Study On In Situ Remediation Of Polycyclic Aromatic Hydrocarbons In Surface Sediments

It is well know of the recalcitrant structural features of polycyclic aromatic hydrocarbons (PAHs) and special environmental properties of surface sediments. This study analyized the pollution level, composition and spatial distribution of PAHs in surface sediments in China. Besides, the most economical, environmentally friendly, and efficient phytoremediation and biochar repairments were also further researched. The main conclusions are as follows:The mean and median values of ?PAHs in surface sediments in our country were 789.1 ng/g and 477.0 ng/g. This indicates that pollution level of PAHs in our country was medium. Content of high molecular weight PAHs accounted for 61.6% of the total PAHs. The median levels of ?PAHs in sediments of lakes and rivers were significantly higher than those of offshore areas. And the median levels of EPAHs in sediments of rivers and lakes were higher in western and northern regions but lower in southern and eastern regions. In sediments of offshore areas, the median levels of ?PAHs in sediments were higher in Yellow Sea and similar among the other sea areas. Climate, energy and industrial structure, hydrodynamic and so on contributed to the difference.A 54-day-long study was conducted to investigate the effect of plant density of Vallisneria spiralis on the remediation of sediments contaminated by polycyclic aromatic hydrocarbons (PAHs). Dissipation ratios of phenanthrene and pyrene in sediments were initially the highest in treatment. However, after a 54-day incubation, no statistical difference was observed in the dissipation ratios under different planting density treatments. Compared with the unplanted sediments, the dissipation ratios of phenanthrene and pyrene in planted sediments increased by 15.2-21.5% and 9.1-12.7%. Considering the sustainability of the ecosystem, lower plant density should be a better selection for phytoremediation of PAHs. Furthermore, dissipation ratio of PAHs was positively related to PAH-degrading bacterial population, suggesting that microbial degradation played a major role in the V. spiralis-promoted remediation. Sediment redox potential positively correlated with PAH-degrading bacterial population. It can be concluded that the enhanced dissipation of PAHs is mainly related to oxygen released by roots.Wheat straw was made into biochar at different temperatures, and it was found that with the temperature increasing, biomass were further carbonized, content of H and N decreased, and ash rised. Since the disappearance of volatile organic components and the formation of secondary micropores, biochar made by high-temperature had significantly higher specific surface area. The elimination of the soluble substance by acid pickling reduced about half of specific surface area of biochar. The pores of biochar modified by ferric chloride were pluged by iron compound, which diminished some surface area. Adding biochar to sediments reduced the degradation of PAHs and proportion of bioavailable PAHs. These effects were especially obvious with biochar made by high temperature. The surface area of biochar and dissipation rate of phenanthrene were positively correlated. Morover, the surface area of biochar also positively correlated with the proportion of bioavailable PAHs. These results showed that the PAHs can employ the large surface area to be bound-form. In addition, the high-temperature biochar could promote the degradation of PAHs in sediments, which may be due to biochar can provide breeding space and nutrients for microbial growth so as to increase the degradation of PAHs.After adding biochar into sediments, Vallisneria spiralis grew better at early stage, but not as good as the control group. This may be due to the availability of nutrients was inhibited. The use of biochar plants reduced the absorption of pollutants to plants, which may be also related to the bioavailability of contaminants is inhibited. Joint application of biochar and plant did not show synergistic degradation of PAHs, which may be due to the strong combined effect of PAHs and biochar suppressed release for organic contaminants.