Estuarine sediment controls on trace metal distributions

Graduation date: 1982Sediments are the major compartment in aquatic environments for many materials of toxic or nutrient concern. The lack of knowledge of the phase associations of toxic metals in sediments is the largest deficiency in our understanding of the environmental hazard posed by metal contamination. This study examined the sedimentary partitioning of copper and cadmium with the goal of predicting the environmental fate of these metals in estuaries. Five solid phases: synthetic hydrous oxides of iron and manganese, montmorillonite clay, a synthetic aluminosilicate gel, and estuarine humic substances (EHS) extracted from a natural sediment were chosen to simulate components responsible for trace metal binding in oxidized sediments. The uptake of copper and cadmium by these model phases in artificial seawater was measured as a function of pH and salinity. Affinity of the model phases for cadmium as measured by K[subscript p], the slope of the linear adsorption isotherm, was in the following order: manganese > iron > EHS > aluminosilicates > montmorillonite. Copper binding affinities were much higher than those of cadmium but the relative order of affinity of the phases was similar except for copper binding by EHS for which K[subscript p] was relatively higher (exceeding that of iron). Metal binding was not influenced by interactions involved in three types of phase complex: iron-clay, iron-humic and clay-iron-humic suggesting that experiments with isolated phases are adequate to predict partitioning. The measured K[subscript p] values were used to calibrate a simple model for predicting partitioning and total metal uptake by sediments. This model which is analogous to speciation in a solution of competing ligands, predicts that cadmium uptake by estuarine sediments is dominated by iron while both iron and organics are important for copper. Results of selective extraction studies and studies of cadmium uptake by natural estuarine sediments are in semiquantitative agreement with these predictions. In anaerobic sediments thermodynamic calculations suggest that copper and cadmium concentrations are determined by equilibria involving solid sulphides and bisulphide and polysulphide complexes. It is expected that the results of this investigation will aid in assessing the environmental significance of metal-contaminated estuaries