Equilibrium and kinetic studies of Se(VI) removal by Mg-Al layered double hydroxide doped with Fe2+

Article abstract of DOI:10.1039/c4ra11645c

Mg-Al layered double hydroxide (Mg-Al LDH) doped with Fe²⁺ was found to be superior to undoped Mg-Al LDH in the removal of Se(vi) from aqueous solutions. For both systems, Se(vi) as SeO₄^2- was removed through anion exchange with intercalated Cl^-. In the Fe²⁺-doped Mg-Al LDH, however, some of the Se(vi) was reduced to Se(iv) upon oxidation of Fe²⁺ to Fe³⁺ in the LDH host layer to produce SeO₃^2-, which was also adsorbed by the Fe²⁺-doped Mg-Al LDH through anion exchange. The reduction of Se(vi) to Se(iv) is advantageous for Se(vi) removal by Fe²⁺-doped Mg-Al LDH due to the larger charge density of SeO₃^2-. The Fe²⁺-doped Mg-Al LDH effectively removed Se(vi) from an aqueous solution because of the anion exchange properties of Mg-Al LDH and activity of Fe²⁺ as a reducing agent. Se(vi) removal occurs through Langmuir-type adsorption, where the maximum adsorption and equilibrium adsorption constant were 1.4 mmol g^-1 and 1.6, respectively. Se(vi) removal is well expressed as a pseudo second-order reaction. The apparent rate constants at 10, 30, and 60°C were 1.2 × 10^-3, 1.5 × 10^-3, and 2.2 × 10^-3 g mmol^-1 min^-1, respectively, and the apparent activation energy was 10.0 kJ mol^-1. The rate-determining step is chemical adsorption through anion exchange of SeO₄^2- and SeO₃^2- with intercalated Cl^-.

Full text of DOI:10.1039/c4ra11645c

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Equilibrium and kinetic studies of Se(VI) removal by
Mg–Al layered double hydroxide doped with Fe2+
Cite this: RSC Adv., 2014, 4, 61817
*
Tomohito Kameda, Eisuke Kondo and Toshiaki Yoshioka
Mg–Al layered double hydroxide (Mg–Al LDH) doped with Fe2+ was found to be superior to undoped Mg–Al
2ꢀ
LDH in the removal of Se(VI) from aqueous solutions. For both systems, Se(VI) as SeO4 was removed
through anion exchange with intercalated Clꢀ. In the Fe2+-doped Mg–Al LDH, however, some of the
Se(VI) was reduced to Se(IV) upon oxidation of Fe2+ to Fe3+ in the LDH host layer to produce SeO3
,
2ꢀ
which was also adsorbed by the Fe2+-doped Mg–Al LDH through anion exchange. The reduction of
Se(VI) to Se(IV) is advantageous for Se(VI) removal by Fe2+-doped Mg–Al LDH due to the larger charge
density of SeO32ꢀ. The Fe2+-doped Mg–Al LDH effectively removed Se(VI) from an aqueous solution
because of the anion exchange properties of Mg–Al LDH and activity of Fe2+ as a reducing agent. Se(VI)
removal occurs through Langmuir-type adsorption, where the maximum adsorption and equilibrium
adsorption constant were 1.4 mmol gꢀ1 and 1.6, respectively. Se(VI) removal is well expressed as a
pseudo second-order reaction. The apparent rate constants at 10, 30, and 60 ꢁC were 1.2 ꢂ 10ꢀ3, 1.5 ꢂ
10ꢀ3, and 2.2 ꢂ 10ꢀ3 g mmolꢀ1 minꢀ1, respectively, and the apparent activation energy was 10.0 kJ
Received 2nd October 2014
Accepted 10th November 2014
DOI: 10.1039/c4ra11645c
molꢀ1. The rate-determining step is chemical adsorption through anion exchange of SeO4 and SeO3
2ꢀ
2ꢀ
www.rsc.org/advances
with intercalated Clꢀ.
reduced to Cr(III) upon the oxidation of Fe2+ to Fe3+, and the
resultant Cr(III) combined with OHꢀ to produce Cr(OH)3. In this
study, the Fe2+-doped Mg–Al LDH was applied to the removal of
Se(VI) from aqueous solutions, as Se also forms toxic
compounds. Se commonly exists as Se(VI) and Se(IV) in aqueous
solutions. Although Se was added as one of harmful substances
in the effluent standards in Japan in 2001, the treatment
method of Se wastewater is not established sufficiently. New
treatment method for Se wastewater must be developed. While
Se(IV) can be removed by co-precipitation with Fe(III),19 similar
treatment of Se(VI) is difficult due to its high stability in aqueous
solutions. In general, for adsorption method, activated carbon
is known to be less effective for the removal of Se(VI) and Se(IV).
