Interactions of positively and negatively charged iron-based engineered nanoparticles with organic matter: The case of humic acid

The widespread release of engineered nanoparticles (NPs) into natural environments raises questions about the interactions of these colloids with natural organic matter (NOM), which directly influence their mobility, reactivity, and fate. While NP surface charge is recognized as a key factor in these interactions, evidence linking charge to molecular mechanisms remains limited. Cationic and anionic NPs were used in this study to investigate how NPs surface charge affects interactions with humic acid (HA), using fluorescence spectroscopy and dynamic light scattering (DLS). The results reveal that cationic NPs exhibit more important interactions with HA than anionic NPs, confirming that surface charge of NPs plays a key role on their interactions with HA. Complementary comparison showed that NP size and surface functionalization do not significantly impact these interactions, reinforcing surface charge as the important factor. Thermodynamic analyses at 270 nm, provide insights that these interactions involve carboxylic acid groups in HA. In addition, these analyses indicate that anionic NPs interact primarily through hydrogen bonding and Van der Waals forces. Finally, HA-NP interactions are more pronounced with an acidic pH than at neutral one, highlighting additional processes associated with charge modification.