KINETICS OF SCORODITE DISSOLUTION

Scorodite (FeAsO₄*2H₂O) is a common weathering product of arsenopyrite (FeAsS), one of the main arsenic-bearing minerals in the earth’s crust. Because scorodite is more soluble than arsenopyrite in low pH environments, its dissolution strongly impacts arsenic release to natural waters in regions of sulfide mineralization. Quantifying scorodite dissolution rates is important because arsenic is both a toxin and a carcinogen. Previous studies on scorodite have focused on its stability and on methods for precipitating scorodite to remove arsenic waste from mine waste. However, little is known about its dissolution kinetics. In this study, we are measuring scorodite dissolution rates using a flow-through reactor (FTR) system. This reactor has been used to study weathering of several minerals, including wollastanite, olivine, and arsenopyrite. The advantage of the FTR is that it can function at a high enough mixing rate to eliminate transport-limited and it can hold a variety of sample volumes. The dissolution rate (R) of scorodite will be determined at three temperatures, 15, 25 and 35 degrees C, to simulate seasonal climatic fluctuations. Rates will be calculated at each temperature using the following equation: R=r_f((m_{i, in}-m_{i, out})/A) where r_f is the rate of flow through the reactor in kg/s; m_i is the molar concentration of arsenic or iron in the influent (in) and effluent (out) in mol/kg; and A is the surface area of scorodite in m². The calculated rates can be used in numerical models that simulate arsenic release to natural waters from scorodite dissolution.