## 3 Rice Industry By-Products
### Properties of Rice Husk and Rice Husk Ash  
Rice husk is generated as a by-product of the rice milling process and it is the outer cover of the rice grain. It is also known as hull and chaff [44]. This is the most common agricultural by-product in agrarian countries. In general, rice husk is utilized as source biomass fuel in rice mills, silica-rich cementitious material, and poultry farming [43; 45; 46]. In addition, a minor amount is used as fertilizer and building material [47]. Due to its low economic value, the availability in huge quantities, higher chemical stability, mechanical strength, and insolubility in water, rice husk is considered as a sustainable absorbent material [48; 49; 50; 51]. However, higher quantities of rice husks are frequently terminated in landfills or open burning, creating huge environmental pollution. The composition and quantity of rice husk depends on several environmental factors such as area, soil type and characteristics of soil, water availability, harvesting and processing technology, and behavior of the market [52; 53; 54]. However, Carbon and Silica are considered as the major components of rice husk for the adsorption process [55]: this is confirmed also by the Energy Dispersive X-Ray Analysis (EDX) performed by the authors and reported in Table 1, where it is evident that Si is the 3 rd element after C and O in wt %. Table 2 represents the organic chemical constituents of rice husk, while the chemical analysis of the rice husk ash obtained after pyrolysis of the husk is represented in Table 3. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{3}{c}{**Elemental Composition (Average wt \%)**} \\ \hline Carbon & 28.22 \\ Oxygen & 53.53 \\ Silicon & 17.00 \\ Potassium & 0.60 \\ Magnesium & 0.05 \\ Calcium & 0.45 \\ Sulphur & 0.11 \\ Chlorine & 0.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Elemental composition of Carnaroli rice husk as determined by Energy dispersive spectroscopy (EDX; X-max 50 mm\({}^{2}\) probe by Oxford Instrument (Abingdon, UK) connected to an EVO MA10 scanning electron microscope by Carl Zeiss (Oberkochen, Germany) performed on a sample after drying at 45 \({}^{\circ}\)C for 6 h in an oven (own source). Figure 3: Conceptual model on the rice waste generated all along the food chain adopted from [42; 43]. Understanding the chemical structure and functional groups present in rice husk is highly important for the adsorption process. Fourier transform infrared (FTIR) spectroscopy and Attenuated Total Reflection (ATR) are commonly used to determine the structure of the backbone carbon chain and whether the backbone consists of linear or branched chains. It also provides information on the characteristics of the functional groups that are bonded with the backbone carbon chain, which aid in the adsorption process for the removal of different pollutants from drinking water. To get a better understanding of the functional groups present in the rice husk surface, the ATR-FTIR spectrum of a real sample is shown in Figure 4. The absorption band at 3293 cm\({}^{-1}\) indicates the stretching vibration of free and hydrogen-bonded hydroxyl groups. The small two peaks at 2918 cm\({}^{-1}\) and 2850 cm\({}^{-1}\) are a signature of the C-H groups of long-chain aliphatic components (alkanes and alkenes structures). Some very weak signals at 2000-2500 cm\({}^{-1}\) can be attributable to C\(\equiv\)C or C\(\equiv\)N bonds or accumulated -C=C=C- and -N=C=O bonds. The band at 1654 cm\({}^{-1}\) is attributable to C=O presence. Finally, the peaks at 1027 cm\({}^{-1}\) can be due to C-O and C--H bonds' vibrations and the bands at 787 cm\({}^{-1}\) and around 500 cm\({}^{-1}\) to Si-O bonds deformations respectively [62]. The right and left shoulders of the peak centered at 1027 cm\({}^{-1}\) can be due to the valence vibrations of C-O-Si or Si-CH\({}_{3}\) bonds [63].