Persulfate Activation by Cobalt-Doped Pyrite Nanoparticles for Oxidative Removal of 4-Chlorophenol

Persulfate-based Fenton-like oxidation is one of the most promising technologies for in situ chemical oxidation (ISCO) remediation of groundwater contamination, yet the mechanisms affecting persulfate activation efficiency remain underexplored. Herein, we investigated the efficiency and mechanisms of peroxydisulfate (PDS) and peroxymonosulfate (PMS) activation by cobalt-doped pyrite (Co-FeS₂) nanoparticles for degradation of 4-chlorophenol (4-CP), a model groundwater contaminant. Notably, the degradation kinetics in the Co-FeS₂/PDS system exhibited a unique "three-stage" characteristic, wherein the 4-CP degradation rate underwent a jump during the 3-5 min phase. This kinetic anomaly stems from the specific generation dynamics of ferryl species (Fe^IV=O), which experienced a 3 min lag phase followed by a rapid burst. Theoretical calculations revealed that surface-accumulated SO₄^2- reduces the thermodynamic energy barrier for Fe^IV=O formation, which accounts for this rapid generation subsequent to the initial lag phase. Furthermore, while hydroxyl (•OH) and sulfate (SO₄^•-) radicals were critical in both systems, •OH concentration was higher than SO₄^•- concentration in the Co-FeS₂/PDS system, whereas the Co-FeS₂/PMS system exhibited the reverse trend. Moreover, homogeneous persulfate activation mediated by dissolved Fe(II) contributed to 4-CP degradation, but to different degrees in the two systems. This study provides mechanistic insights into persulfate-based ISCO processes for groundwater remediation.