Development of Novel Slow-Release Micronutrient Fertilizers Using Mechanochemical Synthesis

Abstract

This thesis investigated the possibility of applying a green mechanochemical synthesis method for the formulation of boron (B) and molybdenum (Mo) fertilizers. More generically, the aim was to combine sustainable engineering and fertilizer manufacturing. The thesis consists of 6 chapters, including an overview of the literature relevant to this project (Chapter 1). Subsequent chapters describe the development of B and Mo fertilizers using mechanochemistry with the aim to produce micronutrient fertilizers or micronutrient-enriched macronutrient fertilizers which would display a range of solubilities of B and Mo and reduce wasteful losses of these nutrients through leaching in high-rainfall environments. Formulations were developed using a range of manufacturing variables, such as reactant species, reactant concentrations and ratios, milling time and post-milling activation treatments (e.g. water assisted reaction). Formulations were characterized by various chemical and spectroscopic techniques, and these characteristics were related to solubility determined in batch and kinetic tests. Finally, agronomic evaluation of the most promising products was undertaken in pot trials using protocols to simulate high leaching environments. Three different types of micronutrient fertilizer were synthesized using mechanochemistry. Firstly, a sparingly soluble zinc borate was successfully synthesized using ZnO and B2O3 as raw materials by a simple mechanochemical process that, when scaled up, would be energy efficient and have a low environmental footprint. Mechanochemical synthesis of zinc borate was compared with conventional wet or hydrothermal methods in terms of waste generation using a sustainability metric called the E(nvironmental) Factor. The E factor for mechanochemically synthesized zinc borate was 1.7 kg waste/kg product compared to 77.5-250 kg waste/kg product for conventional synthesis methods. Secondly, a slow-release Mo fertilizer with a layered structure was mechanochemically synthesized from ZnO, ZnSO4·7H2O and MoO3, with the final formulation containing zinc hydroxide sulfate (ZHS), together with low solubility zinc molybdenum oxide hydrate (Zn5Mo2O11.5H2O) and its less hydrated analogue. Thirdly, a different type of slow-release Mo-Zn fertilizer was synthesized by mechanochemical treatment of ZnO and MoO3. Using these compounds, micronutrient-enriched macronutrient fertilizers were produced by coating or co-compaction. The Mo-containing compounds were not assessed using the E-factor, as no comparable methods which synthesize the same compounds were identified in the literature. However, the mechanochemical procedure used to synthesize the formulations did not generate any waste streams. In addition, the treatment with mechanochemically treated 4% Mo-Zn fertilizer showed significantly less leaching than the control without ball milling and the DAP co-compacted with 4% Mo had a higher plant Mo update compare to other Mo sources. Mechanochemical synthesis methods were successful in producing formulations that demonstrated slow-release characteristics for B or Mo using highly soluble raw materials. The environmental footprint of the proposed mechanochemical synthesis methods was smaller than traditional synthesis techniques. Mechanochemistry therefore offers an opportunity to transform micronutrient fertilizer manufacturing methods. The products made need to be evaluated not only for their manufacturing footprint but also for their agronomic and environmental performance and their compatibility with current nutrient delivery systems.