Kulp & Pratt(2004)による〔『Speciation and weathering of selenium in Upper Cretaceous chalk and shale from South Dakota and Wyoming, USA』(3687p)から〕

『米国サウスダコタおよびワイオミング州からの上部白亜系チョーク(泥灰質堆積岩)および頁岩中のセレンの種形成と風化』


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


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