Utilization of Microbial Biofilms for Soil Improvement: Roles of Biofilm-mineral Interactions in Geomechanical Behaviors of Soils

岩土工程师关注的关键问题之一是如何改善土壤行为以减轻地质灾害。本研究探讨了利用原位微生物生物膜在矿物表面形成作为土壤改良的一种手段。尽管生物膜在广泛的工程应用中得到了广泛的研究，但目前关于地质材料中生物膜-矿物相互作用的知识仍处于起步阶段。提出的研究目标是量化生物膜与土壤矿物质之间的相互作用，并确定这种生物膜-矿物质相互作用对土壤力学特性的影响。主要重点是探索土壤细菌产生的生物膜包裹土壤矿物质并增加土壤的机械强度和刚度的假设，其程度将取决于土壤矿物质的物理化学性质，孔隙水的pH和离子强度，以及围应力。这项研究将是第一次从地质力学角度研究生物膜相关土壤的努力，并将使用各种多尺度实验技术，如原子力显微镜、x射线计算机微断层扫描、地球物理监测和三轴强度测试。这些技术的结合将允许检查矿物学和孔隙水化学对生物膜对矿物质的附着力的影响，孔隙空间中生物膜的生长模式，以及经历生物膜生长的土壤的剪切行为。因此，提出的研究的具体目的是通过表征生长在矿物表面的生物膜的组成和结构来选择一种模式细菌；在纳米尺度上量化模型细菌生物膜与土壤矿物质之间的相互作用；在微观尺度上可视化土壤孔隙中生物膜生长的三维形态模式；并在宏观尺度上确定生物膜的形成对土壤地质力学行为的影响。该研究的成功将通过确定与力学行为相关的最相关因素，调查土壤强度和刚度的增强程度，并为生物膜相关土壤的理论机制模型的发展提供实验数据，从而促进对生物膜形成在增强土壤行为中的作用的理解。社会将受益于对地质介质中生物膜形成的理解的进步，以及微生物生物膜的智能使用的发展，以改善非原位和原位土壤特性。生物膜的应用可以改善水文工程屏障；控制污染物的生物修复方法，以及减轻土壤侵蚀、泥石流和边坡不稳定的可持续土壤改良方法。此外，研究结果将对自然和工程多孔介质的工程应用产生广泛的影响，例如农业/农田近地表土壤中的生物质积累、天然油气生产过程中的生物堵塞、利用选择性堵塞技术提高微生物采收率、微生物燃料电池中的生物膜开发或生物燃料处理。本研究的跨学科性质为研究生和本科生提供了独特的经验和机会，将生物科学，表面物理化学，地球物理学与岩土工程相结合。此外，拟议的与高中教师的教育和推广活动将扩大代表性不足的群体和少数民族学生对工程的参与，加强他们的科学和工程基础，并激发他们对工程的兴趣。研究和教育成果将通过在同行评审期刊和专业会议上发表文章、在区域会议上与高中教师一起展示演示实践模块、以及通过大学网站传播演示实践模块的描述等方式广泛传播给公众和科学界。

One of the key concerns of Geotechnical engineers has been how to improve soil behavior to mitigate geo-hazards. This research addresses the utilization of in-situ microbial biofilm formation on mineral surfaces as a means of soil improvement. Despite extensive research efforts on biofilms in a wide range of engineering applications, current knowledge of biofilm-mineral interactions in geologic materials is still in its infancy. The objectives of the proposed research are to quantify the interactions between biofilms and soil minerals, and to identify the effects of such biofilm-mineral interactions on the mechanical properties of soils. The primary emphasis is to explore the hypothesis that the biofilms produced by soil bacteria coat soil minerals and increase the mechanical strength and stiffness of soils, the extent of which will be dependent on physicochemical properties of the soil mineral, pH and ionic strength of pore water, as well as confining stress. This research will be among the first efforts to investigate biofilm associated soils from a geomechanical point of view, and will use various multi-scale experimentation techniques such as atomic force microscopy, X-ray computed microtomography, geophysical monitoring, and triaxial strength testing. The combination of those techniques will allow examination of the effects of mineralogy and pore water chemistry on adhesion forces of biofilms to minerals, biofilm growth patterns in pore spaces, and shear behavior of soils undergoing biofilm growth. Therefore, the specific aims of the proposed research are to select a model bacterium via characterization of the composition and structures of biofilms grown on mineral surfaces; to quantify the interactions between model bacterium biofilms and soil minerals at the nanoscale; to visualize the three-dimensional morphological patterns of biofilm growth in pore spaces of soils at the microscale; and to identify the effect of biofilm formation on geomechanical behavior of soils at the macroscale. Success of the proposed research will advance the understanding of the roles of biofilm formation in enhancing soil behavior, by identifying the most relevant factor related to mechanical behavior, investigating the extent of enhancement of soil strength and stiffness, and providing experimental data for development of a theoretical mechanistic model of biofilm-associated soils. Society will benefit from the advancement in understanding of biofilm formation in geo-media and the development of the intelligent use of microbial biofilms to improve ex-situ and in-situ soil properties. The use of biofilms may improve hydrologic engineering barriers; controlled bio-remediation methods of contaminants, and sustainable soil improvement methods for mitigating of soil erosion, debris flows, and slope instability. Moreover, the outcome of the research will have broad impact related to engineering applications to natural and engineered porous media, such as biomass accumulation in near-surface soils of agricultural/farm lands, bio-clogging during natural oil and gas production, microbial enhanced oil recovery using selective plugging, biofilm development in microbial fuel cells, or biofuel processing. The cross disciplinary nature of this research gives graduate and undergraduate students unique experiences and opportunities that integrate bioscience, surface physical chemistry, and geophysics with geotechnical engineering. In addition, the proposed educational and outreach activities with high school teachers will broaden the participation of underrepresented groups and minority students in engineering, strengthen their scientific and engineering foundation, and stimulate their interest in engineering. The research and educational results will be broadly disseminated to the public and scientific community through publications at peer-reviewed journals and professional conferences, exhibition of demo hands-on modules with the high school teachers at regional conferences, and dissemination of the description of the demo hands-on modules through university websites.