Digestive ripening and fragmentation of goethite nanorods into nanospheres under natural wet-dry cycles: Unveiling hidden nanocolloid mobilization to water system.

Anisotropic goethite nanoparticles play a critical role in the environmental behavior of nutrient and contaminant due to their abundance, high surface area, and strong binding affinity; however, their transformation into smaller, more mobile nanocolloids under natural setting remains underexplored. Here, we show that natural wet-dry cycles at alkaline pH in the presence of As(V) or P(V) induce extensive goethite nanorod (GtNR) fragmentation into nanospheres (GtNS) and the formation of ferrihydrite single-digit nanospheres (FhSDNS), driven by drying-induced surface reactions, oxyanion-mediated dissolution, reprecipitation, passivation, capillary stress, and defect sites. Under identical conditions, nanospheres of hematite, maghemite, and magnetite exhibit minimal transformation, underscoring the unique susceptibility of GtNR. Passivation by As(V) or P(V) enhances nanocolloid mobility, potentially facilitating long-distance transport of sorbed nutrients and pollutants. This mechanism helps explain persistently low phosphorus availability-even under intensive fertilization-as well as the heightened mobility of As(V) and P(V), posing increased risks to water systems. Furthermore, current analytical methods for phosphorus detection could not address single-digit nanocolloids, complicating efforts to track their mobilization. Collectively, these findings highlight how common agricultural practices, including repeated irrigation and surface fertilization, may inadvertently foster goethite-based nanocolloid formation. Recognizing these mechanisms is crucial for mitigating adverse environmental impacts and improving nutrient-use efficiency.