Probing atomic-scale processes at the ferrihydrite-water interface with reactive molecular dynamics

Interfacial processes involving metal (oxyhydr)oxide phases are important for the mobility and bioavailability of nutrients and contaminants in soils, sediments, and water. Consequently, these processes influence ecosystem health and functioning, and have shaped the biological and environmental co-evolution of Earth over geologic time. Here we employ reactive molecular dynamics simulations, supported by synchrotron X-ray spectroscopy to study the molecular-scale interfacial processes that influence surface complexation in ferrihydrite-water systems containing aqueous molybdate. We validate the utility of this approach by calculating surface complexation models directly from simulations. The reactive force-field captures the realistic dynamics of surface restructuring, surface charge equilibration, and the evolution of the interfacial water hydrogen bond network in response to adsorption and proton transfer. We find that upon hydration and adsorption, ferrihydrite restructures into a more disordered phase through surface charge equilibration, as revealed by simulations and high-resolution X-ray diffraction. We observed how this restructuring leads to a different interfacial hydrogen bond network compared to bulk water by monitoring water dynamics. Using umbrella sampling, we constructed the free energy landscape of aqueous molybdate adsorption at various concentrations and the deprotonation of the ferrihydrite surface. The results demonstrate excellent agreement with the values reported by experimental surface complexation models. These findings are important as reactive molecular dynamics opens new avenues to study mineral-water interfaces, enriching and refining surface complexation models beyond their foundational assumptions.

涉及金属（氧氢）氧化物（metal (oxyhydr)oxide）相的界面过程，对于土壤、沉积物与水体中营养物质及污染物的迁移性和生物可利用性至关重要。因此，此类界面过程会影响生态系统的健康与功能，并在地质时间尺度上塑造了地球的生物与环境协同演化进程。本研究借助同步辐射X射线光谱学（synchrotron X-ray spectroscopy）辅助的反应分子动力学（reactive molecular dynamics）模拟方法，针对含水溶液钼酸盐（aqueous molybdate）的水铁矿（ferrihydrite）-水体系中影响表面络合（surface complexation）的分子尺度界面过程展开研究。我们通过直接从模拟结果中推导表面络合模型，验证了该研究方法的实用性。该反应力场（reactive force-field）能够精准捕捉界面在吸附作用与质子转移（proton transfer）过程中，发生的表面重构、表面电荷平衡以及界面水氢键网络演化的真实动力学行为。模拟结果与高分辨X射线衍射（high-resolution X-ray diffraction）分析均表明，在水合与吸附过程中，水铁矿会通过表面电荷平衡作用重构为更无序的物相。我们通过监测水体动力学过程，揭示了该重构如何使得界面氢键网络与本体水体产生显著差异。本研究采用伞形抽样（umbrella sampling）方法，构建了不同浓度下水溶液钼酸盐吸附以及水铁矿表面去质子化过程的自由能面（free energy landscape）。研究结果与已报道的实验表面络合模型数值具有极佳的一致性。鉴于反应分子动力学为矿物-水界面研究开辟了全新路径，能够在基础假设之上进一步丰富与完善表面络合模型，本研究的发现具有重要意义。