Importance of H-Abstraction in the Oxidation of As(III) Mediated by Superoxo and Peroxo Species on the Surface of NiS ₂

Although various reactive oxygen species (ROS) have been suggested to take part in As(III) oxidation during pyrite-type disulfide dissolution, the corresponding mechanisms remain elusive. In this study, NiS₂, instead of pyrite was used as the model disulfide in that the strong interactions between Fe(II,III) at the surface of pyrite and the produced H₂O₂ make the oxidation mechanism more complicated NiS₂ shows high reactivity in As(III) oxidation by air, accompanied by the production of H₂O₂ under alkaline environments. In the quenching tests and ESR analysis, the contributions of ^•OH, O₂^•-, ¹O₂ and H₂O₂ are excluded in the oxidation of As(III), indicating that superoxo (≡Ni-OO^•) and peroxo (≡Ni-OOH) on the surface of NiS₂ are primary ROS in As(III) oxidation. The 850 and 1088 cm^-1 bands in Raman analysis indicate that the corresponding Ni-peroxo and Ni-superoxo species have been produced on the surface of NiS₂. The linear correlations were observed between the produced H₂O₂ and consumed As(III), as well as between increased production of H₂O₂ and decreasing BDE_O-H values of various phenolic compounds. These correlations collectively reveal that hydrogen abstraction from phenolic compounds/As(III) by ≡Ni-OO^• and ≡Ni-OOH is the central mechanism for As(III) oxidation. Electrochemical analyses indicate the enhanced electron-transfer on NiS₂ in the presence of As(III), confirming the surface oxidation of As(III). DFT calculations further indicate that O₂ is sequentially reduced to ≡Ni-OO^• and then ≡Ni-OOH on NiS₂, via a pathway that concurrently involves H-abstraction from As(III) to form As(V) and H₂O₂. This work provides insight into the central contribution of H-abstraction in As(III) oxidation by NiS₂ and O₂.