Encapsulation of scorodite using crystalline polyferric sulfate precipitated from the Fe(II)-SO42−-O2-H2O system

Abstract

Scorodite was encapsulated by precipitating a crystalline polyferric sulfate (PFS) coating layer on the surface of scorodite particles to restrain the release of arsenic. The encapsulation experiments were carried out at atmospheric pressure, 90 °C, and 1.5 pH by adding 20 g scorodite particles/L to a 0.1-0.4 mol/L ferrous sulfate solution followed by the introduction of pure oxygen to oxidize and precipitate the ferrous ions as PFS. After encapsulation the samples were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and scanning transmission electron microscopy. The results indicated that the coating was crystalline PFS, and the scorodite particles were completely encapsulated by the coating. The stability of the scorodite encapsulated with PFS was evaluated by several toxic leaching tests which use an HAc-NaAc buffer solution of 4.93 pH, a NaOH solution of 9.3 pH, a NaH2PO4-NaOH buffer solution of 8.6 pH, or Na2S solutions of 7.6 and 8.1 pH and a reducing potential (Eh) within the −100 to 0 mV range as leaching solutions. The results of the leaching tests indicated that the scorodite encapsulated with PFS was highly effective at restraining the release of arsenic in both weakly acidic and alkaline solutions under both oxic and anoxic conditions.