Integrating Diffraction, Advanced Microscopy and NMR Methods to Explore NOM-Mineral-Cation Interactions

Combined ⁴³Ca nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and helium ion microscopy (HeIM) results provide novel insight into the interactions among NOM mineral (smectite) surfaces, dissolved ions, and water; and the effect of hydration state and pH on these interactions.  The molecular-scale ⁴³Ca NMR results for a Ca-smectite-Suwannee River NOM-H₂O system suggests that Ca²⁺ in smectite-NOM composites  behaves more like Ca²⁺ in smectites without NOM than Ca²⁺ in NOM alone also support the idea that much of the Ca²⁺ in the composites bridges between the NOM and the mineral surface. The NMR results also show that the NOM protonation state (pH) during composite synthesis has little effect on the local molecular-scale coordination environment and dynamics of Ca²⁺ and that H₂O activity is the most important control of composite basal spacing and ion dynamics. XRD results corroborate the formation of smectite-NOM composites and suggest significant cation-NOM association. With NOM present, there is loss of the (005) smectite reflection; no evidence of amorphous NOM; and growth of a broad peak near 3.5 Å, similar to the spacing observed in the benzene ring stacking of graphene. The XRD results also show that the composite materials expand upon exposure to H₂O, suggesting that H₂O has access to the interlayer spaces and cations. HeIM images offer an explanation as to why there is no pH-dependence in the Ca²⁺ molecular-scale behavior. Ca-smectite-NOM composites formed at pH 12 appear to be a relatively homogeneous on the <~100 nm scale, whereas composites formed by rapidly decreasing the pH from 12 to 2 show both composites and separate NOM aggregates with little porosity at scales >10 nm. It is likely the formation of NOM aggregates is dominated by hydrophobic interactions that exclude Ca²⁺ and thus that ⁴³Ca NMR observes bridging Ca²⁺ similar to that in the pH 12 samples.