Movement of selenite and selenate by saturated and unsaturated flow in east Texas overburden
The objective of this research was to examine the potential mobility of selenite (SeO/sub 3//sup -2/) and selenate (SeO/sub 4//sup -2/) in a composite of east Texas overburden materials. Portions of deep (20-60 m) cores from along the primary lignite outcrop were used. Sorption of selenite and selenate from soil solution and movement by unsaturated water flow were determined as a function of overburden pH and selenium concentration by soil thin-layer chromatography. The greatest sorption of selenite in all experiments occurred at pH 3, near the pK/sub 1/ of selenite (2.75). Almost all (497 ..mu..g/g) of the applied 500 ..mu..g selenite/g of overburden was sorbed from soil solution at pH 3 while 1160 ..mu..g/g was retained against movement by unsaturated flow. Less than 1.2 ..mu..g selenite/g of overburden was retained against movement by unsaturated flow at pH 2 and 9. These pH values are below the pK/sub 1/ (2.75) or above the pK/sub 2/ (8.5) of selenite. Selenite self-diffusion coefficients showed similar trends. The self-diffusion coefficient at pH 2 and 100% of available water was 53.2 x 10/sup -9/ cm/sup 2/sec/sup -1/, 100 times that at pH 3 (4.9 x 10/sup -/exclamation/sup 0/) and three times that at pH 9 (15.7 x 10/sup -9/). Selenate moved with the wetting from under unsaturated flow conditions at pH 7 and 9 (Rf = 0.96) while the Rf value at pH 3 was 0.76. Sorption trends for selenite indicate two mechanisms; an instantaneous precipitation process and a ligand exchange type reaction. Precipitation dominates at pH 2 and 3 where free iron is present, while ligand exchange reactions become more important at higher pH values. Selenate is non-specifically adsorbed at pH values below 7 and is not retained by the overburden at higher pH values.
- OSTI ID:
- 6305647
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
Terrestrial-- Basic Studies-- (-1989)
54 ENVIRONMENTAL SCIENCES
BROWN COAL
CARBONACEOUS MATERIALS
CHEMISTRY
COAL
ENERGY SOURCES
ENVIRONMENTAL TRANSPORT
FOSSIL FUELS
FUELS
GROUND WATER
HYDROGEN COMPOUNDS
LIGNITE
MASS TRANSFER
MATERIALS
MECHANICS
OVERBURDEN
OXYGEN COMPOUNDS
SELENATES
SELENITES
SELENIUM COMPOUNDS
SOIL CHEMISTRY
SOIL MECHANICS
WATER