Effect Of Bacteria(Bacillus Sp.K1) And Biochar Composite On The Remediation Of Cadmium Polluted Soil

The Cadmium（Cd）contamination of farmland in China becomes a permanent problem,which has adverse effects on the food quality and safety.Therefore,it is an urgence to remediate the Cd pollution in soil.In recent years,combination of biochar and bioremediation method for Cd remediation attracted extensive attentions,due to its advantages such as low cost,high efficiency and no secondary pollution.Among various materials,the composite prepared by biochar and Cd-resistant bacteria showed a great potential in the remediation of Cd in aqueous solution and soil.However,previous studies just focused on the remediation effects of the materials on Cd remediation in soil or aqueous solution.The remediation mechanisms are still not clear.No research focuses on Cd remediation effects and mechanisms of bacterial-biochar composite during rice plantting yet.Furthermore,the interactions between inoculated microbes,biochar and indigenous microorganism in soil are poorly understood.Therefore,a Cd-resistant bacteria strain and two kinds of biochar were chosen as raw materials to synthesize bacterial-biochar composite.The aims of the study are to 1)determine the mechanism and effect on Cd remediation in aqueous solution and soil;2)explore the interaction between inoculated microbes,biochar and indigenous microorganism and 3)explain the effects of the composite on the rizhospere soil,growth and Cd accumulation of rice.The study may provide theoretical basis and technical support of utilization of bacterial-biochar composite for Cd remediation in paddy soil.The main results are as follows:（1）Magnetic bacterial-biochar composite（MBB）prepared by Bacillus sp.K1 and magnetic biochar had the best effects on Cd adsorption in aqueous solution and available Cd reduction in soil.It could remove 87.19%of Cd（II）in a 10 mg kg^-1 Cd（II）solution and decreased 90.32%of available Cd in soil with a total Cd concentration of1.6 mg kg^-1.Morphology characterization indicated that the biochar was the skeleton inside,with Fe₃O₄ and bacteria distributed on the surface and pore structures.The sodium alginate formed the layer outside.（2）The results of batch experiments showed that MBB could effectively adsorb Cd（II）in aqueous solution.The maximum adsorption capacity of MBB for Cd（II）was21.50 mg g^-1,which was 230%higher than that of magnetic biochar.MBB adsorbed Cd through precipitation and ion exchange with the surface functional groups.Bacillus sp.K1 provided new biosorption sites such as-NH₂ and-OH on the surface of the composite,thus significantly improves the removal capability of Cd（II）.In addition,hydroxyl complexation on Fe₃O₄ surface was also important for Cd（II）adsorption by MBB.（3）The soil incubation experiment indicated that MBB could effectively reduce the available Cd in soil and alter the fraction Cd in both moist（aerobic）and flooding（anaerobic）conditions.Under aerobic conditions,the effect of Cd remediation in soil by MBB and SBB（straw bacterial-biochar composite）was related to the alkaline properties of the biochars and the biosorption of inoculated bacteria.In the anaerobic condition,MBB was more efficient in soil Cd remeidition.More secondary iron oxides could be formed in MBB treatment,which bound Cd in a stable fraction.The p H was the most crucial factor associated with the disruption changes in the microbial community（especially Bacillus）under aerobic condition.SBB increased the soil p H most,and the relative abundance of Bacillus sp.K1 was also the highest in SBB treatment.Under anaerobic conditions,the correlation coefficient between microbial biomass carbon content and soil microbial community is the highest.MBB increased the soil biomass carbon and the survival and colonization of inoculated bacterium,which significantly changed the structure of soil microbial community.However,the inoculation of exogenic microorganisms with biochar applications may have adverse effect on microbial diversity due to the competition between indigenous microbes and inoculated bacterial strains.（4）1%MBB addtion could significantly increased the p H of rhizosphere soil and improved the DOC for a short time.The acid extracted Cd in rhizosphere soil was converted to residual Cd with MBB treatment,which was related to the formation of second iron oxides under anaerobic conditions.The changes could indirectly reduce the risk of Cd transfer to rice.At the maturation stage,the available Cd and acid-extracted Cd concertration of bacterial inoculation（B）treatment were similar with the control.Under MBB treatment,Bacillus sp.K1 may colonize well in soil,and the abundance of Bacillus was increased by 156.57%compared with the control.MBB slightly increased the biomass and grain weight of rice,and effectively decrease the Cd contents in rice,especially in grains.The content of Fe in the root iron plaque of MBB treatment was significantly increased,while the content of Cd was slightly decreased.The results indicated that the MBB can greatly improve the adsorption capacity of the raw biochar.MBB also reduced the accumulation of Cd in rice.The effects of MBB on rice were 1)Improving nutritional status and promoting rice growth;2)Changing the rhizosphere environment and decrease the Cd mobility and avaliability in the rhizosphere;3)Promoting the formation of root iron plaque and inhibiting the accumulation of Cd by rice roots.The combination of biochar and microorganisms can effectively immobilize Cd in situ,providing a new sight for the utilization of agricultural waste and the remediation of soil heavy metal pollution.