Project

Discipline

Natural organic matter; Humic substances; char; electron transfer reactions; electrochemistry; reactive oxygen species; reduction and oxidation

Lead
Lay summary
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 (ii) Once an electron has been withdrawn from or given to HS, can this step be reversed?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 In analytical electrochemistry, the numbers of electrons transferred to or withdrawn from HS are directly counted (i.e., the electron flow).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.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Title	Year

Number	Title	Start	Funding scheme

1. Summary of the research planHumic substances (HS) play a key role in important biogeochemical redox reactions and in the reductive and oxidative transformation of organic and inorganic contaminants. Despite the importance of HS, a thorough characterization of their redox properties and redox reactivities is yet missing. This proposal aims at overcoming the resulting knowledge gaps through a research plan directed towards improving our understanding of the fundamental redox properties of HS (Part A) and their redox reactions with oxygen (O2) (Part B). The proposed work, which is to be completed by one doctoral student, has the following specific objectives:Part A. Characterization of the fundamental redox properties of HS•Determination of the electron accepting capacities (EAC) and electron donating capacities (EDC) of representative HS as a function of HS origin and solution pH•Identification of the oxidizable and reducible moieties in HS •Determination and parameterization of the dependencies of HS reduction potentials on HS redox state and on solution chemistryPart B. Characterization of the reactivity of reduced HS with O2•Determination of kinetics and extent of the reaction between reduced HS and O2 •Identification and quantification of hydrogen peroxide, H2O2, formed by O2-reoxidation of reduced HSThese objectives will be accomplished through a combination of approaches, including analytical electrochemistry, spectroscopic techniques (electron paramagnetic resonance (EPR), UV-visible spectroscopy), stopped-flow kinetic investigations, and molecular probe methods for selective detection and quantification of H2O2. The proposed work follows a tiered approach in which the EACs and EDCs will be determined in a fast screening step for a large and diverse set of HS, followed by a more time-demanding, in-depth redox characterization of selected HS. The latter will include terrestrial, mixed terrestrial-aquatic, and aquatic HS, whose chemical properties are well characterized and which are commercially available. This will ensure that other research groups working on related questions may study the same HS in the future, which facilitates the advancement of science by allowing for knowledge transfer and discussion between groups. The key anticipated outcomes of the project include (i) the identification and quantification of HS redox properties as a function of the origin, chemical properties, and structural features of HS, and on the solution pH, (ii) establishing the applicability of analytical electrochemistry for the redox quantification of environmental HS samples, and (iii) an improved understanding of electron transfer dynamics at oxic-anoxic interfaces and in intermittently anoxic environments by studying the reactivity of reduced HS with O2. This project will advance our mechanistic understanding of HS redox reactions, and will ultimately improve our ability to unravel the role of HS in biogeochemical and contaminant redox reactions in different environmental systems.The proposed project is designed for the education of one PhD student. In addition, two related master thesis projects are anticipated.