Extraction High Purity Nanosilica Corn Cob by Modied Precipitation Technique

. Corn cob considers the agricultural waste in Iraq. High purity SiO 2 NPs were extracted from corn cob by enhanced precipitation and developed leaching processes. In this study, pre-treatment with 3N HCl has achieved then calcination of the corn cob at 700°C, then follows with the leaching process with (2, 2.5, 3)N NaOH. The characterizations of the prepared SiO 2 NPs were analyzed with atomic force microscopy (AFM), and X-ray fluorescence (XRF). The results were found that the prepared SiO 2 NPs have an amorphous structure with a high purity of 97.13 %. Also, the AFM results indicated that the average diameter of the SiO 2 NPs was 85 nm. It was noted that the leaching processes and pretreatment methods determine the structure, particle size, and quality of the synthesized SiO 2 NPs.


Introduction
High purity nanosilica has unique properties that make it widely used in many applications as photovoltaic devices, electronic devices, solar energy conversion, electric devices, high capacity anodes, and biomedical applications [1][2][3]. Silica exists in the crust of the earth as a crystalline structure or amorphous structure. Corn cob is obtained as a by-product from the process of milling for cobs [3]. In corn cobs, silica considers the main inorganic component [4]. Per year, the corn world harvest is almost 600 million tons. 20% of corn is considered as grains, through the combustion of the cobs, the cobs are converted to ash. Usually, corn cob and the ash of corn cob remained as unutilized industrial waste. Using corn cobs as cheap resource material to obtain significant material like nano-silica produced a big economic benefit because of reducing the cost and utilizing from the wastes. The CCA preparation process was studied with many important investigations [5]. The nature of silica products (crystallinity and microstructure) is highly related to their application because the low reactivity of crystalline silica is limited to its direct applications [6]. Generally, the extracted Silica from CCA has an amorphous structure and also has a high surface area. However, amorphous silica has many chemical applications, such as thermal insulators, catalysts, and absorbents as a result of its proprieties of the high specific surface area [7][8][9]. Various synthetic methods of extracting nano silica from agriculture waste were used like chemical precipitation, ion exchange, solvent extraction, and electrolysis deposition method. Yafei Shen [10] produced nano silica from RHA by chemical method. they found the preparation conditions, time, temperature of combustion have controlled the nature of the produced nano-silica thus the silica with amorphous structure was obtained at 550-800 °C, and the silica with crystalline structure forms at higher temperatures than this. Nittaya thuadaij et al. [11], using the precipitation method to obtain amorphous silica with a specific surface area reaching 656 m2g-1. Carmona et al. [12], extracted amorphous and white silica with both micro and nanometric particles from rice husk using mild acid solutions. They conducted that the prepared nano-silica was prepared by a precipitation process which was extracted from Iraq agriculture sources. Therefore, the main objective of the present investigation is to produce high purity nanosilica from agricultural wastes of corn cob by developed chemical extraction techniques. Many factors are effected on nano-silica purity such as the type of raw material, precipitation temperature, pretreatment steps, and the final purification process. All these parameters determine the quality and quantitative of produced nano-silica.

Preparation corn cob ash (CCA)
For the preparation of corn cob, the first step includes washing the corn cobs by water (three times) for removing the dust and contaminations soluble after that dried for 24 hours at room temperature. The pretreatment process was carried out by soaking 30 g of the dried CC in one liter of 3N HCl solution at 75 °C for 5 hours. The mixture was then subjected to refluxing in the same acid at room temperature for 12 hours. CC was then washed with deionized water and filtered three times to make it free-acid after that dried inside the oven for temperature reaching for 100 °C at 5 hours. The corn cob (CC) pre-treatment produced was burned inside a muffle furnace for 700°C, at the rate of 10 °C/min for 4 hours. The obtained corn cob ash (CCA) becomes almost white color. The residue solution was carefully washed with (100 mL) of boiling water, as shown in Fig. 2. Then sulphuric acid (5N H 2 SO 4 ) was added to precipitate the silica gel as a white gelatinous solid by a gelation process.

Na 2 SiO 3 + H 2 SO 4 → SiO 2 + Na 2 SO 4 + H 2 O (2)
A residue soft gel was aged for 4 hrs, after aging, the slurry was washed and filtration via using the vacuum pump.
NH 4 OH was added to make the pH = 8. Then, it left at room temperature for 4hrs. The product was washed and filtered through whatman (No. 41 ash less) than added 20 ml of warm deionized water many times to become freealkali and dry at 90 °C for 10 hours in the oven. An produced powder was refluxed with 6N HCl for 4 hours and washed with deionized water, after washing, 2.5N NaOH will added and stirred by magnetic stirrer then heated for 70 °C at 3 hours inside covered beaker with capacity (250 ml ) then 5N H 2 SO 4 was added to obtain white nanoparticles with high purity of (SiO 2 ) after washing and filtering by whatman (No. 41 ash less).

Fig. 2.
The high purity nanosilica preparation flow chart by precipitation method.
SiO 2 NPs precipitated was washing with warm deionized water until becomes completely alkali free then filtered through whatman (No. 41 ash less) filter paper and then the produced were collected then dried for 110°C at 12 hours inside an oven. The final product is SiO 2 NPs by precipitation method, as shown in the following sequence steps Fig. 3.   Fig. 3. (a) Corn cobs, (b) cutting and washing for corn cob (CC), (c) corn cob ash (CC) after the treatment with HCl and drying, (d) corn cob ash (CCA) after the calcination process, (e) sodium silicate gel after extraction process, (f) nano-silica.

XRF analysis
The purity of prepared SiO 2 NPs from CC was analyzed by (XRF) X-ray fluorescence. The Table 1 shows a chemical analysis for the prepared silica at (2, 2.5, 3)N NaOH. respectively. This result is an agreement with researches [14][15][16]. The relationship between the degree of purity for extraction SiO 2 NPs and NaOH concentration is show in Fig. 4. It indicates that within an increase in NaOH concentration, the purity increases.

Atomic force microscopy (AFM) and particle Size of Nanosilica
According to AFM analysis, the diameters of SiO 2 NPs extracted at 2N NaOH with range of (21-125 nm) and the average diameter of 77.20 nm. The AFM images of SiO 2 NPs obtained at 2.5N NaOH display keeping the spherical shape of particles with the increase in the size of particles in which the range of particle diameters is 75-100 nm and the average diameter is 79.76 nm as shown in Fig. 6. The 3D surface image of SiO 2 NPs prepared at 3N NaOH appears the surface of SiO 2 NPs has the shape of the hills and valleys. Fig. 6c shows the diameters of prepared nano-silica are in the range of 70 -165 nm and the average diameter equal to 88.93 nm. In general, all these results reveal that the increasing the concentration of NaOH leads to rise in size of the particles. In the other hand, the shape of particles can be changed from spherical which was obtained at NaOH concentration led from 2 -2.5 to crystallike structures at 3N NaOH concentration.

Conclusions
An amorphous SiO 2 NPs was successfully prepared from corn cob by chemically modified precipitation technique. The chemical treatment processes were the main affective parameters for producing high purity nanosilica (97.416%). The current study investigated to find out the best suitable treatment for the extraction of amorphous silica nanoparticles from CC at 3N NaOH solution. The main particle size of the prepared SiO 2 NPs was 40-89 nm. From an engineering point of view, the modified production method is economic, flexible noncomplicated, and produced high-quality amorphous nano-silica. Then, the process could be used for production on an industrial scale.