Catalytic Oxidation Of Mn(?) On The Pyrite Surface And Oxidation Of As(?) By Mn(?)-HCO₃^--O₂

Mn(?)is a common ion in natural aquatic environments,and the redox cycles of different oxidation states of Mn species influence the geochemical cycles of many elements.By means of kinetic experiments and various characterizations,we investigated catalytic oxidation of Mn(?)on the surface of different pyrites and the presence of Mn(?)on the oxidation of different pyrites.In addition,we explored the oxygen carrying ability of the Mn(?)-HCO₃^-species in oxic aqueous environments,and its performance in the oxidation of As(?).Mn(?)always coexists with pyrite in natural environments,however,the catalytic oxidation of Mn(?)by pyrite and O₂ and the effect of Mn(?)on the oxidation of pyrite by O₂have not been investigated yet.In this study,pyrite was used as the heterogenous catalyst to catalyze the oxidation of Mn(?)and the mechanism involving in the process was investigated.Our results show that the oxidation rates of Mn(?)increase with increasing p H values.SEM analysis shows that rod-like Mn Ox can be detected on the surface of pyrite after oxidation.XPS and XRD analyses indicate the coexistence of Mn(?),Mn(?)and Mn(IV)on the surface of pyrite,and no apparent change can be detected in the crystalline structure of pyrite,but secondary Fe minerals can be detected.It is interesting to notice that the oxidation rates of Mn(?)decrease with increasing concentrations of Mn(?),probably due to more Mn(?)occupying the active sites on pyrites,thus inhibiting further reactions between O₂ and pyrite.In addition,oxidation of Mn(?)depends on the surface properties of pyrite.The partial oxidized pyrite accelerates the oxidation rate of Mn(?)and the generated Mn Ox can in turn accelerate the oxidization of pyrite,which also indicates that Mn(?)will improve the development of AMD(acid mine drainage)in the Mn(?)-pyrite system.We also find that the Mn(?)-HCO₃^-complex can carry dissolved oxygen and oxidize As(?)to As(V),and the oxidation efficiency of As(?)decreases with increasing p H.Under the conditions of p H 5.0,0.2 M HCO₃^-and 0.1-1.0 m M Mn(?),the oxidation rate constant of As(?)can reach 1.47×10^-2 min^-1.Quenching experiments indicate that·OH and ¹O₂ are not the primary reactive oxidative species(ROSs)in the oxidation of As(?)in the Mn(?)-HCO₃^--O₂system,whereas O₂^·-and H₂O₂ contribute to some extents.UV-vis analysis further indicates the presence of the Mn(?)O₂⁺-HCO₃^-oxidative intermediate with the characteristic peak at 275 nm in the Mn(?)-HCO₃^--O₂system.This study enriches our understanding on the oxidation of Mn(?)in oxic aqueous environments and on the oxygen carrying ability of the Mn(?)-HCO₃^-species in the oxidation of contaminants.