## 2 Materials and Methods
### Batch Adsorption Experiment  
For the study of the adsorption of arsenic ions (As(III) and As(V)) over the surface of synthesized adsorbents, 500.0 mL stock solutions of both ions were prepared by dissolving Na\({}_{2}\)HAsO\({}_{4}\cdot\)7H\({}_{2}\)O (2.080 g) and NaAsO\({}_{2}\) (0.8670 g) in distilled water to reach the concentration of 1000.0 mg/L. Then, these stock solutions were diluted using deionized water to acquire the required concentration for the batch adsorption experiment. After each experiment, the adsorbent was separated using an external magnet and the solution was examined for the presence of arsenic ions using an atomic absorption spectrometer. The effect of different parameters on the adsorption process was studied including pH, adsorbent dosage, initial concentration, and contact time. The pH effect was studied by changing the pH value in the range of 3.0 to 11.0 by using an initial concentration of arsenic ions of 10.0 mg/L, 0.50 g/L of adsorbent, and solution volume of 100.0 mL. The effect of adsorbent dosage was also studied in the range of 0.50 to 2.50 g/L at pH of 6.70, 30.0 \({}^{\circ}\)C, 150 rpm of shaking for 1.0 day, and solution volume of 100.0 mL. The initial concentration effect was studied in the range of 10.0 to 500.0 mg/L at pH of 6.70, 30.0 \({}^{\circ}\)C, 150 rpm of shaking for 1.0 day, solution volume of 100.0 mL, and adsorbent dosage of 2 and 1.5 g/L for As(III) and As(V), respectively. Additionally, the contact time effect was studied by varying the time in the range of 5.0 to 60.0 min at the same experimental conditions. The removal efficiency can be calculated using Equation (1):  
\[\text{Removal efficiency }(\%)=((\text{C}_{\text{i}}-\text{C}_{\text{e}})/ \text{C}_{\text{i}})\times 100, \tag{1}\]  
The symbols C\({}_{\text{i}}\) and C\({}_{\text{e}}\) denote the initial and equilibrium concentrations of arsenic ions, respectively.  
The adsorption capacity can be calculated using Equation (2):  
\[\text{Adsorption capacity }(\text{q}_{\text{e}})=((\text{C}_{\text{i}}-\text{C}_{ \text{e}})/\text{W})\times\text{V}, \tag{2}\]  
The symbols W and V denote mass of adsorbent (g) and volume of solution (L), respectively.  
The adsorption rate was investigated using the kinetic models of pseudo 1 st order and pseudo 2 nd order according to Equations (3) and (4), respectively:  
\[\text{ln }(\text{q}_{\text{e}}-\text{q}_{\text{t}})=\text{ln }\text{q}_{\text{e}}-\text{k}_{1}\text{t}, \tag{3}\]  
\[\text{t}/\text{q}_{\text{e}}=(1/\text{k}_{2}\text{q}_{\text{e}}{}^{2})+(1/ \text{q}_{\text{e}}), \tag{4}\]  
where k\({}_{1}\) (min\({}^{-1}\)) and k\({}_{2}\) (g mg\({}^{-1}\) min\({}^{-1}\)) denote the equilibrium rate constants and qt denotes the adsorption capacity at t time.  
The adsorption mechanism was investigated using the linearized form of [PERSON]'s isotherm that can be represented by Equation (5):  
\[\text{Log q}_{\text{e}}=\text{Log K}_{\text{F}}+(1/\text{n Log C}_{\text{e}}), \tag{5}\]  
The symbols 1/n and K\({}_{\text{F}}\) denote adsorption intensity and [PERSON]'s constant, respectively.  
Additionally, the adsorption mechanism was investigated using the linearized form of [PERSON]'s isotherm that can be represented by Equation (6):  
\[1/\text{q}_{\text{e}}=(1/\text{K}_{\text{L}}.\text{q}_{\text{max}}.1/\text{ C}_{\text{e}}+(1/\text{q}_{\text{max}}), \tag{6}\]  
The symbols K\({}_{\text{L}}\) and q\({}_{\text{max}}\) denote [PERSON]'s constant and maximum adsorption capacity, respectively.  
The regression coefficient R\({}^{2}\) value was used to determine the best fitting between experimental data and isotherm that determines the mechanism of adsorption.