Quantifying the Structure and Composition of Flocculated Suspended Particulate Matter Using Focused Ion Beam Nanotomography.

Suspended particulate matter (SPM) is present in the natural aquatic environment as loosely bound aggregates or "flocs" and is responsible for the transport and fate of sediment, carbon, nutrients, pollutants, pathogens and manufactured nanoparticles from catchment to coast. Accurate prediction of SPM hydrodynamics requires the quantification of 3D floc properties (size, shape, density and porosity) that span several spatial scales. Yet, current techniques (video camera systems, optical microscopy and transmission electron microscopy, TEM) can only provide 2D simplifications of size and shape with a spatial resolution gap between the "gross" (>100s μm) and nanoscale (<1 μm). Here, we translate 3D-microscopy techniques (focused ion beam nanotomography, FIB-nt) typically used in the biomedical sciences to the study of natural flocculated SPM filling both this spatial and dimensional gap. Fragile 3D floc samples were successfully captured and stabilized, identifying five basic organic and inorganic floc components and quantifying porosity and bacteria numbers. This provides new 3D floc geometric data sets at the nanoscale that will be critical in the development of cohesive sediment transport models. Detailed compositional and structural information could provide novel insights into the association of pathogens and pollutants with SPM and their impact on aquatic life.