| Vertical cutoff walls were usually adopted to solve the leakage of leachate in simple landfills.It is necessary to enhance the barrier performance of cutoff walls in order to improve the standard of contamination control during the reconstruction or expansion of existing landfills.Microorganism has the potential to attach on the surface of porous medium,and reduces the pore size,finally causes the reduction of hydraulic conductivity.However,according to previous research,there was no significant effect of microbes on the hydraulic conductivity towards pure inorganic material,while the attached bacteria quantity is li1ited,and the mechanism of biofilm formation is not clear.Thus,in this study,organic-fiber is introduced to promote the attached microbe quantity and biofilm thickness,in order to reduce the hydraulic conductivity of the fine sand substantially.Based on the fiber type,content and soil conditions,the hydraulic conductivity of biofilm is evaluated,and the relationship between the surface physicochemical properties of materials,distribution of microorganism and hydraulic behavior will be established.Different factors towards hydraulic conductivity of biofilm are taken into account,including soil type,pH,salinity,and water head.Together with the physical,chemical and biological properties as well as the test results,further discussion will be made about the mechanism of biofilm formation and its effect on hydraulic conductivity.Subsequently,an extensive parametric analysis was conducted by finite element software to investigate the transport behavior of contaminant through vertical cutoff walls.The results are summarized as follows:(1)Microorganisms attach to the soil surface and the biomass presented a graduated decrease along the experimental process and low biomass area mainly distributed in the upper and middle part of the specimens and high biomass zones mainly distributed in the lower part of the specimens.The surface of carbon fibers contained the large sirface area and excellent hydrophilicity,which improved the cell adhesion and growth compared with other fiber materials.(2)As the best immobilized material,carbon fibers can significantly improve the generation of microbial membranes in soil.When the fiber content was 0.20%,the biofilm clogging effect was the best,which resulted in the decrease of hydraulic conductivity by 2 N 3 orders of magnitude in a short time.(3)Soil pore size plays a significant role in maintaining microbial activity.The results show that the growth of microorganisms in soil requires a favorable environment(including pore size),and the effect of microbial clogging decreases with the increase of clay content.When microorganisms grow in porous media with small pore size,the growth of microorganisms will be limited not only by space,but also by the number of themselves.(4)The pH change in the environment plays a critical role in determining the growth of microorganisms.The experimental results show that the bio-clogging effect is optirual when the pH value is 7.Once the pH is too high or too low,it is inevitably happened that enzymatic activity and cellular homeostasis are reduced,which is not conducive to microbial uptake of nutrients.(5)Salinity has little inhibition on bio-clogging.At the initial stage,hydraulic conductivity is insensitive to salinity change.In the later stage,the decreasing rate of hydraulic conductivity reduces with the increase of salinity.This is because the salinity stress can cause multifarious adverse effects on microorganism metabolism.(6)Different water head loading modes have significant effect on bio-clogging.The step increased water head experiment showed that the flow shear force with gradient increase can significantly increase the cell adhesion ability.However,a single loading of higher water head is not conducive to cell adhesion,resulting in a large number of microorganisms falling off in a short time.(7)The hydraulic conductivity,cutoff wall thickness,the leachate head,the diffusion coefficient and the retardation factor have great impact on the performance of the vertical barrier system.The significant retardation of contaminant was unable to observe in the fast flowing systems.Therefore,low hydraulic conductivity,low water head and thick cutoff wall,are essential for the vertical barrier systems to achieve desirable barrier performance.Under the condition of low permeability,the effects of diffusion coefficient and retardation factor on contamination transportation cannot be neglected.(8)Compared with the traditional cement-soil cutoff wall,the carbon emission of microbial improved cutoff wall is greatly reduced,which is conducive to the protection of the ecological environment.This is also the specific practice and direction of geotechnical engineering in the process of building a "resource-saving,environment-friendly" society. |
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