Hexagonal birnessite, as the most common and reactive layered manganese (Mn(IV)) oxide, can reductively transform into other Mn mineral phases by reacting with Mn(II). The transformation affects the general environmental roles of Mn oxides. Solution chemistry strongly influences the transformation processes. In this study, we examined the Mn(II)-driven transformation of δ-MnO₂, a turbostratic naturally occurring hexagonal birnessite, in simulated seawater having high ionic strength and containing a suite of alkali (Na and K) and alkaline earth metal cations (Ca and Mg) with Mn(II)/MnO₂ molar ratios of 0.07 – 3.83 at pH 8.2. Results show that the transformation of δ-MnO₂ to triclinic birnessite and 4 × 4 tunneled Mn oxide, as observed previously in NaCl solutions, is highly inhibited in seawater at Mn(II)/MnO₂ ratios of 0.07 and 0.11. Delta-MnO₂ transforms to Mn₃O₄, γ-MnOOH, and δ-MnOOH mineral phases with small amounts of α- and β-MnOOH phases in NaCl solutions at Mn(II)/MnO₂ ratios of 0.50 – 3.83. Increasing ionic strength controlled by NaCl and KCl increases the transformation rate and enhances the crystallinity of Mn₃O₄, γ-MnOOH, and δ-MnOOH. However, the presence of Mg and Ca cations in seawater substantially slows down the transformation and decreases the crystallinity of Mn₃O₄ and MnOOH particles. The inhibition of seawater on the transformation of δ-MnO₂ results presumably from the stabilization of Mg and Ca on δ-MnO₂. This study provides fundamental understanding of the genesis and redox-driven transformation of Mn oxides in oceanic environments.

Kevin Beyer

Appearances