Insights to Selenium Cycling and Remediation Revealed by Stable Oxygen Isotopes

1236182Philip larese - casanovs由于侵蚀、采矿、燃烧、石油加工以及涉及富硒土壤、页岩和矿石的灌溉活动，美国西部地表水和沉积物的硒污染普遍存在。硒在湖泊和沉积物中的积累对野生动物造成了不利的生物影响，对人类的健康构成了威胁，促使需要进行大规模的监测和生物修复工作。硒在水中的流动性和生物利用度在很大程度上取决于硒的化学形态。本项目将利用一种新的同位素技术研究溶解硒氧离子（硒酸盐和亚硒酸盐）的动员机制及其在不同生物地球化学环境下在自然和修复系统中的固定化。在过去的十年中，其他研究人员的稳定硒同位素研究为硒生物地球化学和环境硒源鉴定提供了有价值的见解。然而，目前缺乏的硒氧离子的稳定氧同位素研究将为表征硒氧化还原循环提供一种新的方法，并扩大同位素分馏在以前对硒同位素分馏不敏感的反应中的适用性，例如氧化过程中的氧原子加成。该项目的目标是开发一种稳定的氧同位素方法，并将其应用于解决水硒生物地球化学中的关键知识空白。实验室实验将检查反应途径，包括：(1)描述溶解的硒酸盐和亚硒酸盐如何从母体矿物氧化形成并进入水道；(2)探测氧化阴离子吸附环境金属氧化物过程中发生的界面氧交换机制；(3)组装氧同位素分异指标，特定于氧化阴离子固定途径，如与矿物质、细菌、藻类和真菌的还原和吸收反应。在反应过程中，氧离子、水、氧化物和氧化剂的稳定氧同位素值将使用同位素比质谱法进行监测。氧同位素示踪实验将确定氧阴离子形成和与氧化物相互作用过程中氧结合和交换的可能来源，从而建立更清晰的硒污染物动力学概念模型。与硒同位素一样，测量到的氧同位素分馏值预计对不同的还原途径是独特的，因此可以用来区分化学反应和微生物反应，负责溶解硒的处理。该项目将通过同位素标记的反应路径追踪，扩展对硒氧阴离子在水生系统中形成和转化的科学知识。对硒的化学和微生物控制将有更深入的了解。这些结果有望通过使用直接揭示现场反应的同位素信息，为改进现场监测和补救工作奠定基础。稳定氧同位素评价机制的方法将广泛适用于评价其他无机氧离子污染物，如砷和铬。该项目将为一名研究生、几名本科生、高中生和K-12教师提供研究机会，研究化学、生物和矿物学试剂与金属水污染物的相互作用。除了通过动手实验进行体验式学习外，这些K-12的参与者还将以实地考察演示和课程模块的形式开发教育材料，以指导广大受众关于水污染和处理的原理。

1236182Philip Larese-CasanovaSelenium (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.