The interaction of natural organic matter (NOM) with clay mineral surfaces is central to various geochemical and environmental processes including the biogeochemistry of soils, the fate and transport of heavy metals and organic contaminants, and the global carbon balance. Studies have shown that NOM molecules associate with one another in the bulk solution and that they readily bind with clay surfaces. However, the molecular scale details of the interactions among NOM, the mineral surface and clay charge-balancing cations remain poorly understood, particularly how these structures evolve as a function of time. This presentation discusses the impact that two charge-balancing cations with drastically different hydration properties have on NOM-NOM and NOM-clay surface interactions in a simulated smectite mesopore solution at neutral pH where only the carboxylic acid groups are significantly deprotonated. X-ray diffraction (XRD) data show that NOM binds predominantly at the external surfaces of packets of smectites platelets rather than in the nanoconfined interlayer pores. Simulations of NOM behaviour in these larger, mesopores clearly illustrate that NOM aggregates among itself as dimers and trimers via a hydrophobic mechanism in such mesopore solutions. These small, hydrophobicity-driven clusters remain in the pore solution when Cs⁺ is present and do not experience aggregation or growth over time. In contrast, these NOM clusters aggregate with one another via cation bridging when Ca²⁺ is present in the mesopore solution, largely through a cation bridge between two deprotonated carboxylate groups on adjacent NOM clusters. Eventually, the larger aggregates bind to the clay surface via a quaternary complex (Surface-H₂O-Ca²⁺-NOM). No such stabilizing cation bridges are observed between NOM molecules or between NOM molecules and the clay surface are observed when Cs⁺ is present. Over time in the Ca²⁺ mesopore solution, the NOM aggregates form a larger, supramolecular aggregate that is bound to the clay surface by multiple quaternary bridges. These results are fully consistent with helium ion images of NOM-hectorite complexes suggesting that NOM aggregation extends far into the mesopore fluid through linear linking of NOM supramolecular aggregates in a “chain of pearls” configuration in hydrated Ca-smectites.

Narasimhan Loganathan

Senior Research Associate, Michigan State University

Appearances