Heterogeneous Aggregation of Humic Acids Studied by Small-Angle Neutron and X-ray Scattering

Aggregation of humic acids (HAs) was studied by small-angleneutronand X-ray scattering techniques. The combination of these techniquesenables us to examine the aggregation structures of HA particles.Two HAs with distinctive compositions were examined: a commercialHA (PAHA) and a HA extracted from deep sedimentary groundwater (HHA).While macroscopic coagulation tests showed that these HAs were stablein solutions except for HHA at pH < 6, small-angle neutron scattering(SANS) and small-angle X-ray scattering (SAXS) revealed that theyformed aggregates with sizes exceeding the sub-micrometer length scale.The SAXS curves of PAHA remarkably varied with pD = log a (D+), where a (D+) stands for theactivity of deuterium ions, whereas the SANS curves did not. Withthe help of theoretical fittings, it was revealed that PAHA aggregatesconsisted of two domains: poorly hydrated cores and well-hydratedproton-rich shells. The cores were (dis)aggregated with pD insidethe aggregates of the shells. The SANS and SAXS curves of HHA resembledeach other, and their intensities at low q, where q stands for the scattering vector, increased with a decreaseof pD, indicating the formation of homogeneous aggregates within thespatial resolutions of SANS and SAXS. This study revealed that distinctiveaggregation behaviors exist in humic substances with nm-scale heterogeneousstructures like PAHA, which is important for their roles in the fateof contaminants or nutrients in aqueous environments. Structural understanding of the aggregatesof humic substancesremains elusive. This study demonstrates the heterogeneous aggregationof humic acid by comparing small-angle scattering of X-rays and neutrons.

Automated summary

Aggregation of humic acids (HAs) was investigated through small-angle neutron and X-ray scattering techniques. The results showed that HA particles formed aggregates exceeding the sub-micrometer length scale. It was observed that PAHA aggregates consisted of poorly hydrated cores and well-hydrated proton-rich shells, while HHA formed homogeneous aggregates. This study provides important insights into the behavior and role of HAs in aqueous environments.