Improvement of scorodite stability by addition of crystalline polyferric sulfate

The stability of scorodite and scorodite with crystalline polyferric sulfate (PFS) added was investigated using the Toxicity Characteristic Leaching Procedure (TCLP) and leaching tests using NaOH solutions. The leaching behaviors of iron and arsenic were analyzed using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The results indicated that the addition of PFS reduced the concentration of arsenic dissolved into the leaching solution significantly from 10 to 30 mg/L to below 0.01 mg/L through its adsorption and effect on the equilibrium of scorodite dissolution. The PFS addition facilitated the leaching of iron from the scorodite to precipitate as two-line ferrihydrite that coated the surface of scorodite particles. This potentially slowed down the diffusion of arsenic from the scorodite to the leaching solution. The thermodynamic calculations for the scorodite dissolution process showed that the formation/addition of ferrihydrite was effective in controlling the equilibrium arsenic concentration. With consideration of the ferrihydrite formation in the calculations, the scorodite was shown to be stable between pH 2.0–8.8, which was a wider range than the scorodite stability range of pH 2.0–5.8.