Absorption And Accumulation Mechanism Of Heavy Metals And The Effect Of Phytoremediation By Neyraudia Reynaudiana

Heavy metal is a kind of accumulative pollutant in the soil which with characteristics such as wide source and has great harm to human and environment.Phytoremediation is a new green method in recent years,it uses some accumulators or hyperaccumulators which can absorb or transfer one or several heavy metals,to stabilize or extract heavy metals in soil to make them less harmful to environment.Phytoremediation,as environmentally benign technology,is attracting more and more attentions and become a hotspot in the field of environmental science.In the previous study of our research group found that N.reynaudiana grew well in Changting mined wasteland,and maintain high biomass.N.reynaudiana becomes one of the dominant species in mining wasteland.The study also shows that,N.reynaudiana have capability in Pb/Cd-enrichment,and it has tolerance to Pb and Cd.N.reynaudiana was able to grow normally in Changting rare earth mine wasteland,maintaining a high biomass.But the researches on N.reynaudiana mainly in terms of soil erosion control,the root of the solid earth and its response to drought stress,etc.,and It is unclear that Neyraudia reynaudiana adaptation strategy on heavy metal stress.So,this paper takes N.reynaudiana as the research object,a culture experiment was conducted to explore the mechanism of absorption and tolerance of Pb and Cd,and the mechanism of accumulation and distribution of Pb and Cd was explained.Furthermore,this paper investigated the role of root cell walls in Pb accumulation,and a pot experiment was conducted to study the effects of EDTA and earthworm on the absorption and accumulation of heavy metals in soils of Longyan Pb-Zn Mining Area.The main results are as follows:(1)Here a solution culture erperiment was conducted to examine uptake and subcellar distribution of Pb and Cd in the plant under different metal levels and stress time.Results showed that Freundlich and Michaelis-Menten equations fit the date very well,R2>0.95.It was also revealed that heavy metals were mainly distributed in the root.N.reynaudiana had greater absorption ability for Pb(the highest content reached to 4687.87 mg·kg-1),but higher absorption rate for Cd.For subcellular distribution,Pb and Cd were mainly distributed in the cell wall and soluble constituents,which with the highest rates.Under different concentration of Pb and Cd,this two compartments accounted for 61.98%～100%of the total;under different stress time,it also accounted for 59.88%～100%of the total.Therefore,N.reynaudiana has stronger absorption ability of Pb,and it could change the subcellular distribution of heavy metals to reduce the toxicity of heavy metals for maintain its steady state.(2)In order to study its response to heavy metal stress,a solution culture experiment was carried out at different Pb and Cd concentrations.After 24 hours,oxidative damage,antioxidant enzyme activity,and non-protein thiol compound content or root tissue were assessed.Under Cd and Pb stress,increases in H2O2 and O2-content were accompanied by MDA content increases.At Pb and Cd concentrations of 100 μmol·L-1,the MDA contents respectively reached 6.72 and 16.12 nmol·g-1,and both lipid peroxidation and oxidative damage had increased.Additionally,enzymatic function was effected,with SOD and POD activity initially increasing with metal concentration and then declining under higher Pb stress.SOD activity under Cd stress was similar to that under Pb stress,but POD activity was lower.The non-protein thiol content of roots showed that glutathione and phytochelatins initially increased and then decreased with Cd and Pb concentrations.Comprehensive analysis thus revealed that these two heavy metals at different concentrations have different effects on N.reynaudiana root tissue.Overall,N.reynaudiana can adjust its antioxidant enzyme activity and non-protein thiol synthesis levels in order to reduce heavy metal toxicity and damage.(3)The root cell wall of N.reynaudiana was found to be the most important part of heavy metal localization.In this section,Chemical modification of cell wall,adsorption dynamic experiments and the FTIR spectroscopy analysis were combined to explore adsorption kinetic characteristics and the functional groups of the root cell wall and its components under Pb stress.The results of adsorption dynamic experiments showed that,the amount of Pb absorbed by the root cell wall varied after modified treatments,compared to the unmodified,reducing by 68.1%after esterifying,41.1%via aminomethylation and 48.9%with pectinase.Meanwhile by using Fourier transform infared spectrum,information was characterized about the functional groups of Pb adsorption.The results showed that,cellulose and hemicelluloses,pectin and proteins provide a combination of Pb2+with carboxyl functional groups,carboxyl groups and amino functional groups for binding sites.We come to a conclusion that,root cell wall has high adsorption capacity for Pb,which may be an important mechanism of the high Pb tolerance by N.reynaudiana.(4)High bioavailability of heavy metals in the soil is conducive to improve efficiency of phytoremediation.In order to evaluate the remediation effect and health risks of EDTA-earthworm by N.reynaudiana in lead-zinc mining area,in this part,the effects of the application of EDTA and Earthworm on heavy metals in soil were studied by pot experiment.The results showed that the content of heavy metals in N.reynaudiana increased significantly under the treatment of EDTA-earthworm group.The enriching effects of Pb increased gradually with the concentration of EDTA(combined with earthworm),while low concentration of EDTA(combined with earthworm)can enhance the absorption of Cd and Zn.EDTA can activate heavy metals in the soil,and promote Pb enrichment in earthworm,so it has a major environmental risk to human health.Therefore,the concentration of EDTA should be strictly controlled to reduce the ecological risk.