Prepared by Mahzuzah Zahan, Srivatsan Jayaraman Sridharan, Brett Carlson, Obulisamy Parthiba Karthikeyan, Purushotham Tukkaraja, Venkataramana Gadhamshetty and submitted to the Summer 2024 Currents

Overview of rare earth elements and mining technologies

The group of rare earth elements (REEs) comprises 17 elements from the periodic table, which includes 15 lanthanides along with scandium (Sc) and yttrium (Y). The United States (U.S.) Department of Energy (DOE) identified five REEs as critical, which includes neodymium (Nd), europium (Eu), terbium (Tb), dysprosium (Dy), and yttrium (Y). Recently, REEs have garnered widespread attention owing to their extensive utilization in the production of everyday smart devices. These include the screens of smartphones, computers, and flat-panel televisions, as well as the motors in computer drives, batteries for hybrid and electric cars, and the latest generation of light bulbs. Some of these smart technologies are increasingly being utilized in environmental engineering and water resource applications. They include Internet of Things (IoT) technologies for smart buildings and sensors used for smart water management and water savings—primarily in buildings, with applications including smart monitoring of water level, water consumption, and leakage detection. REEs also constitute the core materials for developing high-efficiency filtration membranes, novel desalination technologies, and electric motors for water pumps and wastewater treatment facilities. Given the increasing prevalence of these REEs in modern electronics and sustainable energy solutions, including renewable environmental engineering and water resource applications, these REEs remain the lifeblood of our critical technologies and industrial innovations.

REEs are not commonly mined but much-needed elements right now. China currently meets nearly 80% of the world's REE demand. The turbulence faced by the U.S. in the REE supply chain in 2019 has awakened the urge to be independent in this field. Agencies such as the Office of Surface Mining Reclamation and Enforcement (OSMRE) are committed to supporting the development of sustainable methods to extract REEs from unconventional sources, including coal waste and coal byproducts. Based on data from the U.S. Energy Information Administration (EIA), the U.S produced a record 1,171.8 million short tons of coal in 2008, marking the highest production level from 1950 to 2022. The U.S. possesses substantial coal resources, with the EIA estimating a demonstrated reserve base of nearly 471 billion short tons, of which around 69% is deemed underground mineable coal. According to EIA reports, as of 2022, there are 242 coal-fired power plants operational in the country. Notably, coal fly ash and coal mine wastes like coal mine tailings, waste rocks, and coal acid mine drainage are recognized as significant, yet untapped, reservoirs of REEs. However, the potential environmental risks, as highlighted by the Kingston coal fly ash spill, underscore the need for responsible management of these resources. There is a clear need for developing sustainable strategies for enabling the use of mine wastes for treatment and simultaneous resource recovery.

Biomining offers significant advantages from environmental and water resource perspective. Traditional mining methods often require substantial amounts of freshwater for ore processing and chemical reactions, leading to water scarcity and pollution in mining areas.  In contrast, biomining uses microorganisms to extract metals, significantly reducing freshwater consumption. Additionally, bioelectrochemical processes in biomining operate at ambient conditions and can recycle water within the system, minimizing wastewater generation and the need for external water sources. This sustainable approach not only conserves water resources but also mitigates environmental impact by reducing the potential for water contamination. Based on this background, this article focuses on utilizing bioelectrochemical processes for extracting REEs from REE-bearing minerals (e.g., bastnasite, xenotime, monazite, and allanite) as well as wastes including coal fly ash and mine solid wastes.

Novel bioprocess methods for REE extraction from coal mine waste at South Dakota School of Mines and Technology (South Dakota Mines) 

OSMRE is supporting the development of new methods to extract REEs from coal waste and byproducts. Our team at South Dakota Mines contributes to the nation's quest for new critical mineral sources, addressing environmental concerns like water pollution in coal communities. Recent funding selected eight projects nationwide, including one at South Dakota Mines, partnered with Disa Technologies, Inc. and Austin Elements Inc. Our focus is on developing a next generation biomining process for extracting REEs from coal mining waste sustainably. Leveraging methanotrophic bacteria and bio-electrochemical methods, we aim to optimize REE accumulation rates and address bioaccumulation bottlenecks, utilizing innovative techniques such as high-pressure slurry ablation for efficient leachate concentrate production without harsh chemicals. Harsh chemicals used for mining activities, directly or indirectly harm the water bodies around it. We are leveraging methanotrophic bacteria that rely upon the commonly discarded greenhouse gas, to sustainably transfer REEs into bacterial biomass. We address bioaccumulation bottlenecks by encouraging methanotrophic bacteria to accumulate REEs faster by leveraging bio-electrochemical methods that can tune exit potentials for the electrons leaving the bacterial cytoplasm rapidly.  

Conclusions   

In conclusion, biomining offers substantial environmental and water resource benefits over traditional mining methods. By using microorganisms for metal extraction, biomining significantly reduces freshwater consumption and minimizes wastewater generation through efficient recycling. This sustainable approach conserves water resources and mitigates environmental impacts by reducing water contamination. However, biomining faces challenges such as slower extraction rates and lower yields compared to traditional methods. Ensuring that selected microbes can thrive in the harsh conditions of coal waste, including toxicity, high metal concentrations, and acidic environments, is crucial. Our research at South Dakota Mines employs a bio-electrochemical process using methane-oxidizing methanotrophs to address these challenges. These methanotrophs are unique in their ability to transfer REEs from water into their biomass through special proteins.

This innovative method not only reduces environmental impact, notably water pollution from harsh chemicals and tailings runoff, but also aligns with a circular economy model by repurposing coal mine waste and methane. By refining this biomining process, we aim to contribute to a more sustainable and environmentally friendly approach to rare earth element (REE) extraction, addressing the pressing need for these critical elements in modern technology and renewable energy applications.

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