Have you ever swum in the ocean and encountered seaweed? You’re likely familiar with Sargassum! This floating seaweed can lead to rapid population growth in coastal waters, adversely affecting water quality, ecosystems, fisheries, and tourism. However, scientists suggest that Sargassum has potential benefits.
It may aid in the extraction of valuable metals known as rare earth elements (REEs).
Rare earth elements are classified by their average atomic mass and weight.
Light rare earth elements
include cerium, praseodymium, and neodymium, while
middle rare earth elements
consist of gadolinium, terbium, and dysprosium.
Heavy rare earth elements
include erbium, thulium, and ytterbium. These metals play crucial roles in magnets, electric motors, wind turbines, spacecraft, and more. Traditional extraction methods, such as open-pit mining and chemical processing, are harmful, leading to habitat loss and toxic waste. Additionally, sourcing REEs globally can be challenging due to access constraints. What if we could harness natural methods for extraction?
Research conducted by scientists from Woods Hole and the University of California, Los Angeles, highlights that Sargassum has the potential to accumulate REEs, forming sustainable solutions to these environmental challenges. They harvested Sargassum samples, storing them in sterile, filtered seawater and aerating them under a controlled 12-hour light/dark cycle to keep the seaweed alive. In addition, they dried another set of samples in the sun under controlled conditions for further experimentation.
Each experimental setup utilized 5 grams (approximately 0.2 ounces) of Sargassum mixed with a variety of nine rare earth elements (cerium, praseodymium, neodymium, gadolinium, terbium, dysprosium, erbium, thulium, and ytterbium) at varying total concentrations: 0.1 micromolar (μM), 11μM, 100–110μM, and 575-600μM. Each experiment was conducted four times, with two trials using fresh seaweed and two trials with rehydrated seaweed.
After one, three, and seven days, researchers extracted one gram (about 0.04 ounces) of seaweed from each solution and dissolved it in acid to release the absorbed rare earth elements. They employed two plasma-based methods to determine the concentrations of these elements in both the seaweed samples and the remaining solutions, allowing them to calculate the bioconcentration factor of Sargassum.
Findings indicated that the bioconcentration factor for raw seaweed exceeded 1,000 after 7 days at a lower REE concentration (0.1 μM). For higher REE concentrations (11μM), the bioconcentration factors ranged from approximately 400 to 700. However, elevated REE levels proved toxic, causing fresh Sargassum to deteriorate after two weeks. Critically high REE concentrations (100–600 μM) were toxic enough to kill the seaweed within a day.
Researchers discovered that once the seaweed perished, it could no longer actively absorb rare earth elements, meaning accumulation could only occur on its surface through a process known as adsorption. Think of it like a powdered donut—once the surface is coated with powdered sugar, no more will stick. This limitation resulted in bioconcentration factors for raw seaweed samples dropping below 250 at the highest REE concentrations. In contrast, hydrated Sargassum demonstrated lower consumption of rare earth elements at low concentrations without toxicity, and it continued to accumulate at higher concentrations.
Ultimately, the research team compared Sargassum to a common industrial method for accumulating REEs, activated carbon. REEs tend to bind easily to activated carbon due to its high surface area and porosity. Both Sargassum and activated carbon accumulated the same amount of REE in 0.1 μM solution, but at 575–600 μM REE concentration, Sargassum outperformed activated carbon, particularly in accumulating heavy rare earth elements.
The researchers concluded that Sargassum is a natural hyperaccumulator, effectively concentrating rare earth elements. However, they recommend future investigations to determine its effectiveness in natural environments. If validated, Sargassum could serve as a sustainable method to extract rare earth elements from wastewater sources such as mine drainage. This approach could expand global access to these metals while addressing the challenges posed by toxic blooms.
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Source: sciworthy.com
