To remove arsenite (As(III)) from wastewater effectively, the catalytic oxidation of As(III) to arsenate (As(V)) and As(V) precipitation with iron ions (Fe(III)) was investigated. The Pt/SiO₂ catalyst functioned as a reaction site for As(III) with oxygen in the atmosphere. The combination of the Pt/SiO₂ catalyst and Fe(III) precipitant improved the removal of As(III) in the precipitate; Pt/SiO₂ worked as both an As(III) oxidation site and precipitation site with Fe(III) precipitant.