The complexation of ‘dissolvedʹ 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 these molecules has remained a mystery. We used a combination of physical and chemical separation schemes to better characterize organic molecules complexing Cu, Zn, Cd, and Pb in Narragansett Bay, RI. Conventionally filtered (<0.2 μm) surface water samples were partitioned by cross-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 have markedly different size distributions. The majority ( 90%) of chelated Zn and Cd, along with their respective unbound ligands, resided in the operationally-defined soluble (<1 kDa) phase. In contrast, 50% of the chelated Cu was colloidal (>1 kDa), with the bulk of it found in the 1–8 kDa colloidal size range. Of the three Cu-binding ligand classes measured, the strongest class occurred mainly in the soluble fraction while the weaker Cu-binding ligand classes were predominantly colloidal (>1 kDa). Approximately 40% of chelated Pb was colloidal but, in contrast to Cu, the bulk of these ligands resided in the larger colloidal size range (8 kDa–0.2 μm). Thus, a continuum of metal complexing ligand size exists, spanning from truly soluble to colloidal, the nature of which differs for individual bioactive metals. These findings support the hypothesis that metal complexation in 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, thereby decreasing metal sorption to particulates, or to facilitate metal removal by sweeping organically bound metals into particulate phases via colloid aggregation.