Rapid decomplexation of Ni-EDTA by microwave-assisted Fenton reaction

Microwave irradiation has been successfully combined with different kinds of advanced oxidation technologies to remove pollutants in aquatic environment due to its special heating mechanism. Here the authors presented a comprehensive study about microwave-assisted Fenton process for the rapid decomplexation of Ni-EDTA. Results showed that under the optimized reaction conditions, microwave irradiation promoted the cleavage of refractory linkages in metallic complexes, achieving 94.0% decomplexation efficiency within 10 min reaction. The decomplexation of Ni-EDTA, manifesting the decrease in the intensities of UV-Vis adsorption spectrums, was accompanied by the spontaneous increase of NH4+-N, inorganic carbon concentration, and solution pH. Radical scavenger tests confirmed that hydroxyl radical ((OH)-O-center dot) was the major reactive species for Ni-EDTA decomplexation, then superoxide anion (O-2(center dot-)), and singlet oxygen (O-1(2)). By comparison experiments carried out in different oxidation processes, microwave was found to improve the (OH)-O-center dot radical generation, and accelerate the ferric speciation transformation from the ionic oligomeric species to the high polymetric species with coagulation effect. In addition, the subsequent X-ray diffraction (XRD) analysis of precipitate obtained after alkalinization treatment, together with element analysis by X-ray photoelectron spectroscopy (XPS), indicated the formation of iron oxides such as Fe3O4, Fe2O3 in the microwave field, which might facilitate the decomposition of H2O2 to (OH)-O-center dot radicals as heterogeneous catalysts. Based on these results, both the thermal and non-thermal effects of microwave were found to be responsible for the rapid decomplexation of Ni-EDTA, and the possible mechanism of microwave-assisted Fenton was further proposed.