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. Previous studies have focused on the stability of scorodite (Dove and Rimstidt 1985) and on methods for precipitating scorodite to remove arsenic from mine waste (Demopoulos et al., 1995). Neither of these studies investigated the dissolution kinetics, which are important for assessing the controls on arsenic release to and transport within natural waters.

The purpose of this research was to examine the kinetics of scorodite dissolution as a function of temperature and pH. The first set of experiments was designed to examine scorodite dissolution rates at temperatures representative of seasonal variation (20, 30, and 40°C). Scorodite was precipitated from a solution of hydrochloric acid, sodium arsenate, and ferric chloride (Demopoulos et al., 1995), which was then heated and kept at 95°C for five hours. After several cycles of washing and drying, the precipitate was analyzed by x-ray diffraction analysis to confirm it as scorodite. Then, the surface area was determined using N₂-BET methods. Dissolution experiments were conducted in batch reactors in a controlled temperature water bath. Samples, collected over a four-hour period, were filtered (0.45 micron), acidified (HNO₃), and analyzed for iron and arsenic. Because arsenic behaved more conservatively then iron during the experiments, it was chosen to be the reaction progress variable. The initial rate method was used with the arsenic data to calculate the scorodite dissolution rate.

Results of the temperature-dependent study revealed that scorodite dissolution rates increase as a function of temperature with rates ranging from 10^-6.00 to 10^-5.53 mol/(m²*sec) over the twenty degree span. Current experiments are being conducted to examine scorodite dissolution rate kinetics as a function of pH.