Methods for making activated carbon-supported transition metal-based nanoparticles include (a) impregnated activated carbon with at least one transition metal-containing compound, and (b) heating the impregnated activated carbon at a temperature and for a time sufficient to carbothermally reduce the transition metal-containing compound. Also disclosed are activated carbon-supported transition metal-based nanoparticles produced by such methods. Further disclosed are methods for treating water and waste streams that include contacting the water or waste streams with the activated carbon-supported transition metal-based nanoparticles.