KINETICS OF ARSENOPYRITE OXIDATIVE DISSOLUTION BY OXYGEN

The objective of our study is to use a mixed flow reactor system to determine the dissolution rate and infer potential mechanisms of arsenopyrite oxidation by dissolved oxygen at 25°C and circumneutral pH. Reaction rates for iron, arsenic and sulfur release are calculated for a variety of initial dissolved oxygen concentrations. The macroscopic rate law for arsenopyrite oxidation as a function of dissolved oxygen is determined using multiple linear regression of log r vs. log P_O2. Preliminary results indicate that arsenopyrite oxidation releases arsenite (subsequent oxidation to arsenate may occur), sulfate, and ferrous iron, though some oxidation to ferric iron is evident. Sulfate is used as the main reaction progress variable for the oxidation reaction, as preliminary experiments showed that it exhibits conservative behavior once it is produced during the oxidation process. Calculated oxidation rates are on the order of 10^-10 mol m^-2s^-1, similar to those reported for pyrite and other arsenic sulfides in air saturated water. This correlation suggests that the rate determining step for arsenopyrite oxidation is oxygen reduction at the mineral surface.

Previous work on arsenopyrite (FeAsS) oxidation has been limited to low pH conditions with ferric iron as the oxidant. However, not all arsenopyrite weathering occurs exclusively in acidic environments. For example, at our field site at an abandoned arsenopyrite mine in Virginia, the pH of ground and surface waters is consistently between 5 and 7. Results of this study will provide important insight to arsenic mobilization processes and rates, consequently aiding in the effort to understand arsenic release and retention in the environment.