Persistent degradation of 2,4-dichlorophenol in groundwater by persulfate synergize with Fe(III)/CaSO3 system: Role of Fe(IV) and 1O2 oxidation

S(IV)-based advanced oxidation processes (S(IV)-AOPs) have been gradually developed in groundwater organic contamination remediation. However, conventional Na2SO3 is extremely soluble and prone to produce high concentrations of SO32− to quench reactive oxide species (ROS), which seriously hinders the practical application of S(IV)-AOPs. In this work, a novel homogeneous iron-based AOPs system consisting of Fe(III), CaSO3, and peroxydisulfate (PDS) was proposed by using CaSO3 instead of Na2SO3 as a slow-released source of SO32−. With the synergistic of PDS, the generated Fe(II) continuously converted to Fe(III), and the kobs of the constructed Fe(III)/CaSO3/PDS system was 8.8 times higher than that of the Fe(III)/CaSO3 system. 96.5 % of 2,4-dichlorophenol (2,4-DCP) was durably degraded by Fe(III)/CaSO3/PDS system at a dose ratio of 1:5:15. ROS quenching experiments, electron paramagnetic resonance (EPR) tests, and probe tests indicated that Fe(IV), SO4−, and 1O2 played a major role in the degradation of 2,4-DCP. The conversion of SO4− to 1O2 in the system was demonstrated. Possible degradation pathways were proposed based on the density functional theory (DFT) calculations combined with LC-MS and GC–MS analysis. The results confirmed that the Fe(III)/CaSO3/PDS system exhibited strong stability and broad-spectrum applicability, which laid the foundation for the future engineering application of homogeneous iron-based AOP systems.