Thermodynamic Consideration on Degradation of Humic Acids by Electrolysis

Abstract

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 S₂O₈^{2 -} , ClO₂ ^- , and ClO⁻. In the presence of both SO₄^{2 -} 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 SO₄^{2 -} /S₂O₈^{2 -} redox couple. The change in absorption spectra with time indicated that the degree of HA degradation by S₂O₈^{2 -} 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 S₂O₈^{2 -} ions. Linear sweep voltammetry also revealed that the formation of the oxidizing chlorine species was fast and comparable to the rate of O₂ generation.