minerals Review Ocean-Floor Sediments as a Resource of Rare Earth Elements: An Overview of Recently Studied Sites Jelena Milinovic 1, * , Francisco J. L. Rodrigues 2 , Fernando J. A. S. Barriga 3 and Bramley J. Murton 4   Citation: Milinovic, J.; Rodrigues, F.J.L.; Barriga, F.J.A.S.; Murton, B.J. Ocean-Floor Sediments as a Resource of Rare Earth Elements: An Overview of Recently Studied Sites. Minerals 2021, 11, 142. https:// doi.org/10.3390/min11020142 Academic Editor: Stefano Salvi Received: 1 January 2021 Accepted: 27 January 2021 Published: 30 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 REQUIMTE-LAQV, Departmentof Chemistry, NOVA School of Science and Technology, NOVA University Lisbon, 2829-516 Caparica, Portugal 2 Department of Communication, Philosophy and Politics, Beira Interior University, 6201-001 Covilhã, Portugal; francisco.jose.rodrigues@ubi.pt 3 IDL-Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, Edifício C1, Campo Grande, 1749-016 Lisboa, Portugal; fbarriga@fc.ul.pt 4 National Oceanography Centre, European Way, Southampton SO14 3ZH, UK; bramley.murton@noc.ac.uk * Correspondence: j.milinovic@fct.unl.pt Abstract: The rare earth elements (REE), comprising 15 elements of the lanthanum series (La-Lu) together with yttrium (Y) and scandium (Sc), have become of particular interest because of their use, for example, in modern communications, renewable energy generation, and the electrification of transport. However, the security of supply of REE is considered to be at risk due to the limited number of sources, with dependence largely on one supplier that produced approximately 63% of all REE in 2019. As a result, there is a growing need to diversify supply. This has resulted in the drive to seek new resources elsewhere, and particularly on the deep-ocean floor. Here, we give a summary of REE distribution in minerals, versatile applications, and an update of their economic value. We present the most typical onshore methods for the determination of REE and examine methods for their offshore exploration in near real time. The motivation for this comes from recent studies over the past decade that showed ΣREE concentrations as high as 22,000 ppm in ocean-floor sediments in the Pacific Ocean. The ocean-floor sediments are evaluated in terms of their potential as resources of REE, while the likely economic cost and environmental impacts of deep-sea mining these are also considered. Keywords: rare earth elements (REE) geochemistry; economic value of REE; offshore exploration of REE; ocean-floor sediments; environmental impact of ocean-floor mining 1. Introduction As developed economies across the world transition away from carbon-based fossil fuels, much of the alternative technology is dependent on an increasing number of relatively scarce yet critical elements. Rare earth elements (REE) belong to the group of critical elements, since they are vital to modern technologies. With the increased development of technology and the constant need for more advanced technical devices, the demand for rare earths is continuously growing worldwide. Considering the unique properties of REE, which can increase efficiency and performance in many technological products and techniques, they are safely considered as the key for a green energy future. REE are crucial components in many advanced technologies, such as electronics, medical and military applications, etc. [1–4]. The annual increase of REE consumption is variable, because it strongly depends on the discovery of new fields of REE application [5]. The leading industrialized countries (e.g., EU/Germany and Japan) established dif- ferent rare earth strategies [4]. As a member of the EU, Germany has no strategy for sustainable REE supply. In general, the EU is strongly dependent on imports of strategi- cally important raw materials and high-tech metals. However, until 2005, the European Commission (EC) did not fully realize that it had a critical dependency on REE. In 2007, a Minerals 2021, 11, 142. https://doi.org/10.3390/min11020142 https://www.mdpi.com/journal/minerals