MnOx biochars as potential mercury sorbents in the context of global mercury emissions.

What role do targeted mercury control technologies play in the context of global anthropogenic mercury emissions reductions and what are the material requirements for a successful mercury sorbent?

Mercury (Hg) pollution is a multi-scalar policy issue. Anthropogenic emissions stem from point sources, but their consequences span the whole globe due to global distribution of the pollutant through the atmosphere, necessitating action across local, global and even temporal scales. To counteract this, the Minamata Convention on Mercury (MCM) aims at reducing levels of `mercury and mercury compounds' in humans and the environment by targeting Hg at different levels of the release cycle, such as trade, use in manufacture, emission sources, and wastes. Against this backdrop, this thesis focusses on different control options for atmospheric Hg emission reduction on a global level, using global scenario modelling, systematic literature review and laboratory studies. Part I of this thesis takes a panoptic view, using global scenario modelling to explore different narratives on future Hg emissions, revolving around a central question:

To what extent are Hg-specific technological interventions necessary to achieve Hg emission reductions globally and where will they be most effective?

Eight different scenarios of energy, climate and clean air policy are considered, all of which influence aspects of Hg emissions. The thesis develops and implements improved modelling representation of Hg removal technologies in the Greenhouse Gas - Air Pollution Interactions and Synergies (GAINS) model. This enables improved representation of Hg-specific control options, as well as co-benefits with traditional air pollutants. Part II is a case study of one such technological solution: a material class with promising Hg sorption and oxidation properties, namely manganese oxide - biochar composites (MnOx-BC). Here, the question is:

Which properties make a MnOx-BC ideal for Hg capture from flue gas - and how can they be achieved through synthesis control?
MnOx-BC material properties are explored using systematic literature review and a laboratory study to underline the relationship between synthesis procedure, biochar feedstock and resulting Mn oxidation state, using X-ray Absorption Near Edge Structure (XANES) to investigate manganese oxidation states. While MnOx-BC sorbents have been studied for Hg capture from coal combustion flue gases, the global modelling shows that this is a sector where much Hg is abated as a co-benefit from particulate matter (PM) and SO2 control. Additionally, the current climate policy and the decarbonisation of the energy system mean that this Hg source will diminish in the future, rendering this emission source less interesting for targeted Hg removal sorbents. Especially the waste sector and manufacturing industries (non-ferrous metals (NFME) and cement) show scope for targeted, further Hg emission removal using novel sorbent technologies in the future, due to their high emission intensity and their continued relevance in a low-carbon, high-population future. The modelling also showed that 17.6% of global Hg emissions could be abated in 2050 compared to 2015 by maximising co-benefits from PM and SO2 control. The most drastic, targeted Hg reduction measures, including a ban on ASGM, would achieve a 79% emission reduction in 2050 relative to 2015, showing that Hg-specific control technologies are crucial to reducing Hg. In terms of material properties, the combination of review and material science established the importance of biochar feedstock and well-controlled synthesis conditions to produce MnOx-BC with optimised porosity, Mn content, Mn distribution and Mn mineralogy. Especially XANES proved to be an excellent method to determine the average Mn oxidation state on the material. This thesis identifies biochars with well-developed macro-and mesoporosity that are impregnated with amorphous Mn(4+) oxides as a promising material. Different synthesis routes using KmNO4 and an oxic calcination step can achieve this.