Electrochemical Analysis of Proton and Electron Transfer Equilibria of the Reducible Moieties in Humic Acids

Abstract

Humic substances play a key role in biogeochemical and pollutant redox reactions. The objective of this work was to characterize the proton and electron transfer equilibria of the reducible moieties in different humic acids (HA). Cyclic voltammetry experiments demonstrated that diquat and ethylviologen mediated electron transfer between carbon working electrodes and HA. These compounds were used also to facilitate attainment of redox equilibria between redox electrodes and HA in potentiometric Eh measurements. Bulk electrolysis of HA combined with pH-stat acid titration demonstrated that electron transfer to the reducible moieties in HA also resulted in proton uptake, suggesting decreasing reduction potentials Eh of HA with increasing pH. This was confirmed by potentiometric Eh-pH titrations of HA at different redox states. Eh measurements of HA samples prereduced to different redox states by bulk electrolysis revealed reducible moieties in HA that cover a wide range of apparent standard reduction potentials at pH 7 from Eh0* = +0.15 to −0.3 V. Modeling revealed an overall increase in the relative abundance of reducible moieties with decreasing Eh. The wide range of HA is consistent with its involvement in numerous environmental electron transfer reactions under various redox conditions