ETBC: Hidden Iron Oxide Redox Processes During Biogeochemical Iron Cycling: Controls on Nanoscale Transformations and the Fate of Contaminants

生物地球化学铁循环主要涉及铁在Fe（II）和Fe（III）氧化态之间的交替。这种铁的氧化还原循环与碳、氧、磷和硫的地球化学循环有关，在控制砷、铀和三氯乙烯等污染物的命运方面发挥着重要作用。水溶液中的Fe（II）和Fe（III）氧化物矿物在铁的地球化学循环过程中经常共存，这些物种之间的二次非生物反应可能会将结晶度差的氧化铁转化为结晶度更高的相，断裂的铁同位素，影响污染物的命运和物种形成，并可能对微生物铁还原起到负反馈作用。由于这些二次过程具有重要的地球化学和环境影响，我们需要获得一个基本的反应，发生在含水Fe（II）和Fe（III）氧化物之间的机械理解。最近的研究已经观察到水溶液Fe（II）和晶体Fe（III）氧化物之间的电子转移和原子交换。初步测量表明，Fe（II）与赤铁矿（#945;-Fe 2 O3）的反应随晶体学取向而变化，（001）表面经历生长和其他表面溶解。在pH 3和7下观察到类似的效果。由于这些反应似乎是独立的Fe（II）吸附，不影响散装矿物学或流体组成，他们是有效的？隐藏？氧化还原过程我们假设Fe（II）起催化作用，铁原子通过溶液以Fe（II）的形式转移到（001）表面，电子通过赤铁矿结构转移离开该表面。本研究的目的是：（1）研究Fe（II）存在下赤铁矿表面氧化还原过程的性质，包括氧化还原过程随溶液条件和时间的变化以及氧化还原过程是连续的还是自限性的;（2）确定常见无机物质铝、磷酸盐和硅酸盐的存在如何影响这种耦合生长和溶解，所有已知的赤铁矿生长调节剂;（3）确定这些过程如何影响结构相容的[Ni（II）]和不相容的[As（V）]污染物的形态;（4）调查这些过程是否可以在硫化物还原溶解过程中激活，这是海洋沉积物中的一个重要过程。这些研究预计将证明一个新的复杂的非生物界面氧化还原过程，发生在地球化学铁循环创建系统与共存的Fe（II）和Fe（III）物种。这些研究也可能为铁生物矿化和纳米颗粒合成提供新的见解。最后，类似的过程可能会发生许多元素，共存于不同的氧化态，具有不同的溶解度，如S，Mn，或U;预期的结果，因此可以作为一个指导，探索复杂的氧化还原过程在其他地球化学系统。更广泛的影响：该项目将促进在地球化学领域培训两名新的研究生。它还将使一些本科研究人员（每年2-3名）接受科学实践教育，培训他们制定研究问题和使用回答科学问题所需的工具和方法。每个研究生将有机会指导一名本科生，作为他们作为未来教育工作者的准备的一部分。这项研究的结果将被纳入PI教授的研究生课程。这项研究也可能产生社会影响，因为这些表面氧化还原反应过程中污染物的命运研究可能为未来开发新的修复策略或方法以补充水过滤系统提供基础。

Intellectual Merit: Biogeochemical iron cycling involves primarily the alternation of iron between Fe(II) and Fe(III) oxidation states. This iron redox cycling is connected to the biogeochemical cycles of carbon, oxygen, phosphorus, and sulfur, and plays an important role controlling the fate of contaminants such as arsenic, uranium, and trichloroethylene. Aqueous Fe(II) and Fe(III) oxide minerals often coexist during biogeochemical iron cycling, and secondary abiotic reactions between these species may transform poorly crystalline iron oxides into more crystalline phases, fractionate iron isotopes, affect contaminant fate and speciation, and possibly acts as a negative feedback on microbial iron reduction. As these secondary processes have important geochemical and environmental implications, we need to obtain a mechanistic understanding of the fundamental reactions that occur between aqueous Fe(II) and Fe(III) oxides. Recent studies have observed electron transfer and atom exchange between aqueous Fe(II) and crystalline Fe(III) oxides. Preliminary measurements reveal that the reaction of Fe(II) with hematite (α-Fe2O3) varies with crystallographic orientation, with the (001) surface experiencing growth and other surfaces dissolution. Similar effects were seen at pH 3 and 7. As these reactions appear to be independent of Fe(II) adsorption and do not affect the bulk mineralogy or fluid composition, they are effectively ?hidden? redox processes. We hypothesize that Fe(II) serves a catalytic role, with iron atoms transferring to (001) surfaces through solution as Fe(II), and electrons transferring away from this surface through the hematite structure. The objectives of this proposal are to: (1) characterize the nature of the dynamic hematite surface redox processes operating in the presence of Fe(II), including how they vary with solution conditions and time and whether they are continuous of self-limiting; (2) determine how this coupled growth and dissolution is affected by the presence of the common inorganic species aluminum, phosphate, and silicate, all known hematite growth modifiers; (3) determine how these processes affect the speciation of structurally compatible [Ni(II)] and incompatible [As(V)] contaminants; and (4) investigate whether these processes can be activated during reductive dissolution by sulfide, an important process in marine sediments. These studies are expected to demonstrate a new complex abiotic interfacial redox process that occurs when biogeochemical iron cycling creates systems with coexisting Fe(II) and Fe(III) species. These studies may also provide new insight into iron biomineralization and nanoparticle synthesis. Finally, similar processes may occur for many elements that coexist in different oxidation states having different solubilities, such as S, Mn, or U; the expected results may thus serve as a guide for exploring complex redox processes in other geochemical systems. Broader Impacts: This project will facilitate the training of two new graduate researchers in the field of biogeochemistry. It will also allow a number of undergraduate researchers (2-3 per year) to be educated in the practice of science, training them in the formulation of research questions and the use of the tools and methods needed to answer scientific questions. Each graduate researcher will be given the opportunity to mentor an undergraduate student as part of their preparation as future educators. Results of this research will be incorporated into a graduate course taught by the PI. This research also may have societal impacts, as the studies of contaminant fate during these surface redox reactions may provide the basis for future development of new remediation strategies or methods to recharge water filtration systems.