Mechanisms Of Lead Contaminated Soil Remediation By Functionalized Bacteria Agentand Alleviation Of Lead Stress On Cucumber Seedlings

Soil is an essential material basis for agricultural production activities and also an important part of the ecological environment.However,anthropogenic activities have led to serious lead（Pb）pollution in agricultural soils with Pb content far exceeding its own self-purification capacity.Phosphate-solubilizing bacteria（PSB）,which secreted organic acids and phosphatases to dissolve insoluble phosphate with Pb to form stable crystalline compoun ds（Pb₅（PO₄）₃OH）,have attracted extensive attention.However,higher Pb concentration inhibited the activity of PSB and is not conducive to the passivation of heavy metals in soil.Therefore,PSB has poor activity and low remediation efficiency under the condition of serious Pb pollution.In this study,PSB was combined with functional iron-based nanomaterials for remediation of Pb contaminated s oil,as well as the ability and mechanism of its to alleviate Pb stress on cucumber were discussed.In this study,the functionalized bacteria agent（CFB₁-P）was prepared by using bone char-supported carboxymethyl cellulose stabilized ferrous sulfide as matrix and coupled with PSB.The different factors affecting the performance of CFB₁-P were optimized,and the mechanism of immobilization of Pb by CFB₁-P was discussed.The results showed that the synthesized CFB₁-P had high Pb immobilization performance when bone char:carboxymethyl cellulose:ferrous sulfide=1:1:1.When the dosage of CFB₁-P was 0.5 g·L^-1 and the temperature was 30 ^oC,the immobilization efficiency for Pb²⁺could reach 65.47%.The efficient passivation of Pb by CFB₁-P was concluded by SEM,XRD,FT-IR and XPS,mainly through biomineralization,electrostatic attraction,surface complexation and chemical precipitation.CFB₁-P was further used in the remediation of Pb-contaminated agricultural soils to investigate the efficacy of CFB₁-P in passivating Pb and its effect on soil microbiology.The results showed that CFB₁-P not only rapidly reduced the concentration of biologically effective Pb in the soil and reached equilibrium at the 10th day,but also converted the exchangeable and carbonate-bound Pb into more stable residual Pb（passivation efficiency of 54.84%）,thus effectively reducing the toxic effects of Pb on plants,animals and humans.In addition,the application of CFB₁-P could increase the content of organic matter,available phosphorus,available potassium and soluble organic carbon in the soil through the determination of soil ph ysical and chemical properties.Meanwhile,soil urease,soil sucrase,acid phosphatase and catalase activities were significantly increased,compared with the control treatment.In addition,application of CFB₁-P increased the diversity of microbial communities and the abundance of functional microorganisms（Actinomycetes,Chlorobacterium and Acidobacteria）in Pb-contaminated soils.Under heavy metal stress,although Pb changed the nutrient composition of soil and increased the toxic stress,application of CFB₁-P could reduce the toxicity of Pb,increase the diversity of soil microorganisms and the abundance of functional strains,then restored the soil functionThe effect of Pb stress on the growth of cucumber seedlings and the effect of CFB₁-P to alleviate the stress were further investigated by pot experiments.The results showed that Pb contamination inhibited the growth and development of cucumber seedlings and the synthesis of photosynthetic pigments,as well as increased the accumulation of Pb in the plants.At the same time,Pb contamination also led to membrane lipid peroxidation damage,massive accumulation of malondialdehyde and proline,and significantly increased in antioxidant enzyme activities in cucumber seedlings.The physiological indices of cucumber seedlings in the treatment with CFB₁-P were significantly higher than those in the CK treatment group:chlorophyll content increased by 1.11 mg·g^-1·FW compared with the CK treatment,malondialdehyde and proline content in leaves decreased to 4.67μmol·g^-1·FW and 6.58μg-g^-1 FW,the accumulation of Pb in cucumber decreased to 6.99 mg·g^-1 and antioxidant enzyme activities decreased.In conclusion,the application of CFB₁-P was able to achieve efficient Pb passivation as well as effective mitigation of Pb stress on the growth of cucumber seedlings,providing a novel approach for the remediation of soil Pb contamination.