Journal of Non-Crystalline Solids 358 (2012) 847–853
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Journal of Non-Crystalline Solids
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Improved method for preparation of anhydrous silica by microwave irradiation withspectroscopic characterization and toxicity assay
Muhammad Ashfaq a,⁎, Rukhsana Tabassum a, Muhammad Mahboob Ahmed a, Karamat Mehmood a,M. Asghar b, Tanveer Hussain c
a Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur, Pakistanb Department of Physics, The Islamia University of Bahawalpur, Bahawalpur, Pakistanc Cholistan Institute of Desert Studies, The Islamia University of Bahawalpur, Bahawalpur, Pakistan
Article history:Received 26 July 2011Received in revised form 20 November 2011Available online 13 January 2012
Keywords:SiO2 (mw)=SiO2 prepared throughmicrowave technique;SiO2 (mwc)=SiO2 prepared throughmicrowave followed by calcination;SiO2 (R)=SiO2 prepared by reported method
Amorphous anhydrous silica SiO2 (mw) (99.99%) is successfully synthesized through microwave irradiationtechnique and time of reaction is reduced up to 1 h. The dehydration phase study of Si–water, Si–OH, Si–O–Si networking, elemental analysis and surface morphology was carried out by FTIR, FTNIR, SEM andEDAX spectroscopic techniques. The broad absorption stretching and bending of Si–OH and H2O at3695.38–2832.96 cm−1, 1638 cm−1 and 1191.20–1017.14 cm−1 completely disappeared and appearanceof new bands at 946.93 and 797.63 cm−1 confirmed the amorphous anhydrous silica with Si–O–Si network-ing. The SEM images of SiO2 (mwc) described the smooth and fine particle texture and confirmed 99.99% Si–O–Si networking of anhydrous silica. The 99.99% purity was verified by EDAX spectra which exhibited sharp sig-nals only for oxygen and silicon. Toxicity against Monomorium minimum and Tribolium castaneum with 100%mortality and LT50 91 min and 7.5 h respectively is being reported. It can be used for long storage of grains inthe future.
Silica covers wide spectrum of uses such as filler in rubber andtires [1–3] anti-caking or free flow agents in powdered or liquid ma-terials, thermal and acoustic insulation, Cerenkov radiation detectors,photo luminescent [4] and radio luminescent [5] devices, comet dust[6] and aerosol particles [7] collectors, adsorption and catalyst sup-ports [8], adhesives, paints and colorants, health care products suchas toothpaste [9], cosmetics and pharmaceuticals [10]. Amorphoussilicon dioxide exhibits a range of unique properties such as low tem-perature linear expansion coefficient, optical transparency from infra-red to ultraviolet radiation, high chemical stability, high resistance onexposure to radiation and neutron flux, stability of dielectric proper-ties over a wide range of frequencies and temperatures. Besides theabove mentioned applications, silica also plays an important role inmany body functions [11,12] and has a direct relationship to mineralabsorption. The average human body holds approximately 7 g of sili-ca, a quantity far exceeding the figures for other important mineralssuch as iron. Both iron and silica are body essential, meaning theyare needed for carrying out ongoing metabolic work that is vital tolife. Silica is essential to the development of the skeleton and miner-alization [13,14], its absence results in skeletal deformities. Hormonaldisturbances in the human organism are often due to a calcium–
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magnesium imbalance. Several studies have shown that silica can re-store this delicate balance. Silica also benefits the assimilation ofphosphorous, calcium andmagnesium [15] which controls the pro-cess of osteoporosis [16–18]. Silica can help to slow down the degen-erative process of connective tissue [19–21]. Silica helps to preventbaldness, stimulates healthier hair growth and assures beautifulshine, luster and strength while silica prevents cavities and preservesteeth as well as nails [22]. The presence of sufficient silica in the intes-tines will reduce inflammation of the intestinal tract, cure stomach andintestinal catarrh and ulcers [23], prevent diarrhea and constipationand also help normalize hemorrhoidal tissues.
Inert dust such as clay, sand, rock phosphate, ashes, diatomaceousearth and synthetic silica have been used as insecticide for thousandsof years and are also used in modern grain storage facilities. The mainadvantage of inert dust is their low mammalian toxicity. In Canadaand the USA diatomaceous earth is registered as an animal feed addi-tive and silicon dioxide as a human feed additive [24].
