Concentration of Mn and separation from Fe in sediments—I. Kinetics and stoichiometry of the reaction between birnessite and dissolved Fe(II) at 10°C

Abstract

Redox reactions between Fe 2+ in solution and Mn-oxides are proposed as a mechanism for concentration of Mn in sediments both during weathering and diagenesis in marine sediments, e.g. the formation of Mn-nodules. If such a mechanism is to be effective, then reaction rates between Fe 2+ and Mn-oxides should be fast. The kinetics and stoichiometry of the reaction between dissolved Fe 2+ and synthetically prepared birnessite (Mn 7O 13·5H 2O) were studied experimentally in the pH range 3–6. Results show a stoichiometry which at pH < 4 conforms to a simple reaction between Fe 2+ and birnessite, releasing Mn 2+ and Fe 3+ to the solution. At pH > 4 FeOOH is precipitated and excess Fe 2+ consumption compared to the theoretical stoichiometry is observed. The excess Fe 2+ consumption is not due to a formation of a quantitative MnOOH layer but rather to adsorption. Reaction kinetics are very fast at pH < 4 and change at pH 4 to a slower mechanism. At pH > 4 the reaction is fast initially until 17% of the bimessite has dissolved and changes then to a slower stage. The later stage can be described by the equation: J = km 0(H +) −0.45[Fe 2+] γ( m m 0 ) β where J is the overall rate of Mn 2+ release, m 0 and m the mass of birnessite at time t = 0 and t > 0, β = 6.76−0.94 pH and γ has values of 0.76 at pH 5 and 0.39 at pH 6. The rate constant k is 7.2·10 −7 moles s −1 g −1 (moles/1) −0.31 at pH 5 and 9.6·10 −8 moles s −1 g −1 (moles/1) 0.06 at pH 6. Diffusion calculations show that the rate is controlled by surface reaction and it is tentatively proposed that the availability of vacancies in octahedral [MnO 6]sheets of the birnessite surface could be rate controlling. It is concluded that reactions between Fe(II) and birnessite, and probably other Mn-oxides, are fast enough to be important in natural environments at the earth surface.