| Abstract
| - Many studies have shown that Cu and other metals innatural waters are mostly bound by unidentified compoundsinterpreted to be strong ligands reversibly complexing agiven metal. However, commonly applied analytical techniquesare not capable of distinguishing strongly but reversiblycomplexed metal from metal bound in kinetically inertcompounds. In this work, we use a modified competitiveligand exchange adsorptive cathodic stripping voltammetrymethod combined with size fractionation to show thatmost if not all of the apparently very strongly (log K ≥ 13)bound Cu in samples from five New England coastalwaters (1−18 nM, 10−60% of total Cu) is actually presentas kinetically inert compounds. In three of the fivesamples examined by ultrafiltration, a significant portionof the 0.2-μm-filtrable inert Cu was retained by a 0.02-μm-pore size filter, suggesting that at least some of the Cuwas kinetically inert because it was physically sequesteredin colloidal material. The rest of the ambient Cu, and Cuadded in titrations, were reversibly bound in complexes thatcould be modeled as having conditional stability constantsof 1010−1013. The Cu-binding ability of these complexeswas equivalent to that of seawater containing reasonableconcentrations of humic substances from terrestrialsources, approximately 0.15−0.45 mg of C/L. Both theinert compounds and the reversible ligands were importantfor determining [Cu2+] at ambient Cu levels in our samples.
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