8153

• UCREFRP Catalog Number: 8153
• Author: Jacobs, e. L. W.
• Title: Editor
• USFW Year: Series
• USFW - Doc Type: 1989
• Copyright Material: YES

SELENIUM IN SELENIFEROUS ENVIRONMENTS 35
vapors are discharged as aerosols to the atmosphere, only to enter the ter-
restrial and aquatic environment by wet or dry deposition.
A Se mass balance study was conducted at the Allen Steam Plant in Mem-
phis, TN (Andren et al., 1975). This plant was equipped with ahigh-efficiency
electrostatic precipitator that allowed only 0.5% of the total incoming fly
ash to be exhausted to the atmosphere. However, as much as 15 to 20% of
the 0.1 to 0.5 µm MMAD particles and a very large portion of the <0.1
µm MMAD particles were lost as aerosols. Of the 2.2 mg Se/kg coal enter-
ing the boiler, 0.3010 exited with the slag, 68% was with the precipitated fly
ash particles, and the remaining 32010 was associated with the vapor phase.
The Se in the slag, fly ash, and vapor phases was identified as elemental Se.
However, some of this Se must occur as or eventually become bioavailable,
because fly ash is a good source of Se for higher plants (see section on Bio-
availability of Fly Ash Selenium, below).
An estimated 6 x 10 ~ 1 and 28 x 1011 kg of coal (having about 11 %
ash) are combusted annually in the USA and world, respectively (Andren
et al., 1975). Perhaps coal consumption has been reduced or data were cal-
culated differently, because Eisenberg et al. (1986) quoted a figure of 4.5
x 1010 kg of fly ash produced in the USA in 1980.
Upon combustion, about 40010 of the ash occurs as bottom ash or slag,
60010 as fly ash, and < 1% as aerosols that escape to the atmosphere (Eisen-
berg et al., 1986). These proportions may differ for other coal-boiler plants,
because of design and operating conditions. Selenium concentrations in these
components at the Allen Steam Plant were 2.2, 0.08, 28, and 88 mg Se/kg
for the bulk coal, slag, fly ash, and aerosol fractions, respectively.
Selenium Enrichment Factors in Aerosols, Particulates, and Precipitation
The Se enrichment of a given fraction relative to crustal rocks provides
a useful index to follow the preferential movement of Se (or other elements)
in the environment. The enrichment factor (EF) for Se is calculated as follows:
EF = (Se/Al)aerosol/(Se/Al)crust
The term (Se/Al)aerosol is the ratio of the Se concentration to that of Al
in the aersol and the (Se/A1)crust is the ratio of the elements in igneous rocks
of the upper continental crust (Wedepohl, 1971). This approach assumes that
AI is conserved in the combustion process and thus the EF for Al is 1.0. The
EF values could also be calculated for the Se enrichment in the ash or other
carrier. Similar EF values of 84 and 132 were calculated for Se in ash from
11 coal-fired power plants in the United Kingdom and 23 in the USA, respec-
tively (Wadge et al., 1986; Furr et al., 1977).
Wadge et al. (1986) separated fly ash into size fractions ranging from
1.25 to 76 µm MMAD. Most of the particles were <20 µm MMAD. The
Se concentrations in the bulk ash from the 11 plants ranged from 2 to 26
mg Se/kg ash with a mean of 5.4 mg Se/kg. These researchers reported that
Se concentrations were inversely related to particle size and further that the