[M-ZZ44] Marine Manganese Minerals: Depositional environments, exploration, and development

[MZZ44-P04] Metal release from submarine manganese deposits assuming mining condition

*Kyoko Yamaoka¹, Misa Toda², Kotowa Sakai², Tetsuro Okamura², Atsushi Suzuki¹ (1.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 2.IDEA Consultants, Inc.)

Keywords:ferromanganese crusts, manganese nodules, environmental impact assessment, dissolved metal

As part of the environmental impact assessment for deep-sea mining, it is necessary to consider the possibility of metal release into seawater. Trace metals in seawater act as essential elements for living organisms, but become toxic when they exceed acceptable limits. Marine manganese oxides such as ferromanganese crusts and manganese nodules are not expected to dissolve in modern oxidative seawater, but the release of each metal in response to changes in water temperature, dissolved oxygen (DO), and pH has not been evaluated in detail. In this study, three possible seawater conditions were set up: 1) deep water (4°C, DO 1 mg/L, pH 7.5), surface water (25°C, DO >5 mg/L, pH 8.0), and onboard storage tank (40°C, DO 0 mg/L, pH 7.0), ferromanganese crusts (Takuyo No.5 Smt.) and manganese nodules (GSJ geochemical standard JMn-1) were used in shaking tests at four solid-liquid ratios (1:1000, 1:100, 1:10, and 1:4). After testing, seawater samples were filtered and the concentrations of dissolved elements were measured using ICP-MS. As a result, the elements were classified into three types: 1) the strongly adsorbed type (Cu, Zn, Pb, and U), whose concentration decreases after the test compared to the original seawater; 2) the leached type (Li, Si, Cr, Ni, Se, Cd, Sb, and Ba), whose elution increases with an increase in the solid-liquid ratio; and 3) the elution-adsorption type (V, Mn, As, Mo, and W), whose elution is determined by the balance of elution and adsorption. Overall, manganese nodules tend to leach elements more readily than ferromanganese crusts, reflecting the difference in elemental speciation due to their forming processes. In terms of ionic form in seawater, strongly adsorbed or leached elements basically take the cation form, while leached-adsorbed elements take the anion form. Since the surface of manganese oxide is negatively charged, cations have higher affinity, among them, elements that form endospheric complexes with manganese oxide surfaces (Cu, Zn, Pb) are explained by stronger adsorption. There is no clear relationship with pH or DO with respect to the amount of elution, and for some elements, the higher the water temperature, the higher the amount of elution. In comparison with the U.S. EPA's national recommended aquatic life criteria (saltwater CCC) for heavy metals of particular environmental concern, none exceeded the standards even at the highest eluted concentrations. Many of the heavy metals in seawater have extremely low concentrations (<1 µg/L), making it difficult to determine the baseline, and many of them are adsorbed or almost not eluted when reacting with manganese oxides. Molybdenum is considered a promising element for monitoring because its concentration is constant at about 10 µg/L regardless of depth and its reaction with manganese oxides is easily detected. This study was supported by the Research Laboratory on Environmentally-conscious Developments and Technologies (E-code), an interdisciplinary research project of AIST.