Strong binding of copper, zinc, and lead to colloids and natural organic matter in rivers

The toxicity and bioavailability of trace metals in aquatic systems are controlled by the speciation of the metal through binding of the metal to organic and inorganic components of the system, including aqueous solutes, natural organic matter (NOM), colloids, and particles. Size speciation of three metals (Cu, Zn, and Pb) in four freshwater rivers was accomplished using tangential-flow ultrafiltration. Chemical speciation (determination of metal-binding ligand concentration and strength) in each size fraction was then determined using voltammetric techniques (both anodic and cathodic stripping voltammetry, or ASV and CSV) and chelating resin column partitioning.; Ultrafiltration using spiral-wound membrane cartridges was assessed for use in trace metal clean situations. Ultrafiltered solutions were used to demonstrate that exchangeable binding of trace metals to natural colloids (those which pass a 0.4-μm filter) can explain the observed decrease in the particle partitioning constant K_D with increasing colloid concentration (the “particle concentration effect.”); Voltammetric techniques measured the concentration and strength of organic metal-binding ligands in several size fractions in four upper-Midwest rivers. In all systems, strong-binding ligand (determined by CSV, log K′ > 12) was present in large excess compared to the trace metals, indicating that, in an equilibrium system, almost all (99%+) of the metal is bound to strong ligands. For all metals and sites, the complexation capacity of the NOM was greater in the colloidal size fractions than in the ultrafiltered (“dissolved”) size fraction, but the dissolved ligands were stronger. Measured ligand parameters and an equilibrium model were able to correctly predict the size fractionation of the metals. Organic ligands represented only a small fraction (generally <0.1%) of the total organic carbon, and the quality of the NOM, with respect to metal binding ligands, differed in the four rivers. Experiments using natural high iron colloid solutions showed that inorganic colloids are also significant metal-binding phases.