Quantitative elucidation of the Role of Natural Organic Matter on the Redox Reactivity of Iron Oxide Minerals

In this project funded by the Environmental Chemical Sciences Program of the Chemistry Division, Professors Lee Penn and Bill Arnold of the University of Minnesota study how iron oxide mineral particles participate in chemical reactions with organic contaminants, with particular focus on how natural organic matter (NOM) affects these reactions. NOM is found in surface water and ground water. It is formed from the decomposition of plant material and produced by algae and other microorganisms, and little is known about how the presence, concentration, and nature of NOM impacts reactions at surfaces. The collaboration between Professors Penn and Arnold improves the ability to predict fate and transport of common organic contaminants. Results are expected to lead to a fundamental, mechanistic understanding of how iron oxide surfaces mediate important reductive degradation reactions in the presence of organic matter.

This project focuses on elucidating how natural organic matter impacts the nature of reactive surface area and thus the kinetics of reductive (and oxidative) reactions involving organic contaminants at the mineral-liquid interface. Rates of contaminant degradation are quantified and solid materials characterized using advanced materials characterization methods before, during, and after reactions. Experiments are performed with and without the addition of model NOM molecules (e.g., catecholates and carboxylates), NOM isolates obtained from the International Humics Substances Society (IHSS), and NOM samples collected from the environment. A particularly novel aspect of the proposed work is direct imaging of the particles in reaction media by way of cryogenic transmission electron microscopy (cryo-TEM) to evaluate aggregation state and quantify in situ accessible surface area. The solid-state (i.e., mineralogical) changes and changes in the aggregation state of suspended mineral particles are quantitatively linked to the observed contaminant degradation kinetics, which enables the development of an integrated model of iron oxide reactivity. The broader impacts of this work include potential societal benefits via improved understanding of the role of abiotic attenuation of contaminants in natural and engineered systems as well as the development of a program aimed at middle and high school students to teach important concepts of water and environmental chemistry.