The invention relates to metal-treated particles, methods for their preparation and methods for using metal-treated particles for, e.g., remediation of process waste-water, sewage, contaminated groundwater aquifers, and soil containing harmful contaminants. Another aspect of the invention relates to a metal-treated particle comprising a ferrosoferric oxide core and a metal supported on the core, where the average diameter or other largest transverse dimension of the core is from about 75 nm to about 990 nm and the amount of metal supported on the core is from about 8% to about 22% by weight, based on the weight of the metal-treated particle.