Photochemistry of Cu complexed with chromophoric dissolved organic matter: implications for Cu speciation in rainwater |
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| Authors: | Melanie Louise Inez Witt Stephen Skrabal Robert Kieber Joan Willey |
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| Institution: | (1) Department of Chemistry and Biochemistry, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403, USA;(2) Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK |
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| Abstract: | Significant quenching of fluorescence by Cu in rainwater samples from southeastern North Carolina demonstrates that chromophoric
dissolved organic matter (CDOM) is an effective ligand for Cu in rainwater. A strong inverse correlation between the decrease
in fluorescence upon Cu addition and CDOM abundance suggests the presence of excess binding sites for Cu in high CDOM samples.
Electroanalytical studies indicate that CDOM extracted from C18 cartridges formed Cu complexes with concentrations and conditional stability constants similar to ligands found in ambient
rainwater. When authentic rainwater samples were photolyzed with simulated sunlight both photoproduction and photodestruction
of ligands were observed, suggesting the photochemical response of Cu-complexing ligands in rainwater is the result of two
competing reactions. The rate of CDOM photobleaching was directly related to changes in strong ligands (KCuL ∼ 1015) whereas weaker ligands (KCuL < 1013) were not correlated, suggesting the photolabile CDOM resides in the strong ligand class. A photolysis study comparing filtered
and unfiltered rainwater samples indicated that Cu-complexing ligands adsorbed onto or otherwise associated with particles
are photodegraded much more rapidly than dissolved ligands. Photolysis with UV radiation appears to be most effective at engendering
changes in Cu ligands, however a significant photochemical response was also observed when samples were exposed to photosynthetically
active radiation with wavelengths greater than 400 nm. Results from this study demonstrate that complexation of Cu by CDOM
has important ramifications for controlling both the speciation of the metal and the reactivity of CDOM in rainwater. |
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| Keywords: | Atmosphere Chromophoric dissolved organic matter Copper Metals Photochemistry Precipitation Speciation |
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