Thermodynamic Consideration on Degradation of Humic Acids by Electrolysis

概要

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The degradation mechanism of humic acids (HAs) by electrolysis in aqueous solutions of Na2SO4 and/or NaCl was examined within a thermodynamics context using potential–pH diagrams. HA degradation is suggested to proceed at the anode through the formation of highly oxidizing sulfuric and chlorine species such as S2O82 - , ClO2 - , and ClO−. In the presence of both SO42 - and Cl− ions, oxidizing chlorine species were found to form preferentially at the anode partly because of the much lower redox potentials of the Cl−/ClO2 - and Cl−/ClO− redox couples compared with that of the SO42 - /S2O82 - redox couple. The change in absorption spectra with time indicated that the degree of HA degradation by S2O82 - ions is higher than that of oxidizing chlorine species, although the degradation rate is much slower. This is attributed to the low formation rate of S2O82 - ions. Linear sweep voltammetry also revealed that the formation of the oxidizing chlorine species was fast and comparable to the rate of O2 generation.