# Adsorption of Arsenic and Heavy Metals from Solutions by Unmodified Iron-Ore Sludge
## 2 Materials and Methods
### Experiments
#### 2.2.2 Adsorption Study  
The adsorption study of the tested contaminants was conducted in batch experiments of both single- (As, Mn, Zn, Cd, or Pb) and mixed-metal (Mn, Zn, Cd, and Pb) solutions. In the single-metal experiments, nitrate salt solutions of Mn, Zn, Cd, and Pb and Na\({}_{2}\)HAsO\({}_{4}\)\(\cdot\)7H\({}_{2}\)O (Kanto Chemical Co. Inc., Japan) were separately diluted by Milli-Q water to obtain the desired concentrations of As and heavy metals. In the mixed-metal experiments, these nitrate salt solutions of Mn, Zn, Cd, and Pb were simultaneously prepared. The mixed-metal solutions excluded As due to the possible reaction between cations and anions in the same solution.  
Different amounts of the adsorbent (10, 20, 40, and 80 g/L) were used to assess the effect of doses on adsorption. In the adsorption kinetic and equilibrium adsorption experiments, 2 g of adsorbent was added to 100 mL of the aqueous adsorbate solution in a 125 mL plastic flask. The initial pH values of the solutions were adjusted by adding 0.1 M HCl and 0.1 M NaOH solutions to reach the desired value. The flask was covered with Parafilm and shaken at 120 rpm on Daihan Labtech LSI-2 orbital shaker at room temperature (25 \(\pm\) 2\({}^{\circ}\)C) for the desired time interval. After the predetermined time period had elapsed, the mixture of adsorbent and adsorbate was immediately separated using a 0.45 \(\upmu\)m syringe filter. A 15 mL sample of the filtered solution was collected and diluted (if necessary) for elemental analysis.  
The amount of adsorbate that was adsorbed at equilibrium, \(q_{e}\) (mg/g), or at time t, \(q_{t}\) (mg/g), was calculated by the following mass balance Equations (2) and (3):  
\[q_{e} = \frac{(C_{o}-C_{e})V}{m} \tag{2}\] \[q_{t} = \frac{(C_{o}-C_{t})V}{m} \tag{3}\]  
where \(C_{o}\) (mg/L) \(C_{e}\) (mg/L) and \(C_{t}\) (mg/L) are the adsorbate concentrations at beginning equilibrium and time t, respectively; m (g) is the mass of used adsorbent; and V (L) presents the volume of the adsorbate solution.  
The adsorption kinetic experiments were conducted on single- and mixed-metal solutions with heavy metal concentrations of 20 mg/L. Glass flasks containing 100 mL of the heavy metal solutions were agitated in a flat shaker at 120 rpm at room temperature (25 \(\pm\) 2\({}^{\circ}\)C) with a constant pH of 5.5. The kinetic studies were conducted for specific durations ranging from 10 to 1440 min.  
Adsorption equilibrium experiments were conducted in various initial concentrations of heavy metals (0-50 mg/L). Approximately 2 g of adsorbent was added to 100 mL of solution with predetermined adsorbate concentration in a 125 mL. Plastic flask. The flasks were immediately covered with a parafilm and shaken at 120 rpm at the same conditions of kinetic experiments.  
Desorption efficiency analysis of the target metal ions was also performed using the adsorbent from the isotherm experiments that had an initial metal concentration of 50 mg/L. This adsorbent was added to 100 mL of solutions with different pH values (4.0, 5.5, 7.0, and 9.0) and was agitated in a flat shaker at 120 rpm at 25 \(\pm\) 2 \({}^{\circ}\)C for 24 h.  
All experiments were conducted in duplicate, and the resulting data were averaged. If the bias of the repeated experiment exceeded 15%, a third run was conducted.