Integrated Molecular and Microscopic Scale Insight into Morphology and Ion Dynamics in Ca²⁺-Mediated Natural Organic Matter Floccs

Combined X-ray diffraction (XRD), helium ion microscopy (HeIM), and ⁴³Ca nuclear magnetic resonance (NMR) results provide novel insight into the nano- and microstructure of flocculated NOM; the molecular-scale interaction among natural organic matter (NOM), dissolved Ca²⁺ ions, and water in NOM floccs; and the effects of pH and ionic strength on these characteristics. Suwannee River humic acid (HA), fulvic acid (FA), and NOM flocculated from Ca²⁺ bearing solutions share similar morphological characteristics on the 100 nm to micron scales, including micron-sized equant fragments and rounded, rough areas with features on the 100 nm scale. HeIM suggests that the NOM floccs are built from a fundamental spheroidal structure that is ~10 nm in diameter, in agreement with published AFM and small-angle X-ray scattering results. Calcium is incorporated into these floccs at 100% relative humidity in a wide range of disordered structural environments, with basic pH leading to shorter mean Ca-O distances and lower mean coordination numbers with respect to floccs formed under acidic conditions. The NMR results show that dynamical processes involving water and Ca²⁺ occurring at frequencies >10⁴ Hz are important for hydrated OM floccs, in agreement with published molecular dynamics simulations of OM in solution. From the NMR results, we find evidence for two Ca²⁺ dynamic averaging mechanisms: one related to rapid exchange (>100 kHz) between surface proximity-restricted (those within 5 Å of a surface) and bulk solution environments when excess Ca²⁺ is present in the pore solution when pore water is unfrozen and a second consisting of intermediate scale (tens of kHz) site exchange among strongly sorbed inner-sphere sites when excess Ca²⁺ is absent and the carboxylic and phenolic functional groups of the NOM are deprotonated.