| In the process of civil engineering construction,the heavy metal contaminated site whose foundation-bearing capacity does not meet the reconstruction requirements,environmental restoration,and foundation reinforcement,must be treated.At present,remediation technologies for contaminated sites mainly include physical,chemical,and biological remediation,which can be implemented by wrapping,removing,solidifying,and stabilizing pollutants.Based on the principle of microbial-induced carbonate precipitation(MICP),the microbial-solidified/stabilized heavy metal-contaminated soil technology has the dual potential of repairing heavy metal-contaminated soil and strengthening soil.It is expected to meet the requirements of environmental remediation and engineering construction at the same time.Compared with other technologies,it is also more lowcarbon and environmentally friendly.In situ stirring process is more conducive to pollution risk control and field application than the multi-round grouting process.Based on the microbial single mixing process achieve powder curing and stability of soil lead pollution into the target,using the powder soil lead pollution concentration,salt,and calcium salt concentration as control parameters,such as through the microbial-gypsum whisker single silt mixing method and treatment of lead pollution,research on microbial powder engineering properties of soil solidification/stabilization of lead pollution,environmental safety,pollutants such as geometrical shape and stability.The main research results are as follows:(1)Microbe-gypsum whisker solidification/stabilization treatment was carried out on leadcontaminated silt with four concentrations of 200,500,1000,and 1500mg/kg by single stirring method,and the influence of different calcium salt types and salt concentrations on the engineering characteristics of lead-contaminated silt before and after treatment was analyzed.The results showed that the unconfined compressive strength of solidified/stabilized lead-contaminated silt was positively correlated with the content and concentration of calcium chloride in calcium salt,and negatively correlated with the concentration of contaminated soil.The unconfined compressive strength of solidified lead-contaminated silt reached 213.18 k Pa.The stiffness of treated samples increases with the increase in strength.When single calcium chloride was used as a calcium source,the strength of the soil solidified/stabilized by microorganisms decreased significantly when the stirring time increased from 30 min to 120 min and 480 min.Gypsum whisker instead of partial calcium chloride as a calcium source can play a retarding role,and can still maintain a higher strength of solidified soil when the stirring time is large.When the curing age is from 7d to 14 d,the strength of the sample increases slightly(especially the gypsum whisker sample),and the strength of the sample does not increase after 14 d.(2)The effects of salt concentration,calcium salt type,and contaminated soil concentration on carbonate production and calcium ion conversion of solidified/stabilized lead-contaminated silt were analyzed,and the results were compared with software simulation results.The results show that the experimental values of carbonate production and calcium ion conversion are slightly lower than the simulated values,and the carbonate production increases with the increase in the salt concentration but decreases with the increase in the lead-contaminated soil concentration.The experimental results show a similar trend to the simulated results.However,it is difficult to reflect the influence of stirring time on carbonate production and calcium ion conversion.Increasing the stirring time does not significantly change the amount of carbonate produced by the samples.The amount of carbonate produced by the samples increases when the curing age increases from 7d to 14 d.After curing for14 days,carbonate production does not change with age.(3)After microbial curing/stabilization treatment,the proportion of available lead in leadcontaminated silt can be reduced,and the level of lead element migration decreases.Specifically,the proportion of exchangeable lead in the sample decreases,while the proportion of carbonatebound lead and Fe/Mn oxide-bound lead increases.With the increase of salt concentration and calcium chloride ratio in calcium salt,the proportion of exchangeable lead and organically bound lead in the sample decreased,the proportion of carbonate bound lead and Fe-Mn oxide bound lead increased,and the proportion of residual lead did not change significantly.By increasing stirring time and curing age,the proportion of exchangeable lead can be reduced to a certain extent,and the proportion of carbonate-bound lead and Fe-Mn oxide-bound lead can be increased,but the proportion of organic bound lead and residual lead has little change.Microscopic analysis shows that the morphology of calcium carbonate generated in lead-contaminated silt is spherical graphite and irregular aggregate,and occasionally a small number of lead carbonate crystals adsorbed on calcium carbonate in the form of radial white lead cluster.(4)The leaching characteristics of lead ions in silty soil under an acidic environment were analyzed by the TCLP leaching test on the samples with microbial solidification/stabilization treatment,and the environmental safety characteristics of the samples with microbial curing/stabilization treatment were explored.The results showed that the leaching rate of lead ions decreased with the increase in salt concentration and calcium-chloride ratio in calcium salt.After curing/stabilization treatment,the lead ion leaching value of silt with a pollution concentration of200mg/kg can be controlled below the maximum limit of TCLP of 5mg/L stipulated by EPA,and the lead ion leaching value of silt with a pollution concentration of 500-1500 mg /kg can also be significantly reduced after treatment.When the stirring time was extended from 30 min to 120 min and 480 min,the leaching value of the samples did not change significantly.After extending the curing age to 14 days,the leaching value of the samples decreased,and the decrease in gypsum whisker samples was more significant than that of calcium chloride samples.When the curing age was extended to 28 days,the leaching value did not change significantly.(5)The inner Merrow index and potential risk index were used to evaluate the pollution level and environmental safety of lead-contaminated silt samples.The results showed that after microbial curing/stabilization treatment,except for a few 1500mg/kg contaminated soil samples with moderate pollution levels,the rest of the samples could be reduced to low pollution levels,and the ecological risk level of all samples was reduced to low level or below. |
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