The incongruent dissolution of scorodite-solubilities, kinetics and mechanisms.

摘要

This thesis reports the results of an investigation of scorodite long-term solubility and stability conducted at fixed pH (5-9) and temperature (22°C, 50°C and 75°C). The decomposition of scorodite at 22°C was found to be extremely slow. At neutral pH, the arsenic concentration stabilized at 5.8 mg/L. As scorodite dissolved, the iron reprecipitated as 2-line ferrihydrite. The growth and re-crystallization of ferrihydrite was delayed by apparent arsenate adsorption. The solubility product of scorodite was calculated as 10-25.5. The dissolution rate of scorodite in a pure system was modeled with a decreasing exponential equation. The pre-exponential factor approached first order vis-a-vis OH- concentration and the activation energy suggested that scorodite dissolution is chemically controlled. A thermodynamic investigation indicated that the stability field of scorodite extends up to pH 6.75 for a molality of the aqueous species of 6.7x10 -5 (5 mg/L As) when in equilibrium with ferrihydrite but only up to pH 2.3 when in equilibrium with goethite. The presence of gypsum led to lower arsenic concentrations; this was attributed to the formation of calcium-iron-arsenate compounds, such as yukonite (Ca2Fe3(AsO4) 4(OH)·12H2O).