Arsenopyrite oxidative dissolution in NaCl solution at high-temperature and high-pressure conditions: kinetics, pathways, dissolution mechanism and geological implications

Arsenopyrite (FeAsS) is one of the sulfide minerals of seafloor massive sulfide deposits. The presence of sodium chloride and high-temperature and high-pressure (HTHP) geological conditions seriously affect the process of arsenopyrite weathering. However, electrochemical oxidative dissolution has never been considered in the context of seafloors, though it has already been shown to increase dissolution significantly in terrestrial deposits. In this work, in situ electrochemical techniques and surface analysis were used to investigate the behaviors of oxidative arsenopyrite dissolution in different concentrations of NaCl at temperatures ranging from 280 to 360 degrees C and pressures ranging from 12.0 to 20.0 MPa. In the initial stage, arsenopyrite was oxidized to S-0, As(III), and Fe(II). The S-0 and As(III) were ultimately converted into SO42- and AsO43- and entered the solution. The Fe(II) was converted into alpha-FeOOH, gamma-FeOOH, and Fe2O3 as a passivation film. The presence of Cl- ions promoted the oxidative dissolution of arsenopyrite without changing its oxidation mechanism. Higher temperatures or greater pressures promoted the oxidative dissolution of arsenopyrite by enhancing charge migration and ion diffusion. Under the experimental HTHP conditions, the oxidative arsenopyrite dissolution rate constant was 8.0 x 10(-5) mol center dot m(-2)center dot s(-1). This work expands the understanding of the geochemical cycles of Fe, As and S and provides an experimental basis for the formation of secondary minerals from arsenopyrite weathering under the hydrothermal solution conditions of the seafloor.

Automated summary

This study investigated the oxidative dissolution behavior of arsenopyrite under different concentrations of NaCl, temperatures, and pressures using in situ electrochemical techniques and surface analysis. The presence of Cl- ions promoted the dissolution of arsenopyrite, while higher temperatures and pressures enhanced the oxidative dissolution rate.