Activated alumina is known to be less effective for the removal
of Se(VI), although it is effective for the removal of Se(IV).
However, the Fe2+-doped Mg–Al LDH can adsorb Se(VI) from an
aqueous solution and reduce Se(VI) to Se(IV), which is advanta-
geous for its removal to be adsorbed. Furthermore, equilibrium
and kinetic studies were conducted to determine the properties
of Se(VI) removal by the Fe2+-doped Mg–Al LDH.
Introduction
Layered double hydroxides (LDHs) are typically represented by
the formula [M1ꢀx2+Mx3+(OH)2] / [M1−x2+Mx3+(OH)2], where
M2+ and M3+ are divalent and trivalent metal ions, respectively;
x denotes the M3+/(M2+ + M3+) molar ratio (0.20 # x # 0.33); and
Anꢀ is, for example, CO3 or Clꢀ.1–4 An LDH consists of stacked
2ꢀ
M3+-bearing brucite-like octahedral layers, where some of the
M3+ is replaced by M2+, in which the charge of the positive layer
is electrically neutralized by interlayer anions. The interlayer
space is occupied by water molecules in the hydration shell of
these intercalated anions. LDHs have been investigated as
promising materials for water preservation and purication.
For example, LDHs can adsorb oxometalates such as arsenite,
arsenate, chromate, selenite, and selenate from aqueous solu-
tions.5–13 We have also examined the removal of antimonate
using LDHs.14–16 On the other hand, magnetic chitosan
composites are known to be a novel material that exhibits good
sorption behavior toward various toxic metal in aqueous
solution.17
In our recent study, a Mg–Al LDH doped with Fe2+ was
prepared by co-precipitation, where some Mg2+ ions in the LDH
host layer were replaced with Fe2+, and used to remove Cr(VI)
from an aqueous solution through anion exchange.18 Cr(VI) was
Experimental methods
Fe2+-doped Mg–Al LDH was prepared through the dropwise
addition of a Mg–Fe–Al chloride solution to a NaOH solution at
a constant pH, as reported in our previous paper.18 The Fe2+-
doped Mg–Al LDH contained 16.6 wt% Mg2+, 6.3 wt% Al3+, 8.7
wt% Fe2+, and 4.4 wt% Fe3+. An undoped Mg–Al LDH was also
Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki,
Aoba-ku, Sendai 980-8579, Japan. E-mail: kameda@env.che.tohoku.ac.jp; Fax: +81-
22-795-7212; Tel: +81-22-795-7212
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prepared through the dropwise addition of a Mg–Al chloride
solution to a NaOH solution at a constant pH, and it contained
25.0 wt% Mg2+ and 6.8 wt% Al3+
.
2ꢀ
An aqueous Se(VI) solution containing SeO4
ions was
prepared by dissolving Na2SeO4 in deionized water. The Fe2+
-
doped Mg–Al LDH and undoped Mg–Al LDH were added to
500 mL of 1 mM Se(VI) solution, and the resultant suspension
was stirred at 10–60 ꢁC for 120 min, with continuous N2
bubbling. Samples of the suspension were collected at different
time intervals and immediately ltered through a 0.45 mm
membrane lter. The ltrates were analyzed for residual Se. In
order to determine the adsorption isotherm of Se(VI) adsorbed
by the Fe2+-doped Mg–Al LDH, 20 mL of 0.5–25 mM Se(VI)
solution and 0.2 g of Fe2+-doped Mg–AlꢁLDH were placed in
50 mL screw-top tubes and shaken at 30 C for 24 h.
