Isothermal Diffusion of Eu and Th in Deep-Sea Sediments: Experimental Results and a Numerical Model

Gerald B. Epstein and G. Ross Heath
Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island 02882
College of Ocean and Fishery Sciences HA-40, University of Washington, Seattle, Washington 98195

Abstract: 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.

Key Words: Deep-sea sediments • Diffusion • Europium • Partition coefficient • Sorption • Thorium

Clays and Clay Minerals; June 1986 v. 34; no. 3; p. 295-306; DOI: 10.1346/CCMN.1986.0340310
© 1986, The Clay Minerals Society
Clay Minerals Society (www.clays.org)