The effects of pH and surface composition of Pb adsorption to natural freshwater biofilms

Two dominant variables that control the adsorption of toxic trace metals to suspended particulate materials and aquatic surface coatings are surface c omposition and solution pH. A model for the pH-dependent adsorption of Pb t o heterogeneous particulate surface mixtures was derived from experimental evaluation of Pb adsorption to laboratory-derived surrogates. The surrogate materials were selected to represent natural reactive surface components. Pb adsorption to both the laboratory surrogates and natural biofilms was de termined in chemically defined solutions under controlled laboratory condit ions. Pb adsorption was measured over a pH range of 5-8, with an initial Pb concentration in solution of 2.0 muM. The surface components considered in clude amorphous Fe oxide, biogenic Mn oxide produced by a Mn(ll) oxidizing bacterium (Leptothrix discophora SS-1), Al oxide, the common green alga Chl orella vulgaris, and Leptothrix discophora SS-1 cells. A linearization of P b adsorption da ta for each adsorbent was used to quantify the relationship between Pb adsorption and pH. The parameters for individual adsorbents wer e incorporated into an additive model to predict the total Pb adsorption in multiple-adsorbent natural surface coatings that were collected from Cayug a Lake, NY. Pb adsorption experiments on the natural surface coatings at va riable pH were utilized to verify the additive model predictions based on t he pH dependent behavior of the experimental laboratory surrogates. Observe d Pb adsorption is consistent with the model predictions (within 1-24%) ove r the range of solution pH values considered. The experimental results indi cate that the combination of Fe and biogenic Mn oxides can contribute as mu ch as 90% of Pb adsorbed on Cayuga Lake biofilms, with the dominant adsorbe nt switching from Mn to Fe oxide with increasing pH.