الفهرس 
Preparation of Nano-silica (SiO2) by different methodsOnly 14 pages are availabe for public view
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Abstract
Nano silica from rice husk ash has attracted developing interest due to its specific structure and various properties. It has promising applications in the fields of industrial catalysis and green chemistry areas as wastewater treatment. SiO2 extracted has been used as an adsorbent material. SiO2 nanomaterials were synthesized using five different methods. Precipitation method (SiOP) which calcined at 500°C for 3h, precipitation method from rice husk ash (SiORHA), combustion method from rice husk ash (SiOC), sol-gel method (SiOSG) which calcined at 500 °C for 3h, and finally (SiOCTAB) which was prepared by surfactant-assisted sol-gel method which calcined at 600 °C for 2h. All synthesized nanomaterials were used as adsorbents in order to remove organic dye (methylene blue MB) from wastewater. The prepared catalysts were characterized by XRD, FT-IR, SBET, and TEM while the obtained (SiOC) was characterized by SEM tool too.
The main conclusions that can be drawn from the obtained results are in the following:
1- XRD of SiORHA, SiOC, and SiOCTAB nanomaterials, respectively indicated the presence of amorphous silica without any arranged crystalline structure and the absence of possible impurities such as sodium sulfate or sodium chloride confirmed the purity of the prepared silica.
2- IR spectra of SiO2 samples showed a presence of characteristic bands due to Si – O – Si, Si-O – H and H – O – H.
3- The adsorption isotherm was classified as type II exhibiting an H3 hysteresis loop in SiOC and SiOCTAB with a very narrow hysteresis loop in SiOCTAB with an upward deviation in SiOC.
4- The SiORHA nanomaterial was classified as type IV isotherm exhibiting an H1 hysteresis loop with two parallel adsorption and desorption branches with cylindrical mesopores.
5- The adsorption isotherm of both SiOSG and SiOp was classified as type IV isotherm exhibiting an H2 hysteresis loop (ink-bottle pore shape) with a wide-body and narrow neck with cylindrical mesopores.
6- The order of BET surface areas for the different SiO2 nanomaterials was as follows: SiOC > SiORHA > SiOSG > SiOCTAB > SiOp.
7- Moreover, the pore volume for different SiO2 was as follows: SiOC > SiORHA > SiOSG > SiOp > SiOCTAB.
8- The average pore diameter for SiORHA, SiOSG, SiOp, SiOCTAB, and SiOC was 10.60, 3.99, 3.99, 3.35 and 3.34 nm, respectively.
9- HR-TEM results of all nanomaterials showed presence of irregular nano-spherical particles with agglomeration.
10- The average particle sizes of the nanomaterials were decreased as follows: SiOP > SiOCTAB > SiOSG > SiORHA > SiOC.
11- SEM showed little black splashes corresponding to the pore spaces as cylindrical holes, the highly soft gel substance agglomerate is surrounded by many pores giving the impression of a sponge that proved porous structure in SiOC nanomaterial makes it exhibits a good adsorption ability.
12- The EDS results of SiOC proved the existence of Si and O elements.
13- Methylene blue quickly adsorbed onto SiOC> SiORHA> SiOSG> SiOCTAB> SiOP adsorbents, respectively at the first 5 minutes of contact time.
14- After the 5th minute dye removal became relatively slow until the equilibrium being arrived with 84.4 %, 87.6 % 90.8 % at 210 min for SiOSG > SiOCTAB> SiOP samples, and 91.6 % at 180 min for SiORHA and 97.6 % at 90 min for SiOC.
15- The percentage dye removal initially increases with increasing the concentration obtaining the maximum removal at 8 mg/L for all adsorbents and then decreased.
16- The percentage of dye removal increases with increasing the amount of SiOC adsorbent from 0.03 g to 0.07 g, by increasing the adsorbent amount thence increasing the active sites participated in the adsorption.
17- The percentage of dye removal gradually increases with increasing the solution pH until arrived at a maximum value at pH 11 with 94.7 % and 99.7 % removal for SiOP and SiOC, respectively.
18- The surface gains a negative charge by increasing the pH of dye solution, thereby increasing the electrostatic attraction between a positively charged dye and negatively charged adsorbent resulting in an increased MB adsorption.
19- The kinetic study showed that the correlation coefficient of a pseudo-second-order kinetic model (R2 > 0.99) is higher than that for the pseudo-first-order kinetic model, suggesting that the MB dye adsorption onto different SiO2 adsorbents perfectly obey pseudo-second-order kinetic model.
20- The intra-particle diffusion model shows multi-linearity, pointing
out that the adsorption of MB dye onto different SiO2 adsorbents
occur from a three-step process and the intra-particle diffusion is
not the rate-limiting step.
21- The correlation coefficient of the Langmuir model was higher than that of Freundlich and Temkin models, which supposes a monolayer coverage onto the adsorbent surface.
22- The RL values of MB adsorption onto different SiO2 adsorbents were found to be between 0 and 1, suggesting that it is a favorable adsorption operation that takes place by the physisorption process. Furthermore, the (SiORHA) and (SiOC) possessed the largest maximum adsorption capacity (qmax) values compared to the other adsorbents.
23-The Freundlich parameter, the adsorption capacity (KF) was found to increase as a following: SiOC > SiORHA > SiOSG > SiOCTAB > SiOP adsorbents, high KF values pointing out to easy uptake of the adsorbate.
24- The values of (1/n) were found to be between 0 and 1, reveals the favorability of MB adsorption onto different SiO2 adsorbents.
25-Temkin parameter, (b) is related to the heat of sorption was found to decrease as a following: SiOC < SiORHA< SiOSG < SiOCTAB < SiOP adsorbents, suggesting a physiochemical mechanism of the adsorption operation.
26- The adsorption heat of (SiORHA) and (SiOC) possessed the lowest value compared to the other adsorbents because they possessed the highest adsorption coverage.
27- The values of standard Gibbs free energy change (ΔGo) were negative at all temperatures, supporting that the adsorption of MB onto different SiO2 adsorbents was spontaneous and thermodynamically favorable.
28- Furthermore, the negative values of ΔHo point out that MB adsorption onto SiOC, SiORHA, and SiOCTAB adsorbents is an exothermic operation.
29- Whereas the positive values of ΔHo suggest that MB adsorption onto SiOP and SiOSG adsorbents is an endothermic operation.
30- In addition, the positive values of ΔSo, implying the high degree of disorder of solid–liquid interface during adsorption and randomness of the MB adsorption onto SiOP and SiOSG adsorbents which demonstrated the affinity of this adsorbent.
31- Whilst, the negative values of ΔSo indicate a decrease in the freedom degree of MB adsorption onto SiOC, SiORHA, and SiOCTAB adsorbents.
32-The adsorption of MB dye onto different SiO2 adsorbents have ΔHo values of > 20.9 kJ mol−1, suggesting that the chemical adsorption is the predominant mechanism, and this is consistent with the conclusion of the adsorption kinetics analysis.