Processing and Characterization of Nano Silica and Iron Oxide Coated Silica Composites Extracted from Rice Hulls

This study examined the effects of adding iron to rice hulls samples (RHs) on the thermal degradation to prepare nano silica and iron oxide-silica mixtures. Iron is precipitated in different amounts within the RH fibers (as hydroxide) by dipping them in a solution of different concentrations of acidic ferrous sulfate and then immersing it in a dilute ammonia solution. The dry RHs were fired at temperatures between 400 °C and 700 °C in a static or limited air atmosphere. Both weight loss and residual weight are determined to follow its thermal degradation. The products of the RH degradation of silica or iron oxide-silica mixtures were characterized by X-ray diffraction and Fourier transform infrared spectroscopy. The surface area, average particle size, and pore diameter of both products were determined. The chemical treatment of RHs before firing accelerates their thermal degradation and leads to obtaining firing products of high purity. Firing RHs in a limited air atmosphere increases thermal degradation and, at the same time, accelerates the catalyst effect of iron on its degradation. Under these conditions, silica and iron oxide bonded silica with an average particle size of 1.31 and 0.07 μm were obtained by firing treated RHs at 600 °C and 500 °C, respectively. Firing in a limited air atmosphere encourages CO2 to react with burned C to form CO, and iron as an accelerator for this reaction completes its degradation. In contrast, iron oxide remained in the ferrous state. Therefore, the bonding between iron and silica was complete.


Introduction
Rice hulls (RHs) are an important agro-industrial byproduct in most of the world and in Egypt, where hulls are separated from rice kernels during the milling operation.Their chemical composition has been widely investigated [1].RHs contain cellulose and lignin and produce on firing a high ash content (13-29 mass%).The ash is largely composed of silica 87%-97% with small amounts of alkali and trace elements [2].The silica obtained from RHs is a hydrated amorphous form [3,4] and similar to silica gel [5].Different methods have been used to extract it from RHs: by dissolution-precipitation technique [6][7][8], changing the rate of heating in oxidizing temperature [9], and acid treatment followed by burning at temperature ˂600 °C [7,10].The extracted silica can be used as a filler in concrete because of its pozzolanic properties [11], filler in a polymer to improve its properties [12], as a catalyst support and adsorbent [13], and as starting material for preparing advanced materials like SiC, Si 3 N 4 , Sialon, Si, and MgSi [14].When the silica extracted from RHs is used in concrete and polymers, the traces of other elements may not be of much significance.However, these elements act as impurities in the preparation of nitrides, carbides, and other inorganics, where RH ash is used as a silica source [1].
RHs are considered a potential raw material for generating energy.In general, there are three methods for producing energy from RHs: a) burning in air where the energy released cannot be stored but must be used as it is produced, and the combustion products may be corrosive or pollutants [15]; b) by burning in the absence of air "pyrolysis" where the volatile matter will boil off leaving a carbon-silica mixture behind and that are recovered after the condensation of volatile matter, oils and tars while the gases are used as a gaseous fuel [16]; c) by burning in a limited supply of air "gasification" where a mixture of gases are produced, consisting principally of CO and H 2 , which are inflammable beside non-inflammable gases as CO 2 and N 2 [15].
The advantage of pyrolysis and gasification against the burning of hulls in excess air is that the two processes produce an easily converting transportable high-grade fuel from RHs.
Some studies reported that iron oxide decreases the activation energy for carbon gasification [17].Therefore, the effect of iron oxide on the thermal degradation of RHs was investigated [18].The addition of iron to RHs was carried out by impregnating RHs in a ferrous sulfate solution and soaking them in an ammonia solution to precipitate iron as iron hydroxide in hull fibers.Thermogravimetric analysis of the treated RHs indicated that the thermal degradation of RHs was completed in two steps, volatilization of volatile matter, and oxidation of carbon.The activation energies for the two steps were 1.042 and 6.457 kcal −1 and 1.198and 7.916 kcal −1 for treated and untreated RHs, respectively [18].The results also showed that iron oxide strongly affects the combustion of carbon of the coked RHs, especially those coked at low temperatures [19].
The current research was carried out as a continuation of that research.Previous research was conducted on a sample containing a specific amount of iron oxide (1.5 mass%).The present research aims to examine the effect of increasing the amount of iron oxide on the thermal degradation of RHs.This was carried out by impregnating the RHs in different concentrations of ferrous sulfate solution, followed by soaking in ammonia solution [19].This study examined the effects of the firing temperature using static air and limited air condition to compare the obtained results from samples fired under different conditions.The loss in weight of burned samples was attributed to volatile matter and carbon oxidation.The physical properties of the iron oxide-silica mixture that remained after burning the RHs were studied because this mixture has been investigated widely as a surface-reactive mixture.
Several studies examined the competitive adsorption of anions and cations onto iron oxide-coated silica.This is similar to what occurred in nature, where iron oxide and silica exist together, bind strongly together, and have adsorption properties.The adsorption properties of an oxide mixture are determined by the relative amount of its components [20].For example, iron oxide present as a coating has a larger specific adsorption capacity for Zn [21], Pb [20], and As [22] than discrete ones.Many studies have shown the importance of these surface coatings in controlling metal distribution in soils and sediments and for their potential application as effective sorbents.These results can be applied to the removal of a variety of elements from industrial wastewater.Similar samples are commonly used to remove As [22] and Ni [23], as listed in Table 1.
The surface hydroxyl groups of silica are involved in the coating process, providing a strong bonding between the coated iron oxide and silica surface [25].The formation of a silica coating on the surface of iron oxide (silica shell) could help prevent aggregation in liquid, improve its chemical stability, and become more suitable for use in column adsorption in water and wastewater treatment.The silica coating with iron oxide particles resulted in a decrease in the particle size of the formed iron oxide and therefore increased its surface reactivity, which is probably due to the expected effect of the silica coating preventing nanoparticle agglomeration [26].
The silica shell in various complex nanostructures can also be used in medical applications as MR contrast agents, biosensors, DNA capturing, bioseparation, and enzyme immobilization [27].
The current research is based on using RHs, a waste material, as a source of silica, which reduced product cost.Furthermore, silica exists as amorphous form, which increases its ability to react with iron oxide.The other researchers depend mainly on using chemicals as starting materials in preparing their iron oxide-coated silica composites, as listed in Table 1.
A reference sample was prepared by impregnating RHs in dilute H 2 SO 4 acid, followed by soaking in an ammonia solution and firing under the same conditions.The obtained silica could also be used as pure active silica.

