During the pyrite oxidation process, aqueous ferrous/ferric ions (Fe 2+ /Fe 3+ ), as well as surface-adsorbed Fe 2+ /Fe 3+ , have been widely recognized to dominate hydroxyl radical ( • OH) generation, while this study reveals that the secondary solid iron species also play nonnegligible roles. Based on the different forms and the presence of sites, the secondary solid iron species were classified as Fe coat (iron-containing coating on the pyrite surface) and Fe dep (ex situ-deposited iron (oxyhydr)oxide that is not in contact with pyrite). Instead of participating in building a stubborn passivation layer on the pyrite surface, Fe coat is easy to fall off from the pyrite surface as the oxidation of pyrite deepens, while large fractions of Fe dep and Fe coat are found to be extractable with nitrilotriacetic acid (NTA). Achieved by cyclically oxidizing pyrite within different NTA levels (0/0.1/10 mM), Fe coat and Fe dep were proved to have distinct redox behavior during the pyrite oxidation process. Amorphous Fe dep , originated from the hydrolyzation of dissolved Fe 3+ , accelerates the nonradical decay of hydrogen peroxide (H 2 O 2 ); as a result, the accumulation of Fe dep always decreases the • OH production during the pyrite oxidation process. However, part of Fe dep adsorbs on the pyrite surface through electrostatic attraction and converts into Fe coat . The electron conduction between Fe coat and pyrite was verified, which accelerates the oxidative dissolution of pyrite, produces reactive Fe(II), and therefore favors • OH generation. This study improves our understanding of the redox behavior of pyrite in complex media such as natural processes and practical engineering systems.

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