Making waves: Reframing heavy metals as dynamic nano-assemblages and chemical mixtures in aquatic environments:...

Heavy-metal research has long been guided by frameworks that emphasize dissolved and free-ion forms in explaining transport, reactivity, and toxicity. Yet metals in natural waters rarely occur only as isolated ions; instead, they are distributed across dynamic assemblages that include colloidal, organically complexed, nanoparticulate, particulate, and sediment-coupled forms interacting with dissolved organic matter, mineral surfaces, and co-occurring contaminants. These multicomponent forms can exhibit chemical behavior, bioavailability, and toxicity that differ from those predicted from ionic species alone. Here, we synthesize advances in nanoscale analytics, speciation-resolved measurements, and mixture toxicology to advance a more integrated framework that builds on established bioavailability concepts while incorporating kinetic behavior, multiphase partitioning, sediment-water exchange, and operationally defined fractions under environmentally variable conditions. We highlight examples involving metal-DOM interactions, particle-associated forms and hybrid contaminant systems, discuss implications for monitoring and risk assessment, and outline future directions linking mixture-aware modeling, colloid chemistry, and ecotoxicology. Framing heavy metals as dynamic nano-assemblages and chemical mixtures provides a more realistic basis for environmental monitoring, regulatory interpretation, and aquatic risk assessment.