Scorodite is an arsenic mineral with the chemical formula FeAsO(4)*2H(2)O. It is the most common natural arsenate associated with arsenic-bearing ore deposits. In the present study we show that the thermoacidophilic iron-oxidizing archaeon Acidianus sulfidivorans is able to precipitate scorodite in the absence of any primary minerals or seed crystals, when grown on 0.7 g L(-1) ferrous iron (Fe(2+)) at 80 degrees C and pH 1 in the presence of 1.9 g L(-1) arsenate (H(3)AsO(4)). The simultaneous biologically induced crystallization of ferric iron (Fe(3+)) and arsenic to scorodite prevented accumulation of ferric iron. As a result, crystal growth was favored over primary nucleation which resulted in the formation of highly crystalline biogenic scorodite very similar to the mineral scorodite. Because mineral scorodite has a low water solubility and high chemical stability, scorodite crystallization may form the basis for a novel method for immobilization of arsenic from contaminated waters with high arsenic concentrations.

No abstract

Bioscorodite (FeAsO 4 •2H 2 O) 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.

The extreme acid conditions required for scorodite (FeAsO4·2H2O) biomineralization (pH below 1.3) are suboptimal for growth of most thermoacidophilic Archaea. With the objective to develop a continuous process suitable for biomineral production, this research focuses on growth kinetics of thermoacidophilic Archaea at low pH conditions. Ferrous iron oxidation rates were determined in batch-cultures at pH 1.3 and a temperature of 75°C for Acidianus sulfidivorans, Metallosphaera prunea and a mixed Sulfolobus culture. Ferrous iron and CO2 in air were added as sole energy and carbon source. The highest growth rate (0.066 h−1) was found with the mixed Sulfolobus culture. Therefore, this culture was selected for further experiments. Growth was not stimulated by increase of the CO2 concentration or by addition of sulphur as an additional energy source. In a CSTR operated at the suboptimal pH of 1.1, the maximum specific growth rate of the mixed culture was 0.022 h−1, with ferrous iron oxidation rates of 1.5 g L−1 d−1. Compared to pH 1.3, growth rates were strongly reduced but the ferrous iron oxidation rate remained unaffected. Influent ferrous iron concentrations above 6 g L−1 caused instability of Fe2+ oxidation, probably due to product (Fe3+) inhibition. Ferric-containing, nano-sized precipitates of K-jarosite were found on the cell surface. Continuous cultivation stimulated the formation of an exopolysaccharide-like substance. This indicates that biofilm formation may provide a means of biomass retention. Our findings showed that stable continuous cultivation of a mixed iron-oxidizing culture is feasible at the extreme conditions required for continuous biomineral formation.Electronic supplementary materialThe online version of this article (doi:10.1007/s00253-011-3460-7) contains supplementary material, which is available to authorized users.

No abstract