Article pubs.acs.org/crystal
Bioscorodite Crystallization in an Airlift Reactor for Arsenic Removal P. Gonzalez-Contreras,* J. Weijma, and C. J. N. Buisman Subdepartment of Environmental Technology, Wageningen University, Bornse Weilanden 9, P.O. Box 17, 6708 WG Wageningen, The Netherlands S Supporting Information *
ABSTRACT: 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 freeflowing nature, avoiding scaling. The biggest and most stable crystals can be harvested by sedimentation, to select the material best suited for final disposal.
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INTRODUCTION During the past decade, arsenic production has increased from about 50 000 to 60 000 metric tons per year due to mineral processing.1 Arsenic waste from mineral processing and coal washing is disposed of in tailings. In the last century, more than 100 significant upstream tailing dam failures have been documented.2 The risk of arsenic contamination from metallurgical effluents, tailing failures, and other anthropogenic sources should be minimized to limit harmful effects on water sources. To achieve this, there is a need for low cost, effective, and simple technologies that guarantee arsenic immobilization. Arsenic is traditionally removed from industrial wastewaters from mineral processing and metallurgical operations by lime neutralization with coprecipitation of arsenic with ferric iron.3,4 Arsenical ferrihydrite precipitation requires a high iron consumption with respect to arsenic, i.e., Fe/As > 4, and thus large amounts of waste material are produced.3,5 More recent technologies focus on arsenic immobilization into scorodite crystals. Scorodite (FeAsO4·2H2O) is a natural mineral that contains 30 wt % arsenic and an Fe/As molar ratio of 1.6−10 Because of its low arsenic solubility, mineral scorodite is the preferred arsenic mineral for disposal in tailing dams or in dry stack facilities. Originally, scorodite precipitation systems required the use of autoclaving technology, i.e., high pressure and high temperatures.11,12 Subsequently, the feasibility of atmospheric scorodite precipitation (
