The interaction of natural organic matter (NOM) with mineral surfaces plays an important role in controlling the binding and transport of naturally occurring and anthropogenic organic and inorganic species in the earth’s near-surface environment and may also play a similar role in deeper geological conditions related to sequestration of CO2. The molecular scale processes controlling NOM-mineral interaction are incompletely understood. We present here the results of combined experimental and computational molecular dynamics (MD) studies of NOM-clay materials that provide significant new molecular scale insight into how OM interacts with mineral surfaces and how this interaction affects the behavior of cations, H2O, and CO2 near the interface. Variable temperature 23Na NMR data for the composites obtained at atmospheric pressure show that Na+ occurs on proximity restricted structural sites and undergoes dynamical processes in the 10 - 100 kHz range. Under water-saturated conditions, 23Na also yields a very narrow peak due to Na+ with solution-like mobility near room temperature that broadens with decreasing temperature. MD simulations of hydrated Na-hectorite-NOM composites modeled at near-neutral pHs (only carboxylic groups deprotonated) show that Na+ is coordinated to the NOM molecules and the clay surface, that the NOM molecules migrate away from the clay surface, and that the Na+ is mobile, as for the clay without NOM. 13C, 133Cs, and 23Na NMR spectra and T1 relaxation rates for Na-hectorite, Cs-hectorite, a Na-hectorite-humic acid composite, and a Cs-hectorite-NOM composite obtained at 90 bars CO2 pressure and 50oC (simulating CO2 sequestration conditions) show that supercritical CO2 interacts more strongly with the composite than with the base clay and that it enters the interlayer galleries. For instance, CO2 causes increased 23Na signal intensity, reduced peak widths, more rapid T1 relaxation rates, and more positive chemical shifts. We interpret these changes to be due to supercritical CO2 increasing the rate of Na+ site hopping, as also shown in MD simulations for hectorite-polymer (PEG) composites containing CO2 under supercritical conditions. MD simulations for hectorite-NOM composites with both H2O and CO2 show that the NOM molecules prefer to occur at the H2O – CO2 fluid interface with the dominantly hydrophobic side of the molecule facing the CO2 and the dominantly hydrophilic side facing the H2O and coordinated by Na+.

NMR and computational molecular dynamics investigation of mineral-NOM interactions at atmospheric and elevated pressures

249th National Meeting (2015)

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