BIOKINETIC PROCESSES OF EXTRACELLULAR POLYSACCHARIDE (EPS) STABILIZATION OF SURFACE SOILS AGAINST DUST GENERATION
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Abstract
Extracellular polysaccharide produced by a oligotrophic and nonpathogenic bacteria, Arthrobacter viscosus, promises to be an effective alternative to the use of chemical substances in dust control on exposed soil surfaces. The feasibility of this biokinetic stabilization approach to dust control depends in part on the capacity of injected microbes to produce EPS that can increase the resistance of soil to drying (desiccation) stresses. Initial laboratory based biokinetic investigations were performed to determine the rate of EPS production by Arthrobacter viscosus in both Haggstrom media (EPS production media) and sterilized samples of silty clay, sandy clay, and sandy silty clay soils and the effects of EPS on dusting resistance indices such as cohesion and retention of intergranular pore liquid. To achieve this objective, both Haggstrom media and the soil samples were inoculated with nutrient broth (20 to 100 ml/mL of Haggstrom media) containing Arthrobacter viscosus and changes in dusting resistance indices (soil cohesion, frictional resistance, and desiccation rate) in response to EPS growth were monitored. It was initially determined through tests that an optimum EPS quantity of 12.5 g/mL of Haggstrom media is produced by microbial broth concentration of 60 ml/mL of Haggstrom media. EPS-CM production rate in soil after initial injection of microbial broth concentrations (5 to 25 mL/g of soil) was tracked using thermogravimetric analysis (TGA), which has been shown to be an effective tool in determining the thermal decomposition of polymeric materials mixed with other composites. TGA results indicate that optimum EPS production in silty clay soil samples occurs at between 48 and 72 hr after soil injection with the highest EPS quantity determined to be 3.8 mg/mg of soil observed when a microbial broth concentration of 20 mL/g of soil is used. In sandy clay and sandy silty clay soils, EPS quantity of 2.5 mg/mg of soil and 3.3 mg/mg of soil occurred in both soils respectively. To further investigate the effectiveness of EPS-CM in surface soil stabilization against dust generation, a direct application of different concentrations (5 to 25 mL/g of soil) of extracted EPS from the Haggstrom media and an indirect application of extracellular polysaccharide-Culture Media (EPS-CM) to the soil through injection of microbial broth with cells of different concentrations (5 to 25 ml/g of soil) for in situ EPS production with time were compared using deionized water as control. Three soil mixes were used, which include silty clay soil (original sample), sandy clay soil, and sandy silty clay soil were prepared from the sieve analyses of the soil samples collected. As part of the characterization of these soil samples, their specific surface areas were determined to be 8.397 m2/g for silty clay soil; 8.121m2/g for sandy clay soil; and 8.193 m2/g for sandy silty clay soil.As an indirect measurement of the potential resistance of the stabilized soil to in situ stresses that can be caused by drying, direct shear and unconfined compression tests were performed on replicates of the treated soil samples. The equations developed in chapter 2 to compare the effects of EPS-CM treatment of soil friability indices, deformation resistance indices, coefficient of soil failure, and effective porosity were evaluated in chapter 8. The results of unconfined compression tests show that in EPS-CM amended silty clay soil samples, a strain of 0.34 to 0.20 from day 1 to day 3 occurred at EPS-CM concentration of 5 mL/g of soil but at higher EPS-CM concentrations, soil strain is observed to fluctuate with time. The least strain (0.25) occurs in silty clay soils treated with EPS-CM concentration of 25 mL/g of soil compared to sandy clay and sandy silty clay soils. Thus soils with higher specific surface and clay minerals can develop cohesion more effectively than coarser-grained soils following EPS-CM amendment. Desiccation tests performed on treated and control soil samples at 34 % relative humidity and temperature of 37 oC show that soil liquid content decreases with time. At relatively high EPS-CM concentrations of 15 to 25 mL/g of soil, EPS-CM-amended silty clay soils retain 5 % more liquid with time than sandy clay and silty clay soils. Fluorescence microscopic imaging of the treated soil samples clearly show the presence of EPS-CM as intergranular pore material and as smears on soil particles in EPS-CM-amended and microbial broth-amended soil samples whereas they are absent in the control samples. The effects of EPS-CM amendment of the following selected indices of soil resistance to dust generation from exposed ground surfaces were investigated (soil cohesion, frictional resistance, effective porosity, desiccation rate). Data show that effective porosity in EPS-CM amended silty clay soil decreases with time due to continued EPS production by A. viscosus while changes in effective porosity with time in sandy clay and sandy silty clay fluctuated with time and EPS-CM production. After a 21-day monitoring with sampling at three 7-day intervals, unconfined compression and direct shear tests indicate that increase in cohesion from 37 to 45 kN/m2 occurs in EPS-CM-amended silty clay soil at EPS-CM concentrations ranging 5 to 25 mL/g of soil. In sandy clay and sandy silty clay soils, maximum cohesion levels of 27 kN/m2 and 24 kN/m2 were observed, respectively, for the same EPS-CM concentrations within this sampling time while control samples show cohesion increments of only 0 to 15 kN/m2. Generally, it is observed that despite cyclical fluctuations in EPS-CM content in response to microbial dynamics in soil, frictional resistance decreases with increase in concentration of EPS-CM. Thus EPS-CM increase in intergranular pore space reduces intergranular friction but enhances cohesion within an overall increase in shear strength especially in fine grained soils that are prone to dusting. Liquid retention capacity, which is known to affect dust generation, improves favorably in EPS-CM-amended soils. With respect to practical use of duct control in the field, this research indicates that mixing of EPS-CM with microbial broth and scarified soil surfaces before compaction can be effective.