PhDThe hydrodynamic behaviour of fine suspended aqueous sediments, and stability of the
bedforms they create once settled, are governed by the physical properties (e.g., size, shape,
porosity and density) of the flocculated particles in suspension (flocs). Consequently,
accurate prediction of the transport and fate of sediments and of the nutrients and pollutants
they carry depends on our ability to characterise aqueous flocs. Current research primarily
focuses on characterising flocs based on their external gross-scale (>1 μm) properties (e.g.,
gross morphology, size and settling velocity) using in situ techniques such as photography
and videography. Whilst these techniques provide valuable information regarding the
outward behaviour of flocculated sediment (i.e. transport and settling), difficulties associated
with extracting 3D geometries from 2D projections raises concerns regarding their accuracy
and key parameters such as density can only be estimated. In addition, they neglect to inform
on the internal micro- and nano-scale structure of flocs, responsible for much of their
behaviour and development. Transmission electron microscope (TEM) and environmental
electron microscope may be used to obtain nano-scale information in, essentially, 2D but
there is a large scale gap between this information and the macro-scale of optical techniques.
To address this issue this study uses 3D tomographic imaging over a range of spatial
scales. Whilst commonly used in materials science and the life sciences, correlative
tomography has yet to be applied in the environmental sciences. Threading together 3D Xray
micro-computed tomography (X-ray μCT) and focused ion beam nano-tomography (FIBnt)
with 2D TEM makes material characterisation from the centimetre to nanometre-scale
possible. Here, this correlative imaging strategy is combined with a non-destructive
stabilisation procedure and applied to the investigation of flocculated estuarine sediment,
enabling the multi length-scale properties of flocs to be accurately described for the first time.
This work has demonstrated that delicate aqueous flocs can be successfully stabilised
via a resin embedding process and contrasted for both electron microscopy and X-ray
tomography imaging. The 3D information obtained can be correlated across all length-scales
from nm to mm revealing new information about the structure and morphology of flocs. A
new system of characterising floc structure can be defined based on the association of
particles and their stability in the structure rather than simply their size. This new model
refutes the postulate that floc structures are fractal in nature.Engineering and Physical Sciences Research Council (EPSRC)
Queen Mary University London (through the Post Graduate Research Fund)
Environment Canad
