Here, we describe surface functionalized, superparamagnetic iron oxide nanocrystals (IONCs) for ultra-high PFAS sorption and precise, low energy (magnetic) separation, considering perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). As a function of surface coating, sorption capacities described are considerably higher than previous studies using activated carbon, polymers, and unmodified metal/metal oxides, among others. In particular, positively charged polyethyleneimine (PEI) coated IONCs demonstrate extreme sorption capacities for both PFOA and PFOS due to electrostatic and hydrophobic interactions, along with high polymer grafting densities, while remaining stable in water, thus maintaining available surface area. Further, through a newly developed method using a quart crystal microbalance with dissipation (QCM-D), we present real-time, interfacial observations (e.g., sorption kinetics). Through this method, we explore underpinning mechanism(s) for differential PFAS (PFOA vs PFOS) sorption behavior(s), demonstrating that PFAS functional head group strongly influence molecular orientation on/at the sorbent interface. The effects of water chemistry, including pH, ionic composition of water, and natural organic matter on sorption behavior are also evaluated and along with material (treatment) demonstration via bench-scale column studies.