Arsenic
Adsorption method Fe(NO) 3 reacts with NaOH to form goethite.The formed oxide was then mixed with silica suspension [23] Nickel Precipitation method Fe(NO3) 3 particles are dissolved in a silica suspension.Then, NaOH solution was added to the system [23] Nickel Modified Sol-gel method Fe 2 O 3 particles are prepared by electrochemical method, and silica is prepared from hydrolysis and condensation of TEOS in ethanol-water mixture.Silica-coated iron oxide composite is formed from both oxides by a sol-gel method [24] Toxic heavy metals

Materials and Methods
A sample of RHs was purchased from the local market in Egypt.The RHs sample was washed off dust with water and dried at room temperature for one day.Subsequently, its components of carbon and volatile matter were determined by firing at 1000 °C for one hour.While the weight loss (88%) represents the amount of the two components, the residue that remained after firing was silica and some metal oxides (12%).Hence, this sample of selected RHs contains a relatively small amount of silica because the selected sample is a mixture of straw and leaves of rice plants crushed together to use for other purposes.
The first objective of the research was to prepare pure silica with a high surface area from the RHs samples.The ash remaining after firing the hulls also contains a small percentage of metal oxides.These oxides affect the purity and color, and the sintering of silica decreases its surface area.Therefore, it is necessary to remove a large amount of these oxides by dipping RHs in a dilute solution of sulfuric acid (10 wt.%) for a specified period (1 hr.), Fig. 1.
After extracting it from an acidic solution, it is placed in a dilute ammonia solution (10 wt.%) for the same period.This step has three goals: neutralize the acidic solution after filtration, help dissolve some impurities, and improve the purity of the prepared silica.Furthermore, acidic and basic solutions are used in the second part of the research when iron is precipitated inside RHs.
The second objective of this research is related to the effect of iron on the thermal degradation of RHs.Therefore, equal amounts of the dried hulls were dipped in acidic iron sulfate solutions of different concentrations (0, 1, 2.5, and 5.0 wt.%) for 1 hr.After draining the solution, the hulls were soaked in an ammonia solution (10 wt.%) for 1 hr. to precipitate iron as iron hydroxide inside the swelled hulls fibers.After washing the RHs with water and drying them for one day in air, the RHs samples were fired at 400 °C, 500 °C, 600 °C, and 700 °C for 1 hr. in different firing conditions.Equal weights of treated of treated and untreated RHs were used.Its quantity is about two thirds of the volume of the porcelain crucible.
The samples placed in crucibles without cover and fired were considered as fired in static air, and the other samples fired in crucibles with loose covers were considered to be fired in limited air.Both crucibles were fired at 400, 500, 600, and 700 °C for 1 hr.The difference in the weight of the samples before and after firing represents carbon and volatile matter, while the remainder contained silica and unburned substances.
The remainder after firing in the first part represented silica while, the remainder after firing in the second part, represents iron oxide silica.

