THE COMPLEXATION OF DISSOLVED CU, ZN, CD AND PB BY SOLUBLE AND COLLOIDAL ORGANIC-MATTER IN NARRAGANSETT BAY, RI

It is widely accepted that the speciation of most bioactive metals in seawater is regulated by natural organic ligands, but the nature of th ese molecules has remained a mystery. We used a combination of physica l and chemical separation schemes to better characterize organic molec ules complexing Cu, Zn, Cd, and Pb in Narragansett Bay, RI. Convention ally filtered (< 0.2 mu m) surface water samples were partitioned by c ross-flow ultrafiltration into < 8 kDa and < 1 kDa size fractions and the chemical speciation of metals was determined in each fraction. The results show that organic molecules which complex bioactive metals ha ve markedly different size distributions. The majority (similar to 90% ) of chelated Zn and Cd, along with their respective unbound ligands, resided in the operationally-defined soluble (< 1 kDa) phase. In contr ast, similar to 50% of the chelated Cu was colloidal (>1 kDa), with th e bulk of it found in the 1-8 kDa colloidal size range. Of the three C u-binding ligand classes measured, the strongest class occurred mainly in the soluble fraction while the weaker Cu-binding ligand classes we re predominantly colloidal (> 1 kDa). Approximately 40% of chelated Pb was colloidal but, in contrast to Cu, the bulk of these ligands resid ed in the larger colloidal size range (8 kDa-0.2 mu m). Thus, a contin uum of metal complexing ligand size exists, spanning from truly solubl e to colloidal, the nature of which differs for individual bioactive m etals. These findings support the hypothesis that metal complexation i n seawater is dominated by distinct, metal-specific ligand molecules. A central question that now emerges is whether these ligand molecules function predominantly to buffer metal ion activities in seawater, the reby decreasing metal sorption to particulates, or to facilitate metal removal by sweeping organically bound metals into particulate phases via colloid aggregation. (C) 1998 Elsevier Science B.V. All rights res erved.