Thermodynamic Consideration on Degradation of Humic Acids by Electrolysis

Abstract

The degradation mechanism of humic acids (HAs) by electrolysis in aqueous solutions of Na 2 SO 4 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 S 2 O 2À 8 , ClO À 2 , and ClO − . In the presence of both SO 2À 4 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 − /ClO À 2 and Cl − /ClO − redox couples compared with that of the SO 2À 4 /S 2 O 2À 8 redox couple. The change in absorption spectra with time indicated that the degree of HA degradation by S 2 O 2À 8 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 S 2 O 2À 8 ions. Linear sweep voltammetry also revealed that the formation of the oxidizing chlorine species was fast and comparable to the rate of O 2 generation.