Collaborative Research: Effects of Iron Redox Processes on Smectite Crystal Structures and Surface Chemistry

粘土在自然沉积物、土壤和其他地质构造中无处不在，它影响着环境中农药、有机污染物、重金属和植物营养物质等化学物质的命运。这种效应与粘土矿物的表面化学性质密切相关，其表面化学性质受其晶体结构中铁（Fe）的电荷或氧化态的影响。我们工作的长期目标是了解铁在这些表面反应中的作用，而本研究的具体目标是确定结构铁氧化态对粘土表面性质影响的潜在原因，并表征与表面物质（如水、金属和有机化合物）相互作用的性质。该研究的中心假设是，通过生物（细菌还原）或非生物（化学还原）还原结构铁，不仅会在粘土表面产生新的氧化还原电位和静电电荷，而且会引起结构铁晶体位置占据的原位变化，并引入结构缺陷，进而进一步改变粘土表面化学。我们计划通过以下四个具体目标来检验我们的中心假设，并实现本研究的总体目标：(a)确定铁还原前后混合Al-Fe-Mg双八面体蒙脱石粘土矿物中结构铁的位置占用；(b)查明铁氧化态变化与表面化学之间的因果关系；(c)确定氧化还原过程的可逆性；(d)表征细菌与无机还原剂对粘土性质的相对影响。在这项研究中获得的基本理解将为利用氧化还原行为来预测和控制粘土表面的化学反应提供有力的工具。

KostkaEAR-0203336 Clays, which are ubiquitous in natural sediments, soils, and other geologic formations, affect the fate of such chemicals as pesticides, organic contaminants, heavy metals, and plant nutrients in the environment. This effect is strongly correlated with the surface chemistry of the clay minerals, which is influenced by the charge or oxidation state of iron (Fe) in their crystal structures. The long-range goal of our work is to understand the role of iron on these surface reactions, and the specific objective of this proposed study is to identify the underlying cause(s) for the effects of structural Fe oxidation state on clay surface properties, and to characterize the nature of the resulting interactions with surface species such as water, metals, and organic compounds. The central hypothesis for the proposed research is that reduction of structural Fe by either biotic (bacterial reduction) or abiotic (chemical reduction) means, not only yields a new redox potential and electrostatic charge at the clay surface, but invokes in situ changes in crystal site occupancy of structural Fe and introduces structural defects, which, in turn, further alter the clay surface chemistry. We plan to test our central hypothesis and accomplish the overall objective of this proposed study by pursuing the following four specific aims: (a) Determine the site occupancy of structural Fe in mixed Al-Fe-Mg dioctahedral smectite clay minerals before and after Fe reduction; (b) Identify the causal relationship between changes in Fe oxidation state and surface chemistry; (c) Determine the reversibility of redox processes; and (d) Characterize the relative effects of bacteria versus inorganic reducing agents on clay properties. The fundamental understanding to be gained during this study will provide a powerful tool for using redox behavior to predict and control chemical reactions at clay surfaces.