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Background, Aim and Scope:
Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental contaminants and contribute to the pollution of soil environment. Soil ingestion is of increasing concern for assessing health risk from PAHs in soils because soil ingestion is one of the potentially important pathways of exposure to environmental pollutants, particularly relevant for children playing at contaminated sites due to their hand-to-mouth activities. In vitro gastro-intestinal tests imitate the human digestive tract, based on the physiology of humans, generally more simple, less time-consuming, and especially more reproducible than animal tests. This study was conducted to investigate the level of PAH contamination and oral bioaccessiblity in surface soils, using physiologically based in vitro gastro-intestinal tests regarding both gastric and small intestinal conditions.
Materials and Methods:
Wastewater-irrigated soils were sampled from the metropolitan areas of Beijing and Tianjin, China, which were highly contaminated with PAHs. Clean (reference) soils samples were also collected for comparisons. At each site, four soils were sampled in the upper horizon at the depth of 0-20 cm randomly and were bulked together to form one composite sample. PAH concentrations and origin were investigated and physiologically based in vitro test was conducted using all analytical grade reagents. The relationship between total PAH concentrations in soils and soil organic carbon (SOC), was assessed.
Results:
A wide range of total PAH concentrations ranging from 1303.63 µg kg-1 to 3368.61 µg kg-1 in soils collected from different sewage irrigated sites in Tianjin, while ranging from 2686.98 µg kg-1 to 4915.77 µg kg-1 in soils collected from different sewage irrigated sites in Beijing, were detected. In general, total PAH concentrations in soils from Beijing sites were significantly higher than that from Tianjin sites, indicating a dominant contribution from both pyrogenic and petrogenic sources. Results indicated that the oral bioaccessibility of PAHs in small intestinal condition was significantly higher (from P<0.05 to P<0.001) than gastric condition. Similarly, the oral bioaccessibility of PAHs in contaminated sites were significantly higher (from P?0.05 to P<0.001) than reference sites. Individual PAH ratios (3-6 rings), a more accurate and reliable estimation about the emission sources, were used to distinguish the natural and anthropogenic PAH inputs in soils. Results indicated that PAHs were both pyrogenic and petrogenic in nature.
Discussion:
The identification of PAH sources and importance of in vitro test for PAH bioaccessibility were emphasized in this study. The oral bioaccessibility of individual PAHs in soils generally decreased with increasing ring number of PAHs in both gastric and small intestinal conditions. However, the ratio of bioaccessibility of individual PAHs in gastric condition to that in small intestinal condition, generally increased with increasing ring number, indicating the relatively pronounced effect of bile extract on improving bioaccessibility of PAHs with relatively high ring numbers characterized by their high Kow values. Similarly, total PAH concentrations in soils were strongly correlated with SOC, indicating that SOC was the key factor determining the retention of PAHs in soils.
Conclusions:
Soils from both Tianjin and Beijing, China, were contaminated with PAHs due to long term wastewater irrigation. PAHs with 2-6 rings showed high concentrations with a significant increase over reference soils. Based on the molecular indices, it was suggested that PAHs in soils had both pyrogenic and petrogenic sources. It was also concluded that the oral bioaccessibility of total PAHs in the small intestinal condition was significantly higher than that in the gastric condition. Furthermore, the bioaccessibility of individual PAHs in soils generally decreased with the increasing ring number in both gastric and small intestinal conditions.
Recommendations and
Perspectives:
It is suggested that more care should be given while establishing reliable soil criteria for PAHs, especially concerning the health of children who may ingest a considerable amount of PAH-contaminated soil via outdoor hand-to-mouth activities.
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