Humic substances (HS) are large and complex organic molecules that are ubiquitous in the terrestrial and aquatic environment, including soils, sediments, and lakes and rivers. HS play an important role in many environmental processes. Among these processes are chemical reactions in which HS either accept or donate electrons from/to living (e.g., microbial) or non-living (e.g., mineral or chemical) reaction partners. These reactions are also referred to as redox reactions. There are many different redox reactions in which HS are involved. For instance, HS may transfer electrons to minerals, which may dissolve as a consequence. HS may also accept or donate electrons from/to organic and inorganic pollutants, which may affect their stability, mobility, and persistence in the environment. HS in fens and bogs may store electrons that otherwise, in the absence of HS, would end up in methane, an unwanted greenhouse gas. Understanding these important reactions requires that the redox properties of HS are known.
The major goal of this research project is to characterize the redox properties of HS. The following aspects will be studied: (i) What are the maximum numbers of electrons that HS can accept or donate? And how are these numbers affected by the origin and composition of the HS? (ii) How easy is it to withdraw an electron from or give an electron to HS? (ii) Once an electron has been withdrawn from or given to HS, can this step be reversed? For instance, can electrons stored in HS be retrieved, for instance by molecular oxygen? These questions have been raised in the past by numerous researchers in the field. However, it was previously difficult to answer these questions because suitable experimental methods were missing. In this project, we will use a novel experimental approach based on analytical electrochemistry that helps overcome limitations of previous methods. In analytical electrochemistry, the numbers of electrons transferred to or withdrawn from HS are directly counted (i.e., the electron flow). Furthermore, electrochemical techniques allow for an accurate control of the reduction potential, which defines whether there is an excess or limitation of electrons in the system.
The results from this project will lead to a better understanding of HS redox properties, which will help bio-geochemists and environmental chemists in assessing the role of HS in electron transfer reactions. This information may help predicting the environmental fate of organic and inorganic pollutants and the emission of the greenhouse gas methane from fens and bogs.