Scorodite dissolution kinetics: implications for arsenic release.

We have measured the rate of scorodite (FeAsO4.2H2O) dissolution over an environmentally relevant range of pH and temperature conditions. Dissolution rates, calculated using arsenic (As) as the reaction progress variable, were slowest at pH 3 and increased with both decreasing and increasing pH. Comparison of the pH-dependence of the dissolution rates with a scorodite stability diagram suggests that our measurements of dissolution rate at pH 2 reflect congruent dissolution, and those at and above pH 3 reflect incongruent dissolution. Because As was used as the reaction progress variable, and recognizing that As may adsorb to iron hydroxides during incongruent dissolution of scorodite, the derived rates may be underestimated. The pH and temperature dependence of scorodite dissolution rates determined in these experiments have implications for the stability of scorodite at field sites and also for the potential use of scorodite to sequester As. Although scorodite dissolution is slow, it can be enhanced by up to a half order of magnitude by increases in pH and temperature.