Abstract: An aqueous medium containing contaminant ions such as hexavalent chromium ions is treated by introducing electrolytically generated ferrous ions which are preferably generated in water acidified to a low enough pH to prevent the formation of flocculent material. The ferrous ions reduce the hexavalent chromium ions to a trivalent state, and thereafter alkaline material is introduced into the medium to raise its pH high enough to cause insoluble chromic hydroxide and ferric hydroxide to precipitate. The apparatus includes a ferrous ion generator for recirculating liquid through an electrolytic cell and a receptacle, from which receptacle liquid for end use can be withdrawn and to which makeup water and acid can be added.