The role of cupric ions in the oxidative dissolution process of marmatite: A dependence on Cu2+ concentration.

Cupric ions (Cu2+) play an important role in the oxidative dissolution process of marmatite in an acidic environment. In this work, dissolution experiments and numerous analytical techniques were utilized to investigate the role of Cu2+ in the oxidative dissolution process of marmatite in sulfuric acid. The dissolution experiments showed that the role of Cu2+ is significantly dependent on its concentration. A low Cu2+ concentration (0.25-750 mg/L) can significantly accelerate marmatite dissolution, and a relatively high Cu2+ concentration (above 1000 mg/L) can hinder marmatite dissolution. Element analysis, synchrotron radiation-based X-ray diffraction (SR-XRD) and Raman spectra of the leaching residues proved that no copper containing mineralogical phase was produced by the reactions between Cu2+ and marmatite. The X-ray photoelectron spectroscopy (XPS) analysis indicated that Cu2+ was first adsorbed on the marmatite surface and then produced Cu-S surface species. An electrochemical measurement further indicated that the adsorption of Cu2+ can remarkably enhance the electrochemical reactivity of the marmatite surface, thus catalyzing the oxidative dissolution process. However, a high percentage of Cu2+ adsorption on the marmatite surface can produce a passivation layer when the Cu2+ concentration is high in the solution, which decreases the electrochemical reactivity, thus resulting in the hinderance of the oxidative dissolution process of marmatite.