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.