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 dm-3.

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The stability constants of the mixed uranyl-hydroxo-peroxo-carbonato species have been calculated. By incorporating those and including stability constants and corresponding equilibria into the developed model of seawater species distribution enables the dependence of uranyl species on pH to be evaluated. The calculations show that at seawater conditions (~H = 8), 88.3% of total uranyl-ion is in the form U02(C03)3 4-, 11% is in the form U02(COs)2(0H)3-, and 0.5% in the U02(COs)(OH)2 2 -mixed-ligand complex form, while in the photic layer, 81.2% of uranyl-ion is in the tricarbonate complex, 10.1% in the form U02(COs)2(H02)3-, 7% as U02(COs)2(0H)3-, 0.5% as U02(COs)(OH)2 4-and 0.5% as U02(02)2 2species.

According to the calculations, in the presence of equimolar concentrations of uranyl-ion and hydrogen peroxide at 5 < pH < 7, the predominant species is IJO,OY complex. Spectrophotometrically, it has been confirmed in the bulk of the solution giving the apparent concentration stability constant K&,!02 = 2 x 10' mol-' L. Electrochemically, normal, differential pulse polarography and cyclic voltammetry at DME and HMDE arc used to elucidate the reduction scheme of the uranyl-monoperoxo complex.