In-situ measurements of rare earth elements in deep sea sediments using nuclear methods

Karlo Nađ,Helge E S Pettersen,Ilker Meric,Jasmina Obhođaš,Milivoj Uroić,Davorin Sudac,Vlado Valković

doi:10.1038/s41598-018-23148-1

Abstract

The prospecting activities for finding new rare earth elements (REE) sources have increased greatly in recent years. One of the main discoveries was announced in 2011 by Japanese researchers who found large quantities of REE on the ocean seafloor at the sea depths greater than 4,000 m. The classic approach to investigate REE in deep sea sediments is to obtain sediment samples by drilling that is followed by laborious laboratory analysis. This is very expensive, time consuming and not appropriate for exploring vast areas. In order to efficiently explore the ocean floor for REE deposits, the further development of affordable sensors is needed. Here, we propose two nuclear techniques for exploring REE in surface deep sea sediments: i) Passive measurement of lutetium-176 radioactivity, appropriate if long-term in-situ measurements are possible, and ii) The use of the neutron sensor attached to a remotely operated vehicle for rapid in-situ measurement of gadolinium by thermal neutron-capture. Since concentrations of lutetium and gadolinium show strong linear correlation to the total REE concentrations in deep sea sediments, it is possible to deduce the total REE content by measuring Lu or Gd concentrations only.

Highlights

REE include 15 lanthanide elements, scandium (Sc) and yttrium (Y) which often occur together in same ore deposits and exhibit similar chemical characteristics
The minimum detection limit (MDL) for 176Lu was obtained experimentally as the smallest fraction of Lu within sea sediment that was possible to analyze with a precision of 3 σ
In this work we have shown that the total REE content in deep sea sediments can be to a first approximation deduced by measuring concentrations of Lu and Gd, by using passive analysis of radioactive 176Lu or by analysis of prompt gamma rays from thermal neutron capture of Gd

Summary

Introduction

REE include 15 lanthanide elements, scandium (Sc) and yttrium (Y) which often occur together in same ore deposits and exhibit similar chemical characteristics. The geostrategic importance of REE supplies came into focus in 2009 when China as a dominant producer started restricting REE export[1]. This clearly indicated the need for diversification of supply channels by development of new REE sources. The very promising new resources of REE are deep sea surface sediments as shown in the study by Kato et al.[2]. REE-rich deep sea sediments do not contain Th and U that are usually associated with land REE deposits causing serious problems of radioactive waste and increasing the price of REE production[2]. The first method is based on passive measurement of naturally occurring radioactive isotope 176Lu, and the second is based on thermal neutron-capture of Gd by using a pulsed 14 MeV neutron generator

Methods

Results

Conclusion