Selenium (Se) pollution of surface waters and sediments is pervasive in the western U.S. due to erosion, mining, combustion, petroleum processing, and irrigation activities that involve Se-rich soils, shales, and ore. Its accumulation in lakes and sediments has lead to adverse biological effects in wildlife and to health threats to humans, prompting a need for large-scale monitoring and bioremediation efforts. The mobility and bioavailability of Se in water strongly depends on the chemical form of Se. This project will investigate mechanisms of mobilization of dissolved Se oxyanions (selenate and selenite) and their immobilization in natural and remediation systems under diverse biogeochemical settings using a novel isotopic technique. Over the past decade, stable Se isotope studies from other researchers have provided valuable insights to Se biogeochemistry and environmental Se source identification. However, stable oxygen isotopic studies of Se oxyanions, so far lacking, will provide a new approach to characterizing Se redox cycling and expand applicability of isotope fractionations to reactions previously not sensitive to Se isotope fractionation, such as oxygen atom addition via oxidation. The objective of this project is to develop a stable oxygen isotopic approach and apply it to answer key knowledge gaps within aqueous selenium biogeochemistry. Laboratory experiments will examine reaction pathways including: (1) describing how dissolved selenate and selenite form from the oxidation of parent minerals and enter waterways, (2) probing interfacial oxygen exchange mechanisms that occur during oxyanion sorption to environmental metal oxides, and (3) assembling oxygen isotope fractionation indicators specific to oxyanion immobilization pathways such as reduction and uptake reactions with minerals, bacteria, algae, and fungi. Stable oxygen isotope values of oxyanions, water, oxides, and oxidants will be monitored during reactions using isotope ratio mass spectrometry. The oxygen isotope tracing experiments will identify likely sources of oxygen incorporation and exchange during oxyanion formation and interaction with oxides, leading to a clearer conceptual model of Se pollutant dynamics. Like Se isotopes, the measured oxygen isotope fractionation values are expected to be unique to different reduction pathways and therefore can be used to distinguish between chemical and microbial reactions responsible for dissolved Se processing.
This project will expand the scientific knowledge of how selenium oxyanions form and transform in aquatic systems through reaction pathway tracing with isotope labeling. A better understanding will be developed for the chemical and microbial controls on selenium mobilization and immobilization. The results are expected to form the basis of improved field site monitoring and remediation efforts by using isotopic information that directly reveal field site reactions. The stable oxygen isotope approach to evaluating mechanisms will have broad applicability to evaluating other inorganic oxyanion contaminants such as arsenic and chromium. This project will provide research opportunities for one graduate student and several undergraduate students, high school students, and K-12 teachers within a multidisciplinary setting that investigates chemical, biological, and mineralogical agents interacting with metallic water contaminants. In addition to engaging experiential learning through hands-on laboratory engagement, these K-12 participants will also develop educational materials, in the form of field trip demonstrations and curriculum modules, for the instruction of broad audiences about the principles of water contamination and treatment.
