Research On The Characteristics And Mechanisms Of Medicago Sativa L. In The Phytoremediation Of Cd And Pyrene Co-Containmnated Soil

Pot experiment was conducted to investigate the phytoremediation of pyrene and cadmium(Cd), as the model pollutants of polycyclic aromatic hydrocarbons and heavy metals in soil byMedicago sativa L.. The strengthened effect and mechanism of different exterior microorganisms(Arthrobacter oxydans, Staphylococcus auricularis and Stenotrophomonas maltophilia) werestudied in this paper. The research aimed to provide appropriate data and technological supporton phytoremediation of heavy metals-organic matters combined pollution in soil.The uptake and accumulation properties of Cd and pyrene in M. sativa were demonstratedand accumulation in shoot of alfalfa was significantly lower than in root. The content anddistribution of Cd in plant cells were in a sequence of cell wall> cell soluble mediators> cellmembrane and organelle. The main chemical forms of Cd were sodium chloride extracted form,acetic acid extracted form and water extracted form. Cd and pyrene single/combined pollutionsboth had inhibiting capacity to alfalfa growth. Meanwhile, enzyme system and antioxidationmatters of alfalfa were stimulated to produce amount of peroxidase, polyphenol oxidase andcatalase, in order to clean up reactive oxygen in alfalfa and protect itself.The combined bioremediation of M. sativa and microorganisms showed that A. oxydans, S.auricularis and S. maltophilia could significantly promote the alfalfa to absorb and removepyrene and Cd. Furthermore, three strains remarkably enhanced the concentration of EXC Cd inrhizosphere and non-rhizosphere soil which could promote Cd uptake by alfalfa. The studyindicated that combined effect of plant-microbes was the main influential factor in pyrenebiodegration. The main reasons behind the phenomenon may include that (1) combined effect ofmicrobe-alfalfa could increase the activities of polyphenol oxidase and dehydrogenase in soil;(2)alfalfa increased species of common microbes in soil, aggrandized Cd-pyrene tolerant bacteriaactivities and enhanced the regularity of community;(3) three microorganisms could significantlypromote the alfalfa to remove pyrene and Cd from soil. BIOLOGECO experiments revealed thatmicrobial community diversity, activity and diversity index in pyrene-Cd contaminated soilincreased remarkably by alfalfa planting and adding exogenous microbes. Principal componentanalysis demonstrated that under different treatments there were differences between the typeand ability of carbon source utilization for microbes in soil. The cluster analysis result couldclassify each treatment reasonably according to removal efficiency of pyrene and the conversionof available Cd in soil.Research on ions metabolism and release exhibited that the concentrations of cationicmetals Na⁺,K⁺,Mg²⁺ and Ca²⁺ in aqueous solution increased to a certain extent over time, exceptfor NH4+under Cd-pyrene single/combined contamination. Kinetic studies illuminated that the release of Na⁺,K⁺,Mg²⁺, NH⁴⁺ and Ca²⁺ by alfalfa best fitted pseudo-second-order equation.Compared with the control, the pH gradual decreased along with the pollutant concentrationincreased. The ternary linear equation results demonstrated that the content of Cd²⁺ absorption byalfalfa was closely related with the release of Ca²⁺,Mg²⁺ and Na⁺. And this exchange was mainlyoccurred among Cd²⁺ and divalent cations. Pyrene removel was strongly linked to concentrationchange of free Ca²⁺ in alfalfa, which was an effect of antioxidative activity and physiologicalaccommodation.The study also proposed the main degradation processes of pyrene were initially attacked bymultiple monooxygenases and dioxygenases. The structure of pyrene changed and decomposedinto some organic compands with1-2phenyl rings, like aromatic acid, aromatic ketone,aromatic aldehyde and saturated hydrocarbon. The pathways of salicylic acid and phthalic acidwere considered to be the major routes of bicyclic aromatic compounds degradation afterconverting to phenanthrene. Under Cd-pyrene combined pollution, pathway of salicylic acid wasonly determined and Cd had inhibition effects on phthalic acid pathway.