Abstract
Worldwide demand for rare earth element (REE) based technologies has drastically increased in recent years within various sectors, such as renewable energy, transportation, and consumer electronics. Earlier studies by the United States Geological Survey (USGS) reported that the State of Idaho contains high REEs dispersed within its topsoil. The unique soil makeup of the Idaho region, with REEs comprising over 3% of the total soil content in some areas, provides a perfect environment to explore the viability of sustainable REE extraction pathways, such as phytomining. Phytomining is an environmental-friendly solution, using plants to extract and store targeted REEs in their various plant parts, such as roots or leaves. The targeted REEs are cerium (Ce), neodymium (Nd), lanthanum (La), yttrium (Y), and praseodymium (Pr) due to their high level of abundance within the Idaho soil. This study focuses on four plant species, which are reed canary grass (Phalaris arundinacea), pokeweed (Phytolacca americana), black nightshade (Solanum nigrum), and brown mustard (Brassica juncea) for REEs extraction from Idaho-sourced REE-rich soil. Particularly, three soil types were used in the greenhouse for growing the selected plant species, including: (a) untreated soil, (b) fertilizer-treated soil, and (c) pinewood biochar-treated soil. Two water treatments were applied throughout the growing process: (a) tap water and (b) tap water mixed with 1% citric acid. After growing to maturity, the plants were harvested, pyrolyzed (heated to a high temperature in the absence of oxygen), acid digested to create an REE-rich liquid suspension, and then analyzed via inductively coupled plasma mass spectrometry (ICP-MS) to measure REE content of each plant. The results show that reed canary grass with fertilizer-treated soil drastically outperforms all other species and treatments, accumulating over 18,000 ppm Ce, 11,000 ppm Y, and 8,000 ppm Nd. It is concluded that phytomining can be a promising net-negative emission solution for mixed REE extraction and has the potential to be applicable at the commercial level.