应用单体碳同位素分析技术探析农田土壤中多环芳烃的植物降解过程

摘要

For the research on degradation and remediation of soil polycyclic aromatic hydrocarbons (PAHs ) pollutants,researchers have been trying to find an effective method to quantitate the dynamic processes.Many approaches have been designed,such as spiked experiment,concentrations comparison before and after the degradation,and modeling calculations.However,all of this culminates in contradictory results and lack of corroboration of the actual environment.Compound-Specific Isotope Analysis (CSIA)technique is used in this study to investigate the phytodegradation process of soil PAHs with maize as the testing plant.Total concentrations of 1 6 PAHs in the blank soil,rhizosphere soil and the controlled soil samples were (299.8 ±37.9)ng/g,(272.2 ±1 1 .6)ng/g,and (281 .5 ±34.7)ng/g,respectively,all lower than the original cultivating soil (380.8 ng/g). The soil PAHs concentrations did not show significant decline tendency during the maize growing period,except for PAHs compounds below 3-ring which was consistent with the accumulation effect of the 3-ring PAHs in maize roots and leaves.The δ13C value of 6 PAHs compounds ranged from -34.31‰to -23.95‰with fluctuations between-9.0‰ --0.6‰,and all δ13 C values turned lighter with maize plants growing except that of pyrene.No significant correlation between δ1 3 C and the concentration of 4-,5-ring PAHs was observed during the maize cultivating process. Most research indicates that PAHs and other macromolecular compounds strongly resist phytodegradation,which illustrates less obvious carbon isotope fractionation.PAHs compounds below 3-ring would degrade more easily and the isotope fractionate more significantly than those of other macromolecular compounds, thus maing CSIA technique more advantageous for understanding the transportation pathway of these compounds.%长期以来，研究者在探讨土壤中多环芳烃（PAHs）的降解及修复过程中，缺乏简便有效的手段对化合物的降解动态进行定量研究。前人尝试用投加实验、对比采用降解措施前后污染物的浓度变化、模型计算等方法研究PAHs的降解过程，其结果常互相矛盾，或不能真实反映复杂的实际环境。本文应用单体碳同位素分析技术对农田土壤中PAHs的植物降解过程进行定量表征，采集了某地农田表土作为供试土壤，选择玉米作为供试作物，开展了作物对土壤中PAHs降解及消除过程的研究。气相色谱－质谱分析结果表明，培养所用的玉米原始土及分4批收集的空白土、根际土、非根际土样品中16种PAHs的浓度总和（∑PAHs）平均分别为380．8 ng／g、（281．5±34．7）ng／g、（272．2±1 1．6）ng／g和（299．8±37．9）ng／g；玉米生长期间，各土壤样品的∑PAHs均比原始土壤有所下降，但除3环以下化合物（萘、苊烯、苊、芴、菲、蒽）外，其他化合物并未随玉米的生长表现出显著趋势。与玉米根、叶倾向于富集低环PAHs化合物相对应，可以判断植物对土壤中的低环化合物去除作用最为显著。各采样时期玉米根际土、非根际土和空白土壤样品中PAHs单体化合物的碳同位素分馏值（δ13C）在－34．31‰～－23．95‰之间，且除芘外的其他化合物的δ13C值随时间呈现逐步变轻的趋势，波动值位于－9．0‰～－0．6‰之间；本文对于PAHs单体化合物，尤其是4、5环化合物，在玉米降解过程中的碳同位素分馏与浓度变化之间未发现明显关系。考虑3环以下的PAHs化合物更倾向于被降解和清除，且其碳、氢同位素分馏情况更容易被观察到，因此稳定同位素分析技术更有助于探明该类单体PAHs污染物在环境中的迁移、转化规律。