The two types of LDHs before and aer removal of Se(VI) were
analyzed by X-ray diffraction (XRD) using Cu Ka radiation. The
Fe2+-doped Mg–Al LDH aer removal of Se(VI) was dissolved in
1 M HCl, and the Fe2+ concentration was determined by
performing UV-Vis spectroscopy at 510 nm using the phenan-
throline method. Furthermore, the oxidation state of Se in the
LDH aer removal of Se(VI) was investigated using X-ray
photoelectron spectroscopy (XPS). For the adsorption experi-
ments, the residual concentration of Se in the ltrates was
determined using inductively coupled plasma-atomic emission
spectrometry (ICP-AES), with an error of 0.1 mg Lꢀ1.†
Fig. 1 Variations in Se(VI) removal over time by the Fe2+-doped Mg–Al
LDH with various molar ratios of Al in the LDH to Se(VI) in solution at
30 ꢁC.
that the removal of Se(VI) from aqueous solution by the LDHs is
due to anion exchange, that is, the intercalation of SeO42ꢀ into
the interlayers of the LDH. The XRD patterns for Fe2+-doped
Mg–Al LDH and undoped Mg–Al LDH aer Se(VI) removal
(Fig. 3(b) and (d)) display (003) reections corresponding to the
basal spacing of the LDH that are broader and weaker than
those before Se(VI) removal (Fig. 3(a) and (c)). These reections
are likely attributable to the structural disorganization that
arises from the intercalation of SeO42ꢀ, which has a sterically
bulky molecular structure. For undoped Mg–Al LDH, the basal
Results and discussion
˚
˚
spacing (d003) increased from 8.1 A to 8.3 A upon the removal of
Se(VI) (Fig. 3(c) and (d)), which conrms anion exchange
occurred between Clꢀ intercalated in the interlayer of the Mg–Al
Fig. 1 and 2 show the variations in Se(VI) removal over time with
Fe2+-doped Mg–Al LDH and Mg–Al LDH. The molar ratios of Al
in the LDH to Se(VI) in the solution (Al/Se molar ratios) were set
at 1 to 3. For both LDHs, Se(VI) removal increased with time for
all Al/Se molar ratios, showing that the LDHs could remove
Se(VI) from aqueous solution. The Se(VI) removal also increased
with increasing Al/Se molar ratios, indicating that increased
amounts of LDH resulted in increased uptake of Se(VI). This was
caused by the increase of adsorption site for Se(VI). However,
Se(VI) removal for the Fe2+-doped Mg–Al LDH was consistently
larger than that for the undoped Mg–Al LDH when Al/Se ¼ 1 and
2. This implies the activity of the Fe2+ as a reducing agent, and is
2ꢀ
LDH and SeO4 in the aqueous solution. On the other hand,
for Fe2+-doped Mg–Al LDH, the basal spacing (d003) decreased
˚
˚
from 8.1 A to 7.8 A upon the removal of Se(VI) (Fig. 3(a) and (b)).
Table 1 shows the oxidation states of Fe and Se in the Fe2+-
doped Mg–Al LDH aer removal of Se(VI). It was found that Fe2+
was oxidized to Fe3+ and Se(VI) was reduced to Se(IV). Therefore,
discussed later. When Al/Se ¼ 3, the Se(VI) removal by Fe2+
-
doped Mg–Al LDH was almost the same as that by undoped Mg–
Al LDH, suggesting that the amount of undoped LDH was
sufficiently high to remove Se(VI) even without the activity of the
Fe2+. The Fe2+-doped Mg–Al LDH had a great advantage in Se(VI)
removal when Al/Se ¼ 1 and 2.
Fig. 3 shows the XRD patterns for the Fe2+-doped Mg–Al LDH
and undoped Mg–Al LDH before and aer Se(VI) removal. The
XRD peaks for all samples were assigned to hydrotalcite (JCPDS
card 22-700),
a naturally occurring hydroxycarbonate of
magnesium and aluminum with the formula Mg6Al2(OH)16
-
CO3$4H2O and structure of a LDH, proving that all samples had
the basic LDH structure. The presence of hydrotalcite suggests
Fig. 2 Variations in Se(VI) removal over time by the undoped Mg–Al
LDH with various molar ratios of Al in the LDH to Se(VI) in solution at
30 ꢁC.
