Effect Of Carbon Nanotubes On The Rhizoremediation Of PAH-Contaminated Soil With Medicago Sativa And Its Preliminary Mechanisms

Polycyclic aromatic hydrocarbons（PAHs）are a kind of persistent organic pollutants commonly found in the environment,which pose a great threat to the ecosystem and human health due to their"mutagenic,teratogenic and carcinogenic"effects.Therefore,the remediation of PAHs contaminated soil has become one of the most important environmental science problems in China.Rhizosphere remediation is a green economic remediation strategy for PAHs contaminated soil.However,there are still some problems such as low efficiency and long repair cycle.Therefore,the research of effective strengthening measures to improve the rhizosphere remediation rate may become an important way to expand the application of rhizosphere remediation.In this study,the indoor pot experiment was conducted to analyze the degradation of PAHs and the response of microbial communities in rhizosphere soil by planting alfalfa（Medicago sativa）and adding different types（multi-walled carbon nanotubes:MWCNTs;single-walled carbon nanotubes:SWCNTs）and contents of carbon nanotubes（CNTs）,with highly PAHs-contaminated soil as the tested soil.The regulation effect of MWCNTs on root exudates of alfalfa was analyzed through sand culture experiment,and the absorption of PAHs in alfalfa was investigated.The effects of MWCNTs on the degradation of PAHs in soil were investigated by microcosmic culture experiment.The research results are as follows:（1）The effect of carbon nanotubes on the removal of PAHs in alfalfa rhizosphere soil was related to its species.The addition of MWCNTs significantly promoted the removal of PAHs in the soil,and the removal rate significantly increased by 35.23%（p<0.05）when the addition amount was 1 g kg^-1.SWCNTs had a certain inhibitory effect on the removal of PAHs in soil,the addition of 0.5 and 5 g kg^-1 SWCNTs significantly reduced the removal rate of PAHs by 3.43%and 6.98%（p<0.05）,respectively.The inhibition effect of SWCNTs on PAHs degradation was mainly derived from 5-6 ring high-molecular-weight PAHs,and the inhibition effect was enhanced with the increase of SWCNTs supplemental level.CNTs（SWCNTs and MWCNTs）had non-toxic effect on alfalfa growth.The results of high-throughput sequencing showed that richness and diversity of soil bacteria had no significant influence after adding CNTs,but significant changes in the soil bacterial community composition.MWCNTs significantly increased the relative abundance of potential PAHs degrading bacteria in rhizosphere soils,e.g.Lysobacter,Sphingomonas,Massilia,Nocardioides,Streptmyces and Haliangium,etc.However,the effect of MWCNTs on PAHs degradation function microorganisms was related to the amount of MWCNTs added.In addition,the higher addition level of SWCNTs(5 g kg^-1)significantly reduced the relative abundance of potential PAHs-degrading bacteria in contaminated soil,including Phenylobacterium,Reyranella,Brevundimonas and Pseudorhodoferax.The results of fluorescence quantitative PCR showed that the addition of 0.1 and 1 g kg^-1 MWCNTs significantly increased the expression levels of GP and GN genes of the PAH-ring hydroxylating dioxygenaseαsubunit（PAH-RHDα）in rhizosphere soil.Spearman correlation analysis further showed that the removal of PAHs in rhizosphere soil was directly related to the potential degradation of PAHs in soil,the contents of nitrogen（N）,phosphorus（P）and potassium（K）,and the soil p H.（2）The results of sand culture experiment showed that the addition of certain concentration of MWCNTs promoted the root growth of alfalfa.In the no PYR additional treatment group,50 mg kg^-1 MWCNTs make root length and root fresh weight of alfalfa increased significantly by 91.92%and 70.73%（p<0.05）,respectively.Meanwhile,adding25 mg kg^-1 MWCNTs significantly increased plant root length by 51.28%（P<0.05）in the PYR additional treatment group.Therefore,the addition of MWCNTs promoted accumulation of pyrene in plant.Metabonomics analysis showed that the addition of MWCNTs significantly affected the root exudates of alfalfa,and the acid substances had the greatest change.In addition,the addition of MWCNTs affected the metabolic pathway.When the supplemental level of MWCNTs was 25 mg kg^-1,the main metabolic pathway affected was the carbon fixation in photosynthetic organismis.When the supplemental level of MWCNTs was 50 mg kg^-1,the main metabolic pathways affected were the citrate cycle（TCA cycle）,amino acid and organic acid metabolism pathways.（3）Under the sterilization condition,when adding 0.05 and 1 g kg^-1 MWCNTs,the removal rates of PAHs in soil were 34.32%and 9.40%,respectively.Under the non-sterilization condition,the removal rates were 51.00%and 47.73%,respectively,which were significantly increased by 15.84%and 12.57%（p<0.05）compared with the treatment without MWCNTs（35.16%）.The results showed that the main reason for the removal of PAHs in soil was the degradation of microorganisms.High throughput sequencing results showed that when the MWCNTs content is 0.05 g kg^-1 has no significant effect on eitherαorβdiversity,but promotes the relative abundance of potential PAHs degrading microorganisms,e.g.uncultured and Sphingobium at the phylum and genera levels.When MWCNTs are added at 1 g kg^-1,they significantly changed the bacterial richness of the soil and promoted the relative abundance of PAHs potentially degrading microorganisms,e.g.Chlorocurbillus,Firmicutes and uncultured.