# Low-Cost Goethite Nanorods for As (III) and Se (VI) Removal from Water
## 2 Experimental Work
### Adsorption Experiments  
A standard stock solution of commercially available Arsenite (As(III)) of 1000 mg/L concentration (Inorganic Ventures, Christiansburg, Virginia, USA) was diluted with distilled water. Se(VI) solutions were prepared by adding appropriate amounts of 1000 mg/L selenate (Inorganic Ventures, Christiansburg, Virginia, USA) to distilled water and used as test solutions. The batch adsorption experiments of As(III) and Se(VI) uptake were designed to analyze the adsorbent dose, kinetics, and effect of pH. Adsorbent dose experiments were performed by adding a different quantity of \(\alpha\)-_FeOOH_ to As(III) and Se(VI) containing water samples prepared in the lab. As(III) or Se(VI) solution (containing 800 mL water, As(III) 515.34 g/L and pH \(\sim\)7) was used for batch experiments. Briefly, 100 mL of As(III) or Se(VI) solution was added to conical flasks. \(\alpha\)-_FeOOH_ powder in the range of 0.05-1 g/L was dispersed in seven flasks, and one flask was kept as a blank reference. The flasks containing the test solutions with adsorbent were shaken uniformly using a laboratory shaker (Make: Remi) at 150 rpm and at room temperature for 6 h. Adsorption pH study was performed at \(\sim\)500 g/L adsorbate ionic concentration, 6 h contact time, and 0.25 g/L goethite dose. The experiments of solution pH were conducted using a pH meter (OAKION, Eutech Instruments), while the variation of solution pH was controlled by using 1 M NaOH or 1 M HCl in the range of 3-12. The measurements were conducted after solutions being shaken for 6 h to attain equilibrium.  
Adsorption kinetics study was conducted in a single conical flask with an \(\alpha\)-_FeOOH_ dose of 0.5 g/L, pH 7.2, and \(\sim\)500 g/L of As(III) or Se(VI) concentrations. Adsorption of As(III) and Se(VI) on \(\alpha\)-_FeOOH_ was monitored as a function of time starting from 1 min to 120 min. Meanwhile, the solution pH was determined by a laboratory pH meter (Make: Thermo). In the final step, samples were filtered using 0.22 um disposable membrane filters. Finally, the filtered water was analyzed for As(III) and Se(VI) using Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS, Perkin Elmer, NexIon 300X, Waltham, Massachusetts, USA) following standard procedures of sample preparation.  
Regeneration experiments are essential for identifying the reusability of an adsorbent. Herein, a modified desorption procedure was conducted by a normal washing process that uses distilled water instead of strong alkaline water. That is to prevent phase transformation. In brief, 100 mL of As(III) or Se(VI) (\(\sim\)8300 g/L) solution was diluted with 1.0 g/L of \(\alpha\)-_FeOOH_ adsorbent at pH \(\sim\)6.7 and room temperature. After equilibration for 3 h, the obtained solution was then allowed to settle down, filtered using centrifugal filtering, and dried at 100 \({}^{\circ}\)C for 2 h. in the next step, the filtrate was analyzed for As(III) and Se(VI) ions. The \(\alpha\)-_FeOOH_ nanorods residues were regenerated by leached out As(III) and Se(VI) ions using only distilled water and then dried at 100 \({}^{\circ}\)C for 2 h. Finally, the obtained powder was weighed using an electronic balance and reused again. The above process was repeated five times and the obtained powder was then characterized for identifying its structure and morphology.