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

Abstract

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 a-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 day-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-offin sorbed + dissolved Eu concentration at the diffusion front. Higher partition coefficients (Kv) 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.

A Brownian-pumping model for oceanic trace metal scavenging: Evidence from Th isotopes

“…Direct experimental evidence linking particle aggregation and metal behavior is needed to distinguish sorption control by coagulation reactions from control by diffusion of the sorbate through surface films or pore spaces (e.g., Enfield et al. 1976;Epstein, 1983;Wu and Gschwend, 1986) or, in the case of radionuclidcs, from slow isotopic exchange (e.g., Johnston, 1977).…”

“…Nash and Choppin (1980) estimated that in natural waters at 0.1 mg 1-1 DOC (humate = 4 x 10-7 eq 1-1) the ratio of [ThHu]/ [Thdiss] would be about 4 x 10 10 . Sequential filtration and ultrafiltration experiments by Santschi et al (\987;1983) have shown that radioisotopes of particle reactive elements are involved in dynamic cycles of aggregation and disaggregation of colloids in the water column of coastal marine ecosystems while Buchholtz et al (1986) demonstrated the aggregation of colloidal radionuclides in laboratory batch experiments using marine sediments. [47,4 b.…”