Promoted elimination of metronidazole in ferrous ions activated peroxydisulfate process by gallic acid complexation

We applied gallic acid (GA) as the complexing agent to stabilizing the regeneration of Fe²⁺ during the Fe²⁺/peroxydisulfate (PDS) Fenton-like reaction for promoting the removal of metronidazole (MTZ). This research evaluated the elimination of MTZ by optimizing the dose of GA and Fe²⁺ and pH condition. MTZ removal reached 83% at the GA: Fe²⁺ molar ratio of 1:1 (30 μM) and initial pH 5 and 6.2 after 120 min, and the kinetics showed two degradation phases (k_obs1 = 0.09636 for the rapid stage and k_obs2 = 0.01056 for the slow stage). The Fe²⁺ and GA complexes could expand the range of pH applicability and effectively stabilize the regeneration of Fe²⁺, which ultimately promoted the decontamination of MTZ. Sulfate radical (SO₄^.-), hydroxyl radicals, and singlet oxygen were proved to exist in this ternary system and contribute to MTZ removal, and SO₄^.- played the dominant role. Furthermore, the possible pathways and mechanisms for MTZ degradation were proposed, and the simulation result indicated that the toxicity of degradation intermediates of MTZ were declined. The GA assisted Fe²⁺/PDS system provided an improved promising advanced oxidation process for organic wastewater disposal.