† Mg–Al LDH, Mg–Al layered double hydroxide; XRD, X-ray diffraction; XPS, X-ray
photoelectron spectroscopy.
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Fig. 4 Schematic diagram for Se(VI) removal by the Fe2+-doped Mg–Al
LDH.
The adsorption isotherm showed Langmuir-type behavior,
Fig. 3 XRD patterns for the Fe2+-doped Mg–Al LDH (a) before and (b)
which was conrmed by arranging the experimental data
after Se(VI) removal and undoped Mg–Al LDH (c) before and (d) after according to the Langmuir equation, expressed as
Se(VI) removal.
qe ¼ CeqmKL/(1 + CeKL),
(1)
where qe (mmol gꢀ1) is the equilibrium adsorption, Ce (mM) is
the equilibrium concentration, qm (mmol gꢀ1) is the maximum
adsorption, and KL is the equilibrium adsorption constant. This
equation can also be expressed as
Table 1 Oxidation states of Fe and Se in Fe2+-doped Mg–Al LDH after
removal of Se(VI)
Fe2+/Fe
0
Fe3+/Fe
1
Se(IV)/Se
0.69
Se(VI)/Se
0.31
Ce/qe ¼ 1/qmKL + Ce/qm.
(2)
the decrease in basal spacing (d003) for the Fe2+-doped Mg–Al
LDH is attributed to the intercalation of SeO32ꢀ, which has a
smaller ionic radius than SeO42ꢀ. Thus, the increased Se(VI)
removal by the Fe2+-doped Mg–Al LDH compared with the
undoped Mg–Al LDH is attributed to the increased charge
Fig. 6 shows plots of Ce/qe versus Ce for the adsorption
isotherm of Se(VI) adsorbed by the Fe2+-doped Mg–Al LDH. The
high linearity indicates that this process follows a Langmuir-
type adsorption. The values of qm and KL, determined from
the slope and intercept of the straight line in Fig. 6, were 1.4
mmol gꢀ1 and 1.6, respectively.
density of SeO32ꢀ, which is more easily exchanged with inter-
2ꢀ
calated Clꢀ than SeO4
.
The schematic diagram for Se(VI) removal by the Fe2+-doped
Mg–Al LDH is shown in Fig. 4. The Fe2+-doped Mg–Al LDH
2ꢀ
adsorbs Se(VI) as SeO4 from an aqueous solution through
anion exchange with intercalated Clꢀ. Se(VI) is reduced to Se(IV)
upon oxidation of Fe2+ to Fe3+ in the LDH host layer, and the
2ꢀ
Se(IV) produced as SeO3 is again adsorbed by the Fe2+-doped
Mg–Al LDH through anion exchange with intercalated Clꢀ.
Se(IV) adsorption does not occur in undoped Mg–Al LDH as only
2ꢀ
SeO4 is adsorbed by this structure. The reduction of Se(VI) to
Se(IV) is advantageous for Se(VI) removal by the Fe2+-doped Mg–
Al LDH due to the increased charge density of SeO32ꢀ. The Fe2+
-
doped Mg–Al LDH can effectively remove Se(VI) from an aqueous
solution because of anion exchange properties of the LDH and
activity of Fe2+ as a reducing agent.
Fig. 5 shows the adsorption isotherm of Se(VI) adsorbed by
the Fe2+-doped Mg–Al LDH where the equilibrium adsorption
increased rapidly with increasing equilibrium concentration.
Fig. 5 Adsorption isotherm of 0.5–25 mM Se(VI) adsorbed by 0.2 g
Fe2+-doped Mg–Al LDH at 30 ꢁC after 24 h.
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of temperature, indicating that Se(VI) removal cannot be repre-
sented by rst-order reaction kinetics. Thus, pseudo second-
order kinetics may be expressed by20–22
dqt/dt ¼ k(qe ꢀ qt)2
(4)
where qt (mmol gꢀ1) is the amount of Se(VI) removed at reaction
time t, qe (mmol gꢀ1) is the amount of Se(VI) removed at equi-
librium, and k (g mmolꢀ1 minꢀ1) is the rate constant for Se(VI)
removal. Integration of eqn (4) gives
2
t/qt ¼ 1/(kqe ) + t/qe.
