Soil minerals and organic matter play critical roles in nutrient cycling and other life-essential biogeochemical processes, yet the structural and dynamical details of natural organic matter (NOM) film formation on smectites are not fully understood on the molecular scale. XRD of Suwannee River NOM–hectorite (a smectite clay) complexes shows that the humic and fulvic components of NOM bind predominantly at the external surfaces of packets of smectite platelets rather than in the interlayer slit pores, suggesting that the key behavior governing smectite–NOM interactions takes place in mesopores between smectite particles. New molecular dynamics modeling of a ∼110 Å H₂O-saturated smectite mesopore at near-neutral pH shows that model NOM molecules initially form small clusters of 2–3 NOM molecules near the center of the pore fluid. Formation of these clusters is driven by the hydrophobic mechanism, where aromatic/aliphatic regions associate with one another to minimize their interactions with H₂O, and charge-balancing cations associated with the deprotonated carboxylate sites are located only at the outer surface of these clusters. Despite hydrophobicity driving the initial clustering, NOM clusters are formed more quickly when high-charge-density cations like Ca²⁺ are present vs low-charge-density cations like Cs⁺, as the former cations more effectively minimize the electrostatic repulsions between the negatively charged NOM molecules. Once the small hydrophobicity-driven NOM clusters form, the simulations show that Ca²⁺ promotes the aggregation of NOM clusters through tetradentate Ca²⁺ bridges involving carboxylate groups on two different NOM clusters. Importantly, our studies indicate that Ca²⁺ plays a crucial role in binding the NOM clusters to the smectite surface, which occurs through multiple quaternary complexes (O_b)–H₂O–Ca²⁺–COO^-_NOM. In contrast, Cs⁺ never forms any coordination or acts like bridges between NOM molecules nor as ion bridges to the smectite surface. Additionally, we observe the formation of a metastable superaggregate involving all 16 NOM molecules several times in a Ca²⁺-bearing mesopore fluid. Superaggregates are never observed in the simulations involving Cs⁺. The modeling results are fully consistent with helium ion microscope images of NOM–hectorite complexes suggesting that NOM surface films develop when preformed NOM clusters interact with smectite surfaces. Overall, the binding of NOM clusters to the outer surfaces of smectite particles and the formation of large NOM aggregates at neutral pH occur through cation bridging, and cation bridging only occurs when high-charge-density cations like Ca²⁺ are present.

中文翻译：

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蒙脱石中孔天然有机物聚集与表面络合机理研究

土壤矿物质和有机物在养分循环和其他生命必不可少的生物地球化学过程中起着至关重要的作用，但在分子尺度上对蒙脱石上天然有机物（NOM）膜形成的结构和动力学细节尚不完全了解。Suwannee河NOM-锂蒙脱石（一种蒙脱石粘土）复合物的X射线衍射表明，NOM的腐殖质和富勒维组分主要结合在蒙脱石薄片包的外表面上，而不是在层间缝隙中结合，这表明控制蒙脱石-NOM的关键行为蒙脱石颗粒之间的介孔发生相互作用。〜110ÅH _2的新分子动力学模型在接近中性的pH值下，O饱和蒙脱石中孔表明，模型NOM分子最初在孔隙流体中心附近形成2–3 NOM分子的小簇。这些簇的形成是由疏水机制驱动的，其中芳族/脂肪族区域相互结合以最小化它们与H ₂ O的相互作用，与去质子化的羧酸盐位点相关的电荷平衡阳离子仅位于这些簇的外表面。尽管疏水性驱动了初始簇聚，但是当存在高电荷密度阳离子（如Ca ^2+）和低电荷密度阳离子（如Cs ^+）时，NOM簇的形成更快，因为前一种阳离子更有效地使带负电荷的NOM分子之间的静电排斥最小化。一旦小的疏水性驱动的NOM簇形成，模拟表明Ca ²⁺通过涉及两个不同NOM簇上羧酸酯基团的四齿Ca ²⁺桥促进NOM簇的聚集。重要的是，我们的研究表明，钙²⁺在NOM簇结合蒙脱石表面，其通过多个季络合物（O发生中起着至关重要的作用_b）-H ₂ O形的Ca ²⁺ -COO ^- _NOM。相反，Cs ⁺从不形成任何配位或行为像NOM分子之间的桥，也不作为与绿土表面的离子桥。另外，我们观察到在带有Ca ^2+的中孔流体中多次形成了涉及所有16种NOM分子的亚稳态超聚集体。在涉及Cs ⁺的模拟中从未观察到超聚集体。建模结果与NOM-锂蒙脱石配合物的氦离子显微镜图像完全一致，表明当预成型的NOM团簇与蒙脱石表面相互作用时会形成NOM表面膜。总的来说，中性pH下NOM团簇与蒙脱石颗粒外表面的结合和大型NOM聚集体的形成是通过阳离子桥接实现的，只有当高电荷密度阳离子（例如Ca）发生时，阳离子桥接才会发生存在²⁺。