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[Display omitted] • A H 2 O 2 mediated oxidation mechanism of pyrite (0 0 1) surface in the presence of oxygen and water is reported based on first principles calculation studies. • Decomposition of the H 2 O 2 release O atom to promote the oxidation of pyrite. • The newly proposed oxidation pathway well explained the earlier isotopic composition experimental findings about the source of O atom in the SO 4 2- during the pyrite oxidation in presence of oxygen and water. Pyrite (FeS 2) is the most common metal sulfide in nature. The oxidation mechanism of the pyrite has attracted intensive research attentions. It has been determined that this process involves multi-step electron transfer reactions between the FeS 2 surface and the adsorbed O 2 and H 2 O. In this process, sulfoxide (such as S 2 O 3 2−, SO 4 2−) and ferrous (Fe2+) are released into solution, and intermediate by-products, such as hydrogen peroxide (H 2 O 2) and other reactive oxygen species (ROS), are produced. However, our understanding of the formation and transformation of these transient species is still limited. In this study, oxidation pathways by O 2 and H 2 O on the pyrite (0 0 1) surface are explored by means of density functional theory (DFT) simulation. The oxidation pathway including the H 2 O 2 forms as the intermediate is reported for the first time. It is found that the H 2 O 2 molecule forms with low activation energy barriers when the O 2 and H 2 O are co-absorbed on the pyrite surface. The H 2 O 2 dissociates and releases the O atom to promote the sulfur oxidation. The product distribution resulted by the H 2 O 2 mediated oxidation pathway agrees with the isotopic composition experiment that a minor amount of O 2 is permanently incorporated into SO 4 2– during pyrite oxidation (O in SO 4 2– is mainly derived from water). [ABSTRACT FROM AUTHOR]