Abstract: A method for the treatment of a contaminated medium including the steps of introducing hydrogen into the contaminated medium, adding micron-sized metal catalyst particles to the contaminated medium, reacting the contaminated medium with the micron-sized metal catalyst and substantially retaining the catalyst particles in the contaminated medium. In another aspect, a system for the ex situ treatment of contaminated fluid including a vessel configured to receive the contaminated fluid, micron-sized metal catalyst particles located in the vessel, a hydrogen source positioned for delivering hydrogen to the contaminated fluid, and a membrane filter positioned to prevent the metal catalyst from eluting from the vessel.