Redox Properties and Reactivity of Sorbed Natural Organic Matter

天然有机物(NOM)是土壤含水层中生物地球化学氧化还原过程的主要驱动力。NOM在电子转移反应中的突出作用是它既可以作为氧化还原缓冲区，也可以作为电子穿梭，促进主体电子供体和受体之间的反应。尽管在天然多孔介质中，NOM主要以吸附状态存在，但氧化还原性质(即电子交换容量和氧化还原状态)对NOM吸附矿物的影响以及吸附对吸附NOM氧化还原性质的影响尚未被研究。我们假设，在没有吸附表面的情况下，吸附的NOM的氧化还原性质将不同于相同类型的NOM。这项研究旨在量化这些变化，为从机理上理解吸附过程如何控制NOM在水体系中的氧化还原性质和生物地球化学功能奠定了基础。由于NOM的吸附还决定了流动和固定NOM的比例及其各自的氧化还原性质，因此需要详细了解NOM的哪一部分可用于污染物转化、微生物呼吸或电子穿梭等过程。我们建议从实验上详细研究决定吸附的NOM的氧化还原性质和氧化还原状态的机理。因此，我们建议解决以下主要研究问题：-NOM本身在固体表面上的吸附(即在没有电子转移的情况下)如何改变其氧化还原性质(电子接受/给予能力、氧化还原状态、EH分布、氧化还原中介能力)？-NOM的氧化还原状态如何影响其吸附行为？-吸附剂和NOM之间的电子转移如何改变被吸附的NOM的氧化还原性质？-NOM的本体性质和预处理(来源、芳香性、酸碱化学)如何影响吸附和电子转移过程？为此，我们将利用新的电化学技术和一系列NOM样品和矿物，在实验室批量实验中对环境相关条件下决定吸附NOM的氧化还原性质和氧化还原状态的机理进行实验和系统的研究。由于NOM涂层在水生系统中普遍存在，对这些问题的回答将使我们能够更深入地了解地下非均相氧化还原过程，并是开发预测矿物-水界面生物地球化学过程的定量模型的先决条件。

Natural organic matter (NOM) is a major diver of biogeochemical redox processes in soils aquifers. The prominent role of NOM in electron transfer reactions arises from is its ability to act as redox buffer as well as an electron shuttle, enhancing reactions between bulk electron donors and acceptors. Although in natural porous media a major fraction of NOM is present in sorbed state, the effect of redox properties (i.e., the electron exchange capacities and the redox state) on NOM sorption to minerals has not been addressed nor the effect of sorption on the redox properties of sorbed NOM. We hypothesize that the redox properties of adsorbed NOM will differ from those of the same type of NOM in the absence of sorbing surfaces. The research proposed here aims at quantifying these changes to provide the basis for a mechanistic understanding of how sorption processes control redox properties and biogeochemical functions of NOM in aqueous systems. Since sorption of NOM also determines the fraction of mobile and immobilized NOM and their respective redox properties, a detailed insight is needed in order to assess which part of NOM will be available and active for processes such as pollutant transformation, microbial respiration or electron shuttling. We suggest to study experimentally the mechanisms in detail that determine the redox properties and redox state of sorbed NOM. Thus, we propose to address the following major research questions in this proposal: - How does sorption of NOM to solid surfaces per se (i.e., in the absence of electron transfer) change its redox properties (electron accepting/donating capacity, redox state, EH distribution, redox mediation capacity)? - How does the redox state of NOM influence its sorption behavior? - How does electron transfer between the sorbent and NOM change the redox properties of sorbed NOM? - How do bulk properties and pretreatment of NOM (origin, aromaticity, acid/base chemistry) affect sorption and electron transfer processes altogether? To this end, we will study experimentally and systematically the mechanisms that determine the redox properties and redox state of sorbed NOM in laboratory batch experiments at environmentally relevant conditions using novel electrochemical techniques and a range of NOM samples and minerals. As NOM coatings are ubiquitous in aquatic systems answers to these questions will allow us to develop a much deeper understanding of heterogeneous redox processes in the subsurface and are a prerequisite for development of quantitative models for predicting biogeochemical processes at the mineral-water interface.