Metallic nodules on the seafloor could pose a headache for mining companies and regulators due to their high levels of radioactivity, according to German researchers.
Nodules are aggregations of metals like copper, nickel, cobalt and rare earth elements which build up over millions of years.
Extractive industries see appeal in deep seafloor mining to obtain metals from these nodules, for processing and use in batteries, electronics and other emerging technologies.
But scientists at the Alfred Wegener Institute’s Helmholtz Centre for Polar and Marine Research, studying nodules in the Pacific Ocean near Mexico and Hawaii, have found some may pose significant health risks.
The International Seabed Authority Assembly will meet from July 24 to 28 to discuss whether to allow seabed mining.
But this new research might setback seabed mining advocates – it shows that nodules are efficient at picking up uranium radioisotopes – unstable forms of uranium which emit high levels of alpha radiation as they decay.
Among the radioactive sources are thorium-230, radium-226, and protactinium-231.
The research, published in Scientific Reports, shows the activity of these nodules to be up to 1,000 times greater than current safety limits.
The researchers suggest that inhalation or ingestion of nodule ‘fines’, radon gas – which was also detected in the form of radon-222 – and exposure to certain high concentrations of radioactive substances present during nodule processing, pose substantial risks to human health.
“Based on previous studies it was already known that the nodules’ outer layer contains natural radioactive substances like thorium-230 and radium-226, which have accumulated at the nodules’ surface from seawater over long periods of time,” says study leader Dr Jessica Volz.
“However, their values had never been considered in the context of radiation protection legislation.
“Our study shows that in the outer layer of these extremely slowly growing nodules, certain substances, which emit alpha radiation, can exceed limits found in radiation protection legislations a hundred- to a thousand-fold.”
The thresholds of radioactivity exposure in Germany, the US and UK were substantially lower than the readings obtained in Volz’s current research, and in previous studies of seafloor nodules.
|Dataset/authority/guidelines (year)||230Th [Bq/g]||226Ra [Bq/g]||231Pa [Bq/g]||222Rn* [Bq/L]||Material|
|This study||7||9||0.55||–||Nodule surface|
|Max. value in literature||64||21||11||–||Nodule surface|
|GER (2018)||0.1||0.01||0.01||–||Solids and fluids|
|US (2018)||0.19||0.19||–||0.19||Upper 15 cm of soil|
|UK (2017)||0.1||0.01||0.01||0.3||Any amount of material|
|IAEA (2014)||1||1||1||0.3||Large amounts of material (> 1 ton)|
Volz suggests these levels may pose obvious concerns for marine environments, even though the radioactive accumulations in these nodules are naturally occurring.
But the human health consequences are particularly important, which is why the researchers are calling for regulators to consider safety limits for the emerging seafloor mining sector.
This research also exposes the potency of radon gas emitted by these materials, which surprised the authors when readings were measured.
“The high accumulation rate of the radioactive noble gas radon was a new finding,” says Dr Walter Geibert, who contributed to the study.
“As such, handling manganese nodules without protective gear can pose a health risk. It is not just through inhaling the dust produced during processing, but also the high radon concentrations that can build up when they are stored in poorly ventilated spaces.
“Some radioactive substances could accumulate in the nodule products during/after processing, such as actinium-227 in the rare-earth elements.”
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