黄土沉积中化学氧化法提取黑碳流程的评估和优化

第四纪黄土沉积物中黑碳主要为当时地表生物质燃料不完全燃烧产生的一系列含碳物质的连续体。由于各含碳物组分间界线不明确, 采取不同的定量提取方法可能导致不同的数据结果。前人采用重铬酸钾化学氧化法提取黄土沉积中黑碳的研究中定量结果存在一个数量级的差异, 影响了研究结果的可解释性和可对比性, 阻碍了黄土高原区域黑碳相关研究的发展。对比分析发现, 先前研究提取过程分别采用了不同的氧化时间, 这可能是数据呈现明显差异的原因。本研究选取黄土高原段家坡剖面特征层位(S₅、L₉和S₉)样品, 并分为多组子样品采用不同氧化时间进行处理, 对不同氧化时间提取物进行剩余碳含量(RC%)、碳同位素组成(δ¹³C)及拉曼光谱测试, 探究氧化时间对重铬酸钾化学氧化法提取黄土中黑碳效果的影响。结果显示, 提取物RC%与氧化时间的变化关系符合质量作用定律, 即达到一定氧化时间后, 提取物RC%趋于稳定, 证明足够的氧化时间可提取黄土中黑碳; 在氧化前烘干研磨样品的条件下, 仅需20 h即可使RC%稳定。但碳同位素组成测试结果显示, 至少100 h后, 所有实验中氧化提取物δ¹³C均保持稳定(变化＜0.5‰)。进一步通过拉曼谱图比较发现, 氧化前烘干研磨样品后, 黄土样品(L₉层)经60 h及以上处理后的氧化提取物及古土壤样品(S₅层)经80 h及以上处理后的氧化提取物变为均一组成, 且有明显的经历火事件的碳质材料一级模特征峰(1340 cm^-1及1580 cm^-1附近), 而不烘干研磨的条件下, 所有样品直接进行100 h氧化反应也可达到相同效果。综合时间效率、结果稳定性和流程统一性等各因素, 提出黄土-古土壤序列沉积物黑碳的提取过程中, 在重铬酸钾化学氧化一步采用100 h的时间进行充分反应, 氧化前无须再烘干研磨。本研究优化了化学氧化法提取黄土中黑碳的实验流程, 对系统开展黄土区域黑碳工作, 重建区域火并探讨其演化规律提供了技术支撑; 为其他沉积环境黑碳流程的优化和评估提供了研究范本。

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.