Reactivity Comparison of Different Ni(II)-Pyrites during Oxidative Dissolution under Acidic and pH-Neutral Conditions

Abstract

Natural pyrite has a strong ability to immobilize Ni(II) impurities. However, the differences in the oxidation reactivity between Ni(II)-adsorbed pyrites [Py*-Ni(II)] and Ni(II)-structurally incorporated pyrites [Py-Ni(II)] are still not clearly understood. In this study, Ni(II)-free pyrite (Py-free), Py*-Ni(II), and Py-Ni(II) were prepared, and their oxidation reactivity were compared. Our results show that Ni(II) can be successfully incorporated into the crystalline structure of Py-Ni(II) by replacing structural Fe(II) with the formation of sulfur defects on the surface of Py-Ni(II). The oxidation reactivity of different pyrites depends on how Ni(II) is immobilized in pyrite and follows the order of Py-0.08 > Py-0.02 > Py-free > Py*-Ni(II) [Py-0.02 and Py-0.08 are named according to the Ni(II):Fe(II) molar ratios in the starting material for the synthesis of Py-Ni(II)], indicating that structurally incorporated Ni(II) enhances the oxidation rate of pyrite, whereas adsorbed Ni(II) does the opposite. Differences in the electrochemical properties also indicate that structurally incorporated Ni(II) enhances the electron-transfer rates at the Py-Ni(II) surface, thus increasing the oxidation rates of pyrite. Variations in H 2 O 2 concentrations confirm that the high electron-transfer rates induced by structurally incorporated Ni(II) enhance the reaction rates between dissolved oxygen and the pyrite surface, producing H 2 O 2 at pHs 2.5 and 7.0. The presence of Fe(III)-S(-II) defects also significantly contributes to the production of H 2 O 2 . Variations of cumulative •OH indicate that structurally incorporated Ni(II) improves the production of •OH at pHs 2.5 and 7.0. The significantly higher concentrations of •OH than those of H 2 O 2 at pH 2.5 indicate that •OH plays an important role in pyrite oxidation under acidic condition. The comparable concentrations of H 2 O 2 and •OH at pH 7.0 suggest that H 2 O 2 , •OH, and even Fe(IV) formed between pyrite and H 2 O 2 contribute to pyrite oxidation under pH-neutral condition.

Differences in the Oxidizing Species on the Surface of Metal-Doped Ferrihydrite and Hematite in the Alkaline S(IV)-O2 Solution

“…Considering the high adsorption capacities of Fh/Hem toward As­(III) and As­(V), , HCl solution was used to extract As­(III) and As­(V) adsorbed on Fh/Hem. − Namely, 1.0 mL of unfiltered sample was mixed with 1.0 mL of 1.0 M HCl for 30 min, and then, the mixture was filtered to analyze the corresponding concentrations of As­(V) by the modified molybdate-blue method . HPLC (LC-16, Shimazu) was performed with a WondaSil C-18 column (250 mm × 4.6 mm, 5 μm).…”