Dissolved organic matter (DOM) represents one of the most mobile and reactive organic matter fractions, controlling a number of physical, chemical and biological processes in ecosystem. Both laboratory and field studies show that litter and humus are the most important source of DOM in soils. Due to the complex molecules structure, a general chemical definition of DOM is impossible. DOM is often defined operationally as a continuum of organic molecules of different sizes, structures and functional properties that is able to pass through a filter with a pore size of 0.45 μm. DOM consists of low molecular weight substances, such as organic acids and amino acids, as well as complex molecules of high molecular weight, such as humic substances and enzymes.
Earlier researchers primarily focused on the complexation and adsorption properties of DOM with heavy metals and organic pollutants, by which their behaviors in soils, such as the adsorption, desorption, and transportation, were greatly affected. Recently, discoveries on the redox reactivity of humus have illuminated a new thought that DOM may also have the same mechanisms. With the aid of numerous analytical techniques, there were multiple lines of evidence for the redox reactivity of DOM. Quinine groups have main contribution to the electron transfer of DOM. Quinones are a versatile class of biomolecules found in numerous substances such as living cells, extracellular material and so forth. Quinones can be cycled among three redox states: oxidized, semiquinone radical, and reduced, thus, the electron transfer processes reversibly take place with the transformation among benzoquinone, semiquinone and hydroquinone. The benzoquinone of DOM can undergo either one-electron reduction to the semiquinone or two-electron reduction to the hydroquinone. DOM can accept electrons from other species when electron donors, especially the humic-reducing bacteria, are available in the environment. Alternatively, the reduced DOM could be oxidized by those substances with high redox potentials, such as oxygen. Previous studies showed that the electron transfer capacity of DOM was closely related to the degradation of heavy metals (e.g. Cr(Ⅵ) and Hg(Ⅱ)) and persistent organic pollutants (e.g. halohydrocarbon and nitroaromatic) in the environments. DOM can function as electron shuttle for continuous electron transfer between the reduced electron donor and the oxidized priority pollutants, displaying a significant effect on the fate and transport of organic and inorganic environmental pollutants. The redox properties of DOM, including electron transfer reversibility, electron acceptor capacity (EAC) and electron donor capacity (EDC), have been provided by chemical and biochemical methods. Nevertheless, the traditional methods (Zn and Fe³⁺ assays) are normally time consuming and usually require high degree of skill and experience to achieve reproducible results. Lately, a novel and rapid electrochemical approach has been presented to investigate the redox properties of DOM, which overcomes the limitations of the methods previously used.
Though the researches on DOM are intensive, our knowledge of the formation and function of DOM is still fragmented and often inconsistent. Thus, a systematic review is benefit to comprehensive understanding of the ecological effects of DOM. The purpose of this paper was to review three main aspects: (1) the mechanism, pathway and capability of electron transfer for DOM; (2) the methods for measuring electron transfer capacity; (3) the ecological effects and research prospects of DOM in the future.