LED3A Enhanced Phytoremediation Of Nickel-Pyrene Co-Contaminated Soil By Alfalfa

In recent decades,heavy metal and polycyclic aromatic hydrocarbons（PAHs）contaminated soil has been rapidly aggravated in China,so it is urgent to develop green and economical soil remediation technologies.The combined chemical-phytoremediation technology can enhance phytoextraction or rhizospheric degradation to remove heavy metal-PAHs complex pollutants from soils through chelation-induced or surfactant-enhanced pathways.Therefore,in this paper,a novel chelating surfactant named N-lauroyl ethylenediamine triacetate（LED3A）was selected as the strengthening agent in combination with alfalfa for remediation of nickel-pyrene co-contaminated soil.The growth of alfalfa,the subcellular structure and morphological distribution of nickel in the plant and the performance of LED3A-enhanced phytoremediation of nickel-pyrene co-contaminated soil were investigated by pot experiments at different LED3A application concentrations(0,300,600and 1200 mg·kg^-1),application methods（1 and 3 times）and duration of action（4,8 and 12 d）.Meanwhile,we analyzed the morphological transformation of nickel and the degradation pathway of pyrene in soil,and the changes of soil enzyme activities and microbial community structure under the action of LED3A,so that explore the effect and mechanism of LED3A in enhancing the remediation of nickel-pyrene contaminated soil by alfalfa.Key findings were as follows:（1）Alfalfa grew well in the treatment with/without LED3A during LED3A potentiation,and the biomass gradually increased over time.In the early（4 d）and mid-term（8 d）of LED3A application,the fresh and dry weights of alfalfa were slightly lower than those of the no-LED3A treatment,and the photosynthetic pigment content of shoots decreased and the accumulation of malondialdehyde（MDA）increased in alfalfa at higher concentrations(>300mg·kg^-1)of LED3A.LED3A significantly increased the activities of polyphenol oxidase（PPO）and superoxide dismutase（SOD）,and the activities of PPO and SOD under three application conditions were higher than those in one application at the same concentration.And the synergistic effect of both peroxidase（POD）and catalase（CAT）made the plants more resistant,and there was no significant difference in the biomass and photosynthetic pigment content of alfalfa at 12 days compared with the treatment without LED3A.（2）LED3A significantly increased the uptake and accumulation of pyrene and nickel in alfalfa,and the enhanced phytoextraction effect showed a continuous increase followed by a slight decrease with the increase of LED3A concentration.The synergistic effect of three applications was better than one application at the same concentration.The 3×200 mg·kg^-1LED3A was the best treatment for the uptake of pyrene and nickel,and the accumulation of pyrene and nickel was 121.2μg·pot^-1 and 2540μg·pot^-1,respectively,which were 364.4%and56.8%higher than those without LED3A treatment after 12 days.LED3A-enhanced phytoextraction is the main pathway for nickel removal from the soil,while the dissipation of pyrene relies mainly on the interplant root degradation effect.The residual concentration of pyrene was significantly reduced by 50.4%～60.6%from 4 days to 12 days compared with the treatment without LED3A,and the removal rate of pyrene in soil with different LED3A treatments reached 95.3%-96.3%at 12 days.Based on the analysis of pyrene degradation products in soil,it was speculated that pyrene was gradually converted into low or non-toxic small molecules through a continuous oxidative ring-opening mineralization process,and decomposed into H₂O and CO₂ by microbial tricarboxylic acid cycle（TCA）.（3）LED3A significantly improved nickel accumulation and transport in alfalfa,and the plants showed super-enrichment of nickel（transport coefficient TF>1）in the middle（8 days）and last stage（12 days）,and the extraction efficiency of nickel increased by 18.9%～69.0%compared with the no-LED3A treatment.LED3A promoted the transformation of nickel from stable fugitive forms（oxidizable+residual forms）to bioavailable forms（exchangeable+reducible forms）in soil.Consequently,the uptake of nickel by alfalfa roots increased obviously.The uptake of nickel by alfalfa roots increased significantly.The nickel was mainly distributed in the plant cell wall,while the application of LED3A weakened the sorption of nickel in the root cell wall,and the nickel was converted to the highly mobile forms（ethanol extracted nickel+distilled water extracted nickel+sodium chloride extracted nickel）,which promoted the transfer of nickel from the roots to the shoots.Due to the effect of LED3A,the proportion of nickel transferred to the shoots increased in the cell wall,and the low active form of nickel（hydrochloric acid extracted nickel+acetic acid extracted nickel+residual nickel）increased,which alleviated the phytotoxicity of nickel.（4）LED3A significantly increased soil dehydrogenase（DHO）and microbial activity but did not affect urease（URE）activity.LED3A increased the relative abundance of the dominant phylum Proteobacteria and the dominant genus Pseudomonas in soil by7.5%～104.7%and 177.3%～894.2%,respectively.Both the dominant phylum and genus were highly efficient pyrene-degrading bacteria with good tolerance to heavy metals.Three times LED3A application was more conducive to activating soil enzyme and microbial activities as well as increasing the relative abundance of the most dominant bacteria than one application.