Limnol. Oceanogr., 44(7), 1999, 1715–1729

The role of the composition of surface-coating materials in controlling trace metal adsorption in aquatic environments was investigated using natural biofilms that developed on glass slides in three New York State lakes and a water-supply well. Adsorption isotherms were obtained for Pb binding to each of the biofilms in solutions with defined Pb speciation at 258C and pH 6.0, with Pb concentrations ranging from 0.2 to 2.0 mM. Adsorption isotherms for Pb binding to laboratory-derived metal oxides and surrogate organic materials were determined under the same conditions. These isotherms, combined with characterization of natural biofilm composition, were used to estimate the relative contributions of the organic and metal oxide surface-coating constituents by assuming additivity of adsorption to discrete adsorbing phases. Cells of a diatom (Navicula peliculosa), a green alga (Chlorella vulgaris), the bacterium Leptothrix discophora, and extracellular polymer of the bacterium Burkholdaria cepacia were tested as laboratory analogs for the organic phase of the biofilms. Amorphous Fe oxyhydroxide, gAl2O3, and a laboratoryderived biogenic Mn oxyhydroxide were used as laboratory surrogates for biofilm minerals. The sum total of predicted Pb binding to the defined surrogates accounted for at least 90% of the total observed Pb binding in the three lake biofilms and 60% of that observed for the well biofilms. For the lake biofilms, Pb adsorption to Fe and Mn oxides was significantly greater than that estimated for organic materials. The use of biogenic Mn oxide as a model component resulted in an estimated Pb adsorption to Mn oxyhydroxides in the lake biofilms up to four times greater than that estimated for Fe oxyhydroxide. Estimated Pb binding by Al oxide was negligible for all four biofilms. These results suggest that Fe and biogenic Mn oxides exert the greatest influence on Pb adsorption in oxic freshwater environments at pH 6.0. The cycling of transition metals in aquatic environments is controlled by adsorptive scavenging, which in turn is controlled by the composition of the aqueous phase and the reactive organic and inorganic components at surfaces (Krauskopf 1956; Jenne 1968; Turekian 1977; Vuceta and Morgan 1978; Shlokovitz and Copland 1982; Murray 1987). The presence of surface coatings such as adherent microorganisms and oxide mineral deposits, the presence of dissolved and adsorbed ligands, the concentration and speciation of the trace metal in the aqueous phase, the concentration of competing trace metals, and solution variables such as pH and salinity all can influence metal adsorption (Muller and Sigg 1990; Giusti et al. 1993). Much progress has been made recently in developing models to 1 Corresponding author.