In this study, tartrazine solutions were oxidized using innovative electrochemical advanced oxidation processes (EAOPs) that combined persulfate (PS) activation with electrogenerated H2O2, cathodic reduction and anodic oxidation at Pt and BDD anodes, and graphite cathode in an undivided stirred reactor. For the Pt anode, SO4·−was generated from a reduction reaction at the cathode and a reaction between the PS and electrogenerated H2O2. For the BDD anode, SO4·−was generated from a reduction reaction at the cathode, an oxidation reaction at the anode, and a reaction between PS and electrogenerated H2O2. Among these activation methods, the activation efficiency of PS by electrogenerated H2O2is much better than other methods. The effects of PS concentration up to 36 mM, applied current density between 6 to 15 mA cm−2,and temperatures between 25 to 45 °C were investigated. For the electro-Fenton process with Pt anode (Pt-H2O2/PS process), the best result for oxidizing 250 mg L−1tartrazine solution was obtained with 37.5 mM Na2SO4+ 9.0 mM Na2S2O8, applied current density at 12 mA cm−2and 45 °C, acquiring total color removal after 30 min reaction. For the electro-Fenton process with BDD anode (BDD-H2O2/PS process), the best result for oxidizing 250 mg L−1tartrazine solution was obtained with 25 mM Na2SO4+ 18 mM Na2S2O8, applied current density at 12 mA cm−2and 45 °C, yielding 100% color removal after 30 min reaction. The main oxidizing agents are SO4·−and OH·in the anodic oxidation process with PS and the electro-Fenton process with PS. It is concluded that the additions of PS tremendously improve the oxidation power of electro-Fenton processes with PS, especially the Pt-H2O2/PS process.Keywords:anodic oxidation;electrogenerated H2O2;persulfate;hydroxyl radical;sulfate radical
Hydrated electron reduction in an ultraviolet (UV)-sulfite system and electrochemical oxidation on a boron doped diamond (BDD) anode both are efficient processes for the degradation of perfluorooctanoic acid (PFOA). However, hydrated electron reduction has inherent deficiency in the mineralization (i.e., converting organic carbon into CO 2 ) of PFOA. Furthermore, toxic byproducts such as partially hydrodefluorinated carboxylic acids (HDCAs) and perfluorosulfonic acids (PFSAs) might give secondary pollution if the wholly hydrodefluorination of PFOA is not achieved. BDD electrochemical oxidation can obtain a high mineralization efficiency, but it leads to a low defluorination (i.e., transforming C−F bonds into F − ions) efficiency due to the generation of toxic volatile fluorinated alkanes. Herein, we report a coupled process that is hydrated electron reduction as pre-treatment and BDD electrochemical oxidation as post-treatment to achieve a complete defluorination and mineralization of PFOA, the results demonstrate that 50 mg/L PFOA can be 100 % deluorinated and mineralized under 5 h UV-sulfite reduction followed by 10 h electrochemical oxidation. This coupled process strongly inhibits the production of the toxic volatile fluorinated pollutants and quickly converts the residual toxic sulfite in UV-sulfite system to the sulfate. Mechanism investigation demonstrates that BDD oxidation is able to transform HDCAs and PFSAs formed by hydrated electron reduction to the perfluorocarboxylic acids (PFCAs) with shorter carbon chain than PFOA, which can be further defluorinated and mineralized.
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