The rice grain, commonly called a seed, consists of the true fruit or brown rice (caryopsis) and the hull, also known as the husk, which encloses the brown rice. The husks are separated from the husked rice through aspiration, making them a byproduct of the rice milling process. The husks are about one-fifth by weight and contain about 20-30% silica, the rest being organic  lignin and cellulose (Chaudhary et al., 2004) of about 70-80% of dry hull (Mohamad, 2007). Other studies report that the husk roughly contains 35% cellulose, 35% hemicellulose, 20% lignin and 10% ash (which is 94% silica) by dry weight basis (Prachayawarakorn and Yaembunying, 2005), with the chemical composition dependent of the different  geographical  conditions  and  location,  type of paddy, climatic variation, soil chemistry and fertilizers used in the paddy growth (Ugheoke and Mamat, 2012). Earlier chemical characterization studies for instance has reported that the husks consists of 66.67% carbon, 22.3% SiO₂, 7.1% H₂O, 0.82% Al₂O₃, 0.78% Fe₂O₃, 1.10% K₂O, 0.78% Na₂O, 0.24% CaO and 0.21 mass% MgO (Chandrasekhar et al., 2003; Babel et al., 2003; Liou, 2004). Therefore, as a result of much contents of cellulose, rice husk is also often considered as cellulosic fiber. The cellulosic fiber tends to degrade at about 200°C and quickly becomes friable with loss of water (Shanks et al., 2004). Besides, these natural fibers have presence of large amounts of hydroxyl groups, which makes the properties of rice husk very much in category of hydrophilic (Mohamad, 2007).

The husk is considered a waste with negligible commercial value, from the agricultural sector. This is due to its high resistance to decomposition in the ground, its low nutritional value to animals and also its being difficult to digest. Rice husks also have low calorific value (3585 kcal kg^–1) and high ash content when burnt (Chuah et al., 2005; Ameri et al., 2019). The ash contains nearly 95% silica and is an important renewable source of silica. In addition, the ash contains potassium, sodium, magnesium, calcium, iron, phosphorus and much smaller quantities of other elements (Bakar et al., 2016), the composition of which again depends on the aforementioned factors like geographical location. Burning is a cheap method of extracting the silica from rice husks for possible commercial use. It has been used as a raw material in the production of rice husk ash by the process of calcination (Kenechi et al., 2016).

Calcination of rice husk ash can give rise to two types of ash depending on the type of combustion; white rice husk ash (WRHA) for complete combustion and carbonized rice husk ash (CRHA) for incomplete combustion. The two types have different reactivity. The reactivity of rice husk ash is based on the properties of amorphous silica and the porous structure of ash (Ramezanianpour et al., 2009). Amorphous silica is obtained between temperatures of 500-700°C and crystallization occurs at temperatures above 700°C (Kang et al., 2019). Amorphous silica has a large specific area and the chaotic formation of the structure is open with holes in the network where electrical neutrality is not satisfied. Crystalline silica structure is formed by the repetition of a basic unit. The structure of crystalline silica reduces the surface area of RHA; thus, reduces its reactivity (Leong, 2015).

Once obtained, RHA and silica from RHA has been used and studied for various applications that includes adsorbents for adsorption of dyes, pigments (Lawagon and Amon, 2019; Shukla, 2020) and heavy metal ions (Maingi et al., 2019) from aqueous solutions; catalytic support and catalyst (Ikhlaq et al., 2019); for manufacturing solar cells for photovoltaic power generation and semiconductors (Zamani et al.,  2019);  in the cement industries as a pozzolone component (Sonat and Unluer, 2019) and fertilizer industries (Sekifuji et al., 2019); in synthesis of advanced materials such as silicon tetrachloride, magnesium silicide, sodium silicate and zeolite (as reported by Genieva et al., 2008); as fillers in rubber (Xue et al., 2019) and plastic (Almirón et al., 2019) composites, due to their low densities, very low cost, non-abrasiveness, high filling levels, recyclability, biodegradability and renewable nature among many others. The aim of the present study is to compare the properties of RHA prepared under controlled conditions that is FRHA with RHA obtained through uncontrolled burning ORHA. The cost and method of obtaining RHA should be as low as possible, to encourage applicability in wide range of applications. The study sought to demonstrate that the properties of the ORHA are not far off different to those of FRHA.

Rice husks to be used were obtained from Nice rice millers dumpsites in Mwea, Kirinyaga County, Kenya.

Incineration of rice husks to get RHA

Rice husks were washed thoroughly with water and dried in an oven at a temperature of 150°C. Calcination of the dried rice husks was done in a muffle furnace (SX2-2-17TP) for 3 h at a temperature of 700°C to produce FRHA (Ong et al., 2019). On the other hand, rice husks were burnt in open air on a hot plate to produce ORHA. The two forms of RHA were then characterized for their composition and properties.

Characterization of RHA

Determination of the composition of RHA

The FRHA and ORHA samples were analyzed in triplicates following the method done by Kamau et al. (1993). For K, Ca, Fe, Mn, Si and Pb analysis, 0.50 g of sample was digested with 9 ml of a mixture of nitric, sulphuric and perchloric acids (3:1:1). The solution was then filtered on ash less filter paper, residue ignited and treated with hydrofluoric acid to expel silica. The digests were topped to the 100 cm³ and analyzed using Atomic Absorption Spectroscopy. In addition, the two types of ash were also characterized using X-ray fluorescence technique for their composition.

