Arsenic removal through adsorption, sand filtration and ultrafiltration: In situ precipitated ferric and manganese binary oxides as adsorbents

This study proposes a process consisting of in situ precipitated ferric and manganese binary oxides (FMBO) adsorption, sand filtration, and ultra-filtration (UF) for arsenic removal. Bench scale studies indicate that the FMBO shows higher capability of removing arsenic than hydrous ferric precipitate (HFO) and hydrous manganese oxide (HMO). This is ascribed to the combined effects of oxidizing As(III) and adsorbing As(V) for FMBO. The continuous experiments indicate that this process is effective for arsenic removal. In the presence of 0.624 mg/L As(III), when the Fe(II) dosage is 3 mg/L and the KMnO₄ dosage is equivalent to the sum of As(III) and Fe(II), the residual As concentration is as low as 29.2 μg/L. The adsorption of arsenic onto FMBO is fast, and the hydraulic retention time (HRT) of 45 s is enough for the adsorption unit. Sand filtration leads to more than 90% of arsenic removal, and UF further removes the particulate arsenic that passes through the sand filter. During the backwashing of the sand filter, the maximal aqueous arsenic concentration is 0.105 mg/L (at 150 s), and the dissolution of arsenic from FMBO is neglectable. The main operating cost of this process is as low as 0.355 RMB/m³, which is acceptable in rural areas for arsenic removal in engineering.