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

Abstract

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.