Rapid reaction of nanomolar Mn(II) with superoxide radical in seawater and simulated freshwater

Superoxide radical (O₂⁻) has been proposed to be an important participant in oxidation−reduction reactions of metal ions in natural waters. Here, we studied the reaction of nanomolar Mn(II) with O₂⁻ in seawater and simulated freshwater, using chemiluminescence detection of O₂⁻ to quantify the effect of Mn(II) on the decay kinetics of O₂⁻. With 3−24 nM added [Mn(II)] and <0.7 nM [O₂⁻], we observed effective second-order rate constants for the reaction of Mn(II) with O₂⁻ of 6 × 10⁶ to 1 × 10⁷ M⁻¹·s⁻¹ in various seawater samples. In simulated freshwater (pH 8.6), the effective rate constant of Mn(II) reaction with O₂⁻ was somewhat lower, 1.6 × 10⁶ M⁻¹·s⁻¹. With higher initial [O₂⁻], in excess of added [Mn(II)], catalytic decay of O₂⁻ by Mn was observed, implying that a Mn(II/III) redox cycle occurred. Our results show that reactions with nanomolar Mn(II) could be an important sink of O₂⁻ in natural waters. In addition, reaction of Mn(II) with superoxide could maintain a significant fraction of dissolved Mn in the +III oxidation state.