Insight into influence mechanisms of pyrite and vernadite on the degradation performance of 2,2,5-trichlorodiphenyl in a pyrophosphate-chelated Fenton-like reaction

Pyrophosphate (PP)-chelated Fenton-like reaction (PP-Fe2+/H2O2, PCFR) has been proven as a promising remediation means for polychlorinated biphenyl-polluted soils. Pyrite (FeS2) and vernadite (delta-MnO2) are typically hot-spot soil redox minerals, but whether both of them can promote the degradation performance of PCFR has been still ambiguous. Therefore, this study insightfully investigated the influence degree and corresponding mechanisms of FeS2 and delta-MnO2 on the degradation of 2,2 ',5-trichlorobiphenyl (PCB18) in a PCFR. Results demonstrated that FeS2 slightly increased the degradation efficiency and kinetic rate constant of PCB18 by 1.2% and 17.0%, respectively, but delta-MnO2 significantly decreased them by 51.9% and 93.1%, respectively, in the PCFR. The slight promoting effect of FeS2 was attributed to the homogeneous Fenton-like reaction catalyzed by the released Fe ions from FeS2, rather than the acceleration of aqueous Fe(III/II) cycle through the direct oxidation of FeS2 because the formal potential of PP-complexed Fe3+/Fe2+ pair was lower than that of SO42-/FeS2 pair. As for delta-MnO2, the aggravation of H2O2 disproportionation by delta-MnO2 and the generation of amorphous ferric (hydr)oxide from the adsorbed PP-complexed Fe3+ on the surface of delta-MnO2 played a dominant role in decreasing the degradation performance of PCB18 in the PCFR. Meanwhile, the beneficial effects of Fenton-like reactions catalyzed by surface Mn and released Mn2+ from delta-MnO2 were markedly suppressed by both detrimental pathways. Results of this study revealed that the highly redox activity of FeS2 and delta-MnO2 was unable to efficiently enhance the remediation performance of PCFR.