| Polycyclic aromatic hydrocarbons（PAHs）are a group of persistent organic pollutants that are widely found in the environment,they are of wide concern because they are difficult to degrade in the environment and are mostly carcinogenic,teratogenic and mutagenic.Therefore,sediments are considered to be one of the main environmental destinations for PAHs.The increasingly serious situation of PAHs contamination in sediments,which poses a threat to human health.Therefore,effective control and remediation of PAHs pollution in sediments of the lake estuary is particularly important and urgent to improve the water quality of the lake and its ecosystem.Magnetic loofah sponge biochar（Mag Ls BC）,a novel magnetic carbonaceous material,can be used to remediate PAHs-contaminated sediments by virtue of its rich macroporous structure,high biocompatibility and extracellular electron transfer capability.Therefore,the study of the long-term remediation effect of Mag Ls BC on PAHs-contaminated sediments and the vertical remediation characteristics of PAHs in sediments will provide theoretical support and technical guidance for the in situ remediation of PAHs-contaminated sediments with Mag Ls BC.In this study,four groups of devices T1 to T4 were constructed under laboratory conditions,corresponding to Con（the control group）,Mag Ox（the magnetic iron oxide particles group）,Mag Co AC（magnetic coconut shell activated carbon）and Mag Ls BC（the magnetic loofah sponge biochar）.After 900 days of long-term operation of the device,the changes of PAHs content and physicochemical indicators such as electron acceptors were examined separately to study the efficiency of magnetic carbonaceous materials in remediating PAHs contamination in sediments.The microbial community composition and structural changes in different treatment groups were also characterised to investigate the microbial mechanisms of action.The Con,Mag Co AC and Mag Ls BC groups were set up for in situ remediation in a field environment to study the remediation effect of Mag Ls BC on PAHs-contaminated sediments,and a sediment microbial fuel cell（SMFC）was constructed to study the effect of SMFC in further enhancing the removal of PAHs.The main findings are as follows.（1）After long-term remediation,the addition of magnetic carbonaceous material significantly enhanced the biodegradation of PAHs in both the upper and lower sediments;PAHs were significantly degraded in the Mag Co AC and Mag Ls BC treatment groups,with removal rates of 58.56%and 64.69%.In addition,the bioremediation of 6-ring PAHs in the sediment was more significant in Mag Ls BC.The vertical variation of PAHs after long-term remediation showed that Mag Ls BC had significant and efficient PAHs remediation efficiency in both the upper and lower layers of sediments,with 59.59%and 69.78%removal efficiency respectively,especially in the lower anoxic sediment PAHs biodegradation was more prominent.In the process of PAHs enrichment by PE membranes,the Mag Ls BC group could significantly reduce the bioavailability of PAHs,especially in high-ring PAHs.The microbial metabolic pathway of the Mag Ls BC group indicated that the degradation of Pyrene（PAH）was completed through carboxylation or methylation processes.（2）The remediation effect of PAHs in different treatment groups was correlated with the changes of physicochemical indicators such as electron acceptors(NO3-,Fe3+,SO42-)and electron donors(TOC,Fe2+)in the sediment;the Mag Ls BC treatment group was able to utilize nitrate in the sediment interstitial water and promote microbial metabolic activity under nitrate reduction conditions;the remediation efficiency of PAHs was correlated with nitrate and conductivity showed a significant positive correlation,and the Mag Ls BC treatment group enhanced degradation of PAHs could be achieved by increasing nitrate reducing microbial activity and increasing conductivity.Furthermore,compared to dispersed magnetic particles,the three-dimensional porous mesh structure of Mag Ls BC was able to retain the magnetic carbonaceous material in the target area more permanently during the remediation of contaminated sediments,thus enabling the targeting and biodegradation of contaminants.（3）The distribution of microbial community characteristics showed that Proteobacteria,Nitrospirae,Chloroflexi and Bacteroidetes dominated the microbial composition of each group,and the Mag Ls BC group was more enriched in Chloroflexi phylum bacteria and altered the microbial community abundance of the Euryarchaeota phylum,with the lower sediment containing more dominant bacteria than the upper.Meanwhile,the dominant genera in the Mag Ox and Mag Ls BC groups were broadly similar in the upper and lower sediments,with the emergence of genera capable of degrading hydrocarbons such as Magnetococcus and Candidatus＿Methanomethylicus in the Mag Ls BC group,while the Sva0081＿sediment group and Ignavibacterium became the dominant genera in the Mag Ox group.Based on the analysis of microbial diversity in the sediment,the Mag Ls BC and Mag Ox treatment groups had the lowest species abundance,with the lower layer having a lower number of species and a lower community richness than the upper layer.（4）Differences in sediment microbial communities indicated that the Con and Mag Co AC groups were more similar in species composition structure,and the Mag Ls BC group differed significantly from the other three groups in terms of microbial communities.Analysis of significantly different species revealed that the bacteria FS117＿23B＿02 spp.and the archaeon Thermoplasmata spp.were widespread in the Mag Ls BC group,with the Marine＿Benthic＿Group＿D＿and＿DHVEG＿1 spp.being the most represented in the lower layer of Mag Ls BC treatments.Microbial communities in the Mag Ls BC group were significantly different from those in the other treatment groups by conductivity and electron donor(TOC,Fe2+).While those in the Mag Ox group were positively correlated with the electron acceptor SO42-,indicating that magnetite addition could enhance electron transfer activity under sulphate reducing conditions.（5）The results of in situ tests found that the magnetic carbonaceous materials（Mag Co AC and Mag Ls BC）exerted degradation effects on PAHs,but did not show significant differences in monocyclic PAHs,monomeric PAH,and only lower layer of Mag Ls BC groups had significant effects on Bghi P（6-ring PAHs）.In comparison,the treatment effect was more prominent in the upper layer of Con group.Despite the high-power density（73.49 m W/m2）and low-internal resistance（100.31Ω）of the Mag Co AC-constructed sediment microbial fuel cell（SMFC）,however,the Mag Co AC-treated group and the blank control group had difficulties in locking in PAHs contaminants for a long period of time and could not effectively maintain the PAHs remediation effect.The SMFC constructed in Mag Ls BC further enhanced the degradation of PAHs in the sediment,with the total PAHs reduced from 136.40 mg/kg to 100.53 mg/kg;the levels of 4-ring PAHs and 5-ring PAHs（high-ring PAHs）were also significantly reduced,with removal rates of 25.27%and 27.28%,respectively,and a relatively stable voltage The signal output was relatively stable and can be used to monitor the biodegradation process. |
