A Mechanistic Exploration of Natural Organic Matter Aggregation and Surface Complexation in Smectite Mesopores

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.