Trace metal interactions with microbial biofilms in natural and engineered systems

Substantial evidence is available indicating that microbial films coat most surfaces in natural aquatic environments. Biofilms are also commonly employed in many engineered wastewater treatment processes. Such films, which consist of attached bacteria and their exopolymers, have substantial metal‐binding properties and are likely to play an important role in the partitioning of toxic metals between surfaces and solutions. In this article, we present a review of findings on attachment and growth of bacterial films, with particular regard to the metal‐binding properties of microbial cell surfaces and extracellular polymeric organic compounds. The review focuses primarily upon direct interactions between transition metals and biofilms under oxic conditions. However, some secondary interactions are considered, such as cell‐ and polymer‐mediated precipitation of Fe or Mn hydrous oxides (which are also active metal scavengers). Other possible secondary interactions, such as those that might occur deep in biofilms where anoxic conditions and sulfate reduction may result in the precipitation of metal sulfides, are excluded in order to limit the scope of the discussion. We also review information on alterations in adsorptive behavior of inorganic surfaces which may result from the presence of biofilms and organic surface coatings of microbial origin. The adsorptive properties of such surfaces are relatively well understood in the absence of microbial modifications, but the properties of biofilm‐modified surfaces are poorly understood. Finally, we attempt to integrate some of the presently available modeling approaches which can be brought to bear on this complex problem.