What controls selenium release during shale weathering? (original) (raw)
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The Bowland Shale Formation is anomalously rich in selenium (Se) at levels an order of magnitude greater than other black shales. The Mam Tor landslide, Derbyshire, England, offers an opportunity to measure whether the Se anomaly is conferred to the alteration products formed by oxidative water flow through the shale. Selenium in the shale is concentrated in diagenetic pyrite. Alteration of the shale causes decomposition of the pyrite to iron oxyhydroxide, which is carried in colloidal form (ochre) by springs draining the landslide. The iron oxyhydroxide contains anomalously high Se, and anomalously high Se was measured in water ponded where the ochre precipitated, although not in flowing groundwater. Other trace elements including cadmium and thallium also occur at concentrations higher than in other ochres. Given the widespread nature of the Se anomaly in the Bowland Shale Formation and equivalents across Britain and Ireland, any alteration products derived from workings through the shale should be disposed of with care.
Applied Geochemistry, 2013
Soils derived from black shale can accumulate high concentrations of elements of environmental concern, especially in regions with semiarid to arid climates. One such region is the Colorado River basin in the southwestern United States where contaminants pose a threat to agriculture, municipal water supplies, endangered aquatic species, and water-quality commitments to Mexico. Exposures of Cretaceous Mancos Shale (MS) in the upper basin are a major contributor of salinity and selenium in the Colorado River. Here, we examine the roles of geology, climate, and alluviation on contaminant cycling (emphasis on salinity and Se) during weathering of MS in a Colorado River tributary watershed. Stage I (incipient weathering) began perhaps as long ago as 20 ka when lowering of groundwater resulted in oxidation of pyrite and organic matter. This process formed gypsum and soluble organic matter that persist in the unsaturated, weathered shale today. Enrichment of Se observed in laterally persistent ferric oxide layers likely is due to selenite adsorption onto the oxides that formed during fluctuating redox conditions at the water table. Stage II weathering (pedogenesis) is marked by a significant decrease in bulk density and increase in porosity as shale disaggregates to soil. Rainfall dissolves calcite and thenardite (Na 2 SO 4) at the surface, infiltrates to about 1 m, and precipitates gypsum during evaporation. Gypsum formation (estimated 390 kg m À2) enriches soil moisture in Na and residual SO 4. Transpiration of this moisture to the surface or exposure of subsurface soil (slumping) produces more thenardite. Most Se remains in the soil as selenite adsorbed to ferric oxides, however, some oxidizes to selenate and, during wetter conditions is transported with soil moisture to depths below 3 m. Coupled with little rainfall, relatively insoluble gypsum, and the translocation of soluble Se downward, MS landscapes will be a significant nonpoint source of salinity and Se to the Colorado River well into the future. Other trace elements weathering from MS that are often of environmental concern include U and Mo, which mimic Se in their behavior; As, Co, Cr, Cu, Ni, and Pb, which show little redistribution; and Cd, Sb, V, and Zn, which accumulate in Stage I shale, but are lost to varying degrees from upper soil intervals. None of these trace elements have been reported previously as contaminants in the study area.
