Onboard experiment investigating metal leaching of fresh hydrothermal sulfide cores into seawater

Shigeshi Fuchida,Hiroshi Koshikawa,Jun-Ichiro Ishibashi,Tatsuo Nozaki,Kazuhiko Shimada,Masanobu Kawachi,Yoshitaka Matsushita,Hidenori Kumagai

doi:10.1186/s12932-018-0060-9

Abstract

We observed the initial release rate of metals from four fresh (i.e., without long time exposure to the atmosphere) hydrothermal sulfide cores into artificial seawater. The sulfide samples were collected by seafloor drilling from the Okinawa Trough by D/V Chikyu, powdered under inert gas, and immediately subjected to onboard metal-leaching experiments at different temperatures (5 °C and 20 °C), and under different redox conditions (oxic and anoxic), for 1–30 h. Zinc and Pb were preferentially released from sulfide samples containing various metals (i.e., Mn, Fe, Cu, Zn, Cd, and Pb) into seawater. Under oxic experimental conditions, Zn and Pb dissolution rates from two sulfide samples composed mainly of iron disulfide minerals (pyrite and marcasite) were higher than those from two other sulfide samples with abundant sphalerite, galena, and/or silicate minerals. Scanning electron microscopy confirmed that the high metal-releasing sample contained several galvanic couples of iron disulfide with other sulfide minerals, whereas the low metal-releasing sample contained fewer galvanic couples or were coated by a silicate mineral. The experiments overall confirmed that the galvanic effects with iron disulfide minerals greatly induce the initial release of Zn and Pb from hydrothermal sulfides into seawater, especially under warm oxic conditions.

Highlights

Sulfide minerals, which are economically important metal resources, have become a major source of contamination via the release of metal cations and acids by oxidation [1]
Our experiments clearly demonstrated that metal dissolution from hydrothermal sulfides into seawater may occur without long term exposure to the atmosphere
Metal dissolution rates depended on the sulfide mineral assemblage and their surface area; the presence of high-rest-potential iron disulfide minerals may be a primary factor in inducing dissolution of low-rest-potential minerals

Summary

Introduction

Sulfide minerals, which are economically important metal resources, have become a major source of contamination via the release of metal cations and acids by oxidation [1]. Anthropogenic release of sulfide minerals into marine environments associated with SMS-mining operations may result in the generation of metal and acid contaminated seawater [2,3,4,5]. The results show that the oxidation rates of these minerals change depending on the physicochemical parameters; namely, pH, temperature, redox potential, oxygen pressure, and amounts of other oxidizing agents such as ferric ions. Based on these findings, the oxidation of terrestrial sulfides can be significantly promoted by atmospheric oxygen. The release of metals from hydrothermal sulfides into seawater is considered to be limited in open marine environments because of the alkalescence, high-buffering capacity, and low gaseous oxygen content of seawater [5]

Methods

Results

Discussion

Conclusion