Scorodite encapsulation by controlled deposition of aluminum phosphate coatings

A new stabilization process for scorodite (FeAsO(4).2H(2)O) solids based on the concept of encapsulation by controlled deposition of mineral coatings immune to pH or redox potential variations is described. The stability of the encapsulated scorodite with aluminum phosphates under simulated anoxic and oxic environments is demonstrated. Encapsulation experiments were carried out at 95 degrees C using 50 g/L scorodite in acidic sulphate solution containing 0.16 mol/L of P(V) with Al(III) to P(V) molar ratio of 1 and precipitation pH of 1.7. The encapsulated particles were characterised by XRD, SEM, TOF-SIMS and TOF-LIMS. The coating was crystalline AlPO(4).1.5H(2)O ranging in thickness from 2.5 to 3.5 microm. Encapsulation of scorodite particles with hydrated aluminum phosphate appears to be effective in controlling/suppressing the release of arsenic under both oxic and anoxic conditions by more than one order of magnitude.