- Title
- Effects of tailored peroxide oxidation on yield and molecular structure of humic substances from Victorian lignite
- Creator
- Hood, Andrew
- Date
- 2023
- Type
- Text; Thesis; PhD
- Identifier
- http://researchonline.federation.edu.au/vital/access/HandleResolver/1959.17/192407
- Identifier
- vital:17996
- Abstract
- Soil organic carbon is vital to soil health and productivity. However, its abundance declines as continually greater demands are placed on agricultural food and fibre production. Humic substances are key components within the soil carbon and confer many benefits to soil and plants. Therefore, demand is growing for external amendments to maintain productive soils, including humic substances (HS), specifically humates. There are many sources of these HS, including lignite and Leonardite. Leonardite represents a naturally oxidised lignite that exhibits a rich source of HS. The limitation is the low abundance of Leonardite or difficulties obtaining a reliable source. Lignites are abundant in Victoria (Australia) but lignite derived-humates are often subject to significant compositional and yield variability. Past research has primarily focused on oxidising lignite under low solids loading conditions to improve HS yield and water solubility. This research focuses on oxidising dense run-of-mine (RoM) lignite slurries (20% dry solids loading) that are more suitable for commercial production. The product chemical and molecular structures are compared with a reference Leonardite. Hydrogen peroxide served as the oxidising agent given its availability at a commercial scale and convenience. The controlled set of oxidation experiments showed that the ratio of oxidant to lignite and lignite composition had more impact on HS yields and chemical structure than slurry temperature and pH. Initial oxidation trials utilised an archived sample of carefully blended 2015 RoM (run of mine) lignite from the Loy Yang (LY) mine in Victoria, Australia. Additional fresh LY RoM lignite was obtained in 2021 to enable further larger-scale (10 and 100 L) reaction trials and assess the impact of sample variation and aging on oxidation reactivity. A substantial increase in humic acid yield (29% to 68%) was produced by mild oxidation using a 1:1 (w/w) ratio of the 2015 RoM lignite (db, dry basis) and 35% (w/v) hydrogen peroxide. For the lower quality 2021 RoM lignite, the humic acid content was elevated from 13% to 78% through oxidation with a 2:1 ratio of hydrogen peroxide to lignite (db). Mass and carbon balances, which are often given little consideration, show >64% (2015 RoM) and >84% (2021 RoM) carbon recovery, with the majority of losses attributed to volatiles including CO2 and small organic acids or aldehydes The optimum reaction conditions for humic and fulvic acid production from LY RoM lignite depend on the target products, RoM properties and other commercial considerations. Optimum conditions are likely to be in the region of: - 40°C to 60°C to balance reaction speed and peroxide efficacy. - High humic acid yields require between 1:1 to 1.5:1 peroxide (35% w/v) to lignite (db) ratio. - Minimising humin yields requires a minimum of 2:1 peroxide to lignite. - High fulvic acid yields require between 3:1 to 4:1 peroxide (35% w/v) to lignite (db) ratio. This research shows that Loy Yang RoM lignite can be progressively oxidised with hydrogen peroxide to produce commercially relevant yields of humic and fulvic acids. A large portion of the humin is converted into humic or fulvic acids, and what remained was more oxidation resistant and water-insoluble aliphatic components. Elemental proportions shift within each fraction during oxidation. For the 2015 and 2021 RoM lignite samples; the O/C for the reaction solids increased by 36% and 16% for the respective lignites. The O/C changes for the HAs were 22% and 19% respectively. The humin undergoes a sizeable shift in H/C ratio consistent with aliphatic enhancement. The ratios elevate by 14% with oxidation for both the 2015 and 2021 lignites. Extensive lignite, reaction slurry and reaction product characterisation including slurry pH, spectroscopy (UV-vis, FTIR and NMR), and pyrolysis GC/MS assist in understanding the structural changes occurring during oxidation as well as identify promising on-line reaction monitoring technologies. Elemental (CHNSO), FTIR, NMR and functional group titrations show that the fundamental organic structural changes to the humic acid fraction achieved through controlled oxidation were greater acidity (particularly carboxylic acid content) and a transition from aromatic to more aliphatic character overall. No combination of ratios or slurry conditions tested produced a humic acid with the same chemical structural properties as those extracted from the reference Leonardite. However, it is possible to match some properties under the combinations tested. Larger-scale reactions (10 and 100 L) show that real-time reaction monitoring as well as adequate heating and cooling capacity are essential for commercialising the conceptual lignite oxidation process. Proactively dealing with an inherently variable feedstock and humic product composition is heavily dependent on the intersection of multiple variables. For example, tracking pH, reaction enthalpy, and UV-vis absorbance of prepared samples can provide valuable real-time feedback on the progress of a reaction. Combining these techniques with redox titrations could measure residual peroxide and help manage reaction control parameters or better understand reaction kinetics for process optimisation.; Doctor of Philosophy
- Publisher
- Federation University Australia
- Rights
- All metadata describing materials held in, or linked to, the repository is freely available under a CC0 licence
- Rights
- Copyright Andrew Hood
- Rights
- Restricted access by author for 24 months starting 9 January 2023
- Subject
- Humic acid, lignite; Oxidation; Peroxidation; Victorian lignite; Humic substances
- Thesis Supervisor
- Verheyen, Vincent
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