Deviation of nanoparticle aggregation from XDLVO theory as explained by dissolved organic matter adsorption and fractionation.

The stability of iron and aluminum-based nanoparticles (NPs) in dissolved organic matter (DOM)-rich environments remains unpredictable when evaluated through classical XDLVO theories. This study demonstrates that DOM fractionation during adsorption governs the contrasting sedimentation behaviors of ferrihydrite (Fh) and amorphous aluminum hydroxide (AAH) NPs through non-DLVO mechanisms. Advanced spectroscopic analyses (3D-EEM, FTIR, 2D-COS) revealed that Fh NPs preferentially adsorb aromatic humic components, which accelerate sedimentation through hydrophobic interactions. In contrast, AAH NPs selectively bind high molecular weight (HMW) protein-like substances, enhancing stability via steric hindrance. Contrary to XDLVO calculations, Fh exhibited DOM-induced aggregation, with a sedimentation rate increase of 37-58 %, whereas AAH showed suppressed settling, with a rate reduction of 42-65 %, underscoring the limitations of XDLVO models. Hydrophobic interactions/π-π interactions/hydrogen bonding (Fh-humics) and macromolecular steric effects (AAH-proteins) dominate stability, with DOM fractionation dictated by NP surface chemistry. These findings demonstrate that different DOM fractions mediate different interfacial forces. However, the XDLVO model describes the overall interactions of the bulk DOM, and could not capture the selective adsorption and molecular rearrangement that occur during DOM-NP interaction. Our results call for a new conceptual framework that integrates DOM chemical heterogeneity and non-DLVO interactions to predict colloid transport and carbon cycling in organic-rich aquatic ecosystems.