Granulated lignite (brown coal) is used to provide an adsorption matrix with which sea water is brought into contact long enough to reach a substantially maximum adsorption of uranium, after which the adsorption matrix is removed from the solution and is burned with utilization of its heating value while the heavy metals are concentrated in the combustion residue, after which the thus concentrated heavy metals are recovered in the form of their salts by previously known treatment. An average grain diameter of the granular lignite between 0.5 and 2 mm is used and about 3 mg of uranium per kg of dry lignite is adsorbed with a contact time in the region between one and sixty seconds. Uranium adsorption falls off when there is a substantially greater concentration of foreign ions, particularly calcium, magnesium and sulphate.