Kinetic research of scorodite formation via oxidative coprecipitation from arsenic–bearing solution

Atmospheric oxidation is one frequently–used method to immobilize arsenic-bearing wastewater as nonhazardous scorodite. Its kinetic research is indispensable for improving synthesis on an industrial scale. In this study, the kinetic for the conversion from ionic Fe(II) and As(V) solution to scorodite was elaborately researched and discussed based on temperature–dependent experiments and software calculations. This work was divided into three parts. In experiments, scorodite synthesis was based on the optimal conditions of initial pH 2.0, 20 g/L of As, Fe/As molar ratio 1.4, O₂ flow rate 0.5 L·min⁻¹ at 95℃ for 12 h. Moreover, scorodite is developed from polymerization and oxidation determined by solution pH, residual [As] and [Fe(II)] with the precipitate phase transformation observed by X–ray diffraction and scanning electron microscope. In kinetic analysis, the activation energy of Fe(II)–As(V) polymerization and oxidation varied at 33.81–435.27 kJ·mol⁻¹ and 52.66–599.25 kJ·mol⁻¹, respectively, calculated from Arrhenius equation based on the established matrix equation solved by Matlab software. In synthetic improving, the whole process is comprised of atmospheric polymerization at 90 ℃ for 1.5 h followed by pressurized oxidation at 130 ℃ and P_O2=1.5 MPa for 3 h as the rate constants of polymerization far outweighs that of oxidation. In general, this kinetic research is reliable and can be applied to other arsenic immobilization from arsenic–bearing solution. The improved synthesis for scorodite is more advanced in reaction duration and oxygen utilization for potential industrial application.