Mechanistic evaluation of ferrite AFe2O4 (A = Co, Ni, Cu, and Zn) catalytic performance in oxalic acid ozonation

Spinel ferrites AFe(2)O(4) (A = Co, Ni, Cu, and Zn) synthesized by citrate sol-gel method were tested for their catalytic performance in ozonation of aqueous oxalic acid. The objectives of this work include the evaluation of catalytic activity and ferrites stability in mineralization of oxalic acid together with exploring the activation mechanisms of ferrite catalysts in catalytic ozonation process. The results indicated that CoFe2O4 was the most active in catalyzing oxalic acid mineralization, removing 68.3% of TOC within 120 min in the reaction conditions of initial pH 2.3, oxalic acid concentration 5 mM, inlet ozone concentration 14 +/- 1 mg L-1, catalyst dose 1.0 g L-1. All catalysts exhibited satisfactory composition stability in a batch experiment, although losing up to 4.67% of the catalyst mass in Fe3+ leaching from ZnFe2O4. H-2-Temperature programmed reduction and cyclic voltammetry scan analysis revealed that these spinel ferrites had the potential reducibility and the electron donating ability to ozone molecule, respectively. Additionally, X-ray photoelectron spectroscopy showed that the surface metal ions' valence state and the surface oxygen species played important roles in catalytic ozonation process. The reaction rate constant of oxalic acid mineralization were well fitted to non-linearly depend on variable metal valence and linearly depend on surface hydroxyl group density. The radical-based mechanistic reaction pathways involving A(2+)-A(3+)-A(2+) cycle and positively charged surface hydroxyl groups were proposed.