## 5 Removal of Arsenic
### Rice Husk as Adsorbent for Removal of Arsenic  
Many literature papers have shown that rice husk, rice straw, and rice bran (polished rice) can be used as adsorbent material in the arsenic removal process. [PERSON] et al. [120] have investigated the efficiency of removal of arsenic from aqueous solution by porous carbon material obtained from the rice husk. The study was carried out in different conditions of pH, adsorbent amount, contact time, and initially adsorbed concentration. According to the adsorption analysis results, the removal of arsenic increased up to 85% under the optimum conditions of pH 6 (as reported in Table 10). [PERSON] et al. [121] also investigated the potential of raw rice husk as an adsorbent for the removal of As(V) from drinking water. They have studied the impact of different operating parameters and their effect on removal efficiency in fixed bed columns. Authors have revealed that the maximum removal of As(V) is up to 90.7% under the optimal conditions of pH 8 (Table 10). To compare, commercial granular activated carbon currently used at drinking water treatment plants allowed As(V) removals between 7 and 61% in batch experiments (carbon dose: 5 g/L, pH: 4, contact time: 24 h). While various types of powdered activated carbon showed a range of 5-84% (carbon dose: 1 g/L, pH: 4, contact time: 24 h) [122]. The removal of As(III), in a solution with an initial concentration of 10 mg/L and 2 g/L of activated carbon, reached the maximum value of 32.8% in batch experiments carried out at approximately pH 7.0 and at a temperature of 30 \({}^{\circ}\)C [123]. Another experiment with 24 h of contact time reported an arsenic loading with coal-based carbon (surface area: 1125 m\({}^{2}\)/g, 5.5% ash content) of 4.09 mg/g of carbon [124]. Rice husk can be subjected to different physical and chemical treatments (as shown in Table 11). [PERSON] et al. [125] investigated the arsenic removal ability of activated carbons produced from calcium chloride (CaCl\({}_{2}\)) impregnated rice husks. The highest arsenic uptake (18.2 \(\mathrm{\SIUnitSymbolMicro g}\)/g) was obtained using activated rice husk carbon at initial concentration of 1000 ppb (Table 10). [PERSON] et al. [115] investigated the possibility of using rice husk as an adsorption material without any pre-treatment in the removal of arsenic from aqueous media. Various adsorption and desorption conditions were considered, and the authors highlighted that the adsorption column method shows the complete removal of As(III) and As(V) (Table 10). Iron oxide nanoparticles have a higher capability of removing heavy metals from aqueous solution due to their magnetic property, size, and high surface area [126]. Based on these promising capabilities, [PERSON] et al. [126] investigated the efficiency of arsenic removal by modification of iron oxide nanoparticles with rice husk (Table 11). The study showed promising results of removal of arsenic (up to 95%) under the optimal conditions. [PERSON] et al. [127] have investigated the removal of As(III) and As(V) from water by iron-manganese oxide incorporated active rice husk silica (FMRS). They compared the arsenic removal capacity of both original rice husk ash and FMRS, which is 0.4 mg/g and 11.9 mg/g, respectively. Under the same experimental conditions, the adsorption capacity of FMRS for As(III) and As(V) was found to be 19.1 and 20.3 mg/g at pH 7, respectively.