Characterization of uranium(VI) in seawater1

Abstract

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.

The influence of seawater pH on U / Ca ratios in the scleractinian cold-water coral <i>Lophelia pertusa</i>

“…Within a typical seawater pH range of > 8, most of the aqueous uranium exists in the form of UO 2 (CO 3 ) 4− 3 (Reeder et al, 2000). With decreasing pH the aqueous species UO 2 (CO 3 ) 2− 2 becomes more dominant, and the proportion of bicarbonate UO 2 (CO 3 ) 2− 2 and monocarbonate uranyl complexes (UO 2 CO 0 3 ) also increase (Djogic et al, 1986). Our data suggest that a preferential uptake of bicarbonates and monocarbonate uranyl complexes can explain the inverse relationship between coral U / Ca ratios and seawater pH, which makes L. pertusa an archive for reconstructions of seawater pH.…”

“…In oxygenated aquatic systems, uranium is conservative and exists in the form of different carbonate complexes (Langmuir, 1978). Speciation is controlled by the carbonate ion forming complexes with the uranyl ion UO 2+ 2 (Djogic et al, 1986). Within a typical seawater pH range of > 8, most of the aqueous uranium exists in the form of UO 2 (CO 3 ) 4− 3 (Reeder et al, 2000).…”

Solubility and spectrochemical characteristics of synthetic chernikovite and meta-ankoleite

“…The uranyl ions behave in a similar way by forming uranyl-hydroxo complexes according to the following scheme (Djogic et al, 1986): [u% 1…”

Advances in measurements of particle cycling and fluxes in the ocean

“…Thorium-234 (t 1/2 = 24.1 d) is the particle-reactive daughter of uranium-238 ( 238 U; t 1/2 = 4.468 × 10 9 y), which is highly soluble in seawater (Djogić et al, 1986). Due to the significantly longer half-life of 238 U than 234 Th, the two species are in secular equilibrium (Activity 234 Th/ 238 U = 1) when there are no net removal or addition processes acting on either species.…”