Remediation Of Cadmium-contaminated Soils By Silicon-doped Ferrihydrite Loads Of Cupriavidus Sp. And Its Mechanism

Excessive accumulation of cadmium（Cd）in the soil environment can significantly threaten human health.Various remediation methods,such as physical,chemical,and biological,have been developed and applied to the remediation of soil Cd contamination.However,single remediation methods had varying degrees of limitations.The combined mineral-bacterial remediation method can overcome the shortcomings of a single remediation method based on the advantages of combined chemical and biological remediation,which is significant for maintaining soil quality and human health safety.In this thesis,the Cd-contaminated soil in Dongdagou,Baiyin City,Gansu Province,was used as the research object.High Cd-tolerant bacteria were obtained through bacterial screening,and pot experiments were used to investigate the remediation effect of bacteria combined application with silicon-doped ferrihydrite（SiFh）,humic acid（HA）,hydroxyapatite（HAP）,and biochar（BC）on Cd-contaminated soils.The mechanism of interaction between SiFh loaded with bacteria and Cd was revealed by batch adsorption,modeling,and characterization methods.These results extend the research content of the combined mineral-biological remediation of Cd-contaminated soil and provide a new solution and theoretical basis for the remediation of Cd-contaminated soils.The main research conclusions are as follows:（1）The bacteria strain H200 belonged to Cupriavidus sp.,which had high Cd tolerance and still had high activity in the environment of Cd stress concentration of200 mg·L^-1.（2）The combination of SiFh and bacteria strain H200 had a relatively good comprehensive effect on the remediation of Cd-contaminated soils.The addition of1.0%by mass of HA,HAP,SiFh,and BC,or the addition of bacteria strain H200 to this,did not result in a large change in soil pH（±0.07）and was more environmentally friendly.The results of the improved BCR leaching showed that SiFh in combination with bacteria strain H200 could convert the unstable acid-soluble Cd to the stable residual Cd(residual-Cd content increased by 1.02 mg·kg^-1).The data on Cd content in pakchoi showed that SiFh inhibited the bioeffectiveness of Cd in the soil more significantly than HA,HAP,and BC(Cd content in pakchoi was reduced by 30.69mg·kg^-1)and that SiFh in combination with bacteria strain H200 further reduced the bioeffectiveness of Cd(Cd content in pakchoi was reduced by 31.00 mg·kg^-1).In terms of the effect of soil enzyme activity,the combination of bacteria strain H200with HA,HAP,and BC increased soil sucrase,alkaline phosphatase,and urease activity but reduced soil catalase activity,whereas bacteria strain H200 in combination with SiFh increased or maintained the activity of all four of these enzymes.（3）Bacteria strain H200 significantly enhanced the inhibitory effect of SiFh on the ability of soil solid phase Cd resupply.SiFh alone caused a slight increase in Cd concentration in soil diffusive gradients in thin-films（DGT）sampling(increased by 1.41μg·L^-1),while the combination of bacteria strain H200 with SiFh decreased Cd concentration in DGT sampling(decreased by 1.41μg·L^-1).SiFh significantly reduced the Cd concentration in the soil interstitial water,while the combination with bacteria strain H200 increased the Cd concentration in the soil interstitial water to the same level as the blank treatment.The results of the DGT Induced Fluxes in Sediments（DIFS）model analysis showed that the type of soil solid phase Cd resupply partially replenished type in all treatment groups,SiFh enhanced the resupply capacity of soil solid phase Cd,and the addition of bacteria strain H200significantly prolonged the sorption time of Cd on the DGT device（31,650-fold）,and significantly reduced the partition coefficient,sorption rate constant and desorption rate constant of Cd.（4）The adsorption effect of SiFh loaded with bacteria strain H200（SiFhB）on Cd²⁺was influenced by factors such as pH,ionic strength,and coexisting heavy metal ion concentration.The higher the pH or lower the ionic strength,the better the adsorption effect of SiFhB on Cd²⁺.The presence of Cu²⁺,Pb^2+,or Zn²⁺significantly inhibited the adsorption of Cd²⁺by SiFhB,and the higher the concentration of Cu²⁺,Pb^2+,or Zn²⁺,the stronger the inhibition of Cd²⁺adsorption by SiFhB.（5）There were various types of adsorption sites on the SiFhB surface,and its adsorption with Cd²⁺was consistent with the characteristics of uniform monolayer adsorption,and intraparticle diffusion was not the only constraint of the adsorption process.The whole adsorption process was dominated by chemisorption,which was accompanied by masking and exposure of surface elements.The pseudo-second-order kinetic model more accurately described the adsorption behavior of Cd²⁺on the SiFhB surface,and the Langmuir isothermal adsorption model more accurately described the adsorption process of Cd²⁺on the SiFhB surface,indicating that the adsorption mainly took place in the form of chemisorption and was mainly homogeneous monolayer adsorption.The binding process of SiFhB to Cd²⁺was divided into fast and slow adsorption phases,where the fast adsorption phase was mainly limited by the superficial diffusion of Cd²⁺on the SiFhB surface,while the slow adsorption phase was mainly limited by the intraparticle diffusion of Cd²⁺on SiFhB.The bacteria strain H200 was tightly bound to SiFh both before and after adsorption,and the uniform distribution of Si,Fe,and Cd on the SiFhB surface after adsorption confirmed that Cd was immobilized on the SiFhB surface by uniform monolayer adsorption.