The adsorption of mercury(II) on the surface of silica modified with β-cyclodextrin

Article abstract of DOI:10.1134/S0036024408080190

Multistage chemical modification of the surface of silica with β-cyclodextrin was performed. IR spectroscopy and quantitative analysis of surface compounds were used to prove the structure of modified silica. The adsorption of Hg(II) from dilute solutions

Full text of DOI:10.1134/S0036024408080190

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2008, Vol. 82, No. 8, pp. 1357–1362. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © L.A. Belyakova, A.N. Shvets, A.F. Denil de Namor, 2008, published in Zhurnal Fizicheskoi Khimii, 2008, Vol. 82, No. 8, pp. 1527–1532.
PHYSICAL CHEMISTRY
OF SURFACE PHENOMENA
The Adsorption of Mercury(II) on the Surface of Silica Modified
with b-Cyclodextrin
L. A. Belyakovaa, A. N. Shvetsa, and A. F. Denil de Namorb
a Chuiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukraine
b Faculty of Chemistry, University of Surrey, UK
e-mail: v-bel@mail.kar.net
Received April 17, 2007
Abstract—Multistage chemical modification of the surface of silica with β-cyclodextrin was performed. IR
spectroscopy and quantitative analysis of surface compounds were used to prove the structure of modified
silica. The adsorption of Hg(II) from dilute solutions was studied. The adsorption affinity of silica for mer-
cury ions increased because of the formation of supramolecular structures with chemically immobilized
β-cyclodextrin.
DOI: 10.1134/S0036024408080190
INTRODUCTION
(p-tosyl chloride, TsCl, Merck) was used without addi-
tional purification.
The increased interest of researchers in cyclodex-
trins (cyclic oligosaccharides) stems from their
unique ability to form inclusion host–guest com-
plexes with many organic compounds [1, 2] and the
possibility of using them in various areas of science
and technology, including the preparation of selec-
tive adsorbents, active catalysts, and sensitive sensor
elements [3–5]; the chromatographic separation and
purification of organic compounds with similar
structures and compositions [6]; the extraction and
concentration of traces of toxic compounds [7]; and
the preparation and detoxication of medicines of pro-
longed action [8, 9]. Studies of the adsorption selec-
tivity of cyclodextrins with respect to heavy metal
ions are of obvious interest. The results can be used
in chemical analysis, removal from the environment,
and utilization of these ions [10–12]. Insoluble
cyclodextrins and cyclodextrins immobilized on var-
ious carriers, including inorganic carriers, are most
suitable in practice [1, 2, 13].
The infrared spectra were recorded on a Thermo
Nicolet NEXUS single-beam IR Fourier transform
spectrophotometer over the frequency range 4000–
500 cm–1. The samples were prepared by pressing
adsorbents into pellets of weight ~30 mg at a pressure
1
of 108 Pa. The nuclear magnetic resonance H NMR
spectra were recorded at 25°ë on a Varian-VXR-300
(300 MHz) spectrometer in dimethylsulfoxide-d6
(DMSO-d6) with tetramethylsilane as an internal refer-
ence. Elemental analyses were made on an Elemental
Analyzer EA 1110 instrument.
The concentration of silanol groups in the adsor-
bents was determined from the chemisorption of dim-
ethylchlorosilane, and the concentration of aminopro-
pyl groups, by pH titration and thermogravimetry.
Thermogravimetry was also used to find the content of
immobilized β-cyclodextrin [14]. The adsorption of
mercury(II) ions was performed from solutions with
pH ~ 1 and mercury nitrate concentrations 2.5 × 10–4–
4.0 × 10–3 M depending on the contact time with silica
and equilibrium solution concentration. Adsorption
was studied under static conditions by the method of
separate samples at 22°ë. The content of Hg(II) in the
initial and equilibrium solutions was determined by
atom absorption spectrometry on a Pye Unicam SP-9
instrument and by back trilonometric titration [15].
In this work, we studied the influence of the chemi-
cal immobilization of β-cyclodextrin (β-CD) on the
adsorption properties of amorphous macroporous silica
with respect to Hg(II) ions.
EXPERIMENTAL
Macroporous amorphous silica, silochrom S-120
with a specific surface area of 118 m2/g, mean pore
diameter 40 nm, and silanol group concentration
0.4 mmol/g, was used as the initial silica adsorbent.
β-Cyclodextrin (Fluka, major component content no multistage modification of the surface of silica, includ-
less than 99%, water content 12.7%) was evacuated ing the preparation of β-cyclodextrin modified by tosyl
before use at 105°ë for 7 h. 4-Tolylsulfonyl chloride groups, silica surface modification with aminopropyl
RESULTS AND DISCUSSION
β-Cyclodextrin was chemically immobilized by the
1357

