Photocatalytic oxidation of organic pollutants catalyzed by an iron complex at biocompatible pH values: using O2 as main oxidant in a Fenton-like reaction

A red iron(II) 4,4′-dicarboxy-2,2′-bipyridine complex ([Fe^II(dcbpy)₃]) was investigated as an extraordinary Fenton catalyst capable of activating much more molecular O₂ to mineralize organic pollutants in water at biocompatible pH values under visible irradiation. Eight representative organic pollutants were effectively degraded in the presence of this catalyst with high turnover number (368−2000). The flexible bifunctional coordination mode (N donor for ferrous ion and O donor for ferric form) devoted by the ligand dcbpy should be responsible for the preservation of iron(II/III) catalysis in such a neutral pH condition, whereas any substitution of the 4,4′-carboxylic groups in dcbpy by other groups such as ether, alcohol, nitroyl, or methyl groups resulted in nearly total loss of catalytic stability. More important, the present [Fe^II(dcbpy)₃] catalyst can dramatically change the traditional role ofH₂O₂ as main oxidant in the general Fenton reaction and make molecular O₂ become the main oxidant in the mineralization of organic pollutants instead. Through the simultaneously quantitative measure of the actual consumption of molecular O₂ and H₂O₂ as well as the corresponding mineralization yields of substrates (2,4-DCP and Org II), respectively, we found that the usage of O₂ not only is almost twice the H₂O₂ depletion for the mineralization of substrates but also rigorously accords with the mineralization yield of substrates in terms of stoichiometric relation. No matter whether H₂O₂ is in excess or not, O₂ participates in the mineralization of substrates and acts as the main oxidant. For the same amount of H₂O₂ consumption, the O₂ consumption was only 2.5% and 8.13% relative to the H₂O₂ usage in the controlled general Fenton reaction and UV−Fenton reaction (pH = 3.0), respectively. This clearly indicates that the present catalyst is able to use much more O₂ to eliminate organic pollutants in water under visible irradiation. Visible irradiation is crucial for the oxidative degradation and mineralization process. The extraordinary ability of bifunctional coordination sites offered by this ligand provides a promising design paradigm for Fenton-like catalyst to eliminate organic pollutants by using more solar energy and O₂ in air.