Cadmium Accumulation Of Helianthus Annuus L. In Contaminated Agricultural Soil And Cadmium Removal From Its Straw By Microbial Leaching

Helianthus annuus L.（oil sunflower）is considered as an economic phytoremediation crop because of its ability to phytoextract cadmium（Cd）from soil with safety standards for its edible parts.Yet,as the last step of phytoremediation,how to safely dispose of accumulators’biomass is considered to be an important factor to measure whether phytoremediation is really successful.In this study,oil sunflower was considered as the research object to explore the effects of endophytic bacteria and rhizosphere microecology of oil sunflower on the uptake and accumulate Cd in Cd-contaminated farmland.Endophytic bacteria were screened from oil sunflower and Aspergillus niger was selected from exogenous bacteria bank.The Cd removal and leaching efficiencies from oil sunflower straw by endophytic bacteria and Aspergillus niger were investigated,and the removal and leaching process was optimized to enhance the Cd removal and leaching efficiencies.The main results of this thesis are as follows:（1）The combined effects of tartaric acid secreted by the root of oil sunflower and the unique endophyte and rhizosphere bacterial phyla enhanced the uptake and enrichment of cadmium in oil sunflower.The tartaric acid and oxalic acid secreted from the root of oil sunflower can increase the average relative abundance of Proteobacteria,Firmicutes and Planctomycetes in the oil sunflower rhizosphere.The unique endophytic bacteria and abundant rhizosphere bacteria could enhance the Cd absorption by oil sunflower crops;and the low toxic pectin-and protein-bound cadmium,insoluble cadmium phosphate and cadmium oxalate were reserved in the above-ground of oil sunflower.The bioconcentration factors of leaves and stems were 10.89 and 1.16 respectively.Thus,oil sunflower showed a good capacity to remediate Cd-contaminated agricultural soil.（2）Endophytic bacteria screened from oil sunflower leaves could remove and leaching cadmium from oil sunflower straw to a certain extent.In this study,two endophytic bacteria,Enterobacter sp.（J14）and Kosakonia cowanii（J15）screened from oil sunflower leaves,which at the initial p H value at 6,liquid volume at 75 m L,and temperature at 25℃,respectively,for Enterobacter sp.（J14）,and the initial p H value at 4,liquid volume at 75 m L,and temperature at 25℃,respectively,for Kosakonia cowanii（J15）,could reached considerable Cd removal and leaching efficiencies.The Cd removal rate from oil sunflower straw by endophytic bacteria.However,the removal and leaching effect of endophytic bacteria on cadmium in oil sunflower straw only reach 46%,so the Cd leaching efficiency just by endophytic bacteria is ineffective.（3）Aspergillus niger can improve the Cd removal efficiency from oil sunflower straw by producing citric acid.The single condition of p H value at 3,sucrose content at 100 g L^-1and inoculation size at 10%for Aspergillus niger could realize a considerable removal and leaching efficiencies of Cd from oil sunflower,and the highest Cd removal efficiency by Aspergillus niger could reach 53.73%.The chemical speciation of Cd in oil sunflower straw before and after bioleaching showed that bioleaching is mainly resulted from the citric acid secreted from microorganisms to dissolve the insoluble cadmium phosphate and cadmium oxalate in the oil sunflower straws.However,compared with endophytic bacteria,the maximum removal rate of cadmium from oil sunflower straw by Aspergillus niger increased by 16.80%,but it was still difficult to achieve the ideal effect by using Aspergillus niger alone.（4）Based on the results of single factor test,the conditions for leaching and removal of Cd from oil sunflower straw by endophyte and Aspergillus niger were optimized by response surface methodology.In the condition of initial p H value at 7,liquid volume at 59.52 m L and temperature at 25℃,J14 can remove 46.29%of Cd from oil sunflower straw;in the condition of initial p H value at 5,liquid volume at 75 m L and temperature at 25℃,J15 can remove 43.26%of Cd from oil sunflower straw.Aspergillus niger can remove 68.2%of Cd from oil sunflower straw in the condition of the initial p H value at 3,the sucrose content at76.27 g L^-1and the inoculation amount at 10%.The Cd concentration in the residue of oil sunflower straw obtained from the optimized bioleaching and different pretreatments were all below the standard of 3 mg kg^-1in Requirements for Limits of Toxic and Harmful Substances in Fertilizers of China（GB 38400-2019）,which can be reused as fertilizer raw materials.（5）Under the optimal response surface condition,pretreatment of oil sunflower straw significantly improved the Cd leaching efficiency of Aspergillus niger.Among these processes,the mechanical comminution-high pressure water pyrolysis-Aspergillus niger extraction process had low cost and short flow,and can remove 80.09%of Cd from oil sunflower straw.The Cd concentration in the residue of oil sunflower straw is lower than the standard of 1 mg kg^-1in the Feed Safety Standard of China（GB13078-2017）,and the loss of phosphorus and other nutrients after this process is also less.The residue of oil sunflower straw can be reused as feed material.This thesis is a preliminary study on microbial extraction of heavy metals from contaminated plant biomass,which provide a theoretical basis for the safe utilization of biomass after phytoremediation,and truly realize environmentally friendly,economic and sustainable development purposes.