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Utilization of layered double hydroxide to remove arsenic and suppress pH decrement during ultrasound oxidation of arsenious acid

Ultrasound oxidation from As(III) to As(V) is an effective method to remove As with iron compounds. However, ultrasound irradiation in air atmosphere reduces the pH, causing the dissolution of adsorbent and requiring a neutralization treatment for discharge. Therefore, we investigated the As removal and pH preservation by Mg–Al layered double hydroxide (LDH) after the As(III) ultrasound oxidation. When As(III) solution (10 ppm, 50 mL, pH 5.0) was oxidized to As(V) by sonication, the solution pH decreased from 5.0 to 3.2. However, when LDH calcined at 400 °C (LDH/As = 152) was added to the solution, the pH increased from 3.2 to 7.4. Moreover, 100% of the As was removed. Finally, we investigated As(III) oxidation and As removal simultaneously using ultrasound and LDH calcined at 400 °C. As was removed entirely, which means that LDH does not prevent the ultrasound oxidation in the solution.