1358
BELYAKOVA et al.
groups, and chemical fixation of tosyl-β-cyclodextrin
The appearance of proton signals (Fig. 2) in the 1H
NMR spectrum of modified β-cyclodextrin with chem-
ical shifts δ (ppm) of 2.433 (s, the methyl radical of the
tosyl group), 7.452 (d, benzene ring 3 and 5 protons),
and 7.747 (d, benzene ring 2 and 6 protons) [17] was
indicative of tosyl group grafting.
on the surface of aminopropylsilica.
Tosyl-β-cyclodextrin was prepared as described in
[16]. The interaction of β-cyclodextrin with 4-tolylsul-
fonyl chloride results in the electrophilic substitution of
tosyl groups for alcohol group protons (first, primary
protons in position 6) [2, 16]. The IR spectrum of
β-cyclodextrin (Fig. 1) brought in contact with a solu-
tion of 4-tolylsulfonyl chloride contained not only
β-CD bands but also absorption bands of the tolylsulfo-
nyl group at 1598 cm–1 (benzene ring C=C bond
stretching vibrations), 1366 cm–1 (asymmetric S=O
bond stretching vibrations of the R1–é–SO2–R2
group), and 815 cm–1 (out-of-plane bending vibrations
of aromatic C–H bonds).
The elemental analysis data on modified β-CD,
Element
Found, %
Calculated, %
C
H
S
45.65
45.62
5.97
6.01
2.41
2.48
are evidence of the participation of only one primary
alcohol group in the reaction with 4-tolylsulfonyl chlo-
ride,
6 OH
OTs
6
HO
4
HO
4
O
O
5
5
O
O
O
O
2
3
3
2
1
1
OH
OH
HO
HO
O
O
O
O
OH
OH
HO
HO
HO
OH
OH
OH
OH
O
O
HO
O
Cl S
O
HO
HO
O
O
C5H5N
–HCl
+
OH
OH
OH
OH
CH3
HO
HO
O
O
O
O
OH
OH
O
O
OH
HO
OH
HO
OH
OH
OH
OH
HO
O
HO
O
O
O
O
O
HO
HO
O
O
OH
OH
The yield of synthesized mono-(6-O-tolylsulfonyl)-β-
cyclodextrin (Ts-β-CD) was 35%.
The modification of the surface of silica with amino-
propyl groups was performed following the procedure
described in [18],
C2H5O
+
C2H5O Si (CH2)3 NH2
C2H5O
Si OH
O
+ C2H5OH.
Si O Si (CH2)3 NH2
Chemical composition of the surface layer of silica adsor-
bents and their structure-sorption parameters
O
Ri, mmol/g
ssp,
m2/g
Adsor-
bent
[Hg] : [Ri]
The interaction of aminopropylsilica (table) with
mono-(6-O-tolylsulfonyl)-β-cyclodextrin was per-
formed under the conditions optimum for electro-
philic substitution of β-cyclodextrin groups for ami-
nopropyl group protons on the surface of silica [14].
The IR spectrum of aminopropylsilica (Fig. 3a, spec-
trum 3) modified with a solution of Ts-β-CD in pyri-
dine at 60°ë contained absorption bands of O–H
bond stretching vibrations of secondary alcohol
groups (3373 and 3289 cm–1), β-cyclodextrin C–H
bond stretching vibrations (2948 and 2877 cm–1),
R1
R2
R3
1
2
3
0.40
0.12
0.12
–
–
–
118
111
98
0.25
0.28
3.80
0.28
0.24
0.035
Note: R is the adsorbent functional group (R , silanol; R , amino-
i
1
2
propyl; and R , β-cyclodextrin) and s is the specific surface
3
sp
area.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 8 2008

THE ADSORPTION OF MERCURY(II) ON THE SURFACE OF SILICA MODIFIED
1359
1
2
3
4000
3000
2000
ν, cm–1
1000
Fig. 1. IR spectra of (1) 4-tolylsulfonyl chloride, (2) β-cyclodextrin before interaction with p-tosyl chloride, and (3) β-cyclodextrin
after interaction.
and N–H and C–H bond bending vibrations (1590, absorption bands of C=C benzene ring bonds
1542 and 1458, 1388 cm–1, respectively). The pres- and S=O asymmetric stretching vibrations of R1–
ence of bending vibrations of primary and secondary é–SO2–R2 groups in the IR spectrum of modified sil-
amino groups in the IR spectrum is evidence of the ica proves that the electrophilic substitution of
partial participation of primary aminopropyl groups the tosyl group Ts-β-CD occurs on the surface of
in the chemical grafting of Ts-β-CD. The absence of aminopropylsilica,
Si
Si
O
O
Si O Si (CH2)3 NH2
Si O Si (CH2)3 NH2
O
Si
(OH)7
(OH)7
O
(OH)6
TsO
Si
O
(OH)7
(OH)7
O
+
Si
O Si OH
(OH)6
Si
O Si OH
O
O
O
O
Si
+ TsOH
Si
Si O Si (CH2)3 NH
Si O Si (CH2)3 NH2
The amount of grafted β-cyclodextrin was occupies ~50% of the surface of modified silica. The
0.035 mmol/g silica. Since the contact area of the β-CD structure of the surface of the silica adsorbents studied
molecule is 2.41 nm2, immobilized β-cyclodextrin is shown in the scheme,
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 8 2008

