Publication

Seafloor hydrothermal sulfides, which are expected to be a future resource for metals, could be a potential source for metal contamination in the seawater around mining sites. In this chapter, we illustrate the potential for metal leaching of both non-oxidized (non-exposed to atmosphere; before and during exploitation) and oxidized (exposed to atmosphere; after lifting and recovery) hydrothermal sulfides to seawater under different temperatures and redox conditions. One of the crucial findings was that metal dissolution behaviors differed significantly according to the specific areas and/or the initial oxidation states of the sulfide surfaces. Once the non-oxidized sulfide chips were ground to particulates and mixed with seawater, Zn and Pb were preferentially released even though these metals were included as minor components of the sulfides. For Zn, the dissolution rate increased under the oxic and higher temperature (20 °C) conditions when compared to the anoxic and lower temperature (5 °C) conditions, but the absolute rate was relatively moderate. These findings suggest that instantaneous metal release from sulfides into seawater will not occur before or during the crushing and lifting processes of seafloor mining. In contrast to the non-oxidized sulfides, the oxidized sulfides rapidly released large amounts of various metals and metalloids (e.g., Mn, Fe, Zn, Cu, As, Sb, and Pb) into seawater. The different metal dissolution behaviors between the non-oxidized and oxidized hydrothermal sulfides suggest the importance of the implementation of appropriate environmental measures to prevent leakage of the lifted sulfides to the marine surface.