Characterization of Prepared Samples
The nature of bonding between the two phases was followed under the different firing conditions by Fourier transform infrared (FT-IR) spectroscopy.The potassium bromide KBr disk technique was used to obtain the IR spectra of the fired RHs samples.The sample (2 mg) was mixed with spectroscopically pure KB powder (200 mg).The mixture was then pressed under a vacuum into a transparent window for IR measurements.The spectra were obtained in the range of 4000-400 cm −1 using a Beckmann recording double-beam IR spectrophotometer.
The surface area and particle size distribution of the samples was characterized by Brunauer-Emmett-Teller method (BET) using BEL JAPAN, INC equipment.In this test 0.1 g sample was heated at 300 °C for 3 h under 10-2 vacuum pressure before measurement.
A JEOL JEM-2100 transmission electron microscopy (TEM) instrument, fitted with a high angular dark field detector, and an energy dispersive X-ray spectroscopy device were used to analyze the degree of crystallinity and particle size.

Extraction of Pure Silica
An attempt was made to extract pure silica from RHs.This was carried out by firing two RHs samples: as-received and after dipping in dilute H 2 SO 4 acid and then in ammonia solution.Both hull samples were fired under different conditions in static and limited air atmospheres.
Figures 2 and 3 (curves a and b) show the residual ash that remained after firing the two samples under different conditions.The treatment process increased the thermal degradation of hull samples, where the dipping in acid and then in an ammonia solution leached some metallic ions and opened the RHs cellulose structure facilitating the thermal degradation of hulls.The thermal degradation increased with increasing temperature but varied with different firing conditions.It is more evident in the fired hulls samples when the temperature is raised from 400 °C to 500 °C, where the increase in thermal degradation of the hull samples in limited air was higher than in the sample fired in static air.After increasing the temperature from 400 to 500 °C, the ash in the two samples fired under static and limited air atmospheres changed from 38, 37.5 mass %, and 24.2, 22.5 mass % to 27.5, 25 mass %, and 22.5, 21 mass %, respectively.This means that the relatively reduced atmosphere is suitable for a reaction between carbon dioxide and burning carbon to form carbon monoxide, which increases the rate of carbon volatilization.
The rate of thermal degradation of RHs increases in the two samples fired under different conditions when the temperature was increased to 600 °C.However, the effect of the reduced atmosphere on the thermal degradation of RHs was greater than that fired at the lower temperatures Residual Ash (mass%) FeSO 4 % fired at 500 0 C for 1hr fired at 500 0 C for 3hr Fig. 2 Relationship between the mass of residual ash and temperature for RHs fired in static air (400 °C-500 °C), where the remaining in the two samples decreased from 27.5, 25.0, and 22.5, 21.0 mass% to 11.0, 11.5, and 11.5, 10.2 respectively.This means that the pure silica can be obtained by firing the treated hulls at 600 °C for 1 hr. in limited air, where its thermal degradation was completed at this temperature.

