- The widespread use of engineered nanoparticles (ENPs) in industrial applications and consumer products makes their release to the environment inevitable. Understanding the aggregation behavior of ENPs is crucial to understanding their fate, transport and toxicology in aquatic systems. This study investigated the comparability of experimentally-determined and model-estimated attachment efficiencies for hematite nanoparticles. Aggregation behavior of hematite was investigated in monovalent KCl solutions and polyvalent synthetic freshwater solutions via time-resolved dynamic light scattering (TR-DLS) and nanoparticle tracking analysis (NTA). Experimental findings show that the critical coagulation concentration for KCl with 25 mg L-1 hematite colloids is approximately 33 mM. Rates of aggregation in the synthetic freshwater were significantly faster than in an equivalent ionic strength of KCl, likely due to the influence of pH on the surface charge of hematite. The model-estimated attachment efficiencies at 5 minutes had a strong linear correlation with measured initial aggregation rate, indicating qualitative agreement between the measured and modeled behavior. However, at longer time intervals attachment efficiency varied, indicating that the model was not accounting for all processes. Lastly, a reduced size 3 ODE test system was found to have a stiffness ratio of at least 107 for all non-zero concentration combinations of N1, N2, and N3, indicating that the model may not be operating within the region of absolute stability for Heun’s method.