Environmental behavior of actinides

Since the plutonium concentration in ocean waters is quite low, most of the plutonium deposited in marine waters has been sorbed onto plants and sediments. Actinides in natural waters usually are not in a state of thermodynamic equilibrium for long time periods as their solubility and migration behavior is strongly related to the form in which the nuclides are introduced initially into the aquatic system for long time periods. Their solubility depends on pH (hydrolysis), E _H (oxidation state), reaction with complexants (e.g., carbonate, phosphate, humic acid, etc.), sorption to surfaces of minerals and/or colloids, etc. The primary variable is the oxidation state of the actinide cation. Actinides can be present in more than one oxidation state which complicates modeling actinide environmental behavior.Np(V)O₂ ⁺ and Pu(V)O₂ ⁺ are weakly complexing and resistant to hydrolysis and subsequent precipitation, but both can undergo reduction to the IV oxidation state. The solubility of NpO₂ ⁺ can be as high as 10^?4M, while that of PuO₂ ⁺ is more limited as the very low solubility of Pu(OH)₄ promotes reduction to Pu(IV). The solubility of hexavalent UO₂ ²⁺ in sea water is limited by hydrolysis, but has a relatively high concentration due to carbonate complexation. Americium(III) hydroxocarbonate, Am(CO₃)(OH), is the limiting species for the solubility of Am(III) in sea water. Thorium has a very low solubility due to the formation of Th(OH)₄.