Development of an Encapsulation Process to Extend the Stability of Scorodite Under Wider pH and Redox Potential Range Conditions

Abstract

Scorodite (FeAsO 4 ⋅ 2H 2 O) is suitable mineral carrier for immobilization of arsenic-rich wastes. Its stability is, however, highly pH dependent (typically at 4 ≤ pH ≤ 7) and satisfactory only under oxic disposal conditions. In this work an encapsulation process using mineralized gels of aluminum hydroxy-oxides is developed to enhance the stability of scorodite under wider pH and redox potential range conditions. The encapsulation involves blending and ageing of synthetic scorodite produced by McGill's atmospheric scorodite process with aluminum hydroxyl gels derived from controlled hydrolysis of aluminum salts. The amorphous hydrolyzed Al-gel encapsulates the scorodite particles, which upon short-term aging transforms into crystalline Al (OH) 3 /AlOOH mineral phases providing a robust protection microscopic barrier. Long-term stability testing demonstrates the encapsulation system to be highly effective in suppressing arsenic release under either oxic or anoxic (100 mV < E h < 600 mV) potential and neutral-alkaline pH (7 ≤ pH ≤ 9) ranges.