Chemical Controls over the Microstructure and Morphology of NOM-Smectite Composites
Natural organic matter (NOM) is a complex mixture of bio-derived molecules often intimately associated with inorganic minerals in surface soils, colloids and sediments, and some sedimentary rock types such as shale. The presence of NOM has the potential to change the interfacial behavior of ions and fluids at mineral surfaces, and both NOM and clay are known to interact with water, organic pollutants, inorganic pollutants, and with some types of supercritical fluids (e.g. CO2 sequestration applications). Studying interfacial interactions between NOM and a clay surface thus provides a deeper understanding of the global carbon balance, soil biogeochemistry, and the transport and fate of hazardous contaminants; however, the effects of cation properties, aqueous phase ionic strength, pH, and hydration states on the microstructure of NOM-smectite composites are not completely understood. Here, helium ion microscopy (HeIM), scanning electron microscopy coupled with energy dispersive analyses (SEM-EDS), and X-ray diffraction (XRD) analyses provide new insight regarding the role of these chemical properties in the microstructure and morphology of NOM-smectite composites composed of hectorite and Suwannee River NOM. Composites of Na-, Ca-, K-, Cs-, and Sr-hectorite (a smectite clay) were prepared by ion exchanging San Bernardino hectorite and coating it with NOM either by bringing the mixture of clay and NOM from its natural pH to pH 12 (pH12 composites) or by bringing the pH 12 suspensions back to pH 2 (pH2 composites). Detailed images of composite topography from HeIM and SEM/EDS show that the NOM-hectorite composites created at pH 12 have a more homogeneous surface distribution of NOM and thinner coatings than the pH 2 composites regardless of the cation. For the pH2 samples, the surface of the alkali metal hectorite-NOM composites are characterized by the distinctive curling of packets of clay layers while the Ca-hectorite-NOM composite has a more heterogeneous surface due to the presence of flocculated NOM. The data suggest that both the ion valence state and solution ionic strength influence the formation and nature of NOM coatings as the pH is lowered from basic to acidic. The SEM-EDS results show that the pH2 Ca-hectorite-NOM and Na-hectorite-NOM composites form a NOM coating at least 3 _m thick while pH12 Ca composites form coatings less than 1 _m thick. For the pH2 composites, XRD results under dry conditions suggest that NOM increases the amount of water retained in the interlayer for Na and Ca NOM composites. XRD hydration studies also show that the presence of NOM in the Ca-hectorite pH2 samples decreases the hydration rate of the clay. We also observe that the cation properties play a controlling role over composite hydration behavior at pH12 such that the composite hydration behavior is quite similar to the base clays (clays without NOM). Together, the results suggest that thin surface NOM coatings form at pH12, presumably via ion bridges between the clay and deprotonated NOM functional groups; that NOM flocculates and accumulates at clay surfaces in pH2 composites via a hydrophobic mechanism as the solution is acidified; that NOM is likely not in the interlayer spaces but at the surfaces of clay packets; and that interlayer expansion observed in XRD is likely the result of changing interlayer H2O contents.
Thesis completed in partial fulfillment of the requirements for the Alfred University Honors Program.
Honors thesis, Natural organic matter, Microstructures, Morphology