8153 - Laserfiche WebLink

Home Browse Search

34 MAYLAND ET AL. tilizers and (ii) spraying plants at the midtillering stage (Feekes 4), but not by seed treatment (Table 2-4). On the basis of these investigations, one can conclude that selenite is a better source than selenate when the concentra- tion in plants is to be raised to a sufficient but not excessive level for livestock nutrition. The application of 10 g Se/ha as selenite to pastures has been suggested (Gissel-Nielsen, 1984). The Se is applied with a carrier fertilizer such as cal- cium ammonium phosphate. A method now legally used in New Zealand is the top-dressing of pastures with Se prills (Gissel-Nielsen, 1984). Kork- man (1985) reported that only New Zealand and Finland had approved the addition of Se to fertilizers with which feedstuffs and food were produced. Contrary to the suggestions of Gissel-Nielsen, both countries use the selenate form at rates of about 10 g Se/ha. In New Zealand, 1% Se granulate is mixed with the granulated fertilizer in bulk blending equipment, whereas in Fin- land a 1% Se solution is sprayed into a granulation drum and Se is thus dis- tributed into all fertilizer granules at an equivalent rate of 6 g Se/ha for silage and 16 g Se/ha for cereal. In New Zealand about 0.5 x 106 ha and in Fin- land about 2.3 x 106 ha would be treated annually, beginning in 1985. SELENIUM IN COAL AND MUNICIPAL REFUSE COMBUSTION Redistribution of Selenium Natural weathering of the earth's crustal material results in an estimated 1.0 x 106 kg and 7.2 x 106 kg Se being mobilized annually and transport- ed by rivers draining the land mass in the continental USA and world, respec- tively (Goldberg et al., 1971; Turekian, 1971). In addition, relatively small, but often detectable amounts of volatile Se compounds are released into the atmosphere as a result of biological activity in terrestrial and aquatic ecosystems (Peterson, 1980; Zieve & Peterson, 1984b). This small amount may be quite important on a global basis. Human activities are significantly impacting the global Se cycle. An- dren et al. (1975) concluded that 1.5 to 2.5 times as much Se is mobilized through coal burning as by natural weathering processes. Most coal consumed in the USA is burned at power plants where Se and other elements entering the boiler are partitioned between a bottom ash stream (slag) and aflue-gas stream containing suspended fly ash and the vapors of volatile elements or compounds. The flue-gas stream next passes through electrostatic precipita- tors, or other devices that efficiently remove (> 95% wt./wt.) fly ash parti- cles > 1.5 to 2.0 µm mass median aerodynamic diameter (MMAD). These scrubbers are less efficient in removing vapors and finer particles (Klein et al., 1975). Most of the bottom ash and the ash from the precipitators are transported to land fills as solid residues or flushed with water to ash ponds, where the Se may be leached from the ash and enter the aquatic environ- ment. About 15% of the residue is recycled in building materials, roadways, concrete, etc. (Eisenberg et al., 1986). Small particles, <2 µm MMAD, and