Polymetallic Nodules: Metal Resources Hidden in the Deep Sea

By Danella Trina Callosa

Living in today’s world, one could say that it would be difficult to continue our everyday progression without metallic resources. One way or another, all of our activities can be traced with some sort of metal being used — from our gadgets, transportation, infrastructure, and ironic as it may sound, even green energy utilizes metallic resources. This is why there has always been a need to explore and extract more of these resources.

Majority of what we know is that the deposits of these metals are being extracted on land. However, there are actually multiple commodities found in only one deposit hidden in the depths of our seas. What’s more fascinating is that they just sit on top of the seafloor and can be collected without any drilling or removing any overburden. These metal resources are called polymetallic nodules.

These nodules were discovered in 1868 in the Kara Sea of the Arctic Ocean off Siberia. With the scientific expedition of the HMS Challenger from 1872–1876, they were then found to occur on the abyssal plains of the world’s oceans [2].

Polymetallic nodules, also called manganese nodules, begin their growth with a core. It could be microscopic and can be transformed into manganese through crystallization, or macroscopic such as a small test of microfossil radiolaria or foraminifera, a phosphatized shark tooth, basalt debris, or even fragments of older nodules. Alternating concentric layers of iron oxyhydroxides and manganese hydroxides then precipitate around these cores onto which other metals such as nickel, cobalt, copper, and rare earth elements sorb. Precipitation can either be hydrogenetic or diagenetic [2]. Hydrogenetic precipitation of the layers occurs when dissolved Mn2+ and Fe2+ in oxygen-rich waters are oxidized into Mn4+ and Fe3+ oxide colloids which then accretes on the core. This process is known to be a very slow one with layers forming only a few millimeters per million years! However, it is thought to produce nodules of similar iron and manganese content with a relatively high grade of nickel, copper, and cobalt. Diagenetic precipitation, on the other hand, occurs when the organic matter in the pore spaces in the sediment column is oxidized. Manganese oxides are then reduced resulting in the dissolution of associated elements such as nickel, copper, and lithium. Due to the concentration gradients in the sediments, these metals diffuse upwards and reoxidize when in contact with the oxygen-rich waters, subsequently precipitating at the sediment-water interface. Even though nodules produced from this process are still rich in manganese, they are poor in iron, nickel, copper, and cobalt, unlike those produced from the former process [3].

In terms of their characteristics, polymetallic nodules can also be classified based on their size, shape, and distribution. Sizes can vary from being microscopic to being as large as pellets ranging up to 20 cm across. Specifically, they can be categorized based on their diameters: small (<4cm), medium (4–8), and large (>8cm) [3]. Most of the nodules, however, range from 5 up to 10 cm in diameter which is just about the size of potatoes! In terms of shape, they can be spheroidal, discoidal, botryoidal, or even polyshape [2]. As for their distribution, nodules can be from tens up to thousands per square meter of the seafloor [3].

With all of this given, one might wonder, with all the years it takes for these nodules to form to the usual size of a potato, how is it possible that they are on top of the seafloor? In fact, older nodules actually exist buried beneath the sediments. However, for those that remain on top, it is assumed that particles settling on the nodules are being cleaned and ejected on the sides or even below them by deposit-feeding benthic organisms such as polychaete or echiuran worms, thus preventing burial [2].

Generally, these nodules can be found in all of the world’s oceans and even in some lakes. However, those of economic interest are more localized in three areas identified by industrial explorers with depths of about 4,000 to 5,000 m — the center of the north Central Pacific Ocean, the Peru Basin in the south-east Pacific Ocean, and the center of the north Indian Ocean. Other factors such as abundance and chemical composition are also considered for it to be of economic interest. Nodules must exceed 10 kg/m2 with an average of 15 kg/m2 over several tenths of a square kilometer. Chemical composition, on the other hand, depends on the size and characteristics of the core and the type of manganese mineral. The constituent of nodules that are of economic interest are as follows: 29% manganese, 6% iron, 5% silicon, 3% aluminum, 1.4% nickel, 1.3% copper, 0.25% cobalt, 1.5% oxygen, 1.5% hydrogen, 1.5% sodium, 1.5% calcium, 0.5% magnesium, 0.5% potassium, 0.2% titanium, and 0.2% barium with nickel, copper, and cobalt as most valuable [2].

With these numbers, it is indeed comparable to terrestrial sources which have increasingly low yields that are often below 1%. Also, since they sit on top of the seafloor, collection of these nodules would mean 99% less solid waste and no toxic tailings. To create a clearer picture of the other potentials of its uses, let us take the battery as an example. Batteries use metals such as manganese, nickel, copper, and cobalt which are found in the said nodules. On a global scale, if polymetallic nodules will be used as a source for creating 1 billion electric vehicle batteries, it would generate 75% less CO2 than using ores from land-based mines. As for its societal and cultural impact, since most of the source areas are already in international waters, there would be no displacement of indigenous communities. Child labor will also be out of the picture since more advanced technology is needed for the collection of said nodules. However, in terms of its environmental impact, more studies are still being done to ensure that it will not cause greater damage compared to terrestrial mines [1].

Nevertheless, polymetallic nodules surely pose as a very promising metal resource in our rapidly advancing societies. Living in today’s world, it is indeed just a matter of time before we explore more resources, even if they are hidden in the deep sea.

References

[1] DeepGreen Metals Inc. (2021). Polymetallic Nodules. https://deep.green/nodules/

[2] Hein, J.R., Koschinsky, A. & Kuhn, T. Deep-ocean. (2020). Polymetallic nodules as a resource for critical materials. Nature Reviews Earth & Environment, 1, 158–169. doi.org/10.1038/s43017–020–0027–0

[3] Lenoble, J. P. (2000). Polymetallic nodules [PDF file]. International Seabed Authority. https://isa.org.jm/files/files/documents/eng7.pdf

Article Photo References

© The Green Times — http://thegreentimes.co.za/collecting-polymetallic-nodules-from-the-pacific-ocean-floor/

© Metals Co. — https://metals.co/wp-content/uploads/2020/08/polymetallic-nodule.jpg

© International Mining — https://im-mining.com/2020/08/26/deepgreen-partners-research-institutions-charactise-potential-impacts-pacific-polymetallic-nodule-mining/

© The American Ceramic Society — https://ceramics.org/ceramic-tech-today/environment/exploring-effects-of-deep-sea-mining-polymetallic-nodule-extraction-may-cause-long-term-reduction-of-carbon-flow-throughput