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 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(11) with O-2(-) of 6 x 10(6) to 1 x 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 x 10(6) M-1. s(-1). With higher initial [O-2(-)], in excess of added [Mn(11)], 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 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.