Electron transfer between birnessite and organic compounds (OC) plays a dominant role in the coupling cycle of manganese (Mn) and carbon across diverse environmental settings. While previous studies have extensively investigated individual processes of interface Mn reduction, surface Mn2+ adsorption, and surface-to-interior electron transfer, the dynamic interplay among these reactions and the mechanisms regulating subtle changes in surface and interior Mn states remained poorly understood. Additionally, existing models have not adequately captured electron transfer kinetics in multivariable systems involving pH, Mn2+ concentration, electron donor type, etc. In this study, we investigated the reduction kinetics of birnessite under the influence of multiple environmental variables by employing three typical OC: formic acid (HCOOH), formaldehyde (HCHO), and methanol (CH3OH). Time-series analysis revealed kinetic discrepancy and time lag between the alteration of the average Mn oxidation state (AMOS) within the solid and the release of Mn2+ from the reductive dissolution of birnessite, indicating a two-stage electron transfer mechanism occurring at the interface between birnessite and OC. X-ray absorption fine structure spectra revealed a rapid increase in corner-sharing MnO6 octahedra and a decline in AMOS during the initial stage, followed by a slight decrease in AMOS and substantial mineral dissolution to release Mn2+ in the subsequent stage. The transition point between the two stages is primarily influenced by the concentration of surface MnII under pH regulation, as confirmed by soft X-ray absorption spectroscopy and density functional theory calculations. Based on these findings, the adsorption equilibrium and electron transfer rate were modeled by a machine learning framework (JAX), which is influenced by three main factors: pH, Mn2+ concentration, and OC types. The adsorption equilibrium constant for HCHO was one order of magnitude lower than for HCOOH, yet displayed a faster reaction rate due to higher electron transfer rates. Competitive adsorption of OC and Mn2+ on reactive sites was influenced by both pH and Mn2+ concentrations. Combining these parameters, we created a 3D surface plot that comprehensively considered the interplay between different elementary reactions, including competitive adsorption and redox reaction rates, thereby visualizing the kinetic regulation mechanisms in multivariable systems. Furthermore, a comprehensive rate equation for the reduction of birnessite by OC was developed to predict its behavior in natural settings. With an electron storage capacity of 2.7×1023 electrons/mol Mn before structural decomposition or dissolution, we propose that birnessite can act as a geobattery driving cryptic elemental biogeochemical cycling. Our findings also suggest that the highly reversible redox reactivity of birnessite and the kinetics discrepancy in multi-step electron transfer reactions enable it to facilitate energy conversion among OC, sunlight, and microbes across a variety of temporal and spatial scales.

中文翻译：

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Birnessite/有机化合物界面处的电子转移：结构重排和分解的机制、调节和两阶段动力学差异


伯恩石和有机化合物 （OC） 之间的电子转移在不同环境设置下的锰 （Mn） 和碳的耦合循环中起主导作用。虽然以前的研究广泛研究了界面 Mn 还原、表面 Mn2+ 吸附和表面到内部电子转移的各个过程，但这些反应之间的动态相互作用以及调节表面和内部 Mn 状态细微变化的机制仍然知之甚少。此外，现有模型没有充分捕捉涉及 pH 、 Mn2+ 浓度、电子供体类型等的多变量系统中的电子转移动力学。在这项研究中，我们采用三种典型的 OC：甲酸 （HCOOH）、甲醛 （HCHO） 和甲醇 （CH3OH） 研究了伯恩石在多个环境变量影响下的还原动力学。时间序列分析显示，固体中平均 Mn 氧化态 （AMOS） 的变化与伯恩斯石还原溶解释放 Mn2+ 之间存在动力学差异和时间滞后，表明伯恩斯石和 OC 之间的界面处发生了两阶段电子转移机制。X 射线吸收精细结构光谱显示，在初始阶段，共享角的 MnO6 八面体迅速增加，而 AMOS 下降， 随后 AMOS 略有降低，矿物大量溶解，在后续阶段释放 Mn2+。两个阶段之间的过渡点主要受 pH 调节下表面 MnII 浓度的影响，软 X 射线吸收光谱和密度泛函理论计算证实了这一点。 基于这些发现，通过机器学习框架 （JAX） 对吸附平衡和电子传递速率进行建模，该框架受三个主要因素的影响：pH 值、Mn2+ 浓度和 OC 类型。HCHO 的吸附平衡常数比 HCOOH 低一个数量级，但由于电子转移速率较高，反应速率更快。OC 和 Mn2+ 在反应位点的竞争性吸附受 pH 和 Mn2+ 浓度的影响。结合这些参数，我们创建了一个 3D 表面图，该图全面考虑了不同基本反应之间的相互作用，包括竞争性吸附和氧化还原反应速率，从而可视化了多变量系统中的动力学调控机制。此外，还开发了 OC 还原 Birnase 的综合速率方程，以预测其在自然环境中的行为。在结构分解或溶解之前，伯恩石的电子存储容量为 2.7×1023 个电子/mol Mn，我们提出伯恩石可以作为土工电池驱动神秘的元素生物地球化学循环。我们的研究结果还表明，Birnesite 的高度可逆氧化还原反应性和多步电子转移反应中的动力学差异使其能够在各种时间和空间尺度上促进 OC、阳光和微生物之间的能量转换。