a constant flow
rate. All the column runs were performed under the
same hydrodynamic conditions with the superficial liquid velocity (SLV) and the empty bed contact time
(EBCT) equal to 0.75 m·h−1 and 4 min, respectively.
1. The SEM micrographs of the host exchanger D201
and its Fe-loaded derivative (Figure 2) indicate that
HFO is uniformly distributed within the inner surface of
the sorbent. It was further demonstrated by TEM analysis of the sorbent (Figure 3). The loaded HFO was in
amorphous phase as indicated by the XRD pattern (Figure 4). Amorphous iron (hydr)oxides were previously
reported to gradually transfer to crystalline iron(III) oxides[25]. However, it was not the case in our samples
even after 1-year deposition. The micorpore structure of
D201-HFO may take a significant role in the specific
HFO property, and further study is needed to elucidate
the mechanism.
2.5 Analyses
Before analysis Fe(III) loaded within D201 beads was
firstly extracted by 2 mol/L HCl solution[11]. Fe(III)
content in solution was analyzed by ferrozine spectrophotometric methods[24]. Arsenic analyses of solution
samples were carried out using an atomic fluorescene
(AF) spectrometer with an online hydride generation
unit (model AF-610A, Realy Instrument Co., Beijing).
3 Results and discussion
3.1 Characterization of sorbent D201-Fe
Some important properties of the strong-base anion exchanger D201 and its Fe(III)-loaded derivative
(D201-HFO) are identified and shown in Table 1. Fe(III)
has been successfully loaded on the anion exchanger
according to the Fe(III) variation before and after loading. The loaded HFO blocked some fraction of pores and
resulted in a drop in BET surface area and average pore
diameter of the sorbent. However, it did not affect the
sorption behavior of arsenic species onto D201-HFO, as
demonstrated by the sorption experiments in the following sections. Nitrogen adsorption isotherms of the
D201-HFO sample (Figure 1) exhibit a steep increase in
the curve at a relative pressure of 0.90 < P/P0 < 1.0,
which may be assigned to the macroporous nature.
Compared to the nitrogen-adsorption isotherm of D201,
D201-HFO presented a similar isotherm form, indicating
that the pore structure of D201-HFO is similar to the
host material D201. Volume decrease of N2 adsorbed at
P/P0 = 1 after HFO dispersion is consistent with the loss
of pore volume of the hybrid sorbent, as shown in Table
Figure 1 Comparison of N2 adsorption isotherms onto D201 and
D201-HFO.
Table 1 Salient properties of D201 and its HFO derivative D201-HFO
D201
BET surface area (m2·g−1)
D201-HFO
25.6
13.1
Pore volume (cm ·g )
0.65
0.31
Average pore diameter (nm)
29.3
12.7
3
−1
Fe(III) content (%)
Color
0
11.5
white
brown
3.2 Effect of solution pH on sorption
Effect of solution pH on arsenate removal by D201HFO was examined compared to ArsenX and the results
are depicted in Figure 5. A similar pH-dependent trend
was observed for both sorbents, but D201-HFO exhibited higher sorption capacity for arsenate sorption. Negligible effect of pH on arsenate sorption onto D201-HFO
was observed in the pH range from 5.0 to 8.5, and less
or larger solution pH was unfavorable for arsenate removal. Taking into account the fact that arsenate sorp-
ZHANG QingJian et al. Sci China Ser B-Chem | Apr. 2008 | vol. 51 | no. 4 | 379-385
381
Figure 2 SEM micrograph of inner surfaces of the parent anion exchanger D201 (a), and its HFO derivative D201-HFO (b).
(the iso-electric point of HFO is 8.1), arsenate cannot be
loaded onto D201-HFO effectively. By contrast, HFO is
decomposed into ferrous ion under acidic solution and
therefore loses its sorption capacity.
Figure 3 TEM micrograph of the hybrid composite D201-HFO. The
dark spots represent HFO.
Figure 5 Effect of solution pH on arsenate sorption onto D201-HFO and
ArsenX.0.050 g sorbent and 100 mL solution with initial arsenate content
of 50 mg/L was used in the experiment.
3.3 Sorption kinetics
Figure 4 XRD pattern of the hybrid sorbent D201-HFO.
tion onto HFO was mainly driven by electrostatic interaction and inner complex formation together[9], and
HFO would be negatively charged at alkaline solution
382
Due to the variation of pore size distribution caused by
HFO loading, it is necessary to characterize the kinetic
property of D201-HFO for its practical purposes. The
kinetic behavior of arsenic sorption on D201-HFO is
illustrated in Figure 6. Arsenic sorption equilibrium was
approached in a very short time (sorption fraction is larger than 0.9 in 1 h, while that for Fe(III)-loaded bead
cellulose is 6 h[11] and activated alumina is 3－5 d[26]),
which is very significant for fixed-bed application. The
pseudo-second-order kinetic model was employed to
describe such sorption process as[27,28]
ZHANG QingJian et al. Sci China Ser B-Chem | Apr. 2008 | vol. 51 | no. 4 | 379-385
2−
−
Figure 7 depicts the effect of SO4 and Cl on arsenate
sorption onto D201-HFO at 298 K by comparison with
D201. Though competing effect of both anions on arsenate removal was observed, D201-HFO still exhibited
high selectivity towards arsenate even in presence of the
competing anions at high concentrations. It was attributed to the specific interaction between HFO and arsenate[9]. Results also indicated that more competitive ca−
2−
pacity occurred for SO4 than Cl .
