Speciation and distribution of Group 3 elements

M(C03) 3 - Available equilibrium data indicate that Ac3+ is the most weakly com-plexed trivalent metal in seawater. All Group 3 elements, with the exception of uranium (U), exhibit variable concentrations with depth. 

Although americium (Am) exists in seawater exclusively in the trivalent oxidation state, its profiles in Fig. 12.4 contrast sharply with those of the trivalent lanthanides. Assessments of Nd isotopic ratios in seawater (e.g. Bertram and Elderfield, 1993) indicate that more than 1000 years are required for attainment of steady-state distributions of lanthanides and chemically similar elements in seawater. On such a basis it is expected that, in spite of substantial chemical similarities to the lanthanides, 241Am, a relatively short-lived isotope (half-life 470 years) with variable and recent anthropogenic inputs, will not exhibit profiles similar to those of the lanthanides. 

Thorium (Th), which exists solely in the +IV oxidation state is, like Uff, highly particle reactive. Dissolved Th in seawater should exist principally as Th(OH)4 and Th(OH)3. 

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