Bioscorodite Crystallization in an Airlift Reactor for Arsenic Removal

Bioscorodite (FeAsO4·2H2O) crystals were crystallized in an airlift reactor fed at pH 1.2 and 72 °C. Arsenic removal was limited by the biological ferrous iron oxidation. In continuous operation, the iron oxidation initially was 30% and increased to 80% in few days when the iron and dissolved oxygen concentration were increased. The bioscorodite yield was 3 g/g of arsenic removed. The first precipitates were identified as scorodite having a dipyramidal octahedron habit with an Fe/As molar ratio of 1.55. The stability test (TCLP) classified the crystals as suitable for storage with a leached arsenic concentration of 0.5 mg L–1 after 60 days. Settling rates of bioscorodite crystals between 50 and 140 m h–1 were measured. Size distribution frequency indicates that bioscorodite crystals grew from an average size of 30 µm during batch operation to 160 µm at the end of the continuous operation phase. The morphology and size of the crystals guarantee their free-flowing nature, avoiding scaling. The biggest and most stable crystals can be harvested by sedimentation, to select the material best suited for final disposal.