No abstract

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.

The redox speciation of dissolved iron in open ocean seawater was evaluated during two Pacific Ocean research cruises. Using a highly sensitive flow injection method based on luminol chemiluminescence, vertical profiles of reduced iron concentration (Fe(II)) were obtained at 134 stations. In this paper, sampling and analytical methods are discussed and values obtained for Fe(II) are compared to shipboard measurements of total dissolved iron (Fe DISS). Concentration profiles are evaluated within the context of various proposed source mechanisms and experimental models of Fe(II) oxidation kinetics. Samples were collected from rosette-mounted GO-FLO bottles using trace metal clean equipment and techniques. While this allowed sample collection to depths of 1000 m, the length of time required for rosette retrieval coupled with the potential for rapid oxidative loss of Fe(II) complicates the detection of photochemical production processes that are expected to be operative in the upper water column. Acidification of seawater samples retards oxidation until sample analyses can be completed, but for undetermined reasons it contributes both to the blank response, and to minor instabilities in system response over time that are depth-specific, effects which must be considered and corrected for. Analysis by luminol chemiluminescence is fast and inexpensive, and typically yielded detection limits of 10-15 pM. A requirement of the method is that Fe(II) present in samples be uniquely oxidized in the reaction cell at the same rate as the Fe(II) used for standard preparation. By changing reaction kinetics, strong organic complexes like EDTA can produce false negatives. Also, reduced species other than iron can be oxidized at the high system pH, leading to positive interferences. Vanadium (IV) and vanadium (V) were found to significantly interfere. Because Fe(II) is believed to form weak complexes in seawater, and reduced vanadium has not been detected in oxic seawater, interferences were believed to be minimal. The results from the two cruises suggest a relatively consistent pattern of Fe(II) occurrence and distribution in the Pacific Ocean. Surface water maxima are present in most profiles, with median concentrations of 25-30 pM, accounting for approximately 12% of the total dissolved iron. Concentrations decline with depth to undetectable levels in the upper 100 meters. Below this depth, Fe(II) remains low. However, deepest samples (700-1000 m) frequently contained detectable Fe(II), though at a very low percentage of the total dissolved iron. The concentration profile for the upper water column is consistent with a photochemical production xii mechanism. However, this production should cease upon sample collection, and the rapid oxidative loss predicted for surface waters should remove these higher Fe(II) concentrations prior to sub-sample collection from the GO-FLO bottles. Fe(II) in deep samples was found in association with the oxygen minimum of the profile, possibly due to the remineralization of sinking biogenic particles...

Dark H2O2 production rates were measured in samples collected in the Gulf of Alaska. We used a simple, novel method for determining absolute rates of dark production and decay of H2O2, both of which are occurring simultaneously (presumably as a result of biological activity) in unfiltered samples. [H2O2] vs. time was measured in 24‐h dark incubations of both unaltered samples and the same samples spiked with 100–250 nmol L−1 H2O2. Data were modeled with zero‐order H2O2 production rates and first‐order H2O2 decay coefficients as fitting parameters, with the assumption that addition of [H2O2] to a sample does not change either parameter. H2O2 production rates ranged from < 0.5 nmol L−1 h−1 to 8 nmol L−1 h−1, and generally decreased with depth and decreasing chlorophyll. Comparison of dark production with estimates of average photochemical H2O2 production rates in the top 50 m of the water column indicated that dark production is likely to be a significant source of H2O2. Indeed, many of the unaltered incubations indicated that in situ [H2O2] was close to a steady state between dark production and decay, especially in samples from depths of ≥ 10 m.