In the past decade, varieties of amorphousmaterials have been de-veloped. One of the well-developed amorphous materials is silica.Silica is attractive because it is chemically inert, thermally stable,harmless and inexpensive [25]. In literature various but more compli-cated methods with time and energy consumption have beenreported to synthesize amorphous silica [26–33]. Although the previ-ously reportedmethods are feasible for industry but not applicable forbio-applications due to use of harmful chemicals e.g. ammonia [34,35]or N2H4 [36] as catalysts such problems have been suggested by
Scheme 1. Schematic illustration: preparation of amorphous silica and reaction sequence.
(A) (B)
Fig. 1. Amorphous silica: (a) by improved method, (b) by reported method [30].
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several research groups [37]. The organic template and sol gelmethods have been reported to synthesize silica [28,29] but it is notanhydrous as indicated by their FT IR spectra which showed OH/H2Oabsorbance at >3500–3000 cm−1. The synthesis of amorphous mate-rials has been of great interest due to an ever-expanding list of uses,ranging from chemical sensors to drug delivery [38]. AmorphousSiO2 is selected to improve its method of synthesis due to describedfacts towards its importance almost in every field of life. We here arereporting an improved method of its preparation through microwave
0
5011
015
020
0
R3= 10 hours
R2= 2 hours
R1= 1 hour
R4= 15 hours
Silisic Acid
3500 3000 2500 2000 1500 1000
Wavenumber cm-1
Tra
nsm
ittan
ce [
%]
Fig. 2. Study of synthesis of silica through FTIR, =silicic acid, =SiO2 (R1), =SiO2 (R2),=SiO2 (R3), =SiO2 (R4).
technique followed by calcination. Themain objective of our plan is toreduce the cost and time of reaction with excellent yield and 99.99%quality.
2. Experimental section
2.1. Synthesis
The chemicals used in this study were water glass (sodium sili-cate) from Al-Madina Scientific Store Bahawalpur, Pakistan and HCl
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(37%) from Merck and used as such. Amorphous silica was synthe-sized by an improved wet chemical method. Hydrochloric acid solu-tion (12 M) was added drop wise to sodium silicate (10 ml, 50%)solution with constant stirring until the pH (1–3) was acidic. Thewhite gelatinous mass obtained was washed thoroughly with dis-tilled water to remove NaCl and then heated in microwave oven for1 min. The schematic synthesis is given in Scheme 1. Only 1 hourtime was enough for complete calcinations (600 °C) of silica whichis confirmed by FTIR, FTNIR, SEM and EDAX while as per reportedmethod [30] calcination (600 °C) process needs 15 h. Fine and white
100
8060
4020
0
Tra
nsm
ittan
ce [
%]
3500 3000 2500 2000 1500 1000
mwc6=60 minutes
mwc7=40 minutesmwc6=30 minutesmwc5=25 minutes
mwc4=20 minutes
mwc3=15 minutes
mwc2=10 minutes
mwc1=5 minutes
mw1
Wavenumber cm-1
Silicic Acid
Fig. 4. FTIR spectral study of calcinated SiO2 (mw) silicic acid and SiO2 (mw1),SiO2 (mwc1), SiO2 (mwc2), SiO2 (mwc3), SiO2 (mwc4), SiO2 (mwc5), SiO2 (mwc6),SiO2 (mwc7), SiO2 (mwc8), [see Table 3 for condition detail].
product was obtained through improved method using microwavetechnology (Fig. 1).
2.2. Characterization
The FTIR spectra were recorded using BRUKER Tensor 27 (M15E-PS/09) FTIR spectrophotometer by ATR sampling technique at range of4000–600 cm−1. The FTNIR spectral studies were conducted usingBRUKER Multipurpose FTNIR Analyzer at range of 12,500–4000 cm−1.The surface morphologies and composition of silica prepared byreported method and modified method were studied using Hitachi S-3000H Scanning Electron Microscope operated at an accelerating volt-age of 15 kV combined with Energy Dispersive X-Ray Analyzer (EDAX).
2.3. Silica bioassay
For both anhydrous and hydrated silica 100% concentration wastested against Monomorium minimum while 0.5% w/w (0.05 g silicain 10 g grains of wheat) was tested against Tribolium castaneum. Foreach concentration three replications were prepared for both typesof products. Twelve Petri dishes were prepared by lining them withfilter paper. 0.2 mg hydrated silica and 0.2 mg anhydrous silica foreach dish of three replicates were placed on the filter paper then ametal probe was used to distribute the dust inside the dishes. Thirtyadults of M. minimumwere placed in each dish and mortality percentwas recorded at intervals of 20, 40, 60, 80, 100, 120 and 140 min re-spectively. Similarly six Petri dishes (three replications for each prod-uct) were loaded with grains of wheat mixed with silica. Twenty five
Fig. 5. FTIR spectra of amorphous silica [SiO2 (R)] prepared by reported method calcina-tion time period of 15 h indicate the presence of Si–OH network at 958.14 cm−1 [30].
