Research On The Formation Of Hydrogen Peroxide During Ferrate Oxidation

As the water shortage and contamination deteriorating, ferrate（VI） has received increased attention as a potential water treatment chemical due to its multi functions such as oxidation, disinfection, adsorbtion and coagulation. The fate of perferryl（V） and ferryl（IV） during ferrate（VI） oxidation, the formation of bromate and halogenated byproducts as well as the combined application of ferrate oxidation with other water treatment process are currently the frontier of the ferrate research. Hydrogen peroxide, as an important product of ferrate decay, has played an essential role in the processes mentioned above. Hence, the formation of hydrogen peroxide during ferrate oxidation needs to be examined systematically to provide reference for the theoretical research and technical support for the application of ferrate.The formation of hydrogen peroxide during ferrate oxidation was subject to the ferrate dosage, ratio of ferrate to target compounds, p H value and the complexing intensity of the reaction system. H₂O₂ yield decreased as the ferrate dosage/ratio of ferrate to the target compounds increased because the oxidative consumption of hydrogen peroxide by ferrate was intensified. As the ferrate exposure was raised greatly with increasing p H, which was adverse to the accumulation of hydrogen peroxide, H₂O₂ yield of ferrate self-decay was reduced dramatically. However, the variation of H₂O₂ yield due to p H change during the reaction between ferrate and target compounds depended on the target compounds. H₂O₂ yield of reaction with alkene, sulfoxide and arsenite decreased with the increasing p H, while H₂O₂ yield of reaction with phenolics and amines dropped first and went up after. Generally, the increase of complexing intensity（depends on the complexing ability and concentration of the coexistence components） was favorable for the formation of hydrogen peroxide. Since the complexing agents affected the consumption rate of hydrogen peroxide weakly, it was proposed that the complexing agents could promote the self-decomposition of perferryl（V）, which produced hydrogen peroxide simultaneously, by complexing the in-situ formed perferryl（V）.The oxidation of target compounds by ferrate was inhibited by complexing agent while it promoted the formation of hydrogen peroxide. The stoichiometry of Fe（VI）: ABTS:ABTS· +:H₂O₂ changed from 1:1:1:1 to 1:1.38:1.29:0.44 in the absence of complexing agents, indicating that perferryl（V） preferred self-decomposition rather than oxidizing ABTS in the presence of complexing agents. The reaction kinetics also suggested that complexing agents such as pyrophosphate, phosphate and oxalate would suppress the oxidation of target organics（e.g. phenol, SMX and CBZ） by ferrate. The aminocarboxyl chelating agents such as EDTA and NTA also restrained the oxidation of SMX and CBZ by ferrate because of its complexing ability. However, its reducing property would induce ferrate to yield the active intermediate and seletively enhanced the oxidation of target organics（e.g. phenol）.The background matrix in surface water promoted the formation of hydrogen peroxide as well as the complexing agents did, probably due to the undefined ligands therein. The influence of background matrix on oxidation of target compounds by ferrate was similar to that of aminocarboxyl chelating agents, which exhibited enhancement or suppression that depended on the target compounds.