Molecular-Scale Dynamic Interaction Processes and Aggregation Mechanisms between Natural Organic Matter and...

Polyacrylamide (PAM) is a ubiquitous synthetic flocculant and water treatment agent whose extensive agricultural and industrial applications have sparked increasing concerns regarding aquatic ecosystems. The environmental fate of PAM is fundamentally governed by interfacial interactions within the aqueous phase, where natural organic matter (NOM) acts as a key mediator in biogeochemical processes. Although NOM effectively associates with PAM through multiple functional groups, the microscopic mechanisms underlying NOM-PAM complexation, particularly in terms of molecular-level structural characterization and dynamic association processes, remain insufficiently resolved. Therefore, this study integrates molecular dynamics (MD) simulations with density functional theory (DFT) calculations to systematically investigate the complexation and aggregation behavior between NOM and three PAM variants, namely, nonionic (NPAM), cationic (CPAM), and anionic (APAM). Results indicate that in unitary systems, NPAM exhibits the highest intrinsic aggregation propensity, while electrostatic repulsion hinders the self-assembly of APAM and CPAM. Upon NOM addition, heteroaggregation is markedly enhanced through hydrophobic interactions and cation-mediated interfacial coupling. Specifically, NOM-APAM assemblies form the most compact, alternating layered architecture via multipoint Ca2+ bridging, whereas NOM-CPAM complexes display the loosest, interpenetrating arrangements with free fragments due to competitive Ca2+ coordination. Consequently, NOM complexation significantly restricts the mobility of PAM, with the most pronounced dynamic constraints observed in ionic PAM systems. Furthermore, PAM effectively immobilizes NOM within heteroaggregates, with NOM-APAM complexes imposing the strongest restriction on NOM migration. Collectively, these findings suggest that the formation of NOM-PAM aggregates has profound implications for the environmental fate of natural and anthropogenic polymers in aquatic systems.