Immobilization Of Cadmium And Lead Mediated By Carbonate Mineralizing Bacteria HJ-1 And HJ-2

As industrialization accelerates,heavy metal pollution continues to worsen the ecological environment.Therefore,the treatment and restoration of heavy metal pollution is urgently needed.In response to this problem,the Microbial Induced Carbonate Precipitation（MICP）technology is widely studied due to its highly efficient and economical solidification and stabilization characteristics.This article focuses on two carbonate mineralization bacteria,HJ-1 and HJ-2,screened from polluted soil,and explores their growth characteristics,as well as their ability and related mechanisms to fix Cd²⁺and Pb²⁺.The aim is to provide a theoretical basis for seeking technical solutions to solve heavy metal pollution problems.The research results of this article include the following aspects:From the heavy metal-polluted soil,two strains of bacteria with the ability to induce carbonate precipitation were selected through isolation and purification methods.These strains were identified through physiological and biochemical characteristics,gene sequencing,and scanning electron microscopy.One strain was identified as Sporosarcina ureilytica,which is positive for urease,and named HJ-1.The other strain was identified as Sporosarcina pasteurii and named HJ-2.The growth characteristics of the two strains were similar,with a milky white,smooth,and moist surface on the solid culture medium.The edge was neat,without a halo,and the center was raised with a diameter of about 0.5mm.The bacteria were short rod-shaped and had flagella around the body.The growth influencing factors,physiological characteristics,and Cd²⁺and Pb²⁺tolerance of carbonate mineralizing bacteria HJ-1 and HJ-2 were studied.The results showed that the optimal growth activity of HJ-1 and HJ-2 was achieved at 35℃,p H 9.0,and Ca²⁺concentration of 25 mmol/L.HJ-1 had a stronger urease activity than HJ-2,with a maximum of 1202.32ΔU/ml.HJ-1 had a faster urea decomposition rate,with a residual content of1.27g/L after 72 h.The maximum concentrations of NH₄⁺andCO₃^2-produced by HJ-1 were10378.39 and 18131.04 mg/L,respectively,while those produced by HJ-2 were 9504.94 and17647.39 mg/L,respectively.HJ-1 had a higher precipitation rate of Ca²⁺than HJ-2,and p H increased with the decrease of Ca²⁺concentration.There was a significant difference in the Cd²⁺tolerance between the two carbonate mineralizing bacteria,with HJ-1 having the highest tolerance of up to 50 mg/L,while HJ-2 had a maximum tolerance of 40 mg/L.The tolerance range for Pb²⁺was 0-800 mg/L,and overall,HJ-1 had better Cd²⁺and Pb²⁺tolerance than HJ-2.The characteristics of the products of carbonate mineralization bacteria-mediated fixation of Cd²⁺and Pb²⁺were analyzed by characterization techniques such as scanning electron microscopy（SEM）,energy-dispersive X-ray spectroscopy（EDS）,Fourier transform infrared spectroscopy（FTIR）,X-ray diffraction（XRD）,and Raman spectroscopy.The results showed that most of the products observed by SEM exhibited blocky crystal structures,and EDS analysis indicated the presence of Cd²⁺and Pb²⁺precipitates around the bacterial strains.XRD analysis confirmed that the Cd²⁺and Pb²⁺precipitates were deposited in the form of CdCO₃ and PbCO₃,respectively.Raman spectroscopy analysis showed that the stretching peaks at 150,270,709,and 1083 cm^-1 had the characteristics of carbonate minerals,and FTIR analysis indicated that functional groups such as hydroxyl,amino,and carbonate were involved in the mineralization of heavy metals.The study investigated the fixation of Cd²⁺and Pb²⁺byβ-tricalcium phosphate in conjunction with carbonic anhydrase mineralizing bacteria HJ-1 and HJ-2.The results showed that the two bacterial strains exhibited higher fixation capability toward Cd²⁺and Pb²⁺in the presence ofβ-tricalcium phosphate.Specifically,at a Cd²⁺concentration of 50mg/L,HJ-1 showed a fixation rate of 93.16%within 24 h,compared to 62.62%withoutβ-tricalcium phosphate;similarly,HJ-2 exhibited a fixation rate of 85.69%at a Cd²⁺concentration of 40 mg/L,compared to 68.37%withoutβ-tricalcium phosphate.Moreover,at a Pb²⁺concentration of 800 mg/L,both HJ-1 and HJ-2 achieved fixation rates of above 95%within 72 h,with fixation rates reaching 71.11%and 62.21%within 24 h,respectively.Characterization of the fixation product by XRD revealed that the fixation mechanism ofβ-tricalcium phosphate in conjunction with carbonic anhydrase mineralizing bacteria mainly involves two aspects:one is the adsorption of carbonic anhydrase mineralizing bacteria on the surface ofβ-tricalcium phosphate,where CaCO₃ is formed with the soluble part ofβ-tricalcium phosphate,which co-precipitates with heavy metal cations such as Cd²⁺and Pb²⁺,lowering their concentrations.The other is the recombination of Ca²⁺from the hydrolyzed part of β-tricalcium phosphate withCO₃^2-and OH^-in the solution to produce CaCO₃ and Ca₅（PO₄）₃（OH）,which allows the formation of a chelating state such as Ca_4.03Cd_0.97（PO4）₃（OH）for active Cd²⁺and Pb²⁺in the system.