A significant correlation between kinetics of nitrobenzene reduction by sulfide and electron transfer capacity of mediating dissolved humic substances.

Dissolved humic substances (DHS) are ubiquitous in surface and subsurface aquatic environments and greatly affect the redox transformation of organic contaminants as reactants and/or electron transfer mediators. However, little is known about the quantitative relationship between the mediation efficiency of DHS and the physicochemical properties of DHS. Using sulfide-induced nitrobenzene reduction as a model system, we measured the reduction rate of nitrobenzene in the presence of 12 different DHS (20 mgC·L), including 4 commercial humic substances (Suwannee River humic and fulvic acids and Pahokee Peat humic and fulvic acids) and 8 soil humic substances collected as leachates from a wide variety of soils. In addition to the UV-vis absorption and fluorescence spectra, the electron donating/accepting capacities (EDC/EAC) of the tested DHS were measured using an electrochemical approach. A significant linear correlation (r = 0.99, P < .0001) was observed between the observed pseudo-first-order rate constant (k) of nitrobenzene reduction and the sum of EDC and EAC which is defined as electron transfer capacity (ETC) of DHS. A relatively good positive correlation (r = 0.69, P < .2) was shown between the k and the specific UV-absorbance at 254 nm (SUVA), whereas no good correlation was shown between the k and the fluorescence of the C1-C4 components identified by the excitation emission matrices and parallel factor (EEM-PARAFAC) analysis. This study provides a new framework for accurate prediction of the capability of DHS in mediating the redox transformation of organic contaminants. CAPSULE: A significant linear correlation exists between the kinetics of nitrobenzene reduction by sulfide and electron transfer capacity of mediating dissolved humic substances.