Study On The Effect And Mechanism Of Biocharcoal Immobilized Microorganism Passivating Heavy Metal Cadmium In Soil

Cadmium is one of the most toxic heavy metal which is difficult to degrade spontaneously in nature.It has serious adverse effects on water and soil.Recently,China and international studies have investigated that biochar and microorganisms have great potential in the degradation of heavy metals.Biochar immobilized microorganisms can degrade heavy metals in the soil.Biochar,as a worldwide recognized biomaterial for adsorption and passivation of heavy metals in soil,has been studied extensively in recent years.There are also many scientific reports on the degradation of heavy metals by functional microorganisms.However,there are few reports on the de-activation and degradation of heavy metals by immobilized functional bacteria using biochar as a carrier agent.In the current study,different biochars and related biomaterials were used as heavy metal adsorbents,and their adsorption properties and mechanism were studied in detail.Eight strains of Cd-degrading bacteria were screened,which were LX1,LX2,LX3,LX5,LX6,LX7,LX9,and LX12,respectively.The degradation performance of Cd-degrading bacteria was also studied by designing the effective immobilization method.The role of immobilized functional strains to degrade heavy metals was compared in detail,and the degradation mechanism of immobilized strains to heavy metals was preliminarily explored.The main results of this study are as follows:?1?Cadmium remediation effect by using pine biochar?PN-B?,peanut shell biochar?PT-B?and bentonite?Bent?,and their adsorption effects were compared.It was found that the adsorption isotherms of PN-B and PT-B were best fit to Freundlich isotherm model,while Bent was fit to both the Langmuir model and Freundlich model.The adsorption equilibrium of PN-B and PT-B reached after three hours,while Bent reached maximum adsorption equilibrium at four hours.The adsorption kinetic equations of these three adsorbents were fitted by the pseudo-second order kinetics model.For PN-B and PT-B,the adsorption rate was high at pH=6,while Bent had the highest adsorption rate at pH=5,with 0.1 g adsorbent dose,and the adsorption effect was followed in order:PN-B>Bent>PT-B.?2?The adsorption mechanism of the three adsorbents was analyzed by scanning electron microscopy?SEM?,infrared spectroscopy?FTIR?and X-ray diffraction?XRD?.SEM and XRD spectra showed that the adsorption mechanism of PN-B and PT-B was mainly surface-OH,-C=O,COOH,and CH₂,which reacted with Cd²⁺by ion exchange and complexation.Bent adsorption mechanism was mainly ion exchange reaction.?3?Eight strains of Cd-resistant strains,LX1,LX2,LX3,LX5,LX6,LX7,LX9 and LX12,were screened from the soil behind the school.The degradation rate of strain LX1 with the highest degradation rate was 78.19%.The 16S rRNA sequencing analysis showed that LX1belongs to Pseudomonas and its maximum cadmium tolerance concentration was 150 mg/L.The optimum conditions of cadmium-degrading bacteria were as follows:temperature 30°C,pH=7,inoculation volume 4%,and cadmium concentration 40 mg/L.?4?The PN-B have shown the highest degradation rate as the optimum immobilization carrier,the optimum immobilization method of bacteria was designed.The maximum immobilization rate of biochar to bacteria was 82.36%.The optimum dosage of immobilized bacteria was 6%,and the optimum pH was 7.The time for cadmium adsorption by biochar was shorter than that of bacteria and immobilized microbial agents,but the degradation rate was as follow:biochar immobilized microorganisms>biochar>strains,and biochar immobilized microorganisms degradation rate was 53.48%.Therefore,the degradation of cadmium by biochar immobilized strains is the synergistic effect of biochar carrier and strains.The study showed that the three kinds of adsorbent materials had the strongest adsorption capacity for heavy metal cadmium,and the adsorption mechanism was ion exchange reaction and complexation reaction.The high-efficiency anti-cadmium strain was isolated by screening,and the strain LX1 had the highest removal rate of heavy metal cadmium.The removal rate of heavy metal cadmium in soil by using biochar immobilized high-efficiency anti-cadmium strain is better than biochar and strain.