Potato

RomoloTavani

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Manganese nodules, also known as polymetallic nodules, can be found in all of Earth’s oceans. They come in sizes ranging from that of a potato to a head of lettuce and are composed largely of iron and manganese oxides.

Large concentrations of these 'sea potatoes' can be found in the Pacific and Indian Oceans at depths of up to 21,300 feet (6,500 meters).

The nodules are considered the most important metal deposits of the sea, due to their high content of iron, titanium, copper, nickel and cobalt – elements important to the production of motors, computers, smartphones, and batteries. This has drawn the interest of the electronics and steelmaking industries, in particular, as potential new sources of metals to meet the growing demand.

AWI/OFOS

Deepsea mining companies have found they can collect the nodules by using a hydraulic machine similar to a potato harvester. Recently, the International Seabed Authority (ISA) has issued permits and entered contracts with 19 companies from China, France, Germany, India, Japan, Russia and South Korea, to explore mining the nodules.

However, a new study published in Scientific Reports shows that the possible industrial mining of the nodules, which can take up to three million years to form, could not only have significant impact on the ocean ecosystem, but could also jeopardise the health of miners, processors, and even end-users due to the nodules’ high levels of radioactivity.

The research shows that, as they grow, the nodules accumulate high levels of uranium radioisotopes, which emit large amounts of alpha radiation as they decay. While external exposure to alpha radiation is not as dangerous as exposure to some other forms of radiation, the researchers suggest that nodule processing can lead to inhalation of nodule dust, or ‘fines,’ and radon gas, as well as exposure to high concentrations of other radioactive substances.

In a press release, Jessica B. Volz, Ph.D., the study’s first author and a biogeochemist at the Helmholtz Center for Polar and Marine Research at the Alfred Wegener Institute (AWI) in Bremerhaven, Germany, explained that the team focused on two particular radioactive substances, thorium-230 and radium-226. These were found in nodules retrieved from expeditions carried out in the Clarion Clipperton Zone in the Pacific Ocean between Hawaii and Mexico.

“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 nodule’s surface from seawater over long periods of time. However, their values had never been considered in the context of radiation protection legislation,” she explained.

“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 legislation a hundred to a thousand-fold,” Volz added.

Repeated measurements of the nodules’ outer layer showed more than 5 becquerels per gram (a quantity of radioactive material in which one nucleus decays per second) of radium-226. This is in contrast to Germany’s limit of 0.01 becquerels per gram provided under the German Radiation Protection Ordinance.

Sabine Kasten, Ph.D., one of the study authors and a researcher at the Helmholtz Center explained that the research focused on how deep-sea mining could influence ecosystems in the Pacific Ocean.

“Our new study on the radioactivity of manganese nodules demonstrates that, beyond the consequences for marine ecosystems, there could be human health hazards in connection with mining and processing of manganese nodules, and the use of products manufactured on the basis. It’s imperative that this aspect is considered in all future planning,” she stated.

In an interview with Interesting Engineering (IE) Walter Geibert, Ph.D., geochemist, senior researcher at the Alfred Wegener Institute, and co-author of the study, explained that, despite expecting high radioactivity levels, the team was quite surprised by results.

“In particular, the high accumulation rate of the radioactive noble gas radon was a new finding. 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 nodules products during/after processing, such as actinidum-227 in the rare-earth elements,” said Geibert .

Sabine Kasten/AWI

“The results show that the outer parts of the nodules naturally reach values for certain radioactive substances that exceed some or all legal safety limits. We found values that exceed by far what we normally measure in natural samples. We also found that the nodules release a lot of the radioactive gas radon, so this builds up to high values when the nodules are kept in enclosed spaces,” he added.

Given the findings, should we actually even be thinking of mining the nodules? According to Geibert, we are not the first species that utilise them - a diverse and abundant assemblage of deep-sea organisms lives on or near the manganese nodules and crusts. “And they have been doing so for a long time and probably need them more urgently than we do,” emphasised the scientist.

“I think we humans can learn a lot from manganese nodules about extreme microbial life forms, and about past ocean conditions. Otherwise, I think in the long term, humanity has most benefits from leaving the manganese nodules where they are right now. We don’t really know their function in the ocean chemistry yet, but considering their abundance and the vast area they cover, I believe that they are a critical component of the Earth system in the long term that modulates seafloor-ocean exchange.”

This, however, raises the question of the possible negative consequences for marine ecosystems as well as for humans from extensive mining of manganese modules. “The ecosystem that lives on and around the nodules is permanently destroyed by mining. We know [the nodules] take millions of years to grow, so recovery in this form simply won’t happen. There is no sustainable deep-sea mining,” Geibert emphasized to IE.

“Regarding the consequences for humans, we don’t really know, except that handling the nodules should be done carefully, not only because of the radionuclides they contain and release, but also to avoid exposure to the heavy metals that are the reason for mining them,” the researcher added.

Geibert revealed that his team’s main focus in the future will be to find the answers to why and how the nodules actually grow. “Maybe then we can say something about their function. If there are future projects to study the environmental impact of mining, we might participate again - but I also believe in a way that we already know enough to say that the seafloor in the deep sea will be permanently altered by removing the nodules,” he added.

“I guess that many authorities and companies will now want to make their own analyses. We need more knowledge, training and more analytical facilities to measure natural radioactivity. Most people only think of nuclear power plants when they hear the term “radioactive”, but in fact this is an extremely powerful tool to study the natural environment in so many aspects,” stated Geibert.

HR/AWI

In search for critical elements, polymetallic nodules at the deep abyssal seafloor are targeted for mining operations. Nodules efficiently scavenge and retain several naturally occurring uranium-series radioisotopes, which predominantly emit alpha radiation during decay. Here, we present new data on the activity concentrations of thorium-230, radium-226, and protactinium-231, as well as on the release of radon-222 in and from nodules from the NE Pacific Ocean. In line with abundantly published data from historic studies, we demonstrate that the activity concentrations for several alpha emitters are often higher than 5 Bq g−1 at the surface of the nodules. These observed values can exceed current exemption levels by up to a factor of 1000, and even entire nodules commonly exceed these limits. Exemption levels are in place for naturally occurring radioactive materials (NORM) such as ores and slags, to protect the public and to ensure occupational health and radiation safety. In this context, we discuss three ways of radiation exposure from nodules, including the inhalation or ingestion of nodule fines, the inhalation of radon gas in enclosed spaces and the potential concentration of some radioisotopes during nodule processing. Seen in this light, inappropriate handling of polymetallic nodules poses serious health risks.