Maghemite nanoparticles (γ-Fe2O3) produced using a recently developed, single-step method proved to be highly effective for selective removal of Cu, Ni, Mn, Cd(II), and Cr(VI) from acid mine drainage (AMD) and simulated wastewater. An average particle size of about 14 nm was estimated for the maghemite nano-particles using transmission electron microscopy. Their adsorption characteristics proved to be highly pH dependent, allowing facilitated selective adsorption of the studied metals, all of which followed the Langmuir adsorption model. In both simulated wastewater and AMD, Cr(VI) adsorption was best at 70 °C and pH = 2.6. Adsorption peaked at pH = 8.5 for Ni(II), 10 for Cd(II), 8.5 for Mn(II), and 6.5 for Cu(II).