Bettina M. Voelker

Iron reduction by photoproduced superoxide in seawater

Abstract The reaction of dissolved Fe(III) with photochemically produced Superoxide radical (O 2 − ) was examined as a potentially important source of Fe(II) in sunlit seawater. The relative rates of Fe(III) reduction and Fe(II) oxidation by O 2 − were determined in the presence of 0.7 M Nad and high concentrations of O 2 − . At pH values above 5.5, most of the dissolved iron was present as Fe(II) after steady state was reached. [Colloidal Fe(III) oxyhydroxides did not react with Superoxide radical at appreciable rates.] The effect of organic and inorganic complexation on the relative rates of reactions of Cu(I) and Cu(II) with O 2 − also was examined. Using these results and previously published O 2 − flux measurements in sunlit open-ocean surface water, we calculate that, despite possible competition for O 2 − by copper, the steady-state concentration of O 2 − is high enough to result in significant concentrations of Fe(II). Our calculations indicate that, in the absence of organic complexation of Fe(III), 30–75% of the dissolved iron in the photic zone will be present as Fe(II) during daytime. Consistent with this hypothesis, illumination by simulated sunlight of open-ocean water samples (acidified to pH 7.3), to which 5 nM iron had been added, resulted in the conversion of approximately 60% of the dissolved iron into Fe(II) after 20 min.

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 6×10(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 O2- was somewhat lower, 1.6×10(6) 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.

Use of H218O2 To Measure Absolute Rates of Dark H2O2 Production in Freshwater Systems

Photochemical production is usually considered to be the main source of H2O2 in freshwater systems; here we show that significant dark production also occurs. We used isotope-labeled H2O2 as a tracer to simultaneously determine H2O2 production and decay rates in incubations of unfiltered water samples. Our new technique for H2(18)O2 analysis, requiring only small sample volumes and simple field equipment, allows for preservation of samples in remote locations, followed by gas chromatography mass spectrometry (GCMS) analysis up to six days later. Dark H2O2 production rates of 29-122 nM/h were observed in several lakewater samples. Measured production and decay rates were consistent with pseudo steady-state, early morning [H2O2] measurements made in each water body. Dark H2O2 production is likely to be more important than photochemical production for the total H2O2 budget over 24 h in the freshwater systems we examined. Our results imply that processes usually assumed to be photochemically induced in freshwaters, such as metal redox cycling mediated by H2O2 and O2(-), and production of strong oxidants from the reaction of H2O2 with Fe(II) (Fentons reaction) could also be occurring at significant rates in the absence of light.