SURFACE-CONTROLLED FERRIHYDRITE DISSOLUTION KINETICS AND ASSOCIATED FE AND AS RELEASE

Because of its common association with sulfide minerals, arsenic is often a major contaminant in tailings and other mine wastes. Following its initial release to solution by sulfide oxidation, As is typically a pentavalent oxyanion associated with amorphous iron oxyhydroxides, especially ferrihydrite. Therefore, when mine-derived ferrihydrite dissolution takes place in acidic media such as gastric fluid or acid mine drainage, it may be accompanied by As release into the associated environment. This is a major source of human As exposure, and further study into coupled ferrihydrite and As dissolution kinetics may yield information useful for predicting As bioaccessibility. Previous studies (Stumm & Furrer, 1987) have found that proton-promoted Fe oxyhydroxide dissolution kinetics are typically controlled by the instantaneous reactive surface area of the solid because Fe detachment is the rate-determining step. When associated with amorphous Fe oxyhydroxides, As can be present as an adsorbed or coprecipitated species. Therefore, this experiment quantified the relationship between BET surface area and ferrihydrite dissolution kinetics in the presence of adsorbed or co-precipitated As.

Model solids with differing amounts of adsorbed and coprecipitated As(V), prepared according to methods outlined by Paige et al. (1997), were treated with buffered pH 1.8 solution in separate batch reactions. BET surface area was measured on freeze-dried material and assumed proportional to reactive surface area. Paige et al. (1997) found that ferrihydrite dissolution follows the cube root model (Schwertmann & Cornell 2003), represented by the equation

1 - (1-a)^1/3 = kt

where k is the rate constant and 1-α is the fraction of original solid material remaining so 1-α was expected to fit an x³ power law until dissolution was complete. Similarly, because the surface area and volume of a spherical particle are related by the expression, the instantaneous BET surface area was tested for an x² power regression line, predicted to control Fe and As release until dissolution was complete.

Paige et al. (1997) Water Res. 31: 2370-2382.

Schwertmann & Cornell. (2003) The Iron Oxides: Structure, Properties, Reactions, Occurences, and Uses. Wiley-VCH.

Stumm & Furrer (1987) Aquatic surface chemistry. New York: J. Wiley & Sons, p.197-219.