The long-term effect of Fe₃O₄ in activating persulfate to degrade refractory organic contaminants for groundwater remediation

Highly reactive metal-based materials for persulfate activation have been intensively studied for treatment of groundwater contaminants. However, the long-term efficacy of the readily available metal-oxide additives, which can be more important for groundwater remediation practice, has received minimal attention. In this study, the results of preliminary experiments demonstrated that Fe₃O₄ was superior to seven metal oxides (MnO, Fe₂O₃, MnO₂, CuO, Cu₂O, CoO, NiO) and zero-valent iron for activating persulfate to degrade 1,2-dichloropropane. Experiments were then conducted to examine the long-term effects of Fe₃O₄-activated persulfate degradation of four contaminants, i.e., 1,2-dichloropropane, 1,1,2-trichloroethane, 1,4-dioxane, and methyl tert-butyl ether (MTBE). The system achieved complete removal of the contaminants in approximately 72 h, which was contrasted to a removal efficiency less than 35 % in the absence of Fe₃O₄. Highly efficient use of persulfate for contaminant degradation was observed with insignificant production of soluble Fe before depletion of the contaminants. Production of sulfate radical (SO₄^•−) and hydroxyl radical (HO^•) via surface reactions was likely the process responsible for the degradation. Ten and five cycles of 1,2-dichloropropane degradation experiments were conducted in deionized water and actual groundwater. These results suggested that the Fe₃O₄ could retain 80 % of its original activation capacity after 5 cycles, but partial oxidation of Fe₃O₄ eventually occurred. Production of some Cl-containing degradation-resistant intermediates from degradation of 1,2-DCP was observed and could be mitigated under specific conditions. The results show that Fe₃O₄ has a long-term effect in activating persulfate for contaminant degradation, which is promising for applications such as permeable reactive barriers and nanoparticle injection for groundwater remediation.