Degradation and mineralization of phenol in aqueous medium by heterogeneous monopersulfate activation on nanostructured cobalt based-perovskite catalysts ACoO3 (A = La, Ba, Sr and Ce): Characterization, kinetics and mechanism study

This study reports the heterogeneous catalytic degradation and mineralization of phenol in aqueous solution by means of potassium monopersulfate (PMS or Oxone) process mediated by cobalt perovskite-based catalysts (P-Co). Four nanostructured perovskites oxides with formula ACoO3 (A = Ba, Ce, La, Sr) were synthesized as heterogeneous catalysts by the citrate sol gel method and characterized by means of nitrogen isotherm adsorption (BET), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectrometry (EDX), x-ray photoelectron spectroscopy (XPS), X-ray Diffraction (XRD), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) techniques. All the examined samples had perovskite structure. The influences of PMS dosage, catalyst loading, initial target compound concentration and solution pH on the removal efficiency were studied. The activity showed an order of SrCoO3 > LaCoO3 > BaCoO3 > CeCoO3. LaCoO3 and SrCoO3 catalysts exhibit the better performance in terms of reaction rate and stability for the phenol degradation and mineralization by advanced oxidation technology based on sulfate radicals. Catalyst stability was assessed by means of consecutive reuse cycles. No significant loss of activity was noticed after five consecutive cycles. The role of reactive oxygen species produced in the system, mainly SO4− and OH, in the overall oxidation of phenol was determined by using suitable scavenger compounds. Under optimal conditions (10−4 M PMS, natural pH, 0.3 gL−1 catalyst loading) complete removal of 20 mgL−1 phenol was achieved in 90 min. In terms of organic carbon removal, about 81% mineralization yield was reached in the optimal conditions for 6 h heterogeneous P-Co/PMS system, suggesting an effective process for phenol mineralization. Four organics intermediates were observed and three of them were identified as catechol, hydroquinone and benzoquinone. A reaction sequence was therefore proposed for the degradation according to the detected products. Subsequent attack of these intermediates by SO4− radicals led to the formation of short chain acids such as, acetic, formic and oxalic acid which were identified by ion-exclusion chromatography.