Screening Of Cd²⁺-Resistant Strains And Characteristics Of Cd²⁺ Uptake

The heavy metal Cd is highly biotoxic and not easily degraded,and is easily accumulated and transmitted through the food chain and water bodies,posing a threat to public health.Traditional Cd²⁺pollution treatment technology can remove certain concentration of Cd²⁺,but the input cost is high and there is a possibility of secondary pollution.In contrast,microbial methods are efficient,environmentally friendly,inexpensive and adaptable,and have broad prospects in the study of heavy metal pollution.In this thesis,Cd²⁺-resistant strains ZQ1 and ZQ2 were screened from the soil of the tailing pond of the Daduanshan Throwing Knife Ridge gold mine in Guizhi District,Chizhou City,Anhui Province,China.The biological characteristics of the resistant strains were investigated,and morphological observations and 16Sr DNA sequence analysis were performed.The adsorption characteristics of the resistant strains on Cd²⁺were studied by fitting the proposed primary and secondary kinetic models and Langmuir and Freundlich adsorption isotherm models,and the adsorption mechanism was studied by using XPS,FTIR,SEM-EDS and other characterization means,and the main results were obtained as follows:（1）The strains ZQ1 and ZQ2 were screened for Cd²⁺resistance,among which strain ZQ1 was more tolerant to Cd²⁺and therefore was used as the subject of subsequent experiments.strain ZQ1 was a Gram-positive,rod-shaped bacterium,and 16Sr DNA sequence analysis showed that strain ZQ1 belonged to Yokenella regensburgei.The bacterial body is about 1.02μm long and 0.48μm wide,and the colonies are characterized as creamy white,smooth and slightly elevated.（2）ZQ1 strain was in stagnant phase during the incubation time 0-4 h,entered the logarithmic growth phase at the 6thh,and entered the stationary phase after the 20th h.The growth temperature range of the strain is within 20℃-40℃,and the optimum growth temperature is 30℃.The growth p H range of the strain is 5.0～9.0,and the optimal growth p H is 7.0.（3）ZQ1 strain has a certain adsorption effect on Cd²⁺in the environment.In the range of1.0-150 mg/L for the test concentration,the removal rate could reach 74.14%when the Cd²⁺concentration was 1 mg/L.With the increasing of Cd²⁺concentration,the removal rate of the strain showed a decreasing trend.The removal rate of Cd²⁺by ZQ1 strain was below 50%when the Cd²⁺concentration was greater than 70 mg/L.The adsorption process of Cd²⁺ions by ZQ1 strain was in accordance with the proposed secondary kinetic model（R²of 0.964）and Langmuir isothermal model（R²of 0.995）.The removal rate of Cd²⁺by ZQ1 strain increased with the prolongation of the stress time.the Cd²⁺stress concentration of 5 mg/L was in the rapid removal stage from 0 to 12 h.The removal rate reached 50.15%at the 12th h,and the removal rate was 0.21 mg/h.Then the removal rate increased slowly from 12 h to 96 h,and the removal rate at the 96th h was72.16%.（4）Environmental factors had an effect on the removal of Cd²⁺by ZQ1 strain.Under the condition of Cd²⁺concentration of 25 mg/L,the removal effect of ZQ1 strain on Cd²⁺was better when the temperature was 25℃-30℃,among which the removal effect was optimal at 30℃,and the removal rate was 63.58%.p H value was 7.0-8.0,the removal rate of ZQ1 strain on Cd²⁺remained above 60%,among which the removal effect was optimal at p H value of 7.0,and the removal rate was 63.09%.The removal rate was 63.09%.（5）With the increase of Cd²⁺concentration,the EPS content of ZQ1 strain increased continuously.156.18 mg/L of total EPS was obtained at 150 mg/L of Cd²⁺concentration,and the growth rate(total EPS at 200 mg/L of Cd²⁺concentration minus total EPS in ck group divided by ck group)reached 57.23%.The polysaccharide and protein contents in EPS showed an increasing trend with the prolongation of the stress time.During 12 h-96 h,the polysaccharide and protein contents continued to increase at Cd²⁺concentration of 50 mg/L.While the polysaccharide content at Cd²⁺concentration of 150 mg/L maintained an increasing trend from 12 h-72 h and slightly decreased after the 72ndh,the protein content increased rapidly from 12 h-48 h,from 20.68 mg/L at the 12thh to 79.94 mg/L at the 48th h,and then the protein content showed a decreasing trend.EPS played a role in the removal effect of Cd²⁺by ZQ1 strain.At the Cd²⁺concentration of 5 mg/L,the Cd²⁺content in EPS was 1.713 mg/L,accounting for 48.59%of the total Cd²⁺removal.As the Cd²⁺concentration increased,the percentage of Cd²⁺content in EPS showed an increasing trend.At the Cd²⁺concentration of 25 mg/L-200 mg/L,the Cd²⁺content in EPS all accounted for more than 50%of the total Cd²⁺content removed.The percentage of Cd²⁺content in EPS at Cd²⁺concentration of 200 mg/L was 66%.（6）SEM observed the presence of filamentous linkages and attached granules on the surface of the bacteria under Cd²⁺stress,and EDS energy spectrum and XPS analysis showed that Cd²⁺fugitive forms on the surface of ZQ1 strain were Cd（OH）₂and Cd O.FTIR analysis showed that the strain under Cd²⁺stress-OH,-NH,C-OH,C-O,N-H,-C-N-,and O-Cd-O and Cd-O groups were involved in the adsorption of Cd²⁺by the strain.