Keywords: 
• humic substances /
• benzoquinone /
• redox /
• electron shuttle /
• 3DEEM

Abstract: Humic substances serve as electron shuttles in the soil environment. They not only stimulate the element migration, but also accelerate containment degradation and environment remediation. In this study, a standard humic acid and two extracted humic acids from soil samples were reduced by H2,and three-dimensional excitation emission matrix fluorescence spectroscopy (3DEEM) was employed to characterize these humic acids. The results of 3DEEMs revealed the differences between native and reduced humic acids. Fluorescence peak intensity of humic acids in the reduced state was lower than those in the natural state, which can be explained by the changes of chemical structures of humic acids, such as the loss of π-π* bonds. As a model compound of quinone groups in humic acids, benzoquinone reduction and 3DEEM analysis were carried out under the same conditions. It was observed that there was a decrease of fluorescence with benzoquinone reduction. Potassium ferricyanide method was used to measure the reducing capacity of native extracted humic acids, which was found to be 0.998 and 0.465 meq·g-1 C for Yingtan and Taoyuan soils, respectively. Electron accepting capacities of humic acids were 3.384 and 1.187 meq·g-1 C for Yingtan and Taoyuan soils, respectively. Determination of electron accepting capacities, quantification of redox-active functional groups and prediction of redox reaction processes in soil are all important applications of 3DEEM in the investigation of humic acid redox-activities.