The reactivity of iron oxides in sediments: A kinetic approach

Abstract

A kinetic approach is presented that allows a quantitative description of the reactivity of ferric oxides for both synthetic polydisperse ferrihydrite 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 given. The rate of ferrihydrite dissolution was found to be proportional to the initial amount of ferrihydrite in solution, nearly independent of the ascorbic acid concentration within the range 5 to 15 mM, and strongly dependent on the mineral fraction dissolved. The reactivity of ferrihydrite could be described as J/m 0 = 4 · 10 −4 ( m/m 0) 1.1, where J is the overall rate of dissolution (mol/s), m 0 the initial amount of ferrihydrite, and m/m 0 the undissolved mineral fraction. Assemblages of ferric oxides in natural sediments can be described as a reactive continuum with a Gamma distribution of ferric oxide reactivity types. This leads to the same rate law as used to describe ferrihydrite dissolution, although the rate constant and exponential term now only have statistical significance. The reactivity of ferric oxides of an oxidized marine sediment from the Bight of Aarhus is then described by J/m 0 = 7.4 · 10 −3( m/m 0) 4.7, and J/m 0 = 5.3 · 10 −5 ( m/m 0) 2.75 was found for an oxidized aquifer sediment from the island of Rømø. Thus, initial reduction rates varied by more than two orders of magnitude, while the change in reactivity during dissolution of 90% of the ferric oxides present may be almost five orders of magnitude. Chemical extraction techniques for reactive iron cover over a wide range in reactivity.