Effects Of Submerged Aquatic Plant Species On Remediation Of Polycyclic Aromatic Hydrocarbons In Sediments

Surface sediment is an important reservoir of polycyclic aromatic hydrocarbons(PAHs).As a result of the material exchange with the overlying water body,the PAHs in sediments not only pose a threat to benthic organisms and fish,but also can be detrimental to human beings through the food chain.Therefore,carring out the research of PAHs remediation in sediment is of great significance in theory and practice.The aim of this study was to evaluate the role of rhizosphere effect on PAHs polluted sediments by planting four submerged aquatic plants(Hydrilla verticillata,Myriophyllum verticillatum,Vallisneria spiralis,and Potamogeton crispus L.)in laboratory and field.The remediation mechanisms of PAHs were discussed based on the physical-chemical properties of the rhizosphere and microbial community.The main conclusions are as follows:Ability of four submerged aquatic plants to facilitate the remediation of sediments polluted with phenanthrene and pyrene was compared.During a 54-day experiment,all of the four plants grew well.Dissipation of PAHs was in the order of V.spiralis(85.2% and 82.4%)> M.verticillatum(63.4% and 50.0%)> H.verticillata(57.1% and 52.4%)and P.crispus(48.0% and 45.7%)> unplanted(33.9% and 44.5%)for phenanthrene and pyrene.In this study,mass balance calculation showed that plant uptake was weak and plant-promoted biodegradation was the dominant contribution.The result also showed that both butanol-extractable and butanol-unextractable amounts of PAHs decreased during the experiment,but the butanol-extractable amounts of PAHs decreased more significantly.Therefore,it can be concluded that the dissipation of butanol-extractable amounts plays the key role in the degradation of PAHs in sediments.The numbers of PAH-degrading bacteria and PCR-DGGE analysis in sediments were counted.In this study,there was no significant correlation(p>0.05)between PAH-degrading bacteria population and the dissipation ratio of PAHs,suggesting that PAH-degrading bacteria population was not the limited factor for the dissipation of PAHs in sediments.PCR-DGGE results showed that the bacteria community of V.spiralis was identified as the most efficient for phytoremediation of PAH-contaminated sediments,therefore it might be concluded that for the DGGE prifile of other three plants which was more similar to that of V.spiralis got a higher dissipation ratio than other two plants.The physical-chemical properties in the rhizosphere and dissolved organic matter of pore water in sediment were determined.The result showed that there were significantly positive correlations between PAHs dissipation ratios and sediment redox potentials.Therefore,root oxygenation capability can be used as an important parameter to measure the potential of the remediation by submerged plants.By the analysis of the total organic acids amounts of pore water and butanol-unextractable amounts of PAHs,it was found that the enhanced desorption of PAHs by root exudates was weak,and plant-promoted biodegradation might be the dominant factor for the dissipation of butanol-unextractable PAHs.In the field environment,dissipation of PAHs by H.verticillata and V.spiralis was analyzed.During the experiment(3.5 months),all of the two plant species grew normally and external signs of phytotoxicity were not observed.At the end,planting V.spiralis enhanced the dissipation ratios of phenanthrene and pyrene by 21.1% and 23.3%,which were higher than H.verticillata(11.5% and 8.8%).The importance of oxygen released by submerged aquatic plants in improving the dissipation of the PAHs in sediments was proved once again.