Fig. 6. FTIR spectra of silica prepared by improved method indicates no Si–OH networkat 958.14 cm−1.
Fig. 8. FTNIR spectra of anhydrous silica [SiO2 (mwc)].
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adults of T. castaneum were placed in each dish and mortality rate inpercent was recorded at intervals of 1, 3, 6, 9, 12 and 15 h respective-ly. Untreated grains were used as control. LT50 for both products andboth species was calculated by using standard statistical procedureProbit Analysis.
3. Results
Microwave technique is successfully employed to synthesizeamorphous anhydrous silica SiO2 (mw) (Scheme 1) and results arecompared with reported SiO2 (R) [30]. The SiO2 (mw) is finer thanSiO2 (R) (see Fig. 1). Results for microwave irradiated product aregiven in Fig. 3 and Table 2 whereas the symbols (mw1, mw2, mw3,mw4, mw5, mw6 and mw7) indicate time of irradiation. An improve-ment was noted that the time of reaction is reduced from 15 h to20 min only. An excellent improvement with 99.99% pure anhydroussilica was obtained through microwave irradiation (1 min) followedby calcination only for 1 h, see Fig. 4 and Table 3. The symbolsmwc1–mwc8 stand for microwave irradiation followed by calcinationprocess. The calcination process was started from mw1 sample up to60 min for five minute intervals of calcination time to compare the ef-fect of calcination on microwave irradiated product.
The FTIR spectra of SiO2 (mw and R samples) were recorded; the re-sults are given in Figs. 2, 3 and 4. Fig. 2 indicates the synthesis of SiO2
according to the reported method [30] and R1, R2, R3 and R4 stand forcalcination range while other calcination timing ranges in betweenthem were also considered but no specific changes were observedto obtain anhydrous silica. The broad absorption stretching and bend-ing of Si–OH and H2O at 3695.38–2832.96 cm−1, 1638 cm−1 and1191.20–1017.14 cm−1 exhibited the presence of water moleculesalong with silica coordination sphere. The sharp absorption overtonebands at 8000 to 7000 and 6000–5000 cm−1 obtained by FTNIR are
Fig. 7. FTNIR spectra of silica [SiO2 (R)] [30].
given in Fig. 8. EDAX spectra of anhydrous amorphous silica confirmpercentage composition of oxygen and silicon but no impurities(Fig. 9).
Table 4 and Fig. 12 showed the % of mortality and LT50 for M. min-imum in 100% anhydrous and hydrated silica through selected timeintervals of 20, 40, 60, 80, 100, 120 and 140 min.
4. Discussion
4.1. FTIR characterization
During calcination the appearance of new bands at 946.93 and797.63 cm−1 is the indication of conversion of silicic acid in to silica.On calcination for 1 h, the absorption at 1191.20–1017.14 cm−1
became more prominent which indicates the origin of Si–O–Si net-working as well as coordination of water molecules. With thepassage of calcination up to 15 h, conversion to sharp band at1066.01 cm−1 and disappearance of band at 1638.57 cm−1 indicatethe removal of coordination of water molecules (Fig. 2 and Table 1).Another peak as a shoulder at 958.14 cm−1 in final SiO2 indicatestraces of water contents. Similar IR data in literature [39–41] isfound which makes doubt about the purity of anhydrous silica.
The absorption at 3695.38–2832.96 cm−1, 1638.57 cm−1 and958.14 cm−1 due to water molecules in Fig. 2 that seemed to disap-pear in Fig. 3 is a great success to recover anhydrous silica withinonly 20 min. As per IR reaction curves (mw1–mw7) in Fig. 3, the %of water molecules in silica is reduced up to 0.02% (mw7) and silicicacid curve showed 55% water molecules while Fig. 2 curves R1, R2and R3 showed 55% water which indicated that up to 10 h of calcina-tion silicic acid is not completely converted into silica. All theunwanted absorption bands due to Si–H2O and Si–OH network werecompletely absent in Fig. 3 curve mw7 which confirmed 99.80%pure anhydrous silica.
Fig. 9. EDAX spectra of anhydrous silica SiO2 (mwc).
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As per experimental detail given in Fig. 4, all the networking dueto Si–OH and Si–H2O were completely removed and straight line ofabsorption from 4000 cm−1 to 1042 cm−1 in curve mwc8 (Fig.4)confirmed the 99.99% pure anhydrous silica which has never beenseen but already reported in literature [28,29,39–41]. More specifical-ly, Fig. 5 explains and confirms the presence of Si–OH network at958.14 cm−1 in the silica prepared by reported method [30] but nosuch type of Si–OH network is detected in an anhydrous silica pre-pared by our improved method (Fig. 6) which is brighter and99.99% pure anhydrous silica than that given in Fig. 5.
