Mechanisms of Mineral Dissolution: Time-Resolved Synchrotron X-ray Diffraction of Fe-and Mn-oxides with Dissolved Organic Ligands

技术说明。该项目将使用实时、原位X射线衍射法探索铁和锰氧化物被铁载体溶解的情况，以确定这些重要土壤反应的机理和速率规律。我们的初步结果表明，铁载体去铁胺-B(DFOB)在0.1-10 mM的浓度范围内将在数小时内导致层状氧化锰水钠石完全溶解。此外，Rietveld对我们的时间分辨同步加速器X射线衍射数据的分析表明，相对于Mn(IV)，Mn(III)被选择性地从水钠铝石结构中移除。去除20mol%的Mn(III)会导致三斜水钠锰矿的临界不稳定性，当空位浓度超过此值时，结构就会坍塌。这些观察结果使我们推测，铁载体溶解矿物的机制取决于金属价态的不均匀。在混合价金属氧化物中，铁载体介导的溶解是在达到临界的三维空位浓度后通过结构坍塌进行的。相比之下，同价金属(氢氧化物)以更传统的二维表面耗尽方式溶解。我们将通过应用tr-XRD技术在DFOB辅助下溶解各种异价氧化物(例如磁铁矿[Fe3+(Fe2+，3+)2O4]、豪锰矿[Mn3+(Mn2+，3+)2O4]、闪锌矿[Na2Fe2+3Fe3+2(Si8O22)(OH)2]和同价氧化物(例如赤铁矿[Fe3+2O3]、针铁矿[Fe3+O(OH)])来验证这些想法。矿物溶解机制是一系列重要社会问题的基础，包括土壤肥力和临界区金属的循环、污染物的迁移、二氧化碳的封存以及地球表面水的化学？S。以前大多数关于矿物在水溶液中溶解速度的研究都是通过流体化学的变化来监测反应的进展。在这里，我们提出了一种新的补充策略，将流体的化学演化与反应固体中的结构变化相关联。通过这种方法，我们可以将流体的化学动力学与结构矿物转变严格地耦合在一起，极大地扩展了我们对潜在反应机制的理解。这项建议中描述的工作将有助于揭示控制铁载体提取不溶性金属以维持土壤和海洋环境中健康代谢活动的机制和速度的因素。铁载体是一种在土壤和海水中以微摩尔浓度普遍存在的低质量生物化合物，它们由各种微生物、真菌和草类产生，以在严重铁限制的条件下获得选择性优势。铁载体可以从几乎不溶的Fe(III)和Mn(III，IV)氧化物中提取Fe(III)和Mn(III)，具有极高的专一性，将阳离子输送到亲本生物中，以满足营养或氧化还原代谢的需要。从这项工作中获得的见解可能会使我们更好地理解提高缺铁农业区金属生物有效性的方法，并将提高我们对矿物风化的理解，矿物风化是大气二氧化碳排放的主要手段。

Technical Description. This project will explore the dissolution of iron and manganese oxides by siderophores using real-time, in situ X-ray diffraction to determine mechanisms and rate laws for these important soil reactions. Our preliminary results reveal that the siderophore desferrioxamine-B (DFOB) at concentrations ranging from 0.1 to 10 mM will induce complete dissolution of the layered Mn oxide birnessite within hours. Moreover, Rietveld analysis of our time-resolved synchrotron XRD data have revealed that Mn(III) is selectively removed from the birnessite structure relative to Mn(IV). Removal of 20 mol% Mn(III) induces a critical instability in triclinic birnessite, and the structure collapses when vacancy concentrations increase beyond this value.These observations lead us to hypothesize that the mechanism by which siderophores dissolve minerals depends on the heterogeneity of metal valence state. In mixed-valence metal (hydr)oxides, siderophore-mediated dissolution occurs by a structural collapse after a critical 3-dimensional vacancy concentration is achieved. In contrast, homovalent metal (hydr)oxides dissolve by the more conventional mode of 2-dimensional surface depletion. We will test these ideas by applying TR-XRD techniques to DFOB-assisted dissolution of a variety of heterovalent oxides (e.g., magnetite [Fe3+ (Fe2+,3+)2O4], hausmannite [Mn3+(Mn2+,3+)2O4], riebeckite [Na2Fe2+3Fe3+2(Si8O22)(OH)2] and homovalent oxides (e.g., hematite [Fe3+2O3], goethite [Fe3+O(OH)]), in the presence and absence of light.Broader Impacts. Mineral dissolution mechanisms are foundational to a range of societally important issues, including soil fertility and the cycling of metals in the critical zone, contaminant transport, the sequestration of CO2, and the chemistry of Earth?s surface waters. Most prior studies of the rates by which minerals dissolve in aqueous solutions have monitored reaction progress through changes in fluid chemistry. Here we propose a novel and complementary strategy that correlates the chemical evolution of the fluid with structural changes in the reacting solid. By this approach, we can rigorously couple the chemical kinetics of the fluid with structural mineral transitions, greatly expanding our understanding of the underlying reaction mechanisms.The work described in this proposal will help reveal the factors that control the mechanisms and rates by which siderophores can extract insoluble metals to sustain healthy metabolic activity in soil and marine environments. Siderophores are low-weight, biogenic compounds that occur ubiquitously in micromolar concentrations in soil and marine waters, and they are produced by a wide variety of microbes, fungi, and grasses to gain a selective advantage in severely Fe-limited conditions. Siderophores can extract Fe(III) and Mn(III) from nearly insoluble Fe(III) and Mn(III,IV) oxides with extremely high specificity, transporting the cations to parent organisms to satisfy nutritive or redox metabolic needs. Insights gained from this work may lead to a better understanding of methods to increase metal bioavailability in iron-deficient agricultural regions, and they will improve our understanding of mineral weathering, a major means of atmospheric CO2 drawdown.