Figure 6 Kinetic curve of arsenate sorption onto D201-HFO. 0.50 g
sorbent and 1000 mL solution with initial arsenate content of 50 mg/L
were used in the experiment.
dqt / dt = k2 (qe − qt ),
(6)
where qe is the sorption capacity at equilibrium and qt is
the solid-phase loading of arsenic at time t. k2
(g·mg−1·min−1) represents the pseudo-second-order rate
constant for the kinetic model. By integrating eq. (2)
with the boundary conditions of qt = 0 at t = 0 and qt = qt
at t = t, the following linear equation can be obtained:
t
1
1
=
+ t,
(7)
qt V0 qe
V0 = k2 qe2 ,
−1
(8)
−1
where V0 (mg·g ·min ) is the initial sorption rate.
Therefore, the V0 and qe values of kinetic tests can be
determined experimentally by plotting t/qt versus t
(shown in Table 2). Relative coefficient (r) larger than
0.99 indicates that the pseudo-second-order kinetic
model can represent arsenic sorption on D201-HFO
reasonably, which was further supported by the calculated qe value approaching the experimental data.
Table 2 Kinetic parameters of pseudo-second order model for arsenate
sorption onto D201-HFO
V (mg·g−1·min−1)
qe (mg·g−1)
K2 (g·mg−1·min−1)
r
32.8
72.6
2.38×10−3
0.992
3.4 Competitive sorption
The innocuous anions, such as sulfate and chloride, are
always present at concentration several orders of magnitude greater than arsenic species. Therefore, effect of
competing anions on arsenic removal onto D201-HFO
should be examined prior to its application in large scale.
Figure 7 Effect of competing anions on arsenate sorption onto
D201-HFO. Sorbent: 0.050 g; solution: 100 mL; initial arsenate content
2−
−
400 μg/L. (a) SO4 ; (b) Cl .
3.5 Fixed-bed column experiments
Figure 8(a), (b) illustrates an effluent history of a separate fixed-bed column packed with 5 mL D201-HFO for
a feeding solution containing arsenic species [As(III) or
As(V)] and competing anions. D201 was also employed
here for comparison purpose. Better column sorption
results of D201-HFO and D201 indicated the role of
HFO loading onto the inner surface of D201. Satisfactory breakthrough results were observed for both arsenic
sorption on D201-HFO even when the total competing
ZHANG QingJian et al. Sci China Ser B-Chem | Apr. 2008 | vol. 51 | no. 4 | 379-385
383
anions are about 3000 times more than the arsenic species in mass concentration. This further infers with the
improved selectivity for both arsenic species over the
competing anions. Moreover, arsenic sorption on
D201-HFO could result in concentration of this toxic
metalloid element below 10 μg/L, which was the new
maximum concentration limit set recently by the European Commission and imposed by the US EPA and
China. Higher sorption capacity of arsenate than arsenite
is consistent with the findings reported elsewhere[17].
Of a noteworthy observation is that the used D201HFO is amenable to an efficient regeneration for repeated use by NaOH (4%)-NaCl (8%) solution, as depicted in Figure 9. A continuous 5-cycle run demonstrated that no significant loss of sorption capacity was
Figure 9 A complete desorption history of D201-HFO preloaded with
arsenate (313 K).
observed for D201-HFO, and HFO loss of the sorbent
was less than 1% in mass after the repeated tests, which
suggested that D201-HFO is a potential candidate for
arsenic control in water.
4 Conclusion
In the present study hydrous Fe(III) oxide (HFO) was
successfully loaded onto a strongly basic anion exchanger D201 by a newly-developed technique, and we
obtained a hybrid sorbent D201-HFO for enhanced arsenic removal from contaminated water. D201-HFO
presented higher sorption capacity of arsenate by comparison with a commercial sorbent Purolite ArsenX, as
well as a satisfactory sorption kinetics. Furthermore, an
extremely high sorption selectivity towards arsenate was
observed over other innocuous anions, such as sulfate
and chloride. Fixed-bed column experiments showed
that arsenic sorption on D201-Fe could result in concentration of this toxic metalloid element below 10 mg/L,
which was the new maximum concentration limit set
recently by the European Commission and imposed by
the US EPA. Also, the spent sorbent is amenable to an
efficient regeneration for repeated use by dilute NaOH
and NaCl solution.
Figure 8 Breakthrough curves of arsenate and arsenite onto D201-HFO
at 298 K. (a) As(III); (b) As(V).
1
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