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Sedimentary arsenic (As) in the shallow aquifers of Bangladesh is enriched in finer-grained de-posits rich in organic matter, clays, and iron (Fe)-oxides. In Bangladesh, sediment color is a use-ful indicator of pore-water As concentrations. The pore-waters of orange sediments are usually associated with lower As concentrations (<50 µg/L) owing to abundant Fe-oxides which sorb As. Using this color signal as a guide, spectroscopic measurements alongside thermal treatment have been extensively utilized for analyzing the properties of both Fe-oxides and clay minerals. This study uses Fourier transform infrared (FTIR) and diffuse reflectance (DR) measurements along with thermal treatment to evaluate the solid-phase associations of As from sediment col-lected along the Meghna River in Bangladesh. The samples analyzed in this study were chosen to represent the various lithologies present at the study site and included riverbank sands (1 m depth), silt (6 m depth), aquifer sand (23 m depth), and a clay aquitard (37 m depth). The concen-trations of sedimentary As and Fe were measured by X-Ray Fluorescence and the spectroscopic measurements were taken on the samples prior to the thermal treatment. For the thermal treat-ment, sediment samples were placed in a preheated furnace at 600C for 3 hours. The thermal treatment caused a deepening of reddish-brown hues in all samples, and the greatest change of color was observed in the finer-grained samples. The FTIR spectral analysis revealed that the clay minerals were composed primarily of illite, smectite, and kaolinite. The DR results indicat-ed that the majority of Fe in sands was present as goethite; however, in the clay and silt samples, Fe was incorporated into the structure of clay minerals as Fe(II). The amount of structural Fe(II) was strongly positively correlated with the sedimentary As concentrations, which were highest in the finer-grained samples. After thermal treatment, the concentrations of As in the finer-grained samples decreased by an average of 40% whereas the change in the As concentrations of the sand samples was negligible. These findings indicate that significant proportions of solid-phase As may be retained by OM and Fe(II)-bearing clay minerals.
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This content will become publicly available on May 15, 2026
Using Peak Geometry and Shifts in the X‐Ray Spectrum of Carbon from Electron Probe Microanalysis to Determine Thermal Maturity of Organic Matter
During the burial of mudstones, the associated organic matter undergoes gradual thermal maturation, a key process that can influence the reactivity of organic matter during catagenesis, the formation of hydrocarbon deposits and the chemical weathering of mudstones. Conventional methods for assessing the thermal maturity of organic matter often fail to reflect the geochemical heterogeneity between individual organic phases in mudstone samples. Here, we report an alternative, non‐destructive, surficial and micro‐scale (analytical spot size of ~ 300 nm with about 4 μm diffusion depth for micrometre‐size organic grains) method to evaluate the thermal maturity of organic matter in mudstones using the carbonKα X‐ray spectrum measured by field emission‐electron probe microanalyser (FE‐EPMA). Using this method, we observed correlations between parameter values derived from FE‐EPMA spectra, including the peak position, the peak area and the intra‐sample heterogeneity of these measurements, and independently measured vitrinite/solid bitumen reflectance for a suite of mudstones, representing different age, geological context and burial depth. With the increased values in peak area and position, we identified an increase in the carbon mass fraction of organic matter and the mean nominal oxidation state of carbon approaching zero. These trends, which are consistent with aromatisation and graphitisation, provide the rationale for using FE‐EPMA to estimate the thermal maturity of organic matter. To explore some of these trends in more detail, we employed time‐of‐flight secondary ionisation mass spectrometry, X‐ray photoelectron spectroscopy and optical reflectance measurements on a subset of samples.
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- Award ID(s):
- 2141520
- PAR ID:
- 10615875
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Geostandards and Geoanalytical Research
- ISSN:
- 1639-4488
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Sedimentary arsenic (As) in the shallow aquifers of Bangladesh is enriched in finer-grained deposits that are rich in organic matter (OM), clays, and iron (Fe)-oxides. In Bangladesh, sediment color is a useful indicator of pore water As concentrations. The pore waters of orange sediments are usually associated with lower As concentrations (<50 µg/L) owing to abundant Fe-oxides which sorb As. Using this color signal as a guide, spectroscopic measurements alongside thermal treatment were extensively utilized for analyzing the properties of both Fe-oxides and clay minerals. This study uses Fourier transform infrared (FTIR) and diffuse reflectance (DR) measurements along with thermal treatment to evaluate the solid-phase associations of As from sediment collected along the Meghna River in Bangladesh. The samples analyzed in this study were chosen to represent the various lithologies present at the study site and included riverbank sands (1 m depth), silt (6 m depth), aquifer sand (23 m depth), and a clay aquitard (37 m depth). The concentrations of sedimentary As and Fe were measured by X-ray fluorescence, and the spectroscopic measurements were taken on the samples prior to the thermal treatment. For the thermal treatment, sediment samples were placed in a preheated furnace at 600 °C for 3 h. The thermal treatment caused a deepening of reddish-brown hues in all samples, and the greatest change in color was observed in the finer-grained samples. The FTIR spectral analysis revealed that the clay minerals were composed primarily of illite, smectite, and kaolinite. The DR results indicate that the majority of Fe in sands was present as goethite; however, in the clay and silt samples, Fe was incorporated into the structure of clay minerals as Fe(II). The amount of structural Fe(II) was strongly positively correlated with the sedimentary As concentrations, which were highest in the finer-grained samples. After thermal treatment, the concentrations of As in the finer-grained samples decreased by an average of 40%, whereas the change in the As concentrations of the sand samples was negligible. These findings indicate that significant proportions of solid-phase As may be retained by OM and Fe(II)-bearing clay minerals.more » « less
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