8153 - Laserfiche WebLink

Home Browse Search

GEOCHEMISTRY & OCCURRENCE OF SELENIUM in terms of the above figures is given below. More detailed discussions are given in Zingaro and Cooper (1974), Vokal-Borek (1979), and Elrashidi et al. (1987). Selenide (Se2 - ) Thermodynamic data suggest that SeZ- should exist in reducing en- vironments as hydrogen Selenide (HZSe) and as metal selenides (Fig. 1-1). Hydrogen Selenide is afoul-smelling, poisonous gas. It is a weak acid when dissolved in water and easily oxidizes to elemental Se (Fig. 1-2), but is a much stronger acid and is more poisonous than hydrogen sulfide (HZS). Metal selenides tend to be found in metal sulfide ores (e.g., Fe, Cu, Pb). Because the ionic radius of SeZ- is 0.191 nm and that of SeZ- is 0.184 nm (Weast, 1985), substitution of SeZ- for SZ- commonly occurs. Metal selenides, as well as Se-sulfides, tend to be very insoluble (Fig. 1-2; Elrashi- di et al., 1987). Microbial processes can produce volatile methylated derivatives of Se such as dimethyl diselenide (Chau et al., 1976) or dissolved organic Selenide compounds (Cutter, 1982; Cutter & Bruland, 1984). Elemental Selenium (Se°) The Eh-pH diagram indicates that elemental Se is stable in reducing en- vironments (Fig. 1-1). The crystalline forms of elemental Se are alpha- monoclinic and beta-monoclinic Se and are referred to as red crystalline Se. The two amorphous forms of elemental Se are the red amorphous, and glassy or black varieties (Vokal-Borek, 1979). All forms are very insoluble in water and the oxidation or reduction kinetics of the solids are very sluggish (Fig. 1-2). Elemental Se can be oxidized to Se03- and trace amounts of Se04- by certain microorganisms (Sarathchandra & Watkinson, 1981). Selenite (Se03-) Thermodynamic data indicate that the 4 + oxidation state of Se (Se03-) is favored under mildly oxidizing conditions (Fig. 1-1). Selenous acid, HZSe03, is a weak acid and is protonated when pH values are acidic to neutral (Fig. 1-l and 1-2). Most Selenite salts are less soluble than the corresponding selenate salts (Fig. 1-2; Elrashidi et al., 1987). Selenite is reduced to elemental Se either by mildly reducing agents [e.g., ascorbic acid (vitamin C) or sulfur dioxide (SOZ)] in acidic environments (Sarquis & Mickey, 1980) or by microorganisms (Vokal-Borek, 1979). Selenite has a strong affinity for sorption, particularly by Fe oxides such as goethite, amorphous Fe hydroxide, and AI sesquioxides (Hingston et al., 1968; Howard, 1972; Parfitt, 1979; Rajan, 1979; Hingston, 1981; Mott, 1981; Benjamin et al., 1982; Merrill et al., 1986; Balistrieri & Chao, 1987, 1989). Adsorption of Se03 - is a function of pH, particle concentration and com-