Abstract
Rare earth elements (REEs) are vital to a broad spectrum of technologies, from advanced defense systems to everyday consumer electronics. Global production is heavily concentrated in a handful of countries, creating significant supply chain risks. As a result, developing a secure domestic REE supply chain has become a strategic priority for many nations, including the United States. To achieve this, there is an urgent need for environmentally friendly alternatives to traditional REE extraction methods, which are often energy-intensive and generate substantial waste. Leaching REEs using organic acids can offer a more environmentally friendly alternative to conventional methods. This study aims to enhance ultrasound-assisted organic acid leaching of REEs from REE-rich soil by combining it with microwave treatment. The optimal leaching yield (17.9%) was achieved with 900 W microwave treatment for five minutes, followed by two hours of continuous ultrasound-assisted leaching. The combined treatment improved the leaching yield compared to ultrasound treatment alone (0.128–0.133%/min vs. 0.048%/min). Scanning electron microscopy (SEM) revealed that samples treated with the combined method had cracks in the monazite grains that may be partly attributed to increased leaching efficiency. The techno-economic analysis estimated the process cost at $1563/kg leached REEs, with approximately 52% of the cost derived from purchasing citric acid. The environmental impacts assessment indicates that the combined process reduced emissions by 51% compared to ultrasound-assisted leaching alone, but it also showed higher impacts relative to comparable studies using bioleaching or mineral acid systems. This suggests that while the combined approach offers meaningful improvements over ultrasound alone, additional optimization is needed to achieve the lower environmental burdens reported for alternative leaching studies.