Abstract Nano zero-valent iron (nZVI) has been used for in situ groundwater remediation due to its strong adsorption and reaction characteristics. However, oxyanion contaminants in groundwater can ready adsorbed onto the surface of nZVI. This can potentially alter the mobility of nZVI and create a secondary pollution source, but these issues have not yet been systematically investigated. In this study, polyaniline-supported nZVI (PnZVI) and phosphate-sorbed PnZVI (PS-PnZVI) were synthesized in the laboratory. The sedimentation and transport behavior of these two nZVI particles were investigated, compared, and mathematically modeled to better understand the impact of phosphate adsorption on these processes. Results showed that phosphate adsorption can enhance the stability and mobility of PnZVI. Interaction energy calculations that considered van der Waals and magnetic attraction, electrostatic double layer and Born repulsion, and the influence of nanoscale roughness and binary charge heterogeneity were conducted to better infer mechanisms causing nZVI particle sedimentation and retention. Nanoscale roughness and binary charge heterogeneity were found to significantly decrease the energy barrier, but not to low enough levels to explain the observed behavior. The rapid settling of PnZVI was attributed to strong magnetic attraction between particles, which produced rapid aggregation and retention due to straining and/or hydrodynamic bridging. Phosphate adsorption enhanced the mobility of PS-PnZVI in comparison with PnZVI due to a decrease in particle size and aggregation, and an increase in the energy barrier with the porous media. A potential risk of nZVI particles to facilitate oxyanion contaminant transport was demonstrated for phosphate.