Properties of Silica
Figure 4 presents the XRD pattern of pure silica prepared by firing treated RHs at 600 °C for 1 hr. in limited air conditions.The diffused peak with a maximum at approximately 22° 2θ is characteristic of amorphous silica.The XRD patterns of raw RHs, burned RHs, and white ash samples showed diffused peaks with maxima varied from 23° 2θ for raw RHs to 21.8° 2θ for white ash [28].
The physical properties of the prepared silica are shown in Table 2 and Fig. 5.These properties could be compared with those of similar silica samples prepared by the firing process, it is better to compare with the properties of the last sample in Table 3 ref.[7].The two silica samples underwent a similar treatment with a sulfuric acid solution and were fired at 600 °C.
The present silica sample is characterized by a smaller particle size (average pore size 1.31 μm) and confined to a narrow distribution range of 0.3 to 10 μm compared to 0.03 to 100 μm, respectively.On the other hand, the silica sample prepared by ref. [7] had a smaller pore diameter of 0.005 μm, which is less than the third of that prepared in the present research (0.014 μm).
The two prepared silica samples showed different specific surface areas: 60.40 m 2 /g for the present silica and 282 m 2 /g for the silica sample in ref. [7].This difference was attributed to boiling the RHs in a 10% (v/v) H 2 SO 4 acid solution for 2 hrs [7].Thus silica sample also showed different specific surface areas (63, 194, and 321 m 2 /g) according to the change in prefiring treatment with none, water, and hydrochloric acid.In general, the difference in the properties between the silica samples in the two investigations was also This simple method of extracting silica from RHs has some advantages: -The properties of silica obtained from the hulls depend on the method used.Previous studies showed that amorphous silica prepared by physical combustion with a controlled temperature contained only the Si(OSi) tetrahedral unit and is the most reactive silica source compared to the other silica samples prepared by chemical extraction [1].
-Some authors used combustion processes to extract amorphous silica from RHs, but they differed in their firing conditions (Table 3), such as the temperature, time, heating rate, and fire atmosphere.Most research stated that 600 °C is the lowest temperature to obtain carbon-free silica from RHs.This is similar to what was found in the current research.Above this temperature, the surface area of silica decreased, and the sintering of RHs ash began to increase [29].
-Treatment with various chemicals was carried out to obtain amorphous silica with high purity.The chemicals included HCl, H 2 SO 4 , HNO 3 , NaOH, and NH 4 OH.Chemical treatment before combustion was more advantageous [1], where the formation of black particles in the silica from untreated hulls was higher than that from acid-treated hulls.Potassium was shown to cause this phenomenon, which was removed by the acid treatment [19].In the present research, the RHs were first immersed in an acid solution and then in an alkaline solution before firing.
-In addition to preparing pure and active silica at a relatively low temperature (600 °C), the benefit of RHs can be  • Alkali extraction followed by acid precipitation [7] -Average particle size: 25 nm -Specific surface area(SSA): 274 m 2 g −1 -Average pore diameter: 1.46 nm • Non-isothermal decomposition with an oxidizing atmosphere at temperatures between 300 and 1000 K (with a heating rate of 5 K/min) [9] -SSA: 235 m 2 /g -Average particle size: 60 nm -Average pore diameter: 5.4 nm • Mild acid chemical treatment followed by burning at less than 600 °C [10] -Nano and microparticles with large particle size distribution (ranged from 182.6 to 265.6 nm) • Precipitation method (alkaline extraction followed by acid precipitation) [6] -SSA: 650 m 2 g −1 -Particle size: 50 nm (agglomerated) • Dissolution-precipitation technique (Silica obtained using a sodium silicate route followed by an acid precipitation technique) [8] -SSA: 634 m 2 g −1 -Particle size: 5-30 nm -Pore diameter: 3-9 nm -Pore volume: 0.811 cm 3 /g • HCl, H 2 SO 4 , or NaOH pre and post-treatment, followed by heating at 600 °C [7] -Particle size from 0.03 to 100 μm -SSA: 321 m 2 g −1 -Pore diameter: 0.0045 μm increased by exploiting it in energy production.Firing RHs in a reduced atmosphere at 700 °C for a sufficient heating time resulted in the removal of most volatiles.According to Boucher et al. [30], the volatile matter can be considered 30.2% gas and 69.8% vapor, leaving 52% water and 48% oil in the liquid product.Therefore, the total heat value of oil and gas from 100 kg green RHs was calculated to be 613 kJ.

