『Abstract
In geologic materials, petroleum, and the environment, selenium
occurs in various oxidation states (VI, IV, 0, -II), mineralized
forms, and organo-Se complexes. Each of these forms is characterized
by specific chemical and biochemical properties that control the
element's solubility, toxicity, and environmental behavior. The
organic rich chalks and shales of the Upper Cretaceous Niobrara
Formation and the Pierre Shale in South Dakota and Wyoming are
bentoniferous stratigraphic intervals characterized by anomalously
high concentrations of naturally occurring Se. Numerous environmental
problems have been associated with Se derived from these geological
units, including the development of seleniferous soils and vegetation
that are toxic to livestock and the contamination of drinking
water supplies by Se mobilized in groundwater.
This study descries a sequential extraction protocol followed
by speciation treatments and quantitative analysis by Hydride
Generation-Atomic Absorption Spectroscopy. This protocol was utilized
to investigate the geochemical forms and the oxidation states
in which Se occurs in these geologic units. Organic Se and di-selenide
minerals are the predominant forms of Se present in the chalks,
shales, and bentonites, but distinctive variations in these forms
were observed between different sample types. Chalks contain significantly
greater proportions of Se in the form of di-selenide minerals
)including Se associated with pyrite) than the shales where base-soluble,
humic, organo-Se complexes are more prevalent. A comparison between
unweathered samples collected from lithologic drill cores and
weathered samples collected from outcrop suggest that the humic,
organic-Se compounds in shale are formed during oxidative weathering
and that Se oxidized by weathering is more likely to e retained
y shale than by chalk. Selenium enrichment in bentonites is inferred
to result from secondary processes including the adsorption of
Se mobilized by groundwater from surrounding organic rich sediments
to clay mineral and iron hydroxide surfaces, as well as microbial
reduction of Se within the bentonitic intervals. Distinct differences
are inferred for the biogeochemical pathways that affected sedimentary
Se sequestration during periods of chalk accumulation compared
to shale deposition in the Cretaceous seaway. Mineralogy of sediment
and the nature of the organic matter associated with each of these
rock types have important implications for the environmental chemistry
and release of Se to the environment during weathering.』
1. Introduction
2. Geologic setting and composition of the Smoky Hill and Sharon
Springs Members
3. Methods
3.1. Sample preparation
3.2. Concentrations of major and minor elements
3.3. C and S determination
3.4. Total selenium determination
3.5. Sequential extraction overview
3.6. Sequential-extraction methods
3.6.1. Water extraction (Duplicate samples)
3.6.2. Phosphate extraction (Duplicate samples)
3.6.3. NaOH extraction (Duplicate samples)
3.6.4. Sodium sulfite extraction (Duplicate samples)
3.6.5. Acetic acid extraction (Duplicate samples)
3.6.6. Cr(II) reduction/volatilization (Single sample)
3.6.7. Nitric/perchloric acid digestion of residues (Both residues)
3.6.8. Speciation treatments
4. Results
4.1. C, S, and total Se
4.2. Sequential extraction
5. Discussion
6. Conclusions
Acknowledgments
References