The mineralogic transformation of ferrihydrite induced by heterogeneous reaction with bioreduced anthraquinone disulfonate (AQDS) and the role of phosphate

Bioreduced anthraquinone-2,6-disulfonate (AH(2)DS; dihydro-anthraquinone) was reacted with a 2-line, Si-substituted ferrihydrite under anoxic conditions at neutral pH in PIPES buffer. Phosphate (P) and bicarbonate (C); common adsorptive oxyanions and media/buffer components known to effect ferrihydrite mineralization; and Fe(II)(aq) (as a catalytic mineralization agent) were used in comparative experiments. Heterogeneous AH(2)DS oxidation coupled with Fe(III) reduction occurred within 0.13-1 day, with mineralogic transformation occurring thereafter. The product suite included lepidocrocite, goethite, and/or magnetite, with proportions varing with reductant: oxidant ratio (r:o) and the presence of P or C. Lepidocrocite was the primary product at low r:o in the absence of P or C, with evidence for multiple formation pathways. Phosphate inhibited reductive recrystallization, while C promoted goethite formation. Stoichiometric magnetite was the sole product at higher r: o in the absence and presence of P. Lepidocrocite was the primary mineralization product in the Fe(II)(aq) system, with magnetite observed at near equal amounts when Fe(II) was high [Fe(II)/Fe(III)] = 0.5 and P was absent. P had a greater effect on reductive mineralization in the Fe(II)(aq) system, while AQDS was more effective than Fe(II)(aq) in promoting magnetite formation. The mineral products of the direct AH(2)DS-driven reductive reaction are different from those observed in AH(2)DS-ferrihydite systems with metal reducing bacteria, particularly in presence of P. (C) 2011 Elsevier Ltd. All rights reserved.