Trends of radionuclide sorption by estuarine sediments.: Experimental studies using 133Ba as a tracer

Sediments play an important (but still poorly understood) role in the dispersion and final fate of radionuclides and other hazardous materials in aquatic systems. Adopting an experimental point of view, the present work deals with the transfer of a radioactive tracer from water to sediments. Sediments and waters were sampled in the Odiel and Tinto estuaries (South-West Spain) with anthropogenic-enhanced Ra-226 concentrations. Ba-133 was used as a tracer since it is a gamma emitter and a good analogue of the environmental behaviour of Ra-226. Laboratory experiments have been carried out to quantify the uptake kinetics of Ba-133 by sediments in aqueous suspensions and by sediment cores under a water column at rest. Depth distributions of Ba-133 in sediments have been studied with different contact times and using sediment samples with different grain sizes. The results reveal an important and fast uptake by suspended sediments (up to 40% in 10 min with a 20 g l(-1) suspension) and sediment cores (up to 70% in a few minutes). The kinetics of the uptake by suspended sediments could be reasonably described by a model of two parallel and reversible reactions followed by a weakly-reversible reaction. The total uptake and the rate of reaction decreased with the increment of grain size. Furthermore, uptake following diffusion through the interstitial water was hardly detectable beyond 1 cm depth. For the case with more experimental results, the depth distribution could be reasonably described by the analytical solution based on the use of an effective diffusion coefficient (4.2 X 10(-12) m(2) s(-1)) and the measured intrinsic distribution coefficient (k(d) = 63 +/- 6 l kg(-1)). (C) 2003 Elsevier B.V. All rights reserved.