THE REACTIVITY OF IRON-OXIDES IN SEDIMENTS - A KINETIC APPROACH

A kinetic approach is presented that allows a quantitative description of the reactivity of ferric oxides for both synthetic polydisperse fe rrihydrite and assemblages of ferric oxides found in natural sediments . Results from reductive dissolution experiments with ascorbic acid at pH 3, and ferrihydrite or ferric oxide contained in sediment are give n. The rate of ferrihydrite dissolution was found to be proportional t o the initial amount of ferrihydrite in solution, nearly independent o f the ascorbic acid concentration within the range 5 to 15 mM, and str ongly dependent on the mineral fraction dissolved. The reactivity of f errihydrite could be described as J/m(o) = 4.10(-4) (m/m(o))(1.1), whe re J is the overall rate of dissolution (mol/s), m(o) the initial amou nt of ferrihydrite, and mime the undissolved mineral fraction. Assembl ages of ferric oxides in natural sediments can be described as a react ive continuum with a Gamma distribution of ferric oxide reactivity typ es. This leads to the same rate law as used to describe ferrihydrite d issolution, although the rate constant and exponential term now only h ave statistical significance. The reactivity of ferric oxides of an ox idized marine sediment from the Bight of Aarhus is then described by J /m(o) = 7.4.10(-3)(m/m(o))(4.7), and J/m(o) = 5.3.10(-5)(m/m(o))(2.75) was found for an oxidized aquifer sediment from the island of Romo. T hus, initial reduction rates varied by more than two orders of magnitu de, while the change in reactivity during dissolution of 90% of the fe rric oxides present may be almost five orders of magnitude. Chemical e xtraction techniques for reactive iron cover over a wide range in reac tivity.