Metal immobilisation by biofilms: Mechanisms and analytical tools

Abstract

In biofilm environments, heavy metal and radionuclide pollutants are removed by a variety of mechanisms, including biosorption, precipitation as sulfides or phosphates and microbial reductive precipitation. Even if the elemental composition and localization of the precipitate trapped in the biofilm is well described thanks to spectroscopic and microscopic techniques, this review highlights that little is known about metal immobilisation mechanisms in microbial biofilms, i.e., mass transfer of metals, mechanisms involved in (bio)sorption and precipitation and the influence of physicochemical micro-environments within the biofilm matrix. The review shows the advantage of using a combination of different techniques to evaluate the fate of metals within microbial biofilms. By combining a variety of techniques (e.g., selective extraction, microscopy, spectroscopy and miniaturised sensors ...), it is possible to gain high-resolution structural and chemical information of biofilms on a level of the individual cell. This approach will facilitate the characterization of the metal immobilisation sites and the metal sorption and (bio)crystallisation mechanisms in biofilms. The results provided by the combination of these techniques will allow to predict the amount of metal accumulation in biofilms as well as their chemical speciation. This review demonstrates that an interdisciplinary approach is required to study metal fate within the biofilm matrix.