Mechanism for the elimination of pollutants from aqueous solutions adopting NiR2O4 (R = Fe, Cr and Al) with microwave energy

• The residual NaOH over NiFe2O4(cp) greatly affected the elimination of BG.
• We failed to verify the so-called hot-spots over NiFe2O4(cp) within MICO process.
• Reactive oxygen species were not captured with salicylic acid within MICO process.
• NiFe2O4 failed to compete for MW energy with polar solvent in MICO process.

The microwave-induced catalytic oxidation (MICO) process has demonstrated promise for the treatment of dye contaminants in water but its mechanism has not been fully elucidated so far. In this study, NiFe2O4 catalysts were synthesized using sol-gel and chemical co-precipitation (cp) methods and applied to decolorizing a series of pollutants in a MICO process. NiFe2O4(cp) calcined at 100 °C or 300 °C decolorized over 60% of brilliant green (BG) in 10 min, but the decolorization efficiency decreased quickly when the catalyst was reused. NiFe2O4(sol-gel), however, did not achieve any significant removal of BG. Moreover, the solution pH rose to 8.6 when suspensions containing NiFe2O4(cp-100 °C) were irradiated with microwave (MW) energy. Control experiments show that addition of NaOH achieved efficient decolorization of aqueous BG solution. This indicates that MW irradiation may promote the release of OH− residues and enhance the removal of dyes. Furthermore, in suspensions containing organic polydimethyl siloxane fluid and NiFe2O4(100 °C), the MW energy distribution could be greatly changed, accompanying with quick increase of temperature. Variations of XRD patterns for NiFe2O4(100 °C), however, were not observed, even if suspensions contained carborundum (SiC). This indicates that so-called hot-spots and reactive oxygen species could be hardly formed in suspensions containing inorganic solvent H2O.

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