Activation of Peroxydisulfate by Ferrite Materials for Phenol Degradation

Persulfates such as peroxydisulfate (PDS) are among the most widely applied oxidants for breaking down organic contaminants in water. The oxidation power arises from conversion of persulfate to sulfate radical or other reactive oxidants. Ferrite materials are good candidates for catalytic activation of persulfate owing to its ability to incorporate a variety of transition metals in the structure, stability against aqueous dissolution, and magnetic susceptibility allowing catalyst separation and reuse. In this study, ferrite spinels incorporating zinc, nickel, cobalt, or copper were synthesized with an epoxide-driven sol–gel method and were annealed at 350 and 700 °C, respectively. The particles were evaluated for activating PDS using phenol as a model organic contaminant. Cu-ferrite annealed at the low temperature (350 °C) was identified to be the most active ferrite for PDS activation. This solid consists of predominantly CuFe2O4, while at the higher annealing temperature, decomposition of CuFe2O4 to Fe2O3 and CuO and significant increase in particle size resulted in severe loss of PDS activation ability. Remarkable increases in phenol oxidation rate were observed above pH 9.0 and were attributed to PDS activation by phenoxide. The presence of methanol, bicarbonate, or chloride ion (1–5 mM) significantly slowed down phenol oxidation, whereas the addition of tert-butyl alcohol did not affect the degradation rate, indicating the dominant oxidant is sulfate radical. Comparison of Cu-ferrite against reference metal oxides suggests that the catalytic performance of Cu(II) sites in the ferrite phase is comparable to those in the highly active but leachable CuO, and Cu-ferrite demonstrated good reusability during repeated phenol oxidation experiments.