Study On Separation And Screening Of Highly Active Calcium Carbonate Mineralizing Bacteria And Fixation Of Heavy Metals In Lead-zinc Tailing

With the pace of ecological civilization construction in recent years,the ecological environment problem has been paid great attention to,people are also paying more attention to environmental pollution brought by tailings and heavy metal spills.The heavy metal leakage from slag is a serious environmental pollution,which will have a continuous impact on human production and life.Biomineralization is a process in which organisms participate in the induction of mineralization.Recent studies have found that heavy metal ions can be fixed by microbial mineralization,and urease and carbonic anhydrase produced by bacteria are considered to be the two key enzymes involved in the induction of mineralization by bacteria.However,the research on the fixation of heavy metal ions by carbonic anhydrase producing bacteria is still in the stage of exploration and supplement.In order to explore the fixation effect of carbonic anhydrase-induced mineralization on heavy metal ions in lead-zinc tailings,two strains of bacteria with high carbonic anhydrase activity were isolated and screened,and their basic physical and chemical properties（growth curve,Gram stain,extracellular polysaccharide and protein）and tolerance to Pb²⁺,Zn²⁺and Cd²⁺were measured.The effects of Ca²⁺concentration and initial p H on the induced mineralization of two strains of bacteria were compared,and the fixation effects of Pb,Zn and Cd mineralization in solution were compared.The effects of two strains of bacteria on the occurrence state,available state change and stability of Pb,Zn and Cd elements in the tailings of lead-zinc mine were discussed.The main research results are as follows:1.Two strains of Gram-positive bacteria with high carbonic anhydrase activity were screened from Sichuan Huanglong Travertine pools and named as Z-2 and HL-1.Bacillus Z-2 was rod-shaped with white colony.It produced spores with short growth cycle and reached a stable period after 8 h.It was identified as Bacillus sp.HL-1 was a short rod-shaped colony with yellow,smooth and long growth cycle,and reached a stable stage 30 h.The identified results were Brachybacterium sp.2.Ca²⁺concentration and initial p H of culture medium affected the growth and induced mineralization of the two strains of bacteria.The results show that low concentration of Ca²⁺can promote the growth of bacteria,and the crystals of the mineralized products exist in the form of calcite and aragonite crystals.High concentration of Ca²⁺can inhibit the growth of bacteria,and the mineralized crystals are mainly calcite.Higher initial p H would advance the time of bacterial decay and affect the contents of protein and polysaccharide in bacterial EPS.The crystal parameters show that the higher the initial p H,the more calcite diffraction peaks are formed,while the lower p H,the mineralization products are dominated by vaterite.Extracellular proteins and polysaccharides secreted by bacteria are also affected by Ca²⁺and p H.3.The induced mineralization of the two strains of bacteria had significant effect on the fixation of Pb,Zn and Cd in solution compared with bacterial adsorption（P<0.05）.Compared with the control,the fixation effect of bacterial Z-2 mineralization on Pb,Zn and Cd reached93.63%,83.30%and 95.63%,respectively.The removal effect of bacterial HL-1 induced mineralization on Pb was 95.37%,Zn 55.60%and Cd 35.29%.Bacterial Z-2 mineralization products are mainly irregular calcite and spherical spheroid aragonite,while bacterial HL-1mineralization products are short rod-shaped bacterial nucleation,forming rod-shaped crystals and spherical crystals with more holes distributed on the crystal surface.XRD and SEM+EDS analysis showed that the three heavy metal elements either participated in nucleation,formed carbonate and metal minerals,or were adsorbed on the surface of Ca CO₃ crystal in the form of extracellular sacs.4.The leaching p H of lead-zinc slag used in this study is 8.5±0.23,with Ca element accounting for 47.80%.The total amount of Pb,Zn and Cd elements was 499.693±23.221mg/Kg,988.955±32.112 mg/Kg and 4.526±0.983 mg/Kg,respectively.Among them,Pb element is mainly reduced state,accounting for 43.09%of the total.The main element of Zn is exchangeable state,accounting for 41.41%of the total amount.Cd element mainly exists in oxidation state,accounting for 35.87%of the total.The leaching of Pb,Zn and Cd was affected by slag particle size,p H of leaching agent and leaching temperature.The smaller the particle size and the lower the p H of the extract,the higher the content of heavy metal elements.5.Bacterial induced mineralization changes the available states and occurrence states of Pb,Zn and Cd elements in the slag.After 30 days of induced mineralization,the effective state fixation of Pb elements in slag was extremely significant（P<0.01）,and the effective state fixation of Zn and Cd elements also reached a significant level（P<0.05）,and the temperature of 35℃had the best effect on the effective state fixation of the three elements.The improved BCR continuous extraction method shows that the residual state of Pb element in slag increases from 9.76%to more than 60%,and the proportion of exchangeable state decreases gradually with the decrease of mineralization temperature,from 35.62%to 26.89%.For Zn,the proportion of residuals and exchangeable states increased from 31.36%to more than 40%.The residual state of Cd element increased from 22.90%to more than 50%.The interchangeable state of Pb elements increased by 0.99%（Z-2）and 4.65%（HL-1）in the 7-cycle wet-dry cycle.The freeze-thaw cycle reduced the reduceable state of Pb by 6.32%（Z-2）and 4.21%（HL-1）,and the residual state of Zn by 6.67%（Z-2）and 4.84%（HL-1）,respectively,while Cd did not change much.To sum up,microbial induced mineralization has certain feasibility for the fixation of heavy metals in tailings,which can effectively reduce the content of the effective state of heavy metals,change the occurrence state of heavy metal ions,lead to the increase of residual state,and thus reduce the risk of heavy metal leakage in tailing.