4.2. FTNIR characterization
FTNIR sharp absorption overtone bands at 8000 to 7000 and6000–5000 cm−1 supports the presence of Si–O–Si network only and
Fig. 10. Scanning Electron Micrograph (SEM) of SiO2 (R), (A
Fig. 11. Scanning Electron Micrograph (SEM) of SiO2 (mwc). (C)
the results are given in Fig. 6 while FTNIR spectra (Fig. 7) are showingthe presence of Si–OH network at 5000–4000 cm−1 combinationband offering an analogous result as given in Fig. 5.
4.3. SEM/EDAX characterization
EDAX spectra confirmed only Si and O moieties in Si–O–Si net-work (Fig. 9) while the SEM photographs of both the products de-scribed the smooth and fine particle texture of SiO2 (mwc) (Fig. 11)which confirmed Si–O–Si network of 99.99% anhydrous silica of par-ticle size 2 μm to 8 μm while coarse and diffused texture of SiO2(R)
particle size existed up to 24 μm (Fig. 10) which is definitely due tothe presence of water molecules as Si–OH network.
4.4. Insecticidal activity
Anhydrous silica showed excellent results as compared to hydrat-ed silica. Mortality of M. minimum was seen at 80 min of exposure inanhydrous silica while hydrated silica exhibited toxicity at 100 min ofexposure. All the organisms under experiment were died within timeof 140 min with LT50 91 min while hydrated silica showed 54% mor-tality with LT50 132 min. As per reported mechanism [24,42] silicondioxide based inert dust damage insect's cuticle to death from desic-cation. Insects die when they have lost approximately 60% of theirwater or about 30% of their body weight. Another insect T. castaneumwas kept under test with silica to find out % mortality and LT50 as well(Table 5 and Fig. 13). Encouraging and excellent results of 100%
) 15 kV and 200×, 200 μm (B) 15 kV and 700×, 50 μm.
at 15 kV and 200×, 200 μm, (D) at 15 kV and 700×, 50 μm.
Fig. 12. Mortality % age of Monomorium minimum exposed to silica. Fig. 13. Mortality % age of Tribolium castaneum exposed to silica.
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mortality with LT50 7.5 h of T. castaneum after 15 h of incubation withanhydrous silica was obtained while hydrated silica shows 52% mor-tality with LT50 14.5 h. According to literature L. Arnaud et al. in 2005reported [43] 88% mortality of T. castaneum in 3 weeks but we arereporting 100% mortality of T. castaneum and LT50 7.5 h. The resultsshowed linear relationship between the mortality percentage and ex-posure time in the case of both insects. As per literature study it isreported [44,45] that as insects move through the grain or across atreated surface, they pick up dust particles on their cuticle, arethought to absorb the cuticular waxes damaging the insect cuticle,and causing death by desiccation. As per our investigation, ourreported anhydrous silica will be applicable for the economical longstorage of grains like wheat, rice, cereals etc. to save a large amountof food for the nation. Now the study is in progress to explore theexact concentration of silica per kg of grains storage.
5. Conclusions
Amorphous anhydrous silica SiO2 (mwc) is successfully synthesizedusing microwave technique followed by calcination. The time of reac-tion to produce anhydrous silica is reduced up to 1 h with 99.99%purity. The appearance of new absorption bands at 946.93 and797.63 cm−1 confirmed Si–O–Si networking in amorphous anhy-drous silica. The SEM image (50 μm) described the smooth and fineparticle texture. EDAX spectra exhibited sharp signals only for oxygenand silicon. NIR supported the same. The Si–O–Si networking and99.99% purity were verified by all techniques. As per literature study[1–10], 99.99% anhydrous silica is needed today in the world specifi-cally in the electronic field. Toxicity test against M. minimum and
Table 5Toxic effect on Tribolium castaneum.
Time(h)
% Mortality(0.5%w/wanhydrous silica)
LT50(h)
% Mortality(0.5%w/whydrated silica)
LT50(h)
1 0 g 7.5
0 g 14.5
3 0 06 19 09 53 1212 85 1715 100 52
T. castaneum showed tremendous potential and proved that it is awonderful insect killer and can be used for long storage of grains inthe future.
Acknowledgments
Dr. Muhammad Ashfaq corresponding author is gratefully thank-ful to Central Laboratories and Department of Chemistry, The IslamiaUniversity of Bahawalpur, Pakistan for providing spectroscopic facili-ties for the completion of this research project.
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