3-dimensional floc structure and dynamics

悬浮颗粒物（SPM）在水生环境中沉积物、污染物、病原体、营养物质和人造纳米材料的影响和最终命运中起着重要作用。悬浮颗粒物对水生生态系统至关重要，它将物质从集水区转移到海岸，对营养物的生物地球化学循环有重要贡献，是海洋中向下碳通量的主要载体。水柱中悬浮沉积物过多可视为污染物；通过增加养分负荷和降低溶解氧浓度来降低水质，从而影响生态状况和英国对欧盟水框架指令的遵守。一旦沉淀下来，细小的沉积物会减少水生生物的多样性，例如通过窒息生物和产卵砾石。泥沙还会堵塞河道和港口，影响航行。例如，在英国，我们每年疏浚约4000万吨沉积物，花费数百万英镑。因此，为了制定基于证据的沉积物管理决策，迫切需要了解并准确预测所有水生环境中SPM的命运和迁移。悬浮颗粒物以絮凝体的形式存在于水体系统中。絮凝体是一种非常脆弱、松散结合的聚集体，由细小的沉积物颗粒、细菌、有机物和充满流体的孔隙空间组成。絮凝体具有非常复杂的形状和结构，密度非常低。当浮冰从悬浮状态中沉淀下来时，它会不断地改变形状、大小和结构，因为它会在湍流的水中多次分解和重组。这些絮凝体的微妙性质和它们的大小（几微米到几毫米）意味着它们很难取样和分析。目前的分析技术，如光学或电子显微镜，既可以观察整个絮凝体，但细节有限，也可以观察絮凝体的亚微米部分，而且它们只能观察二维截面。因此，许多预测细粒沉积物输运的数值模型依赖于复杂的三维形状和结构的二维简化，或者必须使用数学方法来描述絮凝体结构，例如，它们假设絮凝体是分形的，并且它们的结构与尺度无关。因此，我们对絮凝体结构和行为的理解是非常有限的，除非我们能够在一定的空间尺度上对絮凝体进行采样和分析，否则我们既不能支持也不能挑战这些数学假设。我们将填补这一研究空白，并首次在三维和从0.10纳米到毫米的多个相关空间尺度上取样、观察和量化沉积物絮团的微观结构。这将解决三维絮体微观结构如何影响悬浮颗粒物运输的基本问题。该项目整合了来自生物医学科学的采样技术，这些技术更常用于检查脆弱的生物细胞组织，以及材料科学和地球科学中用于分析混凝土、合金和岩石微观结构的分析（聚焦离子束纳米断层扫描FIB-nt/SEM和x射线微断层扫描）。然后，我们将把这种新方法应用于水生环境中精细絮凝沉积物的研究。

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.