Modeling Oligotrophic Biofilm Formation and Lead Adsorption to Biofilm Components

Abstract

Mechanisms governing trace metal interactions with biogenic materials in oligotrophic freshwater aquatic environments were investigated with a laboratory biofilm reactor system using lead as a model compound. Use of controlled laboratory conditions, including a chemically defined medium and an axenic bacterial culture, permitted the development of a mechanistic model to describe lead distribution via the integration of biological models, lead adsorption isotherms, and a chemical speciation program. Concentrations of suspended and attached cells and their extracellular polymers were accurately modeled by defining the processes of growth, cell attachment, and polymer production. Specific extracellular polymer production rates were observed to be similar for suspended and attached cells, but the polymer produced by attached cells appeared to remain in the biofilm matrix, while the dissolved polymer produced by suspended cells washed out of the reactor. Extracellular polymer constituted up to 80% of the total biofilm organic material (based on chemical oxygen demand), and its specific binding capacity for lead was three times higher than that of bacterial cells. Thus, predicted lead binding to extracellular polymer overshadowed lead binding to cells in the biofilms. The integrated model overpredicted lead binding to biofilms by about 30%, indicating the possibility of some masking of available adsorption sites in the biofilm.