Abstract
Across the globe there is a growing demand for products and technologies that rely on the use of rare earth elements (REEs). Traditional mining practices are environmentally damaging, polluting natural resources with toxic and carcinogenic contaminants, reducing biodiversity, and rendering mined land unavailable for future use, which poses a threat to human and ecosystem health. This creates a need for environmentally friendly and low-emission extraction methods to meet worldwide REE demand. One such solution is phytomining, a process that employs metal-tolerant hyperaccumulator plant species to recover metals from surface soils. This study evaluates the benefits, challenges, and knowledge gaps within the field of phytomining through a comprehensive narrative and systematic literature review. Additionally, this study aims to investigate the feasibility of REE phytomining in Idaho, USA through experimentation with various plant species, varieties, and treatments, as well as sustainability evaluation using life cycle assessment (LCA) and techno-economic analysis (TEA). The literature review describes key challenges (e.g., long growth cycle, project duration, species selection, and post-harvest processing), identifies potential advancements (plant and soil treatments, selective breeding, and genetic engineering), and highlights the role of machine learning and artificial intelligence in improving efficiency, addressing challenges, and guiding future commercial applications by, for example, aiding in species selection, yield prediction, and sustainability modeling. Experiments were conducted where plants were grown from seed in REE-rich soil sourced from the Diamond Creek region in Idaho, then the biomass was harvested, dried, pyrolyzed, acid digested, and characterized. Phalaris arundinacea and Pseudoroegneria spicata emerged as the most effective REE hyperaccumulators with an average of 26,044 and 27,761 g/g total mixed REEs in their shoot tissues, respectively. Dry biomass and bio-ore yields were significantly impacted by species, variety, and soil amendments with fertilizer and biochar. REE hyperaccumulation was significantly impacted by species and tissue type (root vs. shoot). The LCA revealed electricity demand of the greenhouse and electricity demand of the pyrolysis reactor contributed the most to greenhouse gas emissions of a theoretical greenhouse and field operation, respectively. The TEA predicted that growing P. arundinacea for six weeks on a one-acre field area would cost USD 6213, with 39% of that cost derived from cultivation, 22% from biomass processing, 21% from soil treatment with fertilizer, and 18% from pyrolysis, respectively. It is concluded that phytomining, especially on REE-rich soil sourced in Idaho, is a sustainable, environmentally friendly approach with the potential to supplement global REE supply.