Effect Of Nucleating Agent On Calcium Carbonate Formation Induced By Different Carbonate Mineralizing Bacteria

Microbial-induced calcium carbonate precipitation（MICP）is the use of urease produced by the metabolism of microorganisms to react with urea and calcium chloride to generate calcium carbonate precipitation,which plays the role of solidifying heavy metals.It is a promising heavy metal curing repair technology.This method is also applicable to the pollution remediation of water and soil contaminated by heavy metals.The disadvantage of this technology is that the microorganisms have a long mineralization time and slow crystallization speed.Therefore,it is very necessary to find a method to accelerate mineralization.In this study,Bacillus pasteurii and Staphylococcus cohnii with mineralization ability were taken as the research objects,the adaptability evaluation of the growth of the two mineralized bacteria was carried out,nucleating agents capable of accelerating mineralization were selected,and nucleating agents were discussed.Sodium citrate regulates the effects of single and mixed bacteria on calcium carbonate precipitation.The main findings are as follows:（1）The effects of pH,urea concentration,and Ca²⁺concentration on the growth of Bacillus pasteurii and Staphylococcus cohnii were investigated through a single factor test,and the optimal culture conditions and tolerance range of the strain were determined.The pH tolerance range of Staphylococcus cohnii was 6-11,and the salt tolerance was less than10%;The pH tolerance range of Bacillus pasteurii was 5-12,and the salt tolerance was less than 8%;The optimal concentration of calcium ion of Staphylococcus cohnii was 30 g/L,and the optimal concentration of urea was 35 g/L;The optimal concentration of calcium ion of Bacillus pasteurii was 27 g/L,and the optimal concentration of urea was 15 g/L;The average hydrolysis rate of urea was 3.677 mg/L·min.（2）The growth of Bacillus pasteurii and Staphylococcus cohnii under different concentrations of Pb,Cr and Cd was studied by single factor test.The tolerance of Bacillus pasteurii to Pb concentration was 5 mmol/L,Cd concentration was less than 1 mmol/L,Cr concentration was 5 mmol/L;the tolerance of Staphylococcus cohnii to Pb concentration was 5 mmol/L,Cd concentration was 3 mmol/L,Cr concentration was 3 mmol/L.（3）Six nucleating agents,namely sodium citrate,magnesium oxide,magnesium chloride,chitosan,silica and ethanol,were selected to investigate the crystal crystallization rate by polarizing microscope.The crystallization rate of the six nucleating agents was:sodium citrate>magnesium oxide>magnesium chloride>chitosan>silica>ethanol.Compared with the blank,the amount of precipitation almost increased under the action of nucleating agents,and the amount of precipitation increased most under the action of sodium citrate;The removal effect of nucleating agent on lead was improved,and the removal rate of lead after adding sodium citrate reaches about 90%;The morphology and size of the final product are different under different nucleating agents and contain polymorphs.The rainwater immersion method was used to study the stability of mineralized products under different nucleating agents.After soaking in rainwater,the lead leaching rate of the mineral formed by sodium citrate in the solution was the lowest,indicating that the stability of lead in the precipitate was the best under the action of the nucleating agent sodium citrate.（4）Sodium citrate has an important effect on the morphology and polymorphism of mineralized products in the process of biomineralization.The polarizing microscope is used to monitor the growth process of minerals in real time,and the weight of mineralized products is calculated by weighing.It is shown that sodium citrate can increase the amount of mineralized precipitation and accelerate the crystallization speed of crystals when adjusting MICP and MICP-Binding;The morphology of mineralized products was analyzed by SEM,the phases and groups of mineralized products were analyzed by XRD and FTIR spectroscopy.It is determined that the mineralized product is vaterite,and sodium citrate can refine the star particles,can increase the nucleation site of CaCO₃,promote crystal crystallization,and make the crystal polymorphization and crystallization speed faster;Atomic absorption spectrophotometer（AAS）was used to determine that the removal rate of lead could be improved when sodium citrate modified MICP and MICP-Binding;By measuring the conductivity and pH of the bacterial solution,it was determined that the sodium citrate modified MICP and MICP-Binding had no effect on pH and conductivity.The experimental results prove that the proposed sodium citrate can regulate the precipitation of calcium carbonate by single bacteria and mixed bacteria.