(5)
The pseudo second-order reaction can predict the adsorp-
tion behavior by assuming that the rate-determining step
consists of chemical adsorption involving valence forces
through the sharing or exchange of electrons between the
adsorbent and adsorbate.20,21,23–25 Fig. 9 displays the pseudo
second-order plot for Se(VI) removal at various temperatures,
which shows good linearity at all temperatures and conrms
that Se(VI) removal can be represented by pseudo second-order
reaction kinetics. The apparent rate constants at 10, 30, and
Fig. 6 Plots of Ce/qe versus Ce for the adsorption isotherms of
0.5–25 mM Se(VI) adsorbed by 0.2 g Fe2+-doped Mg–Al LDH at
30 ꢁC after 24 h.
Fig. 7 shows the variations in Se(VI) removal by the Fe2+
-
doped Mg–Al LDH over time at various temperatures. Se(VI)
removal increased with time and increasing the temperature
from 10 to 30 ꢁC, but was almost constant when the temperature
60 C were 1.2 ꢂ 10ꢀ3, 1.5 ꢂ 10ꢀ3, and 2.2 ꢂ 10ꢀ3 g mmolꢀ1
ꢁ
minꢀ1, respectively. Thus, the apparent rate constant clearly
increased with increasing temperature. An Arrhenius plot of the
rate constants, determined from the slopes of the lines in Fig. 9,
is shown in Fig. 10 and yields an apparent activation energy of
ꢁ
increased from 30 to 60 C. These results suggest that higher
temperatures enhance chemical adsorption (i.e. anion exchange
2ꢀ
2ꢀ
of SeO4 and SeO3 with intercalated Clꢀ).
Next, the kinetics of Se(VI) removal by the Fe2+-doped Mg–Al
LDH were examined based on the data shown in Fig. 7. First-
order kinetics, which depend on the concentration of Se(VI),
may be expressed by
10.0 kJ molꢀ1
.
There are two main types of adsorption: physical and
chemical. Typically, the forces involved in physical adsorption
are weak and have activation energies of no more than 4.2 kJ
molꢀ1. Chemical adsorption, however, is highly specic and
involves forces much stronger than those in physical adsorp-
tion. With chemical adsorption, the reaction rate varies with
temperature according to a nite activation energy (8.4–83.7 kJ
molꢀ1) in the Arrhenius equation.25–27 The apparent activation
energy of 10.0 kJ molꢀ1 for Se(VI) removal is within the nite
activation energy range of chemical adsorption. Therefore, this
ꢀln(1 ꢀ x) ¼ kt,
(3)
where x is the degree of Se(VI) removal, t (min) is the reaction
time, and k (minꢀ1) is the rate constant for Se(VI) removal. Fig. 8
presents the rst-order plots of Se(VI) removal at various
temperatures. None of the plots show good linearity, regardless
Fig. 7 Variations in Se(VI) removal by the Fe2+-doped Mg–Al LDH over Fig. 8 First-order plot of Se(VI) removal by the Fe2+-doped Mg–Al LDH
time at various temperatures for Al/Se ¼ 1.
at various temperatures.
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Fe2+ as a reducing agent. This process was considered to occur
through Langmuir-type adsorption, where the maximum
adsorption and equilibrium adsorption constant were 1.4 mmol
gꢀ1 and 1.6, respectively. Se(VI) removal could be well expressed
as a pseudo second-order reaction. The apparent rate constants
at 10, 30, and 60 ꢁC were 1.2 ꢂ 10ꢀ3, 1.5 ꢂ 10ꢀ3, and 2.2 ꢂ 10ꢀ3
g mmolꢀ1 minꢀ1, respectively, and the apparent activation
energy was 10.0 kJ molꢀ1. The rate-determining step in Se(VI)
removal by the Fe2+-doped Mg–Al LDH was conrmed to be
2ꢀ
chemical adsorption involving anion exchange of SeO4 and
2ꢀ
SeO3 with intercalated Clꢀ.
Acknowledgements
This research was partially supported by the Ministry of
Education, Science, Sports, and Culture, Grant-in-Aid for Chal-
lenging Exploratory Research, 23651061, 2011–2013.
Fig. 9 Pseudo second-order plot of Se(VI) removal by the Fe2+-doped
Mg–Al LDH at various temperatures.
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