Dual-pathway persulfate activation by mackinawite (FeS) for contaminant degradation: Overlooked role of Fe(IV) species and QSAR model.

Mackinawite (FeS) can effectively activate peroxydisulfate (PDS) for pollutant degradation via synergistic radicals (SO4•- and HO•), yet the role of nonradical species (Fe(IV)) has been overlooked. PDS activation by FeS was investigated on the basis of operating parameters, reactive species, the quantitative structure-activity relationship (QSAR), and environmental applications. Compared with radicals, Fe(IV) played a secondary role in contaminant degradation. The radical and Fe(IV) contributions were assessed: the Fe(IV) contribution became more distinct with decreasing [PDS]/[FeS] ratio, initial pH, and operating temperature. A QSAR model regarding degradation rate (k) and molecular descriptors of fifteen organic compounds was developed and assessed. The PDS activation capability suffered kinetic retardation with coexisting natural matters (e.g., humic acid and anions) and solvent exchange with real water matrices. The repetition test showed that the treatment efficiency started to decrease after the first three runs then gradually declined when the catalytic cycle increased, which was ascribed to the Fe(II)-to-Fe(III) and S(-II)-to-SO42- conversions on the surface of FeS during PDS activation. The binding energy of PDS on FeS was -9.471 eV based on density functional theory (DFT) method. These findings provide both meaningful insights into nonradical reactive species produced during persulfate activation and a QSAR model for predicting contaminant removal.