土壤中多环芳烃的稳定碳同位素特征及其对污染源示踪意义

Stable carbon isotopic composition of polycyclic aromatic hydrocarbons in soil and its implications for the pollutants tracing

分析了天津市不同功能区土壤中多环芳烃的稳定碳同位素组成特征.土壤中菲、甲基菲、荧蒽和芘的δ13C值范围分别为-29 5‰～-23 2‰、-39 8‰～-23 4‰、-27 2‰～-23 6‰和-28 1‰～-22 6‰,不同功能区稳定碳同位素组成的差异反映了PAHs来源的差异.稳定碳同位素组成特征表明,在研究区内,化石燃料燃烧产物的干-湿沉降是土壤PAHs的最主要来源之一,其它可能的来源有污水携带的油污、农作物茎杆及薪柴不完全燃烧产物等.就具体地点而言,土壤PAHs以二元混合输入为主,据此,运用稳定碳同位素组成的二元复合数值模型对不同来源PAHs的相对贡献率进行了估算.

Since polycyclic aromatic hydrocarbons (PAHs) are the main types of organic pollutants in the soil environment, it is imperative to study their distribution patterns and sources in soil from different functional zones in order to assess the soil environment quality and prevent the organic pollution. Take Tianjin for example, the specific compounds of PAHs were previously analyzed. By analyzing the stable carbon isotopic composition of individual PAHs, the probable sources of the PAHs and its contributions in the total pollutants were discussed. The aromatic fraction were extracted from the soils through Soxhlet extractor, then were separated and purified through TLC plate. The Agilent 6890GC-5973MSD gas chromatographic-mass spectrometer was used to detect PAH compounds. Eighty compounds were detected in the soil extracts, mainly consisting of phenanthrenes, anthracene, methylphenanthrene, fluoranthene, pyrene and chrysene. The samples also contained fused heterocyclic aromatics, such as benzo([a])anthracene, benzofluoranthene, benzo([e])pyrene, benzo([a])pyrene, perylene, benzo([g,h,i])fluoranthene and benzo([g,h,i])perylene. Besides that, heterocyclic compounds like methyl dibenzfuran and dibenzothiophen were found in samples from the urban and suburban areas. The stable carbon isotopic composition of PAHs was analyzed with Finnigan MAT DELTA^(plus) XL IRMS, which showed that theδ^(13)C values of phenanthrenes, methylphenanthrene, fluoranthene and pyrene in the soils range from -29.5‰ to -23.2‰, -39.8‰ to -23.4‰, -27.2‰ to -23.6‰ and -28.1‰ to -22.6‰ respectively. The isotopic composition obviously varied with functional zones and it was considered to be related to its sources and the way of formation. Dry and wet depositions of combustion products of fossil fuels might play the most important role on the PAHs inputs. Other sources might involve oil wastewater, and products from the incomplete burning of crop stems or woods. The accumulation of the C-3 vegetation leavings or burned ashes resulted in lighterδ^(13)C values in the samples from the non-sewage-irrigated areas, while the contribution of C-4 vegetation leaded to heavierδ^(13)C values in the samples from the mountain areas. It showed that the PAHs of different origins in the Tianjin were blended in a certain spot, so the δ^(13)C values would be characterized by multi-source contributions due to the re-contribution and fractionation of isotope. Judged from the probable origins, the PAHs in the soil of this area might be accumulative in a typical situation of binary mixed inputs. A universal input was the products of fossil fuels combustion. And the other might be the oil contaminated sewage, or the C-3 or C-4 vegetation. It was possible to make a semi-quantitative analysis of the contributions from different PAHs sources in accordance with the stable carbon isotopic composition of the mixtures. In such areas as Tianjin, whose PAHs are mostly input from two different sources, a simple binary numerical model was used to estimate the relative contributions of individual source to the PAHs in the soil. Tentative calculation of the data from the typical spots in the areas under discussion indicated that fossil fuels combustion contributed as high as 80% of PAHs in the soil from the urban. In the sewage-irrigated area, oil fuel makes up 20%～55% of PAHs in soil. In the non-sewage-irrigated area the C-3 vegetation-burning products reached up to 82%. In the northern mountains, the contribution rate of fossil fuel combustion was 83% while that of C-4 vegetation was 17%.