Effect of iron Oxide Addition
The second part of the research was concerned with studying the effects of adding iron oxide in different amounts to RHs samples on its thermal degradation.This was achieved by impregnating an equal amount of RHs in an acidic solution of different iron sulfate concentrations.An ammonia solution with a fixed concentration precipitated iron in hull fibers as iron hydroxide.After drying, the hull samples were fired at 400, 500, 600, and 700 °C for 1 hr. in limited and static air atmospheres.Figure 2 (curves c, d, and e) shows the change in the mass of both volatile matter and residual ash with temperature for RHs samples fired in static air.The increase in the iron oxide content causes an increase in the mass of volatile matter; hence, the amount of residual ash decreases.Its effect is strong with increasing temperature from 400 °C to 600 °C and decreases by increasing the temperature to 700 °C. Figure 3 (curves c,  d, and e) shows the change in the mass of both volatile matter and residual ash with temperature for the RHs samples fired in limited air.The mass of volatile matter was increased by increasing the temperature from 400 °C to 600 °C in the same way as that taking place with RHs samples fired in static air.The mass of residual ash is affected by the temperature and iron oxide content in an opposite manner to volatile matter.The increase in temperature from 600 °C to 700 °C causes a lesser effect on the thermal degradation of RHs.
The two figures show the following: -When the RHs samples are fired at the lowest used temperature (400 °C), the effect of firing on the thermal degradation of RHs in static air is greater than that in limited air conditions.The amount of residual ash reached 17-19 mass% compared with 32-35 mass% depending on the iron oxide content.-When the temperature was increased to 500 °C, the thermal degradation of RHs increased and became relatively rapid and detectable at this temperature.The amount of residual ash after firing the two samples under static air and limited air conditions was 14-17.5 mass% and 21-24.5 mass%, respectively.This means there is a relatively larger increase in the thermal degradation of RHs when firing them under limited air conditions.This occurred even though the effect of iron content appears to be larger in the sample fired at 400 °C under static air conditions.
-The effect of iron oxide on the thermal degradation of hulls was partially reduced by increasing the temperature to 600 °C.This was observed in the hull samples containing relatively high amounts of iron oxide and fired under the two different firing conditions.Increasing the temperature to 700 °C showed that iron oxide does not affect the thermal degradation of hulls where it is almost completed.-By increasing the firing time of the RHs samples at 500 °C from one to 3 hrs, the amount of residual ash after firing the sample in limited air decreased significantly compared to the other sample fired in static air.The amount of residual ash reached 11-13 mass% after firing the sample in limited air compared to 11.5-13.5 mass% for that fired in static air, Figs. 2 and 3.This means that a silica-iron oxide mixture can be obtained carbon-free by firing RHs containing iron oxide at 500 °C for 3 hrs.in limited air conditions.
The following experiment was also conducted on RHs samples fired under limited air conditions for 1 hr.at 400 °C, 500 °C, 600 °C, or 700 °C by refiring them in static air at the same firing temperature for 1 hr.(Fig. 6).The effect of iron oxide on the thermal degradation of hulls depends on the used firing temperature of RHs, where the amount of iron oxide accelerates the thermal degradation of hulls fired at 400 °C.On the other hand, the opposite result was obtained from hulls fired at 500 °C and above, where the thermal degradation of hulls decreases with increasing iron oxide content.This was attributed to the effect of iron oxide on decreasing the crystallinity of carbon and silica; hence, their fixation increases [19].Fig. 6 Relationship between the mass of residual ash and the temperature for RHs samples fired in limited and then in static air at the same temperature The same results were obtained when a hull sample was coked (in the absence of air) at 900 °C and then fired in static air at a relatively low temperature (350 °C).The increase in the coking temperature favors the fixation of carbon in residual ash [19].Therefore, the change in firing condition from limited air to static is not useful when firing in limited air at relatively high temperatures.

