Environ. Sci. Technol. 2000, 34, 1036-1042
Chemistry of Superoxide Radical in Seawater: Reactions with Organic Cu Complexes B E T T I N A M . V O E L K E R , * ,† DAVID L. SEDLAK,‡ AND OLIVER C. ZAFIRIOU† Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, and Department of Civil and Environmental Engineering, University of California, Berkeley, California 94720
Organically complexed Cu can be a significant sink of photoproduced superoxide (O2-) in seawater. Using pulse radiolysis, we examined the rate constant of catalytic dismutation (kcat) of O2- in the presence of Cu complexed by strong (L1) and weaker (L2) organic ligands present in coastal seawater samples and produced by cultures of Synechococcus sp. In the coastal samples examined, CuL2 complexes were almost as reactive towards O2- as inorganically complexed Cu species (kcat ) 2.9-8.1 × 108 M-1 s-1). Furthermore, kcat was invariant with added Cu (560 nM), implying that the added Cu formed a single type of Cu complex or a mixture of complexes with very similar O2- reactivities. Significant differences between kcat values of estuarine and coastal samples were observed, suggesting an effect of organic ligands from terrestrial sources. CuL1 complexes produced by Synechococcus sp. were found to be less reactive (kcat < 1 × 108 M-1 s-1). The high reactivity of CuL2 complexes decreases estimates of steady-state concentrations of O2- in sunlit marine waters by at least a factor of 10.
Introduction Superoxide radical (O2-), a major product of the photooxidation of colored dissolved organic matter (CDOM) in sunlit surface waters (1, 2), may be an important participant in the redox cycling of metal ions. Inorganically complexed Cu and Fe are known to react rapidly with O2-. A fast photoredox cycle of these metals is often observed in natural waters and attributed to reduction of organic complexes of Cu(II) and Fe(III) by photo-induced ligand to metal charge-transfer reactions and subsequent reoxidation of inorganically complexed Cu(I) and Fe(II) by O2 and/or H2O2. However, at sufficiently high O2- steady-state concentrations, both reduction and oxidation of the metals by O2- could become dominant redox processes (3, 4). In the case of other metals whose redox transformations occur on a much slower time scale (e.g., Mn, Co, or Hg), even slow reactions with low steady-state concentrations of O2- could be significant processes. * Corresponding author present address: Room 48-419, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. E-mail:
[email protected]; phone: (617)253-3726; fax: (617)-258-8850. † Woods Hole Oceanographic Institution. ‡ University of Berkeley. 1036
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 6, 2000
To be able to assess the role of superoxide in metal redox processes, we must understand the factors controlling its steady-state concentration ([O2-]ss), given by the balance of its rates of production and destruction. While direct measurements of O2- photoproduction rates have been made in only one study of Caribbean waters (1), studies of H2O2 photoformation rates are abundant and provide a reasonable estimate of this parameter in a variety of sunlit waters (5). The reactions controlling the rate of destruction of O2- in natural waters have not yet been identified. While a number of sink reactions, all one-electron transfer processes, have been hypothesized in marine waters (2, 6, 7), so far only the rates of two have been quantified: bimolecular dismutation (i.e., the reaction of O2- with its conjugate acid HO2) and reactions with the inorganically complexed Cu species present in seawater [Cu(II)-carbonato and Cu(I)-chloro complexes] (5, 8). Although most of the Cu (generally >99%) in coastal and open waters is complexed by organic ligands, our previous study (8) found that the small inorganically complexed Cu fraction is a more important sink of O2- than bimolecular dismutation in most marine waters. Our results also showed that up to 25% of inorganically complexed Cu would be maintained in the reduced state by [O2-]ss exceeding 10-12 M. Organically complexed Cu species that react rapidly with O2- are likely to be a far more important O2- sink; in addition, these reactions could represent a significant photosource of Cu(I). The purpose of this study is to assess the reactions of organically complexed Cu species as sinks of O2- and sources of Cu(I) in seawater. Sink reactions of O2- with a redoxcycling component of dissolved organic matter are discussed in a companion paper (9). We examined the ability of organically complexed Cu species present in coastal seawater samples and in Synechococcus culture media to act as catalytic O2- sinks by relating O2- decay rates to Cu concentration and speciation. O2- was produced non-photochemically using pulse radiolysis, and O2- decay rates were observed directly using kinetic spectrophotometry. Cu concentration and speciation were measured using cathodic stripping voltammetry. Cu(I) formation was observed in solutions exposed to a constant flux of O2- produced by continuous radiolysis.
Methods Samples. Coastal seawater samples for pulse radiolysis were collected during May, July, and October 1995 from three sites in Cape Cod, MA: Vineyard Sound, Eel Pond, and Waquoit Bay. Vineyard Sound is the body of water between Cape Cod and the island of Martha’s Vineyard. Eel Pond is a small tidal pond receiving little freshwater input and open to Vineyard Sound. Waquoit Bay is a shallow bay with tidal flushing into Vineyard Sound; it receives freshwater inputs from both groundwater and the Childs River. Sampling sites are described in further detail, including a map and extensive Cu speciation measurements, in Moffett et al. (10). Samples of Eel Pond water and Vineyard Sound seawater (VSSW) were collected from docks in Woods Hole, MA, using a pole sampler (10). Salinity of Eel Pond samples did not differ significantly from those of Vineyard Sound (30.6 ppt). Samples were collected by pole sampler from a small aluminum boat at three sites in Waquoit Bay: near the middle of the bay (“bay”, salinity 27.9 ppt), close to the outlet of a salt marsh (“marsh”, salinity 28.7 ppt), and near the mouth of the Childs River (“river”, salinity 25.4 ppt). All samples were filtered (0.2 µm, Nuclepore) within 1 day of collection and stored in acid-washed Teflon bottles in the dark at 4 °C 10.1021/es990545x CCC: $19.00
2000 American Chemical Society Published on Web 03/13/2000
until use (no longer than 1 month). A sample of Sargasso seawater, collected using trace metal clean techniques, was obtained from James Moffett. Samples of Synechococcus sp. exudate were obtained from Larry Brand, who prepared the cultures according to the techniques described in Moffett and Brand (11). Pulse Radiolysis-Kinetic Spectrophotometry. Pulse radiolysis experiments were conducted at the Center for Fast Kinetics Research (CFKR) at the University of Texas in Austin. The general method for performing pulse radiolysis in seawater has been described in detail previously (5, 8). Briefly, a pulse of primary radicals (e-aq, H•, HO•) was generated in a flow-through quartz cell by radiolysis of water. Methanol (HPLC grade, 0.3 M) was used to convert HO• to O2- within ∼10 µs; the other radicals were converted to O2- on similar time scales by reaction with O2. Pulses generated 1-10 µM O2-. Experiments were performed by monitoring the kinetics of [O2-] decay as a function of Cu(II) added to the water samples. [O2-] was measured spectrophotometrically at 230 or 250 nm. The baseline absorbance before the pulse and after the complete decay of O2- did not differ significantly, indicating that there was no interference due to radiolysisinduced changes in other light-absorbing substances in the water samples. At the low Cu concentrations used in these experiments (
