Bioscorodite Crystallizationin an Airlift Reactorfor Arsenic Removal.

Bioscorodite (FeAsO4·2H2O)crystalswere 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% andincreased to 80% in few days when the iron and dissolved oxygen concentrationwere increased. The bioscorodite yield was 3 g/g of arsenic removed.The first precipitates were identified as scorodite having a dipyramidaloctahedron habit with an Fe/As molar ratio of 1.55. The stabilitytest (TCLP) classified the crystals as suitable for storage with aleached arsenic concentration of 0.5 mg L–1after60 days. Settling rates of bioscorodite crystals between 50 and 140m h–1were measured. Size distribution frequencyindicates that bioscorodite crystals grew from an average size of30 μm during batch operation to 160 μm at the end of thecontinuous operation phase. The morphology and size of the crystalsguarantee their free-flowing nature, avoiding scaling. The biggestand most stable crystals can be harvested by sedimentation, to selectthe material best suited for final disposal. [ABSTRACT FROM AUTHOR]