The concentration of uranium in seawater is remarkably constant at 3.3 mg U/m3. The main uranium species in seawater is the stable anionic tricarbonate-uranyl complex, UO,(CO,):- (Saito and Miyauchi, 1981, 1982). Marine uranium displays no detectable deviation from the norrnal terrestrial U-235/U-238 isotope ratio. 'The total volume of the oceans has been estimated to be 1.37 x 10" m3, and the dissolved uranium should amount to 4.5 x lo9 tons, which is almost a thousandfold larger than the terrestrial resources of reasonable concentrations. Thus, the ocean is virtually a limitless reservoir of dissolved uranium in a well-defined chemical environment.
A recovery program was begun in England in the early 1960's (Davies et al., 1964). Later some studies were made by universities and by private companies in various countries, including Japan and West Germany (Schenk et al., 1982;Astheimer et al., 1983). Many methods of recovery have been suggested: coprecipitation, adsorption, ion flotation, solvent extraction, and others. Of these, only the adsorption process using a suitable adsorbent seemed to be feasible (Kanno, 1981).
Considerable effort has been directed toward developing such adsorbents that originate from inorganic or organic materials. Hydrous titanium oxide was identified as the most promising inorganic adsorbent. Organic adsorbents were classified into three types: a chelating ion-exchange resin containing amidoxirne functional groups (Egawa et al., 1980a,b), a macrocyclic compound bounded to a resin (Tabushi and Kobuke, 1981), and a cellulose resin immobilized by polyphenol compounds (Saka-Correspoodencc concerning this paper should be addressed to Kyoichi Saito.