minerals Article Characterization of Fines Produced by Degradation of Polymetallic Nodules from the Clarion–Clipperton Zone Mun Gi Kim 1 , Kiseong Hyeong 1 , Chan Min Yoo 2 , Ji Yeong Lee 3 and Inah Seo 4, *   Citation: Kim, M.G.; Hyeong, K.; Yoo, C.M.; Lee, J.Y.; Seo, I. Characterization of Fines Produced by Degradation of Polymetallic Nodules from the Clarion–Clipperton Zone. Minerals 2021, 11, 205. https://doi.org/10.3390/min11020205 Academic Editor: Pedro Madureira Received: 31 January 2021 Accepted: 13 February 2021 Published: 15 February 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 Global Ocean Research Center, Korea Institute of Ocean Science and Technology, Busan 49111, Korea; mgkim@kiost.ac.kr (M.G.K.); kshyeong@kiost.ac.kr (K.H.) 2 Deep-Sea Mineral Resources Research Center, Korea Institute of Ocean Science and Technology, Busan 49111, Korea; cmyoo@kiost.ac.kr 3 Department of Earth and Environmental Sciences, Pukyong National University, Busan 48513, Korea; jylee@pukyong.ac.kr 4 Department of Earth and Environmental Sciences, Jeonbuk National University, Jeonju 54896, Korea * Correspondence: inahseo@jbnu.ac.kr Abstract: The discharge of fluid–particle mixture tailings can cause serious disturbance to the marine environment in deep-sea mining of polymetallic nodules. Unrecovered nodule fines are one of the key components of the tailings, but little information has been gained on their properties. Here, we report major, trace, and rare earth element compositions of <63 μm particles produced by the experimental degradation of two types of polymetallic nodules from the Clarion–Clipperton Zone. Compared to the bulk nodules, the fines produced are enriched in Al, K, and Fe and depleted in Mn, Co, Ni, As, Mo, and Cd. The deviation from the bulk composition of original nodules is particularly pronounced in the finer fraction of particles. With X-ray diffraction patterns showing a general increase in silicate and aluminosilicates in the fines, the observed trends indicate a significant contribution of sediment particles released from the pores and cracks of nodules. Not only the amount but also the composition of nodule fines is expected to significantly differ depending on the minimum recovery size of particles at the mining vessel. Keywords: polymetallic nodules; deep-sea mining; tailings; elemental composition 1. Introduction The economic potential of polymetallic nodules (also called ferromanganese nodules) has attracted attention for more than half a century [1], but actual commercial mining has yet to take place. While the technical issues hampering their exploitation have been partially resolved over time, environmental issues have emerged [2–4]. The assessment of environmental risks is now a prerequisite for a potential polymetallic nodule mining practice. This is especially true since the nodule fields with high economic value are mostly found in areas beyond national jurisdiction [5] and thus subject to the International Seabed Authority (ISA) regulations. A mining system design for polymetallic nodules consists of the following parts in general: a miner at the seafloor, a mining platform/vessel near the sea surface, a riser in between, and a discharge system at some depth in the water column [6]. It is envisaged that the biggest environmental impact will be caused by the miner, and that the mining platform and the riser will result in relatively minor disturbances [7]. Disturbance from the discharge system is another important yet poorly understood factor that may harm broad areas from the surface to the seafloor, both inside and outside the mining block [8,9]. A mixture of bottom water, sediment, benthic biota, and unrecovered nodule fragments will be released back to the ocean after separation processes at the surface platform [10,11]. One preferable strategy to avoid the most damaging outcome is to release them below the oxygen-minimum zone and the thermocline [12], but this is certainly insufficient to alleviate growing concerns on the environmental impacts of tailings discharge. Minerals 2021, 11, 205. https://doi.org/10.3390/min11020205 https://www.mdpi.com/journal/minerals