Rapid reaction of nanomolar Mn(II) with superoxide radical in seawater and simulated freshwater
Superoxide radical (O 2 − ) 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 O 2 − in seawater and simulated freshwater, using chemiluminescence detection of O 2 − to quantify the effect of Mn(II) on the decay kinetics of O 2 − . With 3−24 nM added [Mn(II)] and <0.7 nM [O 2 − ], we observed effective second-order rate constants for the reaction of Mn(II) with O 2 − of 6 × 10 6 to 1 × 10 7 M −1 ·s −1 in various seawater samples. In simulated freshwater (pH 8.6), the effective rate constant of Mn(II) reaction with O 2 − was somewhat lower, 1.6 × 10 6 M −1 ·s −1 . With higher initial [O 2 − ], in excess of added [Mn(II)], catalytic decay of O 2 − 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 O 2 − in natural waters. In addition, reaction of Mn(II) with superoxide could maintain a significant fraction of dissolved Mn in the +III oxidation state.