Hydroxyl and sulfate radicals activated by Fe(III)-EDDS/UV: Comparison of their degradation efficiencies and influence of critical parameters

Abstract

In the present study, comparison of activation efficiencies of hydrogen peroxide (H2O2) and persulfate (PS, Na2S2O8) induced by Fe(III)-Ethylenediamine-N,N'-disuccinic acid (EDDS) under polychromatic irradiation UVA and visible region at same conditions was studied for the first time. The effects of pH, Fe(III)-EDDS concentration, H2O2 and PS concentrations were investigated. p-hydroxyphenylacetic acid (p-HPA) was taken as a model pharmaceutical intermediate pollutant to estimate the oxidative process efficiency. In these two systems, the degradation rate of p-HPA increased (not linearly) using higher concentrated solution of H2O2 and Na2S2O8. However, when Fe(III)-EDDS concentration exceeding 250 μM, the degradation efficiency of p-HPA began to decrease. Surprisingly, results of pH effects showed that Fe(III)-EDDS/H2O2/UV system presents much higher degradation efficiency than Fe(III)-EDDS/PS/UV whatever the solution pH’s, especially in neutral and alkaline solutions. In the Fe(III)-EDDS/H2O2/UV reaction, p-HPA degradation rate (Rp-HPA) increased fast from pH 2.5 to 7.5, then it began to decrease when pH increased to 9.0. While Rp-HPA started to decrease with pH increase to 3.9 in Fe(III)-EDDS/PS/UV system. To explain this phenomenon, the second order constant of p-HPA (for both molecular and mono-anionic forms) with HO and SO4− radicals were determined by laser flash photolysis (LFP) experiments for the first time. Results showed that kp−HPA,HO• was higher than kp−HPA,SO4•− for both anionic and molecular forms of pollutant. These results demonstrated that iron-complex induced photo-Fenton process is more efficient than activation of persulfate process, particularly at environmentally closed pH values and sun-simulated wavelengths (λ > 300 nm).