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Evaluation of a dichromate oxidation method for the isolation and quantification of black carbon in ancient geological samples

Zhu, Mengbo, Li, Meiju, Wei, Siye, Song, Jianzhong, Hu, Jianfang, Jia, Wanglu, Peng, Ping'an
Organic geochemistry 2019 v.133 pp. 20-31
charcoal, organic carbon, oxidants, oxidation, sediments, stable isotopes, sulfuric acid
The dichromate oxidation method is widely used in measurements of black carbon (BC) in geological sediments. The separation of organic carbon (OC) and BC is a critical step in the procedure. A comprehensive evaluation of the dichromate oxidation method was conducted using three types of kerogen with different maturities and two charcoal samples. The residues of each kerogen/BC oxidized under different conditions were quantified to determine the remaining C content. The results showed that the traditional dichromate oxidation method (0.1 M K2Cr2O7/2 M H2SO4) was reliable when the geological sediments contained a Type I kerogen with low maturity (Ro ≤ 0.50%). We compared the dichromate oxidation method under different oxidation times and oxidant concentrations for the separation of OC and BC. The extension of oxidation time improved the removal of all three types of kerogen, but Type I kerogen with high maturity and Types II and III kerogen, still significantly interfered with the isolation of BC in geological sediments. The concentrations of oxidant necessary were tested and indicated that an increase in oxidant concentration significantly improved the separation of BC and OC, and a high concentration of 0.5 M K2Cr2O7/2 M H2SO4 and 300 h oxidation time at 55 °C was effective for the isolation of BC in geological samples. The results of this study also indicated that the stable C isotope values of the remaining C were not affected by the isolation of BC with the dichromate method and that the method can therefore be used for the stable C isotope measurements of BC in geological sediments.