Diffusion of Neptunyl(V)- and Pertechnetate Ions in Marine Sediments

Abstract

The diffusion of the ²³⁵NpO₂⁺ and ^95mTcO₄⁻-ions has been measured directly in sample cylinders of two different sediments from the floor of the deep sea. In smectite-rich sediment not shielded from contact with atmospheric oxygen the following values were obtained for the effective diffusion coefficients of neptunyl- and pertechnetate-ions, respectively: D_eff(NpO₂⁺) = 1.5 × 10⁻¹² m²s⁻¹ and D_eff(TcO₄⁻) = 3.2 × 10⁻¹⁰ m²s⁻¹. Under anoxic conditions in sediment with known reducing properties, the pertechnetate ion appears to undergo slow reduction and the effective diffusion coefficient of the reduced species of D_eff(Tc,red)= 1.1 × 10¹⁴ m²s⁻¹ reflects a substantial decrease of the mobility of the lower-valent technetium.