Electron-optical characterization of nano- and micro-particles in raw and treated waters: an overview.

State-of-the-art information is presented on the analysis, by transmission electron microscopy (TEM), of aquatic colloidal particles in the size range of 3 to 500 nm least dimension, with a focus on nanoparticles (1-100 nm). Case studies include selections from both natural waters and waters undergoing treatment. The "species" of nano-particles receiving the greatest attention are: humic substances, polysaccharide fibrils, hydrous iron oxides, viruses, clay minerals, refractory cell debris, and heavy metal agglomerates on biological surfaces. Artifacts and how to both detect and minimize them are outlined. Correlative use of TEM with other imaging techniques is emphasized, along with associated spectroscopy. Noted is the potential of computerized image analysis for quantifying colloids on a "per colloid species" basis, using water samples centrifuged onto electron microscope grids.

Analysis of environmental particles by atomic force microscopy, scanning and transmission electron microscopy.

Due to their large specific surface and their abundance, micro and nano particles play an important role in the transport of micropollutants in the environment. Natural particles are usually composed of a mixture of inorganic amorphous or crystalline material (mainly FeOOH, Fe(x)Oy, Mn(x)Oy and clays) and organic material (humics and polysaccharides). They all tend to occur as very small particles (1-1,000 nm in diameter). Most natural amorphous particles are unstable and tend to transform with time towards more crystalline forms, either by aging or possibly, by dissolution and re-crystallization. Such transformations affect the fate of sorbed micropollutants and the scavenging properties are therefore changed. As these entities are sensitive to dehydration (aggregation, changes in the morphology), it is highly important to observe their morphology in their natural environment and understand their composition at the scale of the individual particles. Also for the understanding and optimization of water treatment technologies, the knowledge of the occurrence and behavior of nano-particles is of high importance. Some of the possible particle analysis methods are presented: aggregation processes, biomineralization, bacterial adhesion, biofilms in freshwaters, ferrihydrite as heavy metals remover from storm water. These examples demonstrate the capabilities and focus of the microscopes. Atomic Force Microscopy (AFM) allows to analyze the particles in their own environment, meaning in air or in the water. Thus, native aspects of particles can be observed. As well, forces of interactions between particles or between particles and other surfaces such as membranes will be highly valuable data. Scanning Electron Microscopy (SEM) and for higher lateral resolution, Transmission Electron Microscopy (TEM) allow measurement of the morphology and composition. Especially, TEM coupled with Electron Energy Loss Spectroscopy (TEM-EELS) is a powerful technique for elemental analysis. Finally, general guidelines for the effective use of microscopic techniques are provided.

Analytical electron microscopy and focused ion beam: complementary tool for the imaging of copper sorption onto iron oxide aggregates.

Nanometre-scale electron spectroscopic imaging has been applied to characterize the operation of a copper filtration plant in environmental science. Copper washed off from roofs and roads is considered to be a major contributor to diffuse copper pollution of urban environments. A special adsorber system has been suggested to control the diffusion of copper fluxes by retaining Cu with a granulated iron hydroxide. The adsorber was tested over an 18-month period on facade runoff. The concentrations range of Cu in the runoff water was measured between 10 and 1000 p.p.m. and could be reduced by between 96% and 99% in the adsorption ditch. Before the analysis of the adsorber, the suspended material from the inflow was ultracentrifuged onto TEM grids and analysed by energy-filtered transmission electron microscopy (EFTEM). Copper was found either as small precipitates 5-20 nm in size or adsorbed onto organic and inorganic particles. This Cu represents approximately 30% of the total dissolved Cu, measured by atomic emission spectrometry. To locate where the copper sorption takes place within the adsorber, the granulated iron oxide was analysed by analytical electron microscopy after exposure to the roof run-off water. A section of the granulated iron hydroxide was prepared by focused ion beam milling. The thickness of the lamina was reduced to 100 nm and analysed by EFTEM. The combination of these two techniques allowed us to observe the diffusion of Cu into the aggregate of Fe. Elemental maps of Fe and Cu revealed that copper was not only present at the surface of the granules but was also sorbed onto the fine particles inside the adsorber.

Analytical electron microscopy as a tool for accessing colloid formation process in natural waters.

Analytical electron microscopy was used to characterize aquatic iron-rich colloids. We focused our attention on a redox transition medium in the drainage water of a peat soil. In the anoxic peat water, observations by transmission electron microscopy and associated energy dispersive analyses (TEM-EDS) highlight the presence of spherical entities (approximately 100-600 nm), containing only traces of iron. The increase of dissolved oxygen concentration favours the formation of iron oxy(hydr)oxides. In the oxygenated drain, particles with the same morphology and size range are present. Statistical TEM-EDS analyses show that they represent the only colloidal form of iron in the drain samples. Nevertheless, although Fe-K peaks appear clearly on EDS spectra, the proportion of iron in these colloids reaches at most 4% at. (whereas C + O > 90% at.). Structural information completes this study. Both electron spectroscopic imaging (ESI) and electron energy-loss spectroscopy (EELS) reveal the disparity between element distributions within the drain entities. Iron and calcium are preferably distributed on the outer sphere of the particle, whereas carbon and oxygen follow the theoretical variation of the signal intensity within a plain sphere. The implication of organic matter as nucleation site for iron precipitation is spectacularly demonstrated by the presence of nanometre-sized iron-rich phases highlighted by EELS line scans.