Real water remediation is an important issue that requires the development of novel adsorbents with remarkable adsorption properties, permitting reusability. In this work, the surface and adsorption properties of bare magnetic iron oxide nanoparticles were systematically studied, before and after the application of a maghemite nanoadsorbent in two real Peruvian effluents severely contaminated with Pb(II), Pb(IV), Fe(III), and others. We were able to describe the Fe and Pb adsorption mechanisms that occurred at the particle surface.57Fe Mössbauer and X-ray photoelectron spectroscopy results together with kinetic adsorption analyses gave evidence for two involved surface mechanisms: (i) surface deprotonation of maghemite nanoparticles (isoelectric point of pH = 2.3), forming Lewis sites bonding Pb complexes; and (ii) the formation of a thin inhomogeneous secondary layer of iron oxyhydroxide and adsorbed Pb compounds, as favored by surface physicochemical conditions. The magnetic nanoadsorbent enhanced the removal efficiency to values of ca. 96% and provided adsorptive properties with reusability due to the conserved morphological, structural, and magnetic properties. This makes it favorable for large-scale industrial applications.