Manganese oxides are well known in marine geochemistry as efficient scavengers for trace elements in seawater. This property was even applied to preconcentrate radionuclides from the marine water samples. Actually, while occurring all along the water column, this mechanism is leading to a natural impressive enrichment in alpha emitting radionuclides in the slowly growing polymetallic nodules (or manganese nodules). In their study, Volz and co-authors (2023, see reference below) demonstrate that radioisotopes in the nodules mostly exceed exempt activity levels established for naturally occurring radioactive materials (NORM) such as uranium ore. In comparison with these exempt levels, activity concentrations in the nodules are up to three orders of magnitude higher.
The consequence is that the radioisotopes relevant during handling and processing of polymetallic nodules and deep-sea sediments emit alpha radiation (i.e., alpha particles) during decay. Alpha particles have high energies (MeV), and alpha emitters are considered particularly harmful if they enter the body through inhalation or ingestion. While processing these mining products (as grinding for example, inhaled fine particles might reach into the deep lungs and remain there. There, incorporated alpha emitters can damage cells, and thus, impose a great radiological hazard for the human body.
The authors strongly recommend that the collection, transport, storage and processing of polymetallic nodules (and of deep-sea sediments) should be considered under radiation safety aspects in future activities.
Figure: Exempt radioactivity levels have been established, e.g., by the International Atomic Energy Agency (IAEA), to ensure public and occupational health and safety (A).
Various naturally occurring radioactive materials (NORM) are found in the environment and include materials where humans have increased the potential for exposure, e.g., through mining (B).
Volz and colleagues have produced new radioactivity data for manganese nodules from the most extensive manganese nodule field in the Clarion-Clipperton Zone in the Pacific Ocean. Together with existing data from historic studies (C), they show that manganese nodules often exceed the values from (A) and (B).
Jessica B. Volz, Walter Geibert, Dennis Köhler, Michiel M. Rutgers van der Loeff and Sabine Kasten. Alpha radiation from polymetallic nodules and potential health risks from deep-sea mining. Sci Rep 13, 7985 (2023). Acces the paper: https://dx.doi.org/10.1038/s41598-023-33971-w
You can also read the new item published by AWI: https://www.awi.de/en/about-us/service/press/single-view/awi-forschende-weisen-hohe-natuerliche-radioaktivitaet-in-manganknollen-nach.html