Spectroscopic Studies and Particle Size Distribution
Four samples were prepared by the usual method and fired at 500 °C for 3 hrs.: two in static air and two in limited air.One of the samples contained a higher amount of iron oxide and the other a lower amount.
The XRD pattern of samples showed pure silica as a diffused peak with a range from 20 to 30° 2θ characteristic of amorphous silica Fig. 7.The iron oxide remained from formation of iron oxide coated silica was not detected, it may be present in the form of amorphous iron oxide.
The TEM images of the two samples fired at 500 °C in static air and limited air are shown in Fig. 8.Both samples reveal agglomeration of iron oxide particles but with well distribution in the sample fired in limited air.Its iron oxide particles are smaller in size than that fired in static air.This confirmed the results obtained from particle size determination.On the other hand, silica appears to be formed in amorphous state.
The nature of bonding present in the formed iron oxidesilica was studied using FT-IR spectroscopy (Figs. 9 and 10).
The strongest IR absorption bands arose from SiO bonds, three main absorption bands at approximately 762, 802, and 1092 with a shoulder at 1382 cm −1 .These bands were assigned to bending, symmetric, and asymmetric stretching vibrations of silica [31,32].Furthermore, the energy absorbed at 882 cm −1 was also characteristic of the Si-O stretching vibration [33].The two bands at 3426 and 1631 cm −1 in the spectra of the two samples were attributed to the stretching modes and bending vibration of free or adsorbed water, respectively [34].
The absorption at 2972 cm −1 is associated with C-H of the vinyl group [34], while the band at 2926 cm −1 was assigned to the C-H stretching vibration of the ethyl group.The presences of these bands were attributed to incomplete hydrolysis of formed silica [33].However, these two bands were less intense than those in the two samples fired in a limited amount of air (Figs.7 and 8).
Unlike Fig. 9, the IR absorption band at 1092 cm −1 split into two peaks at 1089 and 1052 cm −1 due to the occurrence of an asymmetric vibration band of siloxane (Si-O-Si) in Fig. 8, which confirmed the binding of the silica particles to iron oxide [33].Furthermore, the band at 580 cm −1 indicates the presence of Si-O-Fe [26].Those appear only in the FT-IR spectra of the samples fired at 500 °C for 3 hrs.in a limited air Fig. 7 X-ray diffraction pattern of iron oxide coated silica composite atmosphere which proves that an Fe-O-Si bond formed.This means that a Si-O-Fe bond is formed under this firing condition, which is in contradiction to the results obtained in Fig. 9.
Absorption bands of α-Fe 2 O 3 at 800 and 1020 cm −1 were not visible in both figures because they overlapped with the dominant bands of SiO.

Particle Size Distribution of the Fired Samples
Two samples were prepared as usual and fired at 500 °C for 3 hrs.: one in static air and the other in limited air.The particle size distribution of the iron oxide-silica mixture was determined.As shown in Fig. 11, the results showed a significant difference in particle size and size distribution between the two investigated samples.
The mean particle diameter of the samples fired in static air, and limited air were 1212.5 and 689.9 nm, respectively.This means that the sample fired in limited air has approximately half the mean particle diameter of that fired in static air.The particle size distribution of these samples was narrower than in the sample fired in static air.The two samples had standard deviations of 287 and 970 nm representing 41.6% and 80.5%, respectively.
The study on the coating of silica with iron oxide showed that the surface hydroxyls of silica were involved in the coating process providing strong bonding between the iron oxide and silica surface.Silica can be bonded covalently to the iron oxide surface by replacing the OH group, as indicated by FT-IR spectroscopy [25].
The presence of dissolved silica affects the formation of goethite FeO(OH) crystals, which have a smaller size and higher surface area than those formed in the absence of silica [25].This means that the bonding between silica and iron oxide decreases the crystal size of the formed goethite.This is what occurred in the sample fired at 500 °C for 3 hrs.in limited air where its mean particle diameter decreases.This is another conformation besides the FT-IR that iron oxidesilica bonding was formed in this sample.Limited air S tatic air

