Characterization of uranium(V1) in seawater’

The physicochemical characterization of uranium(V1) in seawater is described on the basis of species distribution calculations and experiments using polarography and spectrophotometry in artificial seawater at elevated uranium concentrations. Various dissolved uranium(W) species are identified under different conditions of pH and carbonate concentration. Below pH 4, the hydrated uranyl ion is present in the “free” state (forming “labile” complexes). Above pH 4, a stepwise coordination of uranyl by the carbonate ion occurs. The monocarbonate complex is formed in the pH range 4-5, the bicarbonate uranyl complex between 5 and 6. Above pH 8, uranium is present predominately as the tricarbonate and to a smaller extent as a trihydroxide complex. There is satisfactory agreement between our experiments and the theoretically computed distribution of uranium(W) in seawater based on published stability constants. The experiments done at higher concentrations are justified by theoretical distributions showing that there is no great difference in species distribution between the uranium at concentrations of 1O-4 and lo--” mol dm3. The large quantities of uranium dissolved in ocean waters, even at low concentrations (3.3 pg literl: Spence 1968), have made its recovery from seawater of interest as one possibility to solve the increasing energy demands of the world. To achieve technically and economically acceptable levels for its extraction, we need knowledge of both the concentration and the physicochemical state of uranium in seawater. The extraction methods of uranium from seawater (adsorption, solvent extraction, coprecipitation, electrolysis, and biological concentration) are based mostly on the uranium transport at interface phases. These processes as well as thermodynamic distribution coefficients of uranium between seawater and other phases depend directly on the ionic forms of uranium in seawater. The critically evaluated thermodynamical data show that uranium in natural waters is usually complexed (Langmuir 1978), but despite an extensive literature on the subject, I This work was supported by the Self Management Council for Scientific Research of S.R. Croatia and the bilateral agreement of SFR Yugoslavia and USA on scientific cooperation. the existence of these species has not been unambiguously established experimentally. Most species distribution studies published so far, closely related to uranium extraction from seawater (Ogata et al. 197 1; Yamashita et al. 1980; Schwochau et al. 1976), were based mainly on calculations from the literature using uranyl stability constants (Yamashita et al. 1980). The behavior of uranium(V1) in acidic media has been studied extensively. The electrochemical reduction of uranium(V1) to uranium(V) on the dropping mercury electrode at low pH produces a polarographic wave with a reversible half-wave potential at about -0.2 V vs. SCE (Harris and Kolthoff 1945; Weber et al. 1953). The half-wave potential depends on ionic strength in perchloric (Kritchevsky and Hindman 1949; Elving and Krivis 1959), chloridic (Kritchevsky and Hindman 1949), and sulfuric (Elving and Krivis 1959) solutions. The dependence of the half-wave potential on the concentration of Cland SOd2is due to the formation of labile uranyl complexes. The interaction of uranyl cation in dilute solutions and seawater was also investigated to establish the relation