Coagulation/sedimentation of submicron iron particles in a eutrophic lake

In situ measurements of the coagulation/sedimentation of natural submicron iron particles in the oxic-anoxic interface of a eutrophic lake are described. Such measurements allow to understand the scavenging role of iron oxyhydroxide particles and give an insight on the role of organic matter on the coagulation of submicron colloids. A special sedimentation tube has been designed and built for this purpose. Experimental conditions have been determined to avoid artifacts during sedimentation measurements. An order of magnitude for the sedimentation rate of iron particles in lakes has been determined. Theoretical predictions have been made based on a classical coagulation/sedimentation model where the kinetics of coagulation is described by Smoluchowskiʹs equations and sedimentation is characterized by Stokesʹ law. The nature and morphology of submicron colloid aggregates, in particular of those containing lake-born iron oxyhydroxides, have been investigated by Transmission Electron Microscopy in conjunction with Energy Dispersive Spectroscopy. Experimental sedimentation results are discussed and compared both to model predictions and Transmission Electron Microscopy observations. Both sedimentation rates and Transmission Electron Microscopy results emphasize the important role of organic matter present in lakewaters, particularly of non fulvic organic macromolecules, on the coagulation/sedimentation processes of iron particles.