A method and system for decreasing the concentration of at least one metal in an aqueous solution. The metal may be molybdenum, tungsten, or both. An aqueous solution is introduced into at least one reaction zone, and at least one source of hydroxide ions is provided into the at least one reaction zone in an amount sufficient to precipitate at least some of the mass of the at least one metal. The aqueous solution includes a mass of the at least one metal and a mass of at least one reducing agent. The at least one reducing agent includes at least ferrous iron from at least one source of the at least one reducing agent. A composition of tungsten ferrite or molybdenum tungsten ferrite may be formed. The method may be used for purifying water, for the refining of metals, or to facilitate a chemical analytical determination.