Interparticle migration of metal cations in stream sediments as a factor in toxics transport

Sorption of metal cations by stream sediments is an important process affec ting the movement of released contaminants in the environment. The ability of cations to desorb from one sediment particle and subsequently sorb to an other can greatly affect metal transport rates but rates for this process h ave not been reported. The objective of this study was to determine the rat e at which sorbed metals can migrate from contaminated sediment particles t o uncontaminated sediment particles as a function of the concentration of t he contaminating solution and the duration of the contact with the contamin ating solution. Samples of small sediment particles were exposed to solutio ns containing cobalt, after which they were rinsed and combined with larger uncontaminated sediment particles in the presence of stream water. Initial concentrations of the contaminating solution ranged from 1 ng/l to 1000 mg /l and exposures to the contaminating solution ranged from 6 h to 14 days, The rate of the migration increased with increasing concentrations in the c ontaminating solution and with decreasing times of exposure to the contamin ating solution. Under the conditions of these experiments, the time require d for the migration to reach equilibrium was on the order of months or long er. In separate experiments, the kinetics of adsorption and desorption of c obalt were measured as a function of concentration of the contaminating sol ution. The time required to reach adsorption equilibrium increased with inc reasing concentration in the contaminating solution, Times to sorption equi librium were on the order of months. Desorption was much slower than adsorp tion and, together with intraparticle diffusion, probably controls the rate of migration from contaminated to uncontaminated sediment. The results of this study show that interparticle migration of metal cations can proceed a t significant rates that are strongly influenced by the length of time that the metal has been in contact with the sediment. (C) 2001 Elsevier Science B.V. All rights reserved.