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Hydrous ferric oxide (HFO)-impregnated agarose beads were prepared as an adsorbent to reduce As(III) concentration in the pharmaceutical wastewater effluent generated from the manufacturing process of an arsenic-containing chemotherapeutic drug. The adsorbent was prepared by immersing wet agarose beads into FeCl3 (aq), followed by another immersion into NaOH (aq), resulting in HFO precipitation directly within the agarose beads. The As(III) adsorption rate of HFO-impregnated agarose beads was determined to be external diffusion-limited, and having a Langmuir maximum adsorption capacity of 76.1 mg As(III)/g. Passing 6 L of the pharmaceutical wastewater through a packed column filled with the adsorbent in an upflow mode of 50 mL/min repeatedly for five times resulted in the reduction of As(III) concentration from 250 to 10 μg of As(III)/L of H2O. Column desorption was performed by passing 0.6 L of 0.1 M NaOH (aq) through the column in an upflow mode of 50 mL/min repeatedly for five times to regenerate the As(III)-adsorbed HFO-impregnated agarose beads. Chemical characterizations revealed that the As(III) species were adsorbed on HFO by the "inner-sphere" bidentate complexation via ligand exchange or "outer-sphere" association via electrostatic interaction, while the "outer-sphere"-associated As(III) species were most likely desorbed. [ABSTRACT FROM AUTHOR]