The Effect Mechanisms Of Self-induced PH On Cadmium Biomineralization And Cadmium Chemical Species In Soil By Bacillus Cereus Cd01

The paddy soils contaminated with heavy metals have been causing many"Cadmium-rice" events in southern China,which has great damage to national food security and public health.Based on our national situations of more people and less land,safe agricultural products need be produced on the mild or moderate heavy metal-contaminated paddy soils,which is urgent to develop an economical,efficient,and simple technique for remediation of heavy metal-contaminated paddy soils.Therefore,in this dissertation,Bacillus cereus CdOl was screened out from cadmium-contaminated soil to investigate influences of pH changes on the adsorption,accumulation and biomineralization of cadmium ions on bacterial cells.Also,the mechanism of strain CdOl alter the chemical species of cadmium in the soil was studied.Results would be expect to utilize the biomineralization of cadmium ions by indigenous microorganism in the fallow period,reducing bioavailabity of cadmium ions,and thus achieving safe rice production in mild or moderate heavy metal-contaminated paddy soils.This paper has obtained some valuable research results:(1)Bacillus cereus Cd01(GenBanK accession No.MF351763)with high Cd2+(5 mmol/L)tolerance was screened as a patent strain deposited at the China Type Culture Collection(Accession No.:CCTCC M 2017358).(2)The mechanisms of the adsorption and accumulation of Cd2+ in the bacterial cells by self-inducing medium pH of strain CdOl were elucidated.In the early growth period,the pH decreased from 6.8 to 6.1,which can inhibit the adsorption of Cd2+.because H+ increased the electrostatic repulsion,and the hydrophobicity of the cell interface and the fluidity of the cell membrane decreased.In the late growth stage,the pH increased from 6.4 to 7.4,which can promote the adsorption of Cd2+.Since H+decrease resulted that the electrostatic effect changed from repulsion to gravity,and the hydrophobicity of the cell interface and the fluidity of the cell membrane increased.Cd2+ mainly adsorbed on the cell wall,while the Cd2+ concentration increased from 0.2 to 1.0 mmol/L,the adsorption of Cd2+ on the cell wall increased from 1.19 to 11.11 ?mol,but the accumulation of Cd2+ only increased from 0.35 to 2.76 ?mol in cells.(3)During the later period(48 h-72 h),strain CdOl can biomineralize the information of cadmium sulfide,phosphate and hydroxide on the surface of the bacteria were determined.TEM-EDS and SEM-EDS analysis showed that the formed mineralized particles contained three elements:sulfur,phosphorus and cadmium,XPS analysis confirmed that the mineralized particles were CdS,Cd-PO4 and Cd(OH)2,HRTEM and SAED analysis obtained that the mineralized particles have a crystal structure with d(200)and d(111)lattices,which are mixed crystal structures.Acid leaching experiments showed that there was no dissolution of Cd2+ within 24 h,indicating that the biomineralized particles of Cd2+ are stable in nature.(4)It was confirmed that inoculating strain Cd01 into soils contaminated with cadmium could change the chemical species of Cd in soil and effectively reduce the bioavailability of Cd.Strain Cd01 was inoculated into Cd-contaminated paddy soils,within 10 days,the soil pH increased from 6.4 to 7.4,the soil exchangeable Cd decreased by 11%,and the relative abundance of strain Cd01 increased by 80%in soil,indicating that strain Cd01 could reduce soil exchangeable cadmium content and cadmium bioavailability.Thus,strain Cd01 can survive in the soil and have the ability to gradually become dominant bacteria.