## 4 Fluoride Removal
### Rice Straw as Adsorbent for Removal of Fluoride  
Rice straw can be utilized as an adsorbent material by subjecting it to different physical and chemical pre-treatments. [PERSON] et al. [110] have investigated the potential of activated rice straw for the fluoride removal process from water. The authors of the same study concluded that activated rice straw can produce low-density, highly porous materials. Strong oxidants such as hydrogen peroxide, nitric acid, and potassium permanganate were used for treating the activated rice straw carbon. The results showed significant improvement and higher efficiency values compared to similar activated carbon not treated with strong oxidants. Potassium permanganate showed the highest effect, while nitric acid and hydrogen peroxide had comparably less effect. [PERSON] et al. [81] realized a comparison in fluoride adsorption capacity between rice straw biochar and La/Fe/Al oxides loaded rice straw biochar. The La-Fe-Al composite was synthesized into the rice straw biochar by the co-precipitation method. Then, the modified rice straw biochar was grinded and sieved under 100 mesh. The comparison is shown in Table 9. Additionally, authors have highlighted that the presence of other anions such as Cl\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)\({}^{-}\), and HCO\({}_{3}\)\({}^{-}\), and the pH value of the solution can have little effect on the removal of fluoride by La/Fe/Al rice straw biochar. Therefore, pH from 3 to 11 was suggested as the optimum range. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type of Adsorbent** & **Type of Water** & **Experimental Conditions** &
\begin{tabular}{c} **Adsorption** \\ **Capacity (mg/g)** \\ \end{tabular} & **References** \\ \hline \multirow{4}{*}{**Rice straw biochar**} & & Time = 45 min & \multirow{4}{*}{26} & \multirow{4}{*}{[107]} \\  & & Adsorbent = 0.8 g/L & & \\ \cline{1-1}  & & pH = 4 & & \\ \cline{1-1}  & & Initial concentration = 10 mg/L & & \\ \cline{1-1}  & & Stirring rate = 400 rpm & & \\ \cline{1-1}  & & Temperature = 303 K & & \\ \hline \multirow{4}{*}{**Rice husk-derived silica**} & & Time = 65 min & \multirow{4}{*}{55} & \multirow{4}{*}{[108]} \\  & & Adsorbent = 1.1 g/L & & \\ \cline{1-1}  & & pH = 6.5 & & \\ \cline{1-1}  & & Initial concentration = 10 mg/L & & \\ \hline \multirow{2}{*}{**Rice husk ash**} & & \multirow{2}{*}{Synthetic} & pH = 8 & 2.9 & [109] \\ \hline \hline \end{tabular}
\end{table} 
Table 8: \(Cont.\) \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type of** & **Type of Water** & **Experimental Conditions** &
\begin{tabular}{c} **Adsorption** \\ **Capacity (mg/g)** \\ \end{tabular} & **References** \\ \hline \multirow{4}{*}{**Rice straw biochar**} & & pH = 3 & \multirow{4}{*}{10.9} & \multirow{4}{*}{[81]} \\  & Aqueous & Temperature = 298 K & & \\ \cline{1-1}  & Aqueous & Initial concentration = 60–160 mg/L & & \\ \cline{1-1}  & & Surface area = 2.59 m\({}^{2}\)/g & & \\ \cline{1-1}  & & Adsorbent = 1 g & & \\ \hline \multirow{4}{*}{**Activated carbon-derived**} & & pH = 2.0 & \multirow{4}{*}{15.9} & \multirow{4}{*}{[110]} \\  & & Solution & Initial concentration = 20.0 mg/L & & \\ \cline{1-1}  & activated carbon & & Surface area = 123 m\({}^{2}\)/g & & \\ \hline \multirow{4}{*}{**Alumina impregnated onto the activated carbon derived from rice straw**} & & pH = 6.1 & \multirow{4}{*}{10} & \multirow{4}{*}{[111]} \\  & Aqueous & Initial concentration = 50.0 mg/L & & \\ \cline{1-1}  & solution & Surface area = 151 m\({}^{2}\)/g & & \\ \cline{1-1}  & & Adsorbent = 4 g & & \\ \hline \multirow{4}{*}{**La/Fe/Al oxides**} & & pH = 3–11 & \multirow{4}{*}{[81]} \\  & & & Temperature = 298 K & \\ \cline{1-1}  & & Initial concentration = 60–160 mg/L & & \\ \cline{1-1}  & & Surface area = 95.36 m\({}^{2}\)/g & & \\ \cline{1-1}  & & Adsorbent = 1 g & & \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Adsorption capacities of fluoride by different types of rice straw (adopted from [81, 110, 111]). [PERSON] et al.