Comparative investigation of organic contaminant removal in the synthesized greigite (Fe3S4)- and other iron sulfide-enhanced PDS/Fe(II) systems: Efficiencies, kinetics, and mechanism insights

In this research, greigite (Fe3S4) was innovatively applied for the enhancement of PDS/Fe(II) system. From XRD and SEM analyses, Fe3S4 was successfully synthesized and appeared uniform sphere-like particles with a petallike structure. Trichloroethylene (TCE), as the target, was significantly degraded from 28.5 % to 92.8 % in PDS/Fe(II)/Fe3S4 system. The enhanced mechanisms, differences, and superiority of Fe3S4 against the other three iron sulfides were elucidated as follows: (1) on account of the abundant reduction sites on its surface, Fe3S4 could not only directly release Fe(II) and accelerate Fe(II) regeneration by sulfur substances, but also participate in the process of O2 center dot generation, and (2) the reaction mainly took place on Fe3S4 surface, in which the surface-bound reactive species (RSs) were of great importance for TCE degradation. Through EPR, quantification, and scavenging tests of RSs, HO center dot, SO4 center dot, and O2 center dot were all the dominant RSs responsible for TCE removal. Besides, these enhanced systems had high tolerance on complex water conditions and could remediate various contaminants in actual groundwater. Three TCE degradation pathways were deduced in four iron sulfide-enhanced systems and the toxicities of intermediates were evaluated, recommending that Fe3S4-enhanced system was of the least toxicity and most favorable for application in practice.