The incongruent dissolution of scorodite - Solubility, kinetics and mechanism

This work reports the results of a laboratory investigation on the long-term stability of crystalline scorodite conducted at fixed pH (5-9) and temperature (22 degrees C, 50 degrees C and 75 degrees C). The scorodite used in this work was prepared via a hydrothermal synthesis procedure. The dissolution of scorodite at 22 degrees C was found to be extremely slow. At neutral pH, the arsenic concentration stabilized after 24 weeks at 5.9 mg/L. Analysis of the solubility data as a function of temperature yielded a. scorodite solubility model equation. The solubility product of scorodite was recalculated as 10(-25.4) using the solubility data generated by the study and the geochemical code PHREEQC for solution modelling. As scorodite dissolved, the iron re-precipitated as 2-line ferrihydrite. The growth and re-crystallization of ferrihydrite was apparently retarded by arsenate adsorption. The dissolution rate of scorodite was modeled with a decreasing exponential equation. The initial rate approached first order dependency on OH- concentration while the apparent activation energy suggested that scorodite dissolution is chemically controlled. (c) 2007 Elsevier B.V. All rights reserved.