Electron Transfer Between Sulfide and Humic Acid: Electrochemical Evaluation of the Reactivity of Sigma-Aldrich Humic Acid Toward Sulfide

Abstract

Little is known of potential reactivity and redox properties of reduced dissolved organic matter (DOM), although DOM in anoxic environments, e.g., groundwater, peat soils, or lake sediments, can be expected to differ from DOM of oxidized environments. We therefore investigated the impact of electrochemical and wet chemical [hydrogen (H2)/Pd catalyst] reduction in Sigma-Aldrich humic acid (HA) as a model DOM for high salinity, high ionic strength, or iron-rich systems on its reactivity toward sulfide. Mediated electrochemical measurement showed that the reactivity of HA toward sulfide decreased in the order non-reduced HA > electrochemically reduced (−0.1 V) HA > H2/Pd-reduced HA > electrochemically reduced (−0.4 V) HA. Results indicated that measured initial values of electron-accepting capacities of HA had a strongly positive correlation with the sulfide transformation, except for the H2/Pd treatment of HA. This latter treatment obviously changed HA structures and lead to a different reactivity toward sulfide, limiting a direct comparison to electrochemically reduced organic matter. Our result confirmed that reduced HA was still reactive toward sulfide, although to a lower extent compared with oxidized HA. Compared to electrochemical reduction, H2/Pd pre-treatment of HA alters redox properties and reactivity of organic matter and may therefore lead to results that cannot be transferred to natural systems.