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Abstract
This thesis presents a series of investigations into the production, comparison and end-use application of agricultural biochar that has been impregnated with mineral silicates and salts prior to pyrolysis. A range of characterisation techniques, including mass spectrometry, ultimate analysis and x-ray diffraction, were employed to determine the changes induced in organic carbon due to cumulative additions of ferrous salt, followed by bentonite and kaolinite clays, with respect to increasing pyrolysis temperature. The addition of ferrous salt catalysed degradation of organic carbon. However, this effect was non-linear with respect to temperature, indicating differences in the response of individual ligno-cellulosic components. Modest increases in acidic functional groups were observed, particularly at the surface, while the added ferrous sulphate impregnated deep into the sub-surface. The supplemented iron addition transformed to magnetite, along with ferrous sulphate at intermediate temperatures likely yielding a more redox-active biochar. With clay additions, these effects were enhanced, and significant cation exchange was also evident, as mineral cations impregnated the char subsurface. A mineral-enhanced biochar, derived entirely from municipal solid waste (MSW), was then compared with an engineered wheat straw and basalt char, to discern whether its composition and microstructure would be comparable with a char requiring a more detailed synthesis. Through electron microscopy and energy dispersive spectroscopy, the microstructural interaction of mineral and organic phases in the MSW char matched that of engineered char; with micron-scale pores gradually accumulating layers of cation-rich silicates. Finally, a mineral-enhanced biochar was used to coat an NPK granule to assess whether nutrient leaching through the soil, from the granule, was reduced. It was tested under irrigation for an incubation period of eighty-five days. Nutrient analysis confirmed that N and K nutrient leaching was significantly reduced. Microscopy, spectroscopy and Rietveld refinement of x-ray diffraction data further confirmed that the nature of the char under incubation mimicked that of similar chars in the previous chapters, and that the structure of the remaining biochar was recalcitrant and intimately bonding to the surrounding smectite-rich clay soil. These analyses illustrate the significant potential for mineral-enhanced chars to exhibit multiple advantageous phenomena in soil under optimal heating and processing.