Scorodite, FeAsO4 · 2H2O, is an orthorhombic iron arsenate commonly associated with arsenic-bearing ore deposits as a weathering product. This study investigated the mode of formation and persistence of scorodite in nature, and suggests that its presence controls the arsenate levels in waters draining from oxidizing arsenic-rich ores. Solubility experiments give Kspscor = 10−21.7±0.5 and ΔGfo=1263.52±0.28kJmol1. Scorodite is metastable under most conditions and tends to dissolve incongruently, forming iron hydroxides and releasing arsenate to solution.

First-hand observations at the Brinton Arsenic Mine, Floyd County, VA, and observations by others at similar arsenopyrite deposits show a close spatial relationship between arsenopyrite and scorodite, suggesting that the reaction which forms scorodite is:
FeAsS(aspy)+14Fe3++10H2O14Fe2++SO42+FeAsO42H2O(scor)+16H+
(1)

This process is catalyzed by Ferrooxidans bacteria which oxidize Fe2+ to Fe3 +. The formation of scorodite involves the oxidation of arsenopyrite at low pH to give locally high arsenate and iron activities which lead to scorodite precipitation. After the arsenopyrite is consumed, scorodite dissolves incongruently to give goethite and aqueous arsenate. Scorodite has a stability field at pH values near 2 and arsenate activity greater than 0.01 molal. Analysis of the surface and ground waters at the Brinton Arsenic Mine site showed that scorodite seems to control maximum dissolved arsenate levels.

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