3-dimensional floc structure and dynamics

Suspended particulate matter (SPM) plays a fundamental role in the impact, and eventual fate of sediment, pollutants, pathogens, nutrients and manufactured nanomaterials in aquatic environments. Suspended particulate matter is important to the aquatic ecosystem, transferring material from the catchment to coast, contributing significantly to the biogeochemical cycling of nutrients, and is the main vehicle for downward carbon flux in the ocean. Too much suspended sediment in the water column can be considered a pollutant; reducing water quality by increasing nutrient loads and reducing dissolved oxygen concentrations with impacts for ecological status and the UK's compliance with the EU Water Framework Directive. Once settled, fine sediment can reduce aquatic biodiversity, for example by smothering organisms and spawning gravels. Sediment can also block river channels and ports with impacts for navigation. For example, in the UK we dredge c. 40M tonnes of sediment every year costing millions £GBs. Consequently, in order to make evidence-based sediment management decisions, there is an urgent need to understand and make accurate predictions of SPM fate and transport in all aquatic environments.

Suspended particulate matter exists in aquatic systems as flocs. A floc is a very fragile, loosely bound aggregate of fine sediment particles, bacteria, organic matter and fluid-filled pore space. Flocs have very complex shapes and structures, and very low density. As a floc settles out of suspension it continually changes shape, size and structure as it breaks apart and reforms many times in the turbulent water. The delicate nature of these flocs and their size (a few microns to a few millimetres) means that they are very difficult to sample and analyse. Current analytical techniques e.g. optical or electron microscopy can either look at whole flocs, but with limited detail, or sub-micron sections of a floc, and they can only look at a 2-dimensional cross section. Therefore, many numerical models that predict fine sediment transport rely upon 2-dimensional simplifications of complex 3-dimensional shapes and structures, or have to use mathematical approaches to describe floc structure e.g. they assume flocs are fractal and that their structures are self-similar irrespective of scale. Therefore, our understanding of floc structure and behaviour is very limited and unless we can sample and analyse flocs at a range of spatial scales we can neither support nor challenge these mathematical assumptions.

We will address this research gap and sample, observe and quantify sediment floc micro-structure for the first time in 3-dimensions and at multiple, correlated spatial scales from c. 10 nanometres to millimetres. This will address the fundamental question of how 3-dimensional floc micro-structure influences suspended particulate matter transport. This project integrates sampling techniques from the biomedical sciences that are more commonly used to examine fragile biological cell tissues and analysis used in materials science and the earth sciences to analyse the microstructure of concrete, alloys and rocks (focussed ion beam nanotomography FIB-nt/SEM and X-ray microtomography). We will then apply this novel approach to the study of delicate, flocculated sediment in the aquatic environment.

The immediate beneficiaries of this research will be those who directly utilise or develop software for cohesive, mixed sediment and flocculation transport models. The improved understanding of suspended particulate matter (SPM) behaviour will be of great importance to those who manage and predict SPM fate and transport in environment, those who assess its impact on environmental pathways, as well as those who generate SPM through their activities. This includes the Environment Agency and Defra in the UK, environmental and engineering consultancies specialising in the aquatic environment, river and coastal managers, fisheries/aquaculture, dredging companies, ports authorities and offshore installations (e.g. windfarms, marine drilling).

Outputs, including the identification and quantification of critical micro-structural parameters that best predict settling velocity, have the potential to revolutionize computational model predictions of cohesive sediment dynamics and will be of extreme importance to sediment transport modellers. Specifically, quantification of parameters such as floc density and porosity will enable more accurate calibration of present sediment transport models, as this will significantly improve our quantitative understanding of how the sedimentary mass that comprises flocculating suspension is distributed across populations within different aquatic environments. Accurately reproducing complex sediment dynamics requires a multi-physics approach, and these processes are of considerable importance to any freshwater, coastal or marine infrastructure project (e.g. those involving dredging, dispersion of plumes, natural fine sediment transport processes), as they affect environmental impact and infrastructure performance. Scientists and engineers who work in commercial settings (e.g. dredging and deep sea mining) that develop software for cohesive, mixed sediment and flocculation models will benefit from an improved understanding of floc formation, composition and subsequent dynamical behaviour. This research offers the potential for a more precise and refined numerical model calibration and verification. This is superior to the simple fractal approximations that assume a self-similar distribution of particles within a floc and which often struggle to mimic adequately naturally flocculating sediments that comprise a mixture of mineral particles (size and mineralogy) interlaced with biological matter.

The visualisation of 3-D sediment micro-structure at nanometre resolutions is a powerful tool for exploring floc composition, specifically as our staining approaches enable the differentiation of organic and inorganic components and if coupled with EDS (energy dispersive spectrometry) there is also the potential to provide elemental analysis. This approach can reveal how pollutants, bacteria, pathogens and nanoparticles are associated with and distributed within different flocs, thus providing an indication of pollutant transport 'efficiency' for eco-toxicological environmental modelling and improving the design and efficiency of engineered water treatment and remediation schemes (e.g. artificial wetlands).

Our experience so far (e.g. at conferences and with students) has indicated that the imagery produced is an exciting, informative and powerful visual tool. Therefore, this also offers excellent education opportunities and the potential to engage students and the public, particularly regarding the scale of environmental processes and the important functions that sediment plays in the aquatic environment e.g. habitat, food source, regulating water quality and the cycling of nutrients.