Applications of novel 3D nano- and microtomography to explore the influence of sediment composition and settling history on floc structure

Within aquatic environments suspended particulate matter (SPM) is exposed to hydrodynamic and biogeochemical regimes that promote aggregation. The settling behaviour of resulting aggregates (flocs), is controlled by their physical properties (e.g. size, shape, density). Accurate quantification of floc properties is therefore critical for predicting the fate of SPM and associated contaminants and nutrients. However, floc properties vary both in time and space, depending on the characteristics of particles (e.g. clay minerals, bacteria, biopolymers), their depositional history and the environmental conditions (e.g. turbulence, salinity). Our understanding of how these factors influence floc structure remains limited due to our reliance on 2D imaging techniques that cannot provide accurate measurements of 3D geometries.

This study reports the first combined application of X-ray microtomography (μCT) and focused ion beam nanotomography (FIB-nt) to explore the environmental factors influencing floc structure. Synthetic flocs consisting of bentonite clay with varying organic matter (OM) concentrations (Xanthan gum, 0 - 5%) were generated and exposed to 2 cycles of settling, consolidation and resuspension, alongside two natural floc samples. Flocs were sampled and immobilised in agarose gel for μCT scanning at medium resolution (10 μm). This allowed the measurement of gross-scale floc properties (i.e. size, shape) for populations of flocs, and the identification of representative individuals for further analysis. High-resolution (50 nm) FIB-nt volumes were collected from selected floc samples to investigate internal floc structure, providing insights into the particle-particle and structural associations responsible for μm-scale floc geometries.

Preliminary results suggest that variability in size and shape within floc populations increases with greater OM content and longer consolidation periods, with a larger variety of internal structural interactions possible when facilitated by biopolymeric substances. Effects on mean gross-scale parameters appear to be non-linear. This study represents the first detailed investigation of the relation between 3D floc structure and environmental forces, and offers the potential for developing improved process-based flocculation models.