Herein, we synthesized scorodite particles using ultrasound irradiation in a strong sulfuric acidic solution (pH 0.5) and investigated the scorodite particles' size and morphology. The highest crystallinity of scorodite (96%) was synthesized using 200 kHz ultrasonic treatment, and maximum yield of scorodite was obtained in three conditions: 200, 28 kHz, and stirrer. The oxidant generated through 200 kHz ultrasound irradiation enhanced Fe(II) oxidation in the solution resulting in crystal nuclei formation. The crystal nuclei were transformed to scorodite by oxidation with a supply of solute [Fe(III)] from the solution and ostwald ripening. Clustered scorodite (4 μm in size) was obtained via synthesis using 200 kHz ultrasound (pH 0.5). Arsenic could be separated from the clustered scorodite via alkaline treatment; scorodite was changed to porous γ-Fe O comprising nanosized particles. Porous γ-Fe O was evaluated as a cathode in a lithium-ion battery, which showed a 149 mAh g discharge capacity with a 0.5 C rate. 2 3 2 3 -1