Study On The Stability Of Microbe Induced Carbonate Deposition To Heavy Metal Lead

With the development of industry and mining,a large amount of heavy metal lead has been released into the environment during the mining process,and accumulated,transformed and migrated in the soil,posing a threat to the balance of the ecosystem.Therefore,soil lead pollution control has become a global problem.Recent research into soil contamination has tended to use microbial techniques,including microbially induce carbonate precipitation（MICP）,which produces carbonate ions through the hydrolysis of urea.These carbonate ions adsorb and complex with surrounding free heavy metals to produce calcium carbonate precipitation,to immobilize the heavy metals.This is why we have decided to use green,efficient,non-secondary pollution-free microbial remediation as the main research technology for this experiment.However,as the real soil environment is complex and influenced by external natural factors,we carried out simulated ageing experiments to test whether the MICP technology can achieve long-term stabilisation and to investigate the mechanism of action of the MICP technology under extreme conditions.In this paper,four simulated aging conditions were designed for heavy metal contaminated soils:dry-wet cycle,freeze-thaw cycle,salt solution-dry-wet cycle-freeze-thaw cycle and dry-wet cycle-freeze-thaw cycle,and three addition groups of Bacillus pasteurii（C₁）,Bacillus pasteurii+sodium citrate（C₂）and Bacillus pasteurii+biochar（C₃）supplemented with physical adsorbent materials and chemical nucleating agents were set.On this basis,the immobilisation and effects of each group on lead in soil under different ageing conditions were investigated.Moreover,the changes in the basic physicochemical properties,leaching toxicity and calcium carbonate morphological characterisation of the soil were focused on,providing a theoretical basis for the treatment and long-term stability assessment of lead-contaminated soil by MICP technology.The main findings are as follows.（1）Dry and wet cycles have an influence on the long-term stability of MICP technology application.MICP technology treatment of Pb-contaminated soil under dry and wet cycle conditions could effectively reduce soil p H,with the C₃group showing the best effect on p H reduction,followed by the C₂and C₁groups.The leaching rate of Pb²⁺in the soil was significantly inhibited by each treatment group,and the leaching rate of C₂group was reduced by 11.37%and 7.08%only at cycle 7and cycle 14 compared with 25.03%and 32.13%of CK group,respectively,and the fixation effect of Pb²⁺was significantly improved,and the content of carbonate bound state in C₁,C₂and C₃groups decreased with the increase of cycle.The increase in the intensity of the anti-symmetric vibration peak of the carbonate in the C₃group indicates that this group can expand the contact surface of the mineralized products with Pb²⁺,increase the adsorption sites of calcium carbonate precipitation,and enhance the hydroxyl hydrogen bonding force to improve the intermolecular binding force.Comprehensive analysis shows that the C₃group has the best curing and stabilization effect under the dry and wet cycling conditions.（2）The freeze-thaw cycles had an effect on the long-term stability of the MICP technology application.With the increase of the number of freeze-thaw cycles,all groups of treatments could effectively reduce the soil p H,among which the C₃group could reduce the soil p H to the optimal stable state.The leaching rate of Pb²⁺in the soil under each treatment group showed a decreasing trend,and the leaching rate of C₃and C₂groups decreased by 44.14%and 35.94%,respectively,compared with that of CK group at the 7th cycle to achieve the best immobilization effect,and the effect of the two treatment groups was more consistent and significantly better than that of C₁group.The biochar injected in the C₃group was damaged by freeze-thaw cycles,which increased the adsorption sites on the surface of mineralized products,expanded the adsorption area,and accelerated the rate of binding with calcium carbonate precipitation,so the peak fluctuation frequency of calcium carbonate precipitation in XRD characterization was higher.Therefore,the C₃group showed the best adsorption and immobilization of Pb²⁺under freeze-thaw cycle conditions.（3）The compound aging cycle conditions had an effect on the long-term stability of the MICP technology application.Under the treatment of compound aging factors,C₁,C₂and C₃groups increased 0.35,0.3 and 0.07 units,respectively,compared with their respective first cycle p H values,with C₃group having a better effect on p H stability.The leaching toxicity of Pb²⁺in the soil was significantly inhibited by all groups of treatments under the compound aging cycle,among which the stabilization of leaching rate was more consistent and better in the C₂and C₃groups than in the C₁group.The increase ratios of the exchangeable state in the C₁,C₂and C₃groups were58.44%,76.47%and 32.20%,respectively,which were lower than that of the CK group（258.31%）,while the decrease rates of the carbonate-bound state were 21.44%,25.85%and 39.75%,respectively,which were lower than the 44.65%of the CK group.The combined analysis showed that the treatment of C₂and C₃groups could maximize the co-precipitation of Pb²⁺with carbonate.