Redox Properties and Reactivity of Sorbed Natural Organic Matter

天然有机质是土壤-含水层地球化学氧化还原过程的主要载体。NOM在电子转移反应中的突出作用来自于其作为氧化还原缓冲剂以及电子穿梭的能力，从而增强本体电子供体和受体之间的反应。虽然在天然多孔介质中，NOM的主要部分以吸附状态存在，但氧化还原性质的影响（即，电子交换容量和氧化还原状态）对NOM吸附矿物还没有得到解决，也没有吸附的NOM的氧化还原性能的吸附效果。我们假设，吸附NOM的氧化还原性能将不同于那些相同类型的NOM在没有吸附表面。这里提出的研究旨在量化这些变化，提供一个机制的理解如何吸附过程控制氧化还原性能和NOM在水系统中的地球化学功能的基础。由于NOM的吸附也决定了部分的移动的和固定的NOM和它们各自的氧化还原性能，需要一个详细的洞察力，以评估哪部分NOM将是可用的和活跃的过程，如污染物转化，微生物呼吸或电子穿梭。我们建议通过实验详细研究确定吸附NOM的氧化还原性质和氧化还原状态的机制。因此，我们建议在本提案中解决以下主要研究问题：-NOM如何吸附到固体表面本身（即，在没有电子转移的情况下）改变其氧化还原性质（电子接受/供给能力、氧化还原状态、EH分布、氧化还原介导能力）？ - 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吸附NOM的氧化还原性质？ - NOM的整体性质和预处理（来源，芳香性，酸/碱化学）如何影响吸附和电子转移过程？为此，我们将在实验室批量实验中，在环境相关的条件下，使用新的电化学技术和一系列的NOM样品和矿物，实验和系统地研究确定吸附NOM的氧化还原特性和氧化还原状态的机制。 由于NOM涂层在水生系统中无处不在，这些问题的答案将使我们能够更深入地了解地下的非均相氧化还原过程，并且是开发定量模型预测矿物-水界面的非地球化学过程的先决条件。

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.