Mechanism
With this method, the iron precipitates in the form of iron hydroxide in the swelled cellulose structure of RHs.With increasing temperature to 700 °C, the iron hydroxide decomposes to iron oxide according to the following equation: At the same time, thermal degradation of the hulls takes place.As a result of this, carbon, hydrogen and hydrocarbons were produced.The following reactions may have occurred between these products and ferric oxide at temperatures below 600 °C [18].These reactions increase the amount of volatile matter and reduce the amount of residual ash.These reactions take ) place more clearly if the burning conditions are more reducing.This is clearly shown in the hull samples fired at temperatures less than 600 °C in limited air, where the thermal degradation of hulls is greater in these samples.As mentioned earlier, the reaction of CO 2 with burning carbon to form CO in a reduced atmosphere plays a vital role in increasing the thermal degradation of RHs.This increases more clearly in the presence of iron because it acts as a catalyst for this reaction.Two mechanisms were suggested for the catalysis of the C-CO 2 reaction by iron to give carbon monoxide.The first is that the vaporization of carbon (by desorption of CO from the surface of carbon) would be enhanced if electrons were transferred from carbon to a catalyst.Iron can catalyze the oxidation of graphite by accepting its electrons due to the nonstoichiometric character of its oxide.The second mechanism is that CO 2 dissociates over iron to give CO and adsorbed O atoms.These O atoms react with carbon to produce gaseous CO [35].
When the hull sample was fired in limited air, the CO 2 -CO mixture produced from firing their inorganic part protected the FeO formed from the mentioned reactions from its oxidation to ferric oxide where the hull sample was placed in this atmosphere from the start of heat treatment.In addition, there was no further reduction of FeO to Fe, even when the temperature was increased to a relatively high degree, because the existence of silica hindered the carbo-thermal reduction of FeO to Fe and facilitated the reaction to Fe-Si-O phases [25,36].Furthermore, amorphous silica of high purity and good reactivity was produced under these firing conditions.In the presence of this silica and ferrous oxide, the Si-O-Fe bond was formed, as indicated by the FT-IR spectra and particle size distribution.This bond formation helps increase the degradation of hulls and facilitates the oxidation of carbon, thereby increasing the volatile matter and reducing residual ash.
Therefore, through obtained results that it was possible to prepare pure iron oxide bonded silica at a relatively low temperature, 500 °C.In the same way, the iron oxide-coated silica with a specific composition can be prepared to meet the required adsorption characteristics.
Many methods were recently developed to synthesize different kinds of iron-coating silica phases.This was attributed to the high adsorption capacity of anions and cations on the iron silica surface.These methods depend on synthesizing iron oxide-coated silica using only chemicals (Table 1), whereas the current research employed RHs as a by-product that reduces the process cost.Iron is precipitated in their RHs swelled fibers, where amorphous silica is present.This accelerates the bonding between the two oxides in a relatively reduced firing atmosphere.
Furthermore, the energy produced by burning CO and H 2 gases resulting from the thermal degradation of RHs is enough to be used in the firing process.The present research also presented pure amorphous silica that can be used as an adsorbent and catalyst support beside the other uses mentioned above.Therefore, the present method is an inexpensive way to use RHs to prepare materials of great use.

Conclusions
-Treating RHs with acid followed by an ammonia solution before the firing process increases its thermal degradation and the purity of the fired products from the silica and iron oxide-silica mixture.-Firing RHs in limited air has many advantages over static air as it increases the thermal degradation of RHs and accelerates the effects of iron in this direction.-The effect of iron on the thermal degradation of RHs increases with increasing content and firing temperature to 550 °C, after which it decreases until it almost, does not affect the samples fired at 700 °C.-An attempt to change the firing condition from limited air to static air is not useful for the thermal degradation of RHs when firing in a limited air atmosphere at ≥500 °C.-Amorphous, pure, and more reactive silica is obtained by firing treated RHs at 600 °C for 1 hr.The iron oxide mixture remaining from firing treated RHs at 500 °C for 3 hrs in a limited air atmosphere has a small particle size and a narrow distribution compared to that obtained under static air conditions.Moreover, FT-IR spectroscopy confirmed Fe-O-Si bond formation in RHs fired in limited air atmospheres.

Fig. 1
Fig. 1 Flow-chart of the preparation of samples

Fig. 3 2
Fig.3Relationship between the mass of residual ash and temperature for the RHs samples fired in limited air

Fig. 5
Fig. 5 Particle size distribution of the prepared silica

Fig. 9 1 )Fig. 10 FT
Fig.9FT-IR absorption spectra of the two samples containing different amounts of iron oxide and fired in static air for 3 hrs.at 500 °C

Fig. 11
Fig. 11 Particle size distribution of the samples fired at 500 °C (a)in limited air, (b)in static air

Table 1
Some published results on the preparation and uses of iron oxide-coated silica composites

Table 3
Some research on silica extraction from RHs