Publication Abstract

Colloidal Behaviour of Actinides in an Oligotrophic Lake

K.A. Orlandini, W.R. Penrose, B.R. Harvey, M.B. Lovett and M.W. Findlay

Numerous environmental studies in water and sediments have made use of trace metals to determine environmental impact, as well as act as indicators of geochemical parameters, e.g., sedimentation rates. Of particular use are actinide markers that can occur from fallout or from local sources. Some of the more useful of these markers include isotopes of plutonium, thorium, americium, and curium, which can be used as indicators of regional and locally sourced materials.

The fate of many trace metal or organic pollutants in natural waters is controlled by their binding to particulate matter. The phase separation resulting from such binding, followed by sedimentation, is an important mechanism by

which particle-reactive pollutants and trace metals can be removed from bulk water. The actinide elements have a strong affinity for particulates, but this affinity is modified by pH, inorganic ions, oxidation state, and the presence of colloidal organic matter. Depending on the circumstances, one or another of these parameters might predominate (1). For example, carbonate can form complexes with U(VI) that bind poorly to particulates. Actinides in the V oxidation state, such as Pu(V) and Np(V), are only weakly particle-reactive, and Am(III), Pu(IV), and Th(IV) are strongly reactive.

Colloidal organic matter can strongly complex trivalent and tetravalent actinides at concentrations commonly encountered in the environment. Nelson et al. (2) demonstrated that concentrations of organic matter existing in many natural waters (1-20 mg/L) can compete with suspended particulates for the available actinides. The binding of actinides to particles (i.e., >_0.45 ,cm) can be reduced by orders of magnitude by the presence of smaller colloidal organic matter. Other forms of colloidal material, such as clays and iron and manganese hydroxides, are also encountered in natural waters, but their roles are less well understood than that of organic colloids.

Recent studies of natural colloidal systems by Santschi and others (3-5) have led to a renewed appreciation of the dynamic nature of these systems. .,Aside from the binding of ions to particulates and colloids, these materials seem to participate in equilibria involving aggregation and disaggregation as well.

We have investigated the distribution of actinides among natural colloidal particles of various sizes. This study was done in an artificial oligotrophic lake located in northwest Wales, United Kingdom, which contains measurable traces of some important actinides.

Reference:

K.A. Orlandini, W.R. Penrose, B.R. Harvey, M.B. Lovett and M.W. Findlay, 1990. Colloidal Behaviour of Actinides in an Oligotrophic Lake. Environmental Science and Technology, 24(5): 706-712.