Insights into the removal of organic contaminants by calcium sulfite activation with Fe(III): Performance, kinetics, and mechanisms

S(IV)-based advanced oxidation process has been applied for contaminants remediation. However, as a traditional source of sulfite (SO32-), Na2SO3 is extremely soluble in water, resulting in a high concentration of SO32- to quench the generated reactive oxygen species (ROS). In this work, CaSO3 was introduced instead of Na2SO3 for its slow-released SO32- ability and Fe(III)/CaSO3 system was established for the removal of trichloroethylene (TCE) and other organic contaminants. The degradation efficiency of TCE reached 94.0% and TCE could be completely dechlorinated and mineralized, while the removal of other contaminants was all over 85.0% at the optimal tested conditions. Through EPR detection, ROS scavenging and probe tests, and quantification of ROS amounts, it was concluded that the dominant ROS in Fe(III)/CaSO3 system were SO4-· and 1O2, of which the transformation mechanism of SO4-· to 1O2 was revealed and demonstrated comprehensively. The synergistic contaminants degradation performance in different sulfur-iron-containing systems and in the presence of oxidants was evaluated. The effects of various solution conditions were assessed and Fe(III)/CaSO3 system was of higher resistance on complex solution matrixes, suggesting the broad-spectrum and application perspective for the remediation of complex contaminants in actual water.