Heterogeneous aggregation of humic acids studied by small-angle neutron and X-ray scattering

Saito, Takumi*; Motokawa, Ryuhei   ; Okubo, Takahiro*; Miura, Daisuke*; Kumada, Takayuki   

Aggregation of humic acids (HAs) were studied by small-angle neutron and X-ray scattering techniques. The combination of these techniques enables us to examine aggregation structures of heterogeneous HA particles. Two HAs with distinctive compositions were examined; a commercial HA (PAHA) and a HA extracted from deep sedimentary groundwater (HHA). While macroscopic coagulation tests showed that these HAs were stable in solutions except for HHA at pH 6, small-angle neutron scattering (SANS) and X-ray scattering (SAXS) revealed that they formed aggregates with the sizes exceeding sub-micrometer length scale. The SAXS curves of PAHA remarkably varied with pD, whereas the SANS curves did not. With the help of theoretical fittings, it was revealed that PAHA aggregates consisted of two domains: poorly hydrated cores and well-hydrated proton-rich shells. The cores are (dis)aggregated with pD inside the aggregates of the shell. The SANS and SAXS curves of HHA resemble each other, and their intensities at low q increased with a decrease of pD, indicating the formation of homogeneous aggregates. This study revealed that distinctive aggregation behaviors exist in humic substances with heterogeneous structures like PAHA, which is invaluable for their roles in the fate of contaminants or nutrients in aqueous environments.