Mn(II)-induced phase transformation of Mn(IV) oxide in seawater

Manganese (Mn) oxides are key components of oceanic and lacustrine Mn nodules and influence metal cycling through oxidation and adsorption processes. Layered Mn oxides (LMOs) are the most common minerals in these nodules and the immediate products of microbially mediated Mn(II) oxidation by O₂. LMOs can transform into tunneled Mn oxides (TMOs), Mn oxyhydroxides (MnOOH), or Mn(II,III) phase (Mn₃O₄). LMOs often concur with Mn(II) in the environment and the adsorption and oxidation of Mn(II) by LMOs can greatly promote the transformation of LMOs to those phases. However, the Mn(II)-promoted transformation of LMOs in seawater—rich in various cations (300 mM Na⁺, 10 mM K⁺, 50 mM Ca²⁺, and 10 mM Mg²⁺) remains poorly understood. We examined the transformation of δ-MnO₂ in artificial seawater (pH 8.2) under anoxic conditions with the Mn(II)/MnO₂ ratio (r) ranging from 0.08 to 3.83. To assess the effect of ionic strength (IS), parallel experiments were conducted in a mixed 530 mM NaCl and 10 mM KCl solution (having seawater ionic strength but without Ca²⁺ and Mg²⁺) and in 100 mM NaCl solution as a control. At low r (0.08), δ-MnO₂ transformed into triclinic birnessite and a 4 × 4 TMO in 100 mM NaCl solution, which, however, was suppressed in seawater due to strong interactions of Ca²⁺/Mg²⁺ with δ-MnO₂. In the mixed 530 mM NaCl and 10 mM KCl solution (the same ionic strength as of seawater), the transformation occurred extensively but the products had lower crystallinity compared to in 100 mM NaCl solution. At the high Mn(II)/MnO₂ ratios (0.5 ≤ r ≤ 3.83), δ-MnO₂ transformed extensively into MnOOH phases and hausmannite (Mn₃O₄) in 100 mM NaCl solution. The seawater suppressed the transformation, but the suppression became weaker with increasing Mn(II)/MnO₂ ratio. For example, the transformation was completely suppressed at r = 0.5 but essentially negligible at r = 3.83. The suppression at these high Mn(II)/MnO₂ ratios was mainly ascribed to the influence of Ca²⁺ and Mg²⁺ rather than of the high IS, and the weaker suppression at the higher Mn(II)/MnO₂ ratio suggests stronger competition of Mn(II) with Ca²⁺/Mg²⁺ for interacting with δ-MnO₂. Moreover, the composition and crystallinity of the transformation products (i.e., the relative abundance of MnOOH (α, β, and γ) and Mn₃O₄) were influenced by both the high ionic strength and the presence of Ca²⁺ and Mg²⁺. Therefore, even though Ca²⁺ and Mg²⁺ concentrations are much lower than Na⁺ in seawater, their impacts on the transformation are dominant. Our study explains why MnOOH phases, hausmannite, and TMOs are less common than LMOs in oceanic environments, partially because seawater chemistry suppresses their formation. LMOs are the most reactive for metal adsorption and oxidation among all Mn oxides. Thus, the high stability of LMOs in an oceanic environment confers the high impacts of Mn oxides on metal cycling in the ocean.