Abstract
Biological extraction of rare earth elements (REEs) has several advantages compared to the current traditional methods. Among these biological methods, organic acid leaching has the potential to provide an environmentally friendly alternative, especially when the organic acids are produced by heterotrophic microorganisms. Despite the increasing interest in organic acid leaching, the process is slower and less efficient than current leaching methods. This dissertation focuses on assessing ultrasound-assisted organic acid leaching to improve efficiency and promote large-scale implementation of sustainable extraction methods. To do so, a review on bioleaching and biosorption of REEs was performed to assess the state-of-the-art. The narrative and systematic reviews showed that REE extraction from ore resources, as well as waste materials, is necessary to keep up with the growing demand for these critical materials. In addition, it showed that low efficiency, long reaction times, and high cost are the major challenges associated with biological methods. Next, the potential of a batch ultrasound-assisted organic acid leaching method was evaluated using REE-rich soil sourced in Idaho. The results were compared to high-temperature and low-temperature methods that are comparable to previous bioleaching studies. The ultrasound-assisted method significantly reduced energy consumption and environmental impact while increasing leaching rate. These results were promising, but the small reaction volume of the batch ultrasound method is not feasible on an industrial scale. A novel continuous ultrasound reactor was designed to increase reaction capacity and the leaching efficiency was tested with two organic acids (gluconic and citric acid). Compared to batch ultrasound, the continuous ultrasound-assisted leaching method improved the leaching yield but increased the environmental impact due to an increase in energy consumption. It was concluded that a low-energy amendment was needed to reduce the reaction time required for a large reaction volume. Subsequently, a combined microwave and continuous ultrasound-assisted leaching method was developed and tested for leaching efficiency using citric acid. At the optimal condition, the leaching yield was improved compared to the continuous ultrasound method alone. In addition, the leaching time, energy requirement, and environmental impact were reduced. Scanning electron microscopy (SEM) was used on the leached solids to investigate the basic physical mechanisms behind the increased leaching yield. The combined microwave and ultrasound treatment reduced the particle size compared untreated soil, suggesting that increased surface area was partially responsible. The monazite grains in the treated samples appeared to have a rough, irregular appearance and the formation of cracking in the particles was observed. Ultimately, the proposed ultrasound-assisted REE leaching methods can help support the development of a domestic REE supply chain in the United States by promoting industrial implementation.