A COORDINATED TEM AND ICPMS STUDY OF METAL SEQUESTRATION IN COLLOIDS AND NANOPARTICLES IN THE LOWER COEUR D'ALENE RIVER VALLEY, NORTHERN IDAHO

Sulfide mine contaminated sediment from five transects along the length of the lower-Coeur d'Alene River Valley in northern Idaho have been evaluated using analytical TEM and a calibrated sequential extraction procedure with subsequent metal analysis using HR-ICPMS (As,Ag,Cd,Mn,Fe,Zn, and Pb) to determine the cycling of trace metals.

The lower CDA river valley consists primarily of lacustrine, palustrine, and emergent fluvial deposits with water levels controlled by the dam at the Lake Coeur d'Alene outlet. Colloidal and/or nano-particulate phases are significant metal sequestering agents in all sedimentary environments, including deep river channel, river edge, levees, overbank and both emergent and submergent palustrine. Colloidal metal sequestration predominates in areas with affected by annual drawdowns. Colloid characteristics depend principally on redox conditions in the sedimentary environment. Various metal oxides and oxyhydroxides predominate in permanently oxidizing levee samples and nano-crystalline and amorphous sulfidic colloids predominate in anoxic (submergent) environments. Amorphous sulfidic and phosphatic colloids carry the primary metal load in environments exposed to annual water-level fluctuations (transient redox conditions). Colloids from these transient redox environments are typically non-stoichiometric, metal-rich phases encrusting bacteria. Colloidal Arsenic is principally associated with either two-line siliceous ferri-hydrite (oxidizing environments) or with the sulfidic non-stoichiometric amorphous biocoatings and nanocrystalline sphalerite (~1 wt% As). Cadmium and Zn have similar affinities in the anoxic samples with a strong tendency toward accumulating in the amorphous non-stoichiometric sulfidic colloids. Silver is widely dispersed in amorphous, non-stoichiometric sulfidic colloids but biocoatings from emergent palustrine environments contain in excess of 20 wt % Ag. Transitional redox environments also contain abundant phosphatic colloidal spheres (~100nm in diameter; ~10 wt % P2O5) with a strong preference for sequestering Pb (60-80 wt % PbO), Fe (~ 5-10 wt % FeO) and trace quantities of other metals.