Partner und Internationale Organisationen
(Englisch)
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Netherlands Institute for Sea Research (NL), Institute of Water and Environment (DK), National Environmental Research Institute (DK), Foundation Alfred Wegener Institute for Polar and Marine Research (D), Goeteborg University (S), University of Liverpool (UK), University of Groningen (NL)
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Abstract
(Englisch)
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During the second COMET phase, we have assessed the following environmental effects on light-induced Cu(I) and Fe(II) formation in water samples from different study sites:1) Effect of salinity on light-induced Cu(I) formation; 2) effect of DOM (dissolved organic matter) size-fractionation on light-induced Fe(II) formation; and 3) effect of total dissolved iron on light-induced Fe(II) formation.
Research question 1. We assessed the effect of salinity on light-induced Cu(I) formation in water samples collected during the NAVICULA cruise (April 2001) at different study sites in the River Scheldt and its estuary, and in the coastal North Sea. Upon irradiation of filtered (0.2 mm) water samples with artificial sunlight, unexpectedly high Cu(I) steady-state concentrations were formed, increasing with increasing salinity: more than 80% of total dissolved Cu was present as Cu(I) (at steady-state) in water samples with 30 and 36‰ salinity. When the water samples with 8, 13, 30, and 36‰ salinity were kept in the dark, Cu(I) concentrations were below the detection limit under otherwise the same experimental conditions. In the non-irradiated water sample with 2.4‰ salinity, however, 40% of total dissolved copper was present as Cu(I), and irradiation had only a minor effect on Cu(I) formation. We interpret these results in terms of (i) stabilization of Cu(I) by Cl-, and (ii) thermal reduction of Cu(II) by ligands, e.g., thiols, present in the water sample with 2.4‰ salinity.
We also determined the concentration ratio of free to total dissolved copper, [Cu2+]/[Cu(II)aq], in water samples with different salinity, using ligand-exchange adsorptive cathodic stripping voltammetry. The ratio [Cu2+]/[Cu(II)aq] increased with increasing salinity, indicating that at low salinity ligands are present that form stable Cu(II) complexes. These results suggest that strong Cu(II) ligands play a significant role in the light-induced redox cycling of copper, either by enhancing Cu(I) oxidation by O2 and/or by decreasing the quantum yield of Cu(II)-ligand photolysis.
Ambient Cu(I) concentrations in the nanomolar range were determined with a new method (Bürge-Weirich and Sulzberger, 2003). This method is based on stabilizing Cu(I) by bathocuproine and masking Cu(II) with ethylenediamine. The novelty of our method is that the Cu(I)-bathocuproine complex is separated from the Cu(II)-ethylenediamine complex by solid-phase extraction.
Research questions 2 and 3. Ultrafiltration (with 1kD cutoff) of DOM yielding high- and low-molecular weigth (HMW and LMW, respectively) DOM was performed on deck during the PELAGIA cruise (May 2001) with water samples collected at two different depths at 6 stations (two in the Mediterranean, three near Gibraltar, and one in the North Sea). Upon irradiation of these DOM fractions with artificial sunlight, higher Fe(II) steady-state concentrations (normalized to DOC) were found in the LMW DOM fractions than in the HMW DOM fractions (Meunier and Sulzberger, 2003). This is in contrast to what we have observed in water samples from the River Tagliamento, suggesting that HMW and LMW DOM fractions exhibit different photochemical reactivity, depending on whether autochthonous or allochthonous DOM is present.
Near Gibraltar water samples were collected at 5 m depth and at depths between 150 and 350 m. The surface layer consisted of Atlantic water, and the deeper layers of Mediterranean water with much higher total iron concentrations (probably due to Fe input from Saharian dust). Consistently higher Fe(II) steady-state concentrations (normalized to DOC) were found in water samples from the Mediterranean layer than in water samples from the Atlantic layer, indicating that iron stemming from aeolian input is photochemical reactive.
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