Tikalon Blog by Dev Gualtieri

Ocean Manganese

September 20, 2013

When I was in graduate school in the early 1970s , there was a lot of press about mining sea floor manganese nodules . These nodules are so-named because they are mostly manganese (27-30%), although they contain quite a number of other useful metals, such as nickel (1.25-1.5%), copper (1-1.4%) and cobalt (0.2-0.25%). They are sometimes called polymetallic nodules, since they incorporate these and other metals as well. I wrote about these nodules in a previous article (The Manganese Conspiracy, June 9, 2010).

Sea floor manganese nodules.

(United States Geological Survey photograph, via Wikimedia Commons.)

These baseball -sized nodules are produced over the course of tens of millions of years by slow geological processes that include precipitation of the metal hydroxides by microorganisms . In any case, the chemistry is simple. If the concentration of a dissolved metal exceeds its solubility , it will leave solution, generally attaching itself to a suitable substrate.

Manganese and nickel are useful elemental additions to alloys . Manganese, which is often used in the formulation of stainless steels , will capture dissolved oxygen and sulfur from the melt and render a better solid. Manganese is used also as an alloying element for aluminium. Manganese at about 1.5% gives aluminum corrosion resistance , and this particular alloy composition is used in aluminum beverage cans . Nickel is used in superalloys .

The base price of manganese is not that high, presently about a dollar per pound . Nickel, however, is an expensive metal, presently about $6.50/pound, with considerable price fluctuation, as shown in the figure. In the 1970s, when the price of nickel was just $1-$2/pound, there was talk about harvesting manganese nodules from the sea floor, not just for the manganese, but the nickel as well.

The price of nickel, 1960-2012, from US Geological Survey data.[1-2]

(Plot by author, using Gnumeric.)

To prevent the usual "tragedy of the commons " scenario with everyone rushing to harvest these valuable nodules from the sea, the United Nations published its Convention on the Law of the Sea in 1982; and it established an International Seabed Authority in 1994. There are presently regulations for nodule harvesting with protection of the marine environment . The United States is not a signatory of the Law of the Sea Convention. Since nodule harvesting is technically challenging enough without governmental interference, most companies do not presently envision nodule harvesting as being economically viable.

The early hyperbole of manganese nodule economics was actually a cover story for a US Central Intelligence Agency operation. What the public thought was an investigation of nodule harvesting by reclusive billionaire industrialist, Howard Hughes, was actually a front for Project Azorian, an attempt to recover K-129, a Soviet submarine that sank in deep water in April, 1968.

The salvage vessel, the Hughes Glomar Explorer , was launched in 1974 with the purported purpose of harvesting manganese nodules from the ocean floor. As the story goes (with the caveat that it's usually not wise to believe everything a government tells you), only a third of the K-129 submarine was recovered. This third did not contain such coveted items as nuclear missiles and code book , but it did contain some cryptographic equipment.

The Hughes Glomar Explorer

(Via Wikimedia Commons.)

There's still some basic research being undertaken on oceanic manganese, as a recent paper in Science proves.[3-4] Scientists from the University of Delaware (Lewes, Delaware ), the Oregon Health and Science University (Beaverton, Oregon ), McGill University (Montreal, Quebec ) and the Université du Québec à Rimouski (Rimouski, Québec ), looked at the oxidation states of manganese in sea water. Manganese is an essential nutrient in photosynthesis . Says paper coauthor, George Luther of the University of Delaware,

"You wouldn't think manganese is that important, but without manganese, we wouldn't have the molecular oxygen that we breathe."[4]

Manganese has several oxidation states, and the research team found that oceanic manganese(III) is far more prevalent than previously thought.[4] In previous studies of this element, only the total dissolved manganese was measured, and it was presumed that it was all manganese(II).[3-4]

The first indication that Mn(III) might be prevalent was a study by Luther in the mid-2000s in which this oxidation state was found in Black Sea waters in which a gradient in oxygen concentration is present. In such water, oxygen levels are relatively high near the surface, and they diminish with depth.[4]

Studies showed that Mn(III) was also present in the sea floor mud, not only at the Black Sea, but also at a Gulf of Saint Lawrence and a Delaware salt marsh having the same oxygen gradient.[4] Soluble Mn(III) is likely stabilized by organic or inorganic ligands , and the research team thinks that it accounts for up to 90% of dissolved manganese.[3] It's produced by oxidation of dissolved Mn(II) and by reductive dissolution of MnO₂ by biological or non-biological processes.[3]

As I wrote earlier, precipitation of metal hydroxides by microorganisms is a process in the formation of manganese nodules, and the published study offers more insight into this mechanism. Says study coauthor, Andrew Madison , who did this research as part of his graduate studies at the University of Delaware,

"In sediments, bacteria prefer to consume molecular oxygen and nitrate first due to their high energy gain... After those are consumed, bacteria then couple organic matter oxidation to manganese oxide reduction, which can produce soluble manganese(III)."[3]

This research was funded by the National Science Foundation and the Natural Sciences and Engineering Research Council of Canada.[3]

Ocean Manganese

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