Determination of the structural properties of RHA using XRD

This was done using Bruker D2 Phasor diffractometer. Finely powdered RHA was mounted on a sample holder and an X-ray diffractogram obtained. The X-ray tube was operated at 30 mA, 30 KV and a scan speed of 2 per minute was used. This was done following the work of Kamau et al. (1993).

SEM analysis of RHA

The morphological characterization of RHA was carried out using a Zeiss  ultra plus  scanning  electron microscope.  The samples were first crushed into powder and then trapped on a tape mounted on a sample holder and then gold coated thrice prior to electron microscopy to give the necessary conductivity. This followed the modified procedure of Ayswarya et al. (2012).

Fourier transform Infrared spectroscopy (FTIR)

FTIR spectra of the RHA were recorded on an ATR Perkin Elmer A100. Samples in the form of powder were used. This followed the method of He et al. (2013a).

Elemental composition of RHA

Both methods of RHA preparation as described produced ash of similar physical looks, as shown in Figure 1. It was also noted that the two types of ash were grey in color and similar by sight. The elemental composition of the two types of ashes done by AAS also showed a lot of similarities. As expected, both had higher content of silicon metal, which is 81% for ORHA and 79% for FRHA as shown in Table 1. Potassium, Manganese, Silicon and Carbon had a higher percentage in ORHA; whereas iron had a higher percentage in FRHA. However, much difference was not noted in the percentages since they differ by less than 2%. Lead metal was not detected in either of the two types of ash. Silicon in RHA exists in the form of silica (silicon dioxide) (Ariffin, 2004). This is confirmed by XRF analysis that shows the levels of silica as 94% for FRHA and 95% for ORHA (Table 2).

From the XRF analysis, the two samples of ash had a slightly different percentage of silica. FRHA was composed of SiO₂-94.85%, MgO-1.99% and K₂O-1.74%. Other elements whose composition was less than 1% were CaO, P₂O_5, Mn and Fe among others. ORHA was composed of SiO₂-95.45% and K₂O-2.46%; while those whose percentages are less than 1% are P₂O_5, S, Cl, CaO, Ti, Cr, Mn, Fe, Cu, Zn and Rb. From the XRF results, MgO is found to be in the FRHA and not in the ORHA. The oxides of K, P and Ca exist in considerably higher percentages than other elements present since these elements are essential plant nutrients and they occur in higher quantities due to preferential uptake by the plant. The uptake of minerals by plants is closely related to the soil conditions. It is also evident that most elements have a lower percentage composition in FRHA as compared to ORHA except Mn and Fe. The difference in composition is mainly attributed to the soil chemistry of the rice growing area and paddy varieties.

Structural characterization of RHA

From the XRD analysis, the ORHA has one broad peak between 18 to 30 coupled two theta degrees. This meant that the open air RHA was amorphous in nature. On the other hand, muffle calcined RHA had major  sharp  peaks at coupled two theta 21.8 and 36.02° and other minor peaks. This meant that the ash was crystalline in nature and this was in agreement with literature. This is shown in Figure 2a and b. This is in agreement with literature where, rice husks calcined at temperatures lower than 700°C produces amorphous silica, while at higher temperatures or equal to 700°C, crystalline silica is formed (Nair et al., 2008). The amorphous silica can be transformed to quartz, tridymite and cristobalite by heating it at high temperature over 900°C (Deshmukh et al., 2012). Under optimized conditions a 99.9% amorphous silica has been formed (Rafiee et al., 2012) and calcination at 1,000°C has been shown to form highly crystalline silica with sharp peaks assigned to cristobalite and tridymite (Deshmukh et al., 2012). It has been reported also that the amorphous silica in RHA is reactive and may be used as a pozzolana (He et al., 2013b); further strengthening the preposition that RHA may be an effective additive in geopolymers (Venkatanarayanan and Rangaraju, 2015).

FTIR analysis of RHA

In the FTIR analysis of RHA shown in Figure 3a and b, greater similarities were noted in the appearance of the peaks. The broad band between 2200.16 and 3398.87 cm^-1 in the ORHA is due to silanol –OH groups and chemically absorbed water. The band at 1051.24 and 1052.95 cm^-1 in the FRHA and ORHA, respectively, correspond to Si-O-Si stretching modes. The bands between 443.24 and 792.35 cm^-1 correspond to O-Si-O bending vibration. The band at 1603.15 cm^-1 in the ORHA correspond to –OH bending vibration. This band was not detected in the FRHA. These findings are comparable to what is found in other studies, and further shows that the bonds present are dependent on the method of preparation (Singh et al., 2019). Both methods of RHA preparation shows slight difference in peak formation, specifically 3398.87, 2200.16 and 1603.15 cm^-1 bands.

Surface textural characterization of RHA

From SEM analysis of RHA in Figure 4a and b, it was clear that the particles of RHA consists of fine particles, which appears to have agglomerated to large group of particles creating a dense formation. These particles are of irregular shapes and morphology. The agglomeration in ORHA may be due to the presence of hydrogen bonding between silanol groups on the surface of rice husk ash, while that in FRHA may be due to the hydroxyl groups.

The authors have not declared any conflict of interests.