Arsenic Complexation with Reduced Organosulfur Moieties in Soil Organic Matter: Implications for Arsenic Oxidation via Biotic and Abiotic Pathways

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Supported by the Environmental Chemical Sciences Program in the Division of Chemistry, Professor Matthew Reid and his students are studying interactions between inorganic arsenic and dissolved organic matter (DOM). DOM is a complex mixture of biomolecules derived from plant residues and microorganisms, and is an important component of all natural environments. DOM binds arsenic to form dissolved complexes with significantly different properties than unbound inorganic arsenic molecules. These reactions influence arsenic concentration in water and uptake into plants, with important implications for human exposure to arsenic, a human carcinogen. However, the functional groups of DOM that react with arsenic are poorly understood. In this project, researchers explore how the structure of DOM affects its reactivity with arsenic, and evaluate how arsenic-DOM binding influences arsenic behavior in soils. Throughout this research, undergraduate and graduate students are trained in experimental methods and characterization techniques to address fundamental problems in environmental chemistry. A graduate student is partnering with outreach staff from the Cayuga Nature Center to develop hands-on activities with a portable X-ray fluorescence instrument. This outreach strengthens the student's science communication skills and engages high school teachers and the public with soil chemistry topics of societal importance. In this project, standard humic substances as well as hydrophobic and hydrophilic DOM fractions isolated from wetland soils with a range of sulfur concentrations are reacted with arsenite (As(III)) in order to explore As(III)-DOM coordination reactions. In the first phase of the research, wet chemical analysis and synchrotron-based XAS are integrated with laboratory dialysis equilibrium experiments to quantify conditional distribution coefficients, characterize sulfur speciation in DOM, and determine the As coordination environment in DOM-bound As. In the second phase of the research, impacts of As(III) binding by DOM on As(III) oxidation kinetics via abiotic and microbial pathways are evaluated using laboratory batch studies. These research activities are designed to test the hypothesis that thiol functional moieties are high-affinity binding sites for As(III), and that As(III) bound with thiol functionalities in DOM will be stabilized in its trivalent oxidation state during oxidizing conditions. Research outcomes are expected to improve geochemical modeling of As speciation in DOM-rich environments and provide new insight into arsenic oxidation reactions in redox dynamic environments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.