1360
BELYAKOVA et al.
Si
Si
Si
O
O
O
O
O
Si O Si OH
Si O Si (CH2)3 NH2
Si O Si (CH2)3 NH2
O
Si
Si
Si
O
O
O
(OH)7
(OH)7
Si
O
Si OH
Si
O
Si OH
Si O Si OH
(OH)6
O
O
O
O
O
Si
Si
Si
O
Si O Si OH
(adsorbent 1)
Si O Si (CH2)3 NH2
Si O Si (CH2)3 NH
(adsorbent 2)
(adsorbent 3)
and their structure-sorption parameters and the chemi- tion coefficient for adsorbent 3 is 625. These results are
cal composition of the surface layer are presented in the in agreement with the data obtained using IR spectros-
table.
copy. The IR spectrum of hydroxylated silica does not
change substantially after the adsorption of mercury(II)
(Fig. 3b, spectrum 1). After the adsorption of mercury
on aminopropylsilica (Fig. 3b, spectrum 2), the IR
bands corresponding to bending N–H bond vibrations
in primary amino groups (1571 and 1542 cm–1)
slightly shift to lower frequencies (1526 cm–1), likely
because of complex formation on the surface of the
adsorbent [19].
Adsorption equilibrium is established in 1 h for
adsorbents 1 and 3 brought in contact with solutions of
mercury nitrate and in 2.5 h for adsorbent 2 (Fig. 4).
This is evidence of the absence of diffusion limitations
also after the modification of the surface of
macroporous silica. Hydroxylated silica (adsorbent 1)
virtually does not absorb mercury ions from ~10–4 M
solutions of Hg(NO3)2 (Fig. 5). At a Hg(II) concentra-
tion higher by an order of magnitude, only 25% of sil-
anol groups on the surface of adsorbent 1 participate in
adsorption.
The IR spectrum of adsorbent 3 (Fig. 3a, spec-
trum 3) contains not only absorption bands of amino-
propyl groups but also O–H bond stretching vibration
Similar results were obtained for aminopropylsilica frequencies of secondary alcohol groups of β-cyclo-
(adsorbent 2), whereas the modification of the surface dextrin (3373 and 3289 cm–1), and the intensity of
of silica with β-cyclodextrin substantially increased the stretching and bending C–H bond vibrations (2948,
affinity of the sorbent for Hg(II), and the static capacity 2877 and 1458, 1388 cm–1) increases compared with
was already attained in dilute solutions. The distribu- aminopropylsilica. After the adsorption of mercury
(‡)
(b)
(c)
3
2
3.0
2.5
δ, ppm
2.0
8.0
7.5
Fig. 2. NMR spectra of β-cyclodextrin (a) before and (b, c) after its interaction with p-tosyl chloride.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 8 2008

THE ADSORPTION OF MERCURY(II) ON THE SURFACE OF SILICA MODIFIED
1361
(‡)
(b)
1
1
2
3
2
3
4000 3500 3000 2500 2000 1500
4000 3500 3000 2500 2000 1500
ν, Òm–1
Fig. 3. IR spectra of adsorbents 1–3 (a) before and (b) after adsorption of mercury(II).
Γ, mmol/g
Γ × 102, mmol/g
0.16
3
3
12
0.12
0.08
0.04
1
1
8
2
2
4
0
0.5 1.0 1.5 2.5
3.0
3.5
4.0
cequil × 103, mmol/ml
0
1
2
3
4
τ, h
Fig. 4. Adsorption of mercury(II) ions on silica adsorbents
1–3 depending on contact time with a 0.01 M aqueous solu-
tion of mercury nitrate.
Fig. 5. Isotherms of the adsorption of mercury(II) ions on
silica adsorbents 1–3 from aqueous solutions of mercury
nitrate of concentration c
.
equil
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 8 2008

1362
BELYAKOVA et al.
3. T. N. T. Phan, M. Bacquet, and M. Morcellet, React.
(Fig. 3b, spectrum 3), the absorption bands of β-cyclo-
dextrin stretching vibrations become less pronounced,
and the absorption bands of N–H and C–H bond bend-
ing vibrations shift to lower frequencies (1523, 1381,
and 1320 cm–1). It follows that the centers of mer-
cury(II) adsorption in adsorbent 3 are chemically
anchored β-cyclodextrin molecules and primary ami-
nopropyl groups unreacted with mono-(6-O-tolylsulfo-
nyl)-β-cyclodextrin (table). Since the total concentra-
tion of active centers of adsorbents 1–3 is constant
(table), and only the chemical composition of their sur-
face changes, an increase in the adsorption of Hg(II) is
related to the immobilization of β-cyclodextrin. The
[Hg] : [functional group] ratio is smaller than one for
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ACKNOWLEDGMENTS
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This work was financially supported by the Euro-
pean Commission (grant ICA2-CT-10052) and the
Complex Program for Basic Research of the National
Academy of Sciences of Ukraine “Nanostructured Sys-
tems, Nanomaterials, and Nanotechnologies” (project
no. 0103U006289).
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