Synthesis and Characterization ofNanosilica

and Adsorption of Metal Ion Tserenadmin Dagiisurent, Erdenebileg Enkhturt, Lkhanaajav Nyam-Ochir\ Khishigjargal

Tegshjargalt, Chimed Ganzorigt, and Namsrai Javkhlantugs*t tCenter for Nanoscience and Nanotechnolgy & Department of Chemical Technology, School of Chemistry and

Chemical Engineering, National University of Mongolia, Ulaanbaatar 14201, Mongolia :1:Department of Theoretical and Experimental Physics, School of Physics and Electronics, National University of

Mongolia, Ulaanbaatar 14201, Mongolia *corresponding email: javkhlantugs@num.edu.mn

Abstract-In present work, the nanosized silica particles

were synthesized using acidic hydrolysis of sodium

silicate by precipitation under controlled pH conditions.

The synthesized nanosized silica particles were

characterized by using Scanning Electron Microscopy

(SEM), Atomic Force Microscopy (AFM), and X-ray

diffraction (XRD). The nanosized silica particles have

been demonstrated the various properties according to

its purity, shape, size and distribution. One of the

applications of synthesized nanosized silica particles is

the adsorption of heavy-metal ions, which is

investigated using UV-Vis spectroscopy. The adsorption

energies of heavy metal ions on nanosized silica particles

were calculated using ab-initio method and compared

the binding energies of metal ions onto silica.

I. INTRODUCTION

Nano sized silica particle has a large surface area and environmentally stable. Therefore, nano sized particles use in various applications as filler in rubber and plastics, absorbent, drying powder, anticorrosion agent, etc. One of the applications of silica particles is the adsorption to remove the aqueous heavy-metal ions [1-7]. The adsorption kinetics of copper (ll) and lead (ll) onto grafted silica were investigated and modeled to fit the experimental data [1]. Amino functionalized silica particles were synthesized and investigated the adsorption of heavy metal ions onto silica particles were studied [4]. Also sulfano and polyvinyl alcohol hybrid silica was investigated to adsorption of ions to remove from the aqueous solution [6, 8].

There exist many methods to synthesize the nano sized silica particles. Sodium silicate is a cost effective source of silica compared to other raw materials. The syntheses of nano sized silica particles were reported previously using some stabilizers [8-14]. Moreover, the expensive, toxic raw materials, and heavy method could be avoided to synthesize the nano sized silica particles. In this work, the nano sized silica particles was synthesized by acidic hydrolysis of sodium silicate using "bottom-up" nanotechnological approaches. The total potential energy of metal ions onto silica surface was calculated by using ab-initio method.

II. MATERIALS AND METHODS

A. Synthesis ofnanosilica

Nanosilica was synthesized by acidic hydrolysis of sodium silicate by 2 N hydrochloric acid using method of Tapasikotoky and S. K. Dolui [8]. 10 % sodium silicate solution was prepared from the pure powder (>97 %, Sigma-Aldrich, Tokyo, Japan) in distilled water. Hydrochloric acid was added to it slowly by 0.05 mlls with stirring at ambient temperature until pH becomes 2. The solution was stirred at ambient temperature for 30 min to carry out for complete hydrolysis. The sol-gel mixture was washed three times to remove the all the ions. It was dried at 40°C during 24 h and then was calcined at 600°C during 24 h.

B. SEM and AFM measurements

The pattern of synthesized silica powder was obtained using Scanning Electron Microscopy (SEM) (Semtrac mini, Tokyo Japan) with 15 kV and 1000X magnification. Atomic force microscopy (AFM) has provided new opportunities for studying surfaces with submicrometer and sub-nanometer resolution, by scanning over them very lightly with a tip [15]. The samples were measured by using AFM as for our previous study [16]. That is, the AFM tips which were used the pyramidal V -shaped cantilevers with nominal spring constant of 0.064 N/m (SPI 3700 with SPA 300, Seiko Instruments, Japan) in non-contact mode. A scan speed of � 1 Hz was used throughout.

C. X-ray diffraction (XRD)

X-ray diffraction (XRD) patterns were performed by using Siemens D500 instrument (Siemens, Cherry Hill, New Jersey) with the 1.5418 A of CuKa radiation.

D. Ultra violet (UV) spectroscopy

Ultra violet (UV) spectra were measured to determine the adsorption of metal ion at neutral pH. In UV measurements, the decrement of concentration of metal ion can be obtained from the absorbance of the UV spectra.

Fig. I. Snapshot of cluster model of silica with metal ions.

When metal ions were adsorbed on nanosilica particles, they are removed from the solution. This allows us to evaluate the time course of metal ion adsorption by observing the metal ion decay evaluated by UV absorbance values. Measurement of UV spectra were performed on a UV-VISTBLE spectrophotometer (SHTMAZU UV-256FW, Tokyo, Japan) using quartz cuvette with a path length of 1.0 cm. UV spectra were recorded at a wavelength of range of 300-900 nm with step of 1 nm. The concentrations of metal ion of 0.1 M and 1 mglml were used. The measurements of UV spectra were performed at ambient temperature.

£. Computational procedure

The cluster model of silica (Fig. \) was used same as previous report [17] and the calculation was performed using the Gaussian03 [18] software with basis set of dgdzvp for metal ions. All calculations were done using density functional theory (DFT) with the B3L YP functional [\9].

TTT. RESULTS AND DISCUSSION

SEM and AFM patterns

The SEM pattern of the synthesized silica sample is shown in the Fig. 2a. It looks clear that the size of the synthesized silica (next it will be denoted as nanosilica) was in within micrometer. Fig. \ b shows the AFM pattern of the synthesized silica. AFM pattern also was shown that the synthesized silica has small particle size with nanometer.

XRDspectra

X-ray diffraction (XRD) spectra showed that the 2theta of synthesized nanosilica without (Fig. 3a) and with washing (Fig. 3b) were 22.8° same as previous reports using surfactants [10, 12, 13]. Fig. 3b shows that the NaCI was removed after washing but NaCI specra were obtained in without washing sample (Fig. 3b).

Fig. 2. SEM (aj and AFM (bj patterns of synthesized nanosilica.

UV spectra

UV spectra were measured to determine the adsorption of metal ion at neutral pH. In UV measurements, the decrement of concentration of metal ion can be obtained from the absorbance of the UV spectra. When metal ions were adsorbed on nanosilica particles, they removed from the solution. The solution of CUS04 looks as light blue by our eyes which means that the blue portion (450-500 nm) of the visible region is not absorbed (Fig. 4a). The blue

2+ h b .

color is from the Cu(H20)4 complex. T e a sorptIon of the solution clearly shows that the wavelength in the red region (about 650 nm) was absorbed (Fig. 4a).

a) '"

UI

.� , ..

�

"

•

b) '"

'"

/:' .� � '"

oS

"

" .u jI ".

lthch " ..

Fig. 3. XRD spectra of without washed (bj and washed (cj

synthesized nanosilica. The spectra were lined with smoothing

factor 10 %.

a)

Joo 4DO :'jot) (joo 700 800 900

b) I

� 0.8

� 0,6

� 0.4

D.'

O � 300 400 .00 GOO 100 800 m

Wlil'!::ltll]::lh(nril)

Fig. 4. UV spectra of the 0. 1 M CUS04 solution before (a) and after (b) treated by nanosilica

After CUS04 solution treated by nanosilica, the absorbent of solution was measured which was decreased because of decrement of concentration of ion after removed onto nanosilica surface from the solution (Fig. 4b).

Fig. 5 shows that the absorbance of metal ion solution as a function of time. Until 30 min, copper ion is adsorbed strongly onto silica surface but after 30 min, it is desorbed slowly into solution till it becomes equilibrated state.

Binding energy between metal ion and silica cluster

Main interest in the present work was for adsorption of metal ions onto silica surface. Adsorption energy of metal ion onto silica surface was calculated as below:

Fig. 6 shows that adsorption energy of Cu2+ and

Ag + onto silica surface. The Cu2+ strongly adsorbed

onto silica surface compared with Ag + which was good agreement with our experimental results.

V. CONCLUSIONS

Nanosilica was successfully synthesized by acidic hydrolysis of sodium silicate which was an expensive and toxic avoided method. The synthesized nanosilica could remove the aqueous heavy metal ion. Additionally, binding energies of metal ions were calculated ab-initio method to compare the adsorption. In future, we need analyze the adsorption activity of heavy metal ions by experimental and theoretical methods to confirm both of the methods.

lI.ll

0.2

0.18

0.16

0.1"

0.12

0.1

0.011

ndsoq)'ioll

t desorption

t equilibnltiOn

20 40 60 8(1 100 120 Timc(min)

Fig. 5. Time dependence of UV absorbance of I mg/ml CUS04 solution.

a) ·0,1

S' ·015

..:i, ·0.2

� " § ·0.25 " '" e. .0.3

'"

� -0.35

·0.'

b) 0.8

S' 0.6

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� 0.4

= " =

0.2 '" . ." Q. ... �

'" ...:

-0.2 1 .5 2.5 3.5 4.5 5.5

Distance (angstrom)

Fig. 6. Adsorption energy ofCu2 (a) and Ag (b) onto silica

surface.

ACKNOWLEDGMENT This research was supported by Asian Research

Center in Mongolia and the Korea Foundation for Advanced Studies within the framework of the Project # 1 (2013-2014). We would like to thank Ms. L. Ganchimeg of Center for Nanoscience and Nanotechnology, School of Chemistry and Chemical Engineering, National University of Mongolia for her help to prepare the samples and devices.

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