A process for making a reduced size oxidant particle is provided. The process forms reduced sized oxidant particles by feeding a coating material and oxidant particles of an initial size into a mill. The coating material may be oleophilic, hydrophobic, siliphilic, hydrocarbon soluble, or a combination thereof. The oxidant particles are then milled, reducing the particle size of the oxidant particles. During the milling step, the coating coats a substantial portion of each reduced sized oxidant particle reducing reagglomeration during the milling step. Optionally, the oxidant particles produced by the process of the instant invention are encapsulated. Alternatively or additionally, the oxidant particles may be placed in suspension. An outermost encapsulant may be substantially non-reacting, impermeable and non-dissolving with water. The oxidant particles formed by the process of the present invention may be placed directly into or in the path of the contaminant(s) in the environmental media to be treated. Processes for using the encapsulated reactant in environmental media are also hereby disclosed.