Adsorption-desorption reactions and bioturbation transport of Ra-224 in marine sediments: a one-dimensional model with applications

The distribution of trace metals in sediments and their exchange between se diments and overlying water is governed by multiple processes including mol ecular diffusion, bioturbation (porewater advection, porewater mixing, and particle mixing), chemical reactions and adsorption-desorption. To understa nd these processes and their relative contributions, a one-dimensional mode l was built, which includes bioturbation and adsorption-desorption processe s, to describe the transport of Ra-224. Because Ra-224 is adsorbed on MnO2, Ra-224 may serve as a proxy for trace metal transport. Three sites were sa mpled and both dissolved and adsorbed Ra-224 were analyzed and modeled to u nderstand the transport and exchange processes. It was found that particle transport of adsorbed Ra-224 followed by desorption at the sediment/water i nterface typically represents the dominant flux. We have further been able to define conditions where the porewater transport for adsorption reactive metals like Ra-224 (and other metals) may be out of the sediments whereas t he active scavenging of Ra-224 from the water column at the sediment water interface via adsorption reactions can result in a flux of Ra-224 into the sediment. These processes are both predicted by the model and observed in s ediment samples. (C) 2001 Elsevier Science B.V. All rights reserved.