EVALUATION AND OPTIMIZATION OF A DICHROMATE OXIDATION METHOD FOR EXTRACTING BLACK CARBON FROM LOESS DEPOSITS
Black carbon(BC)originates from incomplete combustion of vegetation and fossil fuels.It can be recognized as a continuum with ambiguous boundaries between its components,which poses challenges in its extraction and quantification.Previous studies employing the dichromate oxidation method to extract BC and quantify its content in loess deposits on the Chinese Loess Plateau(CLP)yielded results differing by an order of magnitude.These disparities are likely attributed to the different oxidation times utilized in the two studies.Our research utilized parallel samples of characterized loess/paleosol layers(S5,L9 and S9)from Duanjiapo section(34.188° N,109.233° E)on the CLP to examine the effect of different oxidation times on the efficacy of dichromate oxidation-based black carbon extractions from loess deposits.We run residual carbon content(RC％),δ13C test and Raman spectroscopy on the parallel samples which went through different oxidation durations.The results reveal a relationship between RC％and oxidation time that complies with the law of mass action.The relationship indicates that sufficient oxidation time can extract the stable fraction of loess deposits,which can be interpreted mostly as BC.Under the conditions of pre-oxidation sample drying and grinding,the oxidation reaction curve reaches the plateau after 20 hours,which means an oxidation duration of 20 hours suffices for the measure of black carbon content in loess deposits.However,under such condition,the δ13C values of extractions remain stable(variation＜0.5‰)not until 100 hour.Further comparative analysis through Raman spectroscopy reveals that,following pre-oxidation sample drying and grinding,both L9 samples after 60 h-oxidation and S5 samples after 80 h-oxidation exhibit homogeneous compositions.Notably,they display distinct characteristic peaks in first-order region(around 1340 cm-1 and 1580 cm-1)associated with carbonaceous materials that have undergone combustion events.When samples are not pre-dried and ground,similar results can be achieved with a 100-h oxidation reaction.We conclude that to measure BC content and δ13CBC,should be used on loess deposits without pre-oxidation sample drying and grinding.Notably,based on the results of our research,we underline the paradigm that we should select,evaluate and optimize the precise methodology for specific BC-related research.
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维普
