Batch data for the sorption of Eu and Th on pelagic sediments may be represented by equations of the form: ln M = A ln Cs + B/T + D, where M = concentration of sorbate on sediment, Cs = concentration of sorbate in solution, T = absolute temperature, and A, B, and D = constants. Thermodynamic interpretation of this equation leads to an expression for the true thermodynamic equilibrium constant of K = m/CsA and for the enthalpy change, ΔH, of d ln(M/CsA)/d(1/T) = −ΔH/R, where R = universal gas constant.

Experimentally, the sorption of Eu onto clay-rich sediments was very rapid in the first few seconds and slowed over an interval of minutes to hours. Rate curves were similar in shape to those of α-iron hydroxide, rather than of the oxalate-extracted residual sediment, indicating the importance of oxyhydroxide-like phases in the uptake of Eu onto red-clay sediments. For clay-rich sediments, numerical modeling reproduced the general features of a series of diffusion experiments. To a first approximation, the penetration of Eu into a sediment proceeded by saturation of the sediment to the depth of penetration and produced a sharp drop-off in sorbed + dissolved Eu concentration at the diffusion front. Higher partition coefficients (Kp) resulted in greater sorbed + dissolved concentrations, but reduced penetration. For calcareous sediments, however, Eu concentrations at the surface were much higher than at depth, presumably due to the formation of an insoluble carbonate.