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A lack of understanding of the human and environmental health implications of nano-materials has deterred public and scientific support for nanotechnology evolution across industries. Nano-silver in particular has gained increasing popularity due to its biocidal properties in the garment industry to create odor-free clothing. An increase in “nano-litter” release to the environment is expected from erosion by product use and land application of nano-litter enriched wastewater sludge. The faith of nano particles once the enter into the environment is uncertain and in particular how they move through the soil to ground and drinking water. This main aim of the project was to investigated the transport of nano-silver in soils through laboratory experiments using a novel 3-dimensional soil model technique that creates reproducible replicates of real soil pore networks, and simulations that combine non-equilibrium statistical physics of particle solid interactions with information of Lattice-Boltzmann flow fields. The objectives are to: i) determine the effect that soil structure has on the transport of nanoparticles, ii) assess the capacity of soils under different land management practices (and therefore different soil architectures) to filter out suspended nano-litter from groundwater, and iii) develop a modelling tool to help industry forecast the propagation of nano-enabled products and their derivatives through soil environments from product use and disposal.
Although there is a broad scientific acceptance that soil structure affects particle transport, this parameter is often omitted from laboratory experimental designs due to the numerous physical, chemical, and biological factors that are difficult to control for in real soils. Conventional methods to investigate particle transport typically involve column experiments, where effluent concentration is measured and breakthrough curves are obtained as a representation of an average behavior of particle transport within the column’s porous medium. Unfortunately, such methods do not discriminate between the spatial and temporal conditions that affect particle transport.
In this project novel experimental systems were developed and technologies applied to quantify transport characteristics of nano-particles through heterogeneous media such as soil. X-ray micro CT was used to characterise the internal geometry of the pore space at microscopic scales for soil under different management schemes to characterise the different pathways through which nano-particle transport will take place. The results displayed substantial difference in pore geometry between management operations, but also showed huge variability between replicated soil samples from the same field. As such heterogeneity would provide complications in conducting replicable transport experiments novel 3D printing technology was explored. It was shown that 3D printing can reproduce realistic soil structures. This technology, developed in part at Abertay University was demonstrated on the BBC news as a scientific advancement. However, complications occurred for smaller pores which were difficult to free from residual printing material which made this technology at its current state less suited to study the transport of nano-particles. Alternative media in the form of glass beads and Nafion were subsequently used to produce surrogate structures. The experimental systems enabled manipulation of pore geometry in experimental systems with minimal variability between replicates.
X-ray CT was used to characterise the geometry of the samples and the characteristics of the flow pathways, formed by connected water films in unsaturated columns (Fig 1). Xray CT allowed to identify non destructively the microscopic distribution and location of solid, water and air within each sample, as well as allowed for the identification of retention sites of nano particles. Transport processes were studied in columns with nano-particles added and its deposition in the heterogeneous medium was quantified with Xray CT at the end of each experiment. Various water-air-solid interfaces were considered and the location of retained nano particles in each of the sites was used to estimate the probability of retention within porous media. It was demonstrated that in unsaturated media it is critical to consider interfacial areas as retention sites. Nano-particles were in particular deposited in wedge-like shaped areas with a high preference of the water-liquid-solid interface (63%) followed by the solid-solid interface.

(see attached document for figure)

In addition, a novel experimental and analysis tool was developed to follow nano-particles as they are transported through 3D porous media. Given the nature of the data collection and the time required this could not be done with Xray CT and a new experimental system was developed using Nafion in custom build flow-through chambers. The chambers were filled with a liquid with a matching Refractive Index making Nafion ‘invisible’ the 3D transport columns transparent to light. A monochromatic Ar Ion laser source was used to illuminate the cell so that dispersed light from the nano-particles could be used to track them as they have a distinctly different Refractive Index. The development of the new experimental system replaced the need to use lattice Boltzmann modelling to identify flow paths for nano-particles as the mobility and disposition of nano particles could be followed directly. The deposition and flow patterns and pathways determined in the experimental systems demonstrated non Fickian flow and identified pathways and aspects of the pore geometry that might induce accelerate transport or retardation of nano-particles in porous media (Fig 1).
The work conducted has resulted in 3 high impact journal publication to date and delivered the development of novel experimental tools to study the transport of nano-particles and contributed to a step chance in our mechanistic understanding of the faith of nano particles in the environment which is a pre-requisite for predictive understanding. It is anticipated that the findings of the research will ultimately feed into British, European, and even global policies regarding soil and water quality, nano-technology sustainability, and public health in that they allow for more accurate assessment of risk which has be identified to relate to both the structure of the soil pore network as well as the wetness, and in particular the microscopic distribution of water within the soil. The developed theories have implications for colloid retention and contributed to a publication and media exposure related to the effect of hydrofracking on colloid transport.

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