Copper and Cadmium Uptake by Estuarine Sedimentary Phases Robert J. Davles-Colley,t Peter 0. Nelson," and Kenneth J. Wllllamson Civil Engineering Department, Oregon State Unlversity, Corvailis, Oregon 97331
An understanding of the distribution of toxic trace metals and other materials of environmental interest among the different sedimentary phases is necessary to assess the hazard associated with pollution of aquatic sediments. A simple model, analogous to speciation in solution among soluble ligands, was developed to enable the distribution of metals within sediments to be estimated. The model requires the conditional metal binding constants (slopes of the linear portions of the adsorption isotherms) to be evaluated for a small number of model sedimentary phases in isolation, as well as knowledge of the composition of the sediment and water. The model has been applied to the distributions of copper and cadmium in estuarine sediments and predicts that iron and organic matter are the sedimentary constituents of greatest importance for these metals. Manganese phases may contribute to cadmium binding, but clay minerals and (probably) aluminosilicates are insignificant sinks for both copper and cadmium. The model predictions for copper and cadmium in estuarine sediments are in good semiquantitative agreement with the results of selective extraction studies on natural sediments reported in the literature and studies reported here of cadmium uptake by natural sediments.
H
Introduction
Sediments are the major compartment in the estuarine environment for trace metals and other toxic materials. Thus, an understanding of the interaction of such materials with sedimentary phases is necessary to assess ecological impacts. The most common approach to examining the phase associations of metals in aquatic environments is by application of so-called "selective extraction" sequences (1-3) to sediments and associated waters. The sediment samples are subjected to a sequence of physical or chemical procedures that extract one or more sedimentary components (or the materials bound to those components) at each step. Severe problems are encountered with the use of chemical extraction schemes with natural sediments for examining trace metal distributions because there is always a tradeoff between selectivity (for the target phase) and efficiency (proportion of metals extracted from that phase) at each selective extraction step (I, 4). Furthermore, metals extracted from one phase may adsorb on the other phases present ( 4 , 5 ) . 'Present address: Water and Soil Division, Ministry of Works and Development, Hamilton, New Zealand. 0013-936X/84/09 18-0491$0 1.50/0
A more useful approach to the problem of phase association of metals in sediments may be to estimate metal distribution from the known composition of the sediments and the measured affmity of each of the constituent phases for the metals (6). This was the approach adopted in the present study for examining the behavior of the toxic metals copper and cadmium in estuarine environments. The study was confined to consideration of surface (adsorption) reactions since that fraction of the total metal which is incorporated into mineral lattices is most unlikely to have any ecological significance. The study involved performing simple adsorption experiments with model sedimentary phases in order to calibrate a model (outlined below) for metal partitioning among competing consitutent phases in (aerobic) sediments. Predicted copper and cadmium distributions in aerobic estuarine sediments are then compared with the general pattern of results of reported selective extraction studies (1-8) and with some direct measurements of uptake by natural sediments reported here. Model for Trace Metal Distribution in Estuarine Sediments
Since "trace" metals (by definition) are present at low concentrations, binding sites on surfaces of sedimentary phases are in great excess, and adsorption is independent of metal concentration. This greatly simplifies analysis of metal distributions since the mass balance for all the surfaces and ligands can be neglected and only the mass balance on the metal need consideration. Oakley et al. (9) have presented a simple model for the partitioning of a trace metal among a number of different phases comprising an aerobic sediment. The model is analogous to solution phase metal speciation in a singlemetal multiple monodentate ligand system. It is assumed that all the sedimentary phases behave independently; i.e., metal uptake by each phase depends only on mass concentration of that phase and not on state of dispersion or interaction with other phases. Consider a sediment comprised of N phases at any degree of dispersion (whether settled or suspended). The dimensionless concentration of the nth phase, Fn,is given as
n=l
where TS is the total solids content of the sediment slurry
0 1984 American Chemlcal Society
Environ. Sci. Technoi., Vol. 18, No. 7, 1984 491
Table I. Ranges of Concentration of Selected Sedimentary Phases in Aerobic Estuarine Sediments
component (1)hydrous iron oxides (as iron) (2) hydrous manganese oxides (as manganese) (3) clay minerals (
