The Geochemical Processes Controlling Vacancy and Mn(III) Concentrations in Birnessite Structure

土壤、沉积物和自然水域可能会受到人为来源的有毒金属的污染，如采矿和工业废物、汽车尾气、铅酸电池、油漆等，威胁公众健康和生态系统。水钠锰矿是环境中广泛分布的锰(锰)氧化物矿物的主要类型，是一种金属清除剂，可以容纳相当于其体重的几个百分比的有毒金属，因此是一种有效的解毒剂。水钠锰矿清除金属的能力以及其他性质，高度依赖于其结构和化学成分，而这些结构和化学成分受到未知的地球化学过程的影响。这项资助研究的主要目标是确定地球化学过程，并发现它们是如何一步一步地发生的，它们发生的速度有多快，以及它们是否以及如何受到常见环境条件的影响。这些结果不仅有利于关键的地球化学和矿物学过程的基础研究，而且还将有助于锰氧化物的工业应用，如制备高性能的水钠锰矿，作为污染控制的吸附剂和氧化剂，以及作为太阳能光伏产业中收集太阳能的半导体。这项拟议的研究将对学生进行不同程度的培训。国际地球科学协会和一名高中教师将共同为高中生开发一门地球科学课程。水钠铝石的非凡性质是由其结构中的锰空位和锰(III)浓度决定的。因此，迫切需要确定那些控制水钠锰矿空位和Mn(III)浓度的地球化学过程。在缺乏这种知识的情况下，评估污染物的去向和迁移，特别是在富锰环境中，将仍然具有挑战性。我们假设，Mn(II)与水钠石的反应控制着空位和Mn(III)的浓度，并且反应受溶液化学和水钠石生成速率的影响。将使用微生物中介反应系统和化学合成反应系统。除了湿化学分析外，还将使用X射线吸收光谱、衍射和原子对分布函数分析以及高分辨率电子显微镜来表征水钠铝石样品。预计结果将促进目前的理解，同时产生关于氧化锰矿物学和地球化学及其对生物地球化学循环的影响的新知识。

Soils, sediments and natural waters may become contaminated by toxic metals from anthropogenic sources, such as mining and industrial wastes, vehicle emissions, lead-acid batteries, paints, etc., threatening public health and ecosystem. Birnessite, the dominant type of wide-spread manganese (Mn) oxide mineral in the environment, is a metal scavenger and can hold several percentages of toxic metals of its body weight, hence acting as an effective detoxifying agent. The ability of birnessite to scavenge metals, as well as other properties, highly rely on its structure and chemical composition that are affected by unknown geochemical processes. The main objective of this funded research is to identify the geochemical processes and discover how they take place step by step, how rapidly they happen, as well as whether and how they are affected by common environmental conditions. Results will benefit not only the fundamental research on critical geochemical and mineralogical processes, but also industrial applications of Mn oxides, such as fabricating high-performance birnessite as sorbents and oxidizers for pollution control and as semiconductor for harvesting solar energy in the photovoltaic industry. The proposed study will train students at different levels. The PI and a high school teacher will work together to develop a curriculum in earth science for high school students.The extraordinary properties of birnessite are determined by Mn vacancy and Mn(III) concentrations in its structure. There is, therefore, a critical need to identify those geochemical processes that control birnessite vacancy and Mn(III) concentrations. In the absence of such knowledge, assessing contaminant fate and transport particularly in Mn-rich environments will remain challenging. We hypothesize that the reactions of Mn(II) with birnessite control the vacancy and Mn(III) concentrations, and that the reactions are subject to influences of solution chemistry and birnessite formation rates. Both microbially mediated and chemically-synthetic reaction systems will be employed. X-ray absorption spectroscopy, diffraction and atomic pair distribution function analysis, and high resolution transmission electron microscopy will be used to characterize birnessite samples in addition to wet chemical analyses. Results are expected to advance the current understanding while generating new knowledge of Mn oxide mineralogy and geochemistry and their impact on biogeochemical cycles.