The aim of this study is to further investigate the mechanism of peroxodisulfate (PS) activation by iron sulfide crystals (FeSx) in the surface-mediated process in the aqueous environment. Various iron sulfide crystals were synthesized through co-precipitation and etch coating methods and screened for the intended use. XRD was employed to elucidate differences in crystal structure. PS activation by the synthesized iron sulfides produced about 70 times more oxidation equivalents than is possible by homolytic PS activation, suggesting that homolytic cleavage and a catalytic pathway contribute significantly. Reactive species generated during PS activation were identified as SO4•−, •OH, and FeIV=O2+. The predominant reactive species varied depending on the crystal structure of FeSx. The investigation of the PS activation process reveals that FeSx containing pyrite performed longer-term homolytic PS activation and higher stability compared to troilite and mackinawite.