Zinc Bearing Particles

Transmission electron microscopy (TEM), coupled with energy dispersive spectroscopy (EDS), has been shown to be an effective, powerful tool for examining geochemical question on the individual particle scale. When used together in a quantitative sense, the term analytical electron microscopy (AEM) is often used. In particular, AEM is well suited to provide information on the size, morphology, crytallinity, and elemental composition of single particles as well as on their physical and spatial associations (Leppard et. al., 1988; Buffle et. al., 1989). Some recent studies have used AEM to examine mineral associations in aquifers (Swartz et. al., 1997), heavy metal sulfide precipitation in soils (Barnett et. al., 1997), chemical speciation of metals in lakes Figueira et. al., 1999) as well as weathering and diagenetic reactions in lake sediments (Banfield et. al., 1991b; Banfield et. al., 1991a). The real power of the technique arises from the ability to make measurements directly, i.e. by making direct observations of the particles themselves. This eliminates the averaging over many thousands of particles that may occur when looking at bulk properties of the sediments, although a large number of samples must be examined for the results to be statistically significant. Thus many of the insights that can be realized through AEM can be missed when making inferences from bulk chemical data. Thus, by combining AEM with other analytical techniques, a more developed chemical picture can be developed of the sedimentary environment. 86 Electron microscopy of the sediments of Lake DePue reveals the existence of several different colloid classes present in nearly all core samples. These classes can be identified through a combination of their size, shape, electron-opacity, and elemental composition as determined by STEM-EDS. The principal constituents were bacteria, particles containing primarily Mg and Ca exclusively, although found in small amounts at all collection sites, were found to be a minor component among the other major particles observed. X-ray EDS analyses showed that zinc was most prevalent on particles close to the effluent source. The details of these zinc-containing phases are discussed below. Distinct trends of zinc associations on colloidal particles can be observed between the various collection sites. Figures 6.1a and 6.1b show the electron micrographs and Figures 6.2a and 6.2b present their associated EDS spectra of particles collected from the site C 1. This site is characterized by a low volume, oxic, effluent stream transporting up to 300 µM Zn over sediments that contain …