Reactivity Of Different Ni(Ⅱ)-pyrites During Oxidative Dissolution Under Acidic And PH-neutral Conditions

Natural pyrites contain various toxic impurities,and oxidative dissolution of natural pyrites is an important geochemical process that triggers the formation of acid mine drainage（AMD）and enhances the mobility of toxic contaminants in aquatic environments.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（Py-free）,Py*-Ni（Ⅱ）and Py-Ni（Ⅱ）were prepared,and their oxidation reactivity were then compared.Our results show that Ni（Ⅱ）can be successfully incorporated into crystalline structure of Py-Ni（Ⅱ）by replacing structural Fe（Ⅱ）with the formation of sulfur defectson 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 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 Py-Ni（Ⅱ）surface,thus increasing the oxidation rates of pyrite.Variations in H₂O₂ 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 H₂O₂ at p Hs 2.5 and 7.0.The presence of Fe（III）-S（-II）defects also significantly contributes to the production of H₂O₂.At p H 2.5,more production of?OH by the structurally-incorporated Ni（Ⅱ）plays important roles in pyrite oxidation,whereas at p H 7.0,H₂O₂,Fe（IV）as well as?OH are the primary oxidants that mediate pyrite oxidation.However,the detected cumulative?OH at p H 7.0 is still significantly less than that detected at p H 2.5,indicating that contribution of?OH at p H 7.0 is not as significant as that at p H 2.5.The significance of this study is to reveals that the surface properties of pyrites are the key factors that lead to differences in the oxidation reactivity of different pyrites,which are highly important to better understand oxidation differences in natural pyrites.Moreover,the mechanistic understanding of such differences would be beneficial for accurately predicting the formation and development of AMD in natural environments.