Laboratorial Experiments On Remiadation Of Lead-contaminated Soil With Micp Method

Microbial induced carbonate precipitation?MICP?has been explored for soil stabilization and heavy metals immobilization in soil and groundwater.Sponsored by China Natural Science Foundation Grant?Grant No.41472258?,and based on laboratorial experiments and theoretical calculations,this study?1?studied influence of concentration of Pb contamination on bacterial activity and efficacy of Pb immobi-lization in a wider range of Pb concentration,as well as its patterns and mechanism,?2?identified main controlling factors of microbially-induced carbonate precipitation for Pb immobilization,and evolution patterns of precipitation.Major results of this study are listed below:?1?Influence of heavy metal concentration on changes of biochemical properties of the bacteria over time was revealed through the bacterial activity test.The results showed that electrical conductivity and pH values changed continuously in the test period when Pb concentration was no higher than 30 mM.When Pb concentration was higher than 30 mM,conductivity and pH change rapidly over a short period of time and then remain almost constant.The changes in the viable cell number and the urease activity reflected that the threshold concentration of bacteria resistance on Pb toxicity was 30 mM.?2?Through immobilizing Pb contamination tests via MICP in the mixed solution of three cases,properties reflecting the effects of urea hydrolysis and Pb immobilization were measured,and the effects of different calcium sources and the bacterial concentration on efficiency of urea hydrolysis Pb immobi-lization were compared.The results showed that range of Pb immobilization rate was 80-95%in Case 1?CaCl₂+low bacterial concentration?,20-80%in Case 2?Ca?Ac?₂+low bacterial concentration?,and95-100%in Case 3?Ca?Ac?₂+high bacterial concentration?.Results of viable cell number,change in ammonium concentration and total precipitation mass further confirmed the difference of Pb immobili-zation rate in three cases.According to the co-relationship between Pb immobilization rate and viable cell number,Pb immobilization rate in solution increased with the increase of bacterial concentration in the three cases.?3?Visual MINTEQ software was used to calculate and analyze the forming process of precipitation induced by hydrolysis of urea,while Pb immobilization rates were modeled,and main precipitation types were revealed.The results showed that Pb immobilization rate in simulation rapidly increased to 100%with the increase of the degree of urea hydrolysis.At the beginning of the experiment,when process of microbially-induced carbonate precipitation has not been completely facilitated,the precipitation types were mainly abiotic.With the gradual increase of the degree of urea hydrolysis in the simulation,early precipitation such as PbCl₂ and Pb?OH?₂ in the three cases were gradually replaced by PbCO₃,together with the formation of CaCO₃.Based on the results,this study presented mechanism of heavy metal con-taminated soil immobilization via MICP:with bacterial cells located at the center,the precipitation formed a multi-layer structure from the inside to the outside.?4?Soil column was prepared by mixing contaminated soil with bacterial suspension,in order to form a column.After incubation,soil column was and the viable cell number in soil and urease activity of soil pore liquid were measured.The results showed that the viable cell number and urease activity in soil column decreased with the increase of Pb concentration,and the threshold concentration of bacteria resistance on Pb toxicity was 100 mM.The viable cell number and urease activity along the depth of soil column were homogeneously distributed in general,but the viable cell number and urease activity were relatively higher in the upper part of soil column under when Pb concentration was lower than 50 mM.