The effects of cation properties, solution ionic strength, and pH history on the micro-structure of natural organic matter (NOM)-smectite composites are poorly understood, yet developing an integrated micro- and molecular-scale picture of NOM-mineral interactions is essential to fully understand the global carbon balance, soil biogeochemistry, and the transport and fate of organic and inorganic pollutants. Here, we present helium ion microscopy (HeIM), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analyses that provide new insight into how these chemical controls influence the formation and structure of NOM-smectite composites composed of the clay mineral hectorite and Suwannee River NOM. Composites were prepared by centrifuging suspensions of Na-, Ca-, K-, and Cs-hectorite with NOM under basic conditions (BC; natural pH to pH 12 ± 0.5) and basic-acidic conditions (BAC; natural pH to pH 12 ± 0.5 to pH 2 ± 0.5). HeIM images, EDS spectra, and XRD patterns show that the NOM-hectorite composites created under BC have a thin (&lt; 1μm) coating of NOM distributed homogeneously at surfaces. There is XRD evidence of NOM in interlayer spaces in Ca- but not Cs- or K-hectorite. It appears that H2O activity remains the primary control over the basal spacing in the BC composites. In contrast, the BAC composites consist of homogeneous composite material mixed with NOM-rich regions consisting of faceted objects and platelets sitting on the composite surface. The BAC samples exhibit many regions with NOM coatings of 3 μm or greater thickness and XRD patterns that suggest the NOM affects the hydration/dehydration of interlayer cations. These samples also exhibit a loss of the (005) peak, suggesting the hectorite phase in the BAC composites is composed primarily of platelet stacks ≤ 5 layers thick separated by NOM. Often, the loss of the (005) peak is accompanied by growth of a broad, low intensity reflection at 3.5 Å similar to that assigned to pi-pi aromatic stacking in graphene. Together, the results suggest that thin NOM coatings form at BC, presumably via ion bridges, and that NOM flocculates in BAC composites via a hydrophobic mechanism as the solution is acidified.  Increasing suspension ionic strength that accompanies acidification dominates the flocculation behavior, though the cation charge density also appears to exert some influence over flocculation.

Chemical controls of the micro-structure of NOM-smectite composite materials

249th National Meeting (2015)

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