Distribution of strategic key metals in deep-sea polymetallic nodules and their controlling factors

Deep-sea polymetallic nodules are widely recognized as potential resources in future for strongly enriching in many strategic key metals for high-technology applications and economic prosperity. By summarizing previous studies, the contents, occurrence, enrichment mechanism, migration, and evolution of Co, Cu, Li, Mn, Mo, Ni, and Tl distributed mainly in manganese oxides, and REY, Te, Ti distributed mainly in iron oxyhydroxides in different types and settings of polymetallic nodules were analyzed. The surface sorption drove these metals to enrich into polymetallic nodules first, in which Mo, Ni, REY, and Ti could achieve high enrichment in this stage alone. Subsequently, the oxidation of Ce, Co, and Tl, and the structural incorporation of Co, Cu, Li, Ni, and Te continued to be enriched in these strategic metals in polymetallic nodules. When the polymetallic nodules were buried by abyssal sediments, and the surrounding environment changed from oxic conditions to suboxic conditions, the large-scale mineralogical transformation could lead to the enrichment of Co, but strongly depleted in Ni, REY, Mo, and Li compared to surface nodules. When buried polymetallic nodules were finally in reduced conditions, the mineral crystal lattice of these nodules would dissolve and collapse completely, perhaps only some iron oxyhydroxides component of the former nodule could remain. Future sub-micron and in-situ high-precision experimental research work will improve our deep understanding of the distribution, enrichment history, and controlling factors of these strategic key metals in nodules, especially low-content metals of Te and Tl, and help the exploration and utilization of deep-sea polymetallic nodules.