Oxidation of As(III) by pressurized oxygen and the simultaneous precipitation of As(V) as scorodite in acidic sulfate solutions

•Simultaneous oxidation and removal of As(III) to form scorodite.•Arsenic removal efficiency reach 97% for high As(III) concentration of 15 g/L.•A critical role of HO•, Fe(III) and Fe(IV) in the oxidation of As(III).•A synthetic transformation mechanism of scorodite via a four-stage process. Due to the lack of affordable and green disposal technologies, high arsenic-containing waste acid from the heavy nonferrous metallurgical smelter causes serious environmental pollution and threatens human health. In this study, a cost-effective and environmentally-friendly solution for simultaneous oxidation and precipitation of arsenic as well as formation of scorodite by adding ferrous sulfate as an iron source is proposed under elevated temperature (170 °C) and pressurized oxygen (1 MPa) conditions. And that the total reaction can be interpreted as 4Fe2+ + 4HAsO2 + 3O2 + 10H2O → 4FeAsO4·2H2O + 8H+. Consequently, the final oxidation and removal efficiency of arsenic reached 98.95% and 97.42%, respectively, resulting in scorodite with an arsenic leaching concentration of 1.4 mg/L in the TCLP leaching test. The results prove that the HO•, Fe(III) and Fe(IV) produced by the oxidation of Fe(II) is of capable oxidizing As(III) to As(V). Most importantly, the synthetic transformation mechanism of scorodite via a four-stage process is revealed, including the oxidation of As(III), formation of a mixture of amorphous hydronium jarosite and iron arsenate as precursors, partial dissolution of the precursor as well as nucleation and growth of scorodite particles.