Hetero-aggregation of goethite and ferrihydrite nanoparticles controlled by goethite nanoparticles with elongated morphology

Abstract The dispersities of goethite nanoparticles (GTNPs) and ferrihydrite nanoparticles (FHNPs) affect the transport and retention of nanoparticle-associated contaminants. However, the effects of interaction on nanoparticle stability under varying environmental conditions have not been previously investigated. This study utilized settling experiments, a semi-empirical model, and the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory to study the homo-aggregation and hetero-aggregation of GTNPs and FHNPs. The pure system of GTNPs tended to aggregate more easily than that of FHNPs, especially under the conditions of high pH (7.0–9.0), high ionic strength (IS, 10 mM), and low concentrations of humic acid (HA) (2 mg L−1). This aggregation was attributed to the elongated morphology of GTNPs, which contributed to surface heterogeneity. The GTNPs and FHNPs mixtures rapidly coagulated, particularly under the surface-charge disequilibrium caused by an increase in negative charges or IS. Hetero-aggregation increased with increase in the GTNPs ratio, indicating that the elongated GTNPs dominated the coagulation of the Fe mineral nanoparticle mixture, which was attributed to the surface heterogeneity and high probability collisions between the GTNPs. Although DLVO neglects the influence of heterogeneity on the nanoparticle surfaces, SEM revealed that hetero-aggregation of GTNPs and FHNPs occurred. The results obtained in this study provide novel and valuable insights into the behaviors of GTNPs and FHNPs mixtures and suggest that during the gradual transformation of FHNPs to GTNPs in soil or aquatic environments, the hetero-aggregation of GTNPs and FHNPs may be enhanced, thus promoting contaminant immobilization.