Influence of pH on the reductive transformation of birnessite by aqueous Mn(II).

We investigated the effect of pH (5.5-8.5) on the mineralogical transformation of hexagonal birnessite induced by reaction with aqueous Mn(II) (50-2200 μM), using batch sorption experiments, X-ray diffraction analyses, X-ray absorption and infrared spectroscopic measurements. Samples reacted at pH < 7.0 exhibited disrupted stacking of birnessite sheets, but no mineralogical transformation products were observed. At pH 7.0 and 7.5, reaction with Mn(II) under anoxic conditions caused reductive transformation of birnessite into manganite (γ-MnOOH), whereas at pH 8.0 and 8.5, conversion into hausmannite (Mn3O4) occurred. Feitknechtite (β-MnOOH) is a major transformation product at low Mn(II) inputs at pH 7.0-8.5, and represents a metastable reaction intermediate that is converted into manganite and possibly hausmannite during further reaction with Mn(II). Thermodynamic calculations suggest that conversion into hausmannite at alkaline pH reflects a kinetic effect where rapid hausmannite precipitation prevents formation of thermodynamically more favorable manganite. In oxic systems, feitknechtite formation due to surface catalyzed oxidation of Mn(II) by O2 increases Mn(II) removal relative to anoxic systems at pH ≥ 7. The results of this study suggest that aqueous Mn(II) is an important control on the mineralogy and reactivity of natural Mn-oxides, particularly in aqueous geochemical environments with neutral to alkaline pH values.