Molecular Insights into the Interaction Mechanism Underlying the Aggregation of Humic Acid and Its Adsorption on Clay Minerals

Quantitativecharacterizations of the molecular interactionsof HA-HA and HA-clay mineral systems in aqueous mediaat the nanoscale are of great importance for unraveling the aggregationand adsorption mechanism of humic acid in both terrestrial and aquaticenvironments. Humic acid (HA) is ubiquitous in both terrestrial andaquatic environments,and understanding the molecular interaction mechanisms underlyingits aggregation and adsorption is of vital significance. However,the intermolecular interactions of HA-HA and HA-claymineral systems in complex aqueous environments remain elusive. Herein,the interactions of HA with various model surfaces (i.e., HA, mica,and talc) were quantitatively measured in aqueous media at the nanoscaleusing an atomic force microscope. The HA-HA interaction wasfound to be purely repulsive during surface approach, consistent withfree energy calculation; during retraction, pH-dependent adhesionwas observed due to the protonation/deprotonation of HA that influencesthe formation of hydrogen bonds. Different from the mica case, hydrophobicinteraction was detected for the HA-talc system at pH 5.8,contributing to the stronger HA-talc adhesion, as also evidencedby adsorption results. Notably, HA-mica adhesion strongly dependedon the loading force and contact time, most likely because of theshort-range and time-dependent interfacial hydrogen bonding interactionunder confinement, as compared to the dominant hydrophobic interactionfor the HA-talc case. This study provides quantitative insightsinto the fundamental molecular interaction mechanisms underlying theaggregation of HA and its adsorption on clay minerals of varying hydrophobicityin environmental processes.

Automated summary

Quantitative characterizations of HA-HA and HA-clay mineral systems in aqueous media at the nanoscale are important for understanding the aggregation and adsorption mechanism of humic acid in both terrestrial and aquatic environments. However, the molecular interactions of these systems in complex aqueous environments are still not well understood.