Controls on the composition and extraction of rare earth elements and yttrium (REY) in deep sea polymetallic nodules and sediments

Abstract

Rising demand for metals is driving a search for new mineral resources and mining of seafloor deposits is likely to commence in the next few years. These include polymetallic nodules and crusts that are highly enriched in Mn, Co, Ni, Cu, Mo, Li and Te, and deep-sea clays that can contain high concentrations of the rare earth elements and yttrium (REY). The potential environmental impacts of mining these deposits are, however, poorly constrained and a better understanding of the processes that control metal enrichment in marine resources is essential. In support of this, I have undertaken detailed geochemical investigations of polymetallic nodules and crusts and deep sea sediments. 
Analyses of deep-sea sediments recovered from the Atlantic Ocean reveal that REY concentrations are highest (up to ~510 ppm) in slowly accumulating pelagic red clays that contain ferromanganese micronodules. Micronodules that have a hydrogenous source, characterised by lower Mn/Fe, have higher REY concentrations than micronodules that have a diagenetic source. REY concentrations in pelagic red clays from the Atlantic are ~4 times lower than concentrations reported for Pacific clays; the area of seafloor required to extract ~10% of the global annual REY demand is ~100 km2, assuming removal of the upper 1 m of sediment.
The bulk chemical composition of polymetallic nodules from different areas (UK Claim and APEI-6) of the Clarion Clipperton Fracture Zone in the eastern Pacific Ocean is rather similar, but high resolution in situ analyses show that the composition of individual layers within nodules is highly heterogeneous. Nodules from the UK Claim area contain a higher proportion of layers with high Mn/Fe and low Co that accumulate relatively rapidly; these layers likely accumulated in the Miocene when the UK Claim area was located closer to the equator, in the zone of high primary productivity.
Leaching of sediments and polymetallic nodules with dilute oxalic acid is effective for extracting the transition metals but is ineffective for releasing the REY. The REY are effectively extracted from Pacific nodules and sediments with dilute HCl, but the extraction efficiency is lower for samples from the Atlantic Ocean (~45% vs >~80% in the Pacific).Return of leached residues to seawater has the potential to increase the metal content of seawater above a mining site by up to four orders of magnitude. This may represent a source of toxicity to the benthic ecosystem in a mining area.

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ORCID for Rachael James: orcid.org/0000-0001-7402-2315

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