A host of transition metal ions (Co2+, Cu2+, Ni2+, RuX+, RhX+, Pd2+, Ag+, and Pt4+) were sequestered from aqueous media upon exposure to FexOy@Fe nanoparticles. Concentrations were lowered from approximately 100 ppm to below 40 ppb. This technique was extended to the removal of catalytic ions from the products of a "Click" cycloaddition and a Heck coupling in organic solvents. Residual metal concentrations in organic reaction products matched or exceeded pharmaceutical standards.