Selenium Speciation of Marine Shales, Alluvial Soils, and Evaporation of California
1997
The surface exposed shales of the Moreno and Kreyenhagen Formations are considered to be the sources of selenium (Se) that have enriched soils of the west-central San Joaqnin Valley of California. Although the total Se content for the source shales and the resulting alluvial, seleniferous soils has been reported, the distribution of Se oxidation states has not been determined in these materials. This study investigates the Se speciation trends in the seleniferous source shales, the adjacent alluvial soils, the San Luis Drain sediment to the Kesterson evaporation pond soils. The surface-exposed Moreno shales (5.21 mg total Se kg-') had half of the Se content of the less exposed Kreyenhagen shale (10.94 mg total Se kg-'). The elemental Se (Se') concentration was similar in the two shales, hut the selenate (Se+ VI), selenite (Se + IV), and selenide (Se-II) concentrations were lower in the Moreno shale as compared with the Kreyenhagen shale. The Se in the San Luis Drain sediment (83.8 mg total Se kg-') was enriched in the Se' and the Se-II fractions (91.4% of Se inventory). The soils of the Kesterson evaporation pond 4 (47.8 mg total Se kg-'), 7 (6.7 mg total Se kg-'), and 11 (5.4 mg total Se kg-') were also enriched in Se with 86, 76, and 48% of the Se inventory present in the Se" and Se-II fractions, respectively. The organic C content also decreased in the sequence, San Luis Drain, Kesterson pond 4, 7, and 11, from 33.3, 26.7, and 19.3 g C, to 15.1 g C kg-' soil material, respectively, suggesting a relationship between Se concentrations and organic C levels. A linear relationship between the sum of (Se-II and Se')-Se and organic C levels (Rr = O.%*; significant at the 0.05 level) was noted for the analyzed San Luis Drain sediment and the Kesterson soils. For the nine soil materials, an exponential relationship (R' = 0.96) was determined for the sum of (Se-II and Se')-Se and organic C contents, suggesting that the Se in the materials tested was highly associated with the soil organic matter fraction. An additional 12 Se analyses and organic C contents from Se contaminated semiarid and irrigation drainage water evaporation basin soils obtained from the literature closely fit the exponential function established for the nine soil materials. The results suggest that an initial release of organicassociated Se would be expected with increased C oxidation, along with a slower, long-term release of the refractory Se, due to the reversion of the former Kesterson wetland evaporation pond ecosystem back to a native semiarid grassland.
Selenium Speciation of Marine Shales, Alluvial Soils, and Evaporation Basin Soils of California
Journal of Environment Quality, 1997
Although the total Se content for the source shales and the resulting alluvial, seleniferous soils has been reported, the distribution of Se oxidation states has not been determined in these materials. This study investigates the Se speciation trends in the seleniferous source shales, the adjacent alluvial soils, the San Luis Drain sediment to the Kesterson evaporation pond soils. The surface-exposed Moreno shales (5.21 mg total Se kg -') had half of the Se content of the less exposed Kreyenhagen shale (10.94 mg total Se kg-'). The elemental Se (Se') concentration was similar in the two shales, hut the selenate (Se+ VI), selenite (Se + IV), and selenide (Se-II) concentrations were lower in the Moreno shale as compared with the Kreyenhagen shale. The Se in the San Luis Drain sediment (83.8 mg total Se kg-') was enriched in the Se' and the Se-II fractions (91.4% of Se inventory). The soils of the Kesterson evaporation pond 4 (47.8 mg total Se kg-'), 7 (6.7 mg total Se kg-'), and 11 (5.4 mg total Se kg-') were also enriched in Se with 86, 76, and 48% of the Se inventory present in the Se" and Se-II fractions, respectively. The organic C content also decreased in the sequence, San Luis Drain, Kesterson pond 4, 7, and 11, from 33.3, 26.7, and 19.3 g C, to 15.1 g C kg-' soil material, respectively, suggesting a relationship between Se concentrations and organic C levels. A linear relationship between the sum of (Se-II and Se')-Se and organic C levels (Rr = O.%*; significant at the 0.05 level) was noted for the analyzed San Luis Drain sediment and the Kesterson soils. For the nine soil materials, an exponential relationship (R' = 0.96) was determined for the sum of (Se-II and Se')-Se and organic C contents, suggesting that the Se in the materials tested was highly associated with the soil organic matter fraction. An additional 12 Se analyses and organic C contents from Se contaminated semiarid and irrigation drainage water evaporation basin soils obtained from the literature closely fit the exponential function established for the nine soil materials. The results suggest that an initial release of organicassociated Se would be expected with increased C oxidation, along with a slower, long-term release of the refractory Se, due to the reversion of the former Kesterson wetland evaporation pond ecosystem back to a native semiarid grassland.
Environmental Earth Sciences, 2010
Selenium (Se) is essential in the human diet, but has a low threshold for toxic concentration. It is recommended that nutrients such as Se should be consumed through foods as part of a normal diet. Se concentrations in crops and meat depend on the amount of labile Se in the soil where crops and forage are grown. Therefore, managing agriculture for optimal Se in grain crops and forage requires an understanding of the distribution and mobility of Se. Elevated concentrations of Se occur in waters, soils, and forage 120 km west of Pierre in west central South Dakota, USA. The research site lies in an elevated, dissected plain where soils developed on gently dipping Pierre Shale. Soils were sampled along catena transects and waters collected from soil, ponds, and shallow borings in areas of known elevated forage and crop Se. Soil extracts from saturatedpaste extraction and acid (aqua regia and hydrofluoric acid) extraction were analyzed. Selenate was the dominant Se species in both acid and saturated-paste extracts; selenite and organic Se were below detection (\0.2 ppb) in the same soil extracts. On average, 98% of soil Se was not water-soluble.
Chemical Geology, 2018
The selenium enrichment in groundwater may be caused by the contact with sedimentary formations. In many French regions, geogenic sedimentary sources of Se were identified as responsible of the Se concentrations exceeding the European limit in many wells designated to produce drinking water. This study focuses on the black argillaceous infillings of the Hydrogeological Experimental Site of Poitiers (HESP) and aims to better understand the Se release mechanisms at the interface between these infillings and aqueous media using batch experiments with variable pH and contact time. The Se behavior as function of pH was similar in the ultrapure water and in the HESP water and can be discriminated in three main domains. For very acidic domain the speciation of the released Se, that can be associated to the dissolution of some mineral phases, was majorly unidentified. For 2<pH<7.5, Se is mainly released as oxyanions which can be associated to interfacial processes. For basic pH, the maximal quantity of Se was released as oxyanions and unidentified species and is accompanied with the Fe and organic carbon mobilization. The kinetic data showed that the HESP water enhance the Se mobilization as function of time. The kinetic release of Se can be modeled as the release of three main fractions that cannot be directly correlated to the Se speciation: an immediate fraction followed by the release of two fractions corresponding to the pseudo first order kinetic. These results coupled with some in-situ data confirm the diversity and the complexity of the release mechanisms of Se that is not associated with a specific carrier phase but disseminated within the sedimentary matrix.
Chemical Geology, 2012
Several Se-bearing minerals have been identified in Se-rich stone coal, spoils, and their adjacent organic-rich soils in Yutangba of Enshi, China, where human Se toxicity occurred in the 1960s. These minerals mainly include native Se (Se 0), krutaite (CuSe 2), klockmannite (CuSe), mandarinoite (Fe 2 (SeO 3) 3 •6H 2 O), Se-bearing chalcopyrite (CuFe(Se, S) 2), and pyrite (Fe(Se,S) 2). The assemblage of native Se, krutaite, klockmannite, and Se-bearing pyrite and chalcopyrite is primarily present in the stone coal near a fault plane, while the assemblage of native Se and krutaite is found in the Se-rich carbonaceous mudstone and organic-rich soils which are 60 m away from the stone coal exposure. The assemblage of mandarinoite and native Se is present in abandoned stone coal spoils, where natural combustion occurred. Native Se is quite extensive in the stone coal spoils and nearby soils derived from them. The co-occurrence of Cu-Se minerals and native Se indicates that these minerals could have formed under a relatively acidic and reducing environment below 220°C, and suggests that Cu could play a significant role in fixing reduced Se ions in the acidic, organic-rich surface environment. Furthermore, the occurrence of mandarinoite suggests that iron-oxides constrain the geochemical behavior of Se in oxidizing environments. Our observations provide new insights into the mechanisms of Se fixation and accumulation during weathering of Se-rich rocks.