We designed a new technique for in situ sampling of fine particulate matter (PM) including colloids in natural environments. The technique is based on a microlysimeter which is easy to move between field and laboratory and can be routinely used for in situ monitoring. The design of the device aims at limiting bias and artefacts encountered with current sampling methods based on successive field collection of liquid samples, transport, storage and filtration in the laboratory. Samples are directly collected on transmission electron microscopy (TEM) grids, thus totally eliminating the modifications classically due to handling of samples from collection to analysis. Detailed physical and chemical microscopic studies of individual particles can then be performed in order to fully characterize PM in natural media such as soil solutions, atmospheric fallout, rainfall and any aquatic system. This technique can be applied in a number of research fields, such as the characterization and determination of pollution vectors, the tracing of sources of emissions, or investigations on particle interactions.

Herminiimonas arsenicoxydans and Acinetobater calcoaceticus are natural bacteria isolated from As-contaminated environments. One noteworthy characteristic of the bacteria is their ability to oxidize arsenic. In order to investigate the interactions between those bacteria and arsenic we used transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDX) to observe the behavior of the bacteria treated with As[III]. Results showed that a substantial amount of the anion is fixed to the cell wall and into the extracellular polymeric substance (EPS). This sequestration appeared as a dense fibrillar net surrounding the cells. As a comparison, observation of a non As-oxidizing mutant of H. arsenicoxydans showed the same behavior meaning that the precipitation of As is not reliable to the As-oxidizing capacity of the bacteria. Moreover, we propose that such a precipitation into the EPS is due to a passive binding with protonated groups and possibly to a substitution between P and As. Finally, it appears that the tested bacteria are good candidates for As remediation